Drawing. Chain transmissions Toothed chains applications

Moreover, the chain itself includes numerous moving links. They are connected to each other in the form of a closed circle.

Typically, the number of teeth on a sprocket and the number of link elements in chains are determined by a mutually prime number. Thanks to this, the most uniform wear of the mechanism as a whole is ensured.

Advantages and disadvantages of chain transmission

In addition to chain drives, there are also belt drives. However, in most cases they resort to chain ones, since they have a number of important advantages:

No slippage, as happens in belt drives under certain conditions.
Can be provided high degree compactness of the mechanism.
The average gear ratio is at a constant level.
Due to the absence of such a phenomenon as pre-tension, there are no secondary loads on key components of the mechanism.
Even if the speed drops, the power figures remain quite high.
Chain transmissions practically insensitive to humidity and temperature changes.
You can quickly adapt such a transmission to almost any mechanism by adding or removing a chain link.
If necessary, you can transmit torque to several sprockets at once using just one chain.
It is possible to organize the transmission of torque over fairly long distances - up to 7 meters.
Chain transmission has a high efficiency - about 98 percent.
If necessary, failed links, the chain itself or sprockets can be quickly replaced.

However, chain drives also have certain disadvantages:

With prolonged intensive use, the hinges in the chain links wear out, which leads to stretching of the plates and an increase in the overall length of the chain.
The gear can be applied without the need to stop the movement during the reverse stroke.
The chain in some types of mechanisms is quite difficult to lubricate.
You can observe unevenness in the gear ratio and, as a consequence, unevenness in speed. This effect is especially noticeable if the sprocket does not have a large number of teeth.

All of the above should certainly be taken into account when making a choice between chain and belt types of transmissions.

What characteristics do chain drives have?

Among the most important characteristics of almost any chain transmissions are:

Chain pitch indicator - this parameter affects the smoothness and accuracy of movement. When decreasing this parameter accuracy and smoothness increases.
The number of teeth on the driving and driven sprockets.
Radii of the inscribed and circumscribed circles of stars.
The ratio of the radii of the driving and driven sprockets. Accordingly, the larger the diameter of the drive sprocket in relation to the driven one, the easier it will be to transmit movement.
The distance between the centers of the circles of the sprockets - for example, the length of the chain will depend on this.

All these points also need to be taken into account.

What does a chain drive consist of?

Chain drives are fairly simple mechanisms in terms of design. However, it will not be superfluous to know what elements they consist of.

Star. Typically, chain drives are designed with only two sprockets (although there are options). One of them acts as a leader, and the second as a slave. The stability and efficiency of operation of chain transmissions will largely depend on their quality and production accuracy: compliance with the dimensions (down to the millimeter) used in the manufacture of the material.

It is worth noting that the sizes and shapes of the sprockets will be determined by the quantitative characteristics of the chains (and not vice versa, as some people think), the number of gear ratios, and the number of teeth on the smallest drive sprocket in the mechanism. Parametric and other characteristics of sprockets are determined by GOST 13576 - 81. Characteristics of sprockets for roller and bushing chains are determined by GOST 591 - 69.

The sprockets must be made of sufficiently strong and wear-resistant materials that can be used for a long time under significant mechanical loads, including shock ones. According to GOST, such a material can be steel grades 40, 45, 40X and other types with a hardening degree of HRC 50 - 60. Sprockets not intended for high-speed mechanisms can be made from modified types of cast iron grades SCh 15, SCh 20.

Today you can find sprockets with tooth tips made from various types plastic. Such products are characterized by a reduced degree of wear and quiet operation.

The other component of chain drives is, of course, the chain. Chains are produced on industrial production lines. Their parameters are strictly regulated by relevant standards. Today the industry can offer such types of chains as:

Cargo - intended for raising and lowering loads and for hanging them. Such chains are usually used on various kinds forklifts.
Traction - they serve to move goods and are used in transport devices.
Drive - serve to transmit mechanical energy from one sprocket to another. A striking example of the use of such a transmission is the most ordinary bicycle and other types of vehicles.

The main elements of a standard circuit are shown in the figure below.

Circuit classification

Since drive chains are the most common type, it makes sense to take a closer look at what types of chains exist.

Roller chains (position III in the figure) include internal and external links. Those, alternating with each other, form mobile relative to each other serial connections. Each link includes two plates pressed onto axial or bushing supports. The bushings are put on the link axes, forming a hinge joint. To avoid increased wear on the sprockets, a roller is usually put on the bushing, which should replace sliding friction with rolling friction.

The ends of the chain can be connected to each other:

By means of connecting links - with an odd number of links.
Through a transition link - with an even number of links.

If the transmission must operate in intensive mode for a long time, then a multi-row roller chain is used. This allows you to reduce the size of each sprocket and its pitch.

Roller chains can also be made with curved plates on each link (position IV in the figure). This type is used if the connection is expected to be used under conditions of high shock loads. Thanks to the special shape of the plate, the impact force is significantly dampened.

Bush chains (position V) are structurally no different from roller chains, but do not have rollers. Thanks to this, the production of such chains becomes cheaper and their weight is reduced. But this also contributes to faster wear of the teeth.

Silent toothed chains (position VI in the figure) include special plates equipped with teeth. The plates themselves have a hinged connection. Thanks to this design, it is possible to ensure a low noise level of the mechanism, as well as smooth operation. In this case, the teeth are located at an angle of 60 degrees. These types of chains are used in mechanisms with high operating speeds. Therefore, the plate should be made of hardened steel with a hardness of H RC 40 - 45. The disadvantage of such chains can be considered their relative high cost, as well as the need for special care.

Hook chains (position VII). They include links of a special shape without any additional elements.

Bushing-pin chains (position VIII in the figure) - in them the links are connected using pins. This type of chain is used in a variety of fields agriculture and mechanical engineering.

Since any chain will stretch over time during intensive work, its tension should be periodically adjusted. This is achieved by moving one sprocket or two at once, depending on the design features of the adjustment mechanism. It allows, as a rule, adjustment if the chain has stretched by only one or two links. If the degree of stretching is greater, then the chain is simply replaced with a new one.

Don’t forget about timely lubrication of any chain. The duration of its work will directly depend on this. If the speed of movement of the chain is not too high - up to 4 meters per second, then lubrication is allowed using a regular manual oiler. At speeds up to 10 meters per second, a dropper oiler is used.

For deeper lubrication, the chain is immersed in a container filled with oil. The degree of chain immersion should not exceed the width of each plate.

If you have to deal with powerful high-speed mechanisms, then circulating jet lubrication using pumps is used.

When choosing one or another lubrication method, you must rely on design features each specific type of mechanism, as well as the nature of energy loss during friction. Friction losses occur due to the friction of hinge joints, plates against each other, between teeth and chain elements, and in supporting elements of the structure. In addition, there are losses due to splashing of the lubricant. True, they are significant only if lubrication is carried out by immersing the chains in lubricants and when operating at speeds close to the maximum permissible.

Application areas of chain transmission

It is noteworthy that this type of transmission has been known to mankind for quite a long time. At least in theory. A study of the works of the famous inventor and artist Leonardo da Vinci showed that he was thinking about various options the use of chain drives in all kinds of mechanisms. In the pictures you can see prototypes of modern bicycles and many other mechanisms known today. True, it is not known for certain whether the great Leonardo was able to put his ideas into practice. The industry of that time did not allow the manufacture of mechanisms with the required degree of accuracy.

For the first time in practice, it was possible to use this type of transmission only in 1832. It is worth noting that on appearance modern bicycle, as well as its technical and operational characteristics, were largely influenced by the fact that in 1876 the inventor Lawson came up with the idea of using a chain drive. Until then, the wheels were driven either directly through the pedals, or the rider had to push off the ground with his feet.

This type of gear in various modifications is used today extremely widely in various fields mechanical structure. Transport, industrial machine tools, agricultural units - it is not possible to list all the mechanisms in which types of chain transmission are used, without exception.

They also resort to it when the interaxle distances are sufficiently large. In these cases, the use of a belt-type transmission is impractical, and it is impossible to use gear ones due to the significant complication of the design and the increase in the mass of the mechanism. Don’t forget about the friction force, which increases in direct proportion to the number of gears in the mechanism. In the case of chain drives, as already noted, there is a rolling friction force, which is several times less than the sliding friction force.

You can also find this type of gear in technology that uses a chain as a direct working element, and not as a drive element. These, for example, include snow removal units, elevator and scraper mechanisms, and similar ones.

As a rule, they resort to open-type chain drives, which, if necessary, are lubricated manually. In such structures there is either no moisture and dust protection at all, or it is present at a minimal level, as is the case with a bicycle.

Typically, certain types of chain transmissions are used if it is necessary to transfer powers of up to 120 kilowatts at external speeds of no more than 15 meters per second.

A little about stars

The efficiency and longevity of the entire chain mechanism will depend to a large extent on how the sprockets in the mechanism were made. This applies to both compliance with all exact dimensions and materials of manufacture.

The number of teeth is one of the most important characteristics of any sprocket.

The tension sprocket is used where it is necessary to prevent the effect of chain slack. It is usually installed on the driven parts of mechanisms.

