Gears and Cams Chapter 17. 2 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper

Gears and Cams

Chapter 17

2Technical Drawing 13th EditionGiesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart

© 2009 Pearson Education, Upper Saddle River, NJ 07458.

All Rights Reserved.

Objectives

• Define the characteristics of a spur gear, worm gear, and bevel gear

• Calculate the gear ratio and rpm of two mating gears, given the pitch diameters

• Define the principal spur gear terms• Draw a spur gear




Objectives (cont.)

• Describe the relationship between a cam profile and a displacement diagram

• Draw a cam profile, given a displacement profile drawing

• List the types of cam followers




Understanding Gears

• Gears are used to transmit power and rotating or reciprocating motion from one machine part to another

• ANSI/AGMA publishes detailed standards for gear design and drawings




Spur Gears

• Proportions and shapes of gear teeth are well standardized

• The most common geometric form used in gears today is the involute profile• Involute means “rolled inward”







Constructing a Base Circle




The Involute Tooth Shape




Spacing Gear Teeth

• Teeth are spaced around the periphery by laying out equal angles

• The number of spaces should be twice the number of teeth, equal to the tooth thickness at the pitch circle




Spacing Gear Teeth




Rack Teeth

• Gear teeth formed on a flat surface are called a rack




Working Drawings of Spur Gears• Since teeth are cut to a standard

shape, individual teeth are not typically shown




Spur Gear Design

• Spur gear design normally begins with selecting pitch diameter

• The size of the teeth (diametral pitch) depends on:• Gear speeds• Gear materials• Horsepower to be transmitted• Tooth form




Worm Gears

• Worm gears are used to transmit power between nonintersecting shafts that are at right angles to each other

• A worm is a screw with a thread shaped like a rack tooth

• Worm wheels are similar to helical gears cut to conform to the shape of the worm




Worm Gears




Working Drawings of Worm Gears• In assembly drawings, gear teeth

are omitted and the gear blank represented conventionally

• On detail drawings, the worm and gear are usually drawn separately




Bevel Gears

• Bevel gears transmit power between shafts whose axes intersect at any angle

• Bevel gear teeth have the same involute shape as teeth on spur gears but are tapered toward the cone apex




Bevel Gears




Bevel Gear Terminology




Working Drawings of Bevel Gears• Working drawings for bevel gears

give only the dimensions of the gear blank

• Data necessary for cutting teeth are given in a note or table




Cams

• Cams can be used to produce irregular motions• A shaft rotating at uniform speed

carries the cam• A reciprocating member, called the

follower, presses a roller against the curved surface of the cam

• Rotating the cam causes the follower to reciprocate a cyclic motion according to the cam profile




Cams




Displacement Diagrams

• A displacement diagram is a curve showing the displacement of the follower as ordinates on a base line that represents one revolution of the cam

• The motion of the follower as it rises or falls depends on the shape of the curves in the displacement diagram




Displacement Diagrams




Cam Profiles




Cylindrical Cams

• When the follower movement is in a plane parallel to the cam shaft, a cylindrical cam must be employed




Cylindrical Cams

Documents

Gears and Cams Chapter 17. 2 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper