KEVOS® MAKEOS · Gears
Plastic Gear Drive Check (Power Rating and Backlash at Temperature)
Will this plastic gear carry the power, and will the mesh still have backlash when the gears warm up and absorb moisture inside a metal housing?
Check of a plastic spur, helical or bevel gear in a metal housing: power rating by the handbook plastic gear formula with the service factor (as the plastic gear power tool) against the power required, the growth of the gear pitch radii from temperature and moisture swell against the growth of the housing centre distance (the plastic–metal mismatch, as the plastic thermal mismatch tool), the resulting loss of backlash through the pressure angle, and the backlash left at the hot, wet condition.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Gear type | choice | Gear type for the rating formula. |
| Face width | mm | Face width. |
| Lewis form factor | Lewis form factor for the tooth count. | |
| Module | mm | Module. |
| Safe stress for the plastic | MPa | Safe bending stress of the plastic at the running temperature (your value from the resin maker). |
| Pinion pitch diameter | mm | Pitch diameter of the plastic pinion (the gear rated). |
| Gear pitch diameter | mm | Pitch diameter of the mating gear (taken as plastic too). |
| Pinion speed | rpm | Speed of the pinion. |
| Service factor | Service factor for the duty. | |
| Cone distance (bevel) | mm | Cone distance for bevel gears. |
| Power to transmit | W | Power the drive must carry. |
| Backlash | ||
| Pressure angle | ° | Pressure angle. |
| Backlash at assembly | mm | Circular backlash set at assembly at room temperature, dry. |
| Plastic expansion coefficient | µm/m·K | Expansion coefficient of the gear plastic. |
| Housing expansion coefficient | µm/m·K | Expansion coefficient of the housing that sets the centre distance. |
| Temperature rise in service | K | Rise from assembly to running temperature. |
| Linear swell from moisture | % | Linear growth from moisture absorption (nylons swell; acetal very little). |
| Least backlash allowed | mm | Least backlash that must remain. |
Method and formulas
- Spur rating P = F Y m S V ÷ [327 (3.05 + V) C_s] kW-form (helical and bevel variants) with V the pitch-line speed (as the plastic gear power tool).
- Centre distance a = (d₁ + d₂) ÷ 2; gear growth = a (α_p ΔT + swell); housing growth = a α_m ΔT (as the plastic thermal mismatch tool).
- Backlash change Δj = −2 tan α × (gear growth − housing growth); backlash in service = assembly backlash + Δj.
- Example: the pinion is rated 143.19 W against 100 W; warming and swelling take 0.2103 mm of backlash, leaving 0.089698 mm.
Limitations
- Handbook rating formula; resin makers’ data give better limits for modern materials.
- Uniform temperature of gears and housing; gears run hotter than the housing in practice.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a nylon or acetal gear pair
- Set the cold backlash so a plastic mesh does not bind
- Check a plastic gear for a power increase
Who it is for
- Design engineers
- Product designers
- Plastics engineers
What you get
- Power rating against the power required
- Change in backlash from temperature and moisture
- Backlash left in service
Interpreting the result
- Nylon gears in an aluminium housing lose backlash as they warm and soak: set enough cold backlash or use a lower-swell resin.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 604–606, 578–580. Topic reference; no source tables or recommended settings are reproduced.
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Open the Plastic Gear Drive Check (Power Rating and Backlash at Temperature) calculator
