A plain bearing may be installed with the correct nominal clearance, yet still fail in service because the running clearance changes with temperature, load, and material expansion.

Running clearance is the clearance between the shaft and bearing surface under actual operating conditions. It is not the same as the clearance measured before installation.

This guide explains how to distinguish installation clearance from running clearance, how press fit and thermal expansion affect bore size, how different bearing materials influence clearance design, and how to calculate and verify running clearance as part of a bearing selection process.

Quick Answer

Running clearance depends on the initial installation clearance, press fit effect, thermal expansion of the bearing and shaft, and operating load. It cannot be determined from a single catalog value. Material, temperature, fit, and application conditions must all be considered.

What Is Plain Bearing Running Clearance?

Running clearance is the clearance between the bearing bore and the shaft under operating conditions. It is a function of the bearing's initial dimensions, the installation fit, the operating temperature, and the applied load.

For a cylindrical radial plain bearing, clearance is typically expressed as:

c = D_bore − D_shaft

Diametral Clearance and Radial Clearance

Two definitions are commonly used in engineering practice:

• Diametral clearance: c = D_bore − D_shaft

• Radial clearance: c_radial = c_diametral / 2

This article uses diametral clearance unless otherwise stated. Mixing the two definitions can double or halve the calculated result.

Three Clearance States

Plain bearing clearance should be evaluated in three distinct states, and the same reference condition must be used consistently throughout any calculation:

A catalog ID, a free-state bore, or a nominal dimension does not automatically equal the actual installed bore after press fit.

The diametral running clearance described in this article is an engineering estimate used for comparison and design evaluation. It does not represent the local gap at every position and is not equivalent to the minimum oil film thickness in hydrodynamic bearings.

How Press Fit Changes Bearing Clearance

Press fit is used to retain the bearing in the housing and to prevent movement under load. However, press fit also affects the bearing bore.

When a bearing is pressed into a housing with interference, the outer diameter is compressed. Depending on the bearing construction, this compression can partially transfer to the bore, reducing the installed bore diameter and therefore reducing clearance.

The reduction in bearing bore size depends on the bearing construction, housing stiffness, interference fit, and installation conditions; it should not be assumed to equal the nominal interference.

Important factors include:

Verification of installed bore size should be based on the bearing manufacturer's specified housing tolerance, interference fit, and installation requirements, using a measurement or verification method appropriate to the specific bearing structure. Re-measurement is not the only verification method for every bearing construction.

How Temperature and Material Affect Running Clearance

Temperature has a significant influence on running clearance. As the bearing and shaft heat up during operation, their dimensions change according to their respective thermal expansion behavior.

Simplified Thermal Estimation

Define the initial clearance at the same reference temperature used for both diameters:

c₀ = D_b,0 − D_s,0

where:

• D_b,0 = effective bearing bore diameter at the reference temperature

• D_s,0 = shaft diameter at the reference temperature

The change in clearance due to free thermal expansion can be estimated as:

Δc ≈ α_b × D_b,0 × ΔT_b − α_s × D_s,0 × ΔT_s

where:

• α_b = effective thermal expansion data applicable to the bearing bore dimensional change (not simply the backing material coefficient)

• α_s = shaft material thermal expansion coefficient

• ΔT_b = bearing temperature rise relative to the reference temperature

• ΔT_s = shaft temperature rise relative to the reference temperature

The estimated clearance after free thermal expansion is:

c_thermal ≈ c₀ + Δc

This estimate represents only the free thermal expansion effect. It does not include press fit effects, load deformation, or other operating factors.

Note on reference conditions: For an installed bearing, use the effective bore diameter and clearance at the same reference condition. If c₀ is the pre-installation clearance, then press fit effect must be evaluated separately before thermal change is applied. If c₀ is already the installed clearance, do not subtract the press-fit effect a second time. The thermal estimate remains approximate because housing constraint and composite-layer interactions may affect the actual bore change.

For multilayer composite bearings, the effective thermal behavior of the bore should not be assumed to equal the thermal expansion coefficient of the metallic backing alone.

