Gear and Gear Train

What is Gear?

• A gear is a wheel with teeth on its outer edge. • The teeth of one gear mesh (or engage) with the

teeth of another.

Applications of Gears:

• Toys and Small Mechanisms – small, low load, low

cost

• Appliance gears – long life, low noise & cost, low to

moderate load

• Power transmission – long life, high load and speed

• Aerospace gears – light weight, moderate to high load

• Control gears – long life, low noise, precision gears

Applications animation

4

Conveyor/Counting Gear train

Gear PumpWatch gear wheels

Advantages of Gear Drive:1. It transmits exact velocity ratio.2. It may be used to transmit large power.3. It has high efficiency.4. It has reliable service.5. It has compact layout.

Dis-advantages of Gear Drive:6. The manufacturing of gear require special tool and

equipment.7. The error in cutting teeth may cause vibration and

noise during operation.

Classification of toothed wheel:

1) According to the position of axes of the shafts:a) Parallel b) Intersecting c)Non-intersecting and

non-parallel (Skew Shaft)

2) According to the peripheral velocity of the gear:

a) Low velocity: The gear having less than 3 m/s

b) Medium velocity: The gears having velocity

between 3 to 15 m/s

c) High velocity: The velocity of gears is more than 15

m/s

3) According to the type of gear:

a) External gearingb) Internal gearingc) Rack and pinion

Example of External, Internal & Rack and Pinion

Rack and Pinion(Jack)

Internal Gear(Pump)

External Gear

4) According to the position of the teeth on the gear surface:

a) Straightb) Inclinedc) Curved

Inclined gear

Spiral gear

Worm and Worm Wheel

Heringbone gearsOr Double Helical gears

Terms used in Gear

Addendum

Dedendum

Addendum CircleFace Width

Pitch circleBased Circle

Tooth ThicknessWidth of space

Circular Pitch Clearance

Top Land

Bottom Land

Face

Flank

Working Depth

Whole Depth

DedendumCircle

• Pitch Cylinder: It is an imaginary friction cylinder which by pure

rolling, transmit the same motion as that of a pair of gear.• Pitch surface: The surfaceOn which teeth are cut to ensure positive drive is called pitch surface

• Pitch circle: The intersection of pitch surface with a plane perpendicular to the axis of rotation is called the pitch circle.

• Pitch point: The contact point of two pitch circle is called the pitch point.

• Addendum circle: It is the circle which bounds the outer end of the teeth. In other words, it is the diameter of a blank on which teeth are cut.

• Addendum: The radial distance between the pitch circle and addendum circle is called the addendum. Generally, this distance is kept equal to one module in a 20° full depth involute teeth gear and 0.8 times the module in 20° stubbed teeth gear.

• Dedendum circle: It is a circle which bounds the bottom of the teeth.

• Dedendum: The radial distance between the pitch circle and the dedendum circle is called the dedendum. The standard value of dedendum is 1.25 times module in 200 full depth teeth gear and that is 0.8 times module in 20° stubbed teeth gear.

• Total depth of teeth: The sum of addendum and dedendum is called the total depth of the tooth.

• Clearance: The difference between dedendum and addendum of a mating gear teeth is commonly referred to as clearance.

• Base circle: A circle from which the tooth profile curve is generated is known as base circle.

• Tooth thickness: The chord length measured along the pitch circle between the two opposite faces of the same tooth is called tooth thickness.

• Top land: It is the surface of the top of the tooth. • Bottom land: The surface at the bottom of the tooth

between the adjacent fillets.• Face: Tooth surface between the pitch circle and the top

land is known as face.• Flank: Tooth surface between the pitch circle and the

bottom land is known flank.• Circular pitch: The distance measured along the pitch

circle from a point on one tooth to the corresponding point on the adjacent tooth is called the circular pitch. It is denoted by p and given by the following relation:

Where d1, d2 = pitch circle diameters of pinion and gear respectively And Z1, Z2 = number of teeth on the pinion and gear respectively • Module: The ratio of the pitch circle diameter to the

number of teeth is called module and is denoted by m.

1 2

1 2

πd πd = = Z Z

p

1

1

dm = Z

• Diametral pitch: The ratio of number of teeth in the pitch circle diameter is called the pitch. It is reciprocal of the module.

• Gear Ratio: It is the ratio of number of teeth on the gear to that on the pinion.

• Velocity ratio: It is the ratio of the angular velocity of the driven gear to the angular velocity of the driving pinion.

1

1

Z 1DP = = d m

2

1

ZG = Z

2 2 1 1

1 1 2 2

ω N d Z 1VR = = = = = ω N d Z G

Law of Gearing:

Q

O1

O2

T

T

N

M

β

αV

V1

V2

P

α

β