GEC Motor Bearings for Lathe Motor

Hi Andy

By the looks of the recent picture there is room for a ballbearing adaption as IanT, probably more work and perhaps beyond the size you can turn but a worthwhile repair.

IMO ZZ seals are better than the rubber seals as they tend to create heat by continuous contact on the bearing inner and outer parts.

Emgee

I still dont want to do that but you did get me thinking,,, that the recess in the photo is kinda ball race shape and sized.

The diameter of the recess varies between 22 and 21.4mm and it is about 10.5mm deep. A quick search for a sample 8mm ball race seems to be 22mm OD and 7mm wide so it would go in there with a minimal amount of machining.

I think it might just about swing in the gap of the Cowells bed if mounted on the face plate... but only if I can get the inner cap off because that stands maybe 5 or 6mm proud of the casting.

I also reckon I could temporarily drive the Cowells by holding its motor pulley in the chuck of the Unimat. Never tried it but I think it would have enough grunt for a small amount of alloy machining at 22mm diameter..

How concentric the existing recess is and how I would find a reference for clocking the motor end when mounted on the faceplate remain unresolved. Clocking inside an unevenly worn bearing doesn't seem like the right answer. Maybe inside the step where it mounts to the main motor 'tube' is the least bad option.

But whatever happens... getting the inner end caps off needs to be the next step.

Clocking the locating diameter of the end bell that engages the motor housing tube would probably give a good result. Unless your motor was manufactured different to the majority of all small motors made. Generally the end bells are machined for bearing bore and locating diameter to housing in the same setup. (or were several places I worked, who made motors)

Thanks. I was looking at SKF 608-2Z.

The web site quotes the nominal dimensions and then has a popup table which attempts to convey information about tolerances. I can't make head nor tail of it but I would like to know the upper and lower limit of the bore size so that I can figure out if it will slide onto the shaft... or need gentle or extreme persuasion.

http://www.skf.com/my/products/bearings-units-housings/ball-bearings/deep-groove-ball-bearings/deep-groove-ball-bearings/index.html?designation=608-2Z

Regards, Andy

To avoid concentricity problems it will be best if you can position some width of the new bearing on a part of the shaft undamaged by running in a worn bush.

Emgee

JS would have built up the shaft with weld, then turned an oversize journal at each end.

Neil

Yes Neil, and I seem to vaguely remember that he also turned it down whilst the weld was still smoking hot. Thought about doing that (just for an instant) and decided that I wasn't quite ready to try it just yet (and probably never will be).

IanT

For the moment you seem to be up and running.

Catch 22 is using your lathe to turn up new bushes, (Did you measure the bush OD and Shaft Diameter, while the motor was apart?) or to modify the end housings to take shielded ball races. As a jury rig, use a pistol drill as a temporary power source?

P S Ideally, you should have marked the end covers so that they were refitted in the same place as originally, (Maybe the previous owner did not, and this is why the rotor rubbed on the stator)

Howard

Edited By Howard Lewis on 03/10/2018 23:08:11

Yes the motor is back together and driving the lathe again while I try to make the expanding puller thing... which like most jobs is taking a good deal longer then I expected. It will probably make not much more progress now until a week hence because I have various things going on for the next few days.

I measured the shaft at various points before reassembling the motor. I got 0.3168 inches pretty consistently on the parts that run in the bearings (multiple measurements at different angles too.

This seems like a smidgin over 8mm... which presumably means that ball races or off the peg bushes would just about not fit.

I now suspect the rotor is not actually hitting the stator. The new theory is that the noise is caused by vibration of the shaft in the bearings.

Some more progress on the motor bearings with photos.

I made the ‘expanding puller’. It took me a good deal longer than expected but not to worry. The core is a 6mm anchor bolt with the conical ‘expander’ from the same anchor bolt. I made a stepped bush from brass with an internal taper to match the cone and sawed it into four quarters. It worked a treat. The ‘indoor’ cap was still quite reluctant to shift but a bigger hammer did the business.

I also made a stepped punch to drive out the bearing bush. With a 12mm socket supporting the other side, I squeezed the whole thing in the vice and it came out easily once the initial ‘stiction’ had been overcome.

So the bush is clearly not oilite, nor is it anything bronze related. It is magnetic, so most likely either iron or steel. It has some sort of logo stamped on the outside. The bush is 376 thou (9.55mm) long and the OD is 422 to 423 thou (10.73mm).

The motor shafts mike at 316.8 thou where they run in the bearings – a fraction over 8mm… and too big to fit inside an 8mm ball race I think. There does not seem to be any measurable wear on the shafts which is somewhat surprising but perhaps indicates that the bushes are a good deal softer than the shaft.

I’m a bit disappointed that the bush does not seem to be a standard size… unless there is another standard out there somewhere that I have not found. Does anybody recognise these bushes?

So at the moment my least bad idea is to get some 8mm x 11mm oilite bushes and machine a small amount from both the internal and external diameters – ID first (with a boring bar) and then make a mandrel to allow me to machine the OD.

Regards, Andy

That's an ingenious take on the expanding anchor bolt based puller. Something I shall certainly make a note of although, so far at least, using the anchor bolt or stud complete with an alloy adapter to bring it up to bearing size has worked fine for me.

