Tom Bishop, P.E. 
EASA Technical Support Specialist
 
Much of what will be discussed in this article applies to all rolling bearings, both ball and roller. Our focus, though, will be on issues that relate mostly to ball bearings used in electric motors. The intent is to address some of the fundamentals of rolling bearing enclosures, internal clearances and fits. We deal with rolling bearings every day, but we don’t always consider some of these fundamentals until there is a bearing related problem. 

Note: Rolling bearings were previously referred to as antifriction bearings. The American Bearing Manufacturers Association (ABMA), formerly the Antifriction Bearing Manufacturers Association (AFBMA), now terms antifriction bearings as “rolling bearings.” 

Bearing enclosure 
The type of bearing enclosure for a rolling element bearing will be open, shielded or sealed. The open enclosure simply means that there is no enclosure for the balls or rollers and cage.

Variations of the shielded enclosure would be either single or double shields. Sealed enclosures may use contact or non-contact seals. Figure 1 illustrates the shielded and sealed bearing enclosures. 

The enclosure of the bearing is not normally changed from the original unless there is an application issue that suggests a different enclosure should be used. For example, if lubricant contamination is an issue, a change to a sealed bearing may increase bearing life by keeping the contaminated lubricant out of the bearing. If the contamination particles are large, a shielded bearing may prevent or reduce the entrance of contaminants into the bearing. 

Protection options 
Another option that would prob­ably perform better than changing to a shielded or sealed bearing would be to add a sealing arrangement to the labyrinth path between the bearing chamber and the exterior of the machine. That will not only protect the bearing, it will also help prevent contamination of the lubricant. 

If the contaminant is water or some other corrosive material, changing to a sealed bearing may extend life, but will not necessarily eliminate contamination. If the contaminate can become a gas, it can be drawn into the sealed bearing when the bearing cools down, i.e., it may condense on the interior of the bearing and cause corrosion of the ball or roller paths. In this example, a contact-type sealed bearing has a slight advantage over the non-contact type sealed bearing. 

Open, shielded or non-contact seal enclosure bearings have the same limiting speeds for operation; thus a change from one type to another should not be a concern as long as the maximum rated speed remains as original. If a motor is to be used with a variable frequency drive (VFD) and operated above its original speed rating, or if the motor is redesigned for increased speed, check with the bearing manufacturer as to the limiting speed for each bearing. Likewise, when changing to a contact seal enclosure bearing, check with the bearing manufacturer for the limiting speed. 

For example, in general, a 3600 rpm rating would limit contact sealed bearing size to either a 306 or a 207 series ball bearing. The non-contact sealed bearing generates less friction than the contact-type sealed bearing. This is especially important when a bearing is operating at or near its maximum speed rating. 

Bearing internal clearance 
Another aspect of rolling element bearings is the internal clearance. Bearing internal clearance is the amount of radial and axial internal clearance a bearing has before being installed on a shaft or in a housing. Prior to T-frame motors, most ball bearings were of “normal” internal clearance. The higher operating temperatures and greater temperature differentials in the T-frames necessi­tated a change to the “C3” internal clearance. The C3 internal clearance has more internal clearance than the normal clearance. Bearing internal clearance designations, from least to greatest internal clearance, are C2, Normal (sometimes termed “0” or “C0”), C3, C4 and C5. 

Although most ball bearings used in motors are C3 internal clearance, some motor manufacturers have gone to C4 and even C5 fits because of differences in temperature between shaft and housing. Inspect bearings carefully for bearing manufacturer notations as to the internal clearance, and if in doubt, check with the motor manufacturer. 

In some cases the motor manufac­turer may specify a bearing with heat stabilized steel, capable of higher than normal (typically about 250° F or 120° C) operating temperatures. That is another reason to check with the motor manufacturer. The higher the operating temperature, and the higher the operating speed, the more likely the bearing may be of greater than normal internal clearance. For example, a 2-pole motor driving a compressor may have a C4 rather than a C3 internal clearance bearing. 

When a motor is operating, the internal clearance of the bearing is typically less than the clearance prior to mounting. The interference mounting of one side of the bearing, usually the shaft fit, reduces the internal clearance. Differences in shaft and housing temperatures in operation can further reduce the internal clearance. As a general rule, the bearing outer race will operate at 5-10º C less than the inner race or other rotating parts of the bearing. If the internal clearance is too large, the bearing may be noisy and will probably have increased temperature and friction; and it may fail prema­turely. In theory, to maximize life, the optimum operating internal clearance for a bearing is a slight negative clearance (interference fit) after the bearing has reached normal operating temperature. The correct internal clearance, along with proper mounting fits, will result in maxi­mized bearing life and reduce friction and the generation of heat. Table 1 provides examples of the internal clearances in various bore sizes of bearings. 

