A Gear is a simple circular ring /wheel having teeth all along its circumference and usually found in sets of more than two. The basic function of gears is transmission of power and motion.Gears are used when positive drives are necessary and when the center distances are relatively short. Gears can be used for transmitting power from parallel, intersecting or skew shafts.
Can openers, clocks ,cars and many other devices use gears in their mechanisms to transmit power through rotation.
The most commonly used forms of Gear teeth are:
1. Involute 2.cycloidal
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| Gear |
Can openers, clocks ,cars and many other devices use gears in their mechanisms to transmit power through rotation.
The most commonly used forms of Gear teeth are:
1. Involute 2.cycloidal
How Does Gears Work:-
Gears are used for transmitting power from one part of a machine to another. Bicycles with the help of chain by pedaling took power to back wheel and makes bicycle to run and in the similar way in case of motor cars gears transmit power from the crankshaft to the driveshaft running under the car that ultimately powers the car wheels .
You can have any number of gears connected together and they can be in different shapes and sizes. Each time you pass power from one gear wheel to another, you can do one of three things:
Bicycle gears:-
Increase speed: If you connect two gears together and the first one has more teeth than the second one (generally that means it's a bigger-sized wheel), the second one has to turn round much faster to keep up. So this arrangement means the second wheel turns faster than the first one but with less force. Looking at our diagram on the right (top), turning the red wheel (with 24 teeth) would make the blue wheel (with 12 teeth) go twice as fast but with half as much force.
Increase force: If the second wheel in a pair of gears has more teeth than the first one (that is, if it's a larger wheel), it turns slower than the first one but with more force. (Turn the blue wheel and the red wheel goes slower but has more force.)
Change direction: When two gears mesh together, the second one always turns in the opposite direction. So if the first one turns clockwise, the second one must turn counterclockwise. You can also use specially shaped gears to make the power of a machine turn through an angle. In a car, for example, the differential (a gearbox in the middle of the rear axle of a rear-wheel drive car) uses a cone-shaped bevel gear to turn the driveshaft's power through 90 degrees and turn the back wheels.
Various types of gears commonly used are described as under:-
Spur Gear : cylindrical gear whose tooth faces are straight lines parallel to the gear axis and can be used only for connecting shafts having axes parallel only.
Rack : Rack is a segment of a gear of infinite diameter.The tooth can be spur or helical depends on the application.This type of gearing is used for converting rotary motion into translatory motion and visa versa.Its pitch line is straight line and pitch surface is a plane surface.
Straight bevel gear: A gear whose tooth traces are straight line generators of the cone .It is conical in form operating on intersecting axes usually at right angles.
Spiral bevel gears: The tooth traces of such gears are curved and oblique lines but thrust in these is greater .These can take more load , run quietly, but thrust load in these is greater.
Hypoid gears: these are similar to spiral bevel gears but the axes of their shafts do not intersect.
Worm gear pair: The worm and mating worm wheel have their axes non-parallel and non-intersecting. These are used when high speed reduction (more than 10:1) is required.
Spur Gear : cylindrical gear whose tooth faces are straight lines parallel to the gear axis and can be used only for connecting shafts having axes parallel only.
Rack : Rack is a segment of a gear of infinite diameter.The tooth can be spur or helical depends on the application.This type of gearing is used for converting rotary motion into translatory motion and visa versa.Its pitch line is straight line and pitch surface is a plane surface.
Helical gear: It is a cylindrical gear whose tooth traces are straight helicles i.e teeth are inclined at an angle
to the gear axis.
Herringbone gear: in the case of spur gears ,the tooth forces act only normal to the gear axis ,whereas in case of helical gears, an additional component of force along the gear axis also acts. Its effect,however, can be eliminated by using two gears of opposite helix together; or a gear may be fabricated such that half of its width is cut with helix in one direction and the other half of the teeth are cut in the opposite direction .Such a gear is called the Herringbone gear. Straight bevel gear: A gear whose tooth traces are straight line generators of the cone .It is conical in form operating on intersecting axes usually at right angles.
Spiral bevel gears: The tooth traces of such gears are curved and oblique lines but thrust in these is greater .These can take more load , run quietly, but thrust load in these is greater.
Hypoid gears: these are similar to spiral bevel gears but the axes of their shafts do not intersect.
