BIZOMAT MAKEOS · Power Transmission
Bearing and Gear Losses with Circulating Oil Flow
How much heat do these bearings and gears make, and how much circulating oil carries it away?
Heat from rolling bearings by the friction-coefficient method (torque = μ P d ÷ 2, as the bearing friction tool) plus other losses such as gear mesh or seals, and the circulating oil flow to carry that heat away at an allowed oil temperature rise (as the coolant heat-flow tool with oil properties).
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of bearings | Number of bearings on the circuit. | |
| Equivalent load per bearing | N | Equivalent load on each bearing. |
| Bearing bore | mm | Bore diameter of the bearings. |
| Bearing friction coefficient | Friction coefficient referred to the bore (about 0.001 cylindrical roller, 0.0015 deep groove ball, 0.002 tapered and spherical roller). | |
| Speed | rpm | Shaft speed. |
| Other losses (gear mesh, seals, churning) | W | Other heat going into the oil. |
| Allowed oil temperature rise | K | Temperature rise of the oil through the machine. |
| Oil specific heat | kJ/kg·K | Specific heat of the oil. |
| Oil density | kg/m³ | Oil density. |
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: Number of bearings
- What it is
- How many rolling bearings on the oil circuit run under the same load, bore and speed as entered below.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 100. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bearing power loss is directly proportional to the number of bearings.
- Get it right
- If the bearings differ in size or load, run the tool for each group and add the heat.
What this means: Equivalent load per bearing
- What it is
- The equivalent load on one bearing, combining its radial and axial loads into a single value as in the bearing maker’s method.
- 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
- Friction torque and bearing heat rise in direct proportion to load.
- Get it right
- Enter the load per bearing, not the total load on the shaft.
What this means: Bearing bore
- What it is
- The bore diameter of each bearing, which the friction coefficient is referred to in the torque formula.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 3000 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.
- Effect on the result
- Friction torque is directly proportional to the bore.
- Get it right
- Use the bore, not the outside or mean diameter, because the friction coefficient is defined at the bore.
What this means: Bearing friction coefficient
- What it is
- A constant friction coefficient for the bearing type, referred to the bore, that turns load into friction torque.
- Units and format
- Enter a plain number with no unit. The form accepts 0.0005 to 0.02. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.0015 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bearing heat is directly proportional to the coefficient.
- Get it right
- Take it from the bearing maker for the bearing type; it varies with speed, load and lubrication in practice.
What this means: Speed
- What it is
- The rotational speed of the shaft and bearings in steady running, which turns friction torque into heat.
- 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
- Bearing heat is directly proportional to speed.
- Get it right
- Use the speed of the shaft these bearings sit on, not the motor speed if there is gearing in between.
What this means: Other losses (gear mesh, seals, churning)
- What it is
- Heat going into the same oil from sources other than the bearings: gear mesh losses, seal friction and oil churning.
- Units and format
- Enter a number in W: watts. The form accepts 0 W to 10000000 W. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 W as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to the total heat and the oil flow needed.
- Get it right
- Enter heat in watts, not the power transmitted; it is usually a small share of that power.
What this means: Allowed oil temperature rise
- What it is
- How much the oil is allowed to warm between entering and leaving the machine. A larger rise means less flow carries the same heat.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0.000001 K to 60 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Oil flow is inversely proportional to the allowed rise.
- Get it right
- This is the rise through the machine, not the oil’s working temperature.
What this means: Oil specific heat
- What it is
- The heat needed to warm one kilogram of the oil by one kelvin, from the lubricant data sheet.
- 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.
- Effect on the result
- Oil flow is inversely proportional to specific heat.
- Get it right
- Check the unit is kJ per kg·K, not J per kg·K, which would be a thousand times larger.
What this means: Oil density
- What it is
- The mass of one cubic metre of the oil at its working temperature, from the lubricant data sheet.
- 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.
- Effect on the result
- Oil flow is inversely proportional to density.
- Get it right
- Enter kg/m³, not kg/L or g/cm³, which are a thousand times smaller.
Method and formulas
- Friction torque per bearing M = 0.5 μ P d.
- Bearing loss = M ω × number of bearings; total heat = bearings + other losses.
- Oil flow Q = heat ÷ (ρ c_p ΔT).
- Example: the bearings lose 58.905 W; with other losses the oil needs 0.93976 L/min at a 10 K rise.
Limitations
- Constant friction coefficient; at high speed and light load, lubricant churning dominates.
- Oil flow for heat only; bearings also need a minimum flow for lubrication.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a circulating oil system for a gearbox or spindle
- Estimate bearing heat for a thermal check
- Check whether splash lubrication is enough
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Bearing friction torque and power loss
- Total heat
- Oil flow required
Interpreting the result
- If the flow is very low, splash or grease lubrication may carry the heat without a circulating system.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2365–2440. Topic reference; no source tables or recommended settings are reproduced.
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