crankshaft front bearing type

[steve davidson]

Hi all,

One for the boffins maybe.

I’m wondering if anyone knows the technical reason why on the front of the 1”1/8” crankshaft there is one roller and one ball bearing?  I’m not looking to do anything different just looking to understand what roll (pardon the pun) each different type of bearing is doing at this location.  What would happen if you used two rollers or two balls?

Thanks,

Steve

[bob46320]

As I understood it, the rollers give you axial support (Ie front and rear roller bearings) and the ball race gave you longitudinal support. The thinking is that the roller bearings give a better load support than ball bearings as there is more bearing contact to take the thrust of the piston rods etc.

[Chris KC]

Radial, not axial Bob - but basically correct. At least that's the theory, but crankshaft whip among other things clouds the picture slightly. There's quite a bit on alternative bearing configurations in the 750 companion. Two rollers would have little capacity to support end load. A pair of self-aligning ball races can and indeed does replace the ball and roller set up successfully as they are designed to take both axial and radial loads (also a degree of misalignment which a roller race can't handle so well). n.b. It's rather early in the morning so I'll probably realise I've written something stupid later...

[Tony Press]

As I understood it,  the rollers give you axial support (Ie front and rear roller bearings) and the ball race gave you longitudinal support.  The thinking is that the roller bearings give a better load support than ball bearings as there is more bearing contact to take the thrust of the piston rods etc.

Yes- radial not axial for the roller bearings and of course standard roller bearings have no axial capacity.

The sometimes suggested self-aligning ball bearings are very poor in thrust and size for size much lower radial capacity compared to roller bearing loads 

The later crankshaft has two 'angular contact' ball bearings at the front - because of the internal design, these bearings have a greater complement of balls than the corresponding single row ball bearing- they can take greater thrust load and mounted as a pair greater radial load than a pair of ball bearings.

No standard Austin 7 crankshaft bearing assembly has allowance for crankshaft 'bending'.    

Cheers, Tony.

[Bob Culver]

Some other makes with 2 bearing cranks got by with a single bearing at each end, one a ball to locate and one a roller to allow for expansion. The fatigue life of rolling races was hard to predict in automotive applications and the Seven errs on the side of caution. The ratings of modern bearings are much higher than 1920s, hence the meagre bearings in moderns. I am sure two ball races in the front would do fine, at reduced cost.

[steve davidson]

Some other makes with 2 bearing cranks got by with a single bearing at each end, one a ball to locate and one a roller to allow for expansion. The fatigue life of rolling races was hard to predict in automotive applications and the Seven errs on the side of caution. The ratings of modern bearings are much higher than 1920s, hence the meagre  bearings in moderns. I am sure two ball races in the front would do fine, at reduced cost.

Thanks Bob

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Radial, not axial Bob - but basically correct. At least that's the theory, but crankshaft whip among other things clouds the picture slightly. There's quite a bit on alternative bearing configurations in the 750 companion. Two rollers would have little capacity to support end load. A pair of self-aligning ball races can and indeed does replace the ball and roller set up successfully as they are designed to take both axial and radial loads (also a degree of misalignment which a roller race can't handle so well). n.b. It's rather early in the morning so I'll probably realise I've written something stupid later...

Thanks for the reply, now that you mention it I do remember some notes in the companion, I'll have a read and learn some more!