Self-Locking

1. Without, with static, or with dynamic self-locking?

Self-locking is generally defined as the ability of a mechanical system to hold a position without requiring an additional force or brake. This means that the system doesn't move on its own when no external torque or force is applied. More specifically, a distinction is made between static and dynamic self-locking.

To assess self-locking in Screw Jacks and Lifting Cylinders, the lead angle and the type of thread are considered. Self-locking can affect both the worm gear and the trapezoidal thread. However, in lifting systems, only the Trapezoidal Thread is considered for evaluating self-locking.

2. Degree of self-locking based on lead angle

A clear statement about the type of self-locking is provided by the lead angle φ according to the following criteria:

1. Lead angle > 4.5° 🠖 No self-locking
2. Lead angle 2.4° < φ < 4.5° 🠖 Static self-locking *
3. Lead angle < 2.4° 🠖 Dynamic self-locking *

(* Requires vibration-free operation)

3. Definition of Static and Dynamic Self-Locking

Static Self-Locking: In this case, the system remains in a fixed position even when an external axial force is applied to the spindle. The gearbox can't start moving on its own through the worm wheel in a steady, vibration-free state. Static self-locking is usually not considered a braking function in risk analyses for automated applications. Exceptions include manually operated and, in some cases, horizontal systems. However, vibrations or shocks can cancel out static self-locking, causing the gearbox to move again if there's a load on the output side.

Dynamic Self-Locking: With this type of self-locking, the system stops immediately or after a short time once the drive torque is removed, even if an axial force is still acting on the spindle. This means the gearbox comes to a stop on its own after the motor is switched off. Dynamic self-locking can be considered a braking system in safety-related functions.

4. Dependence on material, surface, and lubrication

Dynamic self-locking heavily depends on the friction coefficient of the material pairing. The friction between the contact surfaces of the spindle and nut plays a crucial role, as higher friction values enhance self-locking. Surface quality is also important—a rougher surface can increase friction and improve self-locking. Additionally, the type and amount of lubrication affect friction. Excessive lubrication can reduce self-locking by decreasing friction between the contact surfaces.