BIZOMAT MAKEOS · Gears
Plastics Gear Power Rating
How much power can this nylon, acetal or other plastics gear transmit?
Power rating of plastics spur, helical and straight bevel gears by the handbook’s Lewis-type formulas in metric form, from face width, tooth form factor, module, safe bending stress, pitch-line velocity and service factor, with the corresponding torque.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Gear type | choice | Gear type. |
| Face width | mm | Face width. |
| Tooth form factor Y | Lewis tooth form factor for the tooth count and system (from the handbook table or the material supplier). | |
| Module | mm | Module (normal module for helical gears). |
| Safe bending stress for the plastic | MPa | Safe bending stress of the plastic at the operating temperature and lubrication (from the material supplier). |
| Pitch diameter | mm | Pitch diameter, for the pitch-line velocity. |
| Speed | rpm | Speed of the gear. |
| Service factor | Service factor for the load type and running hours. | |
| Pitch cone distance (bevel) | mm | Pitch cone distance of a bevel gear (must exceed the face width). |
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: Gear type
- What it is
- The kind of plastics gear being rated: spur (internal or external), helical, or straight bevel. Each uses a different rating formula.
- Units and format
- Choose one of 3 options from the list: Spur (internal or external); Helical; Straight bevel.
- Example
- The form opens with “Spur (internal or external)” selected as a worked example. Choose the option that matches your case.
- Get it right
- The pitch cone distance is used only for straight bevel gears; for helical gears give the normal module.
What this means: Face width
- What it is
- The face width of the gear teeth: the length of the tooth measured along the gear axis, where it meshes with its mate.
- 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 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 spur and helical gears the rating is directly proportional to face width.
- Get it right
- Use the width actually in mesh; if the mating gear is narrower, use its width.
What this means: Tooth form factor Y
- What it is
- The Lewis tooth form factor, a number that describes how strong the tooth shape is in bending for the tooth count and tooth system.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.308 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The power rating is directly proportional to Y.
- Get it right
- Take it from the handbook table or the material supplier for the actual tooth count and pressure angle; it is not the same as the AGMA factor J.
What this means: Module
- What it is
- The module of the gear: pitch diameter divided by number of teeth. It sets the tooth size and so its bending strength.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 20 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
- The power rating is directly proportional to the module.
- Get it right
- For helical gears use the normal module, measured square to the teeth, not the transverse module.
What this means: Safe bending stress for the plastic
- What it is
- The safe bending stress of the plastic at the gear’s operating temperature, moisture and lubrication, from the material supplier.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 500 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The power rating is directly proportional to it.
- Get it right
- Plastics lose strength as temperature and moisture rise; use the supplier’s value for the service condition, not room temperature.
What this means: Pitch diameter
- What it is
- The pitch diameter of the gear being rated, used with its speed to find the pitch-line velocity.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 45 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 pitch diameter raises the pitch-line velocity and, with it, the power rating.
- Get it right
- Use the pitch diameter (module × teeth), not the outside diameter.
What this means: Speed
- What it is
- The rotational speed of the gear whose pitch diameter you entered, used to find the pitch-line velocity.
- 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 600 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher speed raises the power rating, but less than in proportion.
- Get it right
- Pair the speed with the same gear as the pitch diameter; pinion and gear speeds differ by the ratio.
What this means: Service factor
- What it is
- The service factor, which reduces the rating for the type of load (smooth, moderate or heavy shock) and the daily running hours.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.25 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The power rating is inversely proportional to the service factor.
- Get it right
- Take it from the handbook’s service factor guidance or the material supplier for your duty.
What this means: Pitch cone distance (bevel)
- What it is
- The pitch cone distance of a straight bevel gear: the distance along the pitch cone from its apex to the outer end of the teeth. Used only for bevel gears.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 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
- For bevel gears, a longer cone distance relative to the face width raises the rating.
- Get it right
- It must exceed the face width; ignored for spur and helical gears.
Method and formulas
- V = π d n ÷ 60,000 m/s.
- Spur: kW = F Y m S_s V ÷ [327 (3.05 + V) C_s].
- Helical: kW = F Y m S_s V ÷ [179 (5.56 + V) C_s]; straight bevel: the spur value × (C − F) ÷ C.
- The metric constants match the US-customary forms (55 (600 + V) P C_s and 423 (78 + V) P_n C_s).
- Example: a 10 mm wide, module 1.5 spur gear at 600 rpm is rated about 143.19 W.
Limitations
- Bending strength only; wear, heat build-up and creep often govern plastics gears.
- Plastics strength falls with temperature and moisture: use the stress for the service condition.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check whether a plastics gear can replace a metal one
- Size face width or module for a plastics gear train
- Rate a moulded gear at a new speed
Who it is for
- Design engineers
- Product engineers
- Gear designers
What you get
- Pitch-line velocity
- Power rating
- Torque at the rating
Interpreting the result
- Plastics gears running against metal pinions wear less and run cooler than plastic-on-plastic pairs.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 604–606. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Design printed gears that mesh
In Printed Mechanisms That Run
What it covers: Choose module and tooth counts a printer can make, lay out a planetary stage that assembles, check tooth bending against measured coupon strength and add the backlash printing needs.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.