Causes of Crane Hook Overrun

Causes, solutions, and preventive measures for crane hook top-out

A crane hook overrun occurs when, during lifting operations, the hook (or hook block) rises beyond its maximum design limit and collides violently with the drum, trolley frame, or other crane components. In severe cases, this can lead to wire rope breakage and result in the hook and load falling.

01

Reasons for the hook hitting the top

There are multiple causes for crane hook overrun accidents, which can primarily be analyzed in terms of equipment-related and management-related factors. Equipment-related factors include missing or malfunctioning hoisting limit switches and the absence of zero-position protection; management-related factors include a lack of routine inspections and maintenance, operator error, and improper lifting techniques such as pulling at an angle or lifting at an oblique angle. Simply put, it occurs when the crane loses control while hoisting to its maximum limit position.

Hoist limit switch malfunction (equipment-related)

The direct causes of a malfunctioning crane upper limit switch include the following:

1) Sticking of the upper limit switch contacts

The crane’s height limiter is a key component responsible for controlling the hook’s movement during hoisting and lowering operations. If routine inspections of the height limiter are not thorough enough, its contacts are likely to stick. Once the contacts stick, the limiter effectively fails, causing the main hook to continue rising and eventually strike the top of the hoist.

2) Screw-type hoisting limiters: poor installation accuracy, improper maintenance, and severe wear

Screw-type hoisting limiters operate by the drum shaft driving the screw via a universal joint or flexible coupling, causing the slider on the screw to move axially. When the slider reaches the preset position, it contacts the limit switch, cutting off the control circuit and thereby disconnecting the power supply to control the hoisting height.

Screw-type hoist limiters are accurate and reliable, but the following points should be noted:

a) If there is a deviation between the centerline of the drum shaft and the centerline of the limiter screw during installation, the rigidly connected drum shaft and screw head will generate additional bending and shear stresses during operation. Over time, the screw head is prone to snapping, causing the limiter to fail.

b) After each wire rope replacement, the limit switch’s stopping position must be readjusted to prevent accidents.

c) Due to prolonged wear and improper maintenance, the displacement accuracy of the slider may deteriorate. If the switch cannot be forced to actuate at the preset position, the limit switch will fail.

3) Vibration and swaying of the hook block, drum movement, etc.

The counterweight-type hoisting limit switch consists of a limit switch and a counterweight. It operates by lifting the counterweight when the hook block (lifting device) reaches the specified limit position, thereby activating the limit switch to cut off the power supply, stop the mechanism, and halt the upward movement of the hook block.

The primary cause of failure in a counterweight-type hoisting limit switch is vibration and swaying during crane operation. When the hook block reaches the limit position, the counterweight may not be lifted due to unstable swaying, causing the limit switch to fail and the hook to strike the top limit.

Additionally, drum runout may also lead to damage or malfunction of the hoisting limit switch.

No Zero-Position Protection (Equipment-Related)

The zero-position protection system is primarily used to control the zero-position start of the cam controller (interlocking control console), preventing the circuit from closing when the cam controller (interlocking control console) handle remains in the operating position after the power supply is cut off. If zero-position protection is absent and power is restored, the motor will start running on its own, potentially causing an accident.

1) Failure to install an upper limit position limiter or a malfunctioning limiter, causing the hook to continue rising until the hoisting wire rope is twisted or pulled to the point of breaking.

2) Malfunction of the hoisting mechanism’s main contactor (e.g., blown main contacts, delayed release of main contacts due to mechanical failure or excessive residual magnetism in the solenoid core), resulting in the inability to cut off the hoisting power in time, until the hoisting wire rope is twisted or pulled to the point of breaking

Management Factors

Crane-using units lack safety operating procedures and management systems for crane operations, and there are many shortcomings in daily maintenance and upkeep. Crane maintenance and operating personnel prioritize usage over maintenance and lack safety awareness, thereby creating potential accident hazards.

Generally speaking, hook over-travel is primarily caused by improper operation. For example, operators frequently reach the limit switch without decelerating or stop by bumping into the limit switch; pulling at an angle or lifting at an oblique angle can damage the limit switch and cause it to malfunction; operators may be distracted while operating the crane, leading to errors such as mistaking the hook descent command for ascent, or failing to return the controller handle to the neutral position. In cases where the limit switch fails, if the emergency stop switch is not activated, this can result in the hook over-traveling.

02

Solution

Identify and analyze the cause, replace damaged components, and resolve the issue completely. At the same time, crane operators should conduct regular inspections and maintenance of the crane’s safety devices and major components as required, with particular attention to the lifting limit switch contacts, which should be inspected or replaced on a regular basis.

03

Precautions

Strictly adhere to crane operating procedures, perform daily inspections and maintenance, and ensure that operators check the limit switches for proper function before each shift. If any issues are detected, they must be reported immediately, and the crane should be shut down for replacement

1. Safety and Technical Controls

Analyze the service life, maintenance status, technical condition, and working environment of the cranes in use, and implement corresponding safety and technical measures. Control critical areas prone to accidents to ensure the reliability and safe operation of safety protection devices and major crane components;

Implement safety and technical measures such as distance protection, time protection, personal protection, the principle of addressing weak links, and the warning and alert principle to eliminate potential hazards and prevent accidents. For example, install a “hook anti-overrun system” on the crane to monitor in real time and prevent overrun accidents.

2. Human Behavior Control

Control human operational errors and reduce non-compliant behavior, such as incorrect operation, carelessness, and failure to use personal protective equipment and safety devices as required. Ensure both human safety and operational safety.

Strictly adhere to the training system for special operations personnel and insist on certified personnel only. Strengthen crane operators’ safety awareness and improve their operational skills and ability to handle emergencies. Consciously comply with safety operating procedures to enhance operational accuracy and reliability, thereby achieving safe operations.

3. Equipment Management Control

Establish and improve various management systems. Based on the correct use, inspection, and maintenance of cranes, establish a safety inspection system for lifting equipment, safety operating procedures, safety work standards, shift handover protocols, and an information feedback system.

Conduct regular inspections and pre-use checks on cranes, incorporating usage, maintenance, and inspection into standardized management. Clarify responsibilities by assigning specific duties to personnel involved in management, operation, and maintenance. Each individual must fulfill their respective responsibilities, conduct daily safety inspections, and operate in strict accordance with procedures. All activities must be properly documented.

Strictly enforce a robust information feedback system. For issues identified during inspections, implement feedback and corrective actions in accordance with system requirements based on their nature and severity, and maintain detailed records of these actions. Report any discovered issues promptly to the supervisor for immediate handling.

In general, hook-over-top accidents are widespread and highly likely to occur. The renowned American safety engineer Heinrich concluded from numerous accident reports and statistical records across various industries that, among 330 similar accidents involving the same object, 300 resulted in no injuries, 29 resulted in minor injuries, and 1 resulted in a serious injury. This statistical pattern is known as the 330 Accident Model.

Of these, 90% were near-miss incidents, less than 10% resulted in injuries, and only 0.3% resulted in serious injuries. This is known as Heinrich’s Law, also referred to as the “Heinrich Safety Law.” It illustrates the widespread nature of accidents—that is, there are far more incidents than the injuries we actually observe—and this holds true for hook-to-ceiling collisions as well.