Revolving centre, how to dismantle?

Had this no 2 live centre for years, its compact accurate and runs smoothly but of late it has started to lose its grease. I am assuming an O ring has given up the ghost.

Trouble is I am unsure how it comes apart? The end cap will unscrew revealing an INA bearing but nothing else is obvious.

Any ideas?

Alan

I have dismantled a couple of revolving centres several years ago and they were knocked apart using a soft drift on the shaft in the end that you show, although mine didn't have a bearing right in the end of the taper like yours.

Make a blank cap with centre thread and screw in a bolt?

So obvious I didn't think of that! A scientific approach.

Alan

A threaded cap with a bolt through the middle is a great idea, wish I had thought of that. But this is one of the revolving centres I dismantled and unfortunately it does not have a thread inside the end, the end plug is just pushed in and held with a rubber O ring. The bearings inside the body were a radial needle roller and a ball thrust bearing.

It may be worth noting that if replacing those INA needle roller bearings, the equivalent Koyo-Torrington needle roller has a higher load rating due to having twice as many rollers and no cage, so may last longer. They do when used as camshaft bearings in Harley engines.

Nobody has offered any suggestions yet as to how to remove the bearing

Roy

Bearing puller, slide hammer, hammer and drift? If at all a proper design that should be obvious, easy and straight forward - once the MT shaft is out of the way.

Edited By not done it yet on 14/03/2021 15:11:42

The design of live centres differs widely, that one may only be dismantled by destroying the bearing. There will also be a thrust bearing behind the needle roller.

You may have to bore a steel plate or ring that will support the rotating end from the MT side and press the centre out.

Another way would be to make a replica of the front end, drill and fit a grease nipple and force grease through the bearings without further dismantling.

Edited By old mart on 14/03/2021 15:24:04

Edited By old mart on 14/03/2021 15:27:17

Edited By old mart on 14/03/2021 15:30:32

Screwcut a threaded plug to fit the end of the centre, tapped M6 through the middle for a 'push out' screw. Gripped the tapered shank in a set of soft jaws and attempted to push it apart with the M6 bolt, by Jove it's tight.

Did it up as tight as I dared but stopped for fear of wrecking it, I got the impression it was assembled but not designed to be dismantled.

It runs fine at the moment so have decided to leave well alone until it shows signs of unhappiness! I am hoping that a ring of synthetic oil I applied around the accessible joint will run into where it's needed.

Alan

Try the grease method, take out the 6mm bolt and replace it with a grease nipple and gently pump grease through. The grease will work its way through all the bearings until it emerges at the back.

Thanks Oldmart, I quite like that idea, at least I can replace the grease that works it's way out.

Alan

That bearing is an open needle roller bearing with no inner race. **LINK** The hardened shaft runs straight on the rollers, as a slightly loose running fit. So the shaft does not need pressing out of the bearing as you would do with a ball bearing with an inner race that is a press fit.

So there must be something else holding it all together. Possibly a pin through the outer housing that locates in a groove in the shaft or something similar. There will be a radial needle roller in behind that visible bearing to take axial load but that too will not stop the main part from coming off in the opposite direction.

Some of those centres with the tapered outer body have a concealed join somewhere along that taper and the body is pressed together at the factory. Each half goes either side of an integral flange on the shaft that bears on the radial roller thrust bearing. Not really designed for dismantling once together. I think they machine the taper after the two halves of the body have been pressed together, so it is really hard to see the join. Or the join may be where the taper meets the parallel section of the outer body. Careful inspection with a glass might reveal?

So be wary of trying to press it apart with bolts or even grease pressure. You may distort the flange on the inner shaft that the radial thrust bearing runs on. Heat from a torch may expand the end half of the body enough to come away from the inner half? Kind of operating in the dark though if you cant see that join on the taper and be certain.

As the bearing is open onto the central shaft, with no seal, standing the unit up vertically and pouring oil down into the bearing space might be enough to free it up. Would only need a couple of drops. You don't want to wash the factory grease out. Or melt a little grease in a tin an pour a small amount in instead of oil.

You could then pack the cavity with a bit of grease, leaving some air space, and screw the centre in with some gasket sealer on the thread to stop the grease coming out.

Edited By Hopper on 16/03/2021 02:28:42

Edited By Hopper on 16/03/2021 02:37:45

Thanks for your thoughts.

The centre is running fine, no tight spots or free play and it is dead smooth (and accurate) it's only problem is it can now lose grease between it's moving and stationary parts. I guess an O ring has seen better days.

There is no obvious joint where it looks like it could be dismantled, hence my original question. If I can introduce grease 'through' the centre via the grease nipple on the end idea above I can at least replace the grease that is being lost.

Alan

Yes that might do it without needing disassembly. It's construction will be an enduring mystery.

AJW,

What make is the live centre? I see there is a 3 ring imprint on the head of the tool but can't make out the wording. If it is a reputable make can you not ask the manufacturer / distributor / agent for advice on its assembly?

I wouldn't be surprised if your live centre turned out not to have any seals fitted, there is not much room for swarf to enter through the rear.

Oilyrag, not got a name as such just 3 rings as you see with the words Trade and Mark either side. Obviously their logo but haven't been able to track it down. Still have the box which says made in Japan. Bought it probably 35+ years ago at a MEX.

Oldmart, you could well be right regarding the O ring although it's strange the grease has only recently started to emerge?

Alan

Stick a thin feeler gauge up the gap and feel if there is rubber or metal in there?

If there is no o-ring or seal, maybe the grease is gravitating downwards while the centre is stored in the usual manner, nose upward. Maybe keep it nose downward between use and drain the grease back inwards again?

I imagine if you put a grease nipple on a threaded plug and fill it chockers with grease, it will ooze out all the more. Plus not good for the bearing to run in 100 per cent grease with no air space. Tends to overheat the bearing as it tries to pump the grease around in there, which then destroys the grease which then lets the bearing wear prematurely. Usually aim at 50/50 grease and air in a bearing cavity. Or even less. And needle rollers dont really like grease. They are happier with a thin oil. But maybe in this low stress low rpm application grease is ok if it is a thin grease like white lithium Optimol etc?

Needle roller bearings may not ‘like’ grease but they are successfully used in an awful lot of greased applications - universal joints, drills, jack hammers, clutch spigot shaft bearings (cars) - to name a few I have come across.

Obviously UJs should not rotate at great speed and clutch-to gearbox bearings (should) only run intermittently but the drill and jack hammer bearings are no particular problem in use.