BIZOMAT MAKEOS · Power Transmission
Ratchet and Pawl Design (Pitch, Tooth Force, Bearing and Pin)
What pitch and diameter does this ratchet wheel need for the torque, what force does the pawl take, and are the tooth face and pawl pin strong enough?
Design of a ratchet and pawl holding a torque: circular pitch at the outside diameter by the handbook crane-ratchet rule and the outside diameter (as the ratchet wheel pitch tool), the tooth force at the outside diameter, the bearing stress where the pawl tip meets the tooth face, and the shear stress in the pawl pivot pin, against your allowables.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Torque held | N·m | Torque the ratchet must hold. |
| Face width of the teeth | mm | Width of the teeth (and of the pawl tip). |
| Safe stress for the ratchet rule | MPa | Safe stress used in the handbook ratchet rule for the material (your value; low values for cast iron). |
| Number of teeth | Number of teeth on the ratchet wheel. | |
| Pawl engagement depth on the tooth face | mm | Depth of the tooth face the pawl tip bears on. |
| Allowable bearing stress on the tooth face | MPa | Allowable bearing (contact) stress between pawl and tooth. |
| Pawl pin diameter | mm | Diameter of the pin the pawl pivots on. |
| Pin shear planes | choice | Whether the pin is in single or double shear. |
| Allowable pin shear stress | MPa | Allowable shear stress of the pin. |
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: Torque held
- What it is
- The torque the ratchet wheel must hold against the pawl, for example the hoist load times the drum radius.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 10000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The required circular pitch grows with the square root of the torque.
- Get it right
- Newton metres, not N·mm.
What this means: Face width of the teeth
- What it is
- The axial width of the ratchet teeth, also taken as the width of the pawl tip bearing on them. It enters both the pitch rule and the face bearing area.
- 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 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A wider face reduces both the pitch needed and the face bearing stress.
- Get it right
- If the pawl is narrower than the wheel, only the width in contact carries load; use that width.
What this means: Safe stress for the ratchet rule
- What it is
- The safe stress for the ratchet wheel material used in the handbook crane-ratchet rule for pitch, taken from the handbook or your design practice for that material.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 1000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower safe stress gives a larger pitch and outside diameter.
- Get it right
- It is a safe working stress for this rule, not the yield or tensile strength.
What this means: Number of teeth
- What it is
- The number of teeth round the ratchet wheel. It also selects the tooth-number factor used in the pitch rule.
- Units and format
- Enter a plain number with no unit. The form accepts 6 to 200. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 16 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The tooth-number factor changes in steps above 12 and above 20 teeth, so results jump across those counts.
What this means: Pawl engagement depth on the tooth face
- What it is
- How far down the tooth face the pawl tip bears, measured from the tooth tip. With the face width it gives the contact area for the bearing stress.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 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
- Face bearing stress is inversely proportional to the engagement depth.
- Get it right
- It must be less than the circular pitch, or the tool rejects the inputs.
What this means: Allowable bearing stress on the tooth face
- What it is
- The largest contact (bearing) stress you allow between the pawl tip and the tooth face, from the softer material’s data and your design practice.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Pawl pin diameter
- What it is
- The diameter of the pin the pawl pivots on, which carries the full tooth force in shear.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 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
- Pin shear stress falls with the square of the pin diameter.
What this means: Pin shear planes
- What it is
- How many cross-sections of the pin carry the tooth force: one for a pawl on a cantilever pin, two for a pawl held between two side plates.
- Units and format
- Choose one of 2 options from the list: Single shear (pawl on a cantilever pin); Double shear (pawl between two plates).
- Example
- The form opens with “Single shear (pawl on a cantilever pin)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Double shear halves the calculated stress only if both plates support the pin firmly.
What this means: Allowable pin shear stress
- What it is
- The largest shear stress you allow in the pawl pivot pin, from the pin material’s data and your design margin.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Circular pitch P = √[F M ÷ (L S N)] with the tooth-number factor F (as the ratchet wheel pitch tool); outside diameter D = P N ÷ π.
- Tooth force at the outside = 2M ÷ D; bearing stress = force ÷ (L h).
- Pin shear = force ÷ (planes × π d² ÷ 4).
- Example: a 16-tooth wheel needs a 29.58 mm pitch (150.65 mm outside); the pawl takes 2,655.1 N.
Limitations
- Static holding torque; shock engagement can multiply the load several times.
- Pawl body bending is not checked.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design a ratchet backstop for a winch or hoist
- Check a pawl pin for a higher torque
- Choose tooth count and width for a ratchet
Who it is for
- Design engineers
- Machine designers
- Maintenance engineers
What you get
- Pitch and outside diameter
- Tooth force and face bearing stress
- Pawl pin shear stress
Interpreting the result
- More teeth give finer engagement but a smaller pitch: the face width must grow to keep the bearing stress down.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2215–2217, 1535. Topic reference; no source tables or recommended settings are reproduced.
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Open the Ratchet and Pawl Design (Pitch, Tooth Force, Bearing and Pin) calculator