FIGURE 7.21 Bearings for double-helical gears. 197 9563_C007.fm Page 198 Wednesday, July 25, 2007 4:13 PM 198 Machine Elements: Life and Design 1.5 1.4 1.3 h W K 1.2 1.1 1.0 20 25 30 35 40 45

FIGURE 7.22 Overload factor for a double-helical gear. Use of appropriate bearings is only a part of a proper design. If the bearing is tted too tightly to the shaft, so that there is no internal clearance between the rings and rollers, the shaft may be squeezed in the bearing. To cut out that possibility, the shaft should be checked by hand for free axial movement (after the gear is completely assembled). Any devices that produce additional load on the bearings, such as heavy ywheels, belt-drive pulleys, and so on, should be avoided on self-aligning shafts. Extra accuracy of the gear mesh helps to decrease the amplitude of the pinions oscillating motion. Owing to this, the speed and acceleration of this motion are reduced, and the unevenness of load distribution, which is caused by friction forces and inertia, decreases. Information about the resistance of cylindrical roller bearings to axial displacement under load is given in Chapter 6, Subsection 6.2.3, Equation 6.5.

When a double-helical gear is properly designed, the overload factor for a half-gear KWh may be approximately taken from Figure 7.22 depending on helix angle . From the chart, we see that the helix angle should be big enough to keep the overload factor within reasonable limits. The reader may ask, why on earth do they make herringbone gears? It is much easier to make helical gears, is it not? That is right, and smalland medium-sized gears are usually helical. But the axial forces in these gears cause asymmetric deformations of the gears and housing. For big gears, symmetry of all deformations is a very important principle that helps to keep the teeth aligned and decrease the KW value. In double-helical gears, the axial forces are balanced and dont load the bodies of gears and housing, so they provide the needed feature.

Another merit of the double-helical gear is that the total face width of the two halves is greater than can be made in a helical gear (because of lesser inuence of the pinion twisting deformation on the load distribution across the face width). Owing to that, the diameters of gears may be reduced, and this is very important for large reduction gears, in which the diameters come up to 56 m (20 ft).