The main parametric characteristics of sprockets are described in the relevant paragraphs of GOST 13576-81.

Chain transmissions are a truly highly efficient and, at the same time, economical type of mechanism. They are used in many areas of transport and mechanical engineering.

Types of chain transmission

Today you can encounter a variety of classifications of this type of transmission. It all depends on what specific criteria are used to classify:

According to their purpose, transmissions can be traction, drive, or cargo.
Complex or simple - if you classify by the total number of sprockets in the mechanism. Complex mechanisms are usually classified as those that contain more than two sprockets.
Also, transmissions can be master and slave.
If we classify gears based on the direction of rotation, then they can be direct and reverse.
According to the principle of arrangement, they can be closed, horizontally or vertically located.
Also, the sprockets can be centered differently. In this case, it is customary to distinguish between horizontally located and vertically located gears, as well as at a certain angle.
Low and high gears - according to the speed.
Open and closed transmission types - depending on whether they are placed in dustproof housings or not. Closed-type gears can also be placed inside a mechanism, the housing of which protects them from the penetration of dust and moisture.
Finally, according to the method of introducing lubricant, transmissions can be manual, oil and circulating. Their specifics have already been mentioned a little above.

Each of these types is used in certain areas of technology.

§ 1. GENERAL INFORMATION

A chain drive consists of a drive and driven sprocket and a chain that surrounds the sprockets and engages with their teeth. Chain drives with several driven sprockets are also used. In addition to the main elements listed, chain drives include tensioners, lubrication devices and guards.

The chain consists of links connected by hinges, which provide mobility or "flexibility" of the chain.

Chain transmissions can be performed in a wide range of parameters.

Chain drives are widely used in agricultural and lifting transport vehicles ah, oil drilling equipment, motorcycles, bicycles, cars.

In addition to chain drives, mechanical engineering uses chain devices, i.e. chain drives with working elements (buckets, scrapers) in conveyors, elevators, excavators and other machines.

The advantages of chain drives include: 1) the possibility of use in a significant range of center distances; 2) smaller dimensions than belt drives; 3) no slipping; 4) high efficiency; 5) small forces acting on the shafts, since there is no need for a large initial tension; 6) the ability to easily replace the chain; 7) the ability to transfer movement to several sprockets.

At the same time, chain drives are not without drawbacks: 1) they operate in the absence of fluid friction in the joints and, consequently, with their inevitable wear, which is significant due to poor lubrication and the ingress of dust and dirt; wear of the hinges leads to an increase in the pitch of the links and the length of the chain, which necessitates the use of tensioning devices; 2) they require higher accuracy of shaft installation than V-belt drives, and more complex maintenance - lubrication, adjustment; 3) transmissions require installation on crankcases; 4) the speed of the chain, especially with a small number of sprocket teeth, is not constant, which causes fluctuations in the gear ratio, although these fluctuations are small (see § 7).

Chains used in mechanical engineering are divided into two groups according to the nature of the work they perform: drive and traction. The chains are standardized and produced in specialized factories. The production of drive chains alone in the USSR exceeds 80 million m per year. More than 8 million cars are equipped with them annually.

Roller, bushing and toothed chains are used as drive chains. They are characterized by small steps (to reduce dynamic loads) and wear-resistant hinges (to ensure durability).

The main geometric characteristics of chains are pitch and width, the main power characteristic is the breaking load, established experimentally. In accordance with international standards, chains are used with pitches that are multiples of 25.4 mm (i.e. ~ 1 inch)

The following drive roller and bushing chains are manufactured in the USSR in accordance with GOST 13568-75*:

PRL - single-row roller of normal accuracy;

PR - high precision roller;

PRD - long-link roller;

PV - sleeve;

PRI - roller with curved plates,

as well as roller chains in accordance with GOST 21834-76* for drilling rigs (in high-speed gears).

Roller chains are chains with links, each of which is made of two plates pressed onto rollers (outer links) or bushings (inner links). The bushings are put on the shafts of the mating links and form hinges. External and internal links in the chain alternate.

The bushings, in turn, carry rollers that fit into the recesses between the teeth on the sprockets and engage with the sprockets. Thanks to the rollers, sliding friction between the chain and the sprocket is replaced by rolling friction, which reduces wear on the sprocket teeth. The plates are outlined with a contour reminiscent of the number 8 and bringing the plates closer to bodies of equal tensile strength.

The rollers (axles) of the chains are stepped or smooth.

The ends of the rollers are riveted, so the chain links are one-piece. The ends of the chain are connected by connecting links with the rollers secured with cotter pins or riveting. If it is necessary to use a chain with an odd number of links, special transition links are used, which, however, are weaker than the main ones;

Therefore, they usually tend to use chains with an even number of links.

For high loads and speeds, multi-row chains are used to avoid the use of chains with large pitches, which are unfavorable with respect to dynamic loads. They are made up of the same elements as single-row ones, only their edges have an increased length. The transmitted powers and destructive loads of multi-row circuits are almost proportional to the number of rows.

The characteristics of roller chains with increased precision PR are given in table. 1. Roller chains of normal precision PRL are standardized in the pitch range of 15.875...50.8 and are designed for a breaking load that is 10...30% less than that of lump precision chains.

Long-link roller chains of the PRD are performed in double pitch compared to conventional roller chains. Therefore, they are lighter and cheaper than regular ones. It is advisable to use them at low speeds, in particular, in agricultural engineering.

PV bushing chains are identical in design to roller chains, but do not have rollers, which reduces the cost of the chain and reduces the dimensions and weight with an increased hinge projection area. These chains are manufactured with a pitch of only 9.525 mm and are used, in particular, in motorcycles and in cars (camshaft drive). The circuits show sufficient performance.

Roller chains with curved PRI plates are assembled from identical links similar to the transition link (see Fig. 12.2, e). Due to the fact that the plates bend and therefore have increased compliance, these chains are used under dynamic loads (impacts, frequent reverses, etc.).

The designation of a roller or bushing chain indicates: type, pitch, breaking load and GOST number (for example, Chain PR-25.4-5670 GOST 13568 -75*). For multi-row chains, the number of rows is indicated at the beginning of the designation.

Toothed chains (Table 2) are chains with links made from sets of plates. Each insert has two teeth with a cavity between them to accommodate the sprocket tooth. The working (outer) surfaces of the teeth of these plates (the surfaces of contact with the sprockets are limited by planes and inclined to one another at a wedging angle a equal to 60°). With these surfaces, each link sits on two teeth of the sprocket. The sprocket teeth have a trapezoidal profile.

The plates in the links are spaced apart to the thickness of one or two plates of the mating links.

Currently, chains with rolling joints are mainly manufactured, which are standardized (GOST 13552-81*).

To form hinges, prisms with cylindrical working surfaces are inserted into the holes of the links. The prisms rest on the flats. With special profiling of the holes of the plates and the corresponding surfaces of the prisms, it is possible to obtain almost pure rolling in the hinge. There is experimental and operational data that the service life of gear chains with rolling joints is many times higher than that of chains with sliding joints.

To prevent the chain from sliding sideways from the sprockets, guide plates are provided, which are ordinary plates, but without recesses for the sprocket teeth. Use internal or side guide plates. Internal guide plates require a corresponding groove to be machined into the sprockets. They provide better guidance at high speeds and are of primary use.

The advantages of toothed chains compared to roller chains are lower noise, increased kinematic accuracy and permissible speed, as well as increased reliability associated with a multi-plate design. However, they are heavier, more difficult to manufacture and more expensive. Therefore, they have limited use and are being replaced by roller chains.

Traction chains are divided into three main types: plate according to GOST 588-81*; collapsible according to GOST 589 85; round-link (normal and increased strength), respectively, according to GOST 2319-81.

Leaf chains are used to move goods at any angle to the horizontal plane in transporting machines (conveyors, elevators, escalators, etc.). They usually consist of plates of simple shape and axles with or without bushings; they are characterized

large steps, since side plates are often used to secure the conveyor belt. The speed of movement of chains of this type usually does not exceed 2...3 M/S.

Round link units They are mainly used for hanging and lifting loads.

There are special chains that transmit movement between sprockets with mutually perpendicular axes. The rollers (axes) of two adjacent links of such a chain are mutually perpendicular.

§ 3. BASIC PARAMETERS OF DRIVE CHAIN TRANSMISSIONS

The power for transmission of which chain transmissions are used varies in the range from fractions to hundreds of kilowatts, in general mechanical engineering usually up to 100 kW. Center distances of chain drives reach 8 m.

Sprocket rotation speeds and speeds are limited by the magnitude of the impact force generated between the sprocket tooth and the chain joint, wear and gear noise. The highest recommended and maximum speeds of sprocket rotation are given in table. 3. Chain speeds usually do not exceed 15 m/s, but in transmissions with high-quality chains and sprockets with effective lubrication methods they reach 35 m/s.

Average chain speed, m/s,

V=znP/(60*1000)

where z is the number of sprocket teeth; n speed of its rotation, min-1; R-

The simplest chain drive (Fig. 3) consists of two sprockets (1 and 2), each attached to its own shaft, the smaller of which is most often the driving one, and an enclosing chain 3, composed of many rigid links that can rotate relative to each other friend.

