Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
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 axial load. Spring-loading allows
the spindle to float laterally during temperature changes without
appreciably increasing or decreasing the original spring axial
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 Points of Runout
The correct use of the high point of runout etched on the bearing
components allows the accuracy of the spindle to be optimized. The
components should be mounted in the housing and on the spindle
so that the high points are aligned with each other. In other words,
the inner ring is fitted on the spindle so the high point of the rear
ring is aligned with the high point of the nose bearing. Similarly, the
high points of the outer ring are aligned in the housing.
To obtain maximum precision, and when the high points of runout
of both the spindle and the housing are known, the respective high
points of the bearing components should be 180 degrees opposite
to those of the spindle and the housing. This will tend to neutralize
the eccentricity and minimize the effect of the high spots of all
components. The figures to the right show typical examples of the
correct and incorrect use of the high point of runout of bearings.
The greatest accuracy can be provided by grinding the spindle nose
after the bearings are installed. This procedure will produce spindle
runout considerably smaller than the bearing runout.
Correct: high points of runout in line.
Incorrect: high points of runout not in line.
Correct: bearing having largest runout at rear.
High points of runout in line.
Incorrect: bearing having largest runout at rear.
High points of runout not in line.
Incorrect: bearing having largest runout at nose.
High points of runout in line.
Fig. 92. The effect of bearing
runout high point locations
on spindle accuracy.
Incorrect: bearing having largest runout at nose.
High points of runout not in line.
100 TIMKEN MACHINE TOOL CATALOG