Offseason WCD with 3 CIM inverted transmission design

[Oblarg]

Interesting compromise with the inverted CIM design to allow different wheel sizes.

I only see two spacers at the bottom running between the gearbox plates. Are you sure that's stiff enough?

Originally Posted by Oblarg

Interesting compromise with the inverted CIM design to allow different wheel sizes.

I only see two spacers at the bottom running between the gearbox plates. Are you sure that's stiff enough?

There are two others you can't see in the model (hidden by the motors).

[asid61]

I would highly recommend you do not cantaliever any gears. My second gearbox I designed, which we neglected to test this year under load and plopped right onto the WCD, had gear mating issues on the second stage gear because it was cantalievered.
Keep in mind this was with two bearings on it, one on the wheel block and one right next to the gear in the gearbox. The gear still had massive mating issues.
Instead, try putting the cims offset by 1/2" or so to allow for the gears to be inside the gearbox and have bearings on them on both sides.

On a side note, that's a pretty sweet gearbox. I like the way you are holding the cims with the gearbox. Seems to save a lot of space.
Keep in mind those holes will need to be a bit larger than 2.536", the OD of the cim.

[Jared]

Originally Posted by asid61

I would highly recommend you do not cantaliever any gears. My second gearbox I designed, which we neglected to test this year under load and plopped right onto the WCD, had gear mating issues on the second stage gear because it was cantalievered.
Keep in mind this was with two bearings on it, one on the wheel block and one right next to the gear in the gearbox. The gear still had massive mating issues.
Instead, try putting the cims offset by 1/2" or so to allow for the gears to be inside the gearbox and have bearings on them on both sides.

I wouldn't worry about the cantilevered gears too much. We had problems when we cantilevered gears on our output shaft, but we had no issues cantilevering our 2nd stage of reduction (30t to 56t gears). The gears cantilevered on this gearbox are rotating quickly so they have less torque, and less force on the shafts than a cantilevered gear on the output shaft.

The only thing I'd worry about is the four gears (pinion, two idlers, and one more) that are engaged in a square. That center pinion might not fit in there. If it engages with one gear, the other gear may never line up. Unless you've designed for it, both gears that engage with the center pinion could always be slightly misaligned. A simple solution might be to replace one of the idler gears with the versa hub pattern with a smaller gear, so that the center pinion only engages with one gear.

Originally Posted by Jared

I wouldn't worry about the cantilevered gears too much. We had problems when we cantilevered gears on our output shaft, but we had no issues cantilevering our 2nd stage of reduction (30t to 56t gears). The gears cantilevered on this gearbox are rotating quickly so they have less torque, and less force on the shafts than a cantilevered gear on the output shaft.

The only thing I'd worry about is the four gears (pinion, two idlers, and one more) that are engaged in a square. That center pinion might not fit in there. If it engages with one gear, the other gear may never line up. Unless you've designed for it, both gears that engage with the center pinion could always be slightly misaligned.

It is designed in there.


Also in the second iteration the idler gears will be bearing bore gears on dead axle shafts that extend through both plates in the gearbox.

[Aren Siekmeier]

Originally Posted by Andrew Lawrence

Also in the second iteration the idler gears will be bearing bore gears on dead axle shafts that extend through both plates in the gearbox.

I like this idea, giving the option to use these dead axles as another pair of standoffs holding the gearbox together.

However, tagging along with Roger and Jared, really pay attention to the mesh on those 4 gears meshing in a loop. You'll have to make sure the tooth offsets given by the interior angles (with alternating sign) add up to a whole number around the loop, so you get the correct mesh all around. But even with that, a small tolerance on gear placement could mean that the driven gear is only contacting one idler at any given time, or even that you still get a gear collision.

[Aren Siekmeier]

Also, how are you planning to attach the bumper rail to its supports and the supports to the main frame?

The side supports will do just fine, though I would recommend adding a lip to them retain the bumper rail horizontally, reducing the load on any weld joint or bolt/gusset. For lack of a better picture, here's a screenshot of a Youtube video as an example http://i.imgur.com/VhdqrmC.png.

If you secure the front/back supports with gussets or brackets, I can tell you from experience that they will bend from a forward or backward impact (the brackets, that is, the tube will be just fine). Even welding, I would think you'd be risking breaking a weld. You can get creative with the geometry of that support piece so that in rotating backward it both compresses some geometry of its own and reacts against the top surface of your main frame. Edit: for example

[Travis Schuh]

Backing up a bit, why did you choose 6 CIMs over 4? For me, there is a very real cost to going with a 6 CIM drive, yet after two seasons of there being bots with 6 CIM drives, I do not see strong evidence that 6 CIMs has a large advantage over 4 CIM. The tradeoff is that you can't use the CIMs elsewhere, they add weight, and they greatly increases breaker tripping risks. We ran 4 CIMs this year after trying 6 last year, and were very pleased with our decision to do so (it would have been much much harder to build the robot we did if we had not done such).

Also, why belts in a WCD? My rough estimate is that you are saving less than 0.5lb by using belts over #25 chain, yet a WCD drive does not have a good answer of how to change a broken belt between elimination matches. If you are going to bet on the belts not breaking, I would recommend sizing up to the 15mm wide. We have broken belts on our robot (albeit most frequently on our practice robot and in the after season), and find that it is an important design priority to be able to quickly change belts. Our calculations have also shown that with 9mm wide GT2, we are a running the belts a good bit out of the belt's specs.

