Bearing damages from ultrasonic vibrations?

[Comaqc]

Our company manufactures roll heating systems using 25 kHz induction heaters. Because we have measured that our induction roll heating equipment generates ultrasonic vibrations in the roll where induction is applied, we are asking ourselves whether the presence of ultrasonic vibrations on the roll could cause an eventual bearing failure. Some of our customers are experiencing premature roll bearing failures which can be linked to the installation of induction heating being used to raise roll temperature. The presence of 25 kHz electrical current through the bearings has been confirmed, and bearings analysis by manufacturers has established that electrical current caused the failures. However, I remain sceptical of the root cause of the problem for the following reasons.

The current version of our induction roll heating systems has been in operation on 40 systems beginning in 2002. No bearing problems have manifested themselves until the recent installation of twelve identical induction systems on paper machine calenders. Because the bearing problems are evidently linked to the installation of induction, we went back and measured the levels of bearing currents at sites which have been running for as many as 5 years without problems. We found the level of electrical current to be of the same magnitude on systems installed earlier as on newer systems linked to bearing failures. Our measurements indicated that the levels of bearing currents are the same regardless whether bearing problems are present.

In our search for an explanation for this phenomenon, we measured the level of mechanical vibrations on the roll, the bearing housings and the beam supporting our induction heating workcoils and discovered the presence of the same frequencies that are used for induction heating on a pilot roll in our lab. Thus the roll vibrates at 25 kHz, in sync with the electrical current flowing through the workcoils. Because the systems causing bearing failures all used the same workcoil mounting to their supporting beam and to the machine frame, we confirmed that the systems with different workcoil to machine mounting showed a level of ultrasonic vibrations about 10 times lower.

My question is this: could the presence of ultrasonic vibrations on the roll shaft initiate mechanical damage leading up to an eventual bearing destruction by the presence of electrical current?

 

[Comaqc]

Unclesyd,

Personally, I don't think your question is dumb. The rolls we are applying induction to are cast iron rolls, not designed to be heated above 60 °C by the normal calendering process. Because we are applying almost 0.5 MW of heating energy to the rolls we apply induction to, bearing positioning on the shaft was one of our initial worries. We measured a 150 °C increase in roll temperature when induction is applied. Since the cast iron roll is 8.3 meters long (and 500 mm diameter), rough calculations estimated the axial elongation to 12 mm. The mill maintenance crew have assured us (without showing numbers or drawings) that the roll can easily slide in its bearing housings to take up the expansion. Having no choice but to believe the mill M&R manager, we drop further questioning. As you can guess, the mill M&R crew refrained from laughing in our face when we questioned them on bearing positioning. I guess the mill people believe so much that bearing currents are the only real cause of bearing failures that it becomes difficult for them to consider other possibilities.

 

[unclesyd]

This is a classic example of "tunnel vision".

I get about 15.5 mm expansion in the roll shaft using the worst case for CI. I would question the amount of float they would allow in a bearing for a roll that is supposedly to remain stationary during operation.

If a bearing is running tight any damage mechanism is enhanced.

I asked the question because we had a half million dollar bill for overhauls due the lack of allowable float available in a bearing. The interference with the stop ring was only about 1.5 mm assuming equal distribution between bearings. The bearing was set up with 1/2" float and we needed 5/8".

One other question have you looked at the temperature rise in the bearings when you start heating the roll.

CI is an excellent dampening material. The dampening capacity runs from around 28% for a 20,00 TS iron to 7% for a 40,000 TS iron. This dampening capacity maybe affecting your vibration readings. quote: The dampening capacity of CI is little affected by temperature, vibrational frequency, or stress history. from "The Properties of Grey Iron"; Iron Founders Society

 

[Tmoose]

Some of the articles on bearing currents make distinctions between a few types of shaft current. I believe The proper bearing insulation strategy (both vs single) vary with the origin of the current and thus type of machine.

My earlier comment on the lubrication condition (kappa) was in quest of providing a substantial EHD oil film and thus-
1 - guaranteeing a certain minimal amount of bearing insulation. I guess your test with insulated bearings was at least as good.
2 - guaranteeing high frequency vibration could not bring balls and races in contact. The descriptoin of the bearing damage as fluting seems to indicate bearing currents are the cause, but the addition of a mechanical event syncronized with
induction seemed potentially worse.

When TIg welding, a high frequency start is sometimes used to get some initial ionization going. At welding currents that can create arcs as surprising unintended locations, like thru compressed felt.