Meshing of Gears

8/6/2019 Meshing of Gears

1/19

INTRODUCTION : -

A gear is a rotatingmachine part having cut teeth, or

cogs, which mesh with another toothed part in order to transmit torque. Two or more

gears working in tandem are called a transmission and can produce a mechanical

advantage through a gear ratio and thus may be considered a simple machine. Geared

devices can change the speed, magnitude, and direction of apower source. The most

common situation is for a gear to mesh with another gear, however a gear can also

mesh a non-rotating toothed part, called a rack, thereby producing translation instead

of rotation.

Fig. :- 1 Two meshing gears transmitting rotational motion. Note that the smaller gear is rotating more quickly. Although

the larger gear is rotating less quickly, its torque is proportionally greater.

The gears in a transmission are analogous to the wheels in apulley. An

advantage of gears is that the teeth of a gear prevent slipping.

When two gears of unequal number of teeth are combined a mechanical

advantage is produced, with both the rotational speeds and the torques of the two

gears differing in a simple relationship.

In transmissions which offer multiple gear ratios, such as bicycles and

cars, the term gear, as in first gear, refers to a gear ratio rather than an actual

physical gear. The term is used to describe similar devices even when gear ratio is

continuous rather than discrete, or when the device does not actually contain any

gears, as in a continuously variable transmission.


2/19

DEFINITION OF GEAR : -

Gears are machine elements that transmit motion by means

of successively engaging teeth. The gear teeth act like small levers.

TYPES OF GEARS : -

(1)SPUR GEARS : -

Spur gears orstraight-cut gears are the simplest type of gear.

They consist of a cylinder or disk, and with the teeth projecting radially, andalthough they are not straight-sided in form, the edge of each tooth thus is straight

and aligned parallel to the axis of rotation. These gears can be meshed together

correctly only if they are fitted to parallel axles.


3/19

Fig. :-2 Spur Gear

An external gearis one with the teeth formed on the outer

surface of a cylinder or cone. Conversely, an internal gearis one with the teeth

formed on the inner surface of a cylinder or cone. For bevel gears , an internal gear is

one with the pitch angle exceeding 90 degrees. Internal gears do not cause direction

reversal.

Fig. :-3 External Contact Fig. :-4 Internal Contact

(2)HELICAL GEARS : -


4/19

Helical gears offer a refinement over spur gears. The leading

edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Since

the gear is curved, this angling causes the tooth shape to be a segment of a helix.

Helical gears can be meshed in a parallelorcrossedorientations. The former refers

to when the shafts are parallel to each other; this is the most common orientation. In

the latter, the shafts are non-parallel.

Fig. :-4 Parallel Helical Gear Fig. :-5 Double Helical Gear

(3) BEVEL GEARS : -

A bevel gear is shaped like a right circular cone with most of its tip cut

off. When two bevel gears mesh their imaginary vertexes must occupy the same

point. Their shaft axes also intersect at this point, forming an arbitrary non-straightangle between the shafts. The angle between the shafts can be anything except zero

or 180 degrees. Bevel gears with equal numbers of teeth and shaft axes at 90 degrees

are called miter gears.


5/19

Fig. :-6 Spiral Bevel Gear Fig. :-7 Straight Bevel Gear

(4) HYPOID GEARS : -

Hypoid gears resemble spiral bevel gears except the shaft axes

do not intersect. The pitch surfaces appear conical but, to compensate for the offset

shaft, are in fact hyperboloids of revolution. [8][9] Hypoid gears are almost alwaysdesigned to operate with shafts at 90 degrees.

Fig. :-8 Hypoid Gear


6/19

(5) W0RM GEARS : -

Worm gears resemble screws. A worm gear is usually meshed with

an ordinary looking, disk-shaped gear, which is called thegear,wheel, orworm

wheel.

Worm-and-gear sets are a simple and compact way to achieve a high gear ratio.

