What Are Bearing Spacer Washer? A Selection Guide

In bearing assembly and shaft alignment applications-such as CNC machine tool spindles, servo feed axes, gearbox output shafts, automation modules, and precision inspection platforms-standard flat washers and inexpensive stamped washers have significant shortcomings in terms of bearing preload adjustment, load distribution, end-face compensation, and vibration-induced loosening prevention. Extensive on-site maintenance statistics show that low-end, ordinary stamped washers are not surface-ground, resulting in flatness deviations of 0.04 mm or more. Once locked in place, this causes uneven force distribution on the bearing end faces, tilting of the bearing inner ring, and a 2–4-fold increase in the radial runout of the shaft system; Ordinary low-carbon steel washers lack sufficient hardness; after bearing preload is applied, the washers undergo plastic deformation, causing preload torque to decay by more than 35%. After a period of operation, this results in increased bearing clearance and excessive axial play; Simple, thin washers have insufficient load-bearing area; under alternating loads, they experience localized crushing, causing indentations on the bearing end faces. Under high-speed conditions, this triggers shaft system vibration and abnormal noise, and degrades the surface finish of machined workpieces; Some washers that have not undergone aging treatment deform under temperature fluctuations, causing the bearing preload to drift. After 1,500 hours of service, the bearing fatigue failure rate increases by more than 65%, failing to meet the stringent operational requirements for precision bearings regarding long-term stability, controllable preload, and end-face compensation.

Bearing Spacer Washer, also known as bearing adjustment washers or bearing spacer washers, are precision shim components specifically designed for rolling bearing assembly. They serve to fine-tune preload, evenly distribute end-face loads, compensate for clearances, and provide isolation and protection. The mainstream products are divided into three major series: PW precision flat washers, WW corrugated elastic washers, and SW spherical self-compensating washers. Their specifications are compatible with bearing inner diameters ranging from Φ5 to Φ120 for general applications, and they are available in two precision grades: standard and precision (P-grade). They are manufactured from spring steel or bearing steel that has undergone quenching and tempering, stress-relief aging, and double-sided precision grinding, allowing for precise control of shim thickness tolerances; some elastic models retain a controlled margin for slight deformation. Thanks to the combined characteristics of high-flatness load-bearing support, precise thickness compensation, and elastic cushioning, these washers evenly distribute the locking pressure on the bearing end face, fine-tune bearing preload and clearance, and compensate for end-face machining deviations. They effectively suppress uneven load wear on bearings, stabilize the rotational accuracy of shaft systems, and prevent typical failures-such as bearing preload failure, end-face damage, shaft system vibration, and accuracy drift-caused by incorrect selection of standard washers. With thickness tolerances of ≤0.003 mm, stable flatness, uniform load distribution, and minimal thermal deformation, these washers are widely used in transmission applications that demand extremely high precision in bearing preload control and uniform end-face load distribution-such as CNC machine tool spindles, ball screw support blocks, gearbox shaft systems, and precision inspection slides. This article systematically explains the performance of Bearing Spacer Washer based on the GB/T 97.1 Flat Washer Standard, GB/T 94.1 (Specification for Elastic Washers), and GB/T 307.1 (Tolerance Standard for Rolling Bearings), combined with actual measurement data from shaft system assemblies and failure case studies, systematically explains the performance characteristics of Bearing Spacer Washer, load compensation mechanisms, differences in structure and materials, operating condition suitability limits, and precision assembly specifications. This helps engineers select the appropriate inner diameter, accuracy grade, washer series, and operating condition versions, thereby avoiding engineering issues such as bearing preload failure, end-face damage, shaft system vibration and noise, and premature bearing failure caused by mismatches.

