Self-locking Worm Gear
A self-locking worm gear is a type of worm gear that does not allow the interchangeability of the input and output gears. As you know, in spur gear trains you can interchange the driving gear and the driven gear but the same is not possible for the self-locking type of worm gears. For this type of gear, the worm always acts as a driving gear and the spur gear as a driven gear- vice versa is not possible. If you try to run it otherwise, it will lock automatically.
As a matter of fact, most of the worm gear trains used in industry are of the self-locking type. But you can of course design a non-self- locking type of worm gear. Approximately, if the tangent of the helix angle of the worm gear is less than the coefficient of friction between the worm and the gear, then the worm gear train should be a self-locking type. A more precise governing equation to ensure the self-locking feature of the worm gear train is:
f < cos θn * tanΛ……………………Eqn.1
f – Coefficient of friction between the worm and the gear.
θn- is the pressure angle of the gear train
Λ- Helix angle of the worm.
Advantages of Self-Locking Worm Gears
You can achieve a large reduction ratio (as large as 200:1) from a self-locking worm gear without increasing the size of the gear box. How? A 360 degree rotation of a single start worm causes the meshing spur gear to rotate by one tooth. So, if a 10 teeth spur gear is meshed with a single start worm then you will get a reduction ratio of 10:1 straight away. Whereas, for achieving the same reduction ratio by using a spur gear train, you have to use another 100 teeth spur gear with that 10 teeth spur gear. So imagine the comparative size reduction.