COTS & Custom Worm Gearboxes

We are working on designing a shooter gearbox that can pull back a spring, release it quickly, and adjust up or down as necessary. In order to hold the spring load the gearbox must be non-backdriveable.

The common way of preventing backdrive is to use a ratchet. The downside to this is that if you dial up your shooter and then decide you want to lower it back down, you can’t. You could if your gearbox used a worm gear that had a low enough pitch angle to be non-backdriveable, then you could spin the input motor either direction to adjust tension. It wouldn’t be particularly efficient, but that is the tradeoff.

It seems there are only two COTS worm gearboxes, the AndyMark WormBox and RawBox. I saw a post about the WormBox stating that it is easily backdriveable, I havne’t seen much info on the RawBox.

Can anybody with a RawBox comment on it’s backdriveability?

Can anybody with custom worm gearbox experience comment on the process, and the parts sourcing?

Please forgive the lack of specifics in this post.

I seem to recall that some of the legal motors do not backdrive - I am thinking of window motors, but maybe not. Explore there.

In 2005, we used a van door motor (powerful) to drive our arm’s elbow, and a window motor as a ‘brake’ to prevent backdrive.

We built our own worm drive in 2011 and it worked really well. Got the gears from McMaster Carr. They are heavy and we could have gotten by with lighter gears but these were easy to get. Stock Drive Products has a good selection of worm and gear combinations. See

We made a simple box structure to hold the gears from aluminum bar stock and bronze sleeve and flanged sleeve bearings which work fine in this application.

We used a CIM attached to an AndyMark micro gear box with the worm mounted directly on the output shaft of the gearbox. The shaft of the wheel gear carried a bicycle sprocket mounted on a plate attached to a shaft collar. A bicycle chain ran our very effective long arm to scoop up and hang the tubes in that game.

If we use a similar arrangement again this year, I would probably design it around a channel to hold the worm, with uprights to hold the wheel to reduce the machining.

Dr. Bob
Chairman’s Award is not about building a robot, every team builds a robot.

Worm Gear Box.JPG

Worm Gear Box.JPG

This is very interesting, I hadn’t thought of pairing a high power motor with an anti-backdrive motor to get the best of both worlds. I remember a good post by Paul Copioli awhile back on pairing dissimilar motors that I’ll have to find.

Does anyone happen to know if the snowblower motor has locking or anti-backdrive pins in it? I don’t have one to inspect. I know that the window motors do for sure.

We made a custom worm gearbox for our robot’s ‘Shoulder’ last year, and considering that it was done mid-season, it worked really well.

The Gearbox had a total 480:1 Reduction and used a COTS 16DP Worm and Worm Wheel From Boston Gear. (Purchased Through Amazon Supply.) The initial stage consisted of a 15T 32DP Pinion Driven by an RS-550, to a 60T 32DP Gear From AndyMark. From there, the 60T Gear drive a Single Start Steel Worm, which in turn drove a 40T Brass Worm Wheel. There was an additional 20DP Spur Stage after the Worm Wheel that allowed us to Vary the boxes overall reduction should we have ever needed to.

In the case of AndyMark’s Worm Boxes (Both the Worm Box and Raw Box), I believe they both use multiple start Worm Gears which will have less back drive resistance than a single start worm. Depending on how exactly they’re integrated into specific setup, you may be able to get away the amount of resistance provided, or you may not. I wouldn’t be surprised if there’s some other COTS worm gear pairing that is the same Pitch and approximate overall dimensions that swaps into AM’s boxes without too much work.

If you decide to go down the Custom Worm Gear path, here’s a worthwhile document from Boston Gear:

I found that most of Boston’s common sizes (12DP, 16DP, 24DP) were readily available through a variety of online resources, one of those being Amazon Supply/Amazon. If I remember correctly, the gears that we purchased were a ‘prime’ item, so they shipped in 2 days for free.

Can anybody with a RawBox comment on it’s backdriveability?

While I do not have very much experience with the AndyMark RAW Box, I did play with it at champs and it is based on our 2012 gearbox. Our gearbox was designed to be very high efficiency (for a worm) and be backdrivable so we used a 4 start worm which (IIRC) AndyMark used in their version. The RAW box is therefore most probably backdrivable.

Worm drives are backdrivable, but it takes some torque. The four/two start worms will backdrive pretty quickly after being worn in, and if you load a 1 start worm enough, it’ll back drive. I’d be willing to guess that this is for some surgical tubing launcher thing, and my suggestion is to use a transmission from a dewalt drill (remember those?). They don’t backdrive.