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Ideas on how to fasten to a motor shaft

  1. Jun 19, 2014 #1
    I have a rubber wheel that has a 1/2" plastic bore with a key slot in it (see attachment). I want to spin that wheel with a motor that has an 8mm straight shaft. I need some ideas on how to fasten the motor shaft to the wheel. I want to have a bearing opposite of the motor to keep the shaft from wiggling. My idea was to get a custom hollow shaft made with a key slot in it. That way the hollow part will slip over the motor shaft and the key slot will will utilize the slot in the wheel. Is that a good idea? Is so, where are some good places to get custom shafts made? If not, what are some better ideas.

    Attached Files:

  2. jcsd
  3. Jun 19, 2014 #2


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    Welcome to PF.
    You can do the entire job in almost any metal lathe. Ask around to find someone local who has a lathe.
    Preferably a hobbyist, maybe a school, garage or ask a machine shop who has one in your area.

    1. Maybe start with a solid steel 5/8” or greater diameter extension shaft. The larger diameter will provide more wall thickness for clamping to the motor shaft, it will also stop the wheel sliding one way along the extension.

    2. Bore a 5/16” hole (=7.9375 mm) in one end to accept the motor shaft. Hold the job in the chuck, with the drill in the tailstock. Drilled holes are often oversize, hence the 62.5 micron allowance. Check the fit with the motor shaft, polish out the bore to enlarge it if needed by using abrasive paper on a rod or wooden dowel. Drill a couple of holes and thread them to take a copuple of clamping screws to the motor shaft.
    Note: If you cannot get a good fit to the shaft you might be better using some form of flexible coupling at the motor shaft junction.

    3. Reverse the extension shaft in the lathe chuck. Turn down the shaft to just over 1/2” diameter over the section that will carry the wheel. Keep the fit tight over the part where the wheel will seat. The wheel's plastic hub will not handle a huge internal pressure so the key is needed to stop rotation. Make a step to prevent the wheel sliding along towards the motor.

    4. Machine a step at the far end of the extension shaft to be a snug fit inside the free end support bearing.

    5. Cut a square key groove in the shaft for the wheel hub key by running a tool steel cutter backwards and forwards in the lathe saddle, with the chuck clamped to prevent rotation. Advance the cross slide very slowly as you cut the key groove progressively deeper. File a rectangular key to fit in the wheel and shaft keyways.

    6.You will need to find a way to stop the wheel sliding, off the key, towards the bearing, maybe make a sleeve to fit between the bearing inner race and the wheel.

    Good luck.
  4. Jun 19, 2014 #3


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    Depending on how much torque you need loc-tite may be all you need instead of cutting a keyway.
  5. Jun 20, 2014 #4
    Wow, that was a fast response! Physics Forums is great! Thanks for the tips guys! I figured that finding a local machine shop is the way to go.
  6. Jun 20, 2014 #5


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    The wheel looks like a mass produced commercial product, so it will almost certainly fit on a standard size of keyed shaft.

    And you can probably buy a connector or adapter to join two standard sizes of shaft.

    Making a one-off solution can be a fun project if you do it yourself and don't put a money value on your own time, but paying somebody else to make a one-off is unlikely to be the cheapest solution.
  7. Jun 21, 2014 #6
    Thanks for the tip AlephZero. Having never done something like this before, I thought that extra cost of a custom shaft would be worth it. How much more, I don't know right now. I just wanted to try an avoid using a coupling and keep my connections to a minimum. I don't have access to a metal shop nor do I have a lot of money to stick into a custom shaft that may be expensive. The wheel is a mass produced commercial product and after checking the specifications, a 1/2 inch keyed shaft fits this wheel. Those shafts are in abundance but are there couplings that can connect a 1/2 inch keyed shaft to an 8mm straight shaft? Is that something that will have to be custom made? Keep the ideas coming guys! I really appreciate all the advice,
  8. Jun 25, 2014 #7


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    Shaft couplers are produced commercially. I doubt you need to have one machined. An 8x12 mm flexible coupling costs about $4.
  9. Jun 27, 2014 #8
    Looking at the wheel I am guessing that it will be used to drive a combat robot. The best method would be to use a chain drive instead of directly coupling the motor and the wheel. For a chain drive you can mount a sprocket on the motor shaft and mount a bigger sprocket on the driven shaft which has wheel attached to it. That driven shaft should have key-way and shoulder to avoid relative movement between the shaft, wheel and sprocket.
    This driven shaft (which has sprocket on it) can be supported by bearings as you like.
    This page has many pictures of it implemented:
  10. Jul 3, 2014 #9


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    The problem with indirect drive systems is hysteresis - the time lag between input and output reaction. Belt drives are generally simpler and more efficient in this respect than chain drives.
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