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# Worm System

General Survey

Monday 30 November 2009, by

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 !

## Forum posts

• Very good article Nico! Using worm gears with XL motors is alos very bad, because it makes the truck go too slow and inefficient.

• First of all, I like your trucks Nico.
Also, I like your work gear comparison.
There is one thing I wonder about though...
You claim that the power transfer quota is about 0.35 for a wormgear, and about 0.90 for a traditional spur gear. While I don’t dispute these numbers (I agree that the worm gear is loosing a lot of power) I wonder how you came up with them?

• Hi, In fact, I am a student engineer in Mechanical engineering so I know the general effeciency of gear.

Effeciency of worm depend of two parameters : irrevesability and lubrification.
For a reversible worm, we have an effeciency from 0,4 to 0,8 and more than 0,85 if it is lubrified. So worm are not bad but irreversible worm are bad (less than 0,4).

http://www.roymech.co.uk/Useful_Tables/Drive/Worm_Gears.html

• I guys, exelent articles, I’am use worm system witch two m motors and I have one good result

View online : http://www.brickshelf.com/cgi-bin/g...

• Hello Marcelo, I am the admin.

The link you are giving is not properly written but i’ll modify this. Why don’t you become a member and present your truck ? It would be nice to know a little bit more about your truck and its characteristics.

• hi ! Good information, but if you try the sur gear, you will notified that much power are available on the wheel axles.

Don’t hesitate to present us your truck ;)

• thanks raus for the pics and video ! you can also become a member and give more details on the truck (motorization, steering ...)

• hi nico nice article
i’m using worm screws for steering in a truck with power puller wheels but the gears keep jumping in the casings, what do i do???

• thats the one i used, but i rebuilt the front axle and it works fine now

• I recently built a Unimog (my own design, though I do love this one), and used a worm gear. Although slow, it worked well.
I wanted some more speed, so I used gears and removed the worm, resulting in a 1:1.429 gear ratio. Problem is, it would move about an inch, then the gears would slip.
I checked the transmission, and all the gears were still in place, the structure hadn’t failed. Also, it is not 4WD, so could this have been one of the factors causing it to slip?
Any suggestions?

Many thanks!