Meccano Gears - 5 - Worm Gears

Meccano Gears - 5

Worm Gear

The Meccano worm gear (part 32) is another popular part included with many of the kits. The picture below shows how it is used.

The input shaft contains the worm gears, the other shafts are output shafts.

This gear is very useful, as it has a couple of clever properties.

1. It provides a high reduction ratio in a small space.
2. It is non reversing, ie you can only drive from the worm gear to the meshed gear, not the other way round. This is useful if you are lifting a weight for example, as you won't need any braking system to stop the weight in any given position.

The worm is like a giant screw thread, so each revolution of the worm gear indexes the meshed gear by one tooth. The lower part of the meshing gear is pulled towards you when the worm is rotated clockwise; in the picture, if the worm rotates clockwise, and you view the output shafts from the lower right side, they will rotate anticlockwise. Conversely, anticlockwise rotation of the worm will rotate the output shafts (viewed as before) in a clockwise direction.

The output is always at 90 degrees to the input.

The size of the worm for meshing purposes is the same as a 19 tooth pinion (part 26), so it will mesh at ½" spacing with a 19 tooth pinion (giving a 19:1 ratio). It will also mesh at 1" pitch with a 57 tooth gear (part 27a), to give a 57:1 ratio. The worm will also mesh with the larger 95 tooth (part 27c) and 133 tooth (part 27b) to give a 95:1 and 133:1 ratio respectively.

You can achieve a 60:1 ratio with a 1" pitch by using a 60 tooth gear (27d) and offsetting the meshing gear from the centreline of the worm.

There are a number of useful variations of this part available as reproductions. These are :-

32a - Left Hand Worm - This is the same as the standard part, but the output shaft will be rotated in the opposite direction when compared to the standard part.

32b - Right Hand Narrow Worm - This part is designed to mesh with the 60 tooth gear (27d) at 1" pitch, 

32c - Short Worm - This is the same as the standard part, only shorter! You will have noticed that only 2 or 3 'threads' on the worm are actually needed for meshing under normal circumstances, so this part will help where space is limited.