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17.3 Plastic Gears
These can be produced by the normal hobbing and shaping processes. In addition, they can be produced by various molding techniques. The latter methods are not  accurate as cut gears due to shrinkage, mold variations, and flow inconsistencies.
Regardless of method, the fabrication of plastic gears suffers in comparison with metal gears due
to temperature instability, material flow, and generally poorer cutting qualities. Attainable quality is less than for metals and varies with the particular plastic.
See Section 20.10, which deals with plastic gears in greater detail.

18.0 GEAR INSPECTION
The performance of a gear can be assured only by confirmation of its critical dimensions and parameters. With increasing gear precision, adequate and proper inspection has become a paramount requirement.
There are many aspects of gear inspection and the subject is too large for complete coverage in this discussion. However, two of the most basic and important inspection criteria, which will be discussed in the following paragraph, are total composite error (TCE) and tooth thickness.

18.1 Varlable-Center-Distance Testers
Both TCE and tooth thickness can be measured by means of roll testing with a variable-center-distance fixture. There are many varieties, but essentially all consist of a fixture having two parallel shafts (or precision centers), one fixed arid the other floating on smooth, low-friction ways. The test gear is mounted on one shaft while an accurate known quality master gear is mounted on the other shaft. The pair is held in intimate contact by spring loading or the equivalent. As the test gear is rotated, tooth-to-tooth errors and runout are revealed as a variation in the center distance of the pair. This variation can be sensed, amplified and displayed as a dial reading or recorded on a chart. See Figure 1.48. Sensitivity of the measurement is on the order of 50 to 100 millionths of an inch.
The unique feature of gear roll testing is that the inspection parallels the gear in its actual usage.
Thus, roll testing is a functional inspection.

18.1.1 Total Composite Error — The TCE is dearly revealed in roll testing and its components can be identified. Referring to Figure 1.48, it is evident that the magnitude of runout and TTCE can be extracted. From this, the gear quality can be judged. Also, when parameters are out of tolerance, the fabricator can identify the source of the difficulty and take appropriate corrective action.

18.1.2 Gear Size — If the center distance setting of the roll tester is carefully established, the absolute readings are an indication of tooth thickness. Thus, in Figure 1.48, the mean line of the trace is a measure of tooth thickness. The high and low readings indicate the extreme variation of tooth thickness at the nominal pitch radius. Changes in center-distance are an indirect measure of tooth thickness and must be converted with the aid of equation 22.

18.1.3 Advantages and Limitations of Variable-Center-DIstance Testers — The functional test of a gear is desirable as it reveals characteristics that occur in the real application. Also, the method is rapid and, therefore, suitable for production gear inspection. Ability to obtain a hard copy record is also a distinct advantage.
Rolling of the gears is not usually relied upon for the determination of. pitch radius. For the measurement of TCE and TTCE, however, roil testing gives excellent results. Repeatability arid absolute measure are usually good, being in the order of .0001 inch. On the other hand, size measurement is not as reliable as an absolute measure. This is due to the nature of the fixture and the integration of several error sources in the calibration process. A repeatability of .0002 inch is considered good, and often it is even better.