How to interpret the self-locking mechanism of worm gearboxes

In machinery, robots or mechanical installations the self-locking mechanism of a worm gearbox as a prevention of e.g. back driving can be the essential factor in the choice of gearbox, as it can seem as an opportunity to save the cost of a braking system or the like. Especially in applications that require lifting and/or holding loads the self-locking mechanism can be useful.

In machinery, robots or mechanical installations the self-locking mechanism of a worm gearbox as a prevention of e.g. back driving can be the essential factor in the choice of gearbox, as it can seem as an opportunity to save the cost of a braking system or the like. Especially in applications that require lifting and/or holding loads the self-locking mechanism can be useful.

An important feature of a worm gearbox is therefore often the ability to be self-locking. In a worm gearbox the worm is typically the driving component, turning the worm wheel and thereby the output shaft. Self-locking means that when applying a torque to the output shaft, the output shaft is not able to rotate the worm, and thereby is locked. Hence, when speaking of self-locking it is always in a static situation.

For a worm gear to be self-locking it requires a low helix angle, g. In theory, a worm gear is self-locking when the static friction angle is larger than the helix angle of the worm.

The static friction angle is described as the angle where the load above will start to move.

The static friction angle in a worm gear is derived from the static tooth friction coefficient, which mainly depends on the combination of the material, the roughness of the surface and the lubrication properties (i.e. oil type, viscosity, and temperature). Besides that, other factors in a worm gear box influences the ability to self-locking e.g. resistance in bearings, oil seals and the driving unit.

At BJ-Gear A/S we generally recommend a maximum helix angle of 5-6 degrees for the gearbox to have self-locking abilities. Below table gives an overview of what size of a BJ-gearbox and ratio to choose if the ability of self-locking is needed.

Helix angle

               
I = 5:1 7:1 10:1 15:1 20:1 30:1 40:1 50:1 60:1
Series 42 23.57° 17.75° 13.50° 9.08° 6.85° 4.57° 3.43° 3.00° 2.48°
Series 52   18.43° 14.03° 9.47° 7.85° 4.77° 3.95° 3.02° 2.60°
Series 61   17.53° 15.72° 10.62° 6.02° 5.35° 4.02° 3.53° 2.68°
Series 79   24.03° 17.53° 11.88° 10.62° 6.02° 5.35° 4.33° 3.57°
Series 99   23.60° 17.35° 11.77° 9.49° 5.95° 4.78° 3.99° 3.43

Are self-locking abilities enough to secure the prevention of back driving?

In general, you cannot count on the lead angle alone to secure the self-locking mechanism. As described earlier, multiple factors can affect the ability to self-lock.

It is important to emphasise that in general, you cannot rely 100% on a gearbox to be self-locking because external vibrations or shocks can neutralize the self-locking ability. This is because the friction coefficient in a brief moment goes from being static to dynamic. The dynamic friction coefficient is lower than the static which means that the derived friction angle will also be lower, and the self-locking ability is lost. This means that where safety is involved, a brake to hold the load is mandatory.

If in doubt, please do not hesitate to contact our specialists at BJ-Gear A/S: bj@bj-gear.com