Analysis and preventive measures for the causes of elevator squatting at the bottom and hitting the top


A traction elevator is a special equipment driven by a traction machine, which relies on the friction between the traction wheel and the wire rope to drive the car to run along the vertical track. The safe operation of elevators has always been a concern in the development process of elevators. Although with the increasing updates of elevator technology, multiple safety devices such as electrical and mechanical devices have been installed on elevators to ensure safety, the effectiveness of their safety devices directly affects the safe operation of elevators. This article analyzes the relationship between the effectiveness of safety protection devices and the occurrence of squatting and topping phenomena, and proposes preventive measures from the perspective of strengthening elevator inspection and maintenance. This is not beneficial for improving the work efficiency of elevator inspection and maintenance personnel and reducing the possibility of elevator squatting and topping. 1. The reasons for elevator squatting at the bottom and hitting the top

Elevator squatting at the bottom refers to the phenomenon where the elevator cannot effectively stop and collides with the bottom pit when descending to the entire bottom end station; The top of an elevator refers to the phenomenon where the elevator cannot effectively stop and continues to rush towards the top of the shaft when it reaches the entire upper station. There are several reasons for elevator squatting and crashing: 1.1 Brake failure or insufficient braking force

The brake mainly plays an effective stopping role in the operation of elevators. According to the requirements of 687588-2003 (Safety Code for Elevator Manufacturing and Installation), when the elevator car runs up and down with a load of 125% of the rated load, the brake can make the elevator car reliably stop, and its average deceleration value is less than 9. If the brake fails or the braking force is insufficient, there is a risk of elevator sliding. The main reasons for brake failure or insufficient braking force are as follows:

(1) The brake spring is loose or broken during braking;

(2) The electrical contacts of the brake are stuck or constantly open, causing the brake electromagnet to be unable to release.

(3) The brake clearance is too large.

(4) Friction between the brake shoe and brake wheel causes overheating of the brake shoe and brake wheel, leading to a decrease in braking capacity. Faults such as brake jamming, brake arm, and axle pin breakage can cause the brake to close effectively due to low energy. 1.2 End station protection switch failure

The end station protection is equipped with three types of protection switches: forced speed change, limit switch, and limit switch, also known as overtravel protection switch, to prevent the elevator from exceeding the top or bottom end station and continuing to operate due to faults caused by the control party. If the car overtravel is caused by slipping of the traction rope, brake failure, or insufficient braking force, the end station protection switch is powerless.


The forced speed switch is mainly used to control the deceleration effect of the elevator in front of the entire end; Limit switch is used to cut off the directional relay or contactor circuit in the elevator control system when the forced deceleration switch fails to stop the elevator deceleration, releasing it and stopping the elevator operation. But at this point, only to prevent the elevator from running in a dangerous direction, the elevator can still run in a safe direction; The limit switch is the third protection against over travel. If the elevator cannot stop running even after the limit switch is activated, the limit switch will be triggered to cut off the circuit, causing the drive host and brake to lose power, and the elevator will stop running. The failure of end station protection switches usually manifests as the following situations:

(1) The failure of the forced speed change switch is mainly due to the loosening of the collision device installed on the elevator car or the collision wheel of the speed change switch, as well as the failure of the electrical contact bonding to release.


(2) The failure of the limit switch is mainly manifested by the loosening of the collision plate installed on the car frame, as well as the adhesion or delayed release of the contactor in the direction of the contact, causing the brake to hold or delay in holding time, resulting in the limit switch not being able to act in a timely manner.


(3) Limit switch failure: often manifested as loose limit switch wheels or bumpers, causing the limit switch to fail when the small energy effectively decelerates and stops.

If all three protections fail, it will inevitably lead to the consequences of squatting at the bottom or hitting the top when the elevator operates at an excessive speed; If the forced deceleration switch fails to slow down the elevator, after the elevator exceeds the end station position and the limit switch fails, even if the limit switch is effective, the elevator cannot stop before reaching the buffer, and there is a possibility of squatting or hitting the top of the elevator; If the limit switch and limit switch fail, even if the forced speed change switch is effective, the elevator may still squat or hit the top in the event of brake failure.


1.3 Insufficient traction capacity

A traction elevator is driven by a traction machine, which relies on the friction between the traction wheel and the wire rope to drive the car to run along the vertical track. Insufficient traction force cannot ensure the normal lifting of the car, resulting in a slipping state. So it can easily cause squatting. The main factors for insufficient traction of elevators are as follows:


(1) Wear of traction wheel groove or reduction of traction rope diameter. Due to the fact that friction is not a constant throughout the entire service life of the elevator, as the elevator operates, the friction between the traction rope and the wheel groove continuously reduces the static force of the rope. The traction rope gradually approaches the bottom of the groove, causing the gripping force between the traction rope and the groove (for V-shaped traction rope groove) to gradually decrease, resulting in insufficient friction and a decrease in traction capacity.



(2) The balance coefficient does not meet the standard requirements. The balance coefficient is the proportion of changes in the load of a balanced elevator car to the lateral bearing capacity. According to GBl0058 requirements, the balance coefficient of heterogeneous elevators should be between 40% and 50%. The small value of the balance coefficient only affects the weight of the support and the small balance load of the elevator, and also affects the tension of the wire ropes on both sides of the traction wheel. The magnitude of tension will have an impact on the specific pressure of the traction wire rope in the rope groove. The greater the tension, the greater the specific pressure, and the stronger the traction ability provided by the traction wire rope. Therefore, the value of the balance coefficient not only determines the unbalanced load, but also affects the traction capacity of the elevator. When the maximum unbalanced load is greater than the maximum traction force of the elevator, the traction wire rope will slip in the groove, increasing the possibility of the elevator squatting at the bottom.


