tom unit 3

8/2/2019 TOM UNIT 3

1/8

Theory of Machines & Mechanisms

B y R a j e s h K . R . K h o d r e S i r(Email:[email protected] Mob: 9752716726)

SIGNATURE INSTITUTE OF PROFESSIONAL STUDIESPlot No.92, Top Floor MP Nagar Zone-II (-9630724520)

Theory of Machines and Mechanisms

Unit IIIGears

Gears are machine elements, which are used to transmit motion from one shaft to another or between a

shaft and a slide.A gear or a "gear wheel" is a rotating machine part having cut teeth, which mesh with another toothed

part in order to transmit torque.

Advantages and Disadvantages of Gear Drives

The following are the advantages and disadvantages of the gear drive as compared to other drives, i.e.

belt, rope and chain drives.

Advantages-1. The teeth of a gear prevent slipping.2. It transmits exact velocity ratio.3. It may be used to transmit large power.4. It may be used for small center distances of shafts.5. It has high efficiency.6. It has reliable service.7. It has compact layout.

Disadvantages-1. Since the manufacture of gears requires special tools and equipment, therefore it is costlier

than other drives.

2. The error in cutting teeth may cause vibrations and noise during operation.3. It requires suitable lubricant and reliable method of applying it, for the proper operation of gear

drives.

Classification of GearsThe gears or toothed wheels may be classified as follows:

1. According to the position of axes of the shafts.The axes of the two shafts between which the motion is to be transmitted, may be

a) Parallelb) Intersectingc) Non-intersecting and non-parallel.

2. According to the peripheral velocity of the gears.The gears, according to the peripheral velocity of the gears, may be classified as:

a) Low velocityb) Medium velocityc) High velocity.

3. According to the type of gearing.The gears, according to the type of gearing, may be classified as:

a) External gearingb) Internal gearingc) Rack and pinion.

4. According to the position of teeth on the gear surface.The teeth on the gear surface may be

a) Straightb) Inclinedc)

Curved.

8/2/2019 TOM UNIT 3

2/8




Characteristics of Each Type of Gears1 Spur GearTeeth are straight & parallel to shaft axis. It transmits power & motion between rotating two parallel

shafts.

Features:

Easy to manufacture There will be no axial force Relatively easy to produce high quality gears. The common most type.

Application:

Transmission components.2 Helical GearTeeth are twisted oblique to the gear axis.

The hand of the helix is designated as either left or right. Right hand and left hand helical gears mate

as a set. But they have the same helix angle.

Features:

Have higher strength compared with spur gear. Effective in reducing noise & vibration compared with spur gear. Gears in mesh produce thrust forces in the axial directions.

Applications:

Transmission components, automobile, speed reducers etc.

8/2/2019 TOM UNIT 3

3/8




3 RackThe rack is a bar containing teeth on one face for meshing with a gear. The basic rack form is the

profile of the gear of infinite diameter. Racks with machined ends can be joined together to make any

desired length.

Features: Change a rotary motion into a rectilinear motion.

Applications:

A transfer system for machine tools, printing press, robots etc.4 Internal GearAn annular gear having teeth on the inner surface of its rim. The internal gear always meshes with the

external gear.

Features:

In the meshing of two external gears, rotation goes in the opposite direction. In the meshing of aninternal gear with an external gear the rotation goes in the same direction.

Care should be taken to the number of teeth when meshing a large (internal) gear with a small(external) gear, since interference can occur. Usually internal gear is driven by external (small) gear. Allows compact design of the machine.

Applications:

Planetary gear drive of high reduction ratios, clutches etc.5 Bevel GearOne of a pair of gears used to connect two shafts whose axes intersect, and the pitch surfaces are

cones.

Teeth are cut along the pitch cone. Depending on tooth trace bevel gear is classified:

i. Straight bevel gearii. Spiral bevel gearI. Straight Bevel Gear

A simple form of bevel gear having straight teeth which, if extended inward would come together at

the intersection of the shaft axes.

Features:

Relatively easy to manufacture. Provides reduction ratio up to approximately 1:5

Applications:

Machine tools, printing press etc. especially suitable for a differential gear unit.II. Spiral Bevel Gear

Bevel gear with curved, oblique teeth to provide gradual engagement & bring more teeth together at a

given time than an equivalent straight bevel gear.

