The minimum oil film thickness is the most important value in the design of plane axial neck fluid film bearing. If you're wrong, metal touches metal. Then you score and the bearings quickly fail. If you do it right, the bearings can run smoothly for a long time. This paper introduces what minimum film thickness means, how minimum film thickness is calculated, how minimum film thickness changes, and why most engineers do not pay enough attention to minimum film thickness.
Why Minimum Film Thickness is more important than you think
In ordinary axial neck fluid film bearing, the shaft does not touch the bearing surface when running normally. There is a thin layer of oil between the two surfaces. The thickness of this layer is not always the same. The bottom is thickest and the load highest. The oil is thinnest where it collects.
The thinnest point is the minimum oil film thickness. They call it Humin.
If h_min is less than the roughness of two surfaces, tiny bumps on both surfaces start to collide. This is the line between a good oil film and a bad one. After that the Bearing life will decline rapidly.
There's an old saying tribology books:
h_min ≥ 3 × (W + S)
Here, Sigma is the surface roughness of each part. The number 3 is safety. It comes from test data collected over the years. You can find this rule in standard machine design handbooks.
How Do You Calculate Minimum Oil Film Thickness?
The math comes from the Reynolds equation. This equation describes how pressure works in a thin film of oil. For ordinary Plain Journal Fluid Film Bearing in homeostasis, you can find h _ min using the math in the the Raimondi-Boyd charts or the Sommerfeld number.
Sommerfeld (S) is:
S = (μ × N × r2) / (P × c2)
Location:
mu = oil viscosity (Pa.
N = shaft speed (rev/s)
r = journal radius (m)
P = Load per region (Pa)
c = radial clearance (m)
When you get S, find the eccentricity ratio (epsilon) in standard diagram. Then:
h_min = c × (1 − ε)
The equation shows the whole picture. If the gap is smaller, or if the epsilon is close to 1 (shaft almost touches the bearing), h_min drops very quickly.
This approach comes from Raimondi and Boyd (1958). Their work, published in Journal Lubrication Technology, remains a key reference for bearing design in schools and factories.
What controls the Minimum Film Thickness?
Here are five things that matter most:
| Factor | What Happens to h_min |
|---|---|
| Oil viscosity (μ) | Higher viscosity → thicker film |
| Shaft speed (N) | Higher speed → thicker film |
| Radial clearance (c) | Bigger clearance → thicker film |
| Load (P) | Higher load → thinner film |
| Surface roughness (σ) | Rougher surfaces → need thicker film |
Viscosity and speed are the strongest levers. In many cases, a 50% jump in viscosity can reach twice the speed of h _ min. That's why bearing failures often come from the wrong oil or sudden temperature drops (which thins oil).
Temperature is a big issue. Oil is thinner when it's hot. The bearing is intact at 40°C and at 80 ° C the h_min drops below safe level. So when checking the thickness of the film, be sure to pay attention to the temperature.
Typical Values and what "skinny" really means
For ordinary industrial plane axial film bearings with a diameter of 50 mm, a clearance of 0.1 mm, a speed of 1,500 RPM speed and a moderate load, the minimum film thickness is usually between 5 and 20 microns.
Human hair is about 70 microns thick. So the film is thinner than hair. Usually just a fraction of the width of hair.
But this layer can carry tons of load. This is how hydrodynamic lubrication works.
Common mistakes in practice
Forget temperature. Many designers calculate h_min at room temperature. But bearings get hot when they run. The film thickness at the actual operating temperature is 30–50% lower than the cold volume.
Use nominal headroom instead of actual headroom. Tolerance, heat expansion and wear all change the actual gap over time. If you design a Plain Journal Fluid Film Bearing with a nominal number, you have no margin.
Ignore surface polish. Axis with Ra = 0.8 μm and bearings with Ra = 0.4 μm require at least 3.6 micron h _ min to avoid bump contact. If your math gives you 3.0 μm, you're already in trouble.
These ideas are repeated in textbooks on tribology and machine design textbooks, including those used in university courses and published by major engineering societies. A proper Plain Journal Fluid Film Bearing design relies on understanding these principles.