Low Cost Self-Locking Worm 3

[PLo1984]

Hi guys

I'm designing a Jig for assembly, and I want to be able to rotate the component during building. I want to use a self-locking worm to rotate the assembly, and hold it in place, with as low-a ratio as possible. The gearbox will be operated by hand. I can't for the life of me find an appropriate of-the-shelf solution online. All I've found are gearboxes for valve actuation, which are usually quarter turn and innapropriate, or gearboxes for electric motors, which are too small.

Any suggestions would be very much appreciated.

Thanks

 

[ornerynorsk]

Worm drives, by their nature, are self locking, provided there is not an enormous amount of vibration or factors at play.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Jboggs]

It sounds like you really want just the gearset, not the whole speed reducer. Have you checked gear manufacturers, like Boston Gear or many others?

 

[MintJulep]

gearboxes for electric motors, which are too small.

Considering that electric motors range from miliwatts to Megawatts that really tells us nothing.

Quantify.

 

[MikeHalloran]

You will find that the normal backlash in common wormgear drives is much too great for workholding.

I have used a Browning 80:1 set with a bronze worm approx 3" diameter for other purposes that required low backlash, and was able to reduce the lash a lot by displacing the pinion a few thou along the worm axis. It's a ticklish thing to do, typically requiring you to carefully fit and even lap shims, slightly preload the worm bearings against each other, and basically set up the gears to not quite bind on each other, displaced from their theoretical positions in both directions enough to almost take up the lash.

You might be happier with a very large chain sprocket driving your fixture, and in turn driving that with a worm gear box and a small pinion. If the weight and bulk of a giant sprocket is undesirable, using a timing belt to drive a big disk. The disk only needs two teeth inlaid at 180 degrees to get positive drive and positioning.





Mike Halloran
Pembroke Pines, FL, USA

 

[racookpe1978]

Mike: it seems to me that any chain-linked drive would have much more backlash (movement due to the changing direction of the forces on the shaft being resisted back and forth) than a gear drive.

Granted, that doesn't the original poster an answer yet. I have seen some very elaborate multiple faced gears used in radar tracking systems with spring-loaded tension between the two parallel gear faces - but that seems too elaborate.

 

[Sparweb]

Are you looking for a fixture for machinists to use, or to adapt something your own way?
If you want to make something from a cheap worm-gear drive, any hand-winch will do.
Ratios anywhere from 20:1 to 50:1 are common
Load ratings from 500 pounds to 3000 pounds
try "Dutton Lainson"
Can you live with the backlash?

In the machinist's world, I have used a rotary table to do this. But then, I don't know the size of the machine you're building, either. It may be too big for a typical milling machine fixture to hold.

STF

 

[kingnero]

you can get (or make, or adapt) "braked" gear boxes, either manually or electrically operated, that lock the outgoing shaft in place.

 

[PLo1984]

Hi guys
thanks for the help, yes the post was very vague. I think Brake winches seem the most appropriate. You're right I could use stock gears, but I want to avoid that if possible.

Many thanks

 

[ornerynorsk]

A Geneva mechanism might also be worth your consideration. It achieves an incremental indexing with each revolution of the actuating element. Very easy to put this together from odds'n'ends if you've got access to even a modestly equipped shop. It can be made self-holding very easily, and will be much quicker to index than either a worm drive or a chain/sprocket reduction.

Just my 2 cents.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[racookpe1978]

I don't have all of the logic worked out yet - but it seems to me that somehow you could use - not just a single worm gear driving a bull gear, but two worm gears.

Backlash (movement of the driven bull gear when the bull gear is forced backwards away from the normal resistance of the friction and load) will happen anytime there is "space" between the tooth of the (driving) worm gear and the previous tooth of the (driven) bull gear right? So, if you are machining or touching or pushing the load irregularly, then the bull gear will always and irregularly "push back away from" that tooth on the worm gear that was pushing and in immediate contact. That movement will screw up the finish and the machined surface you want to work on.

But if you had two separate worm gears, and you knew (or could approximately guess at) what that irregular backwards force will be as the machine tool starts biting in for example), it seems to me you could apply that force to the second worm gear all the time and force the first worm gear never from ever disengaging its tooth-to-tooth contact. if the two teeth never loose contact, you never get backlash or movement.

 

[drawoh]

PLo1984,

You could also spring load the worm against the gear. In this application, you do not care about efficiency.

A crude rule of thumb is that any worm driver with a ratio over 30:1 is self locking. I have managed to spec. out a system with a 100:1 ratio that was not self locking, so don't forget that lead angle is what matters, not the ratio.

--
JHG