Chain drives are widely used in machines for general industrial purposes.

Chain drives are widely used in various lifting (for example, multi-bucket elevators) and transport devices. The use of chain drives in these cases simplifies the design of machine components, increases their reliability and productivity. These devices use circuits of a wide variety of design types.

Chain drives are used both to reduce (reduce speed during transmission) rotational motion and to multiply it (increase speed).

Advantages of chain drives: 1. Possibility of transmitting motion over fairly large distances (up to 8 m). 2. The ability to transmit movement by one chain to several shafts. 3. No slippage, and therefore stability of the gear ratio with reduced lateral load on the shafts and their supports. 4. Relatively high efficiency (0.96...0.98 with sufficient lubrication).

Disadvantages of chain drives: 1. Increased noise and vibration activity during operation due to pulsation of chain speed and resulting dynamic loads. 2. Intensive wear of chain hinges due to impact interaction with the sprocket cavity, sliding friction in the hinge itself and difficulty in lubrication. 3. Chain stretching (increasing the pitch between the link hinges) due to wear of the hinges and elongation of the plates. 4. Relatively high cost.

Classification:

Circuits according to their intended purpose can be divided into:

1. traction chains designed to move loads along a horizontal or inclined surface;

2. load chains designed for lifting loads;

3. drive chains designed to transmit motion, most often rotational, in chain drives.

The most widely used drive chains are roller, bushing and toothed chains. These three types of chains are standardized.

8. Gears, diagrams, purpose, advantages, disadvantages, classification.

Gear- a three-link mechanism, including two movable links interacting with each other through a higher gear kinematic pair and forming lower (rotational or translational) kinematic pairs with the third fixed link

Rice. 1. Types of gears

The smaller gear involved in meshing is usually called gear, more – gear wheel, a gear link that makes a linear movement is called a gear rack (Fig. 1, j).

Rice. 2. Gear diagram and its parameters

The purpose of a gear transmission is to transmit motion (most often rotational) with the transformation of parameters, and sometimes its type (rack and pinion transmission). Rotary gears are the most common in technology (Fig. 5). They are characterized by transmitted powers from microwatts (quartz wristwatch mechanism) to tens of thousands of kilowatts (large ball mills, crushers, kilns) at peripheral speeds of up to 150 m/s.

Advantages of gears:

1. High reliability of operation in a wide range of loads and speeds.

2. Great resource.

3. Small dimensions.

4 High efficiency.

5. Relatively low loads on shafts and bearings.

6. Constancy of the gear ratio.

7. Easy to maintain.

Disadvantages of gears:

1. Difficulty in manufacturing and repair (requires high-precision specialized equipment).

2. Regarding high level noise, especially at high speeds.

3. Irrational use of teeth - usually no more than two teeth of each of the meshing wheels are simultaneously involved in the operation of the transmission.

Gear classification:

1. By gear ratio:

1.1. with gear ratio u >1 – reducing (reducers - most gears);

1.2. with gear ratio u<1 – мультиплицирующие (мультипликаторы).

2. According to the relative position of the shafts:

2.1. with parallel shafts - spur gears

2.2. with intersecting shaft axes - bevel gears

(bevel gears with an angle of 90 degrees between the shaft axes are called orthogonal);

2.3. with intersecting shaft axes - worm, screw (Fig. 5, i), hypoid;

2.4. with motion conversion – rack and pinion

3. According to the location of the teeth relative to the generatrix of the wheel surface:

3.1. straight teeth - the longitudinal axis of the tooth is parallel to the generatrix of the wheel surface;

3.2. helical - the longitudinal axis of the tooth is directed at an angle to the generatrix of the wheel surface;

3.3. chevron - the tooth is made in the form of two helical wheels with an opposing inclination of the tooth axes;

3.4. with a circular tooth - the axis of the tooth is made in a circle relative to the generatrix of the wheel surface.

4. According to the shape of the interlocking links:

4.1. with external gearing - the teeth are directed with their tops away from the axis of rotation of the wheel;

4.2. with internal gearing - the teeth of one of the meshing wheels are directed with their vertices towards the axis of rotation of the wheel;

4.3. rack and pinion gearing - one of the wheels is replaced by a straight toothed rack;

4.4. with non-round wheels.

5. According to the shape of the working tooth profile:

5.1. involute - the working profile of the tooth is outlined along the involute of a circle (a line described by a point of a straight line rolling without sliding around a circle);

5.2. cycloidal - the working profile of the tooth is outlined along a circular cycloid (a line described by a point on a circle rolling without sliding along another circle);

5.3. lantern (a type of cycloidal) - the teeth of one of the wheels that engage are replaced by cylindrical pins - lanterns;

5.4. with a circular tooth profile (Novikov gearing) – the working tooth profiles are formed by circular arcs of almost identical radii.

6. According to the relative mobility of the geometric axes of gear wheels:

6.1. with fixed wheel axles - ordinary gears (Fig. 5);

6.2. with movable axles of some wheels - planetary gears.

7. According to the rigidity of the gear ring of the wheels that engage:

7.1. with wheels of unchangeable shape (with a rigid crown);

7.2. including wheels with a crown of changing shape (flexible).

8. According to the circumferential (tangential) speed of the teeth:

8.1. low-speed (Vз< 3 м/с);

8.2. medium speed (3< Vз < 15 м/с);

8.3. high-speed (Vз > 15 m/s).

9. By design:

9.1. open (unframed);

9.2. closed (cased).

The most widely used are reducing gears of rotational motion, including in multi-purpose tracked and wheeled vehicles (gearboxes, final drives, drives various devices). Therefore, the further presentation, unless specifically mentioned, concerns only rotational motion transmissions.

The transfer of energy between two or more parallel shafts, carried out by meshing with the help of a flexible endless chain and sprockets, is called chain.

The chain drive consists of a chain and two sprockets - drive 1 (Fig. 190) and driven 2, operates without slipping and is equipped with tensioning and lubrication devices.

Chain drives make it possible to transmit motion between shafts over a significant range of center distances compared to gear drives; have a fairly high efficiency of 0.96...0.97; exert less load on the shaft than in a belt drive; One chain transmits rotation to several sprockets (shafts).

The disadvantages of chain drives include: some uneven running, noise during operation, the need for careful installation and maintenance; the need to adjust chain tension and timely lubrication; rapid wear of chain joints; high cost; pulling the chain during operation, etc.

Chain drives are most widespread in various machine tools, bicycles and motorcycles, in hoisting and transport machines, winches, in drilling equipment, in the running gears of excavators and cranes, and especially in agricultural machines. For example, the S-4 self-propelled grain combine has 18 chain drives that drive a number of its working parts. Chain drives are often found in textile and cotton industries.

Chain parts

Asterisks. The operation of a chain transmission largely depends on the quality of the sprockets: the accuracy of their manufacture, the quality of the tooth surface, material and heat treatment.

The design dimensions and shape of the sprockets depend on the parameters of the selected chain and the gear ratio, which determines the number of teeth of the smaller drive sprocket. Options and quality characteristics sprockets are installed according to GOST 13576-81. The sprockets of roller and bushing chains (Fig. 191, I) are profiled according to GOST 591-69.

The working profile of the sprocket tooth for roller and bushing chains is outlined by an arc corresponding to a circle. For toothed chains, the working profiles of the sprocket teeth are straight. In cross section, the profile of the sprocket depends on the number of rows of the chain.

The sprocket material must be wear-resistant and able to withstand impact loads. Sprockets are made from steels 40, 45, 40Х and others with hardening to a hardness of HRC 40...50 or case-hardened steel 15, 20, 20Х and others with hardening to a hardness of HRC 50...60. For sprockets of low-speed gears, gray or modified cast iron SCh 15, SCh 20, etc. is used.

Currently, sprockets with a ring gear made of plastic are used. These sprockets are characterized by reduced chain wear and low noise during transmission operation.

Chains. Chains are manufactured in special factories, and their design, dimensions, materials and other indicators are regulated by standards. According to their purpose, chains are divided into the following types:

load chains (Fig. 192,I) used for suspending, lifting and lowering loads. Mainly used in lifting machines;
traction chains (Fig. 192, II), used to move goods in transporting machines;
drive chains used to transmit mechanical energy from one shaft to another.

Let's take a closer look at the drive chains used in chain drives. The following types of drive chains are distinguished: roller, bushing, toothed and hook.

Roller chains(Fig. 192, III) consist of alternating external and internal links, which have relative mobility. The links are made of two plates pressed onto an axle (outer links) or onto bushings (inner links). The bushings are put on the axes of the mating links and form hinges. To reduce wear on the sprockets when chains run against them, rollers are put on the bushings, which replace sliding friction with rolling friction (Fig. 191, II and III).

The axles (rollers) of the chains are riveted and the links become one-piece. The ends of the chain are connected: if the number of links is even, use a connecting link, and if the number is odd, use a transition link.

At high loads and speeds, multi-row roller chains are used to reduce the pitch and diameter of the sprockets.

Roller chains with curved plates (Fig. 192, IV) consist of identical links, similar to the transition link. These chains are used when the transmission operates with a shock load (reversing, jolts). The deformation of the plates helps to absorb shocks that occur when the chain engages with the sprocket.

