Gears

Topics: Gear, Gears, Pinion Pages: 29 (4302 words) Published: August 7, 2013
Machine Design
HES3350
Lecture 2 – Gears Semester 1, 2013
1

Learning Objectives
• • • • • Familiar with gear nomenclature Able to select a suitable gear type for different applications Able to determine gear train ratios Determine the AGMA bending and contact stress Able to select appropriate gears from supplier’s catalogue

2

Outline
• • • • • • • Introduction Gear Nomenclature Gear Ratios Gear Forces Lewis’s Equation AGMA Bending Stress AGMA Contact Stress

3

Introduction
• Defined as toothed members transmitting rotary motion from one shaft to another. • Most rugged and durable, but more costly among other means of power transmission (eg. Chains, belts) Input parameters: • Power • Torque • rpm • Direction Output: Power • Torque • rpm • Direction

Transmission Type and Efficiency :
Energy losses (due to
friction,etc)

4

Introduction
• What Do Gears Do?
• Transmit rotation between (parallel) shafts Pinion (driving)

Spur gears

Gear (driven)

Helical gears


Reduce speed (usually) Increase torque (usually) Same action as spur gears Smoother meshing Generate axial loads

5

Introduction
Gear types
• Spur • Helical • Bevel • Herring bone

• Worm

6

Introduction
Gear types

Spur Gears

• Teeth is parallel to axis of rotation • Can transmit power from one shaft to another parallel shaft • Cheapest

7

Introduction
Gear types
Figure 16.1ab (p. 648) Types of helical gears. (a, d, Courtesy Boston Gear. c, Courtesy Horsburgh & Scott.)

Helical Gears

• • • • • •

Teeth is inclined to the axis of rotation Smoother than spur, less noise Develop thrust load (helix angle) More expensive than spur gear Heavy duty and high speed application More compact transmission drive than spur gear

8

Introduction
Gear types

• Bevel gears • Teeth is cut on conical blank • Connect non-parallel shafts 9

Introduction
Gear types
Worm

Worm gear

• Worm gear • Worm gear set is essentially a screw meshing with a special helical gear. • Large gear ratio, + Self-locking, Inefficient • Frictional heat generation is high

10

Introduction
Gear types

• Planetary or epicyclical gear • Some gear axes can be allowed to rotate about others

11

Gear Nomenclature

12

Gear Nomenclature
• • • • Face width, b = width of the tooth Addendum = distance between top face of the tooth to pitch circle Dedendum = distance between pitch diameter to bottom of the gear Contact ratio : the number of tooth in contact during meshing. Roughly 1.4 to 1.8 for spur gear Circular pitch = distance from a point from one tooth to the corresponding point on the adjacent tooth measured along the pitch circle 13



Gear Nomenclature

14

www.abbyclock.com/gearing4.html

Gear Nomenclature

15

Gear Nomenclature

16

Gear Nomenclature

17

Gear Nomenclature

18

Gear Terminologies
• Pinion and Gear
Pinion (driving) Gear (driven)

19

Gear Terminologies
• Gear Module and Diametral Pitch • Module, m = ratio of pitch diameter and number of teeth, m = d/N • Diametral pitch, P = Used only with English unit, ratio of number of teeth and pitch diameter. P=N/d N = number of teeth d = pitch diameter (inche) when used with Diametral Pitch d = pitch diameter (mm) when used with module

• m = 25.4 / P (this is the relationship between module and diametral pitch)

20

Gear Terminologies
• Circular pitch
pc 

d
N

• Relationship to Diametral Pitch, P: pc P  
• Pc in inches; P in teeth per inch

• Relationship to Module, m:
Pc/m = π

• Pc in millimetres, m in millimetres per tooth

21

Gear Terminologies
• Gear ratio
Gear ratio =

• Centre distance

Angular velocity ratio is inversely proportional to radii of pulleys

 2 r1  1 r2

22

Gear Train
• Speed ratio
d d  N N d g1  d g 2  a ab     g 1 g 2   g 1 g 2   d d  N N  c bc d p1  d p 2  p1 p 2    p1 p 2 

Figure 15.29 (p....
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