Jim Bryan
EASA Technical Support Specialist

Much has been said and much work performed to produce the “perfect” bearing fit. For any single bearing, there is an inner fit to the shaft and an outer fit to the housing. It is required that one of the two fits be able to slide in order to assemble the machine. If the bearing-to-shaft fit (journal) is tight, then the bearing-to-housing (bore) must be loose. Of course tight and loose are relative terms and the quest for the perfect fit must define these terms.

A tight fit, also known as an inter­ference fit, is usually recommended for a motor bearing journal. The range for radial ball bearing journal fits is from j5 to m5, and the housing fit is H6 (see Table 1). These are the “standard” fits and may be different depending on the machine design­ers understanding of the application. Table 1 is derived from Table 2-13 of ANSI/EASA AR100 Recommended Prac­tice for the Repair of Rotating Electrical Apparatus. It shows the relationship of bearing size to fit tol­erances. Generally, as the bearing gets larger, the tolerance widens. The key to this chart is that the journal fit is always interference and the bore fit is always line-to-line to loose. See AR100 for additional radial ball and roller bearing sizes.

Extreme care when measuring
It should also be noted that the “interference fits” and “loose fits” are expressed in tenths, a unit equal to one ten thousandth of an inch (0.0001” or 2.54 µm). Extreme care must be used when measur­ing to this precision. A recent study involving 16 machinists in 9 service centers showed that an experienced machinist with calibrated and well-maintained micrometers should have no problem measuring journals within ± 2 or 3 tenths (± 5.1-7.6 µm). Figure 1 illustrates the effect this can have. The acceptable limits for a 6210 bearing journal are 1.9686-1.9690” (50.002-50.013 mm). Our machinist measures 1.9687” (50.004 mm) and we say it is acceptable. However, if our machin­ist is capable of measuring within ± 2 tenths (± 5.1 µm), our confidence factor that our measurement is in tolerance is reduced to 75%. It is important that we do not exceed the tolerance in the other direction as well. If the bearing is too tight on the jour­nal, the bearing will be preload­ed (the internal clearance is re­duced) increas­ing the friction and temperature and leading to pre­mature failure.

Bearing housing bores
The same is true of bearing hous­ing bores. If the fit is too tight, the bearing can be damaged during an aggressive assembly attempt (read: large mallet). If it is too loose, there may not be enough friction between the outer race and the bearing hous­ing bore to prevent movement. The fit might be within tolerance at ambient temperature, but expand and allow the bearing outer race to move at op­erating temperature, especially in the case of aluminum housings.

This study used certified master rings of known dimension to determine the accuracy that was achieved by the measurement. Twenty-five percent of the measurements taken were between 10 and 20 tenths [0.0010 and 0.0020” (25 µm and 51 µm)] from the certified measurement. Calibration intervals and machinist’s standards were often not used properly in these cases. Typical calibration intervals are one year; you should determine what is appropriate for your service center and write a procedure to follow for calibration. It is easy to compare the tool to the standard gage. The best machinists do this each time they use a different tool. Good practice indi­cates checking each tool used at least once each day it is used. Technique is the other part of the equation. This method verifies both the tool and the user.

In the event of a loose bearing fit, the movement allowed may range from small micro motion to the bearing spinning in the housing or on the shaft. The latter usually results from the combination of a loose fit and an increase in the internal friction of the bearing. 
This increase in internal friction may have several causes; poor or degraded lubrication, bearing race damage and excessive pre­load are a few. If this type of damage occurs, eventually the bearing will spin even if the fit was originally in tolerance.

Effects of small motions 
Micro motion occurs when a vari­able load is applied and there is room to move. The tolerance band is designed to limit the motion, but the housing bore is loose by design and under the right conditions this motion will occur. Radial load tends to limit this motion. A strong, consistent force will “pin” the bearing race to the bore. The lower this force, the more likely the micro motion will occur; a perfectly aligned, direct-coupled ap­plication theoretically would have no radial load. These small movements cause fretting which will appear as small rusty patches on the bearing inner or outer race or the housing or shaft (see Figure 2). Fretting can occur on either fit under the right conditions.

Conclusion
Bearing fits are critical to the reli­ability of rotating equipment. Appli­cation conditions including the type of driven load, the connection to that load (direct coupled or belted) and the proper bearing for the application are all factors to be considered in achiev­ing the correct fit.
Machinists must use properly cali­brated measuring tools and correct techniques to achieve the accuracy needed to measure these precision tolerances. Not only must the gage be calibrated according to the appro­priate schedule, but also it should be compared to the standard regularly — at least daily.
 If these measures are taken and fretting still is a problem, several anti-fretting compounds are available on the market.  Talk to your bearing vendor.

Categories: Bearings

Tags: Bearings

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