for direct drive. good idea or bad? why? any ideas on how best to accomplish? thanks!
What does this accomplish exactly that a keyed shaft and hub or coupling can not?
if you don’t have those things, tapping the shaft works well. We have used all three methods in the past.
I wonder if this would work:
Put the CIM in the freezer for 10 minutes, drill a hole in the piece you want to connect the CIM shaft to, heat that piece nice and hot, and shrink fit it over the CIM shaft.
We’re using keyed hubs mounted to skateboard wheels, and would like the wheels to stay on the axle, so are tapping it to place a washer over the top. Just did one and it worked very well. Thanks!
Hmmm, Interesting idea. I might have to try that sometime.
It’s potentially easier to heat up the part you’re installing on the CIM with a torch. Raising the temperature by 300-600 degrees with heat will get a bigger dimensional change than cooling by 100 deg.
When this sort of thing is done IRL (bearing races, for example) one part is usually put in an oven and heated to 300-500F and the other part is put in an alcohol bath with dry ice at -109F.
As for actually retaining something on a CIM shaft, check out the normal pinion retainer rings available from AndyMark and/or VEXPro. They may seem a little flimsy, but do a pretty great job holding on.
did you miss this part?
Yes, yes I did, clearly. :yikes:
it works just fine my team did it while prototyping. it took a while but it works just be sure not to tighten the screw to much as it can rub and not work as well but keep it tight enough so the wheel doesnt wobble or it will vibrate to much and well in prototyping lets just say i hurt my finger
Please, what does the pronoun “it” refer to?
Your post is linked to this one:
http://www.chiefdelphi.com/forums/showpost.php?p=1216539&postcount=9
Silly question, but is it legal to do this? I know FIRST has very specific rules on modifying motors.
Yes, I think it is legal to tap the axle to mount a screw. It does not change the performance or safety of the motor, which are FIRST’s concerns.
R33
The integral mechanical and electrical system of any motor may not be modified. Motors, servos, and electric solenoids used on the ROBOT shall not be modified in any way, except as follows:
A. The mounting brackets and/or output shaft/interface may be modified to facilitate the physical connection of the motor to the ROBOT and actuated part.
However, the bearings on a CIM don’t like side loads at all. They are only bronze sleeve bearings.
Also, are you drilling and tapping across the shaft or down the center of the shaft?
Down the center.
Whilst not addressing the exact question of the OP, I’ll add here that heat shrinking a pulley/hub/gear to a motor shaft will work, but it is tricky.
The heat from the hub will soak back into the motor shaft which can damage the rotor, and will certainly bleed oil from the front motor bearing if it is sintered bronze.
Also, heat shrinking is a one-shot process, you need to get it just right first time. If you don’t get the hub fully seated down to the correct position before it starts to grip on the shaft, you then have to bash or press the hub into place which will risk bending the shaft. If it won’t budge, you’ll have to turn the hub off the pulley on a lathe, which is now difficult to do because you have the front motor end cap sandwiched in there, making steadying the shaft all but impossible.
I can warn anyone attempting to heat shrink a hub or gear on this way that when it grips and the hub starts to loose heat to the shaft and the shaft starts to warm up, the whole assembly locks up real quick.
A much easier way to do this job is to use a retaining compound such as Loctite 680. I have assembled thousands of gears on motor shafts with 680 and never had a failure. It will fill gaps up to a few thousandths of an inch IIRC. 680 works very well on “reactive” metals such as steel, but will still grip with aluminum.
Graham.
*Some suggestions to address Graham’s concerns for anyone who wants to try shrink fitting:
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put the CIM in the freezer
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make sure the mass of the piece you want to shrink onto the CIM shaft is less than the CIM rotor
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place a heat sink (e.g. a simple slotted piece of steel plate) snugly around the base of the CIM shaft
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If at first you don’t succeed, use a press to push the CIM shaft out
I was interested in how well the shrink-fit was able to transmit torque from the shaft to the hub, so I made a quick calculator to evaluate a sample design.
An aluminum hub with arbitrary length, shrunk over the 8 mm steel CIM output shaft, is able to transmit a maximum of 4.2 Nm for every 1 mm of hub length (or 78 ftlbf every inch). It requires a total radial interference of .009 mm (or .0004 in). Assuming linear thermal expansion, it takes about 190 F of heating to achieve this radial displacement.
Greater amounts of shrinkage can be achieved, but they’ll force you to cut into the safety factor you’re using for the maximum circumferential stress in the hub.
pressFit.xls (33 KB)
pressFit.xls (33 KB)
Interesting Nate. There’s nothing like some hard calculations to assess an engineering problem! Your numbers agree with calculations I’ve done in the past.
However the situation is more complex. With heat shrinking, not only do you need the half thou or so interference to achieve the required torque capacity, but you need some clearance to be able to get the hub over the shaft in the first place. So your 190F is not going to be sufficient. You’ll probably need more than double that temperature, and then the heat soak problems into the motor start to be an issue. For a data point, my experience is that red heat is needed for a steel gear onto a room temperature shaft.
Ether’s heat sink idea might help, but usually in this situation you’re applying a bending load to the motor shaft in operation and you don’t want excessive distance from the motor bearing to the hub. This is especially important when you’re dealing with the relatively small motor shaft of 8mm on the CIM motor. So typically, there won’t be room for a decent heat sink, or for that matter for a plate to facilitate removal of a poorly located hub by using a press.
Heating may also alter the heat treatment of the hub or gear, plus damage the surface finish.
Don’t get me wrong, heat shrinking can be used, and is a standard production method, and works very well in tightly controlled production conditions. However, for school students, using often less than adequate workshop equipment, the Loctite 680 method is quicker, less messy, safer, less risky in terms of certainty of getting it right, probably stronger, doesn’t risk over-stressing the hub, and probably cheaper.
But heating stuff up to red heat with a blow-torch is always a good way to have fun:)
Graham.