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1. Cams and followers
Technology Education 120
2. Types of motion
3. Cams and followers
A cam is a specially shaped piece of metal or hard wearing plastic, which is usually fixed to a rotating shaft.
A cam mechanism consist of three parts: cam, slide and follower.
A follower is held against the cam, either by its own weight or by a spring.
As the cam rotates the follower moves up and down in a reciprocating motion.
The distance and speed at which the follower moves depends on the shape of the cam.
4. Cam motion Cycle: One complete rotation of the cam.
Rise: The section of cam causing lift.
Fall: The section of cam causing drop.
Dwell: The point on the profile of the cam where the follower does not move.
Stroke: The distance between the highest and lowest points.
Crown: The highest point on the cam.
Heel: The lowest point on the cam.
6. Pear shaped cam
Pear shaped cams are often used for controlling valves such as those found in car engines.
This type of cam has a long dwell period during which time the follower does not move.
When the follower is moving the rise and fall times are equal because of the symmetrical shape of the cam.
7. Pear shaped cam
8. Circular cam A circular cam is also known as an 'eccentric' cam.
The centre of rotation of the cam is offset from the geometric centre of the circle.
This type of cam produces a smooth form of motion called simple harmonic motion.
As the cam is symmetrical the rise and fall motions are the same.
This type of cam is often used to produce the motion in pumps and to operate steam engine valves (photos below).
9. Circular cam
11. Heart shaped cam
A heart shaped cam causes the follower to move with a uniform velocity.
Heart shaped cams are used when the follower is required to move slowly at a constant speed.
The bobbin winding mechanism on a sewing machine uses a heart shaped cam to ensure that thread is wound evenly onto the bobbin.
12. Snail / drop cam Eccentric, heart shaped and pair shaped cams generally allow for a slow rise and fall of the follower.
However, a snail drop cam is used where the drop or fall of the follower must be sudden.
The example snail/drop cam shown opposite rotates in an anticlockwise direction.
Rotating in a clockwise direction would probably lead to the entire mechanism jamming.
This highlights one possible disadvantage of using this type of cam profile.
Also, to ensure the rotation is smooth, the vertical centre line of the snail/drop cam is positioned slightly to the left of the slide (see diagram).
13. Snail/drop cam
14. Snail / drop cam
15. Box cam
Box cams have followers that are guided by a groove, machined or moulded into the face or edge of the cam.
The shape of the groove controls the range of movement of the follower.
Box cams can be found on winding mechanisms of fixed spool fishing reels.
16. Box cam
17. The machine cuts slots in steel parts for car engines. The box cam and follower automatically operates the guard.
18. SWASH PLATE CAMS swash plate cam is usually a disc that rotates around a shaft. The disc is set at an angle so that the follower moves up and down as the disc rotates.The example below shows how two followers can be used resting on the same swash plate cam. Each follower has a wheel or roller that allows smooth movement.
19. SWASH PLATE CAMS
20. Linear Cam
Linear cams have reciprocating motion as both the input motion through the cam and also as the output motion through the follower.
These cams are sometimes used on automatic machine tools to control the movement of tool bits.
21. The machine seen above is a mechanical paper punch. As the lever is pushed down a gear system (called a rack and pinion) moves the flat plate profile to the left. In turn this pushes down the followers which punch two holes in a piece of paper / card.
22. Types of followers
Radial arm follower
23. TASK A car engine uses cams to operate the valves that let the petrol into the cylinder and the exhaust fumes out.
Find out how this system works and make a drawing of it.