The core characteristics of bearing shims include precise thickness compensation, even distribution of end-face loads, fine-tuning of preload, resistance to plastic deformation, and excellent long-term dimensional stability. These are the fundamental differences that distinguish them from ordinary stamped shims and simple thin shims. Setting aside marketing rhetoric, we have distilled four practical advantages based on extensive bench testing and real-world bearing operation and maintenance data:

Double-sided precision flat grinding ensures uniform end-face loading without off-center loads. For precision-grade products, thickness tolerances are controlled within 0.003 mm, and the parallelism accuracy of both sides of the shim is high. This ensures uniform circumferential pressure when locking the bearing, preventing localized stress concentration and reducing the risk of bearing end-face indentations and one-sided heat generation and wear at the source.

Standardized thickness gradients enable precise adjustment of bearing clearance and preload. Categorized by micrometer-level thickness gradients, these shims allow for rapid fine-tuning of bearing assembly clearance by adding or removing shims of different thicknesses. This compensates for end-face geometric deviations caused by housing and shaft shoulder machining, eliminating the need for complex machining adjustments and significantly reducing assembly and commissioning time for shaft systems.

Comprehensive coverage across multiple operating conditions, suitable for conventional, vibration-damping, and misalignment scenarios. PW precision flat shims are suitable for conventional rigid preload and stable ambient temperature conditions; WW corrugated elastic shims provide minimal elastic cushioning and are suitable for alternating vibration and frequent start-stop conditions; SW spherical self-compensating shims can offset minor face tilt deviations and are suitable for assembly scenarios where housing machining results in slight misalignment errors.

Comprehensive stress-relief heat treatment ensures long-term dimensional stability. The washer body undergoes quenching and tempering followed by double-sided precision grinding, resulting in a working surface hardness of HRC 42–50. Even after multiple disassemblies and maintenance cycles, thickness deformation remains below 8%. Compared to standard stamped washers, this design significantly reduces preload drift in bearing assemblies and lowers equipment failure rates.

At the same time, clearly define the operating limits for Bearing Spacer Washer: For ultra-high-load and heavy-duty shaft systems, the end-face compressive strength of the washer must be calculated, and thick bearing steel washers should be prioritized; for high-temperature conditions (>120°C), the material's coefficient of thermal expansion must be calculated, and washers made of high-temperature-resistant alloys should be selected; in environments with aggressive cutting fluids or dust splatter, the washer surface must undergo a Dacromet anti-corrosion treatment to prevent rust and thinning; Bearing Spacer Washer must not be used as the primary anti-loosening element; in high-vibration environments, they must be used in combination with a lock nut and washer; when the end face runout exceeds 0.03 mm, compensation cannot rely solely on spherical washers-the bearing housing end face must be corrected first. In conditions involving extreme high-frequency impacts, the bearing's load-bearing area must be calculated to prevent localized crushing and plastic deformation of the washer.

The core transmission mechanism of the bearing washer differs from the logic of ordinary shims, which merely provide isolation and protection, and also differs from the large-deformation anti-loosening mode of spring washers. It employs a three-in-one composite shaft system adjustment mechanism comprising high-precision parallel end-face load-bearing, micron-level thickness gap compensation, and elastic load buffering for even load distribution. This mechanism addresses, from a mechanical perspective, the four major engineering challenges commonly found in precision bearings-uneven end-face loading, uncontrolled preload, compensation for assembly deviations, and crushing under alternating loads-all common engineering challenges in precision bearings-thereby achieving closed-loop control of bearing clearance, end-face pressure, and shaft system rotational accuracy.

Analyzing the mechanical logic of bearing assembly: the inner ring is positioned by the shaft shoulder, the outer ring is constrained by the bearing housing, and the locking nut applies axial preload; Bearing shims are positioned between the bearing end face and the locking mechanism, serving as a medium for pressure transmission. When the locking nut applies axial pressure, the precision-ground end faces of the shims convert concentrated line pressure into uniform surface pressure, preventing the sharp corners of the locking nut from directly damaging the bearing outer ring's end face; By selecting and combining washers of different thicknesses, the axial clearance of the bearing is precisely adjusted to achieve the preset preload condition and compensate for end-face parallelism errors resulting from the machining of the bearing housing and shaft shoulder.