Fig. :-9 Worm Gear

(6) RACK & PINION : -

A rack is a toothed bar or rod that can be thought of

as a sector gear with an infinitely large radius of curvature. Torque can be converted

to linear force by meshing a rack with a pinion: the pinion turns; the rack moves in a

straight line. Such a mechanism is used in automobiles to convert the rotation of the

steering wheel into the left-to-right motion of the tie rod.

Fig. :-10 Rack & Pinion


7/19

TERMINOLOGY : -


8/19

Addendum: The radial distance between the Pitch Circle and the top of the teeth.

Arc of Action: Is the arc of the Pitch Circle between the beginning and the end of the

engagement of a given pair of teeth.

Arc of Approach: Is the arc of the Pitch Circle between the first point of contact ofthe gear teeth and the Pitch Point.

Arc of Recession: That arc of the Pitch Circle between the Pitch Point and the last

point of contact of the gear teeth.

Backlash: Play between mating teeth.

Base Circle: The circle from which is generated the involute curve upon which the

tooth profile is based.

Center Distance: The distance between centers of two gears.

Chordal Addendum: The distance between a chord, passing through the points

where the Pitch Circle crosses the tooth profile, and the tooth top.

Chordal Thickness: The thickness of the tooth measured along a chord passing

through the points where the Pitch Circle crosses the tooth profile.

Circular Pitch: Millimeter of Pitch Circle circumference per tooth.

Circular Thickness: The thickness of the tooth measured along an arc following the

Pitch Circle

Clearance: The distance between the top of a tooth and the bottom of the space into

which it fits on the meshing gear.

Contact Ratio: The ratio of the length of the Arc of Action to the Circular Pitch.

Dedendum: The radial distance between the bottom of the tooth to pitch circle.

Diametral Pitch: Teeth per mm of diameter.

Face: The working surface of a gear tooth, located between the pitch diameter and

the top of the tooth.

Face Width: The width of the tooth measured parallel to the gear axis.


9/19

Flank: The working surface of a gear tooth, located between the pitch diameter and

the bottom of the teeth

Gear: The larger of two meshed gears. If both gears are the same size, they are both

called "gears".

Land: The top surface of the tooth.

Line of Action: That line along which the point of contact between gear teeth

travels, between the first point of contact and the last.

Module: Millimeter of Pitch Diameter to Teeth.

Pinion: The smaller of two meshed gears.

Pitch Circle: The circle, the radius of which is equal to the distance from the center

of the gear to the pitch point.

Diametral pitch: Teeth per millimeter of pitch diameter.

Pitch Point: The point of tangency of the pitch circles of two meshing gears, where

the Line of Centers crosses the pitch circles.

Pressure Angle: Angle between the Line of Action and a line perpendicular to the

Line of Centers.

Profile Shift: An increase in the Outer Diameter and Root Diameter of a gear,

introduced to lower the practical tooth number or achieve a non-standard Center

Distance.

Ratio: Ratio of the numbers of teeth on mating gears.

Root Circle: The circle that passes through the bottom of the tooth spaces.

Root Diameter: The diameter of the Root Circle.

Working Depth: The depth to which a tooth extends into the space between teeth onthe mating gear.


10/19

INDUSTRIAL APPLICATIONS : -

* Primary machine parts. * Diesel engine builders.

* Rotary die cutting machines. * Blow molding machinery.

* Agricultural equipment. * Automotive prototype and reproduction.

* Motorcycle transmissions. * Special gear box builders.

* Heavy earth moving vechicles. * Special machine tool builders.

ADVANTAGE :-

* It transmits exact velocity ratio. * It may be used to transmit large power.

* It has high efficiency. * It has reliable service.

* It has compact layout.

DISADVANTAGE :-

*The manufacture of gears require special tools and equipment.

* The error in cutting teeth may cause vibrations and noise during operations.


11/19


12/19


13/19


14/19


15/19


16/19


17/19


18/19


19/19

Meshing of Gears

Documents

Transcript of Meshing of Gears