pic: 696 Gearbox Transmission

[sanddrag]

[cdm-description=photo]23300[/cdm-description]

[Travis Covington]

Did you use the standoff screws to mount the CIM motor too? Looks good!

[sanddrag]

Did you use the standoff screws to mount the CIM motor too? Looks good!Yes. That's one of the things I like about it. 3 of the 4 standoff screws go all the way through to the CIM motors. The other standoff gets a nut on the motor plate side. There is one bolt in one of the CIMs that is just by itself. You can barely see it in the picture. There may be reduncancy in having the standoff that is closest to the left side of the picture, but eh, whatever.

The gearbox could be narrower by about 3/4" but we have a sprocket inside and are running the chain right out of the box, so, that's where the width is coming from.

By using small wheels (~3.375"), we were able to direct drive and save weight and space. And we can still get on the platform.

[Travis Covington]

What did you end up doing with the 13tooth gear?

[Rafi Ahmed]

How's are you guys getting up the ramp?

[sanddrag]

Ah yes, the 13 tooth gear. Most likely only Travis knows what I'm talking about, but I'll go ahead and say here. We ended up boring out the gear to O size (.316) and broaching in a 3/32 keyway to not quite full depth. So the gear is keyed to a .316 portion of the shaft. The very end of the shaft is turned down to 1/4" Since we already made the plates to fit a .875 OD (.375 ID) bearing, we put the 1/4" end of the shaft in a bronze bushing with .375 OD which rides in the .375 ID. For now, it is working, but we have not extensively tested it. We are in the process of finishing machining for the second robot's drivetrain.

The 13 tooth gear is one of a few things that keep me up at night. It is a tiny little sucker made from not the highest quality materials and it has the potential to undergo considerable stress.

The gearbox has many things that are kind of "on the egde" when it comes to design. For example, the primary reduction gear's teeth are about .025 away from the shifting dog which is spinning in an opposite direction at a much different angular velocity. We have standoffs that are very close to gear teeth. Our ground clearance is extremely low near the wheel. Approximately 3/16" There's quite a few things about it that just plain scare me, but we always overbuild everything, and instinct tells me (and the second set will comfirm) that it'll be okay. There's a lot of stuff that is real close, but it has no means and no reason to move. If it clears now, it will always clear. Anyhow, I don't think two speed dog shifting transmissions come too much smaller or lighter than this.

There is about a 3:1 difference between high and low. And the overall reduction is a bit more than an AndyMark shifter (allowing us to direct drive the wheel).

For getting up the ramp, you'll quickly notice that our wheels do not look capable. But design has speculated and testing has confirmed that indeed they are. Here's the reason why. If you look square on at the face of the wheel, you will not see a good portion of the bottom of the gearbox. In the side view of the robot, the lower portion of the gearbox is completely encapsulated by the wheel. In climbing the ramp at a roughly straight approach, there is no surface that can come into contact with the gearbox because it is in contact with the slighty larger wheel.

[Travis Covington]

None of those things sound scary to me at all. I wouldn't worry about any of it. It looks like you have yourselves a nice robot yet again!

[team222badbrad]

What is the Pitch Diameter of the 13 tooth gear?

We pressed on 11 tooth 20DP Browning gears onto our CIMS.

We did not use a keyway, we just cut four slots onto their hubs and threw some shaft collars on.

[sanddrag]

Originally we had a 20 pitch 13 tooth gear that was supposed to be cut into a shaft so the gear and the shaft would be one. But, due to time constraints, we decided to press it on a 3/8" shaft. Well, that didn't go too well. The "wall" thickness between bore and root diameter was small and we cracked gears trying to press them on.

So, as it currently stands, the gear is on a 5/16" shaft with a 3/32 key and has the potential to be under 15 N-M of torque if somehow something jams up, and the motors stall, and the electrical system is briefly able to provide full current while the motors are stalled. I highly doubt that will ever happen, and I think we'll be alright, but the thought of a big shock load (like going from full forward to full reverse very rapidly) still scares me.

[TriggsJr]

This looks sweet.....Nice Machining job