Sliding Bearing Clearance
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Sliding Bearing Clearance Explained: Types, Fits, and How to Specify the Right Gap
1) What “clearance” means in a sliding (plain) bearing
In a sliding bearing (plain bearing), the shaft slides on a lubricated surface rather than rolling on elements. The critical parameter is the running clearance—the small gap between the shaft journal and the bearing bore.
- Diametral clearance: Cd=Dbore−Dshaft
- Radial clearance: c=Cd/2
This gap determines whether the bearing can:
- Form a stable hydrodynamic oil film
- Operate in mixed or boundary lubrication
- Overheat and seize under unfavorable conditions
Core trade-off:
| Clearance condition | Benefit | Risk |
|---|---|---|
| Too small | High stiffness, reduced shaft vibration | Oil film collapse → heat → scuffing or seizure |
| Too large | Easier assembly, better thermal tolerance | Reduced stiffness, more vibration, weaker oil film pressure |
A useful perspective:
- Hydrodynamic lubrication requires sufficient clearance
- Mechanical stability favors minimal clearance
2) Types of sliding bearings and their influence on clearance
2.1 Journal (sleeve) bearing — radial load
Designed to support loads perpendicular to the shaft axis. Rotation draws lubricant into a pressure wedge that separates surfaces.
- Requires controlled positive clearance for oil film formation
- Clearance directly impacts load capacity and friction behavior
2.2 Thrust bearing — axial load
Handles loads parallel to the shaft axis, common in pumps and rotating assemblies.
- Focus shifts to axial geometry and alignment
- Surface flatness and perpendicularity are often more critical than diametral clearance
2.3 Flanged plain bearing — combined load
Supports both radial and axial loads in a compact design.
- Must balance:
- Radial clearance (rotation and lubrication)
- Axial stability (thrust support)
- Outer diameter fit (to prevent rotation in housing)
3) Fit systems that control clearance
Clearance in plain bearings involves two interfaces:
- ID fit: shaft vs. bearing bore (running clearance)
- OD fit: bearing outside diameter vs. housing
3.1 Clearance fit
Always provides a positive gap.
- Best for high speed and hydrodynamic lubrication
- Advantages:
- Easy assembly
- Accommodates thermal expansion
- Limitations:
- Lower stiffness
- Potential vibration if excessive
| Fit Category | Fit Name | Hole Basis (ISO) | Shaft Basis (ISO) | Typical Application Description |
| Clearance Fit | Loose Running Fit | H11/c11 | C11/h11 | General-purpose fit with large clearance. Easy assembly, no precise positioning required. Not suitable for moving or precision parts. |
| Clearance Fit | Free Running Fit | H9/d9 | D9/h9 | Suitable for moving parts under high speed, large temperature variation, and heavy radial loads. Not ideal for high-precision systems. |
| Clearance Fit | Close Running Fit | H8/f7 | F8/h7 | Used in precision machines with moderate speed and load. Maintains some positional accuracy. Common in plain bearings (e.g., powder metallurgy bushings). |
| Clearance Fit | Sliding Fit | H7/g6 | G7/h6 | Allows smooth sliding and rotation with accurate guidance. Common in gauges, guide pins, and bushings. |
| Clearance Fit | Locational Clearance Fit | H7/h6 | H7/h6 | Provides accurate positioning with small clearance. Suitable for static parts. Easy manual assembly and disassembly. |
| Transition Fit | Locational Transition Fit | H7/k6 | K7/h6 | Balanced fit between clearance and interference. Requires light force for assembly. Common for bearing mounting. |
| Transition Fit | Tight Transition Fit | H7/n6 | N7/h6 | Higher positioning accuracy than standard transition fits. More interference than clearance. |
| Interference Fit | Locational Interference Fit | H7/p6 | P7/h6 | Used where precise positioning and alignment are required. Common for tightly mounted bearings and shafts. |
| Interference Fit | Medium Drive Fit | H7/s6 | S7/h6 | Provides moderate interference. Can transmit torque. Produces noticeable assembly stress. |
| Interference Fit | Force Fit | H7/u6 | U7/h6 | Strong interference fit for high torque transmission. Requires heavy press force. Produces high stress and often used for permanent assemblies. |
3.2 Interference fit
Parts are press-fitted with no gap.
- Best for high load and rigidity
- Advantages:
- Prevents movement or creep
- Improves positional stability
- Limitations:
- Difficult installation
- Risk of overstress due to thermal expansion
3.3 Transition fit
May result in slight clearance or interference.
- Useful for balanced performance
- Offers moderate stability and serviceability
- Requires tight tolerance control
4) Key ISO concept: mean relative bearing clearance (ψₘ)
A more scalable way to define clearance is mean relative bearing clearance (ψₘ), expressed in per mille (‰).
ψm≈（Dbore−Dshaft）/Dshaft×1000
Typical preferred values (‰):
- 0.56, 0.8, 1.12, 1.32, 1.6, 1.9, 2.24, 3.15
Selection process:
- Choose ψₘ based on load, speed, and lubrication
- Match it to an appropriate ISO tolerance fit
- Verify resulting clearance range
5) Geometry and surface finish considerations
Even with correct nominal clearance, poor geometry can disrupt lubrication.
Critical factors include:
- Roundness of shaft and bore
- Straightness of the journal
- Coaxial alignment
- Surface roughness (Ra, Rz)
- Thrust face flatness (for axial bearings)
Relevant standards such as GB/T 39741.2-2021 define tolerances and surface finish requirements, though exact values should be confirmed from official or licensed documents.
6) Quick selection matrix
| Operating condition | Recommended approach | Reason |
|---|---|---|
| High-speed rotation | Clearance fit (ID) | Enables stable oil film |
| Heavy load, low speed | Interference (OD) + controlled ID clearance | Prevents movement and maintains alignment |
| Large temperature variation | Increased clearance | Compensates thermal expansion |
| Frequent maintenance | Transition fit | Easier assembly/disassembly |
| High precision systems | Lower ψₘ + tighter geometry | Improves stiffness and stability |
7) Example: converting ψₘ into actual clearance
For a 50 mm shaft with ψm=1.12‰
Cd≈1.12×50/1000
- Diametral clearance ≈ 0.056 mm
- Radial clearance ≈ 0.028 mm
Before finalizing:
- Check lubrication conditions (viscosity, speed)
- Evaluate thermal expansion effects
- Confirm manufacturability and inspection capability
8) Troubleshooting guide
| Symptom | Likely cause | Recommended action |
|---|---|---|
| Overheating or seizure | Clearance too small or lost due to heat | Measure bore, increase clearance, review thermal design |
| Bearing rotation in housing | Insufficient interference on OD | Correct housing fit or use oversized bearing |
| Uneven wear | Misalignment or distortion | Check alignment and installation conditions |
| Shaft scoring | Poor surface finish or low hardness | Improve grinding and material properties |
| Noise or vibration | Excessive clearance or lubrication failure | Verify clearance and oil supply system |
9) FAQ
What is an ideal sliding bearing clearance?
There is no universal value. Using ψₘ (typically 0.56‰–3.15‰) provides a more reliable method, with the exact choice depending on operating conditions.
Why can a bearing fail even if clearance is “correct”?
Because local geometry errors—such as poor roundness or rough surfaces—can destroy the oil film, leading to localized contact and failure.
Should clearance or interference fit be used?
- Running surfaces (shaft vs. bore): require controlled clearance
- Housing fit (bearing OD): often uses interference or transition fits to prevent movement