Originally Posted by Travis Schuh

Backing up a bit, why did you choose 6 CIMs over 4? For me, there is a very real cost to going with a 6 CIM drive, yet after two seasons of there being bots with 6 CIM drives, I do not see strong evidence that 6 CIMs has a large advantage over 4 CIM. The tradeoff is that you can't use the CIMs elsewhere, they add weight, and they greatly increases breaker tripping risks. We ran 4 CIMs this year after trying 6 last year, and were very pleased with our decision to do so (it would have been much much harder to build the robot we did if we had not done such).

Also, why belts in a WCD? My rough estimate is that you are saving less than 0.5lb by using belts over #25 chain, yet a WCD drive does not have a good answer of how to change a broken belt between elimination matches. If you are going to bet on the belts not breaking, I would recommend sizing up to the 15mm wide. We have broken belts on our robot (albeit most frequently on our practice robot and in the after season), and find that it is an important design priority to be able to quickly change belts. Our calculations have also shown that with 9mm wide GT2, we are a running the belts a good bit out of the belt's specs.

The goal with the six cims was simply a design challenge for myself - not pure intent for use in competition.

The belts were chosen out of lack of experience using 25 chain and sprockets in this kind of setup. Of course the pulleys can be changed out with sprockets if we really wanted to.

I appreciate the advice, though!

[highlander]

Originally Posted by asid61

I would highly recommend you do not cantaliever any gears. My second gearbox I designed, which we neglected to test this year under load and plopped right onto the WCD, had gear mating issues on the second stage gear because it was cantalievered.

Hold on a second; was your shaft 6061, 7075, or steel? I've seen 6061 shafts bend with a longer lever arm, but with 7075 and a short lever arm like in his picture the gears will be fine (as long as he has the two bearings he mentioned above: one bearing is generally a bad idea)


Also, I like the support you are giving the CIMS, but do you need the support from the sides? You could probably save some aluminum by just providing support from the bottom, if that makes sense.

[asid61]

Originally Posted by highlander

Hold on a second; was your shaft 6061, 7075, or steel? I've seen 6061 shafts bend with a longer lever arm, but with 7075 and a short lever arm like in his picture the gears will be fine (as long as he has the two bearings he mentioned above: one bearing is generally a bad idea)

7075. The shaft was not bent, however, some of the shafts on the outer wheels on our WCD were 6061 and got bent. I still think there could be problems though. Looking at the forces involved here, the gear will want to move out of engagement.
Of course, just my opinion; the gear does look closer than the one on my gearbox did to the bearing.

Remember to put some kind of protection over the gears like polycarbonate, else wires will get stuck in there, as well as fingers.

It looks like there are two screws on the bottom of the gearbox that don't go all the way through the nut. Consider switching to thinner locknuts or lengthening the screw just in case.

[highlander]

Originally Posted by asid61

7075. The shaft was not bent, however, some of the shafts on the outer wheels on our WCD were 6061 and got bent. I still think there could be problems though. Looking at the forces involved here, the gear will want to move out of engagement.
Of course, just my opinion; the gear does look closer than the one on my gearbox did to the bearing.

Remember to put some kind of protection over the gears like polycarbonate, else wires will get stuck in there, as well as fingers.

It looks like there are two screws on the bottom of the gearbox that don't go all the way through the nut. Consider switching to thinner locknuts or lengthening the screw just in case.

I'm not understanding something here then; if it wasn't bent, how could the gear possibly come out of alignment? Were the tolerances bad? Were your bearings messed up? Were the bearings really close together?

[asid61]

Originally Posted by highlander

I'm not understanding something here then; if it wasn't bent, how could the gear possibly come out of alignment? Were the tolerances bad? Were your bearings messed up? Were the bearings really close together?

We don't fully understand either.
The bearings had 1" between them at the closest measurement (2x1 WCD). There was also 2 sprockets between the bearing and the gear, so possibly that was the problem. We haven't checked the gearbox shafts, only the outer shafts. I guess the shaft could be bent, but it seems pretty unlikely to me as it has a huge marign of safety.
The bearings were 1/2" hex bearings which seem to have a lot of wiggle room to me, and maybe that combined with the long distance to the gear let to engagement issues.

[Chris is me]

Originally Posted by highlander

I'm not understanding something here then; if it wasn't bent, how could the gear possibly come out of alignment? Were the tolerances bad? Were your bearings messed up? Were the bearings really close together?

Not all deformation is plastic deformation. It's possible that with a cantilevered shaft, the forces of the gears interacting with each other, transmitted to the shaft, caused a small elastic deflection that isn't visible after load is removed (or barely visible under load even).

There was also 2 sprockets between the bearing and the gear, so possibly that was the problem.

This is almost certainly what the problem was and a detail you absolutely can't leave out. You have at least an inch of space between the bearing and the gear which is a much more dramatic cantilever than a gear butted right up against a bearing, plus the tension in the chain running on those sprockets could have contributed to elastic deflection. This setup has cantilevered gears that are much more well supported and thus less likely to lead ot issue.