BIZOMAT MAKEOS · Fasteners
Taper Bush Grip Check (Screw Preload, Wedge Normal Force, Torque Capacity)
For a split taper bush pulled into a hub by its screws, what grip does it put on the shaft, and will it carry the drive torque without slipping?
Check of a taper-lock (split taper) bush or keyless taper clamp: the preload of each tightening screw from its torque and nut factor and its share of proof load (as the bolt torque and preload tool), the total axial force, the normal force on the taper from the wedge action with friction on the taper face, and the friction torque on the shaft at the bore, against the drive torque from power and speed with a service factor.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of tightening screws | screws | Number of screws that pull the bush into the hub. |
| Screw nominal diameter | mm | Screw diameter. |
| Screw pitch | mm | Screw pitch. |
| Screw tightening torque | N·m | Torque applied to each screw. |
| Nut factor K | Nut factor for the screw (lubricated or dry). | |
| Screw proof strength | MPa | Proof strength of the screws. |
| Taper and shaft | ||
| Taper half angle | ° | Half angle of the taper (taper bushes are commonly about 4°–8°). |
| Friction on the taper face | Friction between the bush and hub tapers. | |
| Shaft diameter | mm | Diameter of the shaft. |
| Friction between bush and shaft | Friction between the bush bore and the shaft (clean, dry surfaces). | |
| Duty | ||
| Power | kW | Power transmitted. |
| Speed | rpm | Shaft speed. |
| Service factor | Factor for starting and shock torque. | |
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 tightening screws
- What it is
- The number of screws that pull the split taper bush into the hub, all tightened to the same torque.
- Units and format
- Enter a number in screws: screws. The form accepts 1 screws to 24 screws. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 screws as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Axial force, normal force and grip torque rise in proportion.
- Get it right
- Count only the screws used for tightening, not the extra tapped holes used to jack the bush out.
What this means: Screw nominal diameter
- What it is
- The nominal (major) diameter of each tightening screw thread, used for the nut factor relation and the stress area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For a given torque a larger diameter gives less preload and grip.
- Get it right
- Use the nominal size, for example the 10 of M10, not the head size or hexagon key size.
What this means: Screw pitch
- What it is
- The thread pitch of the tightening screws, used with the nominal diameter for the tensile stress area and the share of proof load.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It changes only the share of proof load, not the grip torque.
- Get it right
- Check the thread designation; coarse and fine threads of the same size have different pitches.
What this means: Screw tightening torque
- What it is
- The tightening torque applied to each screw with a torque wrench, which the nut factor converts to screw preload.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 100000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Preload, axial force and grip torque rise in proportion.
- Get it right
- This is the torque per screw from the bush maker’s instructions, not a total for all screws.
What this means: Nut factor K
- What it is
- The nut factor that links tightening torque to screw preload for the actual thread finish and lubrication.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher nut factor gives less preload and grip for the same torque.
- Get it right
- Use the factor for the actual lubrication state of the screws from the supplier or a test; oiled and dry screws differ.
What this means: Screw proof strength
- What it is
- The proof strength of the tightening screws, which with the stress area gives the proof load used to judge the preload.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 830 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It changes only the share of proof load, not the grip torque.
- Get it right
- Take it from the screw property class standard or the supplier, not the tensile strength.
Taper and shaft
What this means: Taper half angle
- What it is
- The half angle of the taper between the bush and the hub: the angle of one taper face to the shaft axis, half of the included angle.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 1° to 30°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A smaller angle wedges harder, so the normal force and grip torque rise.
- Get it right
- Enter the half angle, not the included angle; take it from the bush maker’s drawing.
What this means: Friction on the taper face
- What it is
- The friction coefficient on the conical face between the outside of the bush and the tapered hub bore, which resists the wedge action.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher taper friction lowers the normal force and the grip torque.
- Get it right
- Use a value for the actual surface condition from the maker or a test; this is the taper face, not the shaft bore.
What this means: Shaft diameter
- What it is
- The diameter of the shaft the bush grips, which sets the lever arm of the friction at the bush bore.
- 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 35 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The grip torque rises in proportion to the shaft diameter.
- Get it right
- Use the shaft diameter, not the hub or bush outside diameter.
What this means: Friction between bush and shaft
- What it is
- The friction coefficient between the bore of the bush and the shaft surface, which turns the normal force into grip torque.
- Units and format
- Enter a plain number with no unit. The form accepts 0.02 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Grip torque rises in proportion to it.
- Get it right
- Use a value for clean, oil-free surfaces only if they will really be clean; oil or grease greatly reduces it.
Duty
What this means: Power
- What it is
- The power transmitted through the bush and shaft, used with the speed to find the drive torque the grip must exceed.
- 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 5.5 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The required drive torque rises in proportion.
- Get it right
- Enter kilowatts; use the power actually transmitted, or the motor rating for the worst case.
What this means: Speed
- What it is
- The rotational speed of the shaft carrying the bush, used with the power to find the drive 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 1450 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A slower shaft carries more torque for the same power, so the required grip rises.
- Get it right
- Use the speed of this shaft, not the motor speed if there is a reduction between them.
What this means: Service factor
- What it is
- A multiplier on the steady drive torque to cover starting torque, shocks and load variation of the driven machine.
- 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 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The required drive torque rises in proportion.
- Get it right
- Take it from the coupling, belt or bush maker’s service factor tables for your machine and duty.
Method and formulas
- Screw preload = T ÷ (K d) (as the bolt torque and preload tool); axial force = screws × preload.
- Wedge with friction on the taper: normal force = axial ÷ (sin α + μ_t cos α).
- Grip torque = μ_s × normal force × shaft radius; drive torque = 9549 P ÷ n × service factor.
- Example: 20 kN axial gives 69.514 kN of grip; 182.47 N·m capacity against 72.443 N·m.
Limitations
- Normal force taken as acting fully on the shaft; bush stiffness and the hub fit take some of it.
- Clean, oil-free surfaces; oil or grease on the bore greatly reduces the grip.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a taper bush pulley on a motor shaft
- Find the screw torque for a keyless taper clamp
- See why a taper bush slips
Who it is for
- Maintenance engineers
- Design engineers
- Millwrights
What you get
- Screw preload and axial force
- Normal force and grip torque
- Against the drive torque
Interpreting the result
- A smaller taper angle multiplies the grip from the same screws, which is why taper bushes use shallow tapers.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1535, 169–171. Topic reference; no source tables or recommended settings are reproduced.
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Open the Taper Bush Grip Check (Screw Preload, Wedge Normal Force, Torque Capacity) calculator