Power at Work: Machinery parts need a good, tight fit

Monday, March 2, 2009

What constitutes a good bearing fit on a shaft? Zero or negative clearance

by KEITH BERGLIND

In our mechanic's world, thread locker chemicals have a close cousin called retaining compounds. These are similar to the thread lockers. The purpose of the retainer compound is to take up small amounts of space and keep things in place.

With many bearing-on-shaft installations, even a space as small as one-half of a thin piece of paper is too much space. As the shaft turns, the inner race of the bearing will wear away the shaft. So, think about the last time you changed such a bearing. Did you run your thumb nail along the shaft and feel that little groove where the old bearing has worn away the shaft?

Here's my opinion of what constitutes a good bearing fit on a shaft: zero or negative clearance. In other words, the bearing needs to be tight on the shaft. This is necessary to prevent wear on the shaft.

I must point out that many farm equipment bearings are the eccentric locking collar type, designed to lock the bearing in place. These bearings also work well with zero clearance for long life. Even when the collar locks the inner race to the shaft, there is still a risk that excessive clearance will allow the bearing to work loose on the shaft.

There are situations where two parts are pressed together and even then a retaining compound may be needed for extra holding power.

In loose-fit  repairs, it is important to identify how much clearance you are dealing with, because there are separate compounds for different gaps. For example, one compound may be useful up to 0.005 inches (0.13 millimetres) and a different compound for gaps up to 0.015 inches (0.38 millimetres). Obviously, the thumbnail test is not accurate within these close ranges.

When there are open splines on a shaft, it is common to find wear, usually on one side of the splined shaft and in the splined hole, because of the constant one-way load. The two parts wear into each other.

A retaining compound can be used to fill up the wear areas, but it often takes quite an effort to get the old parts seated and all the spaces filled with the compound. Still, it's often worth the effort to reduce the amount of future wear. One common trick is to replace at least one of the parts and then fill the remaining gaps with retaining compound. Again, a judgment call.

In many applications, it pays to research the gap filling and shear strength ratings of the compound available. Pick the right tool for the job at hand.

One tip: Watch for the word ANAEROBIC. This is a class of adhesives and sealants that cures only in the absence of air. BF

Keith Berglind is a licensed heavy-duty mechanic.