Bush chains(Fig. 192, V) are no different in design from the previous ones, but do not have rollers, which leads to increased wear of the teeth. The absence of rollers reduces the cost of the chain and reduces its weight.

Bush chains, like roller chains, can be single-row or multi-row.

Toothed (silent) chains(Fig. 192, VI) consist of a set of plates with teeth, hingedly connected in a certain sequence. These chains ensure smooth and quiet operation. They are used at significant speeds. Toothed chains are more complex and more expensive than roller chains and require special care. The working faces of the plates, which receive pressure from the sprocket teeth, are the planes of the teeth located at an angle of 60°. To ensure sufficient wear resistance, the working surfaces of the plates are hardened to a hardness of H RC 40...45.

In order to prevent the toothed chains from slipping off the sprockets during operation, they are equipped with guide plates (lateral or internal).

Hook chains(Fig. 192, VII) consist of identical links of a special shape and do not have any additional parts. The connected links are separated at a mutual inclination at an angle of approximately 60°.

Pin and pin chains(Fig. 192, VIII) are assembled from links using pins made from StZ steel. The pins are riveted, and in the connecting links they are secured with cotter pins. These chains find wide application in agricultural engineering.

To ensure good chain performance, the materials of its elements must be wear-resistant and durable. For plates, steel 50 and 40X is used and hardened to a hardness of HRC35...45, for axles, rollers and bushings - steel 20G, 20X, etc. with a hardness of HRC54...62-, for rollers - steel 60G with a hardness of HRC48.. .55.

Due to the wear of the hinges, the chain gradually stretches. The chain tension is adjusted by moving the axis of one of the sprockets, using adjusting sprockets or rollers. Typically, tensioning devices allow you to compensate for the elongation of the chain within two links; when the chain for a link is extended more, it is removed.

The longevity of the chain largely depends on the correct use of lubricant. When the chain speed (v) is equal to or less than 4 m/s, periodic lubrication is used, which is carried out with a manual oiler every 6...8 hours. When v s 10 m/s, lubrication is used with oil droppers. More perfect lubrication is by dipping the chain into an oil bath. In this case, the immersion of the chain in oil should not exceed the width of the plate. In powerful high-speed gears, circulating jet lubrication from a pump is used.

Chain transmissions: advantages and disadvantages, classification. Drive chain designs

Chain transmission is based on the meshing of a chain and sprockets. The principle of engagement rather than friction, as well as the increased strength of a steel chain compared to a belt, make it possible for the chain to transmit large loads, all other things being equal. The absence of slipping ensures a constant average gear ratio.

The gearing principle does not require pre-tensioning of the chain, which reduces the load on the shafts and supports. Chain drives can operate at smaller center distances and at larger gear ratios, and also transmit power from one drive shaft to several driven ones.

The main reason for the disadvantages of chain transmission is that the chain consists of individual rigid links and is located on the sprocket not in a circle, but in a polygon. This is associated with inconstancy of chain speed within one revolution, wear of chain joints, noise and additional dynamic loads. In addition, the chain is more expensive and more difficult to manufacture.

The main types of drive chains are roller, bushing (GOST 13568-75) and gear chains GOST 13552-81).

The roller chain consists of two rows of outer (1) and inner (2) plates. Rollers (3) are pressed into the outer plates and passed through bushings (4). The bushings are pressed into the holes internal plates. The bushing on the roller and the roller on the bushing can rotate freely.

The use of a bushing allows you to distribute the load along the entire length of the roller and thereby reduce wear on the hinges. Along with single-row chains, two-, three- and four-row chains are produced. They are assembled from the same elements, only the roller goes through all the rows.

Bushing chains are similar in design to roller chains, but they do not have a roller (5). As a result, wear on the chain and sprockets increases, but the weight and cost of the chain decreases.

Toothed chains consist of a set of plates with two tooth-like protrusions. The chain plates engage the sprocket teeth with their end planes. The wedging angle is assumed to be 60.

The design of toothed chains allows them to be made wide and transmit large loads. They operate smoothly with less noise. They are recommended for use at relatively high speeds - up to 35 m/s.

Chain transmissions are transmissions by engagement and flexible coupling, consisting of a leading 1 and driven 2 sprockets and a chain 3 covering them. The transmission also often includes tensioning and lubrication devices, and guards. It is possible to use several driven sprockets. The chain consists of links connected by hinges, which ensures the flexibility of the chain. Gears are used in agricultural, material handling, textile and printing machines, motorcycles, bicycles, cars, and oil drilling equipment.

> Circuit types

Chains are divided into three groups according to their intended purpose:

1. cargo - used to secure cargo;

2. traction - used to move goods in continuous transport machines (conveyors, elevators, escalators, etc.);

3. drive - used to transmit movement.

Main types of chains: round-link cargo chains, lamellar hinged; traction plate; driven single-row roller, double-row roller, roller with curved plates, bushing, gear with internal guide plates, gear with side guide plates, shaped hook link, shaped bush-pin type. Load and traction chains are covered in detail in the Material Handling Machines course; this course focuses on drive chains.

The main geometric characteristic of the chain is the pitch P - the distance between the axes of adjacent hinges. Most standard chains have pitches that are multiples of 1 inch (25.4 mm).

The most widely used are roller chains, which are formed from sequentially alternating internal and external links. The internal links consist of internal plates 1 and smooth bushings 2 pressed into their holes, on which rollers 3 rotate freely. The external links consist of external plates 4 and rollers 5 pressed into their holes. The ends of the rollers are riveted after assembly. Due to the tension in the connections of the outer plates with the rollers and the inner plates with the bushings and the gap between the roller and the bushing, a hinge joint is formed. To increase fatigue resistance, the interference values are taken to be significantly greater than those provided for by standard fits. Plastic deformation of the plates in the area of the holes, inevitable with such large interferences, significantly increases the fatigue resistance of the plates (1.6...1.7 times). Multi-row chains with a number of rows from two to eight are assembled from parts with the same dimensions as single-row chains, except for the rollers having corresponding longer length. The load capacity of the chains is almost directly proportional to the number of rows, which makes it possible to reduce the pitch, radial dimensions of sprockets and dynamic loads in transmissions with multi-row chains.

For large dynamic, in particular, frequent reverses, roller chains with curved plates are used. Due to the fact that the plates bend, they have increased compliance.

When operating chain drives in conditions that cause an increase in friction in the hinges (dusty and chemically active environments), open-joint plate chains are used. Being open, the hinge of such a chain self-cleanses itself from abrasive particles entering it. The outer links of such a chain do not differ from similar links of a roller chain. The internal links are formed from plates 2, having holes in the shape of a figure eight, and shaped rollers 3, replacing the bushing. Roller 4 freely passes through the hole in plate 2 and interacts with the shaped roller 3. Replacing the thin-walled bushing and roller not only reduces the cost of the chain, but also sharply increases the fatigue resistance of the chain parts. Thanks to this, open-joint chains turned out to be much more durable than roller chains when working in heavily loaded gears.

Toothed chains have now been replaced by cheaper and more technologically advanced precision roller chains, which are not inferior to gear chains in kinematic accuracy and noise characteristics. Toothed chains are used primarily to replace broken chains in old equipment. Due to limited application, toothed chains are not considered.

The ends of roller, bushing and open-joint chains are connected into a closed circuit using connecting and transition links. The connecting link, used when there is an even number of chain links, differs from the usual outer one in that one of its plates fits freely onto the ends of the rollers and is secured to the rollers with locks and cotter pins. If it is necessary to use a chain with an odd number of links, curved transition links are used, which are the weak point of the chain.

The designation of drive chains indicates the number of rows of the chain (if there is more than one), the type of chain, its pitch and destructive force. An example of designation in accordance with GOST 13568-75 - 2PR-25.4-114000 - double-row drive roller chain with a pitch of 25.4 mm and a destructive force of 114000 N.

Moscow State Institute

Electronics and Mathematics

(Technical University)

in the course "Machine parts"

and design fundamentals"

"Chain transmissions"

Moscow 1998

§ 1. GENERAL INFORMATION

A chain drive consists of a drive and driven sprocket and a chain that surrounds the sprockets and engages their teeth. Chain drives with several driven sprockets are also used. In addition to the main elements listed, chain transmissions include tensioning devices, lubrication devices and guards.

The chain consists of links connected by hinges, which provide mobility or “flexibility” of the chain.

Chain transmissions can be performed in a wide range of parameters.

Chain drives are widely used in agricultural and hoisting machines, oil drilling equipment, motorcycles, bicycles, and cars.

In addition to chain drives, mechanical engineering uses chain devices, i.e. chain drives with working bodies (buckets, scrapers) in conveyors, elevators, excavators and other machines.

Chains used in mechanical engineering, according to the nature of the work they perform are divided into two groups: drive and traction. The chains are standardized and produced in specialized factories. The production of drive chains alone in the USSR exceeds 80 million m per year. More than 8 million cars are equipped with them annually.

Roller, bushing and toothed chains are used as drive chains. They are characterized by small steps (to reduce dynamic loads) and wear-resistant hinges (to ensure durability).