Material Considerations

Different bearing types respond differently to temperature and load:

Material properties affect clearance design, but the required clearance cannot be determined from material name alone. Specific product data and manufacturer guidance are required.

When evaluating a specific bearing, request the applicable bore dimensional tolerances, recommended housing fit, installation-related bore change where available, operating temperature limits, and clearance guidance for the intended load and motion conditions.

Thermal Estimation Example

The following example uses assumed values for illustration only. Actual values must be confirmed for the specific bearing and application.

Assumptions:

• Bearing bore at reference temperature: D_b,0 = 40 mm

• Shaft diameter at reference temperature: D_s,0 = 39.90 mm

• Initial diametral clearance at the reference temperature, before installation: c₀ = 0.10 mm

• Bearing temperature rise: ΔT_b = 60 °C

• Shaft temperature rise: ΔT_s = 50 °C

• Effective bearing thermal expansion (assumed): α_b = 12 × 10⁻⁶ /K

• Shaft thermal expansion: α_s = 11.5 × 10⁻⁶ /K

This illustrative calculation considers free thermal expansion only. The effect of press fit must be evaluated separately before determining the installed running clearance.

Calculation:

Δc ≈ (12 × 10⁻⁶ × 40 × 60) − (11.5 × 10⁻⁶ × 39.90 × 50)

Δc ≈ 0.0288 − 0.0229

Δc ≈ 0.0059 mm

c_thermal ≈ 0.10 + 0.0059

c_thermal ≈ 0.106 mm

The calculated value is an estimate of clearance change due to free thermal expansion only. It does not by itself confirm that the bearing will operate correctly. If the bearing is a composite structure, the actual bore change may also be influenced by press fit and structural constraint. The allowable running clearance range, measurement conditions, and design requirements must be confirmed with the manufacturer.

What Happens When Clearance Is Too Small or Too Large?

Clearance problems can lead to different failure modes depending on whether the clearance is too small or too large.

Clearance Too Small

When the running clearance is too small, the bearing may experience:

• Increased friction

• Rising temperature

• Further clearance reduction due to thermal expansion

• Risk of seizure

• Accelerated wear

• Localized high pressure

Insufficient clearance can lead to a chain reaction: higher friction → higher temperature → further clearance reduction → seizure.

Common root causes to check include excessive interference fit, unaccounted thermal expansion, tight shaft tolerances, and installation damage.

Clearance Too Large

When the running clearance is too large, the bearing may experience:

• Increased noise

• Vibration

• Uneven load distribution

• Edge loading

• Increased shock loading

• Reduced positioning accuracy

• Accelerated wear

Excessive clearance may not cause immediate failure, but it can reduce bearing life through noise, vibration, and uneven load distribution.

How Motion Type Affects Clearance Requirements

Clearance requirements depend on the actual motion conditions, not only on the motion category itself.

For each motion type, the following factors should be reviewed:

• Start/stop frequency

• Sliding speed and load

• Temperature change during the cycle

• Relative motion and lubrication mechanism between shaft and bearing

• Specific bearing material and manufacturer's design guidance

No fixed clearance value can be applied to a motion type without considering the specific bearing structure and operating conditions.

How to Calculate and Verify Running Clearance

Running clearance should be evaluated as part of a structured engineering process.

Step 1 — Establish Initial Clearance

Confirm the bearing bore, shaft diameter, reference temperature, and the clearance definition (diametral clearance).

Step 2 — Evaluate Installation Fit

Evaluate how interference fit, housing stiffness, and bearing structure affect the installed bore diameter. Note that interference fit is not equal to the reduction in bore size.

Step 3 — Estimate Thermal Change

Use the thermal expansion data, reference dimensions, and actual temperature rise for both the bearing and the shaft. For composite structures, the effective bore dimensional change may be influenced by press fit and structural constraints in addition to free thermal expansion.

Step 4 — Assess Operating Conditions

Consider load, motion type, deformation, and other factors that may affect the actual operating state.

