BIZOMAT MAKEOS · Maintenance
Journal Bearing Oil Check (Viscosity at Temperature, Film and Friction)
At its running temperature, how thick is this oil, does the journal bearing keep a safe film with it, and how much heat does the oil friction make?
Check of a plain journal bearing with a chosen oil grade: kinematic viscosity at the running temperature from the oil’s two catalogue viscosities by the ASTM D341 relation (as the oil viscosity–temperature tool), dynamic viscosity from the density, then the unit load, Sommerfeld number, Petroff friction torque and power loss (as the Petroff tool), with checks of viscosity against your minimum and of the Sommerfeld number against your limit.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Oil | ||
| First catalogue temperature | °C | Temperature of the first catalogue viscosity (usually 40 °C). |
| Viscosity at that temperature | cSt | Kinematic viscosity at that temperature (the ISO VG number at 40 °C). |
| Second catalogue temperature | °C | Temperature of the second catalogue viscosity (usually 100 °C). |
| Viscosity at that temperature | cSt | Kinematic viscosity at that temperature. |
| Running oil temperature | °C | Oil temperature in the bearing film when running. |
| Oil density at running temperature | kg/m³ | Density of the oil at the running temperature. |
| Least viscosity accepted | cSt | Least viscosity you accept for the bearing (from the bearing maker or your practice). |
| Bearing | ||
| Radial load | N | Radial load on the bearing. |
| Journal diameter | mm | Journal diameter. |
| Bearing length | mm | Bearing length. |
| Radial clearance | mm | Radial clearance. |
| Speed | rpm | Shaft speed. |
| Smallest Sommerfeld number | Smallest Sommerfeld number you accept for a safe film. | |
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.
Oil
What this means: First catalogue temperature
- What it is
- The temperature at which the oil supplier quotes the first kinematic viscosity on the product data sheet.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must differ from the second catalogue temperature.
What this means: Viscosity at that temperature
- What it is
- The kinematic viscosity the oil data sheet gives at the first catalogue temperature.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.5 cSt to 100000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 46 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Kinematic viscosity in cSt (mm²/s), not dynamic viscosity in mPa·s.
What this means: Second catalogue temperature
- What it is
- The temperature of the second viscosity on the oil data sheet, needed to fit the viscosity–temperature line.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Viscosity at that temperature
- What it is
- The kinematic viscosity the data sheet gives at the second catalogue temperature.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.5 cSt to 100000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6.8 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Read both viscosities from the same oil grade and data sheet.
What this means: Running oil temperature
- What it is
- The temperature of the oil in the loaded film while running, which is normally higher than the sump or inlet temperature.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A hotter film thins the oil, lowering the Sommerfeld number and the friction loss.
- Get it right
- If it lies outside the two catalogue temperatures, the viscosity is extrapolated and less certain.
What this means: Oil density at running temperature
- What it is
- The oil density at the running temperature, used to convert kinematic viscosity into dynamic viscosity.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 600 kg/m³ to 1200 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 870 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Least viscosity accepted
- What it is
- The lowest kinematic viscosity you accept at running temperature, from the bearing maker or your lubrication practice.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.5 cSt to 10000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 13 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Bearing
What this means: Radial load
- What it is
- The radial load the journal presses onto the bearing. Divided by length times diameter it gives the unit load.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 100000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher load lowers the Sommerfeld number.
What this means: Journal diameter
- What it is
- The diameter of the shaft journal running in the bearing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Bearing length
- What it is
- The axial length of the bearing surface that carries the load.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Radial clearance
- What it is
- The radial clearance between journal and bearing: half the difference between the bearing bore and the journal diameter.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.04 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The Sommerfeld number varies inversely with the square of the clearance.
- Get it right
- Radial, not diametral, clearance: halve the bore minus journal difference.
What this means: Speed
- What it is
- The rotational speed of the journal relative to the bearing.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sommerfeld number and friction torque rise in proportion; power loss rises with the square of speed.
What this means: Smallest Sommerfeld number
- What it is
- The lowest Sommerfeld number you accept as giving a safe oil film, from your design practice or bearing data.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.05 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- log log(ν + 0.7) = A − B log T (T in K) through the two catalogue points (as the oil viscosity–temperature tool).
- Dynamic viscosity μ (mPa·s) = ν (cSt) × ρ ÷ 1000.
- S = (r ÷ c)² μ N ÷ p; Petroff torque = 4π² r³ L μ N ÷ c; power loss = 2π N T (as the Petroff tool).
- Example: the oil thins to 20.623 cSt (17.942 mPa·s) at 60 °C, giving S = 0.087606 and 54.327 W of friction.
Limitations
- Viscosity relation is reliable between and slightly beyond the catalogue points.
- Petroff friction for a lightly loaded concentric journal.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose an oil grade for a plain bearing
- Check a bearing after its oil temperature rises
- Explain bearing distress after an oil change
Who it is for
- Maintenance engineers
- Lubrication engineers
- Design engineers
What you get
- Kinematic and dynamic viscosity at temperature
- Sommerfeld number and friction
- Viscosity and film checks
Interpreting the result
- Viscosity roughly halves for every 15–20 K rise in a mineral oil: cooling the oil is often better than a thicker grade.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2326–2345. Topic reference; no source tables or recommended settings are reproduced.
- ASTM D341 Viscosity–Temperature Charts for Liquid Petroleum Products
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Oil temperature and the heat balance
In Plain Bearings by the Numbers
What it covers: Find a journal bearing's running temperature where the heat its film makes equals the heat its housing sheds, check the oil there with the BIZOMAT tool, and compare grades.
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.
Open the Journal Bearing Oil Check (Viscosity at Temperature, Film and Friction) calculator