In this post we will go through the concept of worm gears and before going through we will learn some terms like velocity ratio and gear reduction which are opposite in sense.
VELOCITY RATIO: It is defined as the ratio of number of teeth on the driving gear to the number of teeth on the driven gear.
GEAR REDUCTION: It is defined as the ratio of number of teeth on the driven gear to the number of teeth on the driving gear.
PARTS OF WORM GEAR SETUP:
There are 2 parts in warm gear setup. They are 1. WORM ( screw ) and 2. WORM WHEEL ( spur gear ).
As you can see in the image that the thread of worm engages with teeth of the worm wheel, every rotation of the worm pushes the worm wheel by one tooth. For example let’s say that worm wheel has “X” no.of tooth then the worm have to rotate “X” times to finish one rotation of worm wheel.
Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.This phenomenon is called self-locking and find its many applications.
Several variations of the basic arrangment are used for better performance and different design intents(purposes) in practice.And one of the variation is…
MULTIPLE START WORM:
Multiple threads can be added to the warm so each thread makes contact with the wheel tooth. So multiple contacts are possible without using smaller pitch (or) a larger wheel. As increase in no of contact increase contact area, that would increase capacity to carry higher loads.
In the following image you can see the 3 start warm. For every one rotation of the worm the lead is 3 times the pitch.
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 two starts if it advances two grooves (in linear direction) in one revolution.
Efficiency of the worm gear is determined by the lead Angle and the no of threads in contact with the worm wheel. A high lead Angle on the worm reduces frictional loss and heat. A low lead Angle reduces worm gear speed or increase torque.
Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater (or) we can say we will get a very low velocity ratios since both are opposite in sense.
We can find its applications in TORSEN DIFFERENTIAL , automobile steering mechanism, in lifts ,in conveyors( like breaking in conveyors because of self-locking when motor turns off ) ,in presses , in gate control mechanisms etc..