Self-locking for threads

The word self-locking is often used in the field of lead screw technology - but what exactly does this term mean and why is it relevant? There are important differences, particularly with regard to high helix and trapezoidal threads. Here we give you a concise explanation.

What does the term self-locking mean??

Self-locking describes a specification in which the lead screw nut and lead screw only move when an external force is applied. Without this external force, they remain in position and prevent unwanted movements. In many applications, this behaviour ensures that loads can be held securely without the need for additional locking elements. In practice, this has great advantages. In many applications, this means that costly and complex braking mechanisms can be dispensed with.

Self-locking is based on the combination of the pitch of the thread and the coefficient of friction between the contact surfaces. Depending on the angle of the thread pitch and the frictional resistance of the materials, a stronger or weaker self-locking effect can develop. This means that a certain pitch or an increased coefficient of friction can result in the nut only moving when a certain force is applied to it.

The difference between self-locking with trapezoidal and high helix lead screws

Single start trapezoidal lead screws are self-locking, i.e. the nut cannot move independently due to the flank angle and sliding friction. However, self-locking is only possible as long as the static friction is not exceeded. The static friction represents the resistance that prevents the nut from moving. If this is exceeded by an external force, the system loses its self-locking function. Multi-start trapezoidal threads offer a so-called "residual inhibition" compared to single-start threads. This means that a certain degree of stability is still present without external force, albeit in a weakened form.

For applications that require greater manoeuvrability, high helix lead screws are often used, which generally do not have self-locking. Here, the nut moves easily on the lead screw, making this type of thread ideal for cases where rapid and repeated adjustments are required.

The direct comparison

A direct comparison of the two types of lead screw illustrates the differences in pitch and the associated behaviour of the lead screw nuts. On the left, we see the high helix lead screw, while the trapezoidal lead screw is shown on the right.

The pitch, i.e. the angle at which the threads rise, is significantly greater with the high helix lead screw than with the trapezoidal lead screw. This greater pitch means that the nut can slide on the lead screw with less resistance and therefore moves relatively easily even without external force.

The pitch of the trapezoidal screw is smaller. As the coefficients of friction of the two nuts against the stainless steel lead screws are identical in this example, the smaller pitch of the trapezoidal lead screw ensures that the nut cannot move by itself without external force.

How can self-locking be influenced?

The effect of self-locking can be influenced by various factors. These include, e.g. the material pairing of the lead screw nut and lead screw: Different material combinations can lead to different coefficients of friction and thus influence the self-locking effect. The surface finish, i.e. the quality and surface finish of the contact surfaces, is also an important factor - smoother surfaces reduce friction and therefore self-locking, while rougher surfaces increase frictional resistance.

The solid lubricants found in all igus materials also play a decisive role. Their special specifications reduce the friction compared to a standard plastic to a certain level and constantly over the service life of the lead screw nut. In addition, the axial load of the screw drive can indirectly influence self-locking. Under higher loads, the pressure of the lead screw nut on the lead screw increases, which can increase friction and thus self-locking.