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20.8 Design Specifications

Basic gear formals have been discussed in Section 11.1, which also included a typical data block for spur gears (Figure 1.47). The information required for plastic injection-molded gears is very similar. Additional information, which would be useful, would include fillet radius and whole depth. Other data, which is pertinent to the function of the gear and which the design engineer may wish to add to the data block, includes the following: material and pitch diameter of mating gear, operating temperature, lubrication, moisture/humidity data, and annealing, if required.

20.9 Backlash

Due to the thermal expansion of plastic gears, which is significantly greater than that of metal gears (see Tables 1 .40 thru 1.43A) and the effects of tolerances, one should make sure that meshing gears do not bind in the course of service. Several means are available for introducing backlash into the system. Perhaps the simplest is to enlarge center distance. This procedure is described in Section 4.10. Care must be taken, however, to ensure that the contact ratio remains adequate.
It is possible also to thin out the tooth profile during manufacturing, but this adds to the manufacturing cost and requires careful consideration of the tooth geometry.
To some extent the flexibility of the bearings and clearances can compensate for thermal expansion. It a small change in center distance is necessary and feasible, it probably represents the best and least expensive compromise.

20.10 Environment and Tolerances

In any discussion of tolerances for plastic gears it is necessary to distinguish between manufacturing
tolerances and dimensional changes due to environmental conditions.
As far an manufacturing is concerned, plastic gears can be made to high accuracy, if desired. For injection-molded gears TCE can readily be held within a range of roughly 0.003" - 0.005", with a corresponding TTCE of about 0.OO1 - 0.002". Higher accuracies can be obtained if the more expensive filled materials, mold design, tooling and quality control are warranted.
In addition to thermal expansion changes there are permanent dimensional changes as the result of moisture absorption. In addition, there are dimensional changes due to compliance under load. The coefficient of thermal expansion of plastics is on the order of four to ten times those of metals (see Tables 1.40, 1.43A). In addition, most plastics are hygroscopic (i.e., absorb moisture) and dimensional changes on the order of 0.1% or more can develop in the course of time, if the humidity is sufficient As a result, one should attempt to make sure that a tolerance which is specified is not smaller than the inevitable dimensional changes which arise as a result of environmental conditions. At the same time, the greater compliance of plastic gears as compared to metal gears suggests that the necessity for close tolerances need not always be as high as those required for metal gears.

20.11 Avoiding Stress Concentration

In order to minimize stress concentration and maximize the life of a plastic gear the root fillet radius should be as large as possible, consistent with conjugate gear actiotn. Sudden changes in section and sharp corners should be avoided, especially in view of the possibility of additional residual stresses, which may have occurred in the course of the molding operation.