Prototype transmission for potential use this year

[sanelss]

Originally Posted by magnets

Neat idea. However, I am fairly certain that your worm wheel will loose teeth when you have the weight of the robot behind it. If your robot is going fast, and suddenly you stop applying power, the worm wheel won't be to turn the worm gear, so you'll just snap a tooth off. It will be different than just testing it on a table.

In the video you talked about having to use a smaller worm wheel in order to get the same reduction. These are pretty fragile, so you could switch to a bigger worm wheel with a two-start worm.

A few other recommendations. Be sure to lubricate your gears! Make sure there isn't any play sliding the worm or worm wheel up and down on their shafts and that the unsupported drive axle doesn't wiggle around, but a little play in rotating the gears (like you showed in the video) is really important. That backlash makes the transmission operate more smoothly, and helps with lubrication, especially when you're using gear like this, than "slide" instead of "roll" on each other.

oh indeed we are aware bench tests are different. Hence why we are prototyping. The sudden stop may or may not be an issue, we don't know. If it is, we can address it by having programmed coast made to prevent such an instance or at least minimize the sudden load. Hell at full speed our robot may topple itself over even! For robot impacts we have bumpers so with those two things in mind I think we will be fine. But we will be testing for it in the upcoming weeks.

Not sure what you're talking about using smaller worm. We didn't need to do such a thing at all, we had to use smaller wheels or we wouldn't have enough torque and we would load the cims too much so we had to use a smaller wheel. Also these worms are already 2 start.

I was thinking aboud greasing them, it would help reduce the contact friction but would also make a mess =\ And yup there's absolutely zero vertical movement on the worm. Everything was spaced down to 0.001 of an inch. Being wedged between a thrust bearing and roller bearing on the other side it's got absolutely no where to go(unless it destroys the roller bearing, but that can be addressed if it happens)

[thefro526]

Originally Posted by magnets

Neat idea. However, I am fairly certain that your worm wheel will loose teeth when you have the weight of the robot behind it. If your robot is going fast, and suddenly you stop applying power, the worm wheel won't be to turn the worm gear, so you'll just snap a tooth off. It will be different than just testing it on a table.

In the video you talked about having to use a smaller worm wheel in order to get the same reduction. These are pretty fragile, so you could switch to a bigger worm wheel with a two-start worm.

Depending on the exact specification of the worm wheel and worm this could be a very likely scenario. Like Magnets said, you could switch to a larger worm wheel and a two/four start worm to lessen the tooth loading, and it'll have the added benefit of being more likely to back drive. Under extreme loading rather than locking up.

Your wheel choice is also going to play a huge role in the overall setup robustness, something on the lower end of the scale should start slipping before the transmission fails, where a high traction wheel might not slip until after the gearbox fails, its hard to say exactly without doing some calculations.

Magnets also brought up the subject of shock loading the worm wheel and worm during deceleration which is a very, very likely scenario. A decent bit of braking code could help to lessen the chance of hard stops during most matches, but the right hit or two could ruin your day. If you really wanted to stick with the worm setup and have it lock, a clutch setup could do some really cool stuff, or even some sort of flex plate/flexible coupling in the system to ease the load on the gear.

[sanelss]

Originally Posted by thefro526

Depending on the exact specification of the worm wheel and worm this could be a very likely scenario. Like Magnets said, you could switch to a larger worm wheel and a two/four start worm to lessen the tooth loading, and it'll have the added benefit of being more likely to back drive. Under extreme loading rather than locking up.

Your wheel choice is also going to play a huge role in the overall setup robustness, something on the lower end of the scale should start slipping before the transmission fails, where a high traction wheel might not slip until after the gearbox fails, its hard to say exactly without doing some calculations.

Magnets also brought up the subject of shock loading the worm wheel and worm during deceleration which is a very, very likely scenario. A decent bit of braking code could help to lessen the chance of hard stops during most matches, but the right hit or two could ruin your day. If you really wanted to stick with the worm setup and have it lock, a clutch setup could do some really cool stuff, or even some sort of flex plate/flexible coupling in the system to ease the load on the gear.

well the one we are using now is already a 2 start worm. We could go bigger but would probably try to find ones made out of harder material before we go bigger in size. The locking up thing is desired though, if it can survive that is.

