MAINTENANCE • MACHINE TOOL SPINDLES
housing shoulder, locknut J and front slinger C are removed. Next, outer-ring cap K and gasket L are assembled and secured in place, this cap being locked evenly, parallel, and firmly against the outer-ring face of the outside bearing. Front slinger C and locknut J are then replaced, temporarily securing the bearings with a spanner wrench.
Thus, the average mounted resultant fit is .0002 inch loose. If possible these clearances should be obtained, since it produces the ideal fit of the bearing in the housing bore.
At the drive-end of the spindle where the floating bearing is located, it is highly desirable to strive for an average fit of about 0.0001 inch looser than for the wheel-end ball bearings. For the drive-end, therefore, this ideal housing fit is 0.0003 inch loose. In operation, the bearing is itself a heat source and the housing functions as a heat sink and radiator. The equilibrium housing temperature is at some level between room temperature and the bearing. Obviously, the bearing will expand at a higher rate than the housing bore. For example: if we assume that the temperature gradient between the bearing and the housing is 10 F, the bearing will expand in diameter approximately 0.0002 inch more than the housing.
After the shaft is in the housing and assembled with the front pair of bearings, thrust collar M is mounted on the shaft from the drive end, as illustrated in Figure 1. This collar is placed with its wide face outward in order to support the preloaded springs. The springs, in turn, are inserted in cage unit N, which is then mounted in position on the shaft. Spring washer O with its greased packed cavity outward, is placed against the springs.
With the burnished spots in line, ball bearings P and Q are similarly assembled in tandem but with the thrust faces of the outer rings inward and toward the wheel-end of the shaft. Again, the bearings are mounted in such a way that the high points of eccentricity of the shaft seat and of the bearing inner rings are diametrically opposed. Assembly of these bearings is shown in Figure 5.
In the case of superprecision, preloaded pairs of ball bearings for high-speed wheelheads, the ideal shaft fit is a line-to-line fit to one with a looseness of 0.0001 inch. This mounted fit is attained if the bearing is assembled on the shaft under a firm thumb pressure applied to the face of the inner ring. Likewise, a good fit is indicated when the bearing can be assembled by applying gentle tapping blows on the face of a suitable mounting sleeve held against the bearing inner ring face. For these wheelhead bearings it is generally recom- mended that the shaft-seat diameter be the same as the bearing bore. The ideal mounting fit may be obtained by cross-matching the shafts and the bearings. The bearing bore is matched with the shaft seat that will result in a mounted fit of 0.0001 inch loose to one of line-to-line. Where single superprecision ball bearings are employed in high-speed wheelheads, and preloading is accomplished by the applica- tion of spring pressure, it is good practice to use the shaft- fitting procedure.
FIGURE 4 – SUB-ASSEMBLY
Bearing and Shaft Assembled with Eccentricities Opposed
With the asteric markings (*) – denoting the high points of eccentricity – on the faces of the inner and outer rings of the bearings positioned in line, the front pair of ball bearings E and F, Figure 1, with their outer ring thrust faces toward the drive end of the shaft, are mounted in tandem on the shaft G. Inner-ring spacer A and outer ring spacer B are placed between them. The mounting is done in such a way that the location of the high points of eccentricity of the shaft seat and of the bearing inner rings are diametrically opposed. This practice is intended to minimize, and not compound the effects of eccentricity of the shaft and inner rings. This sub- assembly is placed in the housing H, as shown in Figure 4.
The ball bearings are then properly seated against the shaft seat shoulder by placing front slinger C against the bearing inner-ring face, and locknut J, having a right-hand thread, is tightened with a spanner wrench. With the outer ring face of the inboard bearing seated against the internal
FIGURE 5 – MOUNTING DRIVE-END BEARINGS