2. Types of Gears
1. Spur Gear
- used to transmit rotary motion between
parallel shafts
3. 2. Helical Gears
- used to transmit rotary motion between
parallel or nonparallel shaft
4. 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.
5. 4. Bevel Gears
- used to transmit rotary motion between intersecting
shafts
Straight Tooth
Bevel Gear
Spiral Bevel Gear
Hypoid Gear
6. 5. Worm Gears
- used to transmit rotary motion between
nonparallel nonintersecting shafts
8. 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
9. • 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
10. • 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
11. • Clearance, c
- addendum, a minus dedendum, b
-c=b–a
• Backlash
- the width of a tooth space exceeds the thickness of
engaging tooth
12. Backlash & Clearance
• Backlash creates looseness between
meshing teeth
• Too small value of backlash can cause
binding
• Clearance and backlash is required to
prevent binding
13. • 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
14. 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 manufactured
2. Interchangeable with gears having same diametral
pitch
3. Provides efficient transmission of power due to sliding
action is minimized
4. Provides linear path of contact producing constant
pressure angle
15.
16. 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.
19. • 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
20. 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
21. Basic Formulas for Spur Gear
• Diametrical pitch, P = N / D
• Circular pitch, p = πD / N
• Addendum, a = 1 / P
• Dedendum, b = 1.25 / P
• Clearance, c = b – a
• pXP=π
where D = pitch diameter
N = number of teeth on gear wheel
22. Speed Ratio
ω p 2 × π × rg Dg
= =
ω g 2 × π × rp D p
ω = speed in rpm
Dg Ng
=
Dp Np
Dp Dg
C = rp + rg = +
2 2
Np Ng N p + Ng
C= + =
2P 2P 2P
