Worm Drive
April 2, 2013 1:27 PM   Subscribe

For a little project I need to use a worm drive, and the best/ cheapest solution seems to be one like this designed for modelmaking. But how do I fix those gears on the shaft/ Axis? Am I supposed to mill a key in it or something like that? I have a Dremel, so I could carve something in the gear, but not in the worm. And then I would need a shaft with a fitted key and those are expensive. Or are they designed to heat-shrink them onto the shaft? Unfortunately there is no information on the site.
posted by SweetLiesOfBokonon to Technology (7 answers total)
Press fit. You get the right shaft and then hammer it on.

It's a good solution if the want the shaft to slip at high torque. If you don't, you will want something like a d-shaft and a gear with a set screw.
posted by BeeDo at 1:31 PM on April 2, 2013

Drill a hole through the hub of the gear (radially in toward the shaft). Tap the hole. Buy the relevant set screw.

Alternately, if all of the parts are metal, you can get a shaft that has a zero clearance fit in the gear and heat'er up.

Lastly, look at places like McMaster-Carr to see if there is anything that might better solve your problem. I do not know what the exact application is, but if you can go with something plastic (normally nylon) it will get a lot cheaper.
posted by milqman at 1:57 PM on April 2, 2013

I'd drill a hole through the collar of the gears and the shafts they go on and then use a cotter pin to hold them in place. You can choose the material of the pin so that it would or wouldn't shear at high torque.
posted by zengargoyle at 2:50 PM on April 2, 2013

You can buy a ready-made worm drive gearbox from ServoCity. I bought one before and it was well-made.
posted by KevCed at 5:17 PM on April 2, 2013

Another source for a worm gear set is from a garage door repair kit. I recently repaired a Sears Liftmaster and found replacement gears on eBay. The gears were a slip fit on the shaft and driven by a roll pin pressed into a hole cross drilled through the shaft (its easy to push a roll pin in with a small vise). I think the worm used the motor bearings as as its thrust bearing.

Maybe re-purpose a 2nd hand garage door opener if the project doesn't need to be pretty and AC power is OK. If the limit switches are a bother they are easy to remove. Bonus: Direction control via a remote!
posted by tinker at 6:30 PM on April 2, 2013

I agree with everyone... you aren't 'supposed' to do anything. You have many options for connecting them, each with various performance and tooling constraints.

Interference/friction fit trades off torque limits for no tooling.
Set screws, roll pins, shaft keys improve assembly torque limits at the cost of tooling/machining complexity.

All depends on your application needs.

Basically, as my deceased father-in-law (many many patents) used to say: "Just hook it up". You get to choose how. You and your wallet and your schedule, that is. AND, you get to be wrong and break stuff. That's how you learn to get better at it.
posted by FauxScot at 6:18 AM on April 3, 2013

After posting I re-looked at the gear set you found... the garage door set is much bigger. I think it was a 1/2 horse motor with ~Ø.50"/12.7mm shaft.

The cross drilled hole with a roll pin thru it is easy machining if you have access to a drill press, a pain but possible with a hand drill. If possible, match drill the parts. That is put the gear on the shaft and drill though both at the same time. The hole diameter is not too critical but I still recommend drilling a touch undersize first and then chasing it with the proper size drill. Alternately, if you have a few bucks and the parts are simple you can get them custom machined at MISUMI USA, INC.

If your loads are light and the shaft is a close fit (less than Ø.006"/0.15mm) to the gear bore you might get away with an adhesive such as Loctite 609. That adhesive is pricey and you only need two drops... fortunately many machine shops have a big/dusty bottle handy to fix their press fit screw-ups, try begging what you need.

I wholeheartedly endorse McMaster-Carr, but they have industrial prices. Good for ideas though and they have an excellent web site. Here are three other industrial sources to get ideas from:

PIC Design
W.M. Berg
Stock Drive
posted by tinker at 9:19 AM on April 3, 2013

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