Author Topic: Choice of Drive Side Main Bearing  (Read 670 times)

Offline Swarfcut

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Choice of Drive Side Main Bearing
« on: 17.11. 2019 10:47 »
Still working away on my basket case Longstroke A7, which has the standard ball race bearing on the drive side. These bearings are relatively cheap compared to the later roller type, and a far simpler way of eliminating/controlling crank end float.

 Apart from the easier splitting of the cases with the A10 design, is there any other reason for a roller bearing being chosen on the later engine?  The early A65 reverted to the ball race type, but was then superseded by a roller type, so I suppose the higher loading on a more powerful design is the critical factor, but for an oldie ridden considerately, a ball race on an early shortstroke type would solve the endfloat problem.

 Why does the inner race have to be a drive fit on the crank, when it is clamped very tightly by a correctly assembled cush drive nut? Or is this the problem that folks don't tighten the nut to the recommended gut busting 65/70 ftLB's. Polishing the crank to be a push fit in the inner ball race would make stripping the engine just as easy as the roller type. 

 Anyone tried a deep groove ball type race on a later motor, rather than wrecking expensive roller bearings on and off to shim the crank?

 Swarfy.

Offline RDfella

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Re: Choice of Drive Side Main Bearing
« Reply #1 on: 17.11. 2019 15:51 »
From an engineering point of view, the bearing should be a push fit on the shaft, not an interference for the reasons you state. On my A10 some clown of a PO had centre-punched the shaft to make it tight!
As for ball / roller - mine has the ballrace and is all that's required for moderate use. A roller will take more load and thereby last longer but, as you say, does not control endfloat. Not many of us are doing 50,000 miles + between rebuilds.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Swarfcut

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Re: Choice of Drive Side Main Bearing
« Reply #2 on: 17.11. 2019 18:19 »
Thanks RD. A good bit of reasoning and experience to reassure my use of cheap bearings.

Swarfy.

Online beezermacc

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Re: Choice of Drive Side Main Bearing
« Reply #3 on: 17.11. 2019 20:17 »
Another advantage of roller over ball is that the roller bearing allows the crankshaft to travel laterally. There shouldn't be more than a couple of thou lateral movement in any case but, if there are forces causing the crankshaft to wander sideways, it is better that it is allowed to do so. Lateral movement of the ball race outer case will eventually wear out the bearing housing.
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Offline Triton Thrasher

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Re: Choice of Drive Side Main Bearing
« Reply #4 on: 18.11. 2019 12:55 »
Another advantage of roller over ball is that the roller bearing allows the crankshaft to travel laterally. There shouldn't be more than a couple of thou lateral movement in any case but, if there are forces causing the crankshaft to wander sideways, it is better that it is allowed to do so. Lateral movement of the ball race outer case will eventually wear out the bearing housing.

There’s already a bush on the timing side, to allow the crankcase to expand laterally.

The crankcase also expands radially with temperature, making the roller or ball bearing outer race a sliding fit.


Offline RDfella

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Re: Choice of Drive Side Main Bearing
« Reply #5 on: 18.11. 2019 16:06 »
TT - yes, the c'case expands with heat, but the interference fit should be sufficient to retain grip of the bearing. That's how valve seat inserts stay put in alloy heads, for example.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Triton Thrasher

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Re: Choice of Drive Side Main Bearing
« Reply #6 on: 18.11. 2019 17:02 »
TT - yes, the c'case expands with heat, but the interference fit should be sufficient to retain grip of the bearing. That's how valve seat inserts stay put in alloy heads, for example.

Valve seats are an example of something different.

Main bearings lose most of their interference fit when the cases are hot.

I have some rough and ready figures off the Internet.  I don’t work this stuff out myself, when it’s been done by cleverer people a long time ago. 

On an engine with main bearing outer diameter 2&13/16 inch, a 75 degrees C increase in temperature will loosen the housing by 0.002-0.0025 inch. Do you even have that much interference in your fit, at room temperature?
The bearing does expand too,  but even if the outer race were 20 C hotter than the case, it only changes by 0.0006".

You’ll get your crankcases hotter than 75 C without even trying.  Your ball or roller bearing outer race moves in the crankcase.

