Question. Instruments and safety devices installed on cranes. What devices ensure the safety of an overhead crane Calculation of jib cranes

Safety devices and devices

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Article topic:	Safety devices and devices
Rubric (thematic category)	Sport

Instruments and safety devices for overhead cranes are designed to prevent overloading of the load-lifting crane and mechanisms, derailment from the crane runway as a result of random factors, inattention and lack of efficiency of the driver; shutting down the crane mechanisms in emergency situations, as well as to protect operating personnel from injury electric shock, falls from a height, etc.

Bridge cranes use two types of safety devices: limiters and alarms.

Limiters are basic safety devices that automatically turn off a mechanism or group of crane mechanisms when a dangerous load occurs or conditions are violated safe operation. There are limiters for the load capacity, the height of the load, the path of movement of the cargo trolley and the bridge, and the misalignment of the bridge (in long-span overhead cranes). The contacts of the limit switches are included in the crane control circuit. Resuming operation of a disabled mechanism is only possible to return the working element (load, trolley, bridge) to a safe position. For example, after the load lifting mechanism has been disabled as a result of an attempt to lift a load that is 25% greater than the crane's rated capacity, the mechanism must be engaged only to lower the load. If the load lifting height limiter is triggered, when the distance between the top of the hook suspension and the bottom of the load trolley becomes equal to 200 mm, the load lifting mechanism can also be turned on only to lower the load.

In order to prevent a bridge or a cargo trolley from leaving the rail track, they install end stops that absorb loads when stopping. To soften a possible impact, the bridge and cargo trolley are equipped with buffers with shock absorbers.

Alarms overhead crane– sound signaling devices designed to notify service personnel about the start of any operation.

Security questions on chapter 4˸

1. Explain the areas of application of overhead cranes.

2. What is the main parameter of an overhead crane?

3. List the main parameters of overhead cranes.

4. List the main mechanisms of overhead cranes.

5. How are the designs of overhead cranes different?

6. List the safety devices and devices for overhead cranes.

7. Explain the principle of operation of the load limiter of an overhead crane.

TOWER CRANES

Safety devices and devices - concept and types. Classification and features of the category "Safety devices and devices" 2015, 2017-2018.

The safety of people involved in crane operations is also ensured by interlocking devices that automatically remove voltage from exposed live parts and de-energize the crane. Typically, the simplest limit switches of the VK type are used for this purpose. On electric bridge cranes they are installed on the exit hatches from the cabin, on the doors in the end fences of the galleries (Fig. 6.4).

Before exiting the cab onto the crane gallery, the crane operator must turn off the main switch - de-energize the crane. If for some reason the crane operator does not turn off the switch, and there is no interlock, then this creates a danger of electric shock. The purpose of the locking device is to eliminate this danger in such a situation.

Figure 6.4 – Locking device at the entrance to the crane gallery

The exit from the cabin to the crane bridge is through a hatch in the gallery floor. When the lid is opened, the switch installed on the hatch breaks electrical circuit and de-energizes the trolleys. The same purpose is used for the locking device on the doors in the end railings of the crane galleries. The door entering the overhead crane cabin must also be equipped with an electrical lock to prevent the crane from operating when the door is open.

Zero blocking. All motor control circuits on cranes have zero blocking. It allows the line contactor to be turned on and voltage supplied to the controllers when all controllers are brought to the zero position. If this condition is not met and any of the controllers is not set to the zero position, then the contactor of the protective panel will not turn on and the mechanism will not be pulled out arbitrarily.

However, practice shows that if safety rules are violated, even if there is an automatic lock, an accident can occur.

For example, an electric overhead crane moves along the bay of a workshop with castings suspended from a hook. At this time, a crane operator is coming to meet him along the crane track. Having reached the crane, without any warning, he opens the door in the end fence and goes out onto the gallery. What happens? When the doors are opened, the lock is activated. The crane stops. But he was moving at high speed, and the load suspended from him, due to inertia, moves first forward due to the oblique tension of the rope, then makes a reverse movement - the load swayed. And people were working in the workshop in the same flight - and the accident became inevitable.

Limit switches and interlocking devices are only effective at low speeds. If the speed exceeds 80 m/min, lever limit switches cannot provide reliable protection. In these cases, other traffic restriction systems are used.

The Ural Polytechnic Institute has created a photoelectric hitchhiker designed to safely stop overhead cranes moving towards each other at high speed. Hitch-hiking; is activated when the cranes approach a distance equal to the sum of the longest braking distances. This device can be mainly used in indoors. Another system for stopping cranes moving towards each other is made using radar sensors, with the help of which the cranes are de-energized when approaching a dangerous distance.