Step 5 — Verify Against Manufacturer Data

Confirm the applicable clearance range, installation tolerance, and verification requirements with the manufacturer. For structures with complex geometry or tight clearance requirements, more specific dimensional change data or engineering analysis should be obtained from the manufacturer.

For related topics on the mechanical design of sliding bearings, see How to Calculate Bearing PV Value and Sliding Bearing Load and Pressure. For shaft and surface conditions that also affect running behavior, see How Shaft Surface Roughness Determines PTFE Bearing Life.

Running Clearance Checklist

Common Mistakes When Determining Running Clearance

Several mistakes can make an apparently correct clearance calculation misleading.

1. Using installation clearance as running clearance — Ignores press fit and thermal effects.

2. Assuming bore reduction equals nominal interference — Ignores bearing construction and housing stiffness.

3. Ignoring thermal expansion — Assumes clearance remains constant with temperature.

4. Using a generic clearance value for all materials — Ignores material-specific expansion behavior.

5. Failing to verify the installed condition — Assumes the installed bore and clearance match the drawing without using an appropriate measurement or verification method.

6. Ignoring motion type and load distribution — Assumes one clearance fits all operating conditions.

When to Request Engineering Evaluation

Manufacturer or engineering support is particularly useful when the operating conditions make a simple clearance estimate insufficient.

An engineering evaluation is recommended for applications involving:

• High or widely varying operating temperature

• Significant interference fit

• Tight clearance requirements

• Unusual material combinations

• High speed or heavy load

• Oscillating or reciprocating motion

• Long service-life requirements

• New OEM designs

Useful information to provide includes:

• Shaft diameter

• Bearing dimensions

• Installation fit

• Operating temperature

• Shaft material and bearing material

• Motion type

• Load condition

• Environment

• Expected service life

Have a specific bearing application? Send us the shaft diameter, bearing dimensions, fit, operating temperature, and motion type. If some parameters are not yet available, you can also send the existing bearing drawing, bearing number, or application description for an initial review. Our engineers can help evaluate the appropriate running clearance.

Request Engineering Evaluation | Send Your RFQ

FAQ

What is running clearance in a plain bearing?

Running clearance is the diametral clearance between the bearing bore and the shaft under actual operating conditions. It differs from installation clearance due to press fit, thermal expansion, and load.

How does press fit affect bearing clearance?

Press fit compresses the bearing outer diameter and can partially transfer compression to the bore. The bore reduction depends on bearing construction, housing stiffness, and interference fit, and is not equal to the nominal interference.

How does temperature change running clearance?

As the bearing and shaft heat up, their dimensions change at different rates depending on material and temperature rise. The resulting clearance change can be estimated using the thermal expansion coefficients and temperature rise of each component.

How do I determine the correct running clearance?

Start from initial clearance, evaluate press fit effect, estimate thermal change, assess operating conditions, and verify against the manufacturer's data for the specific bearing grade.

Conclusion

Running clearance is not a fixed number. It is the result of installation conditions, operating temperature, material properties, and load. A correct clearance evaluation requires distinguishing installation clearance from running clearance, accounting for press fit and thermal effects, and verifying the result against manufacturer data for the specific bearing product.

For applications involving significant interference fits, varying temperatures, or tight clearance requirements, an application-specific engineering review can help establish a more reliable basis for bearing selection.

Need help evaluating your application? Request an Engineering Evaluation

Related Articles

• How Sliding Speed Impacts PTFE-Based Bearing Performance

• PTFE Composite Bearing Selection Guide

• OEM Composite Bearings: Customization, Materials, Manufacturing & Quality Control

Related Products

The following products represent different bearing constructions. Their installation fits and running clearance requirements should be evaluated according to the specific product data and application conditions.

• MG-1 PTFE Composite Bearings

• MG-4 Hydraulic Cylinder Bearings

• MG-800 Bi-Metallic Composite Bearings

• MG-CR Filament Wound Bearings