Mechanisms: levers, linkages and cams
Design & Technology · Key Stage 3 · The School
Three classes of lever
A lever has an EFFORT, a LOAD and a FULCRUM, and the class depends on their order. FIRST class has the fulcrum in the middle — a seesaw, scissors. SECOND class has the load in the middle — a wheelbarrow, a nutcracker. THIRD class has the effort in the middle — tweezers, a fishing rod, and your own forearm. Identifying the class tells you at once whether force is multiplied.
Linkages
A LINKAGE is a set of bars joined by pivots that changes the direction, size or type of a movement. A reverse-motion linkage makes one end go up as the other goes down; a parallel linkage keeps two parts moving together; a bell crank turns movement through ninety degrees. Linkages are how a small input in one place produces a controlled output somewhere else.
Cams
A CAM is a shaped wheel that turns while a FOLLOWER rests on its edge, so rotation becomes an up-and-down motion. The cam's SHAPE decides the pattern: a pear cam gives a smooth rise and fall with a long pause, a snail cam a slow rise and sudden drop. Choosing the shape is choosing the movement, which is what makes cams so useful in toys and engines alike.
SEE-SAW: fulcrum in the middle, effort one end, load the other — first class. Also scissors and pliers. WHEELBARROW: load in the middle, wheel is the fulcrum, you lift at the end — second class, and it always multiplies force. TWEEZERS: effort in the middle, fulcrum at the joint, load at the tip — third class, which multiplies distance and movement instead. Identify the class by the ORDER of fulcrum, effort and load.