Application Notes
In designing plastic gears, the effects of heat and moisture
must be given careful consideration. The related problems are:
1.
Backlash
Plastic gears have larger coefficients of thermal expansion.
Also, they have an affinity to absorb moisture and swell. Good design
requires allowance for a greater amount of backlash than for metal gears.
2.
Lubrication
Most plastic gears do not require lubrication. However,
temperature rise due to meshing may be controlled by the cooling effect of
a lubricant as well as by reduction of friction. Often, in the case of
high-speed rotational speeds, lubrication is critical.
3.
Plastic gear with metal mate
If one of the gears of a mated pair is metal, there will be a
heat sink that combats a high temperature rise. The effectiveness depends
upon the particular metal, amount of metal mass, and rotational speed.
Duracon M90
Duracon gears have less friction and wear when in an oil
lubrication condition. However, the calculation of strength must take into
consideration a no-lubrication condition. The surface strength using Hertz
contact stress, Sc, is calculated by Equation (18-4).
where:
F = Tangential force on surface (kgf)
b = Tooth width (mm)
d1= Pitch diameter of pinion (mm)
u = Gear ratio =z2/z1
E = Modulus of elasticity of material (kgf/mm˛) (see Figure
18-5)
a = Pressure angle
If the value
of Hertz contact stress, Sc is calculated by Equation (18-4)
and the value falls below the curve of Figure 18-6, then it is
directly applicable as a safe design. If the calculated value falls above
the curve, the Duracon gear is unsafe.
Figure 18-6 is based upon data for a pair of Duracon
gears: m = 2, v = 12 m/s, and operating at room temperature. For working
conditions that are similar or better, the values in the figure can be
used.
