Author Topic: main bearing shimming  (Read 1105 times)

Offline sean

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Re: main bearing shimming
« Reply #30 on: 28.09. 2020 13:46 »
Had to shim my crank and did it "my way":
I prefer a tight bearing on the output shaft so I had the shaft laserwelded and ground to the correct diameter. The inner bearing got heated to about
100degr and slipped over the shaft and will stay tight. The outer race was fitted to the cases
and cases assembled to measure the play. As play was only
0,25mm I made a shim washer
of 0,5mm to go behind the outer
 bearing. To correct for the
needed play the outer race was
then removed from the primary
case and ground down 0,27mm
on the outside - thus giving me
0,02mm sideplay cold. The  crankfacing side was ground
another 0,5mm to give more
space for oilmist entering. The
reason for a thicker shim washer
is that it is easier to make for me
and I can adjust always the play
by grinding. The bigger shim
behind the bearings outer race is
a much larger surface so it will
never be beaten to pieces that fly
 around inside the cases.
 
I have read somewhere but my old foggy brain can’t remember where , not to shim the outer race this is the method Harley uses to set the end play on the older models using the Timken tapered bearings on the crank .....the only  difference I could see between the 2 methods of end play shimming would be possibly off setting the crank shaft .025 or what ever shim was used and putting pressure on something on the timing side of the motoR
cheers, Harty

Offline BSAmoto

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Re: main bearing shimming
« Reply #31 on: 29.09. 2020 20:29 »
I will never put a thin shim behind a bearing race on the crankshaft. Have seen too many engines with a lot of internal damage due to thin shim pieces floating around, of course never had a problem when putting the shim behind the outer race in the case. And grinding it to the desired size and sideplay is no big deal for me as I have all the neccessary machinery.
cheers, Harty

Online RDfella

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Re: main bearing shimming
« Reply #32 on: 29.09. 2020 21:15 »
"I will never put a thin shim behind a bearing race on the crankshaft". Whyever not? The bearing is not rotating on the shaft and the shim is compressed between bearing and flywheel. It can't go anywhere. Only possible failure from an engineering perspective is perhaps if the bearing is a press fit on the mainshaft. In such circumstances, there's no guarantee that the cush drive nut is actually moving the bearing hard up against the shim. Another reason to ensure the bearing is a push, not a press, fit.
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Offline BSAmoto

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Re: main bearing shimming
« Reply #33 on: 29.09. 2020 22:39 »
Easy one: shim on shaft is always thin and has small surface. Take a look at a thin shim after it spent a time between the inner bearingrace and the crank web - it will be dished. By tightening the mainshaft nut it WILL sooner or later slim down and creep out between the relatively small flanges of crank and inner bearing race, thus it becomes dished, only way to avoid this would be a trapped shim but I digress.... Having a sliding fit for a main bearing and relying on the clamping force of the mainshafts nut is not the best engineering practice. You will get away with it up to a point but once the powers that are working on the shaft/bearing are too high - it will just hammer itself lose or to pieces - whatever comes first. This RR crank was worn undersize - of course because the stupids that "restored" this engine did everything wrong and did not have a single clue. So while I had it on the machine to grind the laserweld to size I measured up the available play in the mainbearing (which got oversized by plating/grinding its outer race by 1,5/100mm to restore proper grip in the cases) and decided that I want it to grip the shaft on its own. Again - not suggesting that anyone has to follow my practice but I have the means and machinery to make it right and so I do.
cheers, Harty

Online RichardL

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Re: main bearing shimming
« Reply #34 on: 29.09. 2020 22:55 »
Harty,

I, personally, try not to have so little play to correct that only a thin shim will correct it. If I have to make a stack, thicker shims at the ends of the stack. Not having your resources, the shims are behind the inner race. Hasn't failed for about 5K miles since clamping inner race against the web while bearing-retaining  Loctite set up.

Now, to my question. If you pre-shim behind the outer race such that excess height must be ground off that race, how do you measure the amount to grind off?

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020. This year it's a solo or pillion ride in dapper attire. Watch website at https://www.gentlemansride.com for details.


Offline BSAmoto

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Re: main bearing shimming
« Reply #35 on: 30.09. 2020 10:13 »
Hello Richard
I assemble the crank into the cases and measure endfloat with the outer bearing race bottomned in the primary case half, all screws, nuts, bolts clamping the cases together - minus sealer.  Then with a dial gauge set up at the cranks end the endfloat is measured. Lets say it is 0,25mm and we want only 0,05mm it would mean I insert a 0,2mm washer behind the inner race or the outer race of the primary side bearing. If I had the correct washer made of suitable material I would be finished here - but I have no suitable washers as I mainly do Norton and Vincent engines. So I made a washer. The reason I use a thicker washer and grind the bearing down a bit is due to my grinding machines not holding a thin shim on the magnetos - it will just fly away or if it stayed on the magneto it will become a bellville spring due to internal stress. So in our example the washer is 0,3mm too thick - this is the amount that I grind off the bearing.  As the radius on the outer race of the bearing is a lot larger than the radius of the recess in the crankcase there is no reason to fear the bearing and washer behind will not go fully home. As I said before - even lose races on the crank with some glue and some clamping force of the primary nut will hold but the amount of transferable output will be reduced and once the shim behind the race is collapsing the wear starts as there is no clamping force at all. You will first notice that something is amiss when you hear the knock when the lose bearing hammered the crank undersize or when the splines on the crank are worn away a good bit - both needing costly repairs.
cheers, Harty

