I see lots of gearboxes that follow the same basic pattern: two metal plates with bearing in them supporting lots of gears. how those two plates are attached to each other varies.
I want to know three things:
A)What material is used for those plates? It looks like 1.4 inche aluminum or something similar
B)How do you get those bearings into the plates? This has baffled me. It looks like a hole is cut a a bearing just stuck in there. Why isn’t it coming out? Is this a locktite/glue you are using. I realize that side loads would be zero or minimal, but you still could have your boat broadsided by something really fast like team 16’s 2003 robot, and those bearings could fly out . .
C)What degree of precision are those plates cut with? Is it possible without a CNC mill?
And fourth (I lied about only three questions), where does one find a standard-sized gear that will fit onto the CIM’s shaft?
B) Use a somewhat (really precise, eh? ) tight hole for the bearing and punch four small holes in the plate, along the perimeter of the bearing. I don’t know if that sounded clear enough, tell me if I didn’t.
C) We cut the plates with a laser machine, but the design did allow some slack. You can definitely do it with a skilled drill operator.
Well, to answer the question for my team’s gearbox (which is pretty similar to the others you see on here):
A) We used 1/4" aluminum plate for the sides of the gearbox
B) The bearings were press-fit into place. They don’t take that much side-to-side strain, even in a big hit, so it’s not a big deal how they’re in there, if you get a tight press-fit.
C) Ours was pretty precise. We didn’t have the equipment to make it onsite, as we had only a manual mill. I believe ours and the Technokats’ both need CNC. I know Dave Lavery’s gearbox design can be built with manual machinery, however.
D) The CIM shaft is a metric bore and key size. I believe it’s 7mm. You’d need to find a gear with that size bore, or get a non-metric bore gear that’s smaller, and drill a 7mm hole.
This year my team used aluminum plates. Im not sure what thickness.
For the bearings, use a flanged bearing and collars on the shaft.
What degree of precision? Well, I used a hole saw and hand drill. I really,really dont suggest this. We had many gearbox issues. If you have acess to a CNC mill, use it. And make extras.
I believe DART got all of our gears from WM Berg. I think the gear we got for the chippy motor was a little to small, so we powered up the motor and held a file against it. We then rotabroached a keyway into the gear, and put it on. Use locktite to hold the set screw in.
*Originally posted by Kevin A *
** We then rotabroached a keyway into the gear, and put it on. Use locktite to hold the set screw in. **
What exactly is a rotabroached? Is it doable w/o a big machine . . . your fileing is great . . . perfect way to gear the chip . . if you can put a keyway in it. Can I rotabroach something with a table saw, a drill, a drill press, or any other fun things . .
What if we just drill a shallow hole in the shaft and put the setscrew into that? with locktite, of course . . .
As for your question of putting a screw through the gear and shaft, it seems like it would work, but I would definatly test it if I was going to use it.
Ahhh, with our new budget (haha! professional fundraiser!! wooo!!) we should be able to swing for one of thems. Will be handy in the future years of our team.
In our 2002 year we had solid gearboxes that were milled out of a block of aluminum, I believe. I didn’t make them, so I can’t tell you for certain. It was extremely sturdy, and I don’t think we ever had to do any maintenence on it.
Download some of the gearboxes from the White Papers. Besides drawings, most give fabrication and assembly tips.
As for your questions:
A) We use 1/4" aluminum but have used 1/4" and 3/8" lexan (polycarbonate) in the past.
B) I like to use flanged bearings with flange to the inside. This traps the bearing without the need to glue. Motion Industries found some at reasonable prices.
C) Hole sizes are detailed for press fit. The nominal size with pin punch method has also been used when a boring head is not available. All our work is done on a manual mill.
D) We also custom bored and keyed a standard gear for the CIM. I gave a toolmaker the parts and he shimmed a standard bushing and broach for this.
The whit papres, like what Andy said, has a lot of information.
