Gears and Cams

1. Gears and Cams Chapter 17

2. 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. 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. 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. 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”

7. Constructing a Base Circle

8. The Involute Tooth Shape

9. 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. Spacing Gear Teeth

11. Rack Teeth • Gear teeth formed on a flat surface are called a rack

12. Working Drawings of Spur Gears • Since teeth are cut to a standard shape, individual teeth are not typically shown

13. 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. 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. Worm Gears

16. 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. 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. Bevel Gears

19. Bevel Gear Terminology

20. 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. 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. Cams

23. 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. Displacement Diagrams

25. Cam Profiles

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

27. Cylindrical Cams