Friday, 28 November 2008

Miniature, corrosion resistant & motor bearings and re-lubrication of bearings

Bearing Technical Talk... Radial Play

Radial play, or internal clearance, is an important consideration when choosing a bearing. Quite simply, there must be at least enough play in a bearing to allow rotatation without too much resistance after fitting or it will fail quickly. It is normally preferable to only have a small clearance left once the bearing has been fitted to minimize ball skidding and reduce axial play (end play). Selecting the correct radial play can avoid faster wear and reduce unwanted play. Radial play before fitting is called "initial" radial play. "Residual" or "operational" radial play is what exists after fitting.

So what might change the radial play in the bearing? A shaft that is slightly larger than the bearing inner ring (an interference fit or press fit) will stretch the inner ring reducing radial play. The same thing happens if the outer ring is a tight fit in the housing. This can compress the outer ring so a bearing with a greater radial play may be needed. High temperature can also be a problem where the shaft and housing temperatures differ. If a bearing inner ring gets hotter than the outer ring, it will expand more and reduce radial play. If the outer ring gets hotter, radial play can increase. These problems can also occur where inner and outer ring are the same temperature but made of materials with different expansion coefficients.

In most cases, a standard radial play is suitable and, of course, easier to obtain but there are certain conditions where a non-standard clearance is preferred. A tight radial play gives greater rigidity and running accuracy which is important in low noise, low vibration applications. This is why many of our small electric motor bearings are MC3 radial play. In fact, to further eliminate vibration and noise, a small axial preload is often applied to reduce the radial play to zero. Elsewhere, a loose radial play is desirable. For example, a loose radial play should be considered for high axial loads as it increases the bearing's axial load capacity. Also, a loose radial play will better accommodate misalignment between the shaft and housing and cope better with heavy loads or shock loads.

Finally, it is sometimes thought that when there is too much play, a higher precision grade will solve the problem. Actually, the answer is often to use a bearing with a tighter radial play or introduce an axial preload to the bearing to eliminate the play. When a bearing is manufactured, the balls and rings are carefully matched to produce the desired radial play so a higher precision grade will make no difference to the "looseness" of the bearing. Its just as possible for a high precision bearing to have a large radial play as it is for a standard grade bearing to have a tight radial play. Remember, loose does not necessarily mean poor quality!!
For more information, see the "Technical" page on our website and scroll down to the Radial Play section.

We recently added material specification tables in the Technical section of our website. These give details of the different grades of chrome and stainless steels used in the rings, balls, cages and shields in our bearings. For more details, go to the web page at: www.smbbearings/materialtables.htm
We now have stocks of plastic (acetal resin) bearings with glass balls to complement our existing stock of plastic bearings with 316 stainless steel balls.

Miniature & instrument bearings Stainless steel and plastic corrosion resistant bearingsLow noise electric motor bearings Cleaning and re-lubrication of bearings.

See the range and lots more online at www.smbbearings.comor telephone sales on 01993 842555 or fax us on 01993 842666
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