BIZOMAT MAKEOS · Power Transmission
Ratchet Wheel Tooth Pitch
What tooth pitch and outside diameter does a ratchet wheel need to hold this turning moment?
Circular pitch of ratchet wheel teeth at the outside circumference for holding a suspended load, from the turning moment, face width, safe stress and number of teeth with the handbook’s tooth-number factor, and the resulting outside diameter.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Turning moment on the ratchet shaft | N·m | Torque the ratchet must hold. |
| Face width (ratchet thickness) | mm | Width of the tooth face. |
| Safe stress for the tooth material | MPa | Safe working stress you allow for the ratchet material, reduced where shock loads occur. |
| Number of teeth | Number of teeth on the wheel. |
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: Turning moment on the ratchet shaft
- What it is
- The torque the ratchet and pawl must hold on the ratchet shaft, for example the suspended load times the drum radius.
- 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 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 pitch rises with the square root of the turning moment.
- Get it right
- If gearing lies between the drum and the ratchet, refer the torque to the ratchet shaft itself.
What this means: Face width (ratchet thickness)
- What it is
- The width of the ratchet teeth measured along the shaft axis, that is the thickness of the ratchet wheel where the pawl bears.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 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
- The pitch falls with the square root of the face width.
What this means: Safe stress for the tooth material
- What it is
- The working stress you allow in the ratchet teeth for the chosen material, set by your design rules and reduced where shock loads occur.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 10000 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 higher allowable stress gives a smaller pitch, by the square root.
- Get it right
- This is an allowable working stress, not the yield or tensile strength; take it from your design code or material data with your safety margin.
What this means: Number of teeth
- What it is
- The number of teeth around the ratchet wheel. It also selects the handbook tooth-number factor, which steps down above 12 and above 20 teeth.
- Units and format
- Enter a plain number with no unit. The form accepts 6 to 500. 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.
- Effect on the result
- Within each factor band, more teeth give a finer pitch but a larger outside diameter.
Method and formulas
- P = √(F M ÷ L S N), with M in N·mm, L in mm and S in MPa.
- F = 50 for 12 teeth or fewer, 35 for 13 to 20 teeth and 20 for more than 20 teeth (handbook).
- Outside diameter = P N ÷ π.
- Example: 200 N·m on a 16-tooth, 25 mm wide ratchet needs a 29.58 mm pitch.
Limitations
- Crane-design rule for holding ratchets; check the pawl, its pin and the hub separately.
- Shock loading needs a lower safe stress.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a ratchet and pawl for a winch or hoist
- Check an existing ratchet against a higher load
- Choose tooth count and face width together
Who it is for
- Machine designers
- Lifting equipment designers
- Maintenance engineers
What you get
- Circular pitch at the outside
- Outside diameter
- Factor used
Interpreting the result
- A wider face or a stronger material reduces the pitch and wheel diameter.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2215–2217. Topic reference; no source tables or recommended settings are reproduced.
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