Self-Lubricating Bearing Selection Calculation
Self-Lubricating Bearing Engineering Tool: Selection and PV Calculation
Selecting the right self-lubricating bearing requires more than just picking a size. To ensure long-term performance and avoid premature failure, engineers must validate their design using the PV (Pressure x Velocity) method.
This workbench provides the formulas, service factors, and material limits needed for professional bearing selection.
Step 1: Core Dimensions and Shaft Tolerances
Before calculating loads, ensure your housing and shaft meet industry standards for self-lubricating bushes.
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Housing Bore: Recommended H7 tolerance.
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Shaft Tolerance: Recommended h8 or f7 (for standard applications).
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Shaft Roughness: Aim for Ra 0.2 to 0.8 micrometers. A shaft that is too smooth will prevent the formation of the lubricating transfer film.
Step 2: Calculate Specific Pressure (P)
Bearing load pressure P represents the force exerted per unit of the projected area.
Formula for Cylindrical Bushes:
P = F / (d x L)
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P: Load Pressure (N/mm2)
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F: Total Load (N)
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d: Shaft Diameter (mm)
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L: Bearing Length (mm)
Formula for Thrust Washers:
P = [4 x F] / [(D2 – d2) x 3.14]
Step 3: Calculate Sliding Velocity (V)
The sliding speed V is critical because it directly impacts the friction heat generated.
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Rotational Motion: V = (d x 3.14 x n) / (1000 x 60)
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n: Rotational speed (RPM)
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V: Velocity (m/s)
Step 4: The PV Value and Service Factors (fB)
The PV Value is the product of pressure and velocity (P x V). However, theoretical PV must be adjusted using a Service Factor (fB) to account for real-world environments.
Design PV = (P x V) / fB
Service Factor (fB) Reference Table:
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Clean, intermittent operation: 1.0
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Steady load, continuous motion: 0.8
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Dirty, dusty environments: 0.3 to 0.5
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High-frequency vibration: 0.2
Step 5: Material Selection Matrix
Compare your calculated Design PV against the maximum limits of common self-lubricating materials:
| Material Type | Max Load (N/mm2) | Max Speed (m/s) | Max PV Limit |
| PTFE Composite | 140 | 2.5 | 3.6 |
| POM Lined | 70 | 2.0 | 2.8 |
| Solid Lubricant | 250 | 0.5 | 1.5 |
| Steel/Bronze | 150 | 2.0 | 2.5 |
Engineering Troubleshooting: “What if my PV is too high?”
If your calculated PV exceeds the material limit, consider these engineering adjustments:
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Increase Bearing Length: Extending the length (L) reduces the specific pressure (P).
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Increase Shaft Hardness: Use a shaft with hardness above HRC 45 to improve wear resistance.
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Improve Cooling: Enhance heat dissipation if the operating temperature exceeds 100 degrees Celsius.
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Select JDB Material: If the load is extremely high and speed is low, graphite-plugged bronze (JDB) is the safest choice.
Expert Q&A for Bearing Design
Q: Can I use SF-1 bearings in wet environments?
A: Yes, but you must choose a stainless steel or bronze backing to prevent corrosion, as standard steel backings will rust and delaminate.
Q: Why does the L/d ratio matter?
A: We recommend an L/d ratio between 0.5 and 2.0. If the bearing is too long (L/d > 2.0), shaft deflection can cause “edge loading,” leading to localized overheating and failure.
