Spindle angular contact bearings are run with a deliberately negative internal clearance, so all balls stay in contact with both raceways at rest and the spindle becomes a stiff system with low runout [S5].
The preload value is not a free parameter; for a given bearing code it is fixed at the factory by how much the inner and outer races are offset along the contact angle, and most high speed machine tool spindles use the light (L) or extra light (EL) preload class [S4][S6].
Preload force: where the number comes from
Angular contact bearings must be axially loaded in operation because their design geometry only carries thrust in one direction per bearing, and that thrust is carried through the contact line that runs at the contact angle (commonly 15, 25, 30, or 40 degrees) to the radial plane [S3]. Applying preload to a back-to-back pair (DB) of such bearings pulls the inner rings toward each other, eliminates the residual clearance delta-a0 between them, and locks in a preload force Fa0 whose magnitude is determined by the factory race offset [S5].
SKF frames the magnitude as a multiple of the basic static load rating C0: a typical operating preload for machine tool spindles falls in the range k = 0.005 to 0.013 of C0, rising to k = 0.02 when the preload is mainly there to protect the bearing from standstill vibration damage [S2]. For high speed precision spindles, NSK notes that a preloaded bearing shows roughly one-third to one-half the axial displacement of a non-preloaded bearing under the same axial load, which is a 2x to 3x rigidity multiplier that justifies the loss of clearance [S5].
Position preload vs constant pressure preload
Two preload methods are in production use on spindles, and they trade rigidity for speed and tolerance to thermal growth [S4][S5].
Position preload (rigid preload) is set by a fixed axial distance, normally via ground inner and outer spacers or by a precisely torqued locknut, and it gives the higher rigidity of the two methods because the bearing stack is geometrically fixed [S4][S5]. Constant pressure preload, by contrast, uses a calibrated coil or wave spring to push the bearing pair together; the spring deflection grows with thermal expansion of the shaft, which keeps the preload roughly constant as the spindle heats up, and it is therefore the method of choice for the highest speed spindles and for horizontal-shaft thrust arrangements [S5].
What the correct amount of preload looks like in service
Too little preload and the spindle shows up as chatter, poor surface finish, and loss of axial location; too much and the bearings run hot, grease fails early, and fatigue life collapses well below the calculated rating [S4]. The preloaded bearing arrangement stays in the safe zone as long as the operating point stays below the preload loss threshold, meaning the external axial load Fa has not grown so large that it unloads the opposite bearing and its axial displacement delta-aB drops to zero [S5].
Field installation guidance converges on the same answer from two directions: keep the factory specified preload as the baseline, then control the stack with ground spacers, matched inner/outer spacer sets, and a locknut tightened to the bearing maker's torque rather than to a "feel" reading, while monitoring that the indicator on the spindle nose reads near zero axial and radial play with a light hand load [S1][S4].
Bearing arrangement that sets the preload class
Preload class and bearing arrangement are coupled: a single row angular contact bearing can only carry axial load one way, so the spindle stack is built from matched pairs and the back-to-back (DB) arrangement is the dominant one for precision spindles because the long distance between effective load centers raises moment stiffness and resists tilting [S3][S4]. Face-to-face (DF) is reserved for cases where some misalignment tolerance is worth more than moment stiffness, and tandem pairs inside a DB (called DBD or even DBDD) are used when the spindle needs extra radial and axial capacity without changing the preload class [S3][S4].
The choice of arrangement has a direct effect on the right angular contact bearing reference for a given spindle, and the same design vocabulary applies whether the spindle is a high speed ATC unit or a slow surface grinder spindle: preload is a class, not a number you turn on the bench [S3][S4].
Preload is not a setting, it is a specification
For an angular contact ball bearing the preload is a property of the bearing code, expressed in the suffix (L, M, H for light, medium, heavy in many maker systems), and the bearing is supplied as a matched pair so the two halves already have a defined delta-a0 before they ever meet a shaft [S4][S6]. In a ball bearing stack on a spindle the user does not "set preload" by feel; the user fits the ground spacers, follows the locknut torque, and verifies with a low preload drag torque on the spindle nose [S1][S4].
When higher speed, higher temperature, or higher moment load is the target, the design lever is to switch preload method, change to a higher contact angle (toward 40 degrees), or move to a hybrid ceramic bearing pair, while keeping the preload class inside the L/M envelope the maker publishes for that boundary dimension [S3][S5].
Common failure modes tied to the wrong preload
Most spindle bearing failures that are blamed on the bearing are really preload events: thermal growth that walks the stack out of the design point, centrifugal force at speed that reduces effective preload on a DB pair, spacer mismatch that re-introduces clearance, and over-tightened locknuts that crush the race instead of clamping the spacer [S4]. Even the grease choice has to be made against the preload class, because heavy preload raises operating temperature and pushes the lubricant toward its high temperature limit, which is one reason precision grinder and high speed CNC spindles often end up on the same light/extra-light preload envelope [S4][S5].
For a deeper view of how the steel and cleanliness grade of the rings and balls interact with that preload envelope, see the reference comparison of 52100 vs M50 vs 440C bearing steel cleanliness grades and 2026 sourcing, which feeds directly into the fatigue-life side of any preload decision. Two practical signals to track on a freshly reloaded spindle are spindle-nose indicator reading under hand load and spindle cold-to-warm temperature rise during a no-cut warm-up cycle, since both respond to preload class and method within minutes of running.