*Originally posted by Andy Brockway *
**C) Hole sizes are detailed for press fit. The nominal size with pin punch method has also been used when a boring head is not available. All our work is done on a manual mill. **
My experience on S.P.A.M. working on our gearbozes for all four years of high school, we’ve always used 1/4" Polycarbonate sheet. Also we’ve used 1/8" Aluminum as side plates. Now for the bearings, we used an Arbor Press to push in the bearings into place. In 2002, we used pins. Let me tell you something. You’re bound to lose pins if you have to conduct maintenace work on your gearbox (provided you’re taking it apart) and they break a little too easily. In 2003, we used screws instead and it was much better, much better for us who had to put and maintain the gearboxes. It facilitates maintanence.
Not quite. That’s a keyway broach. A Rotabroach ™ cuts a circular hole. Think very aggressive hole saw that can blow through 1/4" thick aluminum in 30 seconds flat. Hougen Manufacturing makes them, see link:
And, to cut the bearing pockets on a manual mill, cut using either a rotabroach or a drill bit slightly undersize (up to about 0.030"), then ream to final size. If you ream about 0.0005" or 0.001" under the bearing’s OD you’d get a tight press fit. Make sure that the bearing case won’t be compressed too much though. the bearing vendor should be able to tell you what is a good press fit for a bearing. a little bit of time invested here will save you a LOT of gearbox troubles.
and don’t underestimate the side loads that helical gears can generate. That crippled 824’s drive train last year.
We have had incredible luck by building our own, both the shifters and swerve boxes. I had no problems using 1/8” aluminum for the side plates as long as you aren’t using the gearbox as a structural member of the robot. If you were, I would then increase its rigidity accordingly (Thicker Aluminum or possibly a 90 degree break on each side (See Wildstang Pics). As for bearings, we used the same as the above posts, flanged pressed in from the inside of the gearbox so they are captured. The amount of press fit, we fond not to be real important, it definitely shouldn’t be so tight you end up crimping the outside of the bearing to get it into place, maybe .005 undersized hole. Now as for creating the gearbox. CNC sure helps but it is in no way required to have CNC to build the side plates. You can easily use a manual mill that has a Digital Readout as long as you have big enough cutters to cut out the bearing holes. Start by squaring up your piece, then using an edge finder locate the exact edges and zero out the display. Now drill & mill out all of the holes. The three fingers on the shifter can be done on an ordinary rotary table, again CNC not required. The most important dimensions to consider are the center to center distance between the gears to keep the meshing correct. We always add .005 to the distance to allow some clearance for the junk that gets in the box. Lastly for the CIM motor, we pressed our gear assembly onto the CIM motor instead of using a keyway. Measure the output shaft, find a drill slightly smaller, then put the gear on with some green locktite. It pretty much is never coming off now but we have found the CIM motor to be pretty robust and it never failed like the drills and FP in competition so it wasn’t a problem.
Last year we built our first gearbox. It was a 1000:1 reducer so the CIM motor could drive our box stacking arm. We started with 1/4" plate but then machined all the areas that were not holding bearings or spacers down to about 0.060". Of course, the CNC machines and software at our sponsor J & F Machine made it much easier. But it could have been done by hand on a conventional mill. You would have just had to be very careful. The weight reduction was substantial.
*Originally posted by Frank(Aflak) *
**
What if we just drill a shallow hole in the shaft and put the setscrew into that? with locktite, of course . . . **
This has been such a recurring problem that Woodie Flowers has addressed it at kickoff meetings more than once
and gone through the torque and pressure calculations to show that there is no way you can put a setscrew on a motor shaft and expect it to stay there without deforming and coming loose.
Or to put it more simply, in his own words, “Set screws suck!”
The next logical step is to put a screw or pin all the way throught the gear and shaft - this is almost just as bad. you have to put a large screw or pin in there, to be able to take the torque, which means you have to drill a large hole in the shaft, which leaves IT weak.
A slot and key is really the best way to go. Pins or set screws will work long enough to make you think your bot is fine. Then you ship it, and in your first match something comes loose or snaps and your motors spin and spin and bot just sits there like a very fancy statue!
there was one other solution that FIRST put in the kits a few years back, a gear that compresses onto a round shaft with a nut on one side. I forget what it was called, but I havent seen them in a while.
) tight hole for the bearing and punch four small holes in the plate, along the perimeter of the bearing. I don’t know if that sounded clear enough, tell me if I didn’t. 
