Bearing spin on shaft

[raisinbran]

Gentlemen,
My background is Electrical Engineering, so excuse me if I don't use the correct terminology.

We use several helical gearboxes at our plant. They are parallel shaft, single reduction units. Each shaft is approx. 6 inches in diameter. Input shaft speed is 1800 RPM. Each shaft is support by tapered roller bearings. Lubrication is spray oil mist, and there is a heat exchanger and filter in the lubrication system piping.

When the boxes are rebuilt (approx. 5 years of service time), there are signs of bearing spin on the shaft but no signs of bearing spin in the housing. The shaft-to-bearing fit has been tightened from m6 to n6 in an effort to resolve the problem. We will not know the results until the box is rebuilt again. In the meanwhile, are there other things that we should be checking?
Thanks,
Raisinbran

 

[FredGarvin]

First thing coming to mind would be to check the alphas of the shaft and the bearing materials. Make sure the bearing isn't growing too much at higher temps.

 

[Heckler]

Were these gear boxes designed by your company or purchased units? If they were designed by your company the RE should have done a tolerance analysis before parts were made. If this was a purchased unit I would contact the manufacture.

Heckler
Sr. Mechanical Engineer
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[Tmoose]

These guys say an m6 shaft should be approximately +0.0005/+0.0015 inch.

http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_286_2s.html

These guys say their bearing bores are -0.0000/+0.0010 inch.

If all the parts WERE in spec it could have gone together with 0.0005 inc clearance, or maybe 0.0005 inch interference.

With the new n6 fit the shaft should be about +.0010/+0.0020, so it might still be line-to-line, if the bearing bores were at maximum (not likely, but....).
I'd probably confirm the new fit. There should have been codes on the bearings indicating actual bore size.