Feeler gage helps technician check out internal clearance on a roller bearing.

The first things you must know are what bearing internal clearance is, and what it is not. Internal clearance is the total distance one bearing ring moves relative to the other radially (radial internal clearance) or axially (axial internal clearance). Bearing clearance is categorized as unmounted and mounted clearance. The bearing manufacturer establishes the initial, unmounted bearing internal clearance. It is the clearance in the bearing right out of the box. Mounted bearing clearance is established after the bearing is fitted onto the shaft and into the housing and when the bearing reaches steadystate operating temperature. You can also think of mounted clearance as operational clearance.

Ideally, for optimum bearing life and machine reliability, bearing internal clearance is zero or slightly negative at operating conditions. Initial bearing clearance is chosen during machinery design and bearing selection to reach this goal.

Rolling bearings are precision machine elements that support and position rotating equipment with a minimum of friction, noise, and maintenance. A rolling bearing includes balls or rollers fitted precisely between an inner and outer ring. These components are made of special heat-treatable steels having high fatigue strength. The rolling elements are spaced within the rings by the bearing cage (retained) made of steel, brass, or reinforced glass fiber filled polyamide materials.

In most applications, two bearings support the rotating component in the equipment: one is the fixed or locating bearing; the other, the floating or nonlocating bearing. Bearing size and type are selected on the basis of parameters such as bearing load rating, magnitude and direction of the applied load or loads, rotary speed, stiffness, and precision. In some cases the locating bearing, which supports the applied axial loads, is also selected on its ability to maintain axial movement or endplay of the rotating equipment within given limits.

Figure 1 shows the concept of bearing internal clearance, both radial and axial.

ISO Standard 5753-1981 (ANSI/AFBMA Standard 20) defines limits for unmounted radial ball and roller bearings. The standard defines five ranges of clearance “Classes” (called “Groups” by ANSI/AFBMA) for radial bearings. Bearing manufacturers denote radial internal clearance groups as “Normal,” “greater than Normal,” (C3 suffix), “less than Normal” (C2 suffix), and so forth. Table 1 gives the radial internal clearance ranges for ball bearings. There are similar tables for cylindrical roller bearings and spherical roller bearings.

Don’t confuse bearing internal clearance with bearing precision. Precision is the dimensional and running accuracy for manufacture of the bearing. It does not influence the initial or unmounted bearing internal clearance. The ISO (ANSI/AFBMA) tolerance standardizes bearing precision. Besides the “Normal” tolerance class, some bearings are produced to high precision such as that of ISO P5 (ANSI/AFBMA ABEC 5 or RBEC 5) tolerance.

Mounted bearing clearance is usually smaller than unmounted clearance due to reductions in internal clearance because of the shaft and housing fitting, bearing clamping, and differential thermal expansion of bearing rings. For instance, internal radial clearance of a deep-groove ball bearing mounted on a solid steel shaft with an interference fit of 0.013 mm (0.0005 in.) may be reduced as much as 0.010 mm (0.0004 in.) by the mounting.

Unmounted internal radial clearances in deep-groove ball bearings are smaller than those in roller bearings. For example, for 50-mm-bore bearings, the unmounted internal clearance in microns is:

• Deep-groove ball, 6 to 23.
• Cylindrical roller, 30 to 60.
• Spherical roller, cylindrical bore, 35 to 55.

Roller bearings are produced with greater unmounted clearance to account for the greater interference fits used when shaft mounting these bearings.

The ISO (ANSI/AFBMA) “Normal” radial internal clearances in unmounted bearings have been established to provide suitable operational clearance when mounted with normally recommended fits and operated in the usual range of temperatures.

Operational clearance affects bearing life. The load that the bearing supports passes from one bearing ring, through the rolling elements, to the other ring. The number of rolling elements supporting the load influences bearing life. If the clearance is too great, too few rolling elements support the load, causing high stresses at their contacts with the rings. That brings premature raceway fatigue and short bearing life. Too great clearance in lightly loaded bearings can allow sliding or skidding of the elements on the rings, causing ring and cage damage and high bearing temperature.

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