Prototype transmission for potential use this year
 

Re: Prototype transmission for potential use this year
 

Looks great. Do you think you could eliminate a lot of this play with a hex hub? You could try to broach the bronze gear to the same shape.

Re: Prototype transmission for potential use this year
 

well the play isn't on the bronze gear, that mating is actually pretty good. it's specifically between the andymark hub, keystock, and axle. Also we don't have the tools for hex shafts and it would be a major pain to deal with the bearings considering how it has to be assembled. but even then there would be some inherent play anyway. One option would be to setscrew the key in place. or get the 3/8 hubs and then bore and broach it ourselves for a tighter fit. The axle was sanded down though so maybe whoever did that went too far. may not be worth messing with though

Re: Prototype transmission for potential use this year
 

Are you able to turn the wheel by hand on the new version?

Re: Prototype transmission for potential use this year
 

not a chance in hell. this drive is 1 way only(cim to wheels). Trying to go the other way will be met with extreme resistance and would cause something to break if you actually could turn the wheel. But in our case this is actually a desired trait. It allows us perfect control of motor speeds at all times and provides extreme braking when someone else is trying to push is around when we don't want them to(providing the wheels grant enough traction)

Re: Prototype transmission for potential use this year
 

Nice work aligning the worm and minimizing lash in the gearmesh. :]

Have you measured the free current?

Re: Prototype transmission for potential use this year
 

it's about 3.5 amps

Re: Prototype transmission for potential use this year
 

This looks great guys. I think that this version will help you in many of the pushing battles that you all faced last year. Your omni-directional drivetrain was very fun advantage to pair up with last year during the elimination rounds and I wish we would have provided you all with some more firepower. Thanks again for selecting our team last year and I hope that we get to work together in Saint Louis again in 2014.

Re: Prototype transmission for potential use this year
 

I don't follow this logic.

Re: Prototype transmission for potential use this year
 

Well just think about it. Any wheel drive where you can move the motor by turning the wheel you will have drifting wheel speeds depending on what else is going on. for example another robot is trying to push you around or you want to slow down but the inertia carries your robot further. With this setup where the wheel can't move the motor. If we tell the motor to go at 100rpm, then it doesn't matter what else is going on that motor is going to draw up to 100A or apply extreme braking to make that motor turn at 100 rpm, not any slower and not any faster. That means if u want the robot to stop, it's going to come to a complete dead stop instantaneously. Also if we meet some resistance(another robot) then the PID loop will keep increasing force to the absolute limits to try to make that motor turn at the set speed. Also if another robot tries to push us around, even if we don't have enough power to overpower them, simply not trying to do anything will provide tons of force and keep them from pushing us around, well as much resistance as the friction of the omni wheels provide. With this design we don't need the force to over power other robots, we just need enough traction and that will prevent anyone from pushing us around.

Re: Prototype transmission for potential use this year
 

I think what he means to say is that it gives them near perfect wheel control. As soon as the motors are stopped so do the wheels while requiring little to no active braking from the motors. Would make for a very jerky robot without proper PID control though.

Re: Prototype transmission for potential use this year
 

not just a full stop, it also prevents any unwanted changes in speed. so if something is trying to accelerate or decelerate us that force is going to be put into the chasis instead of the motor. So as I stated if we want to go a set speed, any changes in speed will be met with extreme resistance other than what we tell it to be. Also I tested a PID loop earlier today that seems to work extremely well, so much so that the only issue was that when loaded down too much it would draw 80-100A and shut the jaguars down. I have a video of my PID loop with the old transmission, and the only difference a load makes is higher current draw, the tracking is just as good.

Re: Prototype transmission for potential use this year
 

Regardless of gearing method, the motor will shift up and down it's curve as the applied load changes (being pushed is an applied load).

If your statement were true, the gear train would magically be creating and dissipating energy.

Re: Prototype transmission for potential use this year
 

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

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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)

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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 :D

Re: Prototype transmission for potential use this year
 

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

Re: Prototype transmission for potential use this year
 

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. :D But we also had much larger worm gears.

Only four regular CIMs allowed that year though.

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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!

Re: Prototype transmission for potential use this year
 

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.

Re: Prototype transmission for potential use this year
 

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

Re: Prototype transmission for potential use this year
 

They also used 4 lead worms for increased efficiency, which made them backdrivable. This let them coast to stops, and even theoretically get pushed

The other thing to keep in mind about worm gears is that most of their non-back drivability (even for 1 lead worms) comes from when they're stopped. When they're stopped, you've got a ton of force pushing the worm gear's teeth into the worm (if the output's under load), and because of the angle of the contact between the teeth, it usually can't slip. However, when the tooth surfaces are turning relative to each other, you're dealing with kinetic sliding friction, which is a lot less. You'll actually see the output pushing the input a bit when some worm gearboxes aren't under power, but moving. Because they no longer have to deal with the huge static coeficient of friction, they become essentially temporarily backdrivable. This means that worm gearboxes can coast for a little while, especially when they've got a fair amount of force on the output.

Re: Prototype transmission for potential use this year
 

That seems brilliant! Do you know how that would work? It seems quite tricky!

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From what I've heard they weren't backdriveable until the gears wore in, and they had no problems.

