Worm drive problem

Guys,

I’ve got a gripper that uses a worm drive to provide the clamping force. It works very well, but I’ve stripped out my first Vex gear :frowning: . The motor turns the worm, and the 24t worm-meshing gear is coupled to the movable fingers. I’ve got some foam material on the insides of the fingers (weather stripping) to help provide grip & cushioning.

When I clamp down with any significant pressure, the axle-hole in the 24t gear experiences tremendous torque, and as a result seems to be prone to failure. The obvious solution is to use more worm-drive assemblies in parallel, but that also has a few issues:

  1. Geometry. Because worm-drives transfer power at right-angles, this requires you to have multiple input shafts (or output shafts). This is awkward to layout, but can be done either using sprockets & chain or idler gears to couple all the input shafts. Worse, since the gripper is at the end of an arm, I’m trying really hard to minimize mass.

  2. Backlash. Coupling all the input shafts as described above introduces a new problem. Because of backlash in the idlers or chains coupling the parallel input shafts, one worm will always slightly lead the others. This means one of the 24t worm-meshing gears will always lead the pack slightly, causing it to wear more than it’s peers. This kind of defeats the point of ganging them in the first place, and at worst just means the 24t gears fail one-at-a-time in series (not really sharing the load).

I don’t know if #2 is really an issue in practice. What I think I may do is try to drill-out the failed 24t gear so I can put screws through the flat part of the gear to anchor it to the fingers directly. This eliminates the axle from the torque-transfer and should greatly eliminate backlash.

My questions now (at long last): Has anybody else come up with a way to transfer more torque through a single worm drive? What about reliably and compactly parallel worm drives? Anybody modified the 24t gear with holes to directly attach metal?

Thanks!

  • Dean

if you’re experiencing a huge stall you are probably using a servo motor instead of a continous rotation motor

I’m definitely using a continuous rotation motor - the stall isn’t because of the motor, it is because the gripper is closing. Trying to keep any significant gripping force on the fingers is what is causing the back-torque that is stripping the axle-hole.

I’m pretty clear on what is going wrong; I’m just seeking community advice on how best to modify the design. Perhaps I should post a picture to help make my description make more sense…

Thanks,

  • Dean

It sounds like a claw we are using is very similar to yours, we drive two 24tooth gears off the worm gear. The claw “fingers” are attached to the 24tooth gears. We have already stripped out the centers of those gears once and the current ones need replacing. I think this is just an inherent problem when you try and transfer this much torque through a plastic gear. As to making it so the gears don’t strip, I think the only solution would be to not clamp so hard on the object you are grabbing.

A picture would help.

i just remembered something, only the worm gear can be driven, it’s “backstopped” so the 24tooth gear cannot turn the worm gear

so if you are gripping the object to hard the fingers attached to the 24 tooth gear will be forced back against the worm gear, causing it to strip

in this case you should use limit switches to make sure you don’t grip it too hard

have you considered a series of bevel gears?

Yes, this is exactly what is happening.

But, I want to grip fairly hard :rolleyes:

I’d like to find a way to get more power through the system rather than artificially limit it. It looks like the 24t axle hub is the weakest point in my gearing by a fairly large margin, so if I can engineer my wy past this I should be OK. For now, I’m going to try drilling the 24t gear, since it is already ruined. I’ll post back in a few days with my results.

What did you have in mind?

I was considering a design that would use bevel gears (8 of them) to gang four 24t gears spaced around the worm gear at 90 degree intervals, but haven’t tried to build it. I’m not sure if I could get all the gears to mesh, or if some of the worm gears would be “out of phase”. It could be the basis of a pretty cool 4-way claw, but all I need for now is a basic clamp (one finger is fixed, the other closes/opens)

Cheers,

  • Dean

one bevel gear horizantal and then another connected to it vertically, then that one is connected to another bevel gear whereever you want the motor, you could add standard gears to the final bevel gear to make a gear train too for more power

just an idea :smiley:

Oh, I see. You mean just not use the worm drive for this at all. No matter what I do, I think the drive to the final axle will have to have multiple gears/sprockets driving it or I’ll just be in the same boat since they all have pretty much the same hub.

I just always assumed I’d use a worm drive for this, since its primary advantage is to provide a low-speed, high-torque drive that can’t be back-driven. It seems like the design of the 24t tooth gear pretty much precludes (easily) using the worm drive in high-torque applications.

I’ll keep experimenting and let you guys know what I come up with.

Thanks

  • Dean

ill make up a cad of what i mean :smiley:

you can replace the 60tooth gear with whatever drivetrain you want

Seems a little overkill considering you could just skip the two bevel gears in the middle and get the same result. The output would still be “up”.

i know but putting a motor that close might block the fingers, and inbetween the two midle bevels they can add things to better support this mechanism and i wanted to exactly imitate the original worm gear setup

Try using multi-bar links/levers to convert a long-distance, low-force motion into a short-distance, high-gripping-force motion.

Blake

basicxman, thanks for the cad drawing. Your description was good enough that I had more-or-less got the same picture in my head!

This would work, but after all the necessary hardware to hold it together, I’m afraid it might be too large and heavy.

If I can get the metal of the fingers directly attached to the 24t gear, I think it’ll work as-is. If not, then I’ll try some of the suggestions posted here.

Here are the pictures I promised.

Cheers,

  • Dean

Looks like a great claw. It doesn’t look to me like there is much you can do. Unfortunately the 24tooth gears don’t have screw holes in them or I would suggest screwing into the gears.

hmmm…if you have a chain and sprocket kit you could use my CAD with just 2 bevels (one to the claw and other to motor) and have the motor quite a ways a way, pretty much like corpralchee said a few posts ago

We had the same problem with worm gears we used to raise an arm to fix it we drilled two holes into the gear and screwed on two metal barlocks one on either side, works extremley well I will post a picture when I get home.

Awesome. This is exactly what I will be trying tonight. I was considering a pair of barlocks as you describe, but the #8 holes fall right at the edge of the gear. If I put screws through the two extra square holes, it would all fit, but I’d either need to use smaller screws or enlarge 2 of the barlock sq holes.

Another option I was considering is to use some pieces of standard vex plate, which has the standard bolt hole pattern with a barlock pattern offset 1/4" from that. That would provide a pair of bolt holes up a bit higher, away from the worm interface, instead of inline with the axle.

http://homepage.mac.com/dean.reece/Sites/Vex/Worm_Example.jpg

Thanks for the offer of the pictures - I can’t wait to see your solution,

  • Dean

thats an idea but we cut the bar lock so that just two square holes and one round hole was left then we just drilled the square hole in the bar lock and put a barlock on either side of the gear

sorry but my camera was charging so I made this: