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Chapter 4 - Gear

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Two meshing gears transmitting rotational motion. Note that the smaller gear is rotating faster. Although the larger gear is rotating less quickly, its torque is proportionally greater. One subtlety of this particular arrangement is that the linear speed at the rim is the same on both gears.

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Kinds or types of Gears

• The major types of gears are:– Spur gears– Helical gears– Bevel gears- Rack and Pinion– Worm/wormgear sets

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Spur gears

• Spur gears have teeth that are straight and• arranged parallel to the axis of the shaft that• carries the gear.• • The curved shape of the faces of the spur• gear teeth have a special geometry called an• involute curve.• • This shape makes it possible for two gears• to operate together with smooth, positive• transmission of power.

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Helical Gear

• The teeth of helical gears are arranged so that• they lie at an angle with respect to the axis of• the shaft.• • The angle, called the helix angle, can be• virtually any angle.• • Typical helix angles range from approximately• 10O to 300, but angles up to 45O are practical.• • The helical teeth operate more smoothly than• equivalent spur gear teeth, and stresses are• lower.

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Bevel Gear• Bevel gears have teeth that are arranged as elements• on the surface of a cone.• • The teeth of straight bevel gears appear to be similar• to spur gear teeth, but they are tapered, becoming• wider at the outside and narrower at the top of the• cone.• • Bevel gears typically operate on shafts that are 90o• to each other.• • Specially designed bevel gears can operate on shafts• that are at some angle other than 90o.Feature:• Bevel gears are widely designed to transmit power and motion between intersecting

axes, generally at right angles. Therefore bevel gears are suitable when the transmitting direction needs to be changed. The teeth on a bevel gear can be straight, spiral or hypoid.

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Rack and Pinion

• When both bevel gears in a pair have the• same number of teeth, they are called miter• gears and are used only to change the axes• of the shafts to 90 degrees.• • No speed change occurs.• • A rack is a straight gear that moves linearly• instead of rotating.• • When a circular gear is mated with a rack,• the combination is called a rack and pinion• drive.

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Worm Gear

• A Worm Gear set is used for transmitting power between two shafts rotating about two non parallel axes.A worm gear set basically consist of a worm and a worm gear.

• Video worm gear

Important Features:

Speed reduction in small space.

Quieter operation and shock-less tooth engagement.

Self locking arrangement.

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Spur Gear Terminology

• Defined below are the common terms used in the analysis of spur gears.• Addendum: The radial distance between the pitch circle and the addendum circle.• Dedendum: The radial distance between the pitch circle and the root circle.• Clearance: The difference between the dedendum of one gear and the addendum

of the mating gear.• Circular pitchp: The width of a tooth and a space, measured on the pitch circle.• Diametral pitch P: The number of teeth of a gear per inch of its pitch diameter. A

toothed gear must have an integral number of teeth. The circular pitch, therefore, equals the pitch circumference divided by the number of teeth. The diametral pitch is, by definition, the number of teeth divided by the pitch diameter.

• Modulem: Pitch diameter divided by number of teeth. The pitch diameter is usually specified in inches or millimeters; in the former case the module is the inverse of diametral pitch.

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Velocity ratio

• We know that the velocity ratio of a pair of gears is the inverse proportion of the diameters of their pitch circle, and the diameter of the pitch circle equals to the number of teeth divided by the diametral pitch.

Determine the VR link?

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Gear train

• A gear train is formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next

Link to simple gear train