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WORM GEARS
V. Ryan © 2002 - 2017
 
 
 
 
 
The arrangement of gears seen above is called a worm and worm wheel. The worm, which in this example is brown in colour, only has one tooth but it is like a screw thread. The worm wheel, coloured yellow, is like a normal gear wheel or spur gear. The worm always drives the worm wheel round, it is never the opposite way round as the system tends to lock and jam.
 
 
 
 
The pictures below show the typical set up of worm gear systems as seen in school projects. As the worm revolves the wormwheel (spur gear) also revolves but the rotary motion is transmitted through a ninety degree angle.
 
 
 

   

The gear ratio of a worm gear is worked out through the following formula:

number of teeth on wormwheel

number of teeth on worm

 

The worm acts as a single toothed gear so the ratio is;

 

number of teeth on wormwheel

1

 

EXAMPLE:

If the wormwheel has 60 teeth:

60

1

 

Gear Ratio = 60:1

(Rotary velocity is also reduced by 60:1)

Quite simply, this means a worm gear reduces the speed of the spur gear by sixty times. If you need a gear system whereby the speed is reduced by a considerable amount - a worm and wormwheel are worth considering.
   

1. Describe how a worm and wormwheel could be used as part of a design for a mechanical device or toy. Use diagrams and notes.

2. If the wormwheel has 80 teeth, what is the gear ratio ?

   
 
   

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