A. Bhatia, B.E.

Course Outline

Gear is a toothed machine part, such as a wheel or cylinder that meshes with another toothed part to transmit motion or to change speed or direction. Gears are a means of changing the rate of rotation of a machinery shaft.

This 4- hour course outlines the basic fundamentals of gear drives which go to make up a power transmission systems. In order to demonstrate briefly the development of gear drives, from first principles through to safety implications, we consider in this course only the spur gears. Knowledge of these is fundamental to understanding the behaviour of geometrically more complex types, including helical gears which permit high speed applications.

The course includes a glossary and multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the reader will:

• Define gear and gear system;
• Understand the gear classification - parallel, intersecting and non-parallel & non-intersecting gears;
• Describe the characteristics and applications of various gears- spur, helical, bevel, miter, herringbone, rack and pinion gears;
• Understand the terminology for standard gears -addendum, dedendum, module, clearance, circular pitch, diameteral pitch etc;
• Understand the equations used to determine the gear characteristics;
• Understand the law of gearing;
• Describe the gear tooth profiles - involute and cycloidal;
• Explain the method of determining gear ratio;
• Describe how gear ratio is related to torque ratio and speed ratio;
• Describe various types of gear trains - simple, compound, epicyclical;
• Understand the application of materials used in gear metallurgy; and
• Learn the design process in selecting the gear systems - Lewis equation for tooth bending stress.
  

Intended Audience

This course is aimed at students, mechanical engineers, automobile engineers, machine designers, factory & workshop personnel, O & M professionals, facility managers, estimators and general audience.

Course Introduction

Gears are wheels which mesh with each other through interlocking teeth. There are various types of gears that may be classified according to the relative position of the axes of revolution. Spur gears and helical gears are the most common type used for transmission of rotary motion between parallel shafts. They offer maximum transmission of power and high efficiency. Bevel gears and spiral gears are typically used for the efficient transmission of power and motion between intersecting shafts at an angle. The third category, worm gears, hypoid gears and crossed helical gears provide an effective answer for power transmission applications requiring high-ratio speed reduction in a limited space using non-intersecting shafts.

Whatever be the application, gears are typically used for two basic purposes; increase or decrease of rotation speed and increase or decrease of power or torque. To increase speed and reduce torque a large drive gear is coupled to a smaller driven gear. To reduce speed and increase torque a small gear turning a larger gear is used. In majority of cases, gears are generally used for reducing speed and hence increasing torque.
This course will review some basic design fundamentals of gear drives and transmission systems.

Course Content

Course Summary

Gears are machine elements used to transmit rotary motion between two shafts. Gears are of several categories and can be combined in a multitude of ways, some of which are meshing circular spur gears, rack and pinion spur gears, and worm gears. Helical and herringbone gears utilize curved teeth for efficient, high-capacity power transmission. Worm gears, driven by worms transmit motion between non-intersecting right-angle axes.

When two gears are connected they rotate in opposite directions. The only way that the input and output shafts of a gear pair can be made to rotate in the same sense is by interposition of an odd number of intermediate gears. Such a gear train is called a simple train. If there is no power flow through the shaft of an intermediate gear then it is an idler gear. The gear that does the driving is known as the driver and the other is known as the driven gear. If two gears have the same number of teeth then one turn of driver gear causes the driven gear to turn once. When the driver gear is smaller than the driven gear then speed is reduced and it amplifies torque in proportion to their teeth numbers. The pinion is the smallest gear and the larger gear is called the gear wheel.

The shape of the gear teeth is important in order to produce a smooth transfer of the motion. When the teeth action is such that the driving tooth moving at constant angular velocity produces a proportional constant velocity of the driven tooth the action is termed a conjugate action. The teeth shape universally selected for the gear teeth is the involute profile.
One essential for the proper meshing of the gears is that the size of the teeth on the pinion should be the same as the size of the teeth on the wheel. The module must be common to both the gears. Pitch circles contact one another at the pitch point and the pinion's pitch line velocity must be identical to the wheels pitch line velocity. At the pitch point develops a tangential component of action-reaction due to contact between the gears.

Gears can either be obtained as standard components from a manufacturer's catalogue or alternatively specially designed and manufactured. American gear manufacturer's association manuals, AGMA 2001-C95 or AGMA-2101-C95 Fundamental Rating factors and Calculation Methods for involute Spur Gear and Helical Gear Teeth provide guidelines to the selection of gears.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.