Worm gears are special gears used in rudders, automobile steering mechanisms, hoists and rolling mills.
A worm gear drive consists of two elements:
- Driving element → Screw
- Driven element → Helical gear
Driving element (screw) is called worm and driven element (helical gear) is called worm gear or worm wheel.
Worm gear drives are typically used for transmission of power between two non-parallel and non-intersecting shafts.
How worm gear works?
The following animation video will show you how a worm drive works
The worm (screw) continuously rotates and drives the worm wheel (meshed with it). Worm and worm gear form a lower pair as they have sliding contact with each other.
In a worm gear drive, power is always transmitted from worm to worm wheel. Power cannot be transmitted from worm wheel to worm. This phenomenon is called self-locking. It is highly useful in many applications.
Velocity ratio is determined by the number of teeth on worm gear and the number starts on worm. Power transmission decreases with increase in velocity ratio.
Note: A screw (worm) is said to have one start if it advances one groove (in linear direction), in one complete revolution. It is said to have have two starts if it advances two grooves (in linear direction) in one revolution. The worm shown in the animation above has four starts.
Advantages of Worm Drives:
- Worm gear drives operate silently and smoothly.
- They are self-locking.
- They occupy less space.
- They have good meshing effectiveness.
- They can be used for reducing speed and increasing torque.
- High velocity ratio of the order of 100 can be obtained in a single step.
Disadvantages of Worm Drives:
- Worm gear materials are expensive.
- Worm drives have high power losses and low transmission efficiency.
- They produce a lot of heat.
Applications of Worm Gear Drives:
Worm drives are used in:
- Gate control mechanisms
- Hoisting machines
- Automobile steering mechanisms