Front view of a low profile 3 cim shifting gearbox I designed. In the future I will lighten the side plates to reduce weight. The Low speed has a 10.7:1 Gear ratio and the high speed has a gear ratio of 5:1. This translates to about 18.5 fps and 8.65 fps. The gearbox is adjustable so that 11,12,13,or 14 tooth cim pinion gears can be used just like the Vexpro DS gearbox. It uses 9mm 30 tooth belt pulleys.
How is this mounted to tubing? It looks like it’s only using the bearing block bolts to hold the gearbox on.
Also, slight tidbit - the ability to use multiple pinion sizes is a design and feature of WCP gearboxes, not the VEX ones. The DS and SS are made by WCP. VEX and WCP are resellers of each other’s products. Just clearing that up so that credit is given where credit is due.
What is the weight?
How thick are the side plates?
Are you using a custom or COTS shifting shaft? If so, which one?
How is this mounted?
Are the cims acting as nuts for the standoffs? If so, consider changing that to just nuts or pemnuts.
How are the shafts constrained? Are they hex turned to round?
That’s all for now. I can’t tell much from this view, but it looks pretty good.
I would assume since this gearbox is modeled like the WCP DS, that the bolts on the bottom part of the GB would go through the tube and bolt to it.
Nice design. I’m a little worried about how the chains or belts will make it out of the gearbox and to other wheels. It looks like they would go right into the lower standoff tubes.
Apart from the the bearing block, the two bottom bolts through the tube and bolt onto it, just like cash4587 said.
You are correct it is the DS and SS gearboxes that make the adjustable cim mounts but I am so used to using components from both that I tend to get them mixed up a lot.
As is the gearbox weighs about 11.2 pounds according to Solidworks. I made the side plates 1/4" thick. The Shifting output shaft I am using is the one COTS shifting shaft and is the same one the is used on the DS.
It is this one:
The shafts are 3/8" hex shaft turned to 3/8" round but I might possibly use 1/2" hex and turn that to 3/8" because there isn’t much material to constrain the shaft against the bearing with the 3/8" hex shaft.
I am in fact using the cims as nuts but I am curious to know why there is a problem with this. Would it be a structural issue?
Not so much a structural issue as a maintenance one. It’s a pain to have to remove the motors to disassemble the gearbox or to disassemble the gearbox to remove a motor.
What do you guys think of the gear ratios? Are they appropriate gear ratios for a 3 cim gearbox?
And also, Sanddrag, I hadn’t noticed that the pulleys would hit the standoffs but I just raised the location of the standoffs to give enough clearance for the pulleys. Thanks for noticing that!
Yeah, we totally missed it through multiple design reviews on one of our gearboxes one year. Didn’t catch it until assembly. We vowed to never repeat that oversight.
That’s a nice shaft.
3/8" hex turned to 3/8" will work fine. Look at the new Thunderhex usage examples to see how small a shoulder you can use. Bearings are very high precision, and you will have about a 1/32" shoulder on six points to constrain it.
As stated above, it’s a maintenance issue. If you lose a CIM somehow, then having to take off the gearbox and reassemble it is a no-no.
Under what conditions would one “lose a CIM”? In 10 seasons I have yet to experience a bad CIM or see one in person.
It’s actually really difficult to just pull a CIM out of a gearbox without taking it apart. If you’re using a 12 or 11 tooth pinion, the retaining ring on the CIM catches on the gear that the CIM pinion mates to. If you’re using a 14 tooth gear, the pinion won’t fit through the .755" hole for the CIM boss.
Also, if you did want to remove the CIM without disassembling the gearbox, I’d be willing to bet that the gearbox wouldn’t just fall apart if one (or even all three) of the upper standoffs were removed.
If you wanted to disassemble a gearbox without removing a CIM, you’ll have to remove one of the mounting bolts from each CIM. From experience, the CIMs don’t fall out of the gearbox when this happens. They can’t go anywhere because the one remaining bolt hold the CIM boss in a tightly fitting hole.
It looks like a solid design, and the gear ratios sound good too.
Are you using an odd size of retaining ring? I’ve never had this problem before with the ring catching on other gears, but maybe I’ve just been lucky.
Also, if you did want to remove the CIM without disassembling the gearbox, I’d be willing to bet that the gearbox wouldn’t just fall apart if one (or even all three) of the upper standoffs were removed.
Take one motor off at a time and add nuts where the motors used to be. Not that hard to do really, it just requires a little thought beforehand.
I have, just once. It was a brand new CIM in 2012. We weren’t able to climb the bridge straight one match, so right after the match we felt all four CIMs, three were the standard moderately warm to touch, one was cold as if it hadn’t been running (which it hadn’t, we checked after removal).
While rare, it can happen so it’s not a bad idea to design for CIM removal.
We managed to damage all 6 CIMs in our drivetrain at Waterloo last year. We were having issues with popping breakers, inconsistent performance and the CIMs getting very hot very quickly. When we removed each CIM from the gearbox we found their no load current was 7-10 amps versus 2-3 amps a stock CIM would have. Replacing all our CIMs fixed our issues.
It always happens to us. We can usually get it far enough out to wiggle it around the other gear, though.
I have right now no less than four cims sitting my garage. I took them home because they were burning up and were no longer used on the team.
Some grease on the insides brought them back to life, but it is possible if they heat up too much.
Even though it is a lot harder to switch out cims, I’m still think I’m going to keep the cims where they are because chances are we won’t burn a motor. As far as the cim pinions go, its doesn’t require the gearbox take apart because I could just take it out and change it.
One thing that does concern me though is the ease of changing belts if they snap.
You have essentially guaranteed that you will now lose all of your cims during elims due to Murphy’s Law.
But seriously, is it that hard to just add one more non-cim standoff? Two standoffs can hold your box together when swapping cims. In that situation, having through bolts to your cims is wise because it becomes easy to remove and add cims.