Brook Crompton Motor Service

[HollowpointParticipant @hollowpoint]

Hello guys

I have been fitting a VFD to a new (to me) Boxford lathe and I am almost complete. I tested the motor and it all runs fine. BUT, I noticed a few faint noises coming from the motor and it just doesn’t seem to run as smoothly as I expected. It’s not terrible by any means but since the bearings are so cheap I have decided to replace them. A good clean inside won’t harm it either!

Anyway, I opened up the motor to fit the bearings and noticed that a couple of the fins are slightly bent. I am not sure what to do about this? Do I try straighten them out and risk snapping them? Or should I leave them well alone? Does it even matter?

Just like to get some opinions. Thanks.

[noel shelleyParticipant @noelshelley55608]

Why or how were they bent ? Somebody dropped the rotor ? I would leave them be ! If fitting new bearings get XXXX C3 type for electric motors.   Noel.

[roy entwistleParticipant @royentwistle24699]

The fins appear to be part of a casting.  Leave them as they are

Roy

[HollowpointParticipant @hollowpoint]

I think someone may have had the motor apart before. (Not me). The bearings were the caged unsealed type.

[bernard towersParticipant @bernardtowers37738]

Maybe someone poked something into the fins to enable the pulley nut to be tightened/loosened?

[not done it yetParticipant @notdoneityet]

Did it turn smoothly by hand (or do the bearings spin freely, now?  If any roughness is felt, it is far more likely to be bearing wear.  They are likely worn. Particularly the one at the drive end.

Personally, I would likely bend the fins back to straight with careful application of force with a small adjustable spanner – but not to the point of excessive force, if they resist moderate persuasion.  They don’t appear to be bent at the root.

[SillyOldDufferModerator @sillyoldduffer]

Um, might have been deliberately bent to improve balance during manufacture.  I’d leave them alone!

Trouble with second-hand machines is not knowing the history.  Bent for balance is smart-good, bent by poking something into the fins to get the pulley off is bodger-bad, as is dropping it.

Bent fins don’t explain “not running as smoothly as I expected”.    Worn bearings would, and those “faint noises”.   Worth changing the bearings, but note Noel’s advice.

Trivia corner!

Ordinary bearings in electric motors wear quickly because the rotor earths through the balls or rollers causing spark erosion, and VFDs are extra sparky.  From memory, all motors after 1970 are fixed – a separate earth bypasses the delicate parts.

VFDs aren’t instant death to ordinary bearings, they last many revolutions before premature wear becomes apparent.  Bearings expected to last 25 years, failing after 10.

The electrical cause was identified by a statistician, who, investigating the unexpectedly high number of complaints about bearings that tested perfectly well in the factory, noticed a correlation with electric motors.  Turned out mechanical wear was started by miniscule pits blasted out of the fine finish by tiny sparks, and then severe mechanical wear covered up the root cause.  Found this story in an article about electron microscopes during the 1960s.  Rather than investigate worn bearings, brand new ones were run for a short time in an electric motor, and then inspected.  Found spark pits before mechanical wear had time to squish them.  No such pits in bearings tested in the usual rig – it didn’t pass electricity through them.

Dave

[Bill DawesParticipant @billdawes]

I would leave the fins alone, they are a crude fan to help keep the rotor cool.

Can’t imagine Brook would have bent one to balance rotor, it’s conceivable that the bent fin may create a little imbalance but unlikely I would have thought. Incidentally I am not an electric motor expert, just speak with over 60 years experience in the industrial fan industry with motors up to 500kw.

Damage to bearings in electric motors is well known but mainly on larger industrial motors used with variable speed control.

Bill D.

[HollowpointParticipant @hollowpoint]

OK, so curiosity got the better of me and I tried to tweak the fins back straight using an adjustable spanner. I was absolutely gob smacked at just how soft the aluminium casting is! The fins bent back remarkably easily with the only the lightest of pressure. They are now much better.

Got the bearings off easily, they were both open cage type. Made by nachi japan. They both felt OK with just a bit of roughness and wobble to the rear one. I replaced them with Timken 6302 ZZ shielded type. The bearing seats were much looser than I would have liked so I used a bit of bearing retainer.

All back together now and it seems to have helped, the weird noises are gone and it seems more smooth. I think it vibrates less too. So all good.

[not done it yetParticipant @notdoneityet]

One bearing was likely designed to move forwards and aft – as the machine heated up – although there may be other means of keeping the shaft in place  as the motor warms up while in use.

[noel shelleyParticipant @noelshelley55608]

The bearings should be retained by a wave washer to allow axial movement due to expansion when warm, the same reason C3 bearings are used.  Noel.

[Ex contributorParticipant @mgnbuk]

The bearings should be retained by a wave washer to allow axial movement due to expansion when warm, the same reason C3 bearings are used.

According to Brook Motors documentation, the wave washer is there primarily to axially preload the bearings to ensure that in an unloaded motor the balls rotate rather than skid when the motor is started.

Same source states that a stored motor should have the shaft rotated every few months while in storage to prevent brinelling of the races due to background vibration & that the bearings of motors that have been in storage for over a year should be changed before putting the motor into service – wonder how many do this ?

C3 bearings are usually used where heavy interference fits are involved to maintain a correct running clearance when mounted, rather than for thermal considerations. In a warming motor, while the inner race would be expanded by a warming shaft, the outer race would slacken due to the expanding end cap.

[HollowpointParticipant @hollowpoint]

There where no wave washers in the motor. The shaft had a circlip and a washer which was proud in the middle to hold the bearings in place.

The lathe had been stood for a few years so perhaps that was the cause.

[MacolmParticipant @macolm]

Bend the fin straight. If it breaks off, break off the opposite fin as well!

[not done it yetParticipant @notdoneityet]

Not good if the two bent fins were not opposite!  Anyway, Macolm, he bent them back into place two days ago (at least).

[Author]

Posts