The following drive roller and bushing chains are manufactured in the USSR in accordance with GOST 13568-75*:

PRL - single-row roller of normal accuracy;

PR - high precision roller;

PRD - long-link roller;

PV - sleeve;

PRI - roller with curved plates,

as well as roller chains in accordance with GOST 21834-76* for drilling rigs (in high-speed gears).

The rollers (axles) of the chains are stepped or smooth.

Therefore, they usually tend to use chains with an even number of links.

The plates in the links are spaced apart to the thickness of one or two plates of the mating links.

Currently, chains with rolling joints are mainly manufactured, which are standardized (GOST 13552-81*).

large steps, since side plates are often used to secure the conveyor belt. The speed of movement of chains of this type usually does not exceed 2...3 M/S.

Round link units They are mainly used for hanging and lifting loads.

There are special chains that transmit motion between sprockets with mutually perpendicular axes. The rollers (axes) of two adjacent links of such a chain are mutually perpendicular.

Sprocket rotation frequencies and speeds are limited by the magnitude of the impact force generated between the sprocket tooth and the chain joint, wear and gear noise. The highest recommended and maximum speeds of sprocket rotation are given in table. 3. Chain speeds usually do not exceed 15 m/s, but in transmissions with high-quality chains and sprockets with effective lubrication methods they reach 35 m/s.

Average chain speed, m/s,

V=znP/(60*1000)

where z is the number of sprocket teeth; n speed of its rotation, min -1; R-

The gear ratio is determined from the condition of equality of the average speed of the chain on the sprockets:

z1n1P=z2n2P

Hence the gear ratio, understood as the ratio of the rotational speeds of the driving and driven sprockets,

U=n1/n2=z2/z1,

Where n1 And p2- rotation speed of the driving and driven sprockets, min -1; z1 and z2 - number of teeth of the driving and driven sprockets.

The gear ratio is limited by the dimensions of the gear, the angles of engagement and the number of teeth. Usually u£7. In some cases, in low-speed transmissions, if space allows, u £ 10.

Number of sprocket teeth. The minimum number of sprocket teeth is limited by joint wear, dynamic loads, and gear noise. The smaller the number of sprocket teeth, the greater the wear, since the angle of rotation of the link when the chain runs onto the sprocket and runs away from it is 360°/z.

As the number of teeth decreases, the unevenness of the chain speed and the speed at which the chain hits the sprocket increase. The minimum number of teeth of roller chain sprockets depending on the gear ratio is selected according to the empirical dependence

Z1min=29-2u³13

Depending on the rotation speed, z1min is selected at high rotation speeds z1min=19...23; average 17...19, and at low 13...15. In transmissions with toothed chains, z1min is 20...30% more.

As the chain wears, its hinges rise along the sprocket tooth profile from the leg to the top, which ultimately leads to a breakdown in engagement. In this case, the greater the number of sprocket teeth, the smaller the maximum permissible increase in chain pitch. Therefore, the maximum number of teeth is limited when using roller chains of 100...120, and gear chains of 120...140.

Preferably choose odd number sprocket teeth (especially small ones), which, in combination with an even number of chain links, promotes uniform wear. It is even more favorable, from a wear point of view, to select the number of small sprocket teeth from the range prime numbers.

Distance between sprockets and chain length. The minimum center distance amin (mm) is determined from the conditions:

no interference (i.e. crossing) of stars

amin>0.5(De1+De2)

where De1 and De2 - outer diameters of sprockets;

so that the angle of the chain around the small sprocket is greater than 120°, i.e., the angle of inclination of each branch to the transmission axis is less than 30°. And since sin30°=0.5, then amin> d2-d1.

Optimal center distances

a = (30... 50) R.

Typically, it is recommended to limit the interaxle distances to

Amax=80P

The required number of chain links W is determined by the pre-selected center distance A, step by step R and the number of sprocket teeth z1 and z2:

W=(z1+z2)/2+2a/P+((z2-z1)/2p) 2 P/a;

the resulting W value is rounded to the nearest integer (preferably even) number.

This formula is derived By analogies with the formula for belt length and is approximate. The first two terms of the formula give the required number of links at z1=z2, when the branches of the chain are parallel, the third term takes into account the inclination of the branches.

The distance between the axes of the sprockets according to the selected number of chain links (without taking into account the sag of the chain) follows from the previous formula.

The chain should have some slack to avoid increased load from gravity and radial runout of the sprockets.

To do this, the center distance is reduced by (0.002... 0.004) A.

The chain pitch is taken as the main parameter of valuable transmission. Chains with large pitches have a greater load-bearing capacity, but allow significantly lower rotation speeds, they operate with high dynamic loads and noise. You should choose a chain with the minimum pitch allowed for a given load. Typically a/80£P£a/25; It is possible to reduce the pitch of toothed chains during design by increasing its width, and for roller chains - by using multi-row chains. Acceptable steps according to the transmission speed criterion follow from Table. 3.

Chain drives fail for the following reasons: 1. Wear of the hinges, leading to elongation of the chain and disruption of its engagement with the sprockets (the main performance criterion for most gears).

2. Fatigue failure of the plates along the lugs is the main criterion for high-speed, heavily loaded roller chains operating in closed crankcases with good lubrication.

3. Rotating the rollers and bushings in the plates at the press-in points is a common cause of chain failure, associated with insufficiently high quality workmanship.

4. Spalling and destruction of rollers.

5. Achieving the maximum sag of the idle branch is one of the criteria for gears with an unregulated center distance, operating in the absence of tensioning devices and in cramped dimensions.

6. Wear of sprocket teeth.

In accordance with the above reasons for the failure of chain transmissions, we can conclude that the service life of the transmission is most often limited by the durability of the chain.

The durability of the chain primarily depends on the wear resistance of the hinges.

The material and heat treatment of chains are critical to their longevity.

The plates are made from medium-carbon or alloy hardenable steels: 45, 50, 40Х, 40ХН, ЗОХНЗА with a hardness of predominantly 40...50HRCe; gear chain plates are predominantly made from 50 steel. Curved plates are usually made from alloy steels. The plates, depending on the purpose of the chain, are hardened to a hardness of 40-50 HRCe. Hinge parts - rollers, bushings and prisms - are made mainly from case-hardened steels 15, 20, 15Х, 20Х, 12ХНЗ, 20ХИЗА, 20Х2Н4А, ZОХНЗА and are hardened to 55-.65 HRCе. Due to high requirements It is advisable to use alloy steels for modern chain drives. The use of gas cyanidation of the working surfaces of hinges is effective. A significant increase in the service life of chains can be achieved by diffusion chrome plating of the hinges. The fatigue strength of roller chain plates is significantly increased by crimping the edges of the holes. Shot blasting is also effective.

In the hinges of roller chains, plastics are beginning to be used to operate without lubricant or with a poor supply of lubricant.

The service life of chain drives in stationary machines should be 10...15 thousand hours of operation.

In accordance with the main criterion for the performance of valuable gears, the wear resistance of the joints and the load-bearing capacity of chain drives can be determined according to the condition, but in which the pressure in the joints should not exceed what is permissible under the given operating conditions.

In calculations of valuable gears, in particular in taking into account operating conditions associated with the size of the friction path, it is convenient to use the simplest power law relationship between pressure r and by friction Pm=C, Where WITH under these limited conditions can be considered as a constant value. Indicator T depends on the nature of friction; during normal operation of gears with good lubrication T about 3 (in conditions of poor lubrication T ranges from 1 to 2).

The permissible useful force that can be transmitted by a chain with a sliding joint is

F=[p]oA/Ke;

Here [p] o - permissible pressure, MPa, in the hinges for average operating conditions (Table 12.4); A- projection of the bearing surface of the hinge, mm 2, equal for roller and sleeve prices dBin|, ; Ke is the operating coefficient.

Service factor Ke, can be represented as a product of partial coefficients:

Ke=KdKaKnKregKsmKrKt.

The coefficient Kd takes into account the dynamic load; at quiet load Kd=1; at load with shocks 1.2. ..1.5; at strong blows 1.8. The Ka coefficient takes into account the length of the chain (center distance); it is obvious that the longer the chain, the less often, other things being equal, each link engages with the sprocket and the less wear in the hinges; when a=(30...50)P take Ka=1; at a Ka=-1.25, at a=(60... 80) R Ka=0.9. The coefficient Kn takes into account the inclination of the gear to the horizon; the greater the inclination of the gear to the horizon, the less the permissible total wear of the chain; when the line of sprocket centers is tilted at an angle to the horizontal up to 45° Kn= 1; when tilted at an angle y more than 45° Kн=0.15Öy. Coefficient Craig takes into account gear adjustment; for gears with adjustable position of the axis of one of the sprockets Kreg=1; for gears with pull-off sprockets or pressure rollers Kreg=1.1; for gears with non-adjustable sprocket axes Kreg=1.25. The coefficient Kcm takes into account the nature of lubrication; with continuous lubrication in an oil pan or from a pump Kcm=0.8, with regular drip or internal lubrication Kcm=1, with periodic lubrication 1.5. Krez coefficient . takes into account the transmission operating mode; during single-shift operation Kreg = 1. The coefficient Kt takes into account the ambient temperature, at –25° 1.

