Mechanisms: Levers & Linkages

Think of a lever as a stiff bar that swings around a fixed point called a pivot or fulcrum. To make a lever work, you apply an input force called the effort to move an output force called the load. Depending on where the fulcrum, effort, and load are placed, we divide levers into three distinct families or classes.

First-class levers have the fulcrum in the middle, like a see-saw or scissors. Second-class levers have the load in the middle, like a wheelbarrow, which makes lifting heavy things incredibly easy. Third-class levers have the effort in the middle, like fishing rods or tweezers; they do not make you stronger, but they give you amazing control and speed.

Imagine trying to pull a stubborn metal nail out of a block of wood with just your bare fingers. It is almost impossible because our fingers cannot create enough force. But if you use the claw of a hammer, the nail pops out easily because the hammer acts as a clever lever.

This helper effect is called mechanical advantage. If you make the distance between your hand (the effort) and the pivot (the fulcrum) much longer than the distance between the nail (the load) and the pivot, you multiply your strength. You have to move your hand a longer distance, but the force applied to the nail is massively boosted.

What if you need to transfer movement from one place to another, or even change its direction? That is where linkages come in. Linkages are strips of card, wood, or metal joined together by pivot pins that allow them to swing and push each other.

A reverse-motion linkage uses a central fixed pivot to turn a push to the left into a pull to the right. A parallel-motion linkage keeps two bars moving in the exact same direction at the same time, like windshield wipers. A bell crank linkage changes the direction of motion through ninety degrees, turning a horizontal push into a vertical lift.

Everything that moves in a machine follows one of four basic paths. Linear motion is movement in a straight line in one direction only, like a paper trimmer cutting a page. Rotary motion is round-and-round movement in a circle, like a spinning bicycle wheel or a clock hand.

Reciprocating motion is a straight back-and-forth movement, like the needle of a sewing machine or a manual hand saw. Oscillating motion is a curved back-and-forth movement that swings along an arc, like a grandfather clock pendulum or a swing in a park.

Sometimes we need to change rotary motion into another type of motion. A cam is a shaped wheel mounted on a spinning shaft. As it spins, a sliding rod called a follower rests on its edge, rising and falling to match the cam's shape, turning rotary motion into reciprocating motion.

Gears and pulleys are used to transfer rotary motion between shafts. Gears have interlocking teeth that mesh together, meaning if one spins clockwise, the next must spin counter-clockwise. Pulleys use grooved wheels and a flexible belt to transfer spin over longer distances, keeping the direction of rotation the same unless the belt is twisted.