Well we are using the vex omni, so i'm sure those will start dragging across the carpet before anything extremely drastic happens.

We are aware of the potential for failure, at least this system will still work with just 3/4 wheels(even 2/4 if opposite corners go out) and are extremely easily replaced, a single bolt, slide axle out, swap gear, and ur back on in less than a minutes worth of time. a clutch setup could help but then you run into complexity and reliability issues. We'll see how far this will take us and if it can survive what we throw at it then we won't be worried.

[magnets]

Originally Posted by sanelss

I was thinking aboud greasing them, it would help reduce the contact friction but would also make a mess )

You really, really need to lubricate this. You'll have little specks of brass dust all over the place if you don't. I agree with the rest of the post though. It's only a prototype, so you don't really know what will happen. It could be that the transmission with backdrive before you brake anything, or that the wheel (omni wheels don't have much grip) will loose traction with the ground before something goes wrong.

Overall, I really like how small this is. It's perfect to fit in the corner of a robot.

[sanelss]

Originally Posted by magnets

You really, really need to lubricate this. You'll have little specks of brass dust all over the place if you don't. I agree with the rest of the post though. It's only a prototype, so you don't really know what will happen. It could be that the transmission with backdrive before you brake anything, or that the wheel (omni wheels don't have much grip) will loose traction with the ground before something goes wrong.

Overall, I really like how small this is. It's perfect to fit in the corner of a robot.

I don't know. so far from the abuse I put it through it's not grinding, it's actually putting a mirror polish finish on the bronze gear so unless grit/dirt gets inside them I don't think there will be any bronze specks lol. But as stated yes it would be better but will make an absolute mess x.x

http://imgur.com/a/zgy5x

are some pictures of a complete drive chasis. So far everything u see in that picture only weighs about 27lbs and that's actual weighed items.

[EricH]

Originally Posted by sanelss

so far from the abuse I put it through it's not grinding, it's actually putting a mirror polish finish on the bronze gear

Got bad news for ya. The only really good way to get a mirror polish finish on metal is to grind it--admittedly with a really fine sandpaper/grinding wheel/buffer/other piece of metal, but you're still grinding it. You're not seeing the specks because they probably aren't visible to the naked eye, but I'm willing to bet they're there. There's a number of factors that could play into whether or not it continues to grind, like how much it's work-hardening or how smooth the other surface is.

But like I said, you're probably already grinding it a little and don't even realize it. That's why you have the offseason--to test things out and to learn what will and won't work and to figure out what you'll need spares of.

[sanelss]

Originally Posted by EricH

Got bad news for ya. The only really good way to get a mirror polish finish on metal is to grind it--admittedly with a really fine sandpaper/grinding wheel/buffer/other piece of metal, but you're still grinding it. You're not seeing the specks because they probably aren't visible to the naked eye, but I'm willing to bet they're there. There's a number of factors that could play into whether or not it continues to grind, like how much it's work-hardening or how smooth the other surface is.

But like I said, you're probably already grinding it a little and don't even realize it. That's why you have the offseason--to test things out and to learn what will and won't work and to figure out what you'll need spares of.

fair point. Considering they both have a machined finish, it's simply the surface finish that's doing the grinding itself. But we'll see how many work hours we can get out of them before wear becomes unacceptable. One year when we tried to do worm alignment by eye/hand we were eating through worm gears almost on a daily basis. It was literally eating away at the gears rather badly but with a few spares we were able to make it through competition, so aslong as they last long enough we should be fine. we always did plan on having some spares anyway. But as you said, that's what off season is for so we should be ready and tweaked out by the time this season starts

[Woolly]

This would be really interesting if applied to a crab module or 6-cim 6-wheel drop center with traction wheels.

[SoftwareBug2.0]

Originally Posted by Woolly

This would be really interesting if applied to a crab module or 6-cim 6-wheel drop center with traction wheels.

