×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Torque Motor Calibration

Torque Motor Calibration

Torque Motor Calibration

(OP)
We have setup a dynamic calibration of a torque motor and system. One end of the torque motor has a constant 3 rpm input from an air motor through a gearbox.  The other end has a copper wire attached to a 12 lb weight.  We record the torque as we raise and lower the weight at a constant speed (altering the direction of rotation of the motor too).  We consistently have ~11.4 in-lb torque when pulling the weight up, and ~10.3 in-lb when lowering the weight (at the same speed controlled by the 3 rpm input).

I am having a brain-freeze as to why such a 'big' difference.  I've drawn many Free Body Diagrams and there's not much too them... Wire Tension Force, mass, gravity.  There is no acceleration to speak of since the input end of the torque motor is controlled to 3 rpm.

Any help matching theory to actual would be appreciated.

RE: Torque Motor Calibration

I expect the hystersis you are seeing is due to the copper wire.  2 things are happening here:

1. If the ratio of the drum diameter to the wire diameter is low enough, the wire is yielding every time it enters or leaves the drum.

2. As the wire enters and leaves the drum, there is a small amount of sliding of the surface of the wire against the surface of the drum.  The wire may also be sliding against wire that is already on the drum.

Try using as thin a wire as possible.  Thinnest possible music wire, lightly lubricated, would probably be best.  Nylon fishing line would probably be better than copper.

RE: Torque Motor Calibration

(OP)
Thanks for the suggestions Philrock.  Am I right in saying there should technically be almost zero difference between the torque pulling the weight going up and down at a constant speed?
Thanks

RE: Torque Motor Calibration

Yes, provided that the torque measurement is accurate and there is no friction in the drum/wire system.

Some torque measuring setups have bearings that can introduce friction/hysteresis.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources