BIZOMAT MAKEOS · Fasteners
Set-Screw Holding Power and Torque
How much power and torque can set screws hold on this shaft, and how many do I need?
Safe power and torque a cup-point set screw can transmit without slipping by the handbook’s empirical formulas, the corresponding holding force at the shaft surface, and the number of set screws needed for a required power.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Shaft diameter | mm | Diameter of the shaft. |
| Set-screw diameter | mm | Nominal diameter of the set screw. |
| Shaft speed | rpm | Shaft speed. |
| Power to transmit | kW | Power the joint must transmit. |
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: Shaft diameter
- What it is
- The diameter of the shaft the set screw bears on, which the empirical rule uses for the holding power and torque.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25.4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Power and torque per screw are directly proportional to the shaft diameter.
- Get it right
- Use the actual shaft diameter at the screw, not the hub bore or outside diameter.
What this means: Set-screw diameter
- What it is
- The nominal diameter of the set screw, the size of its thread.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12.7 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Power, torque and holding force rise with the 2.3 power of the screw diameter.
- Get it right
- Use the nominal thread size, not the point or cup diameter.
What this means: Shaft speed
- What it is
- The rotational speed of the shaft, which turns the holding torque into the power each screw can transmit.
- 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 1000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Power per screw is directly proportional to speed.
- Get it right
- Use the speed of the shaft carrying the set screw; the torque per screw does not depend on it.
What this means: Power to transmit
- What it is
- The power the hub-to-shaft joint must transmit, divided by the power per screw to find how many set screws are needed.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.237 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The number of screws needed rises in steps with the power.
- Get it right
- Use 0 to skip the count. More than two screws draws a warning to use a key instead.
Method and formulas
- Power per screw P = D N d^2.3 ÷ 50 hp, and torque T = 1250 D d^2.3 lb-in, with D and d in inches (converted here).
- Holding force at the shaft surface = T ÷ (D ÷ 2) ≈ 2500 d^2.3 lb (about 500 lb for a ½-in. screw, as the handbook lists).
- Handbook example: a ½-in. screw on a 1-in. shaft at 1000 rpm holds 4.1 hp.
- Example: a 12.7 mm set screw on a 25.4 mm shaft at 1000 rpm holds about 4.0613 hp.
Limitations
- Cup-point set screws seated on a plain shaft; varies with hardness of screw and shaft.
- Vibration and reversing loads loosen set screws: use a key for anything beyond light duty.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a set-screwed pulley, collar or gear on a shaft
- Decide whether to add a second set screw or a key
- Choose set-screw size for a light drive
Who it is for
- Design engineers
- Maintenance fitters
- Machine builders
What you get
- Power and torque per set screw
- Holding force at the shaft surface
- Set screws required
Interpreting the result
- Two screws at 90° hold more securely than one; a flat on the shaft also helps.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1699–1700. Topic reference; no source tables or recommended settings are reproduced.
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