There is nothing new under the sun.

http://www.chiefdelphi.com/media/photos/26656
http://www.chiefdelphi.com/media/photos/27244

By the way, shock loading was more of a problem for our opponents. But we also had much larger worm gears.

Only four regular CIMs allowed that year though.

[themccannman]

Originally Posted by Woolly

This would be really interesting if applied to a crab module or 6-cim 6-wheel drop center with traction wheels.

If you put some wide-grippy traction wheels on this you'd be destroying your worm gears left and right. The only reason I haven't pointed this out is because the limited traction of the omni wheels will impart very little force back into the gears in the case of an immediate stop or getting pushed. I'm pretty sure this transmission would not last you more than one competition on anything other than omni wheels. The other issue with this is that the gears will lock up much easier with slight damage and wear than a normal transmission would. Slight bends in the teeth are much more prone to locking up worm gears than normal gears.

Here's a quote from cory (who knows more about this stuff than 99% of this forum) mentioning the dangers of using worm gears and relatively soft metals for drivetrains. This comment was from the first picture that softwarebug2.0 posted.

Originally Posted by Cory

That grease won't help them if they collide with another robot at high speed... I remember from 2004 and prior about people using the drill motors with their gearboxes, without removing the anti-backdrive pins, and completely destroying the internals during impact with other teams.

[Madison]

Originally Posted by sanelss

indeed it will. I never said it wouldn't however with this system is such that it makes the job much easier on the cim. For example if we were moving forward at a certain speed. A robot behind us is attempting to push us forward faster. It's not going to happen. All him trying to do so would REDUCE the load on our drive motors and the extra force would be directed into our chasis. If we wanted to remain stationary, and someone tries to push us, provided we have enough traction our robot will move absolutely nowhere regardless of how hard they push. or we can advance forward at the rate we want to advance at and not any faster than we want, due to this type of setup. Now granted if we faced head on against another robot, if we don't have more force than they do we won't be able to push them back, but at the same time they wouldn't be able to push us back either and all the cims simply have to do is not move and all the force is transferred into the transmission structure

Forgive me if I missed this, but what kind of feedback are you receiving that allows you to measure actual velocity versus desired velocity?

With an encoder on the worm, you're only able to measure the CIM's speed; you can't empirically determine whether or not the rotation of the worm is actually being transformed into linear motion along the ground. The presence of (or lack of) backlash between the wheel / worm gear / worm doesn't change that.

[sanelss]

Originally Posted by Madison

Forgive me if I missed this, but what kind of feedback are you receiving that allows you to measure actual velocity versus desired velocity?

With an encoder on the worm, you're only able to measure the CIM's speed; you can't empirically determine whether or not the rotation of the worm is actually being transformed into linear motion along the ground. The presence of (or lack of) backlash between the wheel / worm gear / worm doesn't change that.

Yes you are right. I was talking from a more theoretical viewpoint rather than a practical one where assumptions like wheels not slipping aren't necessarily accurate. Although thinking about it, since we know how much and in what direction force is applied to the motors, we can have a feedback loop with the accelerators which would indeed tell us actual acceleration vs attempted. so if wheels are moving but robot isn't well we know we arn't actually going anywhere or if the proportions are off what they should be.

[yash101]

Saneless,
This is a really great design. Is your robot an omni-drive robot? I showed this to my team captain and he seems quite interested! Also, I do not know if this has been discussed yet, but since the worm can turn the gear but the gear can't turn the worm, wouldn't the momentum of the robot cause the gears to strip? I do not see the point of worm gears because one sudden stop could possibly strip the gear!

I think that the internal braking in motors should be enough. With that high of a gear ratio, that momentum is converted into electricity and discarded as heat, casuing no damage to any physical component!

[MichaelBick]

Originally Posted by yash101

Saneless,
but since the worm can turn the gear but the gear can't turn the worm, wouldn't the momentum of the robot cause the gears to strip?

192 ran worm gears and did not have any problems from what I heard. They have the nice benefit of giving you more space in the center of your robot.

[yash101]

Yes! That is one nice reward from using worms! They are very compact but give a great gear ratio! They are also lightweight!