# Thread: Calculating the friction in bearings

1. ## Calculating the friction in bearings

Hey all,

I have a somewhat engineering related question. I am trying to calculate the resistance of bearings on a rotating shaft. Assume for example that its a lathe powered by a stepper motor. The bearings in the lathe spindle are causeing the motor to miss steps. So new bearings are ordered these new bearings are hit and miss some work some don't. Static coeficient of friction has been suscessfully found, but I want to attach a devise that will show me what the dynamic friction is in the bearings at different speeds.

So the device will be mounted on a stationary base. I want to run a second motor from the base to the spindle that turns the spindle via a rod and recordes the force required to do this. That way different bearings can be tested at variable speed the friction can be found for all cases.

The design of the base is not an issuem, but what I am looking for is the proper type of sensor that will take the data I want. Any ideas?

BTW this isn't a lathe its a robot used in a clean room fab, but the lathe is more familiar and works well as a similiar problem.

2. There are instruments in the market which sense friction automatically. You can check out on web & see whether it suits your application. These instruments are called as FRICTION SENSORS

Hope it helps

3. Surely if the only load on the driving motor is the rotating shaft in the bearings you are calibrating you can simply use the motor load to derive the dynamic friction load from the bearings.

Of course the motor itself would contribute some load and you would have to direct couple the driving motor and the shaft to avoid complications from belt friction. The motor's intrinsic load would be easy to determine just by spinning it up not attached to your shaft.

4. I'd say switch to a servo motor and monitor the max torque at the speed you need. If you need a stepper to reduce the cost of your product, you can use the data to size your stepper motor.

• If you can detach the motor housing from the assembly, shaft still coupled to the load, and allow the motor housing to rotate, then you can either rig a torque gage to the back of the motor housing (directly reading the torque reacted by the motor), or put an arm on the motor housing and put a force gage at a known radius from the motor centerline.
This removes any contribution of the motor bearings themselves and will tell you torque at any speed.