Gear
Straight

In a cylindrical spur gear or straight-cut gear, the tooth faces are straight along the direction parallel to the axis of rotation. Any imaginary cylinder with the same axis will cut the teeth along parallel straight lines.

The teeth can be either internal or external. Two spur gears mesh together correctly only if fitted to parallel shafts. No axial thrust is created by the tooth loads. Spur gears are excellent at moderate speeds but tend to be noisy at high speeds.

For arrangements with crossed non-parallel axes, the faces in a straight-cut gear are parts of a general conical surface whose generating lines (generatrices) go through the meeting point of the two axes, resulting in a bevel gear. Such gears are generally used only at speeds below 5 m/s (1000 ft/min), or, for small gears, 1000 r.p.m.
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