Spring Loaded Mountings
For high speed applications, radial and axial rigidity and smooth spindle performance may be obtained by spring loading the ball bearings with a predetermined thrust load. Spring loading allows the spindle to float laterally during temperature changes without appreciably increasing or decreasing the original spring thrust load.
As the inner ring heats up during operation it expands radially. This radial expansion applies an increasing load through the ball and outer ring and finally to the preload springs. The preload springs deflect slightly to compensate for the loads due to thermal expansion and maintain a consistent load on the spindle system.
In some applications, single, spring-loaded bearings are employed at the front and rear locations, mounted in back-to-back arrangement. Other mountings, similarly spring loaded, have a pair of bearings installed in tandem at each end of the spindle in back-to-back arrangement (DT-DB). In either case, the spring pressure is applied to the pulley-end or rear bearing position, placing the shaft in tension between the two bearing locations.
High Contact Angle • High Axial Rigidity
Moderate Radial Rigidity
Low Contact Angle • High Radial Rigidity
Moderate Axial Rigidity
Preloading of precision ball bearings to a predetermined thrust load for universal mounting is accomplished by grinding off a certain amount of stock off faces of the inner and outer rings so that before mounting the bearing on faces on the abutting side are offset an amount equal to the deflection under “preload”. When mounted, these faces are clamped together, the opposite bearing faces become flush and the bearing parts are subjected to compressive forces, bringing the balls into contact with their respective raceways, to take up the initial clearances of the bearings. Thus, the preload built into the bearings is automatically obtained. The condition of a preloaded ball bearing is similar to that of one in operation under thrust load. This initial thrust load serves to decrease markedly the axial and radial deflections when subsequent operational loads are imposed on the bearing assembly.
Bearings are preloaded no more than necessary. Excessive preload adds little to the rigidity of the spindle but appreciably reduces the range of operating speeds by causing bearings to run hot at higher speeds. To meet conditions of speed, mounting arrangement and maximum rigidity consistent with low operating temperatures, Fafnir precision ball bearings are designed and produced with preloads varying from heavy to zero and, in some in- stances, with negative preload.
In many cases, the amount of bearing preload is a trade-off between having the desired degree of rigidity and reducing any adverse effect preloading has on the equip- ment. If the operating speed is high, a heavy preload can lead to excessively high operating temperatures, resulting in early bearing failure. For these reasons, three classes of ball bearing preloads are used – Light, Medium and Heavy.
In certain applications, such as high-speed motorized router spindles, specially preloaded, superprecision ball bearings are required. Such bearings are “zero” preloaded – that is, the faces of the inner and outer rings are ground flush under negligible load.
The Light, Medium and Heavy standard preload values for Fafnir superprecision angular-contact ball bearings and for both high and low contact angles in “Physical Character- istics”.
Axial deflection curves of various preload conditions for duplex pairs of 2MM209WI superprecision ball bearings are shown in figure 10 and the radial deflection curves for the same bearings are shown in Figure 11.
Effect of Preload on Axial Deflection
Axial Deflection in Inches
B C D
Thrust Load in Lbs. A No Preload B Light Preload 40 lbs. C Medium Preload 125 lbs. D Heavy Preload 250 lbs.
Figure 10 – Axial Deflection Curves
Effect of Preload on Radial Deflection
Radial Deflection in Inches
.0006 .0005 .0004 .0003 .0002 .0001
A B C
Radial Load in Lbs. A No Preload B Light Preload 40 lbs. C Medium Preload 125 lbs. D Heavy Preload 250 lbs.
Figure 11 – Radial Deflection Curves