The lead or pitch, PL and PR, on the two sides of the worm thread are not identical. The example in Figure 14-12 shows the case when PR > PL To produce such a worm requires a special dual lead hob.
The intent of Figure 14-12 is to indicate that the worm tooth thickness is progressively bigger towards the right end. Thus, it is convenient to adjust backlash by simply moving the duplex worm in the axial direction.
SECTION 15 GEAR ACCURACY
Gears are one of the basic elements used to transmit power and position. As designers, we desire them to meet various demands:
1. Minimum size.
2. Maximum power capability.
3. Minimum noise (silent operation).
4. Accurate rotation/position.
To meet various levels of these demands requires appropriate degrees of gear accuracy. This involves several gear features.
15.1 Accuracy Of Spur And Helical Gears
This discussion of spur and helical gear accuracy is based upon JIS B 1702 standard. This specification describes 9 grades of gear accuracy - grouped from 0 through 8 - and four types of pitch errors:
Single pitch error.
Pitch variation error.
Accumulated pitch error.
Normal pitch error.
Single pitch error, pitch variation and accumulated pitch errors are closely related with each other.
15.1.1 Pitch Errors of Gear Teeth
1. Single Pitch Error (fpt)
The deviation between actual measured pitch value between any adjacent tooth surface and theoretical circular pitch.
2. Pitch Variation Error (fpu)
Actual pitch variation between any two adjacent teeth. In the ideal case, the pitch variation error will be zero.
3. Accumulated Pitch Error (Fp)
Difference between theoretical summation over any number of teeth interval, and summation of actual pitch measurement over the same interval.
4. Normal Pitch Error (fpb)
It is the difference between theoretical normal pitch and its actual measured value.
The major element to influence the pitch errors is the runout of gear flank groove.
Table 15-1 contains the ranges of allowable pitch errors of spur gears and helical gears for each precision grade, as specified in JIS B 1702-1976.
In the above table, W and W' are the tolerance units defined as:
(15-1)W= 0.56 W + 0.25m (mm) (15-2)
The value of allowable pitch variation error is k times the single pitch error. Table 15-2 expresses the formula of the allowable pitch variation error.
Figure 15-1 is an example of pitch errors derived from data measurements made with a dial indicator on a 15 tooth gear. Pitch differences were measured between adjacent teeth and are plotted in the figure. From that plot, single pitch, pitch variation and accumulated pitch errors are extracted and plotted.
NOTE: A = Max. Single Pitch Error
B =
Max. Accumulated Error
C =
Max. Pitch Variation Error
Fig. 15-1 Examples of Pitch Errors for a 15 Tooth Gear
