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Thursday, 28 November 2013

Types of Gears

Types of Gears Points : Types of Gears, Spur Gears, Bevel Gears, Internal Gears, Helical Gears, Herringbone Gears, Worm and Worm Gears, Rack and Pinions Many types of gears are used in industrial machines. The most common are spur, bevel and miter, internal, helical, and worm gears. 1. Spur Gears Spur gears are cylindrical and have teeth that are parallel with the axis of the gear. They are designed for use on shafts whose axis are parallel with each other; A spur rack has teeth at a right angle to the axis of motion of the rack. Spur gears are the most commonly used type of gear on industrial machines. Normally, they are used under conditions of moderate speed and moderate load.
2. Bevel Gears In bevel gear the teeth are cut on a conical surface, such as would be represented by a truncated cone. Bevel gears are used for transmitting power around comers. The shafts generally are at right angles, but they may be at various angles. Bevel gears on shafts at angles other than 90° are called angular gears. Two bevel gears 'of equal size with equal number of teeth and designed for use on shafts at right angles are called miter gears.
3. Internal Gears Internal gears have teeth on the inner surface of a cone or cylinder. This type may also have spur, bevel, or helical teeth. An advantage n combining internal and external gears is their compactness. Since the centers of the gears are closer together, less space is required. They also possess increased operational efficiency; more teeth are in mesh, the tooth lines curve in the same direction, and friction is reduced.
4. Helical Gears Helical gears are similar to spur gears, except that the teeth form a helix twisting around the body of the gear. The helical teeth provide greater strength and smoother operation at high speeds. The teeth do not hit each other as in the case of spur gears. Instead, they slide across each other, thus reducing noise and vibration. Since several teeth are in contact at the same time, their strength is greater than for spur teeth of the same size. Helical gears may be used for connecting shafts that are parallel, or at an angle with each other, provided their axial lines de not intersect. When used on parallel shafts, they are called parallel helical gears (Left)] and are of opposite hand. This means a right-hand gear is meshed with a left-hand gear. Gears that operate on shafts with crossed axes are called crossed helical gears. They may have gears with teeth of the same hand or of the opposite hand.
5. Herringbone Gears Originally, a herringbone gear consisted of two helical gears of equal size but of opposite hand joined together. Today, most herringbone gears are produced as a single unit on special machines which cut the teeth in two directions at one time.
6. Worm and Worm Gears Worm gears arc meshed with a warm; the gear and the worm constitute a worm gear mechanism. The teeth on the worm gear are helical and conform to the helix angle of the tooth on the worm. The helical tooth on the worm is a form of thread, similar to an acme thread, and it often is called a worm thread.
Worms may have single, double, or triple threads. With the single thread, one revolution of the worm revolves the worm gear a distance equal to that between a point on one tooth and a corresponding point on the next worm gear tooth, or one circular pitch. One revolution of a doublet thread worm revolves the gear an amount equal to two teeth on the gear, and so on.
Worm gearing is used largely for speed reduction. The worm gear cannot turn the worm when a single-thread worm is used. This type of gear mechanism is self locking. Engineers take advantage of this feature when they employ worm gears in steering mechanisms, in hoisting equipment, and in other devices.
7. Rack and Pinions Many types of racks are used in industry. Some have their teeth cut as spur, while others have helical teeth. The latter are called helical racks. Many such racks used for adjusting the position of parts of machine tools. The function of rack and pinion is to transfer circular motion to rectilinear motion. A gear rack is a flat surface on which teeth have been cut. Rectangular stock is commonly used, but square and round stock may be used, as on the side of the shaft. A gear rack, when meshed with a gear, is used to change rotary motion to reciprocating motion.

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