BIZOMAT MAKEOS · Fasteners
Shear Pin Overload Design (Pin Size for a Break Torque, Break Factor, Hub Bearing)
For a shear pin that protects a drive by breaking at an overload, what diameter should it be, at what torque will the chosen pin break, and is the hub strong enough around it?
Design of a shear pin (breaking pin) through a hub and flange or sprocket: the break torque from the running torque and the overload factor, the pin diameter that shears at that torque from the pin ultimate shear strength, the number of pins and shear planes and the pin circle radius, the break torque and factor with the diameter you choose, and the shear and bearing stresses as a check (as the bolt shear and bearing tool at the break force), against your break factor range and allowable bearing stress.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Running torque | N·m | Torque the drive transmits in normal running. |
| Overload factor to break at | Multiple of the running torque at which the pin should break. | |
| Number of pins | pins | Number of shear pins sharing the torque. |
| Shear planes per pin | choice | Shear planes per pin (2 when the pin passes through a hub between two flanges). |
| Pin circle radius | mm | Radius from the shaft centre to the pin centre. |
| Pin ultimate shear strength | MPa | Ultimate shear strength of the pin material (test a sample batch; it varies widely). |
| Pin diameter chosen | mm | Diameter of the pin you will fit (at the shear plane, often a groove). |
| Limits | ||
| Least break factor accepted | Least break torque as a multiple of running torque (to avoid nuisance breaks). | |
| Largest break factor accepted | Largest break torque as a multiple of running torque (to protect the drive). | |
| Thinnest part in bearing | mm | Thickness of the thinnest hub or flange part the pin bears on. |
| Allowable bearing stress of the hub | MPa | Allowable bearing stress of the hub and flange at the break force. |
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: Running torque
- What it is
- The torque the drive transmits in normal running, which the shear pin must carry without breaking.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 100000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the steady running torque, not the motor’s starting or breakdown torque.
What this means: Overload factor to break at
- What it is
- The multiple of running torque at which you want the pin to shear and disconnect the drive. It sets the pin size needed.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10. 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 pin diameter needed grows with the square root of this factor.
- Get it right
- Choose it to sit inside your least and largest break factors.
What this means: Number of pins
- What it is
- How many shear pins share the torque at the same pin circle radius.
- Units and format
- Enter a number in pins: pins. The form accepts 1 pins to 24 pins. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 pins as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Several pins share well only if their holes are accurately spaced and fitted.
What this means: Shear planes per pin
- What it is
- How many places each pin is cut through when it shears. A pin through a hub between two flanges shears in two planes.
- Units and format
- Choose one of 2 options from the list: Single shear; Double shear.
- Example
- The form opens with “Single shear” selected as a worked example. Choose the option that matches your case.
- Get it right
- Count the interfaces between parts that slide past each other along the pin.
What this means: Pin circle radius
- What it is
- The radius from the shaft centre to the centre of the shear pin, at which the torque becomes a shear force.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 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
- A larger radius lowers the pin force and the pin size needed.
- Get it right
- Radius, not pitch circle diameter: halve the PCD.
What this means: Pin ultimate shear strength
- What it is
- The shear stress at which the pin material actually fails, which sets the break torque.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A stronger material needs a smaller pin for the same break torque.
- Get it right
- Test pins from the batch you will use; published strengths vary too much for a reliable break torque.
What this means: Pin diameter chosen
- What it is
- The diameter of the pin you will actually fit, at the shear plane. The tool works out its break torque and break factor.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Break torque grows with the square of this diameter.
- Get it right
- If the pin is grooved at the shear plane, use the groove diameter.
Limits
What this means: Least break factor accepted
- What it is
- The lowest break torque, as a multiple of running torque, you accept, so the pin does not break in normal service.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.8 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must not exceed the largest break factor.
What this means: Largest break factor accepted
- What it is
- The highest break torque, as a multiple of running torque, you accept, so the pin breaks before the drive is damaged.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Set it from the torque the gearbox, coupling or shaft can survive.
What this means: Thinnest part in bearing
- What it is
- The thickness of the thinnest hub or flange part the pin bears on, which carries the bearing stress at the break force.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 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.
- Get it right
- The tool puts the whole break force on this one thickness, so enter the thinnest part that carries the full force.
What this means: Allowable bearing stress of the hub
- What it is
- The bearing stress you allow in the hub and flange holes at the force that breaks the pin, so the holes do not stretch before the pin shears.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Set it from the hub material’s strength; enlarged holes change the break torque.
Method and formulas
- Break torque = running torque × overload factor; break force at the pin circle = torque ÷ radius.
- Pin diameter = √(4F ÷ (π × pins × planes × τ_u)); chosen pin break torque = pins × planes × π d² ÷ 4 × τ_u × radius.
- Shear and bearing stress at the chosen pin’s break force (as the bolt shear and bearing tool).
- Example: a 7.2837 mm pin breaks at 1,000 N·m; the 7.5 mm pin breaks at 1,060.3 N·m (2.1206 × running).
Limitations
- Ultimate shear strength varies between batches; test pins from each batch for a reliable break torque.
- Sudden overloads; slow overloads can fatigue and break pins early.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a shear pin for a conveyor or mixer drive
- Check a replacement shear pin
- Set the break torque of a sprocket shear pin
Who it is for
- Design engineers
- Maintenance engineers
- Agricultural machinery engineers
What you get
- Break torque and pin diameter needed
- Break torque and factor with the chosen pin
- Bearing at the break force
Interpreting the result
- A groove at the shear plane fixes where the pin breaks and makes the break torque more repeatable.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1535, 1720–1735. Topic reference; no source tables or recommended settings are reproduced.
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