Removing PTFE Sleeve Bearings

Hi, I'm new here. I've got a problem in that I need to be able to remove, replace, and potentially resize some 32mm ID/35mm OD PTFE lined aluminium backed bearings. They are fitted to a blind hole and located about 130mm from the opening. Can anyone recommend whether a tool exists for this type of job or whether I would be better having something made up...

Many thanks,

Ben.

Now that's an interesting puzzle, Ben

slippery stuff, PTFE ... so you're almost certan to damage it before any expanding tool will grip.

Question: Is the bearing really at the bottom of the hole, or is there some gap beyond?

The [apocryphal?] 'hydraulic ram' trick might possibly work, but 130mm is a long way!!

MichaelG.

Hi Michael, glad to have been of service!

I did miss some detail earlier, the bearings do have gap beyond probably 30mm or so. Are you aware of any off the shelf engineering tools that can deal with this? One suggestion I received elsewhere was a shaped washer on a rod to be dropped just below the bearing and dragged out carefully with a slide hammer..

Getting new bearings back in and sized could be even more interesting... Maybe a shaped drift. Very fine tolerances though, 32mm +/- 0.001mm for the sizing.

Thanks,

Ben.

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That should make it relatively easy ... I'm not sure about the availability of off-the-shelf kit for this, but, for example, I have one special puller that expands a set of legs behind the bearing. ... The shaped washer should work, and slide hammers are good; but determining the optimum shape & size of the washer could be tricky. Hopefully the old bearings are considered sacrificial.

MichaelG.

Bearings definitely sacrificial on the way out. Will be back in touch if I find out any more.

Can you heat the housing and slide the old bearings out using a suitable bearing puller etc?

Can you heat the housing and slide the old bearings out

Depends on the material. Aluminium fits tighter in steel as temperature rises. Better to freeze it possibly. A dewar of liquid nitrogen would easily do the trick, I suspect.

Aluminium backed PTFE sleeve bearing to a 0.001mm tolerance sounds a very unusual beast. It would help if we knew what is this bearing fitted to and how its is retained? If the housing is aluminium then extracting the aluminium bearing backing sleeve might be difficult because of the two surfaces galling.

How do you intend to do the resizing after fitting the new bearing anyway?

Ian P

Hi, the bearings are fitted to a very old set of bike suspension forks. Not quite a model engine bit figured the use case was pretty similar. The fork lower tubes, where the bearings are fitted, are a magnesium alloy of some kind. Into the bearings slide the fork uppers. The bearings do seem a very tight fit on their outer radius, heating might be possible although would need to be very careful with the alloy.

​I was hoping for some advice on the resizing part of this job, given the material is PTFE I was thinking tapered drifts of varying sizes could be used.

My internet research tells me that installation and resizing tools are very difficult to come by!

​Thanks,

​Ben.

If you can seal the top of the upper fork leg you have a potential ram with which to extract your bearing. Goes something like this. Fill the lower leg to just above the bearing with oil. Fit the upper tube into the lower leg . Fully fill the upper leg and seal the top. Force the upper leg down into the lower leg maybe by thumping with heavy hammer padded with a lump of wood if you cannot arrange a press. The bearing should be forced up the lower leg. Maybe easier if you turned up a flanged bush to sit at the top of the lower fork leg to keep the whole assembly concentric.

regards Martin

Ben

If the replacement bearings are available surely the fork manufacturer will have fitting details? if the forks are so old they are no longer supported then you could research what other manufacturers recommend.

Are these PTFE bushes actually 'aluminium backed' or are they a composite of the two materials? I see that PTFE loaded tin bearings are made so maybe yours use aluminium instead.

Knowing what the application is I cannot understand why a tolerance of 0.001mm is being specified, seems overkill!

Ian P

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... That is, of course, "The [apocryphal?] 'hydraulic ram' trick" that I mentioned earlier.

I have a suspicion that we all "know a bloke who met somebody that knew a man that did this"

... But perhaps, Martin, you are that man

MichaelG.

Edited By Michael Gilligan on 14/06/2016 10:10:24

I should have said as mentioned earlier and I have done in on bushes with a bar end and grease. Don't see why it shouldn't work on a fork leg bush especially if it can be set up without too much trouble. Is there any form of retaining clip on the bush?

regards Martin

Is it apocryphal because the bush is 'hidden' down the fork leg.?

Martin,

I'm impressed to hear that you have actually done this [so you are indeed 'that man']

I do fear, however, that a magnesium alloy fork leg might burst before the bush moves 130mm

MichaelG.

Why do you think that then Michael. Why should the leg burst before the bush moves. Do you actually think you can produce that amount of pressure?

Martin

Michael for nearly 30 years I used to remove oilite bushes from blind holes on printing machines using the hydraulic method    Believe me it works

Roy

Edited By roy entwistle on 14/06/2016 11:41:17

The apocryphal hydraulic ram trick sounds like a neat idea. The tops of the fork leg can be sealed no problem, although I feel that maybe attempting it whilst hiding behind a blast proof screen might be a good idea

Looking at the bearings closer I see one problem with this and that is they're slotted to allow oil to blow past.

The bushes are interference fit with no retaining clips. Interesting point about whether they truly are aluminium backed or PTFE composite - I'm not sure.The original manufacturer is in Switzerland - DT Swiss - and no longer holds spares but did indicate the bearings are not necessarily specific to this industry

It seems that official manufacturer tools are very few and far between - and in most cases their recommendation is to replace the whole lower assembly where the bearings are already pressed in - seems a waste. Unfortunately due to the obsolescence within the cycling industry, forks of this particular size are no longer manufactured and therefore to throw the fork away means to throw the whole bike away...

My subsequent research has found a handy guide provided by another fork manufacturer.

**LINK**

They use generic tools that can cater for a variety of bearing sizes. Looks like the bearing removal and replacement tool is available from USA, and is reassuringly expensive.

I have made glass filled PTFE fork bushes and the stated 0.001mm tolerance is questionable. I usually bore the bushes to plus 0.003" (0.075mm) above the stanchion size. Any less and the forks will not move. I would think the 0.001mm is from the bearing manufactures specification and the stanchions are undersize.

Don't knock the method. Likely not work in ths instance if the cross section of the sleeve is minimal (suspect it may be).

The method was used just two weeks ago on the engine of my daughter-in-law's car. They needed to remove the bush in the flywheel/crankshaft as this exchange engine accepted a smaller spigot shaft, than the gearbox to which it was to be fitted, and it had stymied the mechanics. They actually used bread as the 'hydraulic jacking medium'. The bush was extracted successfully - and quite easily in the end.

Edited By not done it yet on 14/06/2016 14:04:43

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32mm diameter piston, driven hard enough to move a bearing with such a small end-area [wall-thickness is only 1.5mm] suggests some pretty high fluid pressures.

Happy to be wrong; but I think a slide-hammer might be safer.

All depends on how tight the bearing is fitted into the bore of the leg.

MichaelG.