Meachanical Gears

               

Gears are the most common means used for power Gears are the most common means used for power transmission transmission

They could be applied between two shafts which are They could be applied between two shafts which are

      1)Parallel Parallel

      2)Collinear Collinear

      3)Perpendicular and intersecting Perpendicular and intersecting

      4)Perpendicular and nonintersecting Perpendicular and nonintersecting

      5)Inclined at any arbitrary angle

Indian history 

As per our mythological stories was more than 12,000 years old. the knowledge of gears had gone from India to east through some of the globe trotters from China as back as 2600 years BC Primitive gears shown in Fig. 1 were first used in door drive mechanism in temples and caves,and water lifting mechanisms 2600 B.C. in India and elsewhere.

###Aristotle in the fourth century B.C. mentions in his writings that gears were being used

very commonly in many applications. Classical origin of worm gearing was made by

Archimedes 287-212 B.C. Vitruvius a military engineer in his writing in 28 B.C. had described a number of gear applications

Leonard da Vinci used multitudes of gears in various mechanisms developed by him 500 A.D.

CLASSIFICATION OF GEARS

Gears are classified according to the shape of the tooth pair and disposition into spur, helical, double helical, straight bevel, spiral bevel and hypoid bevel, worm and spiral gears

(a) Spur gear,

(b) helical gear,

(c) Double helical gear or herringbone gear,

(d) Internal gear ,

(e) Rack and pinion,

(f) Straight bevel gear,

(g) Spiral bevel gear,

(h) Hypoid bevel gear

(i) worm gear and

(j) Spiral gear

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.  One  such  application . Helical gears are used for parallel shaft drives. They have teeth inclined to the axis . Hence for the same width, their teeth are longer than spur gears and have higher load carrying capacity. Their contact ratio was higher than spur gears and they operate smoother and quieter than spur gears. Their precision rating was good. They are recommended for very high speeds and loads. Thus, these gears find wide applications in automotive gearboxes . Their efficiency was slightly lower than spur gears.  The helix angle also introduces axial thrust on the shaft

DOUBLE HELICAL GEAR OR HERRINGBONE GEAR

Double helical or Herringbone gears used for transmitting power between two parallel shafts. They have opposing helical teeth with or without a gap depending on the manufacturing method adopted, Two axial thrusts oppose each other and nullify. Hence the shaft was free from any axial force. Though their load capacity was very high, manufacturing difficulty makes them costlier than single helical gear. Their applications are limited to high capacity reduction drives like that of cement mills and crushers, one such application was exhibited.

INTERNAL GEAR

Internal gears are used for transmitting power between two parallel shafts. In these gears, annular wheels are having teeth on the inner periphery. This makes the drive very compact .In these drives, the meshing pinion and annular gear are running in the same direction  Their precision rating was fair. They are useful for high load and high speed application with high reduction ratio. Applications of these gears could be seen in planetary gear drives of automobile automatic transmissions , reduction gearboxes of cement mills, step-up drives of wind mills.

RACK AND PINION

Rack was 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. Typical example of rack and pinion applications.

STRAIGHT BEVEL GEAR 

Straight bevel gears are used for transmitting power between intersecting shafts. They can operate under high speeds and high loads. Their precision rating was 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 was in automotive differentials, right angle drives of blenders and conveyors. A typical application of straight bevel used in differential application.

SPIRAL BEVEL GEAR 

Spiral bevel gears are also used for transmitting power between intersecting shafts. Because of the spiral tooth, the contact length is more and contact ratio is more. They operate smoother than straight bevel gears and have higher load capacity. But, their efficiency was slightly lower than straight bevel gear. Usage of spiral bevel gears in an automobile differential.

  
HYPOID BEVEL GEAR

These gears are also used for right angle drive in which the axes do not intersect. This permits the lowering of the pinion axis which was an added advantage in automobile in avoiding hump inside the automobile drive line power transmission. However, the non – intersection introduces a considerable amount of sliding and the drive requires good lubrication to reduce the friction and wear. Their efficiency was lower than other two types of bevel gears. These gears are widely used in current day automobile drive line power transmission.

WORM GEAR 

Worm and worm gear pair consists of a worm, which was very similar to a screw and a worm gear, which was a helical gear . 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 was 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. An industrial worm gear box used for converting horizontal to vertical drive.

SPIRAL GEAR

Spiral gears are also known as crossed helical gears. They have high helix angle and transmit power between two non-intersecting non-parallel shafts. They have initially point contact under the conditions of considerable sliding velocities finally gears will have line contact. Hence, they are used for light load and low speed application such as instruments, sewing machine etc. Their precision rating is poor. An application of spiral gear used in textile machinery.

