Q: What is that on the shaft of the sewing machine motor? Some kind of rubber cap to grip the wheel; but what specifically?
A: The motor came off the sewing machine with a plastic pulley that was driving the belt. So I picked a piece of rubber hose that would go over the pulley for a better grip with the main wheel.
Q: The flat metal length with the spring, what is its job?
A: The metal bar is supporting the motor and the spring serves as a tensioner to make sure that there is contact between the motor pulley and the main wheel at all times. The main wheel is not perfectly round so that's why it requires a tensioner.