timing pulley becoming magnetized (PROXIMITY) issue.

[Jamie]

For what ever reason, we have a piece of equipment that employs
2 hollow shafts both with timing pulleys on the end of each and
the inside pulley gets magnetized.

The inside unit happens to be connected to the outer hollow shaft.
these 2 shafts turn at different speeds and such that we have
prox's mounted on a nonferrous plate aligned vertical into the timing
teeth of these pulleys.

just picture 2 hollow shafts, one fits inside of the other,
both having a pulley connected to them on the same end.

Both Prox are of the same type etc, I discovered a problem where
the inside pulley had critical issues allowing the prox to properly
sense the passing timing teeth. At slow RPM's it seems to work fine
how ever, as RPM's increase the sensor goes into a latched output
effect and simply stays in a constant on state, this is obvious via
the monitoring equipment connected to it and scope I used. Pulling the
prox away from the pulley allows for higher RPM's to develop before the
output turns to a constant one state.

These units are basic N.O. NPN output..

Trying to assess as to why the pulley becomes magnetized which is
causing the problem here.

I guess a mechanical engineer may have an answer to that. I did
observe a mechanical thumping noise that could be a bearing issue on
that shaft that has the magnetized pulley.

Apparently hitting it a few times with a hammer fixes it for a short
period. I'm not a mechanical engineer but I do have some ideas in the
back of my head on how to neutralize the mag in the pulley..


Any idea's?

 

[Tim]

Maybe have the pulley pass through an AC magnetic field to prevent it
from building up magnetic flux? Like a degaussing coil or tape eraser.

- Tim -

 

[Jamie]

Ye's. You don't pluralize with an apostrophe.

And how does that help me with my question?

Oh wait, you're useless. How could I have
made such a mistake!.

My apologies.

 

[Jamie]

<snip>

What sort of proximity sensor? There are quite a few ways of measuring
proximity, and iit would help if we knew which one was being used -
manufacturer and part number would be even more helpful.

we found the problem.

I had to dig out some old reference material I have here and a little
on line probing kind of jog the old memory.

This particular timing pulley has a disc brake attached to it that is
bolted on the back side of it. After a while from use of the brake, the
timing pulley which has it's teeth perpendicular to the surface of the
disc brake with a unsymmetrical pattern causes the pulley to become
magnetized over time, with use of the brake system.

This effect can be seen in the use of drill bits where friction
causes the bit to become magnetized because of the unsymmetrical shape
of the bit at the end.

Getting back to the pulley, when the RPM's start to pick up the small
magnetic field in the pulley effects the prox and causes it to lock on.

We tried inductive and Hall. Both have the same problem. The hall is
worse.

I'll do have an idea that I once did years ago to remove the effects
of a magnetized slide bar and that was a shorted coil being placed near
the moving magnetized metal so that it wouldn't disturb the sensors ahead.

 

[Jasen Betts]

And how does that help me with my question?

it helps you ask it correctly!