25.2.
Types of Spur Gearwheels
Tooth gearing is used for positive transmission of rotary motion from one shaft to another. In spur gear the shafts are parallel, and the gearwheels are cylindrical discs with teeth on their circumference. The gear wheels are usually manufactured from low-alloy nickel chromium molybdnenum steels. Figure 25.4 illustrates gear tooth profile terminology.
Straight-tooth Spur Gear.
In straight tooth spur gears (Fig. 25.5), teeth are cut at right angles to the face and parallel to the axis of the gear wheel. When the teeth profiles contact, the relative motion is a rolling
action at the pitch point, and this changes to sliding once contact occurs on either the face or flank of the tooth. Radial forces between the teeth in contact tend to separate the gears and that must be absorbed by radial type bearings.
Helical-tooth Spur Gear.
In helical tooth spur gears (Fig. 25.6), teeth are cut at an angle both to the face and to the axis of the gearwheel. Thus, contact between meshing teeth takes place along a diagonal line across the faces and flanks of the teeth. Since one pair of meshing teeth remains in contact until the following pair engages, the load on the teeth is distributed over a larger area. This reduces tooth loading and promotes smoother and quieter running. Axial, or end thrust is felt at the shafts and that must be absorbed by bearings. Side thrust may be reduced by using double helical gears.
