shooter testing problem

[Geary]

We're having a problem getting or shooter test done we rigged a CIM to some plywood and put on our pneumatic wheel with the 8mm Key hub from andymark (http://www.andymark.com/product-p/am-0320.htm) directly on to the motor. The only problem is we are finding that the wheel is moving up and down on the CIM's output shaft because there is nothing holding the key hub in place, we tried using shaft collars but we had none that fit the output shaft of a CIM, any idea on how to fix it to the wheel so it wont move up and down?