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06-13-2009, 08:35 AM #1
Bronze bearing running clearances
I'm working on my Do-All saw rebuild this weekend and I've hit a snag. I'm looking for advice on how to re-fit a bronze bearing.
The bearing in question is pressed into the output shaft of the transmission, which carries the band wheel and blade tension load. The bearing stabilizes the output shaft, so that it will not rack the main output bearing. It does this by accepting a portion of the input shaft, which rotates in the opposite direction. Below is a photo of everything laid out:
The existing bearing is bell-mouthed and rather worn. At the widest point the bore is around .025" larger than the shaft. I'm going to clean up and re-polish the input shaft, remove the existing bronze and press in a new one, then bore to fit.
My question is: What clearance should I leave between the bearing and the shaft? The bushing is not directly lubricated, nor bathed in oil. It's sole means of lubrication is via a passageway drilled though the input shaft gear and down the center of said shaft. Oil picked up by the reduction gear "falls though" the input gear into the concentric passage and then into the bushing. Not much lubrication at all, I'd suspect!
The speed differential is probably around 800 RPM, lubricant is 80W gear oil. Bearing bore is 5/8"
I'm thinking a bore .005" to .007" over the journal would be in the neighborhood. I don't want anything too tight or it might seize up with the temperature rise bound to occur with light lubrication.
Any thoughts on this?
06-13-2009, 08:53 AM #2
IMHO, if you bore it for the clearances you have posted, it will end up just as sloppy at it is now. I would use Oilite for the bushing material, bore it .001 over and put a few oil channels in the bushing. Be sure to drill through the new bushing so it has a chance to get any gear lube from the gear box.
06-13-2009, 09:30 AM #3
Yes Oilite could be the best and clearance in the 0.001" to 0.002" region.
06-13-2009, 09:34 AM #4
.0015 at the most for a bearing that small. Agree with the oilite. If you can find one that fits, it will last forever at 800rpm. The old plain bearing skilsaws had oilite 1/2-5/8" oilite bearings running up around 4500rpm.
06-13-2009, 10:38 AM #5
Eight thou is way too large a diametral clearance for a bushing like that. One or two
thou on the diameter would be good as suggested.
Another possibility, would be to purchase an INA needle bearing and replace the
bronze bush with that. Almost looks like there would be enough clearance. Check
out the needle bearing section in the MSC catalog to get the dimensions of
06-13-2009, 10:49 AM #6
Unless the shaft end is 60Rc+, having it run in a needle bearing might be problematic.
I personally wouldn't run any more than .001 clearance on the bronze bushing.
06-13-2009, 11:23 AM #7
Sounds like I was going down the right path before I got worried about tight clearances. I'll follow Machinery's handbook then, .001 to .0015" clearance on the new bronze. I was afraid without pressure lubrication that clearance might be too tight, but then I got to thinking about babbit bearings and wick lubrication... Brain fart.
I know the bearing house stocks oil-impregnated bronze bushings that machine readily, I've used them before. That's probably the route I'll go. I'm not sure I can get an Oillite to fit, especially after cleaning up the shaft stub, which is slightly worn too.
I'm going to increase the oil level in the gearbox at least an inch, so I should get better oiling results.
Should have the transmission back together and another Do-All rebuild thread update by next weekend.