When assessing the value of the service factor Ke it is necessary to at least approximately take into account the stochastic (random) nature of a number of parameters influencing it.

If, according to the calculation, the value of the coefficient Ke>2...3, then it is necessary to take constructive measures to improve the operation of the transmission.

Drive chains are designed based on geometric similarity, so the projection area of the hinge support surface for each chain size range can be represented as A=сР 2, Where With - proportionality coefficient, s»0.25 for single-row chains, except for chains not included in the natural size range: PR-8-460; PR-12.7-400-1 and PR. 12.7-900-2 (see Table 12.1).

Permissible force F of chain with mp rows

F= cP 2 [p]o mp/Ke,

Where tr - chain row coefficient, taking into account the uneven distribution of load across the rows:

zp=1 . . . . 2 3

tp,=1 .... 1,7 2,5

Permissible moment (N*m) on a small sprocket

T1=Fd1/2*10 3 =FPz1/2p10 3

Hence the chain pitch

Р=18.5 3Ö T1Ke/(cz1mp[p]o).

Approximate value of single row chain pitch (mm)

P=(12.8…13.5) 3ÖT1/z1

where the coefficient is 12.8 for PR circuits, and the coefficient is 13.5 for PRL circuits, T\- moment, N*m.

The selection of chain drives is carried out in next order. First, determine or select the number of teeth of the small sprocket and check the number of teeth of the large one. Then the chain steps are set taking into account the rotation speed of the small sprocket according to the table. 12.3 or preliminarily determine the step using one of the above formulas, in particular, by specifying the approximate value of Ke.

Then, as a test calculation, the torque on the small sprocket that the chain can transmit is determined and compared with the specified one. Typically, these calculations are made using several close to optimal combinations of parameters and the optimal option is selected.

The durability of circuits is most realistically assessed using the similarity method based on the transmission life established from operating experience or testing, taken as the reference. According to I. I. Ivashkov, this resource is multiplied by the ratio of the refined correction coefficients for the reference and calculated gears.

Correction factors:

on the hardness of the hinges when working with lubricants and contamination with abrasives: surfaces without heat treatment 2, with volumetric hardening 1, with carburization 0.65;

by pressure in the joints (r/r "o), where with continuous lubrication x = 1.5...2.5, with periodic lubrication without contamination with abrasives x = 1, the same with abrasive contamination with volumetric hardening x = 0.6;

according to operating conditions when lubricated with oil: without abrasive contamination 1, in an abrasive environment 10... 100;

by the nature of lubrication: periodic irregular 0.3. regular 0.1, in oil bath 0.06, etc.

Transmissions by toothed chains with rolling joints are selected according to proprietary data or semi-empirical dependencies from the wear resistance criterion.

When determining the operating factor Ke it is allowed to be limited to taking into account the coefficient of inclination angle Kn and at and> 10 m/s coefficient of influence of centrifugal forces Kv=1+1.1*10 -3 v 2

During operation, the leading branch of the chain experiences a constant load F1, consisting of a useful force F and the tension of the driven branch F2:

F1=F+F2

The tension of the driven branch with a known reserve is usually taken

F2=Fq+Fts

where Fq - tension due to gravity; Fc - tension from the action of centrifugal loads on the chain links.

Tension Fq(Н) is determined approximately as for an absolutely flexible inextensible thread:

Fq=ql 2 /(8f)g cosy

where q - weight of one meter of chain, kg; l - distance between chain suspension points, m; f - sag, m; g - free fall acceleration, m/s 2 ; y- the angle of inclination to the horizon of the line connecting the suspension points of the chain, which is approximately taken to be equal to the angle of inclination of the transmission.

Taking l equal to the center distance A and f=0.02a, we get a simplified dependence

Fq=60qa cozy³10q

The chain tension from centrifugal loads Fc(N) for chain drives is determined by analogy with belt drives, i.e.

Fts=qv 2,

Where v- chain speed, m/s.

Centrifugal force acting along the entire contour of the chain causes additional wear on the joints.

The calculated load on the chain drive shafts is slightly greater than the useful circumferential force due to the chain tension from the mass. She is accepted by RmF. For horizontal transmission, take Rm = 1.15, for vertical transmission, Rm = 1.05.

Chain transmissions of all types are tested for strength by the values of the breaking load Fbreak (see Table 12.1) and the tension of the most loaded branch F1max, determining the conditional value of the safety factor

K=Frazr/F1max,

Where F1max=F+Fq+Ft+Fd (definition of Fd see § 12.7).

If the value of the safety factor K> 5...6, then it is believed that the chain satisfies the conditions of static strength.

When a chain drive is operating, the movement of the chain is determined by the movement of the link hinge that was the last to engage with the drive sprocket. Each link guides the chain as the sprocket rotates one angular step, and then gives way to the next link. In this regard, the speed of the chain during uniform rotation of the sprocket is not constant. The chain speed is maximum in the sprocket position, in which the radius of the sprocket through the hinge is perpendicular to the leading branch of the chain.

In an arbitrary angular position of the sprocket, when the drive joint is rotated relative to the perpendicular to the drive branch at an angle, the longitudinal speed of the chain (Fig. 12.6, a)

V=w1R1cosa

Where w1- constant angular velocity of the drive sprocket; R1 is the radius of the location of the chain hinges (initial circle) of the drive sprocket.

Since the angle a varies from 0 to p/z1, then the chain speed changes from Vmax to Vmax cos p/z1

Instantaneous angular velocity of the driven sprocket

w2=v/(R2 cosb)

where R2 is the radius of the initial circle of the driven sprocket; b- angle of rotation of the hinge adjacent to the leading branch of the chain (relative to the perpendicular to this branch), varying from 0 to p/z2

Hence the instantaneous gear ratio

u=w1/w2=R2/R1cosb/cosa

From this formula and Fig. 12.6, b you can see that:

1) the gear ratio is not constant;

2) the greater the number of sprocket teeth, the higher the uniformity of movement, since then cosa and cosb closer to unity; the main importance is to increase the number of teeth of the small sprocket;

3) the uniformity of movement can be significantly improved if you make sure that a whole number of links fit in the leading branch; subject to this condition, the higher the uniformity, the closer the number of sprocket teeth is to one another; for z1=z2 u=const.

The variability of the gear ratio can be illustrated by the coefficient of uneven rotation of the driven sprocket with uniform rotation of the drive sprocket.

For example, for transmission with z1=18 and z2 =36 e varies within 1.1...2.1%. The smaller value corresponds to a transmission in which the leading branch contains an integer number W1 of links, and the larger value corresponds to a transmission in which W1+0.5 links.

Dynamic loads of chain drives are caused by:

a) a variable gear ratio, leading to accelerations of the masses connected by chain drives;

b) impacts of chain links on sprocket teeth when new links engage.

The impact force when the links enter the engagement is estimated from the equality of the kinetic energy of the impact of the oncoming chain link to the deformation energy of the system.

The reduced mass of the working section of the chain is estimated to be equal to the mass of 1.7...2 links. Abundant lubrication can significantly reduce impact force.

Friction losses in chain drives consist of losses: a) friction in hinges; b) friction between the plates; c) friction between the sprocket and the chain links, and in roller chains also between the roller and the bushing, when the links engage and disengage; d) friction in supports; e) losses due to oil splashing.

The main ones are friction losses in hinges and supports.

Losses due to oil splashing are significant only when the chain is lubricated by dipping at the maximum speed for this type of lubrication v = 10...15 m/s.

The average efficiency values for transmitting the full design power of fairly accurately manufactured and well-lubricated gears are 0.96...0.98.

Chain drives are positioned so that the chain moves in a vertical plane, and the relative height position of the driving and driven sprockets can be arbitrary. The optimal locations for the chain drive are horizontal and inclined at an angle of up to 45° to the horizontal. Vertically located gears require more careful adjustment of the chain tension, since its sagging does not ensure self-tension; Therefore, it is advisable to have at least a small mutual displacement of the sprockets in the horizontal direction.

The leader in chain transmissions can be either the upper or the lower branches. The leading branch should be the top one in the following cases:

a) in gears with a small center distance (a and > 2) and in gears close to vertical, in order to avoid the sagging upper driven branch from trapping additional teeth;

b) in horizontal gears with a large center distance (a> 60P) and a small number of sprocket teeth to avoid contact between the branches.

Chain tension. Chain transmissions, due to the inevitable elongation of the chain as a result of wear and contact compression in the hinges, as a rule, must be able to regulate its tension. Pretension is essential in vertical transmissions. In horizontal and inclined gears, the engagement of the chain with the sprockets is ensured by tension from own strength chain gravity, but the chain slack should be optimal within the above limits.

For gears with an inclination angle of up to 45° to the horizon, the sag f is chosen to be approximately equal to 0.02a. For gears close to vertical, f=(0.01... 0.015)a.

The chain tension is adjusted:

a) moving the axis of one of the sprockets;

b) adjusting sprockets or rollers.

It is desirable to be able to compensate for chain elongation within two links, after which two chain links are removed.