Online RichardL

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Re: main bearing shimming
« Reply #36 on: 30.09. 2020 15:55 »
That explains it. Earlier this year (before the fall of Rome) I tried to grind a thin shim using the surface grinder I had access to at the time. With extremely little experience with  surface grinders, I soon learned of the lack of magnet strength to hold the piece and the potato-chipping of the now-waste-of-time part.

Richard L
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020. This year it's a solo or pillion ride in dapper attire. Watch website at https://www.gentlemansride.com for details.


Online muskrat

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Re: main bearing shimming
« Reply #37 on: 30.09. 2020 21:47 »
G'day Harty.
I beg to differ.
In the original set up a C3 clearance bearing is used meaning both inner and outer race is an interference fit (pressed onto crank, shrink fit in case) to give the correct running clearance in the bearing. When the inner is pressed onto the shaft sandwiching the shim and the cush nut is done up to 65-70 ft/lb the shim can't escape. The shim is under two forms of clamping, interference fit of the inner means it won't move and the clamping of the cush nut makes sure of it.
With the shim behind the outer race it is only relying on the shrink of the case around the race (the alloy case expands more than the steel race). Then add any hammering of the crank/outer race due to the end float (which will be larger when the motor heats up). Then add to that any crank flex working through the rollers onto the outer race we have a lot of forces trying to "rock, rattle & roll" the outer race in the case. Notrun used "superblend" bearings to combat this.
In conclusion. If it's done the way it was originally designed there will be no failure of the shim. Most if not all shim failures are due to the inner bearing being loose on the shaft and/or the cush nut not tensioned correctly.
Just the way I see it.
Cheers
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Offline BSAmoto

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Re: main bearing shimming
« Reply #38 on: 30.09. 2020 23:48 »
Hello Muskrat - I see where you come from. That´s why I had added 1,5/100 mm for proper grip on the outer race to make sure the bearing stays in place. (Yes, I ground the outer race internally to get my desired running clearance as it was closing up due to the shrink fit internally and externally) Still the relatively small surface of a shim behind the inner race is not good practice - only if it is trapped which would mean to machine the inner bearing race and cranks flange as well. As I said before - this will easily work up to 40hp or maybe a little more but once you turn 7200 revs on the german autobahn to find out what the thing will do (238km/h) , especially with a 905 Norton (with third mainbearing added in my new crankcase half that was cast for this motor to help the crank coping with the power) those little shims are not up to the job. Lesson learned. Not that I would give the BSA that punishment - it will fly to pieces long before....If the dimensions of crank and case were done correctly, no shim at all would be needed. For Nortons there was only ever a shim to go behind the outer race available - never one to go on the crank. Seems they knew why. I know now too. Yes, the shim was on the timing side - in case you would ask.
I am not giving advice. I just tell what I do and why. Today I added 3 long lost fins to the cylinderhead (two in the middle of the stack), milled and welded up all 5 posts on the head to get proper metal back for new threads, replacing the bronce inserts with inserted helicoils.... did I tell this engine was "restored" by a superduper shop in Nizza / France ;-) Tomorrow I will make proper valve guides and fit them to the head and voila - no more rattle or smoke.
cheers, Harty
PS: superblends were added when the cases got stiffening ribs - before those ribs we got away with very high power output of the 750 Norton - Mr Axtell knew his stuff and prefered the early cases for his hot engines as they were flexing with the crank... just as the thin BSA primary side does.

Online metalflake11

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Re: main bearing shimming
« Reply #39 on: 01.10. 2020 02:04 »
Harty, have you not considered the ultimate solution to this problem, the needle roller bearing conversion?


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Offline BSAmoto

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Re: main bearing shimming
« Reply #40 on: 01.10. 2020 08:55 »
Hello Metalflake: no need for the needle roller bearing in this bike - the last "mechanic" fitted a big oilpump to compensate the knock on the timing side. Now that I made a proper bearing and ground the TS end to cylindrical and round dimensions again I doubt that I will ever wear out that bearing as this bike is to be used around our back roads and will not be thrashed because of 0,060" pistons and last undersize journals of the crank.
cheers, Harty

Online Truckedup

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Re: main bearing shimming
« Reply #41 on: 03.10. 2020 16:53 »
 Most of my experience is on Triumphs having ball and roller bearings on both mains. The bearings are supposed to be a light press fit and there's also the CN ,C3 etc internal bearing clearance to consider.. 
 So on the A10, use a  C3 bearing with hopefully a light press fit and shim for a few thousands inch clearance ?
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