Gears

PRESENTATION ON GEARS

TOPICS TO BE DISCUSSED

•Gear defination•Gear & wheel•Advantages & disadvantages of gear.•Types of gear•Terminology•Gear trains

GEARGear are used to transmit motion from one

shaft to another or between a shaft and a slide .

Gears use no intermediate link . It transmit motion by direct contact . Two bodies have either a rolling or a

sliding motion along the tangent . No motion is possible along the

common normal .

GEAR AND WHEELIf power transmitted between two shaft is small, motion between them may be obtained by using two pin cylinder or disk 1 and 2 as shown in figure given below .

As the power transmitted increases, slip occurs between the discs and the motion no longer remains definite .

To remove slipping the wheel provided teeth

A toothed wheels is called gears The motion of gears is equivalent to the

friction wheels .

ADVANTAGES OF GEAR DRIVE

It transmit exact velocity ratio .It may be used to transmit large powers It has high efficiency.It has reliable service .It has compact layout

Disadvantages of gear drive

The manufacture of gears require special tools and equipment

The error in cutting teeth may cause vibration and noise during operation .

SPUR GEAR

Teeth is parallel to axis of rotation

Transmit power from one shaft to another parallel shaft

Used in Electric screwdriver, oscillating sprinkler, windup alarm clock, washing machine and clothes dryer

External and Internal spur Gear

EXTERNAL VS INTERNAL GEARS

An external gear is one with the teeth formed on the outer surface of a cylinder or cone.

Conversely, an internal gear is one with the teeth formed on the inner surface of a cylinder or cone.

Helical Gear The teeth on helical gears are cut at an angle to the

face of the gear This gradual engagement makes helical gears

operate much more smoothly and quietly than spur gears

One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees

Helical Gear

SPUR RACK AND PINION

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called the pinion engages teeth on a linear "gear" bar called "the rack“ .rotational motion applied to the pinion causes the rack to move linearly, thereby translating the rotational motion of the pinion into the linear motion of the rack.

SPUR RACK AND PINIONIt is Special case of a spur gear where it is made of infinite diameter so that the pitch surface is a plane .It is used in a lathe in which the rack transmits motion to the saddle .

Double helical or Herringbone gears

A double helical gear is equivalent to a pair of helical gears secured together, one having a right hand helix and the other a left hand helix

The teeth of the two rows are separated by a groove used for tool run out.

Axial thrust of the two rows of teeth cancel each other .

DOUBLE HELICAL OR HERRINGBONE GEARS

These can be run at high speeds with less noise and vibration If the left and right inclinations of a double helical gear meet at a common apex and there is no groove in between, the gear is known as herringbone gears.

intersecting shaft - Bevel gears

Bevel gears are useful when the direction of a shaft's rotation needs to be changed

They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well

The teeth on bevel gears can be straight, spiral or hypoid

locomotives, marine applications, automobiles, printing presses, cooling towers, power plants, steel plants, railway track inspection machines, etc.

Straight and Spiral Bevel Gears

STRAIGHT BEVELGEARSThe teeth are straight, radial to the point of intersection of the shaft axes and vary in cross section throughout their length .Miter gears are mating bevel gears with equal numbers of teeth and with axes at right angles.Use – roller shutter door.

SPIRAL BEVEL GEARA spiral bevel gear is a bevel gear with helical teeth .There exists an axial thrust.

Zero bevel gears –spiral bevel gears with curved teeth but with a zero degree spiral angle .

GEAR TERMINOLOGY

GEAR AND PINION

Gears are the most common means of transmitting power in mechanical engineering.

Gear or wheel. The larger of two interacting gears.

Pinion. The smaller gear in a pair.

Base circle : It is a circle from which involute form is generated. Only the base circle on a gear is fixed and unalterable.

Base pitch (pb): It is the distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the base circle. Sometimes called the 'normal pitch'.

BASE CIRCLE

Pitch is the distance between a point on one tooth and the corresponding point on an adjacent tooth.

Pitch circle. A circle, centered on and perpendicular to the axis, and passing through the pitch point. Sometimes also called the 'pitch line', although it is a circle.

Pitch diameter (D). Diameter of a pitch circle. The nominal gear size is usually the pitch diameter.

PITCH CIRCLE

Circular pitch (p). The distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the pitch circle.

Diametral pitch (Pd). The ratio of the number of teeth to the pitch diameter.

Module (m). The module of a gear is equal to the pitch diameter divided by the number of teeth.

ADDENDUM AND DEDENDUM

Addendum : The radial distance from the pitch surface to the outermost point of the tooth.  

Dedendum : The radial distance from the depth of the tooth to the pitch surface.

A point of contact is any point at which two tooth profiles touch each other.

The pitch point is the point of tangency of two pitch circles and is on the line of centers.

POINT OF CONTACT

The line of action is

also called path of action.

It is the straight line passing through the pitch point and tangent to both base circles.

LINE OF ACTION

LANDS

Bottom land is the surface at the bottom of a gear tooth space adjoining the fillet.

Top land is the (sometimes flat) surface of the top of a gear tooth.

GEAR TRAINS A gear train is two or more gear working together

by meshing their teeth and turning each other in a system to generate power and speed

It reduces speed and increases torque Electric motors are used with the gear systems to

reduce the speed and increase the torque

Types of Gear Trains

Simple gear train Compound gear train Reverted gear train Planetary or epicyclic gear train

Simple Gear Train The most common of the gear train is the gear pair

connecting parallel shafts. The teeth of this type can be spur, helical or herringbone.

Only one gear may rotate about a single shaft axis.

Simple Gear Train

Compound Gear Train

For large velocities, compound arrangement is preferred

Two or more gears may rotate about a single axis

REVERTED GEAR TRAINIf the axes of the first and the last wheels of a compound gear coincide, it is called a reverted gear train .

Such an arrangement is used in clocks and in simple lathes where back gear is used to give a slow speed to the chuck.

PLANETARY OR EPICYCLIC GEAR TRAIN

A gear train having a relative motion of axes is called a planetary or an epicyclic gear train

Important application of epicyclic gears are in transmission, computing devices, wrists watches, hoists, back gear of lathe etc.

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