Straight-Tooth Spur & Helical Spur Gears

Figure shows an example of this type of gear. Straight-tooth spur gears have straight teeth parallel to the axis of rotation. When the teeth engage, they do so instantaneously along the tooth face. This sudden meshing results in high impact stresses and noise. Thus, these gears have been replaced with helical gears in most transmissions. However, these gears do not generate axial (or thrust) loads along the shaft axis. Furthermore, they are easier to manufacture and can transmit high torque loads. For these reasons, many transmissions use spur gears for first and reverse gears. This accounts for the distinctive "whine" when a car is reversed rapidly.

Helical Spur Gears

Figure shows an example of a helical gear. Helical gears have teeth that are cut in the form of a helix on a cylindrical surface. As the teeth begin to mesh, contact begins at the leading edge of the tooth and progresses across the tooth face. Although this greatly reduces the impact load and noise, it generates a thrust load that must be absorbed at the end of the shaft by a suitable bearing.