11.2 Tooth Thickness and Backlash
One of the most important criteria of gear quality
is the specification and control of tooth thickness. As mentioned in Par. 4.10, the
magnitude of tooth thickness and its tolerance is a direct measure of backlash when the
gear is assembled with its mate.
Although it is possible to set the tooth thickness
and tolerance to any value within a wide range, convenient quality classes have been
established by AGMA in Gear Classification and Inspection Handbook (ANSI/AGMA 2000 - A88
). This information is reproduced in Table 1.9. The previous issue of this specification,
(390.02), offered a more detailed table of backlash allowance and tolerance which is still
a useful design guide. See Table 1.10. Although no longer part of current AGMA standards,
it is consistent with Table 1.9.
Note that the data in Table 1.9 is for unassembled spur and
helical gears; i.e. an individual gear. Backlash for a meshed gear pair due to tooth
thickness tolerance will be the sum of two values from Table 1.9, Most often the same
tolerance is applied to each gear of a meshed pair.
11.3 Position Error (or Transmission Error)
In many precision gear applications the transmission of motion from shaft-to shaft must have a high degree of linearity. This is known by several names: transmission linearity, angular transmission accuracy, and index accuracy. Theoretically, involute gears will function perfectly. However, in practice there are deviations from ideal motion transmission due to involute profile variations, spacing errors, pitch line runout, and radial out-of-position. Combinations of all these errors cause a net position error, which is transmitted to the instrument or machine involved.