Gears

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