# DC Motor RPM Drops with Load in Direction of Rotation

I am trying to understand the kinematics of a machine that uses a parallel indexing cam. As you can see, this mechanism has a dwell phase where there is minimal load on the cam, and a motion phase where the follower is quickly accelerated and decelerated to an indexed position by the cam. The machine is driven by a small 12V hobby motor. I've plotted the angular velocity of the cam and follower relative to the angle of the cam based on experimental results. The blue plot is the cam velocity and the red plot is the follower velocity.

As you can see here, the RPM of the motor drops as the follower is accelerated. From what I understand, the RPM is linearly proportional to the load, so this makes sense. What I don't understand is the second drop in the deceleration phase of the the motion. At this point, there is a torque exerted on the motor in the direction of rotation. This is where my limited understanding of DC motors fails me. Would it ever make sense for the RPM of the motor to drop in this case?

One theory I have here is that during this rapid deceleration, the pressure exerted by the bearings on the cam increases, which increases friction in the system. Is this a reasonable explanation, or could something else be going on here?

• It is possible that your index shaft/assembly is not perfectly balanced. This can cause an imbalance which could result in the shaft wanting to return to a lower gravitational potential. Alternatively, it could be the bearings you're using not rolling smoothly and bumping as if it wants to "click" into a new position. Since this is more of a Dynamics question than an electrical one, I suggest asking on the physics or mechanical stack exchange as opposed to here.
– Pxl
Nov 23, 2023 at 19:52
• Yeah, I was planning to go there next. Just wanted to rule out anything that might be going on with the motor itself. I think I should be able to test your theory by remounting the cam on the motor shaft at a different angle. Thanks Nov 23, 2023 at 20:17