Note how the clearance increases with bearing bore and with the different internal clearance fits. For example, a 208 series bearing, with a 40 mm bore, might have a 6 mi­crometer internal clearance with a normal fit, and a six times greater 36 micrometer clearance with a C3 fit. 

Bearing fits 
The vast majority of motor bearings require an interference fit to the shaft and a clearance fit to the housing. Exceptions to this rule exist, such as for vibrator/shaker motors, which may have interference fits to the housings and clearance fits to the shaft. Whenever dealing with an unusual bearing fit, it is good practice to contact the motor or machine manufacturer for the proper bearing fits. Quite often in these special cases, the bearing internal clearance may be unusual. Using the vibrator/shaker motor example again, these motors often use C4 internal clearance bearings. 

The “ideal” way to select a bearing fit is to obtain it from the machine manufacturer. 

However, in most cases if the bearing fit is that of a standard motor application, Tables 2-14 and 2-15 in ANSI/EASA AR100 Recommended Practice For Repair Of Rotating Electrical Apparatus can be used. These provide the necessary bearing to shaft interference fit, and housing to bearing clearance fit. Note that the “tolerance class” for bearing fits is not constant, but changes for certain diameters. The shaft and the housing fit may both vary by application, e.g., a primarily thrust load. Bearing manufacturers as well as ABMA standard 20 list the various bearing journal and housing fits. Fits are given in alphanumeric notation, using lower case letters for shaft fits (e.g., k5) and uppercase letters for housing fits (e.g., H6). 

Further, the fits for radial ball bearings (Table 2-14) differ from those for cylindrical roller bearings (Table 2-15). What is significant about these changes is that one should not assume that a tolerance class that applies to one size bearing applies to all sizes, even under the same operating conditions and for the same application. 

If the bearing fit required is not in one of the ANSI/EASA AR100 tables, how is the fit determined? As an example, we will determine the acceptable housing dimension range for a 7320 angular contact ball bearing for vertical motor with a manufacturer specified H7 fit. The first step is to determine the outer diameter (OD) of the bearing, which can be done by checking a bearing manufacturer’s catalog. We find that the nominal OD of the 7320 bearing is 215 mm, or 8.4646 inches. The American Bearing Manufacturer’s Association (ABMA) Standard 7 provides shaft and housing fits for metric radial ball and roller bearings. The fit tolerance in ABMA 7 for an H7 fit with a bearing OD of 215 mm is “0” (0.0000”) tight to 18 (0.0018”) loose, with the tolerance values expressed in 0.0001”. Thus the resultant size range for the housing will be 8.4646” (8.4646-0.0000) to 8.4664” (8.4646+0.0018). We have now identified the proper size range for the 7320 bearing housing. 

Since the bearing is manufactured to dimensional tolerances, the OD is probably not exactly 215 mm (8.4646”). The manufacturing tolerance for a 7320 bearing OD is +0.0000/-0.0012”, making the acceptable range of the OD from 8.4646” to 8.4634”. The H7 bearing fit tolerance allows a resultant fit between bearing and housing of “0” tight to “30” loose (expressed in units of 0.0001”). 

This is the result of combining the housing tolerance (0/18) with the bearing tolerance (0/12). The bearing fit ranges allow variation of the bearing and housing or shaft dimen­sions such that the resulting values are within tolerance. For example, even if the bore (ID) of the bearing was at the maximum tolerance limit and the shaft journal was at the minimum, the resulting fit would still be within the acceptable tolerance. 

Bearing fits for roller bearings may, and often do, vary from those of equivalent size ball bearings. A common example is the shaft journal fit of a cylindrical roller bearing. Table 2-15 in ANSI/EASA AR100 provides shaft and housing fits for cylindrical roller bearings. Also, Table 2-14 in the same standard provides radial ball bearing journal and housing fits. (These tables can also be found in the Technical Manual Section 9.5.) 

Let’s compare the shaft fit of a 212 (e.g., 6312) radial ball bearing to an equivalent bore size 212 cylindri­cal roller bearing (e.g., NU212). The 212 ball bearing shaft journal diameter range, from Table 2-14, is 2.3628” maximum to 2.3623” minimum. The 212 cylindrical roller bearing shaft journal diameter range, from Table 2-15, is 2.3634” maxi­mum to 2.3626” minimum. 

Note that the roller bearing journal is, on average, larger than the comparable ball bearing journal dimension, about 0.0005” in this case. This illustrates that when changing from one type of bearing to another, verify the proper shaft and housing fits. The bearing journals or housings, or both, may have to be changed in size. 

A common mechanical modifica­tion associated with the example of a conversion from ball to roller bearing would be to increase radial load capability for a belt drive application. Conversely, another relatively common mechanical modification would be to convert a cylindrical roller bearing to ball bearing for a direct-coupled application.

Categories: Bearings, Ball bearings, Roller bearings

Tags: Bearings Bearing fit Rolling element bearing

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