BIZOMAT MAKEOS · Maintenance
Lubricant Viscosity Check (Operating and Cold-Start Viscosity)
Is this oil grade thick enough at the operating temperature and thin enough to circulate at a cold start?
Check of an oil grade for a bearing, gearbox or hydraulic system: viscosity at the operating temperature and at the cold-start temperature from the oil’s viscosity at two temperatures by the ASTM D341 chart relation (as the oil viscosity–temperature tool), the same values in dynamic viscosity and Saybolt units (as the viscosity units tool), and the ratio of the operating viscosity to the minimum your machine needs, against your cold-start limit.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| First data temperature | °C | Temperature of the first viscosity value on the oil data sheet (usually 40 °C). |
| Viscosity at the first temperature | cSt | Kinematic viscosity at the first temperature. |
| Second data temperature | °C | Temperature of the second value (usually 100 °C). |
| Viscosity at the second temperature | cSt | Kinematic viscosity at the second temperature. |
| Oil specific gravity | Specific gravity of the oil (for dynamic viscosity). | |
| Machine | ||
| Operating temperature | °C | Steady operating temperature of the oil in the machine. |
| Least viscosity needed at operating temperature | cSt | Least viscosity the bearings, gears or pump need (from the component maker or a film thickness calculation). |
| Cold-start temperature | °C | Lowest temperature at which the machine starts. |
| Largest viscosity for a cold start | cSt | Highest viscosity the pump or system can start with (from the pump or system maker). |
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: First data temperature
- What it is
- The temperature at which the oil data sheet gives its first kinematic viscosity value.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -60 °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 data temperature.
What this means: Viscosity at the first temperature
- What it is
- The kinematic viscosity of the oil at the first data temperature, from the oil data sheet.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.5 cSt to 1000000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 68 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use kinematic viscosity in cSt (mm²/s), not dynamic viscosity in cP.
What this means: Second data temperature
- What it is
- The temperature at which the oil data sheet gives its second kinematic viscosity value.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -60 °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 the second temperature
- What it is
- The kinematic viscosity of the oil at the second data temperature, from the oil data sheet.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.5 cSt to 1000000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8.6 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use kinematic viscosity in cSt (mm²/s), not dynamic viscosity in cP.
What this means: Oil specific gravity
- What it is
- The density of the oil compared with water, from the data sheet. It is used only to convert the results to dynamic viscosity.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.88 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It does not affect the pass or fail checks.
Machine
What this means: Operating temperature
- What it is
- The steady temperature of the oil in the machine when it is running at normal load.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -60 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70 °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 oil is thinner, so the viscosity ratio falls.
- Get it right
- Measure the oil in the bearing or sump, not the room temperature.
What this means: Least viscosity needed at operating temperature
- What it is
- The least kinematic viscosity the bearings, gears or pump need at operating temperature, from the component maker or a film thickness calculation.
- 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 15 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Cold-start temperature
- What it is
- The lowest oil temperature at which the machine has to start.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -60 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A colder start gives a thicker oil and a higher start-up viscosity.
What this means: Largest viscosity for a cold start
- What it is
- The highest kinematic viscosity at which the pump or system can still start and circulate the oil, from the pump or system maker.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 1 cSt to 1000000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 cSt 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 kelvin) through the two data points (as the oil viscosity–temperature tool).
- Dynamic viscosity = ν × specific gravity; SUS ≈ 4.632 ν (as the viscosity units tool).
- Viscosity ratio = ν at operating ÷ least needed; cold-start viscosity against the limit.
- Example: at 70 °C the oil is 19.966 cSt (ratio 1.3311); at 5 °C it is 678.2 cSt.
Limitations
- Straight-grade mineral and synthetic oils; viscosity-modified oils may shear thin.
- Extrapolating far beyond the two data temperatures loses accuracy.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check an oil grade for a machine running hotter than planned
- Choose a grade for a cold climate
- Compare two grades for a gearbox
Who it is for
- Maintenance engineers
- Lubrication technicians
- Reliability engineers
What you get
- Viscosity at operating and cold-start temperatures
- Dynamic and Saybolt equivalents
- Viscosity ratio and checks
Interpreting the result
- If neither limit can be met by one grade, the machine needs oil heating, cooling or a higher-index oil.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2329–2337, 2425–2426. 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
Choosing a viscosity grade
What it covers: Use ISO viscosity grades, a machine maker's viscosity window and the machine's temperature range to choose an oil grade, check it with the BIZOMAT tool, and avoid a heavier-is-safer choice.
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 Lubricant Viscosity Check (Operating and Cold-Start Viscosity) calculator