Gears and Cams Chapter 17

2 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ 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

3 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

4 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

5 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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”

6 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved.

7 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Constructing a Base Circle

8 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. The Involute Tooth Shape

9 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

10 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Spacing Gear Teeth

11 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Rack Teeth Gear teeth formed on a flat surface are called a rack

12 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Working Drawings of Spur Gears Since teeth are cut to a standard shape, individual teeth are not typically shown

13 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

14 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

15 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Worm Gears

16 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

17 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

18 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Bevel Gears

19 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Bevel Gear Terminology

20 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

21 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

22 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Cams

23 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. 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

24 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Displacement Diagrams

25 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Cam Profiles

26 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Cylindrical Cams When the follower movement is in a plane parallel to the cam shaft, a cylindrical cam must be employed

27 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart © 2009 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Cylindrical Cams