Bearing bore tolerances?
Not asking about shaft diameters, just wondering what the manufacturing tolerance is on a bearing bore. Like a bearing that has a 1" ID, would it be say -.0002/-.0001, or do they also have a plus tolerance? as just say +/-.0001? I'm gonna assume the higher end bearings are kept in the few millionths in tolerance. But just curious if they're ever on the higher side of their nominall size, or if the always try to keep it a tiny bit smaller on ID's. I tried google but only ended up with shafts fits, doesn't tell me much.
SKF table viewer
SKF has a really good website including fits and tolerances for fits for bearings for different applications.
The main page:
You can get to the first link by going through products, and clicking a product type in the left hand column, then product data, then tolerances. There is tons of other stuff there too.
Hmm, don't have 'Machinerys Hand Book' in front of me but IIRC it's a combination of both 'fit' desired AND shaft diameter? IE a perss-fit on a 1" shaft will be a lot different than say a 12" shaft?
One thing the bearing people have lots of is tables. Self contained ball and roller bearings are made accurate enough to serve as gages for most of us. I've been known to grab a handy bearing to check the mike I was using at the moment.
Why the assymetrical tolerances? Who knows? It's been well docemented by the QA people that there is a tendancy for good machinists to machine stuff just inside the tolerance. The bearing people are well aware of this tendency. When dealing with parts quantities that comprize statistical universes it's only sensible to acknowledge that fact and set tolerances to optimise the internal clearance of the ball bearing.
I've done it myself. Back when I was grinding noise quiet motor journals I had the bearing to be used for each journal on hand fresh from the anderometer shop with a paper stating the as-found internal clearance. If the bearing was snug we were to grind the journal deeper into the minus tolerance. Sounds persnickety but its one of the details that made for motors that passed their noise signatures every time.
as an example the timken bearing selection guide shows: stationary housing fits for 1.125" housing bore (S5K ball bearing) is 1.1250/1.1255. a rotating housing 1.1241/1.1246.
guess it all depends on what the bearing conditions are, hopefully this is a starting point.
I prefer the NTN technical documents for ease of use.
Standards & tolerances for ball & roller bearings are here
Rest of the technical data here
NTN Corporation Products and Technology Catalog PDF Download
High end bearings for severe service have tapered bores and clearance is set in the field with a feeler gauge. This method is much easier than getting a tight tolerance on the fit. In real big bearings it is hard to believe but the housing bore for severe service is .002-.007 loose. Talk to your bearing suplier and get a timken products cataloge they are free and contain every thing you need to know.
A lot of imported bearings are way off from there stated size and i have seen them so small that they wont fit on cold rolled. Have to flap wheel them to get them on.
Last edited by Heavey Metal; 07-18-2011 at 01:41 AM.
Please forgive the hijack but Forrest, this being a possible alternate for the OP as well,...... in your experience, is it ever preferable to fit the bushing to the ground journal? And if so, what is the preferred method, reaming, single point boring etc?
Originally Posted by Forrest Addy
I'm faced with that just now and am loath to mess with the "perfect" journals, when my bushings, (1-5/16" OD) fit so nicely in my spindle mounted TG150 collet chuck, to improve concentricity over "as sintered". I was dismayed to find ovality and barrel deviation in the 15/16" bore of new domestic bushings.
These for high speed "floating bearings", such as are found in turbo chargers, operating dependably @ well over 100,000RPM. My use will be less.
I have a new 1" domestic, quality reamer that can be "dulled" (guidence solicited here as well) for bronze and several extra bushings to experiment with. Lot's cheaper than new shafts and housings.
Much thanks, Bob
I find that I have to measure taper roller bearing races: they can be as much as .001" oversize on the nominal dimension for a shaft fit. There might be a logical reason for them being made this way, since the housing fits are usually a thousandth or more interference. This means the assembly guy needs an easier fit on the shaft to get the thing assembled.
Ball bearings, pretty well right on size or a couple of tenths tight.
That would be true of spherical roller bearings but I have never seen any other type with that mounting. They are supplied with a chart to specify the amount of clearance reduction.
Originally Posted by Heavey Metal