Crank Bearing Preload

[go_hercules]

I am having trouble visualizing what supplies preload to the crank bearings. The crank bearings are deep groove ball bearings which need some amount of preload. But looking at the parts assembly drawings i see the left and right bearings in their shouldered holes. The right side has a thrust snap ring. Looking at the crankshaft assembly, I see no shims or spacers. So what sets the preload? When the clutch pack is installed, it looks like it cinches up the inner race of the right bearing to the crank. On the left side, it looks like the crankshaft just floats in the inner race. So with everything buttoned up, what keeps the crankshaft from floating left and right? I'm obviously missing something since a deep groove bearing has to have some preload.

[Bullfrog]

I don't know the intricate details of "deep groove" bearings, but hundreds of thousands of hours of engine operation tend to suggest that the Hodaka crankshaft bearing system simply works. The description you gave of the crankshaft/crankcase/bearing assembly is quite accurate . . . and it works. Even with the clutch disengagement system pushing the crank sideways from time to time!

Ed

[go_hercules]

Hey Ed. In my Clymer manual it says that on an assembled crankshaft, the distance from outer crank flywheel surfaces should be 1.57 inches +/- 0.0008. That is a very tight tolerance which made me wonder how accurate the distance between installed crank bearings could be. Just curious. And like you said, disengaging the clutch pushes on the crankshaft, so I could see where on some bikes that could tighten the bearing clearance, and on others loosen it just depending on how much axial play is in the crankshaft to bearing fit when assembled.

[Hydraulic Jack]

Sounds to me like the referenced clearance of 0.008" is between the flywheel magnets and the stator coil ends, which would not change due to left/right float.

Deep groove ball bearings can not be preloaded. Nor can shallow groove ball bearings. Preload on a bearing implies thrust along the axis of the bearing, and ball bearings should never be pushed against the sides of the groove. Their build tolerances alone account for any ability to float left or right, and those tolerances are a lot smaller than 0.008".

The clutch side assembly is torqued against the crank bearing race. The bearing is captured in its pocket by a snap ring, not that it really needs one. There should be a thin shim between right bearing inner race and engine case IIRC. That shim takes up whatever slack might be present in the bearing pocket/snap ring assembly.

Because the crank is captured against the right hand bearing race, I seriously doubt the crank floats left or right, and certainly not 0.008".

[thrownchain]

I'm at a lose here, why would you "preload" ball bearings? Tapered ones I understand, but not ball bearings. Crank bearings have to be shimmed possibly so the flywheels don't drag on the inner cases when the clutch is in use, but other than that you want the bearings to run as free as possible.

[go_hercules]

Radial ball bearings can handle radial loads and small axial loads. A radial ball bearing has radial clearance built in to allow for thermal expansion and also to allow for change in dimension when squeezed into a bore or when the inner race is expanded onto a shaft. The axial clearance is usually about ten times the radial clearance. If you take a brand new bearing, and spin it, it will feel loose (except for the little oil drag). The bearing requires a small axial load to push the inner and outer races apart to take up the axial clearance which in turn gives the design radial clearance. If you have ever taken apart an electric motor you might have noticed a wave washer against the inner race of the shaft bearing. That is a preload washer. Without it, the motor will be noisy as hell as the bearing balls are bouncing and skidding rather than rolling. When Ed mentioned the clutch pushing on the crank, that really got me wondering what happens to that right bearing since it is being loaded and unloaded. Here is a link to NMB Bearings where they describe ball bearing preload:

http://www.nmbtc.com/bearings/engineering/preload/

I think on most bike engines, the target side to side play is usually around zero to a few thousandths CLEARANCE. Once the engine is warm and expanded, that clearance should actually tighten and preload the bearings slightly. So if a Hodaka engine has too much side to side play when cold, there certainly will be too little preload when warm. Now shove it even more with the clutch and it rattles. For grins, picture the counterbore that the right bearing fits into. Now say it is machined too deep. Put the crank in and it can move left to right. That is what I am trying to describe, and why I was questioning whether there were ever specs for that end play.

[thrownchain]

I think you may be over thinking the whole idea. Just my opinion.

[MWL]

I put new crank bearings in an 03 125 and noticed a noise when the clutch was pulled in. It wasn't bad, just sort of a whirling sound and I wrote it off as typical clutch noise. As I rode a few Ahrma CC events, the noise increased to a point where it was making a squealing sound when the clutch was pulled in. I put a dial indicator on the ignition side of the crank and pulled the clutch in. I was surprised how much play there was. I have since replaced the bearings but not run it yet.
Mike

[Bill2001]

Agreed. The clutch-side main bearing is held in a precise axial location. The magneto-side bearing, although it has a nice press fit in the case on installation, once that aluminum case heats up and the engine starts vibrating, that bearing is going to ease in or out where it is happiest.

[go_hercules]

Mike, that sounds like a good test using an indicator on the magneto. I think I will try that just out of curiosity.