Overhead cranes must be equipped with load limiters that prevent overloading by more than 25%.

If lifting cranes are controlled from the cabin, from a remote control or remotely, they are equipped with a mechanical or electrical signaling device.

General conditions for protecting electrical equipment on cranes from emergency situations

According to their purpose, specificity of work and design features, cranes belong to the category of equipment that has an increased danger, which is explained by the very process of operation of these mechanisms on sites and in premises where people and valuable equipment are located at the same time.

General requirements for the safety of operation of cranes and crane electrical equipment are formulated in accordance with the “Rules for the design and safe operation of load-lifting cranes” and the “Rules for the construction of electrical installations”.

All electrical equipment located in crane control cabins is equipped with grounded metal casings or must be completely closed from the possibility of touching live parts. The control cabin must also contain a device that provides direct or remote shutdown of all power cable routes running along the crane, with the exception of input devices.

Access to crane platforms where electrical equipment and trolleys that are not protected by casings are located can only be achieved through doors and hatches that have a lock that turns off the power to all sources of electricity in the crane.

The section of the main trolleys, the main current collectors and current leads that remain energized when all intra-crane wiring is disconnected. must have reliable protection from accidental touching. This fence must have a lock with an individual key.

Repair and inspection of current leads can only be carried out when the power to the main trolleys or the general input device located outside the crane is turned off. The chains of several cranes are powered from general workshop trolleys, it is provided repair area, where the trolleys can be turned off without interrupting the power supply to the remaining cranes.

Cranes are moving installations and are subject to vibrations and shocks during movement, so the possibility of damage to cables and wires on cranes is relatively higher than with their stationary installation. In addition, on a number of cranes, current transfer to moving parts is carried out using flexible hose cables, the damage of which cannot be completely avoided. Taking this into account, the first task of protection is to protect electrical equipment on taps from short-circuit currents.

Short-circuit currents in individual circuits within the crane will be smaller, the smaller the cross-section of the mounting wires of these circuits and the smaller the sizes of various current junctions and current connectors. Maximum short-circuit currents in control circuits with a wire cross-section of 2.5 mm2 is 1200-2500 A. In this case, to protect the circuits it is possible to use fuses of the PR series for currents of 6-20 A or any types circuit breakers AP 50, AK 63, etc. Short-circuit currents, A, in electric motor circuits can be approximately determined using the formula

Where I kzyuf - current short circuit in the supply phase, line after 0.04 s; s p - wire cross-section in the circuit under consideration, mm2.

Since the current k.e. should not, before turning it off, destroy the switching device located in this circuit, then when choosing devices and wire cross-sections, it is necessary to observe certain ratios that ensure the thermal resistance of the device. Assuming that the thermal resistance of most devices used in crane electric drives is 10I n for 1 s, then the relationship between the maximum permissible wire cross-section, mm2, and the rated current of the device should be as follows:

Where I n - rated current of the device, A.

The last relationship shows that at possible short-circuit currents. on a feeder of more than 8000 A, it is unacceptable to install 25 A devices due to thermal resistance. Devices for currents of 63 A can only be used with cable cross-sections of no more than 6 mm2, and devices for currents of 100 A - with cable cross-sections of no more than 16 mm2.

At possible short-circuit currents. 12,000 A (limit for cranes) devices with currents of 63 A can only be used with cable cross-sections of no more than 4 mm2, i.e., with rated currents up to 30 A. Devices with a current of 100 A can be used with cable cross-sections of no more than 10 mm2, i.e. at rated currents up to 60 A. Thus, for cranes receiving power from feeders of particularly high power, it is necessary either to install devices with currents of at least 100-160 A, or to limit the cross-section of wires to these devices in order to reduce possible currents k.z.

Protection of the crane cable network from short-circuit currents. is carried out using an instantaneous maximum current relay, and if necessary can be carried out by automatic installation machines.

Protection of wires from short-circuit currents. is complicated by the large power range of the electric motors of the mechanisms within one crane. In accordance with the rules for electrical installations, protective devices must be designed for an operating current of no higher than 450% of the continuous current of the protected circuit. By the same rules, for wires and cables operating with intermittent loads, the permissible heating current is determined by the expression

Where I pv and I n - rated cable currents in intermittent and continuous operating modes.

At PV=40% I pv = 1.4 x I n. Thus, the multiple of the protection setting to the permissible current of the wire (cable) should not be higher than 450/1.4 = 320% of the current in the 40% duty cycle mode. Permissible loads on wires and cables within the crane at temperature environment 45°C are given in the reference tables.

