Gears

Types of Gears

1. Spur Gear

- used to transmit rotary motion between parallel shafts

2. Helical Gears

- used to transmit rotary motion between parallel or nonparallel shaft

3. Herringbone Gears

- A herringbone gear consists basically of two rows of helical teeth cut into one gear.

- the axial thrust forces are cancelled by direct subtraction.

- herringbone gears are used for heavy and continuous load applications.

4. Bevel Gears- used to transmit rotary motion between intersecting shafts

Spiral Bevel Gear

Straight Tooth Bevel Gear

Hypoid Gear

5. Worm Gears

- used to transmit rotary motion between nonparallel nonintersecting shafts

Gear Terminology

Important Gear Terminology……

• Pitch Circle

- theoretical circle upon which all calculations are usually based

- its diameter is Pitch Diameter, D

• Circular Pitch, p

- distance measured on the pitch circle from one tooth to adjacent tooth

- p = tooth thickness + width of space

• Pitch diameter, D and Pitch Radius, r

- diameter and radius of pitch circle

• Pitch point

- the point in the imaginary line joining the centers of two meshing gears where the pitch circle touch

• Addendum, a

- radial distance between the top land and the pitch circle

• Dedendum, b

- radial distance from the bottom land to the pitch circle

• Diametral Pitch, P

- ratio of the number of teeth to the pitch diameter, D

- By formula, P = N / D , ( N = number of teeth )

• Module, M

- reciprocal of Diametral Pitch

- M = D / N

• Clearance, c

- addendum, a minus dedendum, b

- c = b – a

• Backlash

- the width of a tooth space exceeds the thickness of engaging tooth

Backlash & Clearance

• Backlash creates looseness between meshing teeth

• Too small value of backlash can cause binding

• Clearance and backlash is required to prevent binding

• Base Circle

- imaginary circle which tooth involute profile is developed

• Pressure Angle, Φ

- angle between the line of action and a line tangent to the 2 pitch circle at the pitch point

Involute teeth profile- Gear tooth profile must be designed to provide smooth

transmission of motion and not interfere with mating tooth

- Advantages of Involute Profile:

1. Easily manufactured2. Interchangeable with gears having same diametral

pitch3. Provides efficient transmission of power due to sliding

action is minimized4. Provides linear path of contact producing constant

pressure angle

Interference and Undercutting- Mating teeth will interfere with each other if contact is made where either of the two surfaces are not involute in shape.

- Interference can be eliminated by machining a concave surfaces into the blank areas which is sufficient to avoid contact in these areas.

- This process is called undercutting which actually weakens the tooth at areas close to the point of maximum bending moment.

Interference and Undercutting

Rack & Pinion

• A rack is a gear whose pitch diameter has become infinite in size

• a straight line for the pitch circle, which is called the pitch line

• An involute of a very large base circle approaches a straight line.

• Therefore, surfaces of the rack teeth are flat, but mesh properly with the involute surfaces of the mating pinion

The Internal Spur Gear (Planetary Gear)

• Circular rings with teeth cut into inner surfaces• Provide much more compact drive system than external

Spur Gear• Since the pinion and internal gear rotate in the same

direction less sliding action and wearing of teeth• Provide larger contact ration and can transmit more

power

Basic Formulas for Spur Gear

• Diametrical pitch, P = N / D• Circular pitch, p = D / N• Addendum, a = 1 / P• Dedendum, b = 1.25 / P• Clearance, c = b – a• p X P =

where D = pitch diameter

N = number of teeth on gear wheel

Speed Ratio

p

g

p

g

N

N

D

D

p

g

p

g

g

p

D

D

r

r

2

2

= speed in rpm

P

NN

P

N

P

NC

DDrrC

gpgp

gpgp

222

22