Worm gear pair: The worm and mating worm wheel have their axes non-parallel and non-intersecting. These are used when high speed reduction (more than 10:1) is required.
Other types of gears are zerol bevel gears ,crossed helical gears etc.
TERMINOLOGY OF GEAR TOOTH
A gear tooth is formed by portions of a pair of opposed involutes .Most of the terms used in connection with gear teeth are explained in fig.
Base circle: it is the circle from which involute form is generated. Only the base circle on a gear is fixed and unalterable.
Pitch circle : it is an imaginary circle most useful in calculations . It may be noted that an infinite number of pitch circles can be chosen , each associated with its own pressure angle.
Pitch circle diameter (P.C.D): it is the diameter of the circle which by pure rolling action would produce the motion as the toothed gear wheel. This is the most important diameter in gears.
Module: It is defined as the length of the pitch circle diameter per tooth. Thus, if P.C.D of gear be D and number of teeth n then module (m)=D/n. It is generally expressed in mm.
Diametral Pitch: it is expressed as the number of teeth per inch of the P.C.D.
Circular pitch (C.P): It is the arc distance measured around the pitch circle from the flank of one tooth to a similar flank in the next tooth.
C.P= πD/n = πm.
Addendum: this is the radial distance from the pitch circle to the tip of the tooth.
Clearance: this the radial distance from the tip of a tooth to is the bottom of a mating tooth space when the teeth are symmetrically engaged. Its standard value is 0.157m.
Dedendum: this the radial distance from the pitch circle to the bottom of the tooth space.
Dedendum= Addendum + Clearance
=m+0.157 m = 1.157m
Blank Diameter: this is the diameter of the blank from which gear is cut . It is equal to P.C.D plus twice the addenda.
Blank diameter = P.C.D + 2m.
=mn + 2m = m (n + 2)
Tooth thickness: this the arc distance measured along the pitch cicle from its intercept with one flank to its intercept with the other flank of the same tooth.
Normally tooth thickness =1/2 C.P = πm/2
But thickness is usually reduced by certain amount to allow for some amount of blacklash and also owing to addendum correction .
Tooth space: it is the width of space between two teeth measured on the pitch circle.
Face of tooth: it iz that part of the tooth surface which is above the pitch surface.
Flank of the tooth: it is that part of tooth surface which is lying below the pitch surface.
TERMINOLOGY OF GEAR TOOTH
A gear tooth is formed by portions of a pair of opposed involutes .Most of the terms used in connection with gear teeth are explained in fig.
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| Terminology |
Pitch circle : it is an imaginary circle most useful in calculations . It may be noted that an infinite number of pitch circles can be chosen , each associated with its own pressure angle.
Pitch circle diameter (P.C.D): it is the diameter of the circle which by pure rolling action would produce the motion as the toothed gear wheel. This is the most important diameter in gears.
Module: It is defined as the length of the pitch circle diameter per tooth. Thus, if P.C.D of gear be D and number of teeth n then module (m)=D/n. It is generally expressed in mm.
Diametral Pitch: it is expressed as the number of teeth per inch of the P.C.D.
Circular pitch (C.P): It is the arc distance measured around the pitch circle from the flank of one tooth to a similar flank in the next tooth.
C.P= πD/n = πm.
Addendum: this is the radial distance from the pitch circle to the tip of the tooth.
Clearance: this the radial distance from the tip of a tooth to is the bottom of a mating tooth space when the teeth are symmetrically engaged. Its standard value is 0.157m.
Dedendum: this the radial distance from the pitch circle to the bottom of the tooth space.
Dedendum= Addendum + Clearance
=m+0.157 m = 1.157m
Blank Diameter: this is the diameter of the blank from which gear is cut . It is equal to P.C.D plus twice the addenda.
Blank diameter = P.C.D + 2m.
=mn + 2m = m (n + 2)
Tooth thickness: this the arc distance measured along the pitch cicle from its intercept with one flank to its intercept with the other flank of the same tooth.
Normally tooth thickness =1/2 C.P = πm/2
But thickness is usually reduced by certain amount to allow for some amount of blacklash and also owing to addendum correction .
Tooth space: it is the width of space between two teeth measured on the pitch circle.
Face of tooth: it iz that part of the tooth surface which is above the pitch surface.
Flank of the tooth: it is that part of tooth surface which is lying below the pitch surface.
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