Squealing motor

I have recently become the owner of a Cowells 90ME lathe. It's about 30 years old, but has been almost unused. It's driven by its original single-phase TEFC synchronous motor. The motor's maker's/ratings plate has been removed. I would guess <1/3hp. It has a capacitor, in circuit all the time (no centrifugal switch), but its value is obscured by paint. The electrical details are hopefully irrelevant, because the problem is that the motor emits a very high-pitched, constant squeal, when run in either direction.

Dismantling reveals the expected construction. One pre-load wavy washer. All is clean - no corrosion, dust or discoloured lubricant weeps. The shielded ball bearings feel 'right'. No evidence of anything rubbing or races turning, etc. The motor feels absolutely fine, when spun by hand. A single oil seal exerts a little drag. Loading the shaft axially and radially, under power, makes no difference.

So, anyone any idea why the wretched thing squeals? I've never come across ball bearings seeming OK, but noisy like this - but what do I know? I doubt that the bearings themselves are at fault. Can shields rub and squeak? I suppose I should replace the bearings, but before that - could it be anything else?

Bearings are cheap. You already have the motor apart. Replace them as a matter of course. Use shielded bearings again as double rubber seal types can overload the motor on start up. The old grease has probably dried out and causing a squeal. Also replace and lubricate the single oil seal you mention.

It's better to do this than have a squealing bearing lock up and spin the outer race in the housing and trash the housing. Much more expensive to fix then.

Edited By Hopper on 06/08/2018 12:01:38

Sorry to be pedantic, but this is more likely an induction motor, not synchronous. Synchronous motors are rare and tend to be rather low power for their size.

John, I'm fine with pedantry. Of course, you're right. Please excuse my terminological inexactitude - it's getting late here on the opposite side of the world. Now that we've got its name sorted out, any idea why it squeals?

If the motor when stattic does not make a noise, that only leaves the rotor/bearing assembly as the culprit.

As hopper eluded to the grease, though the bearings when manually spun may seem ok, it could well be the grease has dried out...You could carefully remove the bearing shields, clean out the old grease, repack/replace the shields and try again....Prior to parting with beer tokens for new bearings...

Edited By John Rudd on 06/08/2018 12:40:38

Brushes need attention maybe?

Induction motor my friend....

No brushes...

If it's belt driven it could be that the belts are slipping.

Brian

If by any chance your motor proves to be beyond restoration, you might consider the conversion shown in the 'First Run' video, in the set that I've just linked for Steve Sherlock. **LINK**

https://m.youtube.com/channel/UCf-ZiCQo0pfvubnP7M48_Fg/videos

MichaelG.

Something foreign or loose between the stator and rotor might explain it. The gap between the two is tiny, possibly something lifts off the rotor as it speeds up and vibrates as it rubs against the stator.

Is it likely? Not normally, but the motor has been in storage for 30 years...

When putting old machines back into service it's a good idea to start with a good clean, and lube.  More work if it's been stored badly.

Dave

Edited By SillyOldDuffer on 07/08/2018 08:44:36

The problem is almost certainly the oil seal and not the bearings. Motor seals run dry.

Rub a little grease into it. Replace when convenient.

Edited By Mark Rand on 07/08/2018 09:44:41

Thanks everyone for your suggestions.

Bearing in mind (apologies...) the age of the bearings, it seemed a good idea to replace them, so in went a pair of Polish NSK 6201ZZCM. The 'M' clearance rating is supposedly tighter than the C3 rating commonly available here, and is considered good for motors - quiet-running. Chinese bearings avoided - some must be fine, but which?

Motor reassembled with well-greased seal. Silence - for 30 seconds, then Squeeeek. Not the bearings then... With a hook probe, I was able to flick the seal's spring off whilst the motor was running - silence! The seal is a muck-keeper-outer, not an oil-keeper-inner, so the spring is accessible on the outside. So Mark is right - it was the seal! The removed bearings spin 'nicely', but with less grease drag than one would expect, so it was a good idea to replace them.

Inspection of the seal with a magnifier revealed that there was a matt cylindrical land, about 1.5mm axial length, in place of the expected sharp lip. The shaft was unmarked. This isn't a seal failure due to wearing-out, because of such little running time, but presumably due to the spring pressure over 30 years having caused the rubber to deform. Perhaps also long periods of disuse allowed the seal to adhere to the shaft, so that its surface was torn off at the next start.

I've not come across this before, but perhaps it's a well-known failure mechanism. Anyway, the thread is now here for anyone who searches for a solution to a squeaky motor.

Lesson learned: Whenever replacing bearings, replace the seals too.

Corollary: Any rubber or plastic items on old machinery more than 20 years old is suspect and on the verge of either going hard, going soft or disintegrating. (This includes brake hoses on old vehicles. You can't see it but the rubber swells up and the hole in the middle gets squeezed shut, leading to some real weirdness like letting fluid flow in one direction under pressure, but not back again, thus jamming brakes on etc.)

'

Edited By Hopper on 11/08/2018 11:53:41

The squeal is because the lip has been worn down on the original seal.

So fit new seals, DON'T forget to lubricate the shaft before fitting. You don't want to rip the lip on initial start up.

The grease should be pushed back into the new bearings, which won't do them any harm. They are ZZ, so only shielded, therefore a seal is needed for each one.

Howard

You shouldn't be putting lots of grease into new bearings - the actual amount should be a small fraction of the free space. The grease isn't supposed to spend its life as if it's inside a mincing machine. NSK recommend 10-15% and they should know.

You'd have been well off going for sealed bearings rather than shielded. Then you wouldn't be reliant on the questionable (in fact buggered) original seal. And as the bearings would be already lubricated by the manufacturer, it would prevent fists of ham trying to "fill" them with grease. I think we can be fairly certain that an induction motor would be capable of overcoming any minimal seal drag at startup.

Murray

Edited By Muzzer on 12/08/2018 12:54:00

Hopper - Absolutely! Some plastics are the wonder, neverlasting material industry's salesforce has been looking for. I've had many plastic components deteriorate or even disintegrate within a couple of decades of purchase. I usually replace seals after any dismantling that will have disturbed their relationship to the shaft, etc. The new bearings went in with the original seal because I couldn't immediately find a replacement that I thought I had in stock*.

Howard - I agree about the seal. What surprised me is that I really don't think the seal wore out 'normally', but that the main problem was that it deformed during 30 years of minimal activity. Perhaps it stuck to the shaft and its surface ripped at each infrequent start. Of course, once it was in a configuration where lubricant was squeezed out, it wore quickly - and noisily.

Muzzer - had I been able to get bearings with non-contact seals, I would have fitted them, if only for better lubricant retention. Some conventional bearing seals exert much more drag than others, but you can't test them in sealed packets very easily, so I stuck with the single, external, easily replaced seal. The poor little motor is rated at only 1/8hp.

* It eventually turned up, of course, exactly where it should have been.