Adjusting sprockets and rollers should, if possible, be installed on the driven branch of the chain in places where it sags the most. If it is impossible to install them on the driven branch, they are placed on the leading branch, but to reduce vibrations - from the inside, where they work as pull-offs. In gears with a PZ-1 toothed chain, the adjusting sprockets can only work as tensioning ones, and the rollers as tensioning ones. The number of teeth of the control sprockets is chosen equal to the number of the small working sprocket or larger. In this case, there must be at least three chain links in engagement with the adjusting sprocket. The movement of adjusting sprockets and rollers in chain drives is similar to that in belt drives and is carried out by a weight, spring or screw. The most common design is a sprocket with an eccentric axis pressed by a spiral spring.

The successful use of chain drives using high-quality roller chains in closed crankcases with good lubrication with fixed sprocket axles without special tensioning devices is known.

Carters. To ensure the possibility of continuous abundant lubrication of the chain, protection from contamination, quiet operation and to ensure operational safety, chain drives are enclosed in crankcases (Fig. 12.7).

Internal dimensions the crankcase must provide the possibility of chain slack, as well as the possibility of convenient transmission maintenance. To monitor the condition of the chain and the oil level, the crankcase is equipped with a window and an oil level indicator.

§ 9. ASTERISES

Profiling of roller chain sprockets is mainly carried out according to GOST 591-69, which provides for wear-resistant profiles without offset (Fig. 12.8, a) for kinematic precision gears and with offset for other gears (Fig. 12.8, b) The offset profile is distinguished by the fact that the depression is outlined of two centers shifted by e=0.03P

The hinges of the chain links, which are in engagement with the sprocket, are located on the pitch circle of the sprocket.

Diameter of the pitch circle from consideration of a triangle with vertices at the center of the star and at the centers of two adjacent hinges

Dд=P/(sin (180 0 /z))

Protrusion circle diameter

De=P(0.5+ctg (180 0 /z))

Tooth profiles consist of: a) a cavity outlined by a radius r=0.5025d1+0.05 mm, i.e. slightly larger than half the roller diameter d1 ; b) an arc outlined by a radius r1=0.8d1+r; c) straight transition section; d) head outlined by radius r2 . The radius r2 is chosen such that the chain roller does not roll along the entire tooth profile, but smoothly comes into contact with the sprocket tooth in the working position at the bottom of the cavity or slightly higher. The sprocket profile ensures engagement with a chain that has a certain degree of increased pitch due to wear. In this case, the chain rollers come into contact with sections of the tooth profile that are more distant from the center of the sprockets.

In the clarification of GOST 591-b9*, the tooth height coefficient varies from 0.48 with a ratio of pitch to chain roller diameter P/d1=1.4...1.5 to 0.565 with Р/d1= 1,8... 2,0.

Width (mm) of the sprocket ring gear for single-row, double- and triple-row b1"0.95Bin-0.15, where Vvn - distance between inner plates.

The radius Rз of the tooth in the longitudinal section (for smooth running of the chain) and the coordinate h of the center of curvature from the circle of the tooth apexes are taken Rз=1.7d1 and h=0.8d1.

At a chain speed of up to 5 m/s, it is permissible according to GOST 592-81 to use a simplified sprocket profile, consisting of a depression outlined in an arc, a straight working section and a rounding along an arc at the apexes. The profile allows you to reduce the set of tools for cutting stars.

Profiling gear sprockets with toothed chains according to GOST 13576-81 (Fig. 12.9) is much simpler, since the working tooth profiles are straight.

3...7 teeth are involved in transferring the payload (depending on total number sprocket teeth), then follows a transition section with unloaded teeth and, finally, 2...4 teeth working with the back side.

The diameter of the pitch circle of the sprockets is determined according to the same dependence as for roller chains.

Protrusion circle diameter

De=P ctg (180 0 /z)

Tooth height h2=h1+ e, where h1 - the distance from the line of the centers of the plate to its base; e - radial clearance equal to 0.1 R.

Chain wedging angle a=60°. Double tooth cavity angle 2b=a-j, tooth sharpening angle g=30°-j, where j=360°/z.

The links of an unworn toothed chain mesh with the sprocket teeth using the working edges of both teeth. As a result of stretching from wear in the hinges, the chain is located at a larger radius, and the chain links are in contact with the sprocket teeth along only one working edge.

The width of the ring gear of the sprockets with the internal direction is B=b+2s, where s is the thickness of the chain plate.

Sprockets with a large number of teeth for low-speed gears (up to 3 m/s) in the absence of shock loads can be made from cast iron grade SCh 20, SCh 30 with hardening. IN unfavorable conditions from the point of view of wear, for example in agricultural machines, anti-friction and high-strength cast iron with hardening is used.

The main materials for the manufacture of sprockets: medium-carbon or alloy steels 45, 40Х, 50Г2, 35ХГСА, 40ХН with surface or general hardening to a hardness of 45...55 NKSe or carburized steels 15, 20Х, 12ХНЗА with carburization of 1...1.5 mm and hardening to NKSe 55...60. If you need silent and smooth operation of power transmissions R£5 kW and v £ 8 m/s it is possible to make sprocket crowns from plastics - textolite, polyformaldehyde, polyamides, which leads to reduced noise and increased durability of chains (due to reduced dynamic loads).

Due to the low strength of plastics, metal-plastic sprockets are also used.

The design of the sprockets is similar gear wheels. Due to the fact that the teeth of sprockets in roller gears have a relatively small width, sprockets in roller gears have a relatively small width, sprockets are often made from a disk and a hub connected by bolts, rivets or welding.

To facilitate replacement after wear, the sprockets installed on the shafts between the supports in machines with difficult disassembly are made detachable along the center plane. The parting plane passes through the cavities of the teeth, for which the number of sprocket teeth must be chosen even.

§ 10. LUBRICATION

For critical power transmissions, continuous crankcase lubrication of the following types should be used whenever possible:

a) by dipping the chain into an oil bath, and the immersion of the chain in oil at the deepest point should not exceed the width of the plate; apply up to a chain speed of 10 m/s to avoid unacceptable agitation of the oil;

b) spraying with the help of special splashing protrusions or rings and reflective shields along which the oil flows onto the chain is used at a speed of 6...12 m/s in cases where the oil level in the bath cannot be raised to the location of the chain;

c) circulating jet lubrication from a pump, the most advanced method, is used for powerful high-speed gears;

d) circulating centrifugal with oil supply through channels in the shafts and sprockets directly to the chain; used when transmission dimensions are limited, for example, in transport vehicles;

e) circulation lubrication by spraying oil droplets in a stream of air under pressure; used at speeds above 12 m/s.

In medium-speed gears that do not have sealed housings, plastic intra-joint or drip lubrication can be used. Plastic internal lubrication is carried out by periodically, after 120...180 hours, immersing the chain in oil heated to a temperature that ensures its liquefaction. Grease is suitable for chain speeds up to 4 m/s, and drip lubrication is suitable for chain speeds up to 6 m/s.

In gears with large pitch chains, the maximum speeds for each lubrication method are slightly lower.

During periodic operation and low chain speeds, periodic lubrication using a manual oiler is permissible (every 6...8 hours). Oil is supplied to the lower branch at the entrance to the engagement with the sprocket.

When using manual drip lubrication, as well as jet lubrication from a pump, it is necessary to ensure that the lubricant is distributed across the entire width of the chain and gets between the plates to lubricate the hinges. It is preferable to apply lubricant to the inner surface of the chain, from where, under the influence of centrifugal force, it is better supplied to the hinges.

Depending on the load, industrial oils I-G-A-46...I-G-A-68 are used to lubricate chain drives, and for low loads N-G-A-32.

Abroad, they began to produce chains for light duty operation that do not require lubrication, the rubbing surfaces of which are covered with self-lubricating antifriction materials.

Currently, modern motorcycles use chains with protective seals on each link. Such motorcycles ride with open chains, which are absolutely not afraid of either water or dirt. Conventionally, based on the shape of the sealing rings, they are called “O-ring”. This chain design, which has continuous advantages, has only one drawback: compared to conventional chains, it has increased friction, which worsens the transmission efficiency in the “joints” with oil seals. Therefore, the “O-ring” is not used in motorcycles for cross-country and road racing (dynamics are extremely important in them, and the life of the chain does not matter due to the short duration of the races), as well as on small-capacity equipment.

However, there are also chains called “X-ring” by the creators. In them, the sealing rings are no longer made in the shape of an educational donut, but have a cross-sectional shape resembling the letter “X”. Thanks to this innovation, friction losses in the chain joints were reduced by 75% compared to the O-ring.

LITERATURE

1. Machine parts: A textbook for students of mechanical engineering and mechanical specialties of universities. – 4th ed., revised. and additional – M.: Mashinostroenie, 1989. – 496 p.