Features:

Have higher contact ratio, higher strength & durability than an equivalent straight bevel gear. Allows a higher reduction ratio. Have better efficiency of transmission with reduced gear noise. Involves some technical difficulties in manufacturing.

Applications:

Automobile, tractor, vehicles, final reduction gearing for ships

8/2/2019 TOM UNIT 3

4/8




6 Miter GearA special class of bevel gear where the shafts intersect at 90 & the gear ratio is 1:1.

7 Screw GearA helical gear that transmit power from one shaft to another, non-parallel, non-intersecting shafts.

Features:

Used in a speed reducer and/or a multiplying gear. Tends to wear as the gear come in sliding contact. Not suitable for transmission of high horse power.

Applications:

Driving gear for automobile. Automatic machines that require intricate movement.8 Worm Gear Pair:Worm is a shank having at least on complete tooth (thread) around the pitch surface; the driver of a

worm wheel worm wheel is a gear with teeth cut on an angle to be driven by a worm.

Features: Provides large reduction ratios for a given center distance. Quite & smooth action. A worm wheel is not feasible to drive a worm except for special occasions.

Applications:

Speed reducers, anti-reversing gear device making the most of its self-locking features, machinetools, indexing device, chain block, portable generator etc.

8/2/2019 TOM UNIT 3

5/8




8/2/2019 TOM UNIT 3

6/8




Terminology

8/2/2019 TOM UNIT 3

7/8




Terms Used In Gears1. Pitch circle

It is an imaginary circle which by pure rolling action, would give the same motion as the actual

gear.

2. Pitch circle diameterIt is the diameter of the pitch circle. The size of the gear is usually specified by the pitch circlediameter. It is also called as pitch diameter.

3. Pitch pointIt is a common point of contact between two pitch circles.

4. Pitch surfaceIt is the surface of the rolling discs which the meshing gears have replaced at the pitch circle.

5. Pressure angle or angle of obliquityIt is the angle between the common normal to two gear teeth at the point of contact and the

common tangent at the pitch point.

6. AddendumIt is the radial distance of a tooth from the pitch circle to the top of the tooth.

7. DedendumIt is the radial distance of a tooth from the pitch circle to the bottom of the tooth.

8. Addendum circleIt is the circle drawn through the top of the teeth and is concentric with the pitch circle.

9. Dedendum circleIt is the circle drawn through the bottom of the teeth. It is also called root circle.

10.Circular pitchIt 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.

11.ClearanceIt is the radial distance from the top of the tooth to the bottom of the tooth, in a meshing gear. Acircle passing through the top of the meshing gear is known as clearance circle.

12.Total depthIt is the radial distance between the addendum and the dedendum circle of a gear. It is equal to the

sum of the addendum and dedendum.

13.Working depthIt is radial distance from the addendum circle to the clearance circle. It is equal to the sum of the

addendum of the two meshing gears.

14.Tooth thicknessIt is the width of the tooth measured along the pitch circle.

15.Tooth spaceIt is the width of space between the two adjacent teeth measured along the pitch circle.

16.BacklashIt is the difference between the tooth space and the tooth thickness, as measured on the pitch circle.

17.Diametral pitchIt is the ratio of number of teeth to the pitch circle diameter in millimeters.

18.Module.It is the ratio of the pitch circle diameter in millimeters to the number of teeth. Face of the tooth. It

is surface of the tooth above the pitch surface.

19.Top landIt is the surface of the top of the tooth.

20.Flank of the tooth

8/2/2019 TOM UNIT 3

8/8




It is the surface of the tooth below the pitch surface.

21.Face widthIt is the width of the gear tooth measured parallel to its axis.

22.ProfileIt is the curve formed by the face and flank of the tooth.23.Fillet radiusIt is the radius that connects the root circle to the profile of the tooth.

24.Path of contactIt is the path traced by the point of contact of two teeth from the beginning to the end of

engagement.

25.Length of the path of contact. It is the length of the common normal cut-off by the addendumcircles of the wheel and pinion.

26.Arc of contactIt is the path traced by a point on the pitch circle from the beginning to the end of engagement of a

given pair of teeth.

The arc of contact consists of two parts, i.e.(a) Arc of approach

It is the portion of the path of contact from the beginning of the engagement to the pitch point.

(b) Arc of recessIt is the portion of the path of contact from the pitch point to the end of the engagement of a

pair of teeth.

tom unit 3

Documents