Under steady-state, high-speed reciprocating operating conditions, the bearing continuously endures alternating feed loads, start-stop impact loads, and minute component deformations caused by temperature rise, making the shaft system highly susceptible to end-face pressure fluctuations and high-frequency micro-vibrations. Bearing shims utilize the slight elastic deformation of their flat load-bearing surfaces or corrugated structures to adaptively cushion pressure fluctuations, consistently maintaining the bearing end-face pressure within the design-allowed range. At the same time, the shims' rigid support structure prevents localized stress concentration at the lock nut, reduces the risk of single-point indentation on the bearing end face, and enhances the dynamic operational stability of the shaft system. Compared to standard stamped washers, their thickness precision is controllable, eliminating the risk of plastic collapse; compared to thick copper washers, they offer higher hardness and are less prone to deformation under long-term pressure.

A comparative analysis of the operational limitations of various washer components reveals the following: Standard stamped flat washers have large thickness tolerances and lack precision grinding, making them highly susceptible to plastic deformation under pressure, which causes rapid preload decay; standard spring washers undergo significant deformation and are unsuitable for fine-tuning the preload of precision bearings, often leading to preload overload; soft red copper shims have low hardness and are suitable only for one-time assembly; repeated disassembly and reassembly cause rapid thickness compression and failure; Direct locking without washers causes the sharp corners of the nut to damage the bearing end faces, resulting in skewed force distribution on the bearing raceways and a sharp increase in radial runout. Precision Bearing Spacer Washer balance thickness accuracy, uniform load transfer, compensation for assembly deviations, and long-term dimensional stability, making them the most well-rounded standardized adjustment components currently available for precision rolling bearing assembly systems.

Their core engineering value can be summarized as follows: through the structure of precision shims, they evenly distribute axial clamping pressure, compensate for end face machining errors, precisely control bearing clearance and preload, eliminate the risk of bearing end face damage and shaft system vibration and wobble, maintain long-term stability in bearing rotational and positioning accuracy, and significantly enhance the operational stability and overall service life of precision shaft systems.

Product Showcase

The bearing shim features an integral annular thin-plate load-bearing structure, precision-machined according to four key dimensions: end-face parallelism, thickness tolerance, load-bearing strength, and long-term dimensional stability. All components undergo multiple rounds of stress-relief aging treatment to eliminate residual stresses from stamping and grinding processes. Core components include the shim body, precision load-bearing working surface, inner bore guide edges, and surface protective coating. Detailed structural parameters are as follows:

Structural Component	Brief Introduction	Core Requirements
Washer Base Body	Main axial load bearing structure	65Mn spring steel / GCr15; HRC42‑50; stress relief treatment
Precision Bearing Surface	Benchmark for pressure transfer	Thickness tolerance ≤0.003mm; Ra≤0.4μm; double-side grinding
Inner Hole Guide Edge	Assembly positioning, prevent eccentric force	H9 tolerance; burr-free edge, no scratch to shaft journal
Surface Protection Layer	Anti-rust, resist cutting fluid and moisture	Zinc plating / Dacromet coating; salt spray resistance ≥500h

In addition to the standard general-purpose design, specialized variants are available: the high-temperature model features an optimized base material suitable for the temperature range of –40°C to 150°C; the heavy-duty, thickened model increases the base material thickness to enhance the end-face load-bearing limit; and the ultra-thin precision model, with a thickness of 0.01–0.1 mm, is used for micron-level clearance fine-tuning.

Base Material Selection Guidelines:

65Mn Spring Steel: A versatile, mainstream material offering excellent value for money. Suitable for conventional automation modules operating at room temperature under moderate loads, as well as for standard CNC feed-axis bearing assemblies.

GCr15 Bearing Steel: Provides enhanced base material hardness and resistance to crushing. Suitable for spindle bearings in machining equipment subjected to heavy-duty reciprocating motion and frequent start-stop cycles.

