Introduction
Crane slipping hook is a high-risk special equipment fault, which refers to the uncontrolled automatic sliding of heavy objects after the equipment stops operation. It is extremely easy to cause falling object accidents, equipment damage and personal injury and death. In accordance with national standards such as Safety Code for Lifting Appliances, Technical Conditions for Bridge and Gantry Cranes, and Electric Hoists (GB/T, JB/T series), this paper systematically sorts out the judgment criteria, all fault inducements, on-site maintenance and disposal schemes of slipping hook, as well as long-term equipment optimization schemes for high-risk scenarios, providing reference for equipment administrators, maintenance personnel and crane operators.
1. Definition of Slipping Hook and National Standard Allowable Maximum Sliding Distance
1.1 Definition of Slipping Hook
After the crane controller handle returns to the zero position to stop operation, the hook and load cannot be reliably locked by the brake, resulting in uncontrolled automatic sliding of heavy objects exceeding the specification limit, which is defined as slipping hook (brake failure, over-standard sliding distance).
1.2 Legal Maximum Braking Sliding Limit for Different Equipment
1.2.1 General Cranes (Bridge, Gantry, Bridge Erecting Crane, etc.)
Braking distance requirement: ≤ 1/65 of the stable lifting height per minute. Example: When the lifting speed is 12.5m/min, the maximum allowable sliding distance is ≤ 192mm.
1.2.2 Wire Rope / Chain Electric Hoist (Stricter Standards)
Under rated load and rated voltage working conditions, the braking sliding distance shall not exceed 1/100 of the lifting speed, with a fixed upper limit of no more than 200mm.
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2. Seven Core Fault Causes of Crane Slipping Hook
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2.1 Contaminated Brake Friction Surface
Oil stains, rainwater and other dirt on the surfaces of brake wheels and brake shoe linings will directly reduce the friction coefficient, greatly attenuate the braking torque, and lead to insufficient locking capacity and slipping hook faults.
2.2 Excessive Wear of Brake Parts
Long-term operation leads to severe wear of brake pads and excessive brake gaps, resulting in insufficient braking force. For single-beam electric hoists, wear of the rear motor brake disc and fracture of the brake shaft are common faults, which will directly cause complete loss of braking function.
2.3 Improper Brake Debugging or Mechanical Jamming
Loose adjustment of brake springs leads to insufficient pre-tightening force, and the braking torque fails to meet the design requirements. Corrosion and foreign matter blockage at brake pin shafts and hinge points cause the brake shoes to fail to close completely, resulting in braking failure.
2.4 Undersized Brake Selection
During the equipment selection stage, the rated braking torque of the brake is lower than the actual load demand, resulting in unreliable braking under full-load working conditions.
2.5 Electrical Control System Failure
Adhesion and jamming of brake contactor contacts cause the brake to fail to brake immediately after power off; random modification of inverter torque parameters leads to insufficient output braking force; faults of DC excitation brake devices and excessive line voltage drop cause the motor output torque to be lower than the load torque; complex control loop design leads to delayed brake signal response and delayed sliding.
2.6 Exclusive Faults of Hydraulic Driven Cranes
Excessively high hydraulic oil viscosity causes slow opening and closing action of the brake and over-standard sliding distance at the moment of shutdown; internal wear and leakage of lifting motors and reversing valves lead to failure of pressure locking after the handle returns to zero, accompanied by dual faults of slipping hook and slipping vehicle.
2.7 Non-standard Manual Operation
Long-term overload operation exceeds the design bearing capacity of the equipment; frequent inching operation makes electrical components fail to keep up with the operation frequency; habitual illegal operation accelerates the wear and loss of brakes and reducers, indirectly inducing slipping hook faults.
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3. Corresponding Fault Rectification and Solutions
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Friction surface contamination: Thoroughly clean oil stains and water stains on brake wheels and brake shoes to keep the braking working surface dry and clean.
Part wear failure: Replace severely worn brake pads and brake discs, repair or directly replace fractured brake shafts.
Debugging and jamming faults: Re-calibrate the pre-tightening force of brake springs, clean the movable hinge points of brakes, and apply lubricating grease to eliminate jamming problems.
Mismatched brake model: Replace with qualified brakes whose rated torque meets the actual load requirements.
Electrical system faults: Inspect and replace adhered contactors, troubleshoot line voltage drop problems, and re-lock the inverter braking torque parameters to avoid random modification.
Hydraulic system faults: Replace high-viscosity hydraulic oil with low-viscosity oil (transformer oil is recommended as an alternative), repair or replace lifting motors and reversing valves with internal leakage.
Non-standard operation management: Carry out special safety training for operators, strictly implement the rated load operation specifications, and put an end to frequent inching and brutal operation.
4. Long-term Improvement Measures for High Safety Level Working Conditions
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For scenarios with extremely high safety requirements such as metallurgical workshops, high-risk lifting operations and crane equipment supporting manned operations, the following protective devices can be configured in advance during equipment procurement to eliminate slipping hook risks from the hardware level:
4.1 Install Independent Safety Brake
As a secondary mechanical safety insurance, it can forcibly lock the drum when the conventional brake fails, which is widely used in metallurgical cranes and other high-risk equipment.
4.2 Adopt Dual-brake Configuration
Dual-brake structure is strongly recommended for lifting mechanisms with working level M6 and above. When a single brake fails, the other set can independently complete reliable braking to avoid slipping hook accidents.
4.3 Install Overspeed Protection Switch
Aiming at uncontrolled sliding caused by electrical faults, the device will cut off the main power supply and trigger the braking action immediately when the heavy object slides at an overspeed, effectively preventing the heavy object from falling at an overspeed and eliminating major safety hazards.
