# Need help choosing a motor.

 P: 834 I just thought of a second component you may need for your torque calculation, depending on how precise you want your spec to be. Above, I'm only talking about the torque of the moment of inertia, its calculated like: $T = \alpha I$ where alpha is the angular acceleration (hence, at a constant rpm you will ideally need 0 torque, but practical losses of the motor require torque to keep it moving). The other torque component will be from the force the ribbon exerts on the reel. Assuming I have the right image in my head of what you are building, there will be a tangent force to the wheel that is based on the tension of the ribbon. This is calculated the easy way since it is tangential (90 degrees, so cross product turns into pure multiplication): $T = F*d$