# Question for the physicists about a spinning disk.

We need to test a windshield wiper motor at a constant RPM in the lab. To test the wiper motor, we need to give the motor a load.

We’re trying to figure out a way to load the windshield wiper motor. One idea for a load is to use a DC motor, and stick a variable rheostat on the output of the DC motor.

My primary question is about another idea… what if we simply attach the shaft of the windshield wiper motor to the center of a large disk? At a constant RPM, would a spinning disk offer a constant load / constant torque to the wiper motor by virtue of its diameter and mass? Or would it only be a load when the angular speed (i.e. RPM) is changing?

A coworker is of the opinion that a spinning disk only offers an appreciable load when the disk is *changing * its RPM; at a *constant * RPM, the windshield wiper motor only has to overcome bearing & air friction. My analytical side tends to agree, but it would still seem that a spinning disk (at a constant RPM) would also offer some resistance simply due to its angular inertia.

And you’re thinking of angular momentum. The angular momentum of a disk spinning at a constant speed is constant. You can either change the speed or tilt the axis to change the angular momentum.

Sorry, but your coworker is correct on this issue. The angular momentum (a product of its rotational inertia and angular velocity) of the disk will keep it spinning at the same rate until some external force (i.e. the motor) changes the speed. In order to measure the torque at a given speed (which I’m understanding as your goal) you need to apply a braking force on the shaft and measure the resultant reaction torque. There should be test equipment commerically available to do this, or you can contact a mechanical testing company like Anderson Labs or a local engineering university. As I recall, measuring torque output on an electric motor was one of our lab exercises in the basic Mechanical Engineering lab course.

Stranger

Not a physicist, but a Prony Brake sounds like exactly the sort of thing you need.

If your analytical side agrees, what would you call the side of you that disagrees?

Assuming that the “spinning disk is not a load” question is now resolved, the question remains how to apply a load. Tevildo’s post looks like it fits the billl, but how much load do you need to apply? Are you trying to determine the load limit for the motor, or do testing under a typical windshield wiper load? I would guess that ww load is pretty minimal. A ww is maybe 2-3 feet long, let’s say you need 3 lbs of force at the tip, times two wipers, so it needs say 18 ft-lbs of torque. I don’t know how you translate that into a load on the motor, but hopefully that’s a step in the right direction.

One way to use the spinning disk is to use a metal disk in a magnetic field. The eddy currents provide the braking force. (Incidentally, home power meters with a spinning disc on them work this way).
Or put a tire on the disk and put it on a treadmill.

ow!

Thanks for all your responses. The Pony Brake looks like it may fit the bill.

What about using a DC motor, or a generator from an old (pre-1970’s) automobile? We could stick a rheostat on the output. For a given resistance, will the torque be constant over RPM?

(And I feel a little embarrassed at asking these questions, me being an EE and all. But I know virtually nothing about electric motors.)

I meant Prony Brake. :smack:

If you just want to run it in an loaded condition, why not just put an large model aircraft propeller on it?

For that matter, why not just attach it to a windshield wiper?

Never simulate reality when you can use the real thing.

Well, if they’re trying to measure constant torque output at a given speed then the windshield wiper isn’t going to give steady output, nor is the motion particularly easy to quantify owing to a number of unconstrained parameters, i.e. flexibility of the blade and arm causing lag-lead behavior, variable friction and stiction on the windshield surface, discontinuity at the turn arounds, et cetera. (Having done kinetic analysis and simulation for an automotive supplier I can speak with some measure of experience in the matter.) But to expand on Chronos’s question, what exactly do you need to quantify that you can’t just simulate? Do you need numbers, or do you just need to know that it can do the job without burning out or being overtorqued? It is the chronic and occasionally near-terminal[sup]*[/sup] illness of engineers to want to overanalyze everything.

Stranger

[sup]*[/sup]As in, and I quote from a former product manager, “If you don’t get this fucking thing on the production line by next week, I’m going to send men to your house to kill you, your family, and your dog.” Ah…to work for the automotive industry again…

A constantly spinning disc WILL give you a constant torque but, depending on the aerodynamics of the disc, you might have to spin it at a unreasonably high RPM to get the torque you want. The easiest way around that would be to add some drag to the disc. Rip apart an old exercise bike or use an airplane propellor or something.

Your dc motor load will work. You didn’t ask for load suggestions, but another way would be to get one of those cheapie water pumps that run from an electric drill. Put a valve in the line to restrict the flow and I think that would be a pretty good load.