Crane electric drives have the following main types of protective devices:

Maximum protection for disconnecting the electric drive from the network when unacceptable currents occur in the protected circuit;

Zero protection to turn off the electric drive when the power supply from the electrical source is interrupted or interrupted. A type of zero protection is zero blocking, which prevents self-starting of the electric motor when power is restored to the supply line if the control is in the operating position

Ultimate protection to prevent movement of moving structures beyond certain permissible limits.

An important task of the protection system is to prevent unacceptable overloads for all types of electric drives of crane mechanisms associated with malfunction of control circuits, jamming of mechanisms, open brake circuits, etc. This is the difference between the requirements for overload protection of crane electric drives and overload protection for electric drives of continuous operation .

Due to the uncertainty of the load of crane mechanisms, changing heating rates of motors, and their operation under conditions of frequent starts and braking, it is not even possible to set the task of protecting electric drives from thermal overloads. The only condition for preventing thermal overloads of crane electrical equipment is its right choice taking into account any pre-calculated operating modes possible in operation.

Thus, overload protection comes down to monitoring the inrush current during step-by-step starting and protection against jamming of squirrel-cage motors or electric drives with current cut-off. With a properly organized start of the electric drive with stepwise acceleration, the starting current should not exceed 220-240% of the current corresponding to the calculated value.

Taking into account the necessary margin for the spread of both the starting current and the maximum relay setting, the latter should be designed to operate at a current of about 250% of the calculated one, which may be equal to or less than the motor current in duty cycle mode = 40%.

According to the above, the maximum current relay in the crane electric drive system has two functions:

1. protection against short-circuit currents. wires (cables) in each pole on direct current and in each phase on alternating current,

2. overload protection, to ensure which it is enough to turn on the relay in one of the poles or one of the phases.

In accordance with the rules, electric drives of cranes must have, i.e., when the power is interrupted, the electric drive must be turned off, and it can be turned on again only after the control element returns to the zero position. This requirement does not apply to floor push-button systems that have self-resetting pushbuttons.

The presence of zero blocking eliminates self-starting of electric drives of cranes, and also eliminates re-starting when various protections are triggered.

Phase loss protection is not used on taps. Analysis possible consequences phase failure outside the tap and an acceptable phase failure protection system showed that, on the one hand, there is currently no satisfactory technical solution for the use of a reliable, cheap and simple apparatus voltage control on the phases, and on the other hand, phase failure within the tap and outside it is unlikely due to the fact that the use of fuses in the main circuit is not currently practiced.

New dynamic braking systems, used instead of counter-switch braking, minimize the risk of a load falling due to phase failure.

Overload protection relay in crane electric drive

To protect the circuits of crane electrical equipment from overloads, an instantaneous electromagnetic relay type REO 401 is used. These relays can be used in both AC and AC circuits. DC. The relay has two designs. In Fig. Figure 1 shows a general view of the REO 401 relay.

The relay consists of two main components: electromagnet 2 and opening auxiliary contact 1. The coil of the electromagnet 3 is located on the tube 4, in which the armature 5 moves freely. The position of the armature in the tube is adjustable in height and determines the value of the relay operating current. When the current in the coil increases above the operating current, the armature rises up and opens the contacts through the pusher of the contact assembly.

In the second version, relay electromagnets in the amount of two to four pieces are attached to a common base, which also has a common bracket that transmits the forces of any individual electromagnet armature to an auxiliary contact installed on the base. Thus, in this design several electromagnets act on one auxiliary contact.

After the current is turned off, the armature returns under the influence of its own weight. The relay has one normally open auxiliary contact. The auxiliary contact is rated for up to 10 A AC switching at 380 V and or 1 A DC switching at 220 V and L/R = 0.05

Rice. 1 . General view relay REO 401

Relay coils for currents above 40 A are made of bare copper. The terminals of these coils are located on a special insulating panel. Coils for currents up to 40 A are insulated. When choosing a relay to install in. complete devices should be guided by the permissible load of the coil in duty cycle mode = 40% and the operating range, taking into account the necessary shutdown settings.

REO 401 relays can perform their functions provided that the starting current of the electric drive is less than the current of the braked electric motor when it is turned on at the rated voltage, i.e., protection of short-circuited electric motors and electric drives with current cut-off using the REO 401 relay is impossible. Protection of such electric motors should be carried out using thermal series TRT.

