Overdriving Worm Gear & Brake Options

[richg1]

Hi,

We are experiencing a problem with an overdriving worm gear box. The worm gear ratio is 100:1 which would suggest that it should (ideally) self-lock. However probably due to the high loads and vibration it is overdriving. It is probably also a function of gear teeth wear as the overdriving is being experienced only after time in service.

Anyway my question is what brake options/configurations are available particularly keeping in mind the drive train is used intermittently with very short drive periods. Ideally I would like a soft stop, if that makes sense. Any recommendations on suppliers would be greatly appreciated.

As a side-bar question do double enveloping worm gears increase self-locking characteristics?

Cheers,

Rich (Mech Eng)

 

[MikeHalloran]

Filling the gearbox with Dexron(r) instead of the recommended oil would probably make it self-lock better, at least to the extent that the overdriving comes from better surface finish as the gears wear in. It probably won't help with whatever contribution the vibration makes.

It will surely void any remaining warranty, and it may kill the gears; the stuff makes wet clutches grab like crazy, but automatic transmissions don't have many sliding surfaces that aren't clutches or brakes ... unlike a worm gear box.

Of course, if there's a life safety issue even remotely involved, I am prepared to retract the above statement, and to deny ever having made it or thought about it. ;-)


A magnetic particle brake on the input shaft should lock it up nicely, and with some electronic bits you can ramp the current to get a 'soft stop' as you want. Of course you need to power (and probably cool) the brake to keep it stopped, so a spring-set brake might be a better choice. If it's released by air, you can figure out how to make it apply softly.



Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

This is a common mistake to assume that a worm wear drive will be self locking in all conditions. I had my share of arguments with other less experienced engineers to convince them not to rely on worm gear alone for self locking especially for aerospace applications where the aircraft has very strong vibrations. The same applies to friction brakes. We use only positive lock pin engaged into a matching hole. When release is needed the pin is pulled using ant means such as solenoids, pneumatic, hydraulic or electrical motor to pull the pin out of the hole.

 

[gearcutter]

What about installing a sprag clutch/one way bearing on the worm shaft?

 

[plasgears]

One simple brake would be a light belleville spring bearing against the gear side. Of course, this would have to be lubricated to counteract heat and wear. A little drag would go a long way.

 

[redtricycle]

I was under the impression you should not rely on a worm gear to self-lock, but I can see how it's useful in some practical applications. Here's a quote about self-lock from Boston Gear:

"Generally speaking, if the worm lead angle is less than 5°,
there is reasonable expectation of self-locking. Again, no guarantee should be made and customer should be advised. If
safety is involved, a positive brake should be used."

 

[israelkk]

plasgears

A standard belleville spring has a linear behavior therefore, as long as it wears the braking force will change. For this to work you need a belleville spring with a range of constant force which is not standard. One more problem is that the ends of the spring will cut into the gear if not wide enough and grinded. The belleville spring hardnesss is usually ~50RC.

 

[plasgears]

israel,

Full details could include wear washers to protect the gear side and ID interface, a thin scallopped belleville spring with curled fingers, etc.

Maybe this is not a std belleville washer.