Basic mechanical engineering: gear

Define spur gear.A spur gear is a cylindrical gear whose tooth traces are straight line generation
of the reference cylinder. They are used to transmit rotary motion between
parallel shafts.

Define addendum and dedendum.Addendum is the radial distance of a tooth from the pitch circle to the top of thetooth.
Dedendum is the radial distance of a tooth from the pitch circle to the bottom of
the tooth.
Define circular pitch.It is the distance measured on the circumference of the pitch circle from a point
of one tooth to the corresponding point on the next tooth. It is denoted by Pc
Circular pitch Pc= P/DT
Where D = Diameter of pitch circle.
T = Number of teeth on the wheel.
Path of contact: It is the path traced by the point of contact of two teeth from the
beginning to the end of engagement.
Length of path of contact: It is the length of common normal cut- off by the
addendum circles of the wheel and pinion.
law of gearing.Law of gearing states that, the common normal at the point of contact between a pairof teeth must always pass through the pitch point.
conjugate action.When the tooth profiles are so shaped so as to produce a constant angular velocity ratioduring Meshing, then the surface are said to de conjugate.
angle of approach.The angle of approach is defined as the angle through which a gear rotates from the instant a pair of teeth comes into contact until the teeth are in contact at the pitch point.
the characteristics of in volute action.
a) Arc of contact.
b) Length of path of contact.
c) Contact ratio.
contact ratio.Contact ratio is defined as the ratio of the length of arc of contact to the circular pitch mathematically.
Contact ratio = length of arc of contact Pc
Where Pc = circular path.
the advantage of in volute gear The most important advantage of involutes gear is that the center distance for a pair of involute gears can be varied within limits without changing the velocity ratio.
the conditions to be satisfied for interchangeability of all gears.For interchangeability of all gears, the set must have the same circular pitch, module,diameter pitch, pressure, angle, addendum and dedendum and tooth thickness must be one half of the circular pitch.
gear tooth system.A tooth system is a standard which specifies the relationship between addendum,dedendum, working depth, tooth thickness and pressure angle to attain
interchangeability of gears of tooth numbers but of the same pressure angle and pitch
cycloid.A cycloid is the curve traced by a point on the circumference of a circle which rolls
without slipping on a fixed straight line.
clearance.The amount by which the dedendum of a gear exceeds the addendum of the mating
gear is called clearance.
in volute interference occurs.If the teeth are of such proportion that the beginning of contact occurs before the interference point is met then the involute proportion of the driven gear will mate a non in volute portion of the driving gear and involute interference is said to occur.
the principle reason for employing non standard gears
a) To eliminate the undercutting.
b) To prevent interference.
c) To maintain reasonable contact ratio.
the advantages and disadvantages of gear drive.
Advantages:
a) It transmits exact velocity ratio.
b) It has high efficiency.
Disadvantages:
a) The manufacture of gears require special tool and equipment.
b) The error in cutting teeth may cause vibrations and noise during operation.
helix angle (b).It is the angle between the line drawn through one of the teeth and the center line of the shaft on which the gear is maintained.
gear ratio.The quotient of the number of teeth on the wheel divided by the number of threads on
the worm.
gear train.A combination of gears that is used for transmitting motion from one shaft to another
shaft is known as gear train.
E.g. spur gear, spiral gear.
velocity ratio.Velocity ratio of a simple gear train is defined as the ratio of the angular velocity of
the first gear in the train to the angular velocity of the last gear.
epicycles gear train.In a gear train when the axes of shafts over which the gears are mounted move relative to a fixed axis is called epicyclic gear train..
the function of differential gear used in the rear drive of an automobile.
a) To transmit motion from the engine shaft to the rear driving wheels.
b ) To rotate the rear wheel of different speeds while the automobile is taking a turn.
bevel gears.The gears which are used to connect shafts whose axes of rotation intersect are called bevel gears.
limited slip differential.The coupling unit which is sensitive to wheel speed causes most of the torque to be directed to the slow moving wheel. This combination is called limited slip
differential

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