Gears

View A Video on Gears - A Quick Introduction

Gear shape and size determines gear performance. When interlocking gears differ in size from one another, the smaller gear will turn faster than the larger gear; the relationship between gear size and speed is called "speed ratio" or "gear ratio." A gear's number of teeth can be used to calculate a gear assembly's ratio. For example, if two interlocking gears have 40 teeth and 20 teeth respectively, the gear ratio is 2:1. Different sized gears are used with one another to increase or reduce a shaft's rotational speed in speed reducers, motors, transmissions, heavy machinery and clocks. Bevel gears, sprockets, rack and pinion spur gears, planetary gears, spline gears and helical gears are crucial in automotive motors and power transmissions, as are rear end gears and differential gears. Gear manufacturers produce a wide variety of gear configurations. Gear applications are variable and highly customizable, as different gear types can be configured in a virtually limitless number of ways to increase speed, reduce speed, transmit power, transmit motion or reduce the amount of force necessary to accomplish a task. From 20 foot diameter industrial gears to small worm gears and plastic gears, gear manufacturers make custom gear assemblies for nearly any application.

The simplest type of gear is the spur gear, or the "straight-cut gear." Spur gears are straight-sided along the gear wheel's axis with straight teeth radiating in alignment with the axis. Spur gears are often used as sprockets, which are thin gears with teeth that lock easily into roller chains. Sprockets act as a non-slip pulley for power transmission applications such as bicycle gears. Splines are cylinders or rods with straight teeth used to fit inside internal gears or devices to transmit motion laterally. Other types of spur gears fit with other parallel aligned gears, transmitting torque and motion laterally; these gears are a main component of planetary gears, or epicyclic gear trains. Planetary gears contain one central spur gear, or "sun gear" surrounded by three or more "planet gears;" the exterior gears interlock with the inward-facing teeth of a larger internal gear, increasing the output speed of the large outer gear through rotational torque applied to the internal sun gear. While spur gears are used in a wide range of simple and complex applications, planetary gears are used in complex automotive transmissions, drivetrains and other applications where complex gear ratios are required for smooth torque transmission.

Not all gears have straight, axis-aligned teeth. Worm gears' teeth are arranged around a cylinder-shaped gear like a screw; spur or helical gears interlock with the screw-like worm gear so that the axes are perpendicular. The worm usually drives the gear, providing a high level of speed reduction for applications such as textile looms, packaging machinery, material handling and conveyor systems. Helical gears are refined spur gears with helically angled teeth; these angled teeth engage more fully than straight-toothed spur gears as well as providing far smoother, quieter functioning. Helical gears can also be aligned parallel or crossed, allowing gears to transmit torque perpendicularly for a broader number of applications. Conically shaped bevel gears allow gears to interlock at perpendicular angles for applications such as rear end wheel torque in cars. Bevel gears may have straight teeth, or they may be spiral bevel gears with curved teeth, similar to helical gears. Bevel gears are used as rear end gears in cars, trucks, other self-powered vehicles and industrial machinery requiring perpendicularly transmitted torque. Automotive differential gears are bevel gears arranged into epicyclic configurations, which allow various attached axles to turn at different speeds.

Gear manufacturers use gear cutting techniques such as gear hobbing to fabricate gears, designing their products to withstand harsh and repetitive use. A gear's number of teeth and its specific gear ratio determine the function, speed and control the gear will have within a larger gear assembly. These factors will determine the gear's durability and strength and also the speed it will be able to engage. Gears may be made from a variety of materials, including most metals and hard plastics. Highly wear-resistant plastics such as nylon and polycarbonate are useful in machinery applications where low weight is a requirement. In demanding applications such as automotive transmissions, gears are made from hard metals such as steel, brass, copper and even titanium. Heat, vibration and the presence of corrosive elements are all possible impediments to gear performance, which is why most gear systems use a form of lubrication. Operations that involve the use of gears are much more likely to be successful if gear composition and configuration are chosen with careful consideration for properties of strength, heat and corrosion resistance. Gears that are properly matched to their applications can be expected to perform reliably much longer than improperly applied gear equipment.