High-Temperature Alloy Steel: Designed for equipment operating under sustained high-temperature conditions; its coefficient of thermal expansion matches that of the bearing housing, preventing preload drift caused by temperature differences.

Key Points for Avoiding Installation Pitfalls: The inner diameter of the shim must match the assembly journal; the clearance between the inner bore and the shaft must not be too large to prevent eccentric assembly; precision-grade bearings must be paired with precision-grade P-class shims, as standard stamped shims will directly compromise bearing preload accuracy; Do not stack too many shims; stacking multiple shims will amplify parallelism errors; during assembly, ensure the parallelism of the bearing housing end faces; end face deviations exceeding 0.02 mm will inevitably cause uneven loading and crushing of the shims; in wet cutting fluid environments, use shims with corrosion-resistant surface treatments; do not use simple shims without rust protection.

Bearing Spacer Washer are specifically designed for precision rolling bearings to support end-face loads, fine-tune preload, evenly distribute loads, and ensure long-term, low-failure operation. They are used in four core fields: CNC machine tools, automation modules, precision testing equipment, and gear reducers:

CNC Machine Tool Spindles: Used in the bearing assemblies of machining centers and CNC lathe spindles to adjust bearing preload and evenly distribute clamping force, effectively preventing bearing end-face damage, spindle vibration marks, and dimensional drift, thereby ensuring consistency in batch machining.

Automated Linear Modules: Bearing assembly for servo slides and lead screw support brackets. This compensates for machining deviations on the housing end faces, stabilizes bearing clearance, resolves issues such as bearing preload decay and operational noise, and enhances the module's repeatable positioning accuracy and operational smoothness.

Precision Inspection Equipment: Bearing assembly for image inspection systems and precision displacement stages. Through high-precision thickness control, it minimizes fluctuations in axial bearing preload, ensuring the stability of high-precision inspection reference points.

Gear Reducer Equipment: Bearing assembly for servo gear reducers and gearboxes, designed to accommodate alternating impact loads, cushion pressure fluctuations during start-up and shutdown, suppress dynamic bearing vibration, and enhance the high-speed operational stability of gear reducers.

In addition, these solutions are widely used in equipment applications with stringent requirements for bearing preload stability and uniformity of end-face loads, such as precision drive shafts in textile machinery, feed shafts in printing equipment, custom-designed precision transmission mechanisms for automation, and rotating shaft systems in precision tooling.

Statistics on bearing and shaft system precision failures indicate that approximately 48% of cases involving bearing preload drift, bearing face damage, and shaft system vibration stem from non-standard bearing washer assembly. Key failure causes include washer eccentricity, excessive stacking of washers, and residual contaminants on the bearing faces. The assembly process strictly adheres to three technical principles: ultra-clean component preparation, coaxial and smooth assembly, and graded preload locking:

Preliminary Cleaning and Parameter Verification

Clean the shaft shoulder, bearing end faces, and both sides of the shims with anhydrous ethanol to thoroughly remove metal shavings, oil residue, and oxidized burrs. Residual hard contaminants can damage the working surface of the shims, causing localized bulging and compromising the uniformity of bearing preload. Verify the shim thickness, precision grade, and material to ensure they meet the bearing clearance adjustment requirements; Inspect both sides of the shim for dents or chips and check the edges for burrs; defective shims must not be installed.

Coaxial Pre-assembly Alignment

Slide the washer horizontally onto the shaft journal, ensuring it remains aligned with the shaft centerline throughout the entire process. Do not tilt or pry from one side to prevent washer deformation or the inner bore from scratching the shaft journal. Once the washer is in place, confirm that it is fully in contact with the end face of the bearing outer ring, with no warping or gaps.

Gradual Diagonal Tightening for Preload

Tighten the lock nuts diagonally in stages, applying the preset preload torque gradually over 2 to 3 passes. Do not apply excessive force in a single tightening operation to avoid instantaneous high pressure causing plastic crushing of the washer. The washer serves solely as a pressure-transmitting and compensating element; do not rely on washer deformation to achieve excessive preload.

