Replacing Oilite Bushes in ML7 Countershaft bracket

I have 4 unused Oilite bushes 3/4" bore, 7/8" OD but 3/4" long, Free of Charge.

If you want them. I will put them in the 'For Sale' column on this forum., contact me there.

As has been said Oilite is a porous sintered material. My usual is to press or tap out, even cut with a saw blade and collapse the bush if needs, Press in the new bush using the shaft as an internal size plug as it goes in. Only machine Oilite if really necessary and cut with a sharp boring tool otherwise the surface material pockets will be destroyed lessening the lubrication ability. All information can be gleaned from the Oilite website.

Oilite bushes can be bored to size, but reaming will close up the pores which defeats the self lubricating idea. I had heard it said many years ago that if a dummy shaft was made with a good surface finish of the same size was used to push the bush in, the bush material would crush slightly and the fit would be perfect. The problem of removing the dummy shaft would then have to be addressed.

Hi William!

I worked for a major tyre manufacturer (Pirelli) for many years (just retired!), And in My earlier career there I worked as a Fitter's Mate in the workshop that maintained the tyre building machines.

These made profuse use of Oilite bushes that needed frequent replacement due to wear, and the procedure We used to change them was to simply push the old bush through with the new bush, usually in the bench vice with a suitably sized socket behind to cover the old bush on the way out.

Probably not the best pure engineering solution, but in worked without problems for the 10 years I was working on the job!

Cheers, Pete

William. it looks like Richard is closer to you as I am up in the Midlands. Keep us posted on how you get on.

Morty,

I expect your method was ‘rough ’n ready’ and clearly worked well enough.

In this instance, the part may not easily be held by a vise and this is a bearing with a relatively thin wall, so your method, using twice the force required (by removing and inserting simultaneously) may not be the best plan.

I would advocate make removing and fitting bushes separate operations. Trying to do both in one operation risks damaging the new bush. It is thin walled and the forces involved might crush it and cause it to distort..

No point in saving ten minutes at the cost of damaging new components

Howard.

I have had to do this for a 7 and S7, and confirm it is not a trivial job, especially if you intend to fit a new shaft. Hammering them in, using G clamps etc is not recommended, oilites will distort in a heartbeat.

You absolutely do need to make up a pulling mandrel, find lengths of threaded bar and have some appropriately sized tube washers and nuts to use as pullers.

Your mandrel should be stepped to conform to the exact external diameter of the shaft (avoid being smaller) with the wider portion say 10 thou less than the id of the bearing recess. The part upon which the bearing sits needs to be slightly (4-5 mm) longer than the bearing. Tap the mandrel for 8 or 10mm threaded pulling rod.

Pulling the old bearings out is easy, pulling the new ones in needs a bit of care on the initial alignment. Soak bearings in oil and use lots of oil when placing bearing on mandrel (it may be tight)

You may need to wring the mandrel back out of the bearing, or even need to pull it out.

My nasty experience came when I discovered the new shaft was a wringing fit in the new bearings. I tired slow running the shaft as a bearing run in, but it was clear that it was seizing.

Now here is the thing, you don't want to ream this type of bearing and even if you did, how would it be possible to do it accurately in situ? I solved this problem by making up a polished olive that was a couple of thou bigger than the shaft id. This is pulled through the tight bearing making it conform to the required size (process may have to be repeated). These bearings have a porosity, so within reason can be 'squeezed' to size.

Eventually, the shaft fitted albeit a little tight, but easily enough that after a bit of running in, all was well. The running in consisted of: run the shaft at min primary speed, as soon as the bearing gets cup of coffee hot, say over 50 C. stop and allow to cool. Repeat until happy that the heat build up is reducing.

With the lathe back in service, keep bearings well oiled and monitor temperature, ceasing operations if getting too hot until well bedded in. A year or so after fitting, my countershaft bearings get just slightly warm after a couple of hours run. I make sure the oil cups are full before every use.

NDIY, since the bush being inserted displaces the bush being ejected at exactly the same rate, wouldn't it remain constant at once the force required? (Ignoring all the other variables.)

George B.

Constant - yes (well maybe, but dependent on the force required for each separate operation being identical), The same - no. Total Work Done would be the same, but all in one operation. Same Displacement, so Force must be increased (WD = F x D).

Edited By not done it yet on 07/02/2021 07:00:43

I haven't done the maths and could yet be caught out by something I haven't thought of, but this is the way I see it:

I would expect the extracting force to be F = Fmax at displacement D = 0, with a straight line reduction to F = 0 at D = Dmax. (Again, ignoring all other variables). If we plot a graph of displacement versus force, then the work done would be the area under the straight line.

Similarly, the insertion force would be F = 0 at D = 0 and would increase linearly to F = Fmax at D = Dmax. Again, the work done would be the areaa under the line.

At any value of displacement D = Dx the total force Fx = F extraction + F insertion = Fmax and total work done is Fmax x Dmax.

So I don't see how the force could ever exceed Fmax, which is the force required to start the extraction.

George B.

Sorry, if you can’t see, I can't help you any further. Perhaps others might.