BIZOMAT MAKEOS · Power Transmission
Gearbox Oil Temperature and Grade Check (Heat Balance, Viscosity Hot)
At what temperature will my gearbox oil settle, does it need a cooler, and is the oil grade still thick enough at that temperature?
Check of a gearbox’s steady oil temperature and its lubricant: heat generated from the power and efficiency, the equilibrium oil temperature from the housing’s dissipation, the thermal power limit, cooler duty and oil flow if needed (as the gearbox heat balance tool), and the oil’s viscosity at the running temperature (the equilibrium temperature, or the limit if a cooler holds it there) from its viscosity at 40 °C and 100 °C (as the oil viscosity–temperature tool), against the least viscosity the gears need.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Input power | kW | Power going into the gearbox. |
| Gearbox efficiency | % | Overall efficiency of the gearbox. |
| Heat-dissipating surface area | m² | Outside surface area of the housing that sheds heat. |
| Heat transfer coefficient | W/m²·K | Combined convection and radiation coefficient of the housing (higher with a shaft fan). |
| Ambient temperature | °C | Air temperature around the gearbox. |
| Maximum oil temperature | °C | Highest oil temperature you allow. |
| Oil temperature drop across a cooler | K | Temperature drop of oil through a cooler, if one is needed. |
| Oil specific heat | kJ/kg·K | Specific heat of the oil. |
| Oil density | kg/m³ | Density of the oil. |
| Oil grade | ||
| Viscosity at 40 °C | cSt | Kinematic viscosity of the oil at 40 °C (from the data sheet). |
| Viscosity at 100 °C | cSt | Kinematic viscosity at 100 °C. |
| Least viscosity the gears need at running temperature | cSt | Least viscosity the gear maker requires at the running temperature. |
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: Input power
- What it is
- The mechanical power entering the gearbox input shaft under the steady running load being checked.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Heat generated is proportional to input power.
- Get it right
- Use the actual running power, not the motor nameplate, if the drive runs below rating; heat follows real power.
What this means: Gearbox efficiency
- What it is
- The share of input power that leaves through the output shaft; the remainder becomes heat in the oil and housing.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% to 99.9%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 92% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher efficiency lowers the heat generated and the oil temperature.
- Get it right
- Type a percentage, not a fraction; small errors matter because the heat follows 100 minus the efficiency.
What this means: Heat-dissipating surface area
- What it is
- The external surface area of the gearbox housing exposed to air that sheds heat, from the maker’s data or measured from the housing.
- Units and format
- Enter a number in m²: square metres. The form accepts 0.000001 m² to 10000 m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The oil temperature rise above ambient falls in inverse proportion to the area.
What this means: Heat transfer coefficient
- What it is
- The combined rate at which the housing surface loses heat to the air by convection and radiation, per square metre and per kelvin of temperature difference.
- Units and format
- Enter a number in W/m²·K: watts per square metre per kelvin. The form accepts 0.000001 W/m²·K to 500 W/m²·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 W/m²·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The oil temperature rise above ambient falls in inverse proportion to this coefficient.
- Get it right
- Take it from the gearbox maker or a heat transfer reference for your mounting and air flow; a shaft fan changes it greatly.
What this means: Ambient temperature
- What it is
- The air temperature around the gearbox in service, including heat from nearby equipment or sun.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -40 °C to 80 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Maximum oil temperature
- What it is
- The highest steady oil temperature you allow; above it the tool sizes a cooler and runs the viscosity check at this temperature.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 20 °C to 150 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Oil temperature drop across a cooler
- What it is
- The temperature drop of the oil as it passes through an oil cooler, used only to size the oil flow when a cooler is needed.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0.000001 K to 50 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It affects only the cooler oil flow; the oil temperature and viscosity results do not depend on it.
What this means: Oil specific heat
- What it is
- The specific heat capacity of the oil, from the oil data sheet; used only to work out the oil flow through a cooler.
- Units and format
- Enter a number in kJ/kg·K: kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/kg·K to 5 kJ/kg·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.9 kJ/kg·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter kJ/kg·K, not J/kg·K: divide a value in joules by 1000.
What this means: Oil density
- What it is
- The density of the oil, from the oil data sheet; used only to convert the cooler heat duty into an oil flow rate.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 1500 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.
Oil grade
What this means: Viscosity at 40 °C
- What it is
- The kinematic viscosity of the oil at 40 °C, printed on the oil data sheet; the ISO VG grade is the nominal value at this temperature.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 1 cSt to 100000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 220 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 or mPa·s.
What this means: Viscosity at 100 °C
- What it is
- The kinematic viscosity of the same oil at 100 °C, from its data sheet; with the 40 °C value it fixes how viscosity changes with temperature.
- 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 19 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be lower than the 40 °C value, and both values must come from the same oil.
What this means: Least viscosity the gears need at running tempe…
- What it is
- The least kinematic viscosity the gear or gearbox maker requires at the running temperature to keep a lubricating film on the teeth.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 1 cSt to 100000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Heat = P (1 − η); equilibrium temperature = ambient + heat ÷ (h A); thermal limit, cooler duty and flow (as the gearbox heat balance tool).
- Running temperature = equilibrium temperature, or the maximum if a cooler is fitted to hold it there.
- Viscosity at the running temperature by the ASTM D341 relation through the 40 °C and 100 °C values (as the oil viscosity–temperature tool).
- Example: the oil settles at 70 °C where the oil is 51.702 cSt against 50 cSt needed.
Limitations
- Steady state; start-up and intermittent duty run cooler.
- Single housing coefficient; sun, enclosures and nearby heat sources change it.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a gearbox for a higher power or hotter site
- Choose an oil grade for the running temperature
- Decide whether a gearbox needs a cooler
Who it is for
- Maintenance engineers
- Lubrication engineers
- Design engineers
What you get
- Heat generated and equilibrium temperature
- Thermal limit, cooler duty and oil flow
- Viscosity at the running temperature
Interpreting the result
- A shaft-mounted fan can double the housing coefficient, often removing the need for a cooler.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3339–3340, 2430–2436, 2329–2337. Topic reference; no source tables or recommended settings are reproduced.
- ASTM D341 Viscosity–Temperature Charts for Liquid Petroleum Products
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Open the Gearbox Oil Temperature and Grade Check (Heat Balance, Viscosity Hot) calculator