THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

14.8 Gears 377The subscripts 1 and 2 refer to the driver and driven gears, respectively. For fulldepth gears with standard pressure angles, the standard addendum is a = m =1/P d .For20 ◦ pressure angle stub teeth, the standard addendum is a = 0.8m =0.8/P d .Gears are selected to meet specified power and speed ratio requirements. Theserequirements affect the selection of gear material, tooth width, module or diametralpitch, number of teeth, and so on. If the number of teeth is increased for each gearin a train, the contact ratio will usually increase, with the result the tooth meshingfrequency and the accompanying noise level will decrease.Helical GearsWhen a pair of spur gear teeth begins to mesh, the contact occurs at once across theentire face width. In the case of a helical gear in which teeth entwine about an axialsurface rather than cut straight across (Figure 14.9), the contact occurs gradually,with the contact beginning at a point and then extending across the tooth face. Thisresults in reduced vibration and lower impact loads that produce tooth meshingfrequency noise. Hence, helical gears on parallel shafts may be substituted forspur gear trains to reduce noise; and the actual number of teeth in contact willalso be substantially increased. In order to ensure smooth and quiet operation, itis recommended that the thickness of the helical gear be 1.2 to 2 times the axialpitch (the distance between corresponding points on two adjacent teeth). Thrust(axial) loads must be taken into consideration when specifying helical gears in thedesign of shafts and bearings. Well-made helical gears cost more than spur gears,but the payback in terms of smoother and quieter operation can more than offsetthe greater expense.Figure 14.9. Helical gears. (Courtesy of Designatronics, Inc., New Hyde Park, NY.).