Clearance Setting for a Tapered Roller Bearing

Premature destruction would be more like it! I'll just say that I have found, like you, that you usually can't run standard taper rollers at speed without some clearance - the sort of value that comes to mind is .0005" to .001" - sometimes more. Ideally, check axial float with a dial gage. I don't think its possible to give a blanket value for this - it depends on the size of the bearings, how much things expand radially versus axially, whether you suddenly go to maximum speed, the exact configuration of the design etc. I would say that you can get away with less clearance the further apart the bearings are - assuming you have two bearings. There used to be a theory that Timken had that if the intersection point of the cones of the bearings was in the same place on the shaft center line, then you wouldn't have expansion problems, but that's clearly not true because the temperature is never unifornm throughout, especially under transient conditions. You could try your local Timken rep for advice. I have a feeling that Diamondjim will answer this post - he apparently is an ex-Timken man so will probably have some valuable suggestions.

One other thing - if you or anyone else really want to get into this sort of thing in detail, and not just rely on bearing manufacturers (who have thier own proprietary programs), there is a computer program known as SHABERTH which allows you to study the thermal expansion effects etc. But it's very old, mainframe or DOS based, and very hard to use, so I wouldn't recommend it. However, under a government contract, Poplawski Associates have recently been developing a more up to date computer program (see which works in conjunction with ANSYS, and solves thermal expansion problems, thermal runaway etc.

EnglishMuffin, you are correct about a Timken Rep giving the numbers. Up until now, that's what we would do, just call one of those guys. But it seems a hassle to try to get ahold of one. We're engineers, we should be able to figure this type of stuff out with no problem. All they usually ask is bearing size and RPM. There's got to be an easy way to calculate. I checked my college text book and it says that tapered roller bearings should have no endplay because they need a thrust load to function. That's true, but again you run into a problem when you get some real-world heat build up.


Oh, the mechanics do use a dial gage to measure the gap.

MEnut:
Actually, in some cases, if you have a very flexible housing, I think you can get away with zero or negative clearance - that's why I don't think there is a simple technically correct answer to this question. It can be calculated - but requires a coupled thermal - mechanical FE analysis. Even then - the heat generation rate is a bit of a nebulous thing to predict theoretically, especially with grease lube. I'm skeptical that even Timken would do this, at least for free within a short time frame. So we are probably talking about a rule of thumb here.

MEnut:
The end play or pre-load of the tapered roller bearing depends on the application. Though there are thumb rules, but when one needs an exact answer it is better to contact the manufacturers. The setting of the bearing to end play or pre-load can change during the operation due to thermal effects. If you want to find out how much should be the clearance, then you need to determine the losses due to thermal effects and set the bearing to so much clearance that during operation the bearing will reach a line to line contact stage, i.e. the rollers are touching the outer race. Note that if it is a two row bearing then losses due to tight fits also needs to be considered. We should also look in the load zone, operating setting and heat generated when selecting a suitable end play or pre-load.

Sometimes for machine tools you set the clearance based on heat generation, i.e. if it gets too hot you loosen it off a bit.

Obviously you have to know the correct target temp to start with!

Lester Milton
NBC Group Ltd, Telford, Shropshire, UK