Helical gears

The treatment here of the principal types of non-spur gears will be relatively brief. A helical gear can be thought of as an ordinary spur gear machined from a stack of thin shim stock, each section is rotated slightly with respect to its neighbours, as seen in figure 9. The teeth are therefore not in line with the axis of rotation but at an angle (helix angle) to this. The transfer of load from one tooth to the other therefore occurs progressively over the width of the face or flank of the tooth. When power is transmitted by a pair of helical gears, both shafts are subjected to a thrust load, along the center of the shaft. This is caused by the helix angle of the gear face or flank (refer : figure 9).

Figure 9 A Hansen Helical gear set.

The thrust load is usually taken up by taper roller or spherical bearings. Under extreme conditions a separate thrust bearing may be fitted to take up axial load on the shaft. The gears on a shaft in a Hansen gearbox are also arranged such that the axial forces tend to be facing opposite directions, thereby reducing the net axial forces on the bearings.

Figure 10 A Hansen gearbox open.

The larger the helix angle of the gear, the smoother the load transfer but the thrust produced increases. In modern day gearing the helix angle is kept as small as possible. In Hansen gearboxes, the technique is applied of reducing the helix angle for the high torque gears (low speed) to minimize excessive axial loads being produced on the housing. On the high-speed gears the helix angle is larger with the transferred torque and thus the resultant thrust being smaller. The need for quiet operation at high rotational speeds is however required.