TRT relays have five sizes in the current range from 1.75 to 550 A. Relays of all types are enclosed in a plastic casing and differ in the shape of the reacting thermal element, the presence of an additional heater and the size of the leads. The fifth size relay is mounted on a current transformer. Invarstal bimetal, flown by current and additionally heated by a heater, is used as a reacting thermal element of the relay. The relay has one normally open contact, rated for switching AC 10 A, 380 V at Cos φ = 0.4 and DC 0.5 A, 220 V at L/R = 0.05.

Technical data of the TRT relay are given in reference books. The timing characteristics of the TRT series relays are shown in Fig. 2. The relay does not operate at 110% of the rated current in continuous mode. At a current of 135% of the rated relay, it operates in 5-20 minutes. At a current of 600% of the rated relay, it operates in a time from 3 to 15 s. The regulator available on the relay allows you to adjust the rated current setting within ±15%. The relay contacts return to the on state 1-3 minutes after the current is turned off.

When choosing a relay, you should be guided by the following conditions:

1) the rms current of the protected circuit must not be higher than the rated current of the heater;

2) with three starts in a row, the relay should not operate;

3) the response time at the starting current should not be higher than the permissible time for the electric motor to remain under current in this mode.

When using the time characteristic of the operation of the TRT relay, it should be taken into account that the possible actual deviations of the operation current are about ±20% of the set current.

Protective panels

In accordance with the requirements, each crane must be equipped with a device designed to supply power to the electric drives of the mechanisms and turn it off, and turning it on, i.e., supplying power, can be carried out after unlocking the switching device using an individual brand key.

Rice. 2. Timing characteristics of TRT series relays.

In turn, the key cannot be removed without performing a shutdown operation. This locking makes it possible to ensure that the crane is brought into a usable state only by a person authorized to operate the crane.

On all types of electric cranes, except construction tower cranes, an individual brand key is used in. For construction tower cranes, the specified key is used to lock the main switch (or circuit breaker) in the tower crane power cabinet to which the flexible power cable is connected.

Rice. 3. Diagram of control circuits of protective panels: a - when controlling cam controllers; b - when controlling magnetic controllers; 1P-ZP - fuses; KB - “return” button; CL - hatch contact; AB - emergency switch; L - linear contactor: MP1, MP2 - maximum relay contacts; KVV, KVN - limit switches; PP - test switch; K12 - zero contacts of controllers.

Depending on the type of crane (overhead, tower, self-propelled jib, etc.) and the type of drive (electric, mechanical), the crane is equipped with a number of instruments and devices that ensure its safe operation. Such devices include:
a) limit switches designed to automatically stop the mechanisms of cranes with electric drive. On cranes with mechanically driven mechanisms, limit switches are not used. Requirements for equipping lifting machines with limit switches are set out in the Crane Rules;
b) blocking contacts used for electrically blocking the entrance door to the crane cabin from the landing platform, the hatch cover for the entrance to the bridge deck and other places;
c) lifting capacity limiters, designed to prevent crane accidents associated with lifting loads with a mass exceeding their lifting capacity (taking into account the hook reach). Installation of the device is mandatory on jib, tower and portal cranes. Overhead cranes must be equipped with a load limiter in cases where their overload cannot be excluded due to production technology. Requirements for installing the device are contained in the Crane Regulations;
d) skew limiters, designed to prevent dangerous skew of metal structures of gantry cranes and bridge loaders due to one of the supports being ahead of the other when the crane is moving. The need to install the device is determined by calculation during design;
e) a load capacity indicator installed on jib-type cranes, in which the load capacity changes with changes in the hook reach. The device automatically shows what the crane's lifting capacity is at the set reach, which helps prevent the crane from overloading;
e) tilt angle indicator for correct installation jib cranes, except those operating on rail tracks;
g) anemometer. Tower, portal and cable cranes should be equipped with such a device to automatically sound a sound signal at wind speeds dangerous for work;
h) anti-theft devices used on cranes operating on surface rail tracks to prevent them from being stolen by the wind. The requirements for these devices are set out in the Crane Regulations;
i) automatic alarm dangerous voltage(ASON), signaling the dangerous approach of the crane boom to live wires of the power line. The device is equipped with jib self-propelled cranes (with the exception of railway cranes);
j) supporting parts that are supplied to overhead cranes, mobile cantilever cranes, tower cranes, portal cranes, cable cranes, as well as cargo trolleys (except for electric hoists) to reduce dynamic loads on the metal structure in the event of breakdown of the axles of the running wheels;
k) stops installed at the ends of the rail track to prevent lifting machines from leaving them, as well as on jib cranes with variable boom reach to prevent it from tipping over;
m) an audible warning device used on cranes controlled from the cabin or from a remote control (if remote control). On taps controlled from the floor, a signaling device is not installed.

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