using a DC motor's power draw to calculate passive friction of rotating system

I have a system that consists of a rotating disc with a sliding linkage (a piston) attached to one end (think locomotive mechanism). When the linkage is disconnected, the disc spins freely for several revolutions. When I attach the linkage, it stops very quickly.

I want to measure how much work the piston is doing to stop the rotation of the disc. I'm wondering if I can use a geared up DC motor (12V DC Motor 251RPM w/Encoder - RobotShop) to see how much poer (current * voltage) is drawn by the free disc, and compare it with how much power is drawn when moving the linked disc?

Only having a vague mental image of what you are doing, I have to think the disk has some inertia.

I don't think a DC motor would be a good measurement device, too many losses. Perhaps if you could calibrate it to something.

Is is not possible to calculate the force needed to stop the disk?

No need for all that. Just attach a spring scale to the piston and measure the "weight" as the piston is pulled. That will let you compute the friction, which is stopping the rotation, except when the crank pin is on dead-center, when no work is being done.

Paul

If your disc only spins a few revolutions by itself, then you have bearing problems to think about as well :slight_smile:

Sure, you can measure and subtract the friction of the system but it’s not going to be linear as the disc rotates. That may invalidate your power calculation although the average may be usable for most purposes, especially so if you run at a constant speed.

I think it would be appropriate to ask is your goal to determine how much work is being done by the piston? If so, a direct measurement of the force is probably the best solution achievable though many methods. Load cell, spring scale, FSR, etc.

A more detailed explanation of your desired goal is usually more helpful than discussing a proposed solution. See the x-y problem for the reason why.

Paul_KD7HB:
No need for all that. Just attach a spring scale to the piston and measure the "weight" as the piston is pulled. That will let you compute the friction, which is stopping the rotation, except when the crank pin is on dead-center, when no work is being done.

Paul

Engineers should not confuse weight with mass! :slight_smile: