pic: Two Motor, 4WD

[Matt_Krass]

In this design I decided to trade a weight/simplicity for ease of use to make 4WD on a robot, with dual outputs and sprockets it’s much easier and neater to connect two wheels to each transmission.

It uses two stages of 1:4 reductions for an overal 1:16 reduction driven by two CIM motors. It uses three 15 tooth and three 60 tooth gears. The cims are bolted on to the side plate and the outputs are 1/2" shafts turned down to fit in the gear bores mounted through two bearings. I offset the sprockets lifting them up so I could run the axle straight through to a second bearing without conflicting with either CIM to increase strength. Comments and critiques welcome.

[Billfred]

Looks interesting.

Any chance of a top view to get a little bit better view of things?

[Matt_Krass]

A top view? Surely.

It’s attached, any questions anyone? Comments? Huge flaws I missed?

[Arefin_Bari]

First of all, great job Matt. It is nice to see a transmission which can be build and put on a robot. Remember the night when we were talking about transmission and mechanisms in general and how you said that you wanted to design something that works? Well here it is… you did it.

Now some questions…

Are you going to add stands between the two plates? How are the two plates attached to the plate on the bottom? I am guessing screws? Even if you put screws on the bottom, personally I would use stands between plates. How thick are those plates?

Do you plan to take out any weight from the gears/plates?

I am guessing that the transmission is about 8” by 4” by 2”… am I right? How much does this thing weigh?

Looking at the Top view… I think you can make the gearbox thinner. It will be a bit of work, because you will have to modify the plate on the bottom.

Can’t wait to see the new version.

[Tristan_Lall]

Do you have the ratios handy? I’m somewhat concerned with the torque that’s going to be passing through the pinions (especially on the centre shaft). If the black 3/16" material is hardened steel, you’d be off to a good start. That centre pinion (20 teeth?) looks like it will die first, if anything has got to go, though.

Also, with thin hubless gears, you’ll need to consider how you want to mount them to the shaft. Lots of ways (e.g. keys, adhesives, welds, hubs, press fits, etc.) are possible, but it all depends on how you want to be able to disassemble it.

I’d consider leaving a little bit of clearance between the side plate and the face of the gears. In an ideal world, there would be exactly zero clearance, and exactly zero friction between them; in real life, the gear will wobble somewhat, and touch the plate, probably producing visible scratches, and yielding correspondingly increased friction (at least until it wears in).

Regarding the CIMs, am I overlooking a spacer somewhere, or have you constrained the front of the CIMs’ raised centre protrusion (you know, the 0.750" one with the shaft in the middle—what’s this called, anyway?) to the surface of the plate? You probably want the faceplate of the motor to sit flush instead, and to leave a hole for the centre protrusion.

Regarding the diameter of the shafting, you’ll probably need to use something torsionally rigid to use that size; almost certainly steel, and pre-hardened, precision-ground would be a bonus. And of course, I assume a nice set of ball bearings will be added in later…

[Tristan_Lall]

Regarding the CIMs, am I overlooking a spacer somewhere, or have you constrained the front of the CIMs’ raised centre protrusion (you know, the 0.750" one with the shaft in the middle—what’s this called, anyway?) to the surface of the plate? You probably want the faceplate of the motor to sit flush instead, and to leave a hole for the centre protrusion.

And upon a second look, I think I see the problem: if the faceplate of the motor were constrained to the surface of the gearbox plate, the tip of the motor shaft might interfere with the second stage of gears. If that’s really the case—and I’m not being fooled by some trick of perspective (it sucks when you can’t just rotate the picture and look underneath!)—you can either use a spacer, widen the gearbox, or cut the shaft. I’d probably go with widening the gearbox, unless space is an issue, because the spacer will likely give you another component to add complexity and misalignment to the assembly, and if you avoid cutting the shaft, it means that when you really, really, need to change a blown motor for the next match, you can just borrow one from me and install it, rather than having to run around trying to find a hacksaw with which to modify the shaft. Actually, there’s another option which may or may not work; that’s to rearrange the gears so that this shaft doesn’t interfere—but this may necessitate a change of ratio, or idlers, or other more involved changes.

[Nikhil_Bajaj]

What are the diametral pitch of the gears and their thicknesses? I’m worried about that 20-tooth as well…

[Arefin_Bari]

What are the diametral pitch of the gears and their thicknesses? I’m worried about that 20-tooth as well…

The thickness for the gears are .25". I suggested him to make them .375". As far the pitch of the gears go… Matt will answer that. There are no 20 tooth gear in that transmission. He is using 3 15 tooth gear and 3 60 tooth gear. Each sprockets has 16 tooth.

[Tristan_Lall]

The thickness for the gears are .25". I suggested him to make them .375". As far the pitch of the gears go… Matt will answer that. There are no 20 tooth gear in that transmission. He is using 3 15 tooth gear and 3 60 tooth gear. Each sprockets has 16 tooth.

Are you sure about the 0.25" thickness for the gears? Compared to the front plate of the CIM (which is definately 0.25"), they seem smaller. Maybe Matt changed them without posting an updated picture? Depending on the material, I think that these narrow gears could be made to work for the first (higher speed) stage; it’s the second stage that will fail first.

And as for 15 vs. 20 teeth, 15 just makes things worse! (Higher stresses on the pinion, and depending on the pressure angle of the gear, the potential for some significantly undercut teeth.)

[Nikhil_Bajaj]

Are you sure about the 0.25" thickness for the gears?

And as for 15 vs. 20 teeth, 15 just makes things worse! (Higher stresses on the pinion, and depending on the pressure angle of the gear, the potential for some significantly undercut teeth.)

I’m going to second the .375 suggestion–that’ll definitely be safe…even our 12 tooth, 20 pitch, .375 gears on our gearbox that used 2 chips and a fish was fine…then again the small gears were early to middle stages in the gearbox so the torque was not so high. But I think .375 should be fine. And if you do go with the wider gears then it may make it easier for you to work out the spacing for the gearbox.

By the way, how are the plates with the bearings and such mounted to the other section? For gearboxes, precise and accurate alignment is absolutely critical. A few thousandths of an inch of misalignment can have a horrendous effect on efficiency. You may want to figure out a way to keep it spaced well…we picked a silly design last year and we ended up having to remake the spacing blocks like three times, and even after that had to use a shim or two.