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Table 7-1 presents equations nor a protile shitted screw gear pair. When the normal coefficients of profile shift xn1 = xn2 =0, the equations and calculations are the same as for standard gears.    Standard screw gears have relations as follows:   (7-7) 7.3 Axial Thrust Of Helical Gears    In both parallel-shaft and crossed-shaft applications, helical gears develop an axial thrust load. This is a useless force that loads gear teeth and bearings and must accordingly be considered in the housing and bearing design. In some special instrument designs, this thrust load can be utilized to actuate face clutches, provide a friction drag, or other special purpose. The magnitude of the thrust load depends on the helix angle and is given by the

expression:    WT = Wttanb                    (7-8) where WT = axial thrust load, and Wt = transmitted load.    The direction of the thrust load is related to the hand of the gear and the direction of rotation. This is depicted in Figure 7-1. When the helix angle is larger than about 20º, the use of double helical gears with opposite hands (Figure 7-3a) or herringbone gears (Figure 7-3b) is worth considering.    More detail on thrust force of helical gears is presented in SECTION 16.

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