2. MOTO No. 7/98, Please good chains, p84...85. Ó “Behind the wheel”, 1998.


§ 1. GENERAL INFORMATION

§ 3. BASIC PARAMETERS OF DRIVE CHAIN TRANSMISSIONS
§ 4. CRITERIA FOR PERFORMANCE AND CALCULATION OF CHAIN TRANSMISSIONS. CIRCUIT MATERIALS
§ 5. LOAD CAPACITY AND CALCULATION OF CHAIN TRANSMISSIONS
§ 6. CONSTANT FORCES IN THE BRANCHES OF THE CHAIN AND LOADS ON THE SHAFT
§ 7. FLUCTUATIONS OF GEAR RATIO AND DYNAMIC LOADS
§ 8. FRICTION LOSSES. GEAR CONSTRUCTION
§ 9. ASTERISES
§ 10. LUBRICATION
§ 11. “O-RING” and “X-RING” CHAINS
LITERATURE

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The subsection contains information about drive and traction chains. Drive chains are used to transmit mechanical energy over medium distances between parallel shafts. Compared to belt drives, chain drives are smaller in size and provide a constant gear ratio, as they operate without slipping. To facilitate the selection of drive roller chains, the table of main parameters includes the projection areas of the bearing surfaces of the hinges. Traction chains are used as a traction element in various conveyors.

Description of chain transmission

A chain transmission is a transmission consisting of two sprocket wheels connected by a chain (Fig. 13). The rotation of the drive sprocket is converted into rotation of the driven sprocket due to the engagement of the chain with the sprocket teeth. It can have both a constant and variable gear ratio (for example, a chain variator).

Rice. 1 — Chain transmission device

The chain consists of moving links. The ends of the chain are connected into a closed ring to transmit continuous rotational motion using a special collapsible link.

As a rule, they try to make the number of teeth on sprockets and the number of chain links mutually simple, which ensures uniform wear: each sprocket tooth works in turn with all chain links.

Characteristics

Chain drives are universal, simple and economical. Compared to gear drives, they are less sensitive to inaccuracies in the location of shafts and shock loads, allow virtually unlimited center-to-center distances, provide a simpler layout, and greater mobility of the shafts relative to each other. The chain drive can be made almost silent in operation, with much greater technological simplicity compared to silent helical gears.

Advantages of chain drives

Compared to belt drives, they are characterized by the following advantages:

no slippage;
compactness (take up significantly less space in width);
constancy of the average gear ratio;
absence of pre-tension and associated additional loads on shafts and bearings;
high power transmission at both high and low speeds;
maintaining satisfactory performance at high and low temperatures;
adaptation to any design changes by removing or adding links.
the ability to transmit movement by one chain to several sprockets;
compared to gears - the ability to transmit rotational motion over long distances (up to 7 m);
relatively high efficiency (> 0.9 ÷ 0.98);
Possibility of easy chain replacement.

Disadvantages of chain drives

elongation of the chain due to wear of its hinges and stretching of the plates;
relatively high cost of chains;
inability to use gear when reversing without stopping;
transmissions require installation on crankcases;
the supply of lubricant to the chain joints is difficult, which reduces the service life of the transmission.
the chain speed, especially with a small number of sprocket teeth, is not constant, which causes fluctuations in the gear ratio.
the chain consists of individual links and is located on the sprocket not in a circle, but in a polyhedron, which causes noise and additional dynamic loads;

Circuit classification

By purpose:

drive chains
traction chains
load chains.

In some mechanisms, lifting chains, such as hand-operated chain hoists, act as drive chains.

Drive chains are classified according to their design:

roller,
bushing,
gear,
shaped.

Roller drive chains

The chain is engaged with the sprocket through a freely rotating hardened roller, which, turning on the bushing, rolls along the tooth of the sprocket, forming a sliding hinge. This design allows you to equalize the pressure of the tooth on the bushing and reduce wear on both the bushing and the tooth.

The plates are outlined with a contour resembling a number 8 and ensuring equal strength of the plate in all sections.
Roller chains are widely used. They are used at speeds v ≤ 15 m/s.

Drive roller chains are produced according to GOST 13568-75. There are:

single row normal (PR),
single-row long-link lightweight (PRD),
single-row reinforced (PRU),
double row (2PR),
three-row (ZPR),
four-row (4PR),
with curved plates (AT).

Of the single-row roller chains, the most common are normal PR. Long link lightweight chains PRD manufactured with a reduced destructive load; permissible speed for them is up to 3 m/sec.
Reinforced chains PRU manufactured with increased strength and precision; they are used under large and variable loads, as well as at high speeds.

Multi-row chains allow you to increase the load in proportion to the number of rows, so they are used in transmission large capacities. Roller chains with curved plates of increased compliance are used under dynamic loads (impacts, frequent reverses, etc.).

Rice. 2 - Single and double row roller chains

Bush drive chains

Bush drive chains are similar in design to roller chains, but do not have rollers, which reduces the cost of the chain, reduces its weight, but significantly increases the wear of chain bushings and sprocket teeth.

A single-row bushing chain (see Fig. 3) consists of internal plates pressed onto bushings that rotate freely on rollers on which outer plates are pressed.
Depending on the transmitted power, drive bushing chains are made in single rows (PV) and two-row (2PV).
These chains are simple in design, light in weight and the cheapest, but less wear-resistant, so their use is limited to low speeds, usually up to 10 m/sec.

Bushing and roller chains are manufactured single-row and multi-row with the number of rows 2, 3, 4 and more. A multi-row chain with a smaller pitch t makes it possible to replace a single-row chain with a larger pitch and thereby reduce the diameters of sprockets and reduce dynamic loads in the transmission.
Multi-row chains can operate at significantly higher chain speeds. The load capacity of the chain increases almost in direct proportion to the number of rows.

The ends of the chain are connected with an even number of links using a connecting link, and with an odd number, with a less strong transition link with curved plates. Therefore, chains with an even number of links are used.

Single-row and double-row bushing chains of the PV type are produced in accordance with GOST 13568-75.

Rice. 3 — Single-row and double-row bushing chains type PV

Traction leaf chains

Traction leaf chains (bushing and roller) are produced in accordance with GOST 588-81; This GOST applies to traction plate bushings, roller and roller chains (with smooth rollers and plain bearings) used in hoisting and transport machines and other mechanisms.

Drive chains

Drive toothed chains are produced in accordance with GOST 13552-81. These chains operate smoothly, with little noise, provide high kinematic transmission accuracy due to uniform pitch changes during operation, and have increased reliability. Toothed chains consist of a set of tooth-shaped plates, hingedly connected to each other. The number of plates determines the width of the chain, which depends on the transmitted power. The working faces of the plates are the planes of the teeth located at an angle of 60°, with which each chain link sits on two sprocket teeth. Thanks to this feature, toothed chains have the smallest possible pitch and therefore allow higher speeds.
To eliminate the lateral drop of the chain from the sprocket, guide plates are used, located in the middle of the chain or on its sides. Toothed chains, compared to others, operate more smoothly, with less noise, and absorb shock loads better, but are heavier and more expensive.

Rice. 4 - Toothed chain

Shaped link chains

There are two types of shaped link chains:

hook;
pin.

Hook chain consists of links of the same shape, cast from malleable cast iron or stamped from 30G strip steel without additional parts.
Assembly and disassembly of this chain is carried out by mutually tilting the links at an angle 60°.

IN pin chain cast links made of malleable cast iron are connected by pinned steel (made of St3 steel) pins.

Shaped link chains are used when transmitting small powers and at low speeds (hook - up to 3 m/sec, pin - up to 4 m/sec), usually under conditions of imperfect lubrication and protection.
The links of shaped chains are not processed. Due to their low cost and ease of repair, shaped link chains are widely used in agricultural machines.

Scope of application of chain drives

Chain drives are widely used in many areas of mechanical engineering, designs of agricultural and road machines, machine tool industry, etc.
They are used in machine tools, motorcycles, bicycles, cars, industrial robots, drilling equipment, lifting and transport, road construction, agricultural, printing and other machines, in oil equipment to transmit motion between parallel shafts over long distances when the use of gears is impractical , but belt ones are impossible. Chain drives are used at relatively large interaxle distances, when gear drives cannot be used due to their bulkiness, and belt drives due to the requirements of compactness or constant transmission ratio. The predominant distribution is open chain drives, operating without lubrication, or with periodic manual lubrication, with single-row bushing-roller chains, directly built into the machines.

Chain transmissions are most widely used for transmitting power up to 120 kW at peripheral speeds up to 15 m/sec.

Asterisks

The operation of a chain transmission largely depends on the quality of the sprockets: the accuracy of their manufacture, the quality of the tooth surface, material and heat treatment.

The design dimensions and shape of the sprockets depend on the parameters of the selected chain and the gear ratio, which determines the number of teeth of the smaller drive sprocket. The parameters and quality characteristics of sprockets are established by GOST 13576-81. Roller and bush drive sprockets are made in accordance with GOST 591-69, sprockets for leaf chains in accordance with GOST 592-81, sprockets for toothed chains in accordance with GOST 13576-81.

Recently, sprockets with a toothed rim made of plastic have been used. Such sprockets are characterized by reduced chain wear and low noise during transmission operation.

Examples of designs and elements of chain drives

Roller drive chains according to GOST 13568-75 (ST SEV 2640-80)

Gear drive chains according to GOST 13552-81

Collapsible traction chains according to GOST 589-85 (ST SEV 535-77)

Sprocket designs, tension sprockets.

Guarding and lubrication of chain drives

Lamellar traction chains according to GOST 588-81 (ST SEV 1011-78)