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A little foray into bearings and clearances

Posted: Tue Feb 19, 2019 2:12 am
by 13Black
I intend to come back to this a little later, in its current form it is quite incomplete and will be by no means instructional - more an interesting little titbit.
Bearing clearances, oil flow and oil choice is more complicated than the generalisations in this post may make it seem.

- - - - - -
Some Fundamentals

The bigger the shaft you need a bearing for, the bigger the gap you'll need between the two.
A general rule for this is a [1 thousands of an inch gap] for a [1 inch diameter shaft], [2 thousands of an inch gap] for a [2 inch shaft], and so on.
For those that don't deal with these small numbers that often and want to picture what that is, pluck a hair out of your head... that's about 1 or 2 thou thick.
Sweeping generalisations:
For smaller gaps you may desire a thinner oil, for a larger gap you may want a thicker oil.
For higher stresses like high RPM and extended race sessions, you may want a larger gap... let's say about 20% more, for now.
10w30 is a fairly standard viscosity for standard engine clearances, for min film thickness reasons and other things.

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Getting it wrong

Now, there's a fairly well known example of manufacturers perhaps getting this a little bit wrong in the form of BMW's S65 engine - a sodding great high-revving 4L V8 as found in their E90 M3. The manufacturer recommended oil for these was a rather gloopy-seeming 20w50.

It wasn't long before the bearings that go between the connecting rod and the crankshaft starting failing.
A few people stripped the engines to find that the clearance between these failing bearings (which we would expect to be approximately [1.20 thou/inch] from our rough rule-of-thumb above) was in fact about half that, around [0.61 thou/inch].
Hmm..! Rather thin gap, rather thick oil.
Some people say that thinner oil might help a little bit, some say the opposite. What seems to be agreed upon is that you should fit bearings with more clearance to solve the problem.

- - - - - -
Oh yeah, rotaries...

With that background, I figured I'd crunch the numbers on RX-8 and FD engines - given the former seemingly have some main bearing failure issues, and the latter don't.

Something to get out of the way initially: rotor bearings on each seem to be as expected. They only have a rough max 6,000 RPM relative to their journal on the eccentric shaft (so nothing too punishing there).
Their clearances are about [1.08 thou/inch] on both - cool.

Main bearings however, for whatever reason seem to be somewhat gaping! We have rather funny engines so this is probably nothing to be alarmed about, but it's a peculiar spot none-the-less.
RX-8 main bearing clearance: [1.74 thou/inch]
FD main bearing clearance: [1.86 thou/inch] - even looser!


How all this plays into the "5w30 oil is too thin" debate, and whether 10w40 should therefore be better ... some might think from these calculations that it is quite obvious (no doubt from how the post has been worded :ninja: ).
However, it should be noted that these are just some very general 'rules' and calculations, and the physics behind it all is far more complex than I will ever be willing to understand without being paid a decent sum of money to gather and apply that knowledge.


Now, fight.


Re: A little foray into bearings and clearances

Posted: Tue Feb 19, 2019 11:51 am
by warpc0il
I recall a discussion I had with the engine designer for the (then new) Triumph Triple, at a meeting of the IMechE, about 30 years ago, on this very subject.

It was his first real foray into "unit" construction*, where the engine and gearbox were a single unit and shared the same oil - as nearly all 4-stroke Japanese bikes had done for some time.

However, this meant a total rethink for the clearances in the gearbox, as even in those days, engine oil was so much thinner that the hypo gear oils used in pre-unit construction.

Most bikes also run their (multi-plate) clutches in oil, which many car owners find unbelievable, but it can cause problems if snake-oil additives are used.

*The Bonneville was also unit construction, designed by a different guy, and not putting out nearly as much power

Re: A little foray into bearings and clearances

Posted: Tue Feb 19, 2019 12:01 pm
by 13Black
The (really) old gearbox I'm rebuilding at the moment that flew apart due to poor clutch material choice also run in the same oil as the engine - not a shared sump as such, but a shared reservoir.
After discussing this with our supplier previously, we decided on a suitable material that has since proved itself to work well - any idea what material they use in those bikes?

I had a clutch to sort on an old quad last year which was simply a steel male cone into a female brass cone and it was slipping tremendously. The problem being that the engine leaked a load of oil out of the mains into the gearbox which should be quite dry. In the end, to save an engine rebuild, I just machined the brass female cone to mimic a modern syncro and cut through any oil film... which seemed to work quite alright so far.

Re: A little foray into bearings and clearances

Posted: Tue Feb 19, 2019 12:31 pm
by warpc0il
13Black wrote:
Tue Feb 19, 2019 12:01 pm
The (really) old gearbox I'm rebuilding at the moment that flew apart due to poor clutch material choice also run in the same oil as the engine - not a shared sump as such, but a shared reservoir.
After discussing this with our supplier previously, we decided on a suitable material that has since proved itself to work well - any idea what material they use in those bikes?
Cork?

Re: A little foray into bearings and clearances

Posted: Tue Feb 19, 2019 12:40 pm
by PeteH
https://www.quora.com/What-are-the-mate ... utch-plate

I always recall them being mostly cork in the early days of my biking. But then I also recall them slipping a lot. It seems nowadays they are Kevlar, and a variety of carbon composites, mostly.

My Ducati has a dry clutch (Ducati are famous for them). You can always spot a dry Ducati clutch because they rattle like a dead rotary when disengaged.

Re: A little foray into bearings and clearances

Posted: Tue Feb 19, 2019 12:52 pm
by 13Black
One of the cone clutches on one of the trikes gearboxes was leather (as was the front brake, which was far too aggro for me) and I do believe a common thing to do is soak them in neatsfoot oil to make them a little less abrupt in their engagement. This one wasn't I presume, and I managed to pop a rather splendid wheelie at Dunsfold, somewhat toward a terrified looking owner.

Re: A little foray into bearings and clearances

Posted: Wed Mar 06, 2019 1:02 am
by 13Black
Interesting snippet from an SAE paper by Idemitsu:

" 4.2 System oil for engine system
outside combustion chamber

(1) Lower base oil viscosity contributes
to improved fuel consumption. However, from
the view point of prevention of failure of
the bearing surface, it is necessary to use
a base stock having a viscosity grade over
SAE 40. "