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- 1. CHAPTER – 8 : GEAR & GEAR TRAIN TYPE OF GEARS : a) Spur gear :- Spur gears have their teeth parallel to the axis and are used for transmitting power between two parallel shafts. They are simple in construction, easy to manufacture and cost less. They have highest efficiency and excellent precision rating. They are used in high speed and high load application in all types of trains and a wide range of velocity ratios. Hence, they find wide applications right from clocks, household gadgets, motor cycles, automobiles, and railways to aircrafts. b) Helical Gear :- Helical gears are used for parallel shaft drives. They have teeth inclined to the axis as shown in Fig. Hence for the same width, their teeth are longer than spur gears and have higher load carrying capacity. Their contact ratio is higher than spur gears and they operate smoother and quieter than spur gears. Their precision rating is good. They are recommended for very high speeds and loads. Thus, these gears find wide applications in automotive gearboxes. Their efficiency is slightly lower than spur gears. c) Bevel Gar :- Straight bevel gears are used for transmitting power between intersecting shafts. They can operate under high speeds and high loads. Their precision rating is fair to good. They are suitable for 1:1 and higher velocity ratios and for right-angle meshes to any other angles. Their good choice is for right angle drive of particularly low ratios. However, complicated both form and fabrication limits achievement of precision. They should be located at one of the less critical meshes of the train. Wide application of the straight bevel drives is in automotive differentials, right angle drives of blenders and conveyors. Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 2. CHAPTER – 8 : GEAR & GEAR TRAIN d) Worm & worm gear :- Worm and worm gear pair consists of a worm, which is very similar to a screw and a worm gear, which is a helical gear as shown in Fig. They are used in right-angle skew shafts. In these gears, the engagement occurs without any shock. The sliding action prevalent in the system while resulting in quieter operation produces considerable frictional heat. High reduction ratios 8 to 400 are possible. Efficiency of these gears is low anywhere from 90% to 40 %. Higher speed ratio gears are non-reversible. Their precision rating is fair to good. They need good lubrication for heat dissipation and for improving the efficiency. The drives are very compact. Worm gearing finds wide application in material handling and transportation machinery, machine tools, automobiles etc. e) Rack & pinion :- Rack is a segment of a gear of infinite diameter. The tooth can be spur or helical. This type of gearing is used for converting rotary motion into translatory motion or visa versa. Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 3. CHAPTER – 8 : GEAR & GEAR TRAIN GEAR TERMINOLOGY : Pitch Circle or Pitch Curve It is the theoretical curve along which the gear rolls without slipping on the corresponding pitch curve of other gear for transmitting equivalent motion. Pitch Point It is the point of contact of two pitch circles. Circular Pitch It is the distance along the pitch circle circumference between the corresponding points on the consecutive teeth. If d is diameter of the pitch circle and ‘T’ be number of teeth, the circular pitch (pc) is given by Pc = ll X d / T Diamental Pitch It is defined as the number of teeth per unit length of the pitch circle diameter. Therefore, diamental pitch (pd) can be expressed as Pd = T / d Module It is the ratio of the pitch circle diameter to the number of teeth. Therefore, the module (m) can be expressed as M = d / T Addendum Circle and Addendum It is the circle passing through the tips of gear teeth and addendum is the radial distance between pitch circle and the addendum circle. Dedendum Circle and Dedendum It is the circle passing through the roots of the teeth and the dedendum is the radial distance between root circle and pitch circle. Full Depth of Teeth and Working Depth Full depth is sum of addendum and dedendum and working depth is sum of addendums of the two gears which are in mesh. Tooth Thickness and Space Width Tooth thickness is the thickness of tooth measured along the pitch circle and space width is the space between two consecutive teeth measured along the pitch circle. They are equal to Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 4. CHAPTER – 8 : GEAR & GEAR TRAIN each other and measure half of circular pitch. Top Land and Bottom Land Top land is the top surface of the tooth and the bottom land is the bottom surface between the adjacent fillets. Face and Flank Tooth surface between the pitch surface and the top land is called face whereas flank is tooth surface between pitch surface and the bottom land. Pressure Line and Pressure Angle The driving tooth exerts a force on the driven tooth along the common normal. This line is called pressure line. The angle between the pressure line and the common tangent to the pitch circles is known as pressure angle. LAW OF GEARING : Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 5. CHAPTER – 8 : GEAR & GEAR TRAIN GEAR TRAIN : a) Simple gear train :- Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 6. CHAPTER – 8 : GEAR & GEAR TRAIN Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 7. CHAPTER – 8 : GEAR & GEAR TRAIN b) Compound gear train :- Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 8. CHAPTER – 8 : GEAR & GEAR TRAIN c) Riverted gear train :- Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 9. CHAPTER – 8 : GEAR & GEAR TRAIN d) Epicyclic gear train :- Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi
- 10. CHAPTER – 8 : GEAR & GEAR TRAIN ~*~*~*~*~*~*~*~*~*~*~*~*~*~*~ Theory of Machine (1MEB25) Prepared By : Prof. Jignesh Dangi

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