If you ever have to do another one its worth remembering that brass is a relatively slippy material. Aluminium alloy usually gives better grip. I suspect that four slots in a one piece expander with a slender section at the end so it can move would have worked as well and be much easier to handle than 4 separate pieces. Separate pieces ought to grip better tho'.

Your magnetic bearing is probably some form of sintered iron bearing. Same sort of thing as oilite, just a different material. One supplier here :- **LINK**.

If you get oilite bushes check the bore size as supplied. Generally they are made a fraction over to allow for shrinkage when pressed in to a housing. If they are sufficiently oversize to work with your shaft in the relaxed state consider using loctite to hold them without press fit.

In a similar situation I used a wrapped steel, Glacier DU dry bearing material lined bush. Due to the open seam down one side playing about with the housing bore let me adjust the inner bore for a good fit on the shaft. Used the dry bearing mostly because fix was needed immediately and that was what I had. Worked fine in that small motor for at least year or so before it all moved beyond my ken.

Clive.

Edited By Clive Foster on 27/10/2018 16:37:00

My puller has a step to engage with the bearing cover rather than relying on friction. I also made a few extra steps in the unlikely event that I ever need to use it for a smaller hole. Before I made the stepped bush I did try just wrapping some lead sheet around the anchor bolt but it didin't do the job - the bearing cover was too tightly attached so the bolt just pulled out of the hole.

Thanks for the link. I'd never heard of sintered iron bearings but if I understand correctly, those ones are made to order so probably not an option for someone who is just after two small bushes.

A split bush might save some mucking about but the 8mm glacier bushes that I've found are all 10mm OD so would be too small to fit my motor.

So at the moment modifying an off-the-peg oilite bush is looking like the most practical option. The tolerances quoted on the oilite site indicate that the bearing is a tad over 8mm but that the shaft is expected to be a tad under. My shaft diameter is more than the max size of the bearing bore so I don't think I will get away without opening up the bore of the bush. Exactly why GEC would make their shaft so close to 8mm but a little over is a mystery to me. It doesn't seem to match any imperial size as far as I can see either.

Regards, Andy

It is quite posible that your bearing is a Oilite bush, some are made of sintered cast iron in a similar fashion to the bronze ones.

Ian S C

the logo on the bush/bearing is Vandervell, better known for big end and main bearibngs for IC engines. Replacements may be available. have a search! the bearing is probably a steel backing with a softer white metal lining.

Andy,

I have a 3/4 hp motor with plain bearings, that to was making a funny noise considering it has rubber mounts. The windings look fine, but it runs hot after 3/4 hour constant use (60C). Checking the winding resistance I got 3ohms, trying a good 1/2hp motor I got 5 Ohms, sounds near enough you'd think. But using a clamp meter the 3/4hp is drawing 10A constant without the belt so there an internal short across one of the windings ( which wont trip the RCD)

Be a shame to fix the bearings and then not bother to check the resistance.

Gosh that's a lot of amps.

I dont think I have the means to accurately and safely measure mains current draw but I do know someone who might be able to help with that.

Does anybody know the HP rating for this motor? I've seen a number somewhere on the Interweb (1/8 or 1/12 I think) but there is no HP number on the motor sticker so I have no idea whether the number I saw is reliable..

In the meantime my plug has a 3 amp fuse so hopefully I can be sure that it's not drawing anything like 10A continuously.

Regards, Andy

Hi Andy

Full load current (FLC) on the motor plate is stated as 1.15A so your 3A fuse is a good choice and in the event of short circuit overcurrent will keep to a minimum any damage to the motor windings.

Emgee

While the fuse may be rated at 3A, it may well not blow unless continually loaded somewhat above that value. It is more likely to blow due to the surge at start up, particularly if under a load.

Andy, I can only applaud your determination to repair this thing. I would do the same. Problem is, I wish I didn't spend so much time, effort and money trying to resurrect the dead or rescue the hopeless cases (even though they turn out to be not-quite dead or not entirely hopeless). I've got to the stage where this sort of battle is no longer a pleasure, but just an annoying distraction from doing what I really want to be doing. Also, I've probably spent more money on rebuilding machines and upgrading than I would have done by appearing extravagant and buying something really good in the first place. And the time spent and the aggravation along the way is incalculable.

As you get older, you realise that time's running out. If you're lucky(?) you may be able to predict fairly confidently that time will run out before the money does. If that's the case, put a load of beer vouchers into a VFD and a 3-ph motor, and enjoy the benefits.

Not telling you what to do, just the reflections of an old one. Good luck with the project, I'm interested to see how it turns out.

Cheers. I dont think the bearing job is terribly onerous. It's definitely worked out to be more complicated than I had hoped but most jobs have a tendency to be like that. I think the motor is basically sound and the Cowells 6 speed drive arrangement gives a sensible set of speeds so I'm pretty happy with where I expect to be when the job is finished. It's also been a good 'shake down' test of the lathe, my (often deficient) tooling and my somewhat rusty lathe skills.

I'm not ruling out re-motoring but my little Unimat would probably be first in the queue there but even then the motor seems sound and the brushes look almost new... but a decent variable speed motor on that one would give some useful speeds that the current arrangement really can't deliver... as well as the ability to use the thing for longer continuous time periods.

I have no doubt though that the re-motoring job on either lathe will also have all sorts of unforseen extras creeping in as the job progressed.