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Shaft coupling from 0.125 D to 0.25 square

Hi, hoping someone can help me with what I think is a pretty basic question:

I have 2 motors with a 0.125inch keyed (or D) shaft.  I need to connect it to a 0.25" square socket (vex tank tread driver).  I assume there's some sort of coupler or adaptor that could help with this?  I've been looking for most of the afternoon and haven't found any good solutions but I imagine it's largely due to me not knowing where to look...this is my first robot so I'm a bit new to the whole connnecting a shaft thing...  many thanks for any suggestions or thoughts.

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I purchsed some 4mm hubs like those when I purchased my motors. I debated whether or not to use them. The problem is, where to mount them to the sprockets. As you suggest, you could fill the inner space with Polymorph and attach them that way. I think this would work. I decided against that approach since it moves the sprockets away from the motor which increases the sideways torque on the motor. I thought minimizing the distance between the sprockets and motor would increase the durability of the robot.

I considered milling out one side of the sprocket to remove the center section and the ribs and mount the hub to the center divider of the sprocket. I decided this was more work than I wanted to perform and I wasn't sure the hub would still reach the motor this way.

I think adding hubs to Polymorph filled sprockets would be about the same amount of work as adding a set screw. I think the desired final geometry of your motor/spocket combo would be the determining factor on which technique to use.

yes the side loading could be an issue, will have to think about it.  Thanks for the help!

yes the side loading could be an issue, will have to think about it.  Thanks for the help!

Are you sure about the 0.25" socket? I thought the Vex square hole was about 1/8" square?

I have a couple of sets of Vex treads. I'm using these motors from Pololu (purchased on their Black Friday Sale).

The motors' shaft is 4mm. I drilled out the Vex sprockets to 4mm. I then drilled an 1/8" hole through the side of the sprocket, close to the edge of one of the spikes, down to the center of hole. I added a 4-40 bolt to act as a set screw. I added one 4-40 nut to the bolt just inside of outer wall of the sprocket so the bolt had something to push against when it was tightened.

With the nut and bolt in position, I then filled the hollow area on the side with the bolt with Polymorph. The Polymorph holds the nut in place and I think it may add some strength to the sprocket which had been slightly weakened by having a larger hole drilled through its center.

I did something similar to mount Vex Mecanum wheels to a Rover 5. If you follow the links on this page, you should find some picture of how I added a set screw to the Mecanum wheels.

If my directions on modifying the Vex tank tread sprockets don't make sense, let me know, and I can take some pictures of what I did.

Thanks Duane.  Your right of course it's 1/8", not sure where I came up with 1/4"...

I already have a couple motors I plan to use (and control coding writen around them, analog feedback) but maybe I can adapt some of the ideas you've noted.

If I buy these hubs
http://www.lynxmotion.com/p-762-universal-hub-18-pair.aspx

then I'm hoping I can just fill the inner drive wheels with polymorf and screw in the 4 bolts from the hub to the drive wheels?  Think it'll hold?