(3) Wear of transmission and reduction mechanisms: The transmission mechanism mainly refers to mechanical components such as the traction machine spindle, bearings, gears, and worm gears. Due to long-term use or insufficient lubrication of these moving parts, excessive wear and aging occur, resulting in a serious decrease in traction.

1.4 Speed limiter and safety gear failure

The speed limiter safety gear system is a safety device that provides protection in the event of elevator overspeed and rope breakage. A speed limiter is a device that limits the overspeed operation of an elevator. When the elevator exceeds the set electrical action speed, it will cut off the safety circuit of the elevator through an electrical switch, thereby cutting off the system power supply. If the elevator continues to exceed speed due to load or inertia, the mechanical action device of the speed limiter will be triggered, causing the speed limit wire rope to stop moving, thereby pulling the safety gear. A safety clamp is a mechanical device that stops the operation of a car or counterweight. It acts on the guide rail and relies on the friction between the wedge and the guide rail to stop the car.


(1) Speed limiter does not operate: According to GB7588-2003, the operating speed of the elevator speed limiter should not be less than 115% of the rated speed. If there is a malfunction in the elevator safety circuit or the speed limiter does not meet the requirements, the speed limiter will not operate. When the elevator exceeds the speed limit, the control circuit cannot be disconnected and the brake cannot be applied.

(2) Although the speed limiter is activated, it is capable of manipulating the safety gear of the elevator car. The reason is that the groove of the speed limiter is worn, which reduces the friction between the rope pulley and the speed limiter rope. When the elevator exceeds the speed limit, although the electrical switch of the speed limiter acts, the insufficient tension of the speed limiter rope cannot make the safety clamp work.


(3) Failure of the linkage switch of the car roof safety gear: Currently, travel switches are commonly used for safety gear switches, which is prone to electrical contact adhesion, causing the electrical switch of the speed limiter rope to fail when pulling the safety gear. After the elevator exceeds speed, the control circuit is not disconnected and the brake is not applied.


(4) The safety gear cannot reach the effective operating position after installation or maintenance. Due to testing and adjustment carried out after installation and maintenance, the action was not in place or the safety clamp did not move synchronously on the curved side. There is a sequence between the safety clamp on one side of the car and the safety clamp on the other side of the car, or the gap between the safety clamp wedge on one side of the car and the side of the guide rail is too large, and the safety clamp on that side fails to stop the car, causing the car to fall.


(5) The surface friction coefficient of the sliding wedge block of the safety gear decreases. If the actual friction force between the safety gear wedge block and the side clamping of the guide rail is less than the force required to act on the guide rail during the operation of the safety gear; The dirt and rust on the safety gear machinery were not repaired and cleaned in a timely manner, resulting in mechanical jamming and inability to move. This kind of situation makes the safety clamp play a small role in stopping the elevator car. 1.5 Elevator overload

According to GB 7588-2003 "Safety Code for Elevator Manufacturing and Installation", overload refers to exceeding 10% of the rated load and at least 75 kg. In the case of overload, there should be an audio and/or luminous signal inside the car to notify the user, and the power driven automatic door should be kept in the fully open position. When the elevator stops for a long time or the environment in the shaft or pit is mixed with moisture, the movable contacts of the overload protection device located at the bottom of the car may rust out. When the elevator is overloaded, the overload device does not act, making it impossible to disconnect the safety circuit, and the car cannot stop on the floor and squat on the bottom.

1.6 Failure of uplink protection device


According to GB7588-2003, traction driven elevators should be equipped with a car up speed protection device, while previously installed in use elevators were all equipped with this device. At present, there are several common types of safety devices, including rope clamps, speed limiters with a car safety clamp, speed limiters with a pair of safety clamps, and permanent magnet synchronous brakes. Among them, the speed monitoring of the upward overspeed protection device for rope clamps, bidirectional speed limiters with a car safety clamp, and strong directional speed limiters with a pair of safety clamps is carried out by the speed limiter. The failure of the upward overspeed protection device is usually manifested as the failure of the speed limiter with a safety clamp.


In summary, when the elevator brake fails and the elevator car is overloaded and descending, the elevator will exceed the speed limit. If the speed limiter safety clamp system exceeds the limit when returning, the elevator car can still effectively stop after its return exceeds 115% of the rated speed. However, if the speed limiter safety clamp system fails, the elevator's squatting accident cannot be avoided. When the traction capacity of the elevator is insufficient or the control system malfunctions, when the elevator car exceeds the bottom end station, the end station protection switch can be used to power off the main engine or brake, and the elevator car can also be stopped; If the end station protection switch fails, it is highly likely to cause squatting accidents when the elevator goes over the distance. When the elevator goes up with a light load, if the brake fails, it will cause the elevator to overspeed when going up. For elevators that are not equipped with an up protection device or have an up protection device that fails, a top collision accident will occur. For elevators with an up protection device that is a strong speed limiter safety clamp system, if the speed limiter safety clamp system fails at this time, it will also lead to a top collision accident. When the elevator approaches the top floor end station due to a control system malfunction, if the end station protection switch fails or the elevator's top floor space does not meet the requirements, there is a possibility of the elevator car hitting the roof.



From the above analysis, it can be seen that only reliable and effective safety components such as brakes in elevators can prevent accidents such as squatting at the bottom and hitting the top of the elevator. Therefore, in addition to conducting inspections and maintenance measures on the safety components of elevators in accordance with relevant regulations during daily maintenance and annual inspections, targeted inspections and maintenance should also be carried out on components that are prone to causing elevator squatting and crashing, in order to prevent accidents from occurring.