Metric Gears

Metric gears are defined by module, which designates the center-to-center distance between successive teeth. There are many different types of products. Examples include spur gears, change gears, cluster gears, helical gears, herringbone gears, miter or bevel gears, hypoid gears, differential end gears, worm gears, worm wheels, gear stock or pinion wire, racks, and pinions. Miter or bevel gear sets and gear stock or pinion wire are also available. Some metric gears are made of metals such as aluminum, brass, bronze, cast iron, carbon, hardened steel or stainless steel. Other metric gears are made of plastics such as acetal, delrin, nylon, or polycarbonate. Combination gears combine the strength and good-fit properties of a metal hub with the quiet-running, closer-fitting tolerances of plastic teeth. 

Specifications for metric gears include number of teeth, tooth form, direction, gear center, and pressure angle. The number of teeth, along with the desired pitch diameter, determines the pitch or module of the gear. Straight teeth are parallel to the axis of rotation. Spiral or helical teeth curve along the face. Spiral, bevel and miter gears have a right-hand or left-hand specification, which describes the direction of the tooth curve. Right-hand gears advance in a clockwise direction. Left-hand gears advance in a counterclockwise direction. Some metric gears contain a circular, center bore for shaft mounting. Others locate the gear on a non-separable, cylindrical center shaft. Pressure angle is the angle between the line of force or action and the tangent to the pitch circle at the point of mesh. The most common pressure angles for gears are 14.5° and 20°; however, some worms and worm gears use a 25° pressure angle. Like other gears, metric gears must have the same pressure angle to mesh. 

Metric gears can mount on a hub, a cylindrical extension of the gear body which allows different shaft mounting options. Hubless gears are typically attached via a press-fit, adhesive, or an internal keyway. Products that use hub clamping screws attach via a fastener that squeezes the inner diameter (ID) of the hub to a tight fit around the shaft. Split hubs are divided into several pieces, each of which is tightened by a separate clamp that grips the shaft. Simple bores are suitable for press-fit or adhesive attachment. Keyways have one or more square-edged cutouts that are designed to mate with a shaft key.

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• Fundamentals of Metric Gear Trains
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• Cool running
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Part Numbers for Metric Gears