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Worm System

General Survey

Monday 30 November 2009, by Nico71

Worm system is very common in lego creation, and very useful, It allows a high gear ration and a bevel construction with irreversibility. But it has a big problem : the general output.

The two exemples shows the most common gear ratio. The first is 1/24 and the second is 1/8. The structure of worm system is easier to build than the normal structure because we can use lot of usefull parts like those on the picture.

To compare with the spur gear, we must put in place 3 * 24/8 to have the same gear ratio.

The worm system permit high gear ratio, compact structure and irreversibility. But the output is bad. The general output of worm system is 0.30 to 0.40 with irreversible worm (the case in lego). While the spur gear has a output of 0.90.

If we compare the output-power of the system, with 2.6watts (a XL motor) on driving axle we have :
- worm : 0.35*2.60 = 0.90 watts (1.7 watts lost)
- spur gear: 0.9*0.9*0.9*2.6 = 1.9watts (0.7 watts lost)

We can see that the spur gear system is twice over efficient than the worm system. It results in a high quantity of lost power with worm system. That is why it is recommended not to use worm system on drivetrain.

If you are not convinced, watch this video and listen the sound of motor at 2min50 for Nico71’s unimog (spur gear) and 3min26 for Maxlemalade’s truck (worm gear). When Maxlemalade’s truck comes on slope, It losts 60% of its power, due to the worm system.

So, keep in mind : Don’t use the worm system on your drivetrain !

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