Worm gears and racks.

I’m not sure how i missed this post i’m sorry. I just wanted to add that worm gear has a slight angle on the teeth that mates up to the wheels. This is caused by the continuous thread. If the thread was not pitched it would end at it’s starting point, Making it a useless as a gear. I too would like to see a worm gear mate to the rack. it would help make an awesome slide. if you can find a way that will not reduce the life of the worm or rack would you please post pics?

Hope your having as much fun as i am.

This is a good thing to point out - Thanks.

My question was about how tightly I could pack a motor-plus-worm combo that would engage a rack. Your point about the worm’s slanted threads not being parallel to the faces of the gear or rack teeth is one to remember also.

On a rack, the worm is going engage with enough of the rack’s teeth simultaneously to make the misalignment between the threads and the teeth become pretty important (except under very light loads).

Alternatively, when engaging a round gear (especially small ones), teeth other than the one or two that are directly in contact with the worm “fall away” from the worm as they follow the gear’s circumference. This helps them get out of the way of the worm’s slanted threads.

I think this means that worms are best used with gears that are not large (in circumference) when compared to the size of the worm gear; because small segments of the circumferences of very large circles/gears start to approximate straight lines (start to look like racks).

I also suspect that for large-diameter worm gears, using gears that are too wide/thick would create problems.

Are there any Mech Engineers, savvy Techs or shade-tree mechanics out there who can confirm or correct these thoughts; and perhaps give us some rules of thumb to obey?


This thinking is correct. When you have gears with straight teeth (spur gears), the larger diameter ones will mate with the worm gears worse than small diameter ones.

Note, the “worm-wheels” that are designed to mate with the “worm-gears” typically have slanted teeth. These slanted teeth form a good interface with the worm, regardless of worm-wheel diameter. The Vex Advanced gear kit includes a 24t worm-wheel paired with each worm gear.

I’m not an expert by any means, but hopefully my understanding of these components will help provide insight.



I had not paid enough attention to the title “Worm Wheel”. Now I understand.

For other other naive folks like me, I recommend that you (VexLabs) update the kit description/advertisement to point out (rub my nose in it) that a “worm Wheel” has slanted teeth, and is different from the other Vex (Spur) gears

Thanks again - Blake

I had actually meant to post information on this last week but just got too busy to post it. I took photos of the gear teeth comparing the worm wheel with a 36t spur gear to show the slant of the teeth on the worm wheel. Someone not familiar or not paying close enough attention to the worm wheel could easlily assume that they are interchangable and fully usable with the spur gears to give them additional gear ratio combinations.

You can probably use the worm wheel in combination with the rack or spur gear for limited applications with success. If you have any significant load however you may experience slippage and/or wearing of the teeth. The same goes for the worm gear and rack combination/spur gear combination.

Here is a photo comparison of the worm wheel and a spur gear. You will notice the teeth on the worm wheel are at an angle.
gear comparison2.jpg