Gears and Cams

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