Photo Courtesy of Commercial Gear and Sprocket Company, Inc.	Photo Courtesy of Omni Gear Machine Corporation

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Gear Types

• Bevel gears are intended to operate on axes that intersect, usually at a 90 degree angle, to work around corners.
  
• Differential gears link two shafts through a covering, forcing the total of the rotational angles of the shafts to be the same as the rotational angle of the covering.
• Gear manufacturers are companies that fabricate torque transmission equipment.
• Helical gears have angled teeth to create a thrust load on the gears when they mesh. Car transmissions often have these.
  
• Herringbone gears conduct power and motion between non-intersecting, parallel axes that may or may not have a center groove, with each tooth making two opposite helices.
  
• Hypoid gears are similar to bevel gears but differ by operating on non-intersecting axes.
  
• Industrial gears are round mechanical components that have "teethed" circumferences, allowing them to interconnect with corresponding teethed gear wheels or parts; these gear pairs transfer/reverse motion, increase/decrease rotational speed and torque or synchronized axes.
  
• Metric gears are defined by the length in millimeters of the pitch circle diameter per tooth.
  
• Miter gears are bevel gears put together with equal numbers of teeth and axes that are usually at right angles.
  
• Planetary gears come in sets of usually two or more that operate on or inside larger industry gears. Planetary gears make drastic gear ratios possible.
  
• Plastic gears are made primarily of plastic, sometimes with both metal and plastic components. These gears are generally cost effective and light; some are injection molded plastic gears.
• Rear end gears are bevel gears arranged into epicyclic configurations which transmit torque and rotation through three shafts.
• Spline gears contain a series of ridges on a driveshaft that even out rotation speed of the companion piece.
• Sprockets are gears with metal teeth that enmesh with chains.
• Spur gears are cylinder shaped gears that have both straight and parallel teeth and work on parallel axes.
  
• Straight bevel gears have straight teeth that radiate from a center point.
  
• Spiral bevel gears are similar to helical gears and have a higher potential load transmission compared to straight bevel gears, which is achieved by cutting the teeth in a curved shape.
  
• Worm gears have a curved and recessed throat that gives the worm access to the worm gear teeth. Usually they are made of bronze, iron or steel.

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Gear Terms

Backlash - The distance the width of a tooth space surpasses the thickness of the engaging tooth on the pitch circles of industry gears.
 
Bore - The diameter of the hole in an industry gear.
 
Bull - The larger gear of a set of industry gears.
 
Center Distance (CD) - The smallest distance between the axes of non-intersecting, mating industry gears.
 
Coupling Sprockets - Used for connecting two non-continuous shaft ends of industry gears.
 
Dedendum (DED) - The distance, radial or perpendicular, between the bottom of the gullet and the pitch circle on industry gears.
 
Gullet - The distance between the teeth of industry gears.
 
Hob - A cutting tool used for cutting teeth in industry gears by using a hobbing machine.
 
Hub - An extension projecting from the side of industry gears that creates width to a part so it can be mounted on a shaft.
 
Hub Style - The type of style of the hubs on industry gears.  Type 'A' indicates there is no hub on the industry gear; type 'B' indicates there is a hub on only one side of the industry gear; type 'C' indicates there is a hub on both sides of the industry gear.
 
Module (MOD) - The pitch diameter's ratio to the number of teeth on the industry gear expressed in millimeters.
 
Mounting Distance (MD) - The distance from the crossing point of the axes to the location surface of industry gears.
 
Pinion - The smaller industry gear of a set of industry gears.
 
Pitch - The size of the teeth of industry gears.
 
Rack - A steel bar with teeth on one side where a pinion can be driven across. Also a
spur gear with an unlimited pitch diameter.
 
Ratio - The distance from the center of industry gears to the point where the teeth or edges come into contact with one other.  It is directly related to the size of one industry gear compared to the other.
 
Teeth - Edges or outside of industry gears have pits and extrusions to interlock with other industry gear parts. The teeth of the industry gears prevent slippage between the gears and promote consistency and regularity in the industry gears.

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