Journal Bearing Design Calculator
Estimate hydrodynamic journal bearing performance: Sommerfeld number, minimum film thickness, friction coefficient, power loss, and basic safety checks.
Input parameters
Results
Operating parameters
Not evaluated- Unit load p
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- Sliding speed U
- –
- PV factor
- –
- Sommerfeld number S
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Film & friction
- Minimum film thickness hmin
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- Film thickness ratio hmin/Rq
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- Friction coefficient f
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- Friction power loss Pf
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Enter parameters and click “Calculate bearing performance” to see results and qualitative safety assessment.
How this journal bearing design calculator works
This tool provides a quick, first-pass design check for hydrodynamic journal bearings using classical short-bearing theory and common engineering correlations. It is not a replacement for detailed tribology or rotordynamics analysis, but it helps you size and compare bearing options in seconds.
Key assumptions
- Hydrodynamic lubrication with a continuous oil film (no metal-to-metal contact).
- Short bearing approximation (L/D ≤ 1) when that option is enabled.
- Steady-state operation at the specified speed, load, and oil viscosity.
- Isothermal oil (no temperature rise calculation in this basic version).
Core equations used
We use a simplified, engineering-oriented formulation around the Sommerfeld number:
Unit load (average bearing pressure):
\[ p = \frac{W}{L \cdot D} \]
Sliding speed at the journal surface:
\[ U = \pi D n \]
where \(n\) is in revolutions per second.
Sommerfeld number (one common definition):
\[ S = \frac{\mu \, n \, (R/C)^2}{p} \]
with \(R = D/2\), \(C\) radial clearance, \(\mu\) dynamic viscosity.
From the Sommerfeld number, we estimate eccentricity ratio and minimum film thickness using empirical correlations typical of design charts:
Approximate eccentricity ratio (heuristic fit):
\[ \varepsilon \approx \frac{1}{1 + k S} \]
with \(k\) chosen so that typical design ranges are reproduced.
Minimum film thickness:
\[ h_{\min} = C (1 - \varepsilon) \]
The friction coefficient in a hydrodynamic journal bearing can be related to the Sommerfeld number. We use a simple correlation of the form:
Friction coefficient (approximate):
\[ f \approx k_f \sqrt{S} \]
where \(k_f\) is tuned to give realistic values (typically 0.001–0.01).
Friction power loss:
\[ P_f = f \, W \, U \]
Safety checks
The calculator performs two simple but useful checks:
- Film thickness vs. roughness: We compute the ratio \(h_{\min} / R_q\). Values > 3–5 are usually considered acceptable for full-film lubrication; lower values suggest mixed or boundary lubrication risk.
- PV factor: \[ PV = p \cdot U \] is compared to the PV limit you provide (typical values for babbitt-lined bearings are around 1–2 MPa·m/s). If PV exceeds the limit, overheating and wear are more likely.
Design tips for journal bearings
- Choose clearance carefully: Too small increases seizure risk; too large reduces film pressure and stiffness.
- Use viscosity at operating temperature: Oil thins significantly as it heats up; using room-temperature viscosity will overestimate film thickness.
- Check L/D ratio: Short bearings (L/D ≈ 0.5–1) are common in high-speed machines; long bearings carry more load but are more sensitive to misalignment.
- Consider start/stop conditions: Hydrodynamic film is weakest at low speed; boundary lubrication or auxiliary systems may be needed.
FAQ
What is a journal bearing?
A journal bearing is a sliding bearing where a rotating shaft (journal) runs inside a stationary cylindrical sleeve. A thin film of lubricant separates the surfaces and carries the load through hydrodynamic pressure generated by rotation.
What is the difference between short and long bearings?
In short bearings (L/D ≤ 1), end leakage dominates and the pressure field is assumed uniform along the axial direction. In long bearings (L/D ≥ 1), side leakage is less important and the pressure varies mainly circumferentially. Different analytical approximations are used for each case.
Can I use this calculator for tilting-pad or foil bearings?
No. This tool is intended for conventional cylindrical hydrodynamic journal bearings. Tilting-pad, foil, and magnetic bearings require different models and design data.
How accurate are the results?
The equations and correlations used here are suitable for preliminary design and comparison of alternatives. For critical machinery, you should validate the design with detailed bearing software, manufacturer data, and, where appropriate, rotordynamic analysis.