BIZOMAT MAKEOS · Maintenance
Lubricant Film Thickness Ratio (Lambda)
Is the lubricant film thick enough to separate these rolling surfaces?
Film thickness ratio lambda of a rolling bearing or gear contact: the working film thickness divided by the composite roughness of the two surfaces, from RMS or average roughness, with the lubrication regime it indicates.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Working film thickness | µm | Minimum film thickness at the operating temperature (from an EHL calculation or the bearing maker’s data). |
| Roughness entered as | choice | Whether the roughness values are RMS or arithmetic average. |
| Roughness of surface 1 | µm | Roughness of the first surface (for example the raceway). |
| Roughness of surface 2 | µm | Roughness of the second surface (for example the rolling element). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Working film thickness
- What it is
- The minimum lubricant film separating the two surfaces in the loaded contact at operating temperature, from an elastohydrodynamic (EHL) calculation or the bearing or gear maker’s data.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0.000001 µm to 100 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.25 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Lambda rises in proportion.
What this means: Roughness entered as
- What it is
- Whether the roughness values are root-mean-square (Rq) or arithmetic average (Ra). Ra values are converted to RMS by the tool before they are combined.
- Units and format
- Choose one of 2 options from the list: RMS (Rq); Average (Ra).
- Example
- The form opens with “RMS (Rq)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Both surfaces must be given on the same basis.
What this means: Roughness of surface 1
- What it is
- The surface roughness of the first contacting surface, for example the bearing raceway or one gear flank.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 50 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.06 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the roughness type selected above; a profilometer report usually states both Ra and Rq.
What this means: Roughness of surface 2
- What it is
- The surface roughness of the second contacting surface, for example the rolling element or the mating gear flank.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 50 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.04 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Lambda falls as either roughness rises, since the two combine as the square root of the sum of their squares.
Method and formulas
- λ = h ÷ √(R_q1² + R_q2²).
- Where only Ra is known, R_q ≈ 1.25 Ra for typical ground surfaces.
- Commonly λ < 1 indicates boundary lubrication, 1–3 mixed, and above about 3 full separation.
- Example: a 0.25 µm film on surfaces of 0.06 and 0.04 µm gives λ = 3.4669.
Limitations
- Film thickness must be calculated or supplied; this tool does not compute EHL film thickness.
- Contamination and debris dents defeat a good λ.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check whether a bearing runs in full-film lubrication
- Judge the effect of a different oil viscosity
- Explain short bearing life from surface distress
Who it is for
- Reliability engineers
- Lubrication engineers
- Maintenance engineers
What you get
- Composite roughness
- Film thickness ratio λ
- Lubrication regime
Interpreting the result
- Raising viscosity at the operating temperature or improving surface finish increases λ and bearing life.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2420–2422. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Useful alongside · BIZOMAT original book
Starts, stops and bearing materials
In Plain Bearings by the Numbers
What it covers: Estimate when a journal bearing's film forms and fails as it starts and stops, judge the regime with the film thickness ratio, and choose between white metal, bronze, aluminium and polymer linings.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.