What Are The Causes of The Inability To Lock Or Failure of Self-locking in Trapezoidal Screw?

The essence of self-locking in trapezoidal screws is that the helix angle is less than the equivalent friction angle (i.e., the friction force is greater than the sliding component force), thus achieving self-locking in static or low-speed conditions. When self-locking fails (unable to lock or sliding), it is usually caused by multiple factors such as design parameters, working conditions, mechanical wear, and system structure. The specific reasons are as follows:

1. Design parameters mismatch

Excessive helix angle: The self-locking condition is "angle of ascent < friction angle". If the screw lead is too large or the pitch is too large, the angle of ascent will be too large, and when the angle of ascent is greater than the equivalent friction angle, self-locking fails, and the load will slide under the action of gravity or axial force.

Too small friction angle: If the friction coefficient of the material pair is too low (such as excessive lubrication of the surface or low friction coefficient between materials), the equivalent friction angle will decrease, unable to resist the sliding component force.

2. Changes in working conditions

Vibration and impact loads: During equipment operation, if subjected to large vibration or impact, it will disrupt the static friction balance between the threads, causing instantaneous "slipping" or self-locking failure.

High speed: The self-locking performance is mainly good in static or low-speed conditions. If the equipment operates at high speed, the inertial force or dynamic load may overcome the static friction force, resulting in self-locking failure.

Excessive load or sudden change: Although an increase in load will increase the normal pressure, thereby increasing the friction force, if the load is too large, the system will deform, or a sudden change in load generates a large sliding component force, which may exceed the limit of the friction force.

3. Mechanical wear and improper lubrication

Mechanical wear: Severe wear of the screw or nut threads will cause an increase in the fit clearance and a reduction in the contact area, resulting in a decrease in friction force and a weakening of self-locking ability.

Excessive lubrication: The trapezoidal screw relies on sliding friction to achieve self-locking. If an efficient lubricant or excessive lubrication is used, the friction coefficient will be significantly reduced, weakening the self-locking performance.

4. System structure limitations

No self-locking design in the transmission system: If the front or rear end of the equipment uses a ball screw (with extremely high transmission efficiency, no self-locking capability) or gear transmission, and no mechanical braking device is added, it will inevitably reverse and slide after stopping the drive.

Solutions: If self-locking fails due to excessive helix angle or too low friction coefficient, the parameters need to be re-verified or a low lead screw should be replaced; if due to harsh working conditions (large vibration) or the system has no self-locking capability, an additional mechanical braking device (such as brake pads, electromagnetic brakes) or anti-reversal mechanism should be added.

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