Q: Can bearing washers be replaced with ordinary stamped flat washers?

A: This is absolutely not recommended. Ordinary stamped washers are not precision-ground on both sides; they have large thickness tolerances and poor flatness, and are highly prone to plastic deformation and collapse under pressure. Once locked in place, the bearing end faces are subjected to uneven forces, which quickly leads to uneven load distribution and wear, as well as a loss of preload. The use of ordinary stamped washers in precision bearing assemblies is strictly prohibited.

Q: What is the main cause of increased bearing clearance after the bearing has been in operation for some time following assembly?

A: There are three primary causes: First, the use of standard low-carbon steel washers, which undergo plastic deformation and collapse under axial pressure, causing the preload to be released; second, residual iron filings or impurities on the washer surface, leading to localized collapse and loss of pressure; third, insufficient washer thickness, resulting in inadequate preload margin. Precision bearing assemblies must use precision bearing washers that have undergone quenching and tempering heat treatment.

Q: How do you select and distinguish between the PW, WW, and SW bearing washers?

A: For standard room-temperature applications with rigid preload and no significant vibration, use PW precision flat washers; for applications with alternating vibration and frequent start-stop cycles that require minimal elastic cushioning, use WW corrugated elastic washers; for applications where the bearing housing end face has slight angular misalignment and self-aligning compensation is required, use SW spherical self-aligning washers.

Q: Can bearing washers be repeatedly disassembled and reassembled for reuse?

A: Precision-grade PW flat washers can be properly disassembled and reassembled up to 8 times without indentations or plastic deformation and can be reused normally; WW corrugated elastic washers experience fatigue loss due to elastic deformation, so they are generally recommended for single-use assembly; once a washer shows indentations, warping, or hardening deformation, it must be replaced with a brand-new washer immediately.

Q: What should be considered when selecting bearing washers for high-temperature operating conditions?

A: For shaft systems operating at temperatures above 120°C, prioritize high-temperature-resistant alloy steel washers and calculate the material's coefficient of thermal expansion. Use high-temperature-resistant Dacromet for surface corrosion protection to prevent coating failure and rusting at high temperatures. Allow for variations in preload caused by temperature changes; do not select washers based solely on room-temperature parameters.

Q: Does a thicker shim result in better bearing preload?

A: Absolutely not. Shim thickness is used solely to adjust clearance; preload is controlled by the torque of the lock nut. Blindly selecting excessively thick shims increases end-face stress and can easily lead to shim crushing. For precision assemblies, prioritize fine-tuning with multiple thin shims, using a single shim as the primary load-bearing shim; stacking more than three shims is not recommended.

Q: Can red copper shims be used as substitutes for precision bearing shims?

A: This is not recommended. Red copper is relatively soft and is highly prone to sustained compression deformation under load; the preload value continues to decrease over time. It is only suitable for low-pressure, one-time sealing assemblies and cannot meet the requirements for long-term, stable preloading in precision bearings.

Q: What causes abnormally high temperature rise on one side of the shaft assembly and operational vibration after assembly?

A: This is most likely caused by eccentric shim installation, residual contaminants on the end face, localized crushing of the shim under pressure, and uneven force distribution on the bearing end face. It may also be due to substandard shim flatness, causing the bearing inner ring to tilt and triggering micro-vibrations in the shaft assembly. The assembly must be disassembled and cleaned; qualified precision shims must be replaced, and the assembly must be reassembled and realigned.

Flat Washers GB/T 97.1‑2002

Technical Requirements for Elastic Washers GB/T 94.1‑2018

Rolling Bearings-Tolerance Specifications for Radial Bearings GB/T 307.1‑2015

Assembly Process Manual for Shaft System Shims in Precision Machine Tools China Machine Tool & Tool Industry Association

Technical Manual on Bearing Shim Failure Analysis and Preload Control Precision Transmission Industry Technology Center

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