Stirnrad

Stirnrad, known in English as a spur gear, is a gear with straight teeth that run parallel to the gear’s axis. When two Stirnräder mesh, their pitch circles engage along a single plane, so the connected shafts rotate with a constant velocity ratio. Spur gears are widely used to transmit power between parallel shafts in machines ranging from clocks to industrial gear trains.

Geometric fundamentals for Stirnräder include the number of teeth z and the module m (metric) or diametral pitch DP (imperial). The pitch circle diameter D equals z times m (or D = z/DP). Tooth height comprises addendum (about 1m) and dedendum (roughly 1.0–1.25m). The standard tooth profile is involute, commonly with a pressure angle of 20 degrees, though other angles appear in different standards. The contact ratio indicates how many teeth are in contact at once, influencing smoothness and load distribution.

Manufacturing and materials typically involve steel or alloy steel, with plastics and cast iron used in lighter applications. Common production methods are milling, hobbing, and shaping, followed by finishing operations such as grinding or lapping to achieve required tolerances. Design and manufacturing adhere to standards such as DIN, AGMA, or ISO, which specify tolerances, backlash, and surface hardness.

Applications and limitations: Spur gears are simple, robust, and cost-effective for transmitting motion between parallel shafts, offering high efficiency at moderate speeds. Their main drawbacks are increased noise and vibration at high speeds and higher radial loads if misaligned. For non-parallel shafts or higher speeds with smoother operation, helical or bevel gears are used.