BIZOMAT MAKEOS · Power Transmission
Overhung (Radial) Load on a Gearbox or Motor Shaft
What radial load does this pulley, sprocket or gear put on the gearbox shaft, and is it within the allowable at its position?
Overhung load check: shaft torque from power and speed, overhung load from the drive element’s pitch diameter and its tension factor (chain, gear, V-belt or flat belt), the catalogue allowable corrected for where the load actually sits on the shaft, and the resulting load on the outer bearing from the lever over the bearing span.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Power at the shaft | kW | Power transmitted through the shaft. |
| Shaft speed | rpm | Speed of the shaft. |
| Drive element | choice | Type of element on the shaft; the factor allows for belt or chain tension above the transmitted pull. |
| Pitch diameter of the element | mm | Pitch diameter of the sprocket, gear or pulley. |
| Catalogue allowable overhung load | kN | Allowable overhung load from the gearbox or motor catalogue. |
| Catalogue load position from the outer bearing | mm | Distance from the outer bearing to where the catalogue allowable applies (often mid-shaft extension). |
| Actual load position from the outer bearing | mm | Distance from the outer bearing to the centre of the hub actually fitted. |
| Distance between the shaft bearings | mm | Centre distance between the two bearings of the shaft. |
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: Power at the shaft
- What it is
- The power passing through the shaft that carries the pulley, sprocket or gear. With the shaft speed it gives the torque the drive element must pull.
- 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 7.5 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque and overhung load rise in direct proportion to power.
- Get it right
- For a gearbox output shaft use the power delivered there, not a larger motor nameplate figure, unless the motor can deliver it in service.
What this means: Shaft speed
- What it is
- The rotational speed of the shaft the element is mounted on. Lower speed means more torque for the same power, and so a larger pull on the shaft.
- 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 100 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Overhung load is inversely proportional to speed.
- Get it right
- On a gearbox output use the output speed, not the motor speed; mixing them gives a load many times too small.
What this means: Drive element
- What it is
- The kind of drive element on the shaft. Each carries a factor for how much more radial pull it puts on the shaft than the transmitted force alone, mainly from belt or chain tension.
- Units and format
- Choose one of 4 options from the list: Chain sprocket (factor 1.0); Spur or helical gear (factor 1.25); V-belt pulley (factor 1.5); Flat belt pulley (factor 2.5).
- Example
- The form opens with “Chain sprocket (factor 1.0)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- The overhung load is multiplied by the factor shown in the option.
- Get it right
- Choose the element actually fitted; tensioning practice can change the real pull, so use the maker’s factor where one is published.
What this means: Pitch diameter of the element
- What it is
- The pitch diameter of the sprocket, gear or pulley on the shaft: the diameter at which the chain, mating teeth or belt actually transmit force.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Overhung load is inversely proportional to it, so a larger element pulls less on the shaft.
- Get it right
- Use the pitch diameter, not the outside or hub diameter; for a V-belt pulley it is the datum diameter.
What this means: Catalogue allowable overhung load
- What it is
- The radial load the gearbox or motor maker allows on this shaft, as given in the catalogue at a stated load position along the shaft extension.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The allowable at the actual position is proportional to it.
- Get it right
- Check the catalogue for the speed and mounting this figure applies to, and note the load position it is quoted at.
What this means: Catalogue load position from the outer bearing
- What it is
- Where along the shaft extension the catalogue allowable applies, measured from the outer bearing. Catalogues often quote it at a set point such as the middle of the extension.
- 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 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The allowable at the actual position is proportional to this distance.
- Get it right
- Measure this and the actual position from the same reference, the outer bearing, so the ratio between them is right.
What this means: Actual load position from the outer bearing
- What it is
- The distance from the outer bearing to the centre of the hub of the element actually fitted, where its pull acts on the shaft.
- 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 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer distance lowers the allowable at that position and raises the outer bearing load.
- Get it right
- Measure to the centre line of the belt, chain or gear face, not to the end of the shaft or the hub face.
What this means: Distance between the shaft bearings
- What it is
- The centre-to-centre distance between the two bearings that support the shaft inside the gearbox or motor. The overhung load levers over this span.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer span lowers the outer bearing load; it does not change the overhung load itself.
- Get it right
- Take it from the maker’s drawing or by measurement between bearing centres, not between housing faces.
Method and formulas
- Torque T = P ÷ ω; overhung load = 2T ÷ D × factor.
- Allowable at the actual position ≈ catalogue allowable × L_c ÷ L_a (equal bending moment at the outer bearing).
- Outer bearing load = OHL × (S + L_a) ÷ S (lever over the bearing span).
- Example: 7.5 kW at 100 rpm on a 200 mm sprocket gives 7.162 kN against 7.5 kN allowed.
Limitations
- Simple lever correction; use the maker’s position factors where published.
- Factors are typical values for the element type; tensioning practice changes them.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a sprocket or pulley on a gearbox output shaft
- Decide whether to move the hub closer or add an outboard bearing
- Compare chain and belt drives for shaft load
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Torque and overhung load
- Allowable at the actual position
- Outer bearing load
Interpreting the result
- Mounting the hub tight against the shaft shoulder is often the cheapest way to cut bearing load.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2487–2556, 2365–2416. Topic reference; no source tables or recommended settings are reproduced.
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Open the Overhung (Radial) Load on a Gearbox or Motor Shaft calculator