BIZOMAT MAKEOS · Fasteners
Set-Screw Hub Retention (Torque, Axial Hold, Screws Needed)
Will set screws hold this hub on its shaft for the power and axial load, how many are needed, and when should I use a key instead?
Check of a hub, collar or pulley held by cup-point set screws: power, torque and holding force per screw by the handbook empirical rule (as the set-screw holding tool), the torque needed with your safety factor, the number of screws needed, the torque and axial capacity of the screws you fit, and the advice to change to a key or clamp collar when more than two screws would be needed.
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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 screws. |
| Shaft speed | rpm | Speed of the shaft. |
| Power transmitted | kW | Power passing through the hub. |
| Safety factor | Safety factor on the holding torque and axial force (higher for shock and reversing loads). | |
| Set screws fitted | Number of set screws fitted (usually one or two, about 90° apart). | |
| Axial load on the hub | N | Axial force trying to slide the hub along 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: Shaft diameter
- What it is
- The diameter of the shaft the hub, collar or pulley is fixed to.
- 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
- Holding torque per screw rises in direct proportion to the shaft diameter.
What this means: Set-screw diameter
- What it is
- The nominal diameter of the cup-point set screws.
- 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
- Holding torque per screw rises with the screw diameter to the power 2.3.
What this means: Shaft speed
- What it is
- The rotational speed of the shaft, used to turn the transmitted power into torque.
- 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
- For the same power, a faster shaft needs less holding torque.
What this means: Power transmitted
- What it is
- The power passing through the hub between the shaft and the pulley, gear or collar.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 kW to 10000 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.
What this means: Safety factor
- What it is
- The safety factor applied to both the torque and the axial force the screws must hold, set higher for shock and reversing loads.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Set screws fitted
- What it is
- The number of set screws fitted in the hub.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 8. 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
- Torque and axial capacity rise in direct proportion.
What this means: Axial load on the hub
- What it is
- The axial force trying to slide the hub along the shaft, for example from helical gears or belt misalignment.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 10000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 if nothing pushes the hub along the shaft.
Method and formulas
- Handbook rule (inch units): holding power hp = D N d^2.3 ÷ 50; torque = 1250 D d^2.3 lb·in, converted to metric (as the set-screw holding tool).
- Torque needed = P ÷ ω × SF; screws needed = torque needed ÷ torque per screw (rounded up).
- Axial capacity ≈ holding force per screw × screws fitted, against axial load × SF.
- Example: each M12.7 screw holds 28.679 N·m; 2 screws give 57.358 N·m against 32.043 N·m needed.
Limitations
- Empirical rule for cup-point screws on a hardened screw and unhardened shaft; flats or spot drills change the hold.
- Vibration loosens set screws: use thread locker or a key for critical drives.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a set-screw hub for a motor or pulley
- Decide between set screws and a key
- Check axial hold of a collar
Who it is for
- Design engineers
- Maintenance engineers
- Machine designers
What you get
- Holding per screw (power, torque, force)
- Screws needed and capacity of those fitted
- Torque and axial checks
Interpreting the result
- Two screws at 90° hold better than two at 180°, and a flat on the shaft stops marking.
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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Open the Set-Screw Hub Retention (Torque, Axial Hold, Screws Needed) calculator