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What do you mean by omni-drive? I think the answer is yes. The sudden stop I don't really think will be an issue, yes the wheels will lock up but worse case the energy will go into the robot frame and might tip the robot slightly or make the wheels just skip over carpeting before anything bad happens. It takes considerable force to strip these teeth but we will see how much of an issue it becomes. We already have several contingency plans in case it becomes an issue(such as a rubber interface as a clutch system that would slip on high force changes).

Re: Prototype transmission for potential use this year
 

Hi sanelss,

Thanks so much for sharing the details of your design; the youtube video was terrific!

We're working on a similar design for sim motor/Jaguar/encoder control/Mecanum wheel. Here're some links that describe gear design that have been discussed in this thread:

Surface hardening and polishing of gear wear-in
Gear tooth strength
Worm gear "anti back-drive" or "self locking"

We looked at worm gear to allow getting the CIM motor 90 deg to wheel but are pursuing a 2-stage design with planetary on the wheel shaft and bevel for 90 deg transition:



Do you have advice on where to source gears? I've only found expensive sets for industrial fabrication and very cheap hobby gears.

Re: Prototype transmission for potential use this year
 

McMaster sells some, or you could go with one from boston gear. I've done a swerve with the L110y. They're pretty expensive.

As for the drive design, bevel gears are going to be more durable than the worm gear setup. However, you do have to take into account that the two gears will be pushing against each other in every way possible, so you'll need thrust bearings, as much support as possible on your shafts, and you should use shorter shafts to minimize bending. It's also advantageous to run the gears at a higher speed if possible. When the gears run slowly, they can bend away from each other and slip.

Also, whenever you use gears, REMEMBER TO LUBRICATE THEM!

Re: Prototype transmission for potential use this year
 

Thanks for the feedback!

I have a couple of L93Y miter gears I bought on Amazon ($17) to play with. Our design has changed and requires a gear reduction and I'm just learning about bevel gear sets. Any guidance on bevel gear selection would be greatly appreciated.

Re: Prototype transmission for potential use this year
 

Ok, there is a lot for me to cover here

First off, please lubricate your gears, and put some sort of cover on the gearbox so the gears don't get covered in debris. Your efficiency is already going to be really low using a two start worm and no lubrication isn't going to help.

I have to agree with many who've already stated this, you are at risk of snapping the worm gear teeth. You have the benefit of using a bigger pitch gear than we did (16 Pitch right? or metric?) but you'll also using a smaller diameter gear and bigger wheels. The gear teeth snapping was one of our main concerns and we would never have used worm gears if they didn't back drive.

As for getting the worm gear in a harder material the only other material that they're made in is cast iron and while thats harder it's also brittle and won't help at all with the teeth snapping issue.

One other comment on the design, you're going to want thrust bearings on both sides of the worm.

Our Gearboxes were always back drivable, we could always push the robot around but it just wouldn't roll by itself early in the season. By champs we could push it and it would roll just like it would if it had a standard gearbox.

We never had any problems with the worm gearboxes, we didn't have to touch them the whole season...or after.

Our worm gears were Boston gear gears's purchased through Motion Industries. Gears with the same specks are available from SDP-SI.
Boston Gear bevel gears are also available through Motion Industries but i would suggest getting Martin Sprocket and Gear gears instead. Martin bevel gears are case hardened and will last longer than the Boston gear ones.
The bevel gears on Mcmaster-carr are Martin but they have a limited selection so we bought ours through Motion Industries.
But yeah they're all pretty expensive...

Re: Prototype transmission for potential use this year
 

I brought up a few of the several concerns here, but I don't have experience with worm gearing for drive parts. My thought is that the teeth will just snap off before the wheel looses traction with the ground. In your gearbox, did you have your worm gear before or after any other reduction?

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I know you were the first one to bring up these concerns and I agree with you completely, I was just adding another voice to the argument.

In our gearbox we had a 1:1.4 spur gear (20 to 28 tooth) reduction from each of the cim motors before the 1:10 reduction of the worm gear (4 start worm and 40 tooth gear). The worm gear was on the output shaft. It can be seen here http://www.chiefdelphi.com/forums/sh...ht=192+gearbox.
The layout is the same as the AndyMark rawbox

Re: Prototype transmission for potential use this year
 

That's really cool. I've never seen a robot with this sort of layout. The gearbox looks pretty small/space saving. Thanks for sharing!

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The gear box that Joey linked to is the first generation of space saving designs that 192 has produced. This year we will be using our third generation of space saving gear box. Each year is different and this gives the students a great engineering project to work thru. The worm gear box was bulletproof.:)

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Joey,

Thanks so much for the info! I hadn't found the Martin company; I've ordered the a Martin set: 3:1, 16P, steel/CA to test. I found them on Amazon where I've have had excellent shipping service. I use Amazon Prime and all Prime orders are free (with subscription) and guaranteed delivery dates usually within 2 days.

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Good to know they're on amazon with prime, ours took a lot longer to get to us from motion. I don't know how much info amazon gave you on the gears but all dimensions and mounting distances are available on their website in the catalogs.
This is the one for their bevel gears http://www.martinsprocket.com/docs/d...s.pdf?sfvrsn=4

Re: Prototype transmission for potential use this year
 

Good point! I almost wrote: Amazon's technical information is very weak and sometime flat-out incorrect. I never select from their info; only part #, and they sometime screw that up.

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We obtain our gears from qtcgears.com They arn't the easiest to navigate but have almost anything you could want and at reasonable cost which fits well into FRC robot budgets. this year we're only spending about $70 per gear set.