Bearing Installation — Fitting Practice
One of the objectives of proper bearing installation is to achieve radial retention of the bearing rings on their respective seats. Neither ring should be allowed to rotate on its seating surface (shaft for the inner, housing bore for the outer). such movement will allow the ring(s) to wear away the seating surface, making subsequent installations more susceptible to this abrasion. For this reason, it is very important to prevent loss of radial retention by proper selection of shaft and housing bore dimensions and tolerances. The recommended method for achieving radial retention is to create an interference, or press fit, between the bearing ring and its seat.By manufacturing the shaft diameter slightly larger than the bearing bore, the inner ring is expanded during installation and thus "grips" the shaft. Likewise, when the housing bore is machined slightly smaller than the outside diameter of the outer ring, a press fit is also accomplished. The pressed ring grips the seat with enough force to overcome rotational torque during bearing operation.
To Facilitate ease of installation and ensure adequate residual radial internal clearance, it is desirable to press fit only the ring most susceptible to rotation in its seat. Bearing load direction and magnitude determine which ring to be press fitted and how much interference is necessary to achieve retention. The chart in the Appendix shows that the general rule is to press fit the ring that rotates in relation to the load direction. Also
BSA Educational Services Committee
Peter Fitzpatrick, Chairman DeWayne Allmon John M. Armolea William Audette Kevin Boyle Thomas A. Brown William M. Dix James Fitzpatrick, Jr. Jeff Ford E. Joseph Fults Craig A. Gipson Bruce Hill Peter Horn Juston Jacobi Michael L. Ketchel James Little Scott A. MacPherson John Masek Charles Nicholson Jeffrey Ramras Douglas Savage Peter Torras R. Scott Waddell Tom Wojcik David Zoesch
The ESC Report is a program of the Educational Services Committee intended to keep members informed about topics affecting the bearings industry. Comments may be sent to the committee c/o BSA, 800 Roosevelt Road, Building C, Suite 20, Glen Ellyn, IL 601375833.
Copyright 2000 by BSA
in the Appendix, the degree of interference is shown based on the type of application, and the ratio of the bearing load to the bearing dynamic capacity. Heavier loads require a greater interference since the rotational torque inside the bearing tends to break the ring free to rotate. Note that the chart in the Appendix recommends a "loose fit" for rings, which must move axially on their seats to allow thermal expansion of the seat without inducing a thrust load on the learing. These rings are allowed to "float" on their seats and hence must not be installed with a press fit.
ISO Coding System
The International Standards Organization (ISO), has established standards for both machining quality (tolerances), and degree of interference/clearance between ring and seat. This allows manufacturers worldwide to communicate fitting practice without confusion. The ISO code consists of a letter (degree of interference/clearance), followed by a number defining the machining quality (amount of tolerance). To prevent confusion, the lower case letters are used to define an outside diameter (shaft OD) and conversely the upper case letters define an inside diameter (housing bore) dimension. The degree of interference furthermore increases as the letters get further down the alphabet (i.e.,”m” is a greater press fit than “j”).The number that follows the letter is a code that defines the quality of machining, or the acceptable spread of diameter tolerances. The smaller the number, the smaller the tolerance. A common fitting practice in use for many bearing applications is j6/H7.
Three different conditions can be achieved by comparing the...