Offline terryk

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Re: Choice of Drive Side Main Bearing
« Reply #7 on: 10.12. 2019 08:46 »
This question has come up years ago the ball main bearing on longstrokes is fine.
1950-53 A10 rigid/plungers, 1958-61 A10 super rockets, 1947-50 A7 longstrokes, 1949 Star twin,
1951-54 A7 plungers, 1940s M21, WDM20s,
1948-50s B33s rigid/plunger/swingarm, 1948-50s b31s rigid/plunger/swingarm

Offline RDfella

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Re: Choice of Drive Side Main Bearing
« Reply #8 on: 10.12. 2019 18:19 »
TT - afraid I have to disagree. If a bearing becomes loose, it will move and wear the case until it's scrap. The interference fit has to allow for the coeff of expansion of the particular alloy and the envisioned operating temperature. That's why we use extra-clearance races, to allow for the closing effect of the interference. And a valve seat insert is no different - I just used the example because it's a fairly extreme one and if ever one of those comes loose it's goodbye engine. If, with our crankcases, the required interference has been lost then the answer is to either fit a larger OD bearing or fit the case with a sleeve.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline berger

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Re: Choice of Drive Side Main Bearing
« Reply #9 on: 10.12. 2019 19:07 »
well I went to the pub so today i'm not very good  *countdown* *beer* and don't know too much about anything, but an interference fit to me is a cold or room temperature fit between two things that cause friction when pressed together, a bearing outer in an alloy case should be fitted with a spit hot casing and a cool bearing so to my mind it is not an interference fit , many bearings turn in cases that have been abused by bashing them in and out cold *bash*. in an ideal world bearing in nitrogen and  a hot case is easy peasy, but I wouldn't call this method of fitment interference *fight* edit-- brain has come back a bit, the bearing in engine case fit must be a shrink fit,  I might not need to see a shrink just yet.   I hope!!

Offline Triton Thrasher

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Re: Choice of Drive Side Main Bearing
« Reply #10 on: 10.12. 2019 22:31 »
TT - afraid I have to disagree

Do you disagree with the typical main bearing interference fit I quoted?  Ok what interference fit do you think is used? Tell us why you think that.

Do you disagree with the expansion figures I quoted?  If so, perhaps your dispute is with Isaac Newton and various Enlightenment physicists.







Offline Swarfcut

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Re: Choice of Drive Side Main Bearing
« Reply #11 on: 11.12. 2019 15:36 »
 I only asked about the choice of bearing. Any one else using cheap ball race on a shortstroke motor?

 I would imagine with a ball race fitted it would be difficult to measure between the timing bush face and the crank, (there will be no end float and access difficult) but as long as the crank pokes through the bush as usual and the cases bolt together without the crank binding when cold, then sufficient axial clearance should exist between the timing bush face and the crank when the motor is hot,  allowing lateral thermal expansion of the crank towards the bush.
 The problem comes with those sideways forces, and I can see that a ball race firmly clamped to the crank will either restrain the crank, or be moved bodily sideways, all dependant on the forces holding the bearing outer race to the crankcase. Maybe this is the reason for the change from ball to roller.

  As for the bearing fit in the case, the coefficient of thermal expansion is known for the differing metals, we know the original size "cold". We know the expected change in temperature so can calculate the change in size with increasing temperature for a hot motor. The trick is to make sure an interference fit still exists at the higher temperature and the alloy case is originally machined to accommodate this. All fine and dandy, but 60 years later don't worry about it too much, just reach for the Loctite Bearing Fit if the case is a bit slack.

Bergs fit I would describe as a shrink fit, but either way we are relying on the friction between the two components. 

Looks like I've got enlightenment as well as  "O" level physics.


Swarfy

Offline Triton Thrasher

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Re: Choice of Drive Side Main Bearing
« Reply #12 on: 11.12. 2019 16:17 »
Loctite is pretty useless for main bearing outers in alloy cases.

Mark the next one you put in and then if you ever split the cases again you can check to see if it rotated in service.

It’s not a problem, but they do move.

Offline Triton Thrasher

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Re: Choice of Drive Side Main Bearing
« Reply #13 on: 11.12. 2019 16:45 »
The loss of bearing interference fit in a hot engine isn’t irrelevant, as the subject of lateral crankshaft movement was being discussed.

Another advantage of roller over ball is that the roller bearing allows the crankshaft to travel laterally. There shouldn't be more than a couple of thou lateral movement in any case but, if there are forces causing the crankshaft to wander sideways, it is better that it is allowed to do so. Lateral movement of the ball race outer case will eventually wear out the bearing housing.

Online muskrat

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Re: Choice of Drive Side Main Bearing
« Reply #14 on: 12.12. 2019 18:50 »
G'day Swarfy.
As you say clearance between the crank and timing bush would be nigh on impossible with a ball bearing fitted. Too much clearance there and oil will escape instead of going to the big ends.
As for interference fit in the c/case, has anyone got the exact factory dimensions? Two and a half thou sounds about right. I have had virgin cases that have taken 200 C to get the outer race to let go.
Cheers
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