The method suits precision spindles, ballscrew supports, metrology equipment, and robotic joints where rigidity, repeatability, friction, and temperature must be balanced rather than optimized by fastener torque alone [S1][S5].
What Shim Adjustment Controls
Angular contact bearings are commonly assembled in opposed pairs with adjustable internal clearance [S1]. Preload is the negative internal clearance condition created when the rings and rolling elements are elastically deformed under internal stress [S1].
A precision shim changes the effective axial distance between bearing rings. SPIROL describes preload adjustment by displacing the gaps with a shim of the required thickness during assembly [S6]. This makes the shim a dimensional control for bearing spacing rather than a general-purpose friction component.
The adjustment sequence moves from positive clearance through zero clearance and then into light preload as additional axial displacement is imposed [S5]. A single-row angular contact bearing cannot be meaningfully preloaded by itself; a second bearing must locate the shaft in the opposite direction [S5].
DB, DF, and Shim Location
Back-to-back, or DB, mounting provides a long distance between effective load centers, high overall rigidity, and strong resistance to tilting or overturning moments [S1][S3]. Face-to-face, or DF, mounting has a shorter load-center distance, lower moment-load capacity, and greater permissible misalignment than DB [S3].
The preload method follows the mounting geometry. DB and DF pairs can be preloaded by introducing controlled axial displacement between the bearing rings [S5]. For a DB pair, the back faces of the inner rings are pressed together; for a DF pair, the front faces are pressed together [S5].
The angular contact bearing designation identifies the functional family, but it does not establish the permissible shim thickness. Bearing dimensions, contact angle, internal clearance, and matched-set tolerances determine the usable adjustment range.
Comparison of Preload Adjustment Methods
Shim adjustment, direct nut tightening, and constant-pressure preload serve different control requirements. Shim adjustment provides repeatable axial spacing, direct tightening provides a compact assembly, and constant-pressure preload suits applications requiring stable axial support at high speed [S1].
Position preload produces greater rigidity, while constant-pressure preload is better suited to high-speed rotation, axial-vibration control, and thrust bearings on horizontal shafts [S1].
Shim Thickness and Measurement Control
Fine adjustment requires shim thickness control, clean seating faces, and a defined reference dimension.
The target dimension must be derived from the installed bearing stack, not from the shim thickness alone. Account for the axial width of both bearing inner rings, the adjacent spacer or shoulder, the target interference, shim parallelism, and the tightening sequence specified for the bearing set [S2][S6].
The measurement should be repeatable at the same radial location and loading direction. For a spindle or ballscrew support, check axial displacement and runout as the assembly is moved through its range, then verify that the indicator does not exceed the functional target established for the machine [S1][S5].
Shims must sit flat and remain stable during tightening. Edge-bonded construction can hold layers together and reduce piece movement, while an unbonded stack introduces an additional variable if the layers shift or fret [S6]. Parallelism and total compressed stack thickness should be checked at multiple points.
Assembly Sequence and Failure Modes
Excessive preload overloads the bearing, raises friction, and negatively affects bearing life [S8]. Over-constraining the rings also creates excessive friction and heat buildup [S3]. Light preload is therefore a controlled condition near the transition from zero clearance, not the maximum force the housing can generate.
Start with the shim stack installed and the bearing rings correctly oriented. Tighten the clamp progressively so the rings seat evenly against the spacers and shoulders, while preserving the designed axial displacement [S2][S5]. Do not convert an unspecified clamp torque into a preload assumption without the bearing maker’s assembly instruction or a validated force-displacement procedure [S2][S4].
Adding axial displacement can also cause preload growth as operating speed and temperature rise. The usable preload therefore needs confirmation under representative running conditions, especially in high-speed spindles where heat can reduce lubricant viscosity and increase the internal load [S5].
A failed preload check can present as excessive starting torque, rapid temperature rise, low-speed control oscillation, or reduced bearing life [S5][S8]. The corrective action is not simply to tighten farther. First verify ring orientation, spacer dimensions, shim seating, and the actual measured displacement.
Application Fit and Sourcing Requirements
Shim adjustment is appropriate when repeatable axial spacing, low assembly variation, and controlled rigidity are priorities. It is less suitable where the housing geometry cannot maintain even contact, where the available axial stack is insufficient, or where thermal growth leaves no room for the intended preload [S3][S4].
The method is especially useful for precision rotary systems, including machine tool spindles, measuring instruments, high-speed angular contact arrangements, and ballscrew support assemblies [S1][S4]. A matched set with factory-defined clearance and preload can reduce the need for manual trial fitting, but replacement of one member of a matched set can alter the pair’s behavior.
Specify the shim by functional location, total thickness, tolerance, parallelism, and material compatibility rather than by a generic washer description [S6]. Where rolling-element life is the controlling requirement, use the ball-bearing designation only as a family reference and retain the angular contact part number, contact angle, internal-clearance class, mounting orientation, and maker’s preload limits.
Track three measurable signals during commissioning: axial displacement under a defined indicator load, shaft runout after assembly, and bearing temperature during a representative run [S1][S5]. Those measurements turn preload adjustment into a repeatable acceptance test rather than an installation judgment based on nut torque or operator feel.