Friction Clutches

                        A clutch is a mechanical device that engages and disengages the power transmission, especially from driving shaft to driven shaft.
Friction clutch is mostly used  in the transmission of power of shafts and machines which must be started and stopped frequently. It is also found in cases in which power is to be delivered to machines partially or fully loaded. The force of friction is used to start the driven shaft from rest and gradually brings it up to the proper speed without excessive slipping of the friction surfaces. In automotives, Friction clutch is used to connect the engine to the gear box. In operating such a clutch, care must be taken such that the friction surfaces engage easily and gradually brings the driven shaft up to proper speed. The proper alignment of the bearing must be maintained and it should be located as close to the clutch as possible.
It may be noted that
               1. The contact surfaces should develop a frictional force that may pick up and hold the load with reasonably low pressure between the contact surfaces.
               2. The heat of friction should be rapidly dissipated and tendency to grab should be at a minimum.
               3. The surfaces should be backed by a material stiff enough to ensure a reasonably uniform distribution of pressure.
             The material of the clutch faces (i.e. contact surfaces) depends upon the allowable normal pressure and the coefficient of friction.

   MOST USUSAL TYPES OF MECHANICAL CLUTCHES

              1. Disc or plate clutches (single disc or multiple disc clutch),
              2. Cone clutches, and
              3. Centrifugal clutches.

 Single Disc or Plate Clutch

              A single disc or plate clutch, consists of a clutch plates whose both sides are faced with a friction material (commonly Ferrodo). It is mounted on the hub which is free to move axially along the splines of the driven shaft. The pressure plate is mounted inside the clutch body which is bolted to the flywheel. Both the pressure plate and the flywheel rotate with the engine crankshaft or the driving shaft. The pressure plate pushes the clutch plate towards the flywheel by a set of strong springs which are arranged radially inside the body. The three levers (also known as release levers or fingers) are carried on pivots suspended from the case of the body. These are arranged in such a manner so that the pressure plate moves away from the flywheel by the inward movement of a thrust bearing. The bearing is mounted upon a forked shaft and moves forward when the clutch pedal is pressed.
              When the clutch pedal is pressed down, its linkage forces the thrust release bearing to move in towards the flywheel and pressing the longer ends of the levers inward. The levers are forced to turn on their suspended pivot and the pressure plate moves away from the flywheel by the knife edges, thereby compressing the clutch springs. This action removes the pressure from the clutch plate and thus moves back from the flywheel and the driven shaft becomes stationary. On the other hand, when the foot is taken off from the clutch pedal, the thrust bearing moves back by the levers. This allows the springs to extend and thus the pressure plate pushes the clutch plate back towards the flywheel.. Single disc or plate clutch.
              The axial pressure exerted by the spring provides a frictional force in the circumferential direction when the relative motion between the driving and driven members tends to take place. If the torque due to this frictional force exceeds the torque to be transmitted, then no slipping takes place and the power is transmitted from the driving shaft to the driven shaft.

Multiple Disc Clutch

             A multiple disc clutch, may be used when a large torque is to be transmitted. The inside discs (usually of steel) are fastened to the driven shaft to permit axial motion (except for the last disc). The outside discs (usually of bronze) are held by bolts and are fastened to the housing which is keyed to the driving shaft. The multiple disc clutches are extensively used in motor cars, machine tools etc.

Cone Clutch

            A cone clutch, was extensively used in automobiles but now-a-days it has been replaced completely by the disc clutch.
            It consists of one pair of friction surface only. In a cone clutch, the driver is keyed to the driving shaft by a sunk key and has an inside conical surface or face which exactly fits into the outside conical surface of the driven. The driven member resting on the feather key in the driven shaft, may be shifted along the shaft by a forked lever provided at B, in order to engage the clutch by bringing the two conical surfaces in contact. Due to the frictional resistance set up at this contact surface, the torque is transmitted from one shaft to another. In some cases, a spring is placed around the driven shaft in contact with the hub of the driven. This spring holds the clutch faces in contact and maintains the pressure between them, and the forked lever is used only for disengagement of the clutch. The contact surfaces of the clutch may be metal to metal contact, but more often the driven member is lined with some material like wood, leather, cork or asbestos etc.

Centrifugal Clutches

             The centrifugal clutches are usually incorporated into the motor pulleys. It consists of a number of shoes on the inside of a rim of the pulley, The outer surface of the shoes are covered with a friction material. These shoes, which can move radially in guides, are held against the boss (or spider) on the driving shaft by means of springs.
            The springs exert a radially inward force which is assumed constant. The mass of the shoe, when revolving, causes it to exert a radially outward force (i.e. centrifugal force). The magnitude of this centrifugal force depends upon the speed at which the shoe is revolving. A little consideration will show that when the centrifugal force is less than the spring force, the shoe remains in the same position as when the driving shaft was stationary, but when the centrifugal force is equal to the spring force, the shoe is just floating.
            When the centrifugal force exceeds the spring force, the shoe moves outward and comes into contact with the driven member and presses against it. The force with which the shoe presses against the driven member is the difference of the centrifugal force and the spring force. The increase of speed causes the shoe to press harder Centrifugal clutch  and enables more torque to be transmitted.

 SOURCE : THEORY OF MACHINES : KURMI

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