Re: Crushed roller bearing?

Posted by cdmn on September 06, 2004 at 21:04:30 from (66.228.228.31):

In Reply to: Re: Crushed roller bearing? posted by Allan in NE on September 03, 2004 at 08:28:34:

In my former life I used to earn a high salary worrying about this sort of thing. A tapered bearing converts some of the radial load to thrust. So you might as well tighten the bearing up to the thrust load, because it's going to see that load anyway. In the work I did, this was carefully done by grinding shims to fit under bearing caps. We had to take into account how much the gearbox itself would deform under load.
The biggest part of the job, though, is to figure out what the thrust load is. (This is complicated by bevel or helical gears.) Part of this involves some philosophy, part of it may be controlled by industry standards, so you don't need to agonize so much. But, the engineer has to take into account how long the thing should last, how much it will cost, how much it will likely be abused, how carefully the owner will maintain it, and even allow for variations in climate, torque wrench accuracy, and lubrication quality. You can over-design the bearing by a lot and this will usually work out, even though the cost might be a bit higher. Sometimes, you have to presume a failure will happen but in such a way that it will allow a "limp-home". Or in such a way that it signals a problem, by noise or smoke, for examples. Sometimes you rebuild after so many hours, whether it needs it or not.
A bearing fails by surface fatigue, and this is a combination of load and cycles. And it's not linear. For example, a 10% increase in load might reduce life by 50%. So if you preload too much, you are artificially creating a load in the bearing that might be a lot higher than the payload would develop.
So, if you can calculate the load, statistically, and then add in the variability of all the other factors, you can come up with a design you can guarantee will last xxxx hours, 90 per cent of the time. It depends upon what your marketing guys and your warranty department want, and what your bean counters will let you spend.
If you deviate from what you once did, or from what your competitors do, and you have an accident, the insurance company lawyers will be all over you.
Minnesota Testing Machines Co has made millions by designing and building machines for OEM companies to put a spectrum of loads, vibrations, shocks, and rpms on their products (all major car makers, for example). They can put 100,000 miles of wear on a design in a relatively few hours.
Since it has become routine for major companies to do this, the insurance companies will deem you to be a high risk if you don't do it too. [I don't know how this all shakes out if you buy your stuff from overseas. About 20 years ago, someone started importing substandard bolts and screws. They caused so much trouble, that there were suggestions of Capital Punishment.]

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