Self Locking Worm Gear

What is a 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.

Self-Locking Worm Gear Features

Using a self-locking worm gear allows you to achieve a very high gear reduction ratio. This is especially useful for industrial applications such as lifts and elevators. However, this feature is not totally reliable. There are other factors that influence its ability to self-lock.

One of the main reasons for its high speed reduction is the fact that a worm gear uses fewer moving parts. This is a very economical solution to speed reduction. Another benefit of worm gears is their quiet operation.

Despite their efficiency, worm gears must be lubricated. The lubrication must be specialized for the equipment being used. Typically, worm gear lubricants have a high viscosity. This lubricant also needs to be filtered and on-site for optimal performance.

Another disadvantage of worm gears is the amount of time it takes to lubricate. The worm gear's sliding motion causes significant frictional losses, and it requires a lot of lubrication.

To determine the durability of a worm gear, the friction coefficient of the tooth is considered. It is determined by the material of the gear and the surface roughness. This is important because it is difficult to predict how long a worm gear will last.

Advantages of Self-Locking Worm Gears

A self-locking worm gear can be used to achieve a large reduction ratio (as large as 200:1) 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,to achieve 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.

You can think of using the self-locking worm gear for the applications where loading against the gravitational force is required and when you have the input and output shafts perpendicular to each other.

Disadvantages of Self Locking Worm Gear

A huge amount of heat generation is one of the concerns for this kind of gear train. The problem however can be diluted by arranging proper heat removal provisions in the design.

High contact stress is another concern for this kind of gear trains. As the contact area between the worm and gear is smaller, so the amount of stress generation is more. The enveloping worm gear is one of the solutions for the problem.