A shovel, which is a tool made up of a wedge and a lever; a wheelbarrow, which is a tool made up of levers, including planes, screws, and wheels and axels; and a bicycle, which is a vehicle made up of wheels and axels, pulleys, screws, levers, and included planes, are three common examples of compound machines.
Compound machines are devices that use more than one method or type of power transmission to generate mechanical advantage. For example, a person using a shovel to move dirt would be using both muscle power and leverage to do so. A motor vehicle uses muscle power plus a combustion engine or electric motor to move itself and its load. A tractor uses two or more tractors connected together to increase their working capacity. This page focuses on non-motorized compounds.
The word "compound" comes from Latin componere, meaning "to make up," and thus it means "made up of." Every compound machine consists of two or more simple machines joined together in such a way that they work as a single unit - i.e., they combine forces to produce a greater force than could be produced by any of the components working separately.
Simple machines include engines or motors that convert energy into mechanical movement (such as belts and gears) and tools that convert movement into energy (such as hammers and saws).
Every day, we come across a variety of compound machineries. These are machines that are constructed from one or more of the six simple machines. A wheelbarrow, for example, is a compound machine. A wheelbarrow, of course, has a wheel and an axle. A pair of scissors is another simple compound machine tool. They have two cutting elements that move in perpendicular plane. One element cuts the inside of the material, the other cuts the outside.
A hammer is another common simple machine. It has one moving part: a handle with which to strike the object. The moving part strikes the object with enough force to cause it to change shape or state. So, a hammer is a striking tool. An axe is a special type of hammer used by woodworkers. It has a sharp edge instead of a face!
A saw is yet another common machine. It has one moving part: a blade that moves relative to a stationary base or frame. The moving part cuts the material placed between it and the base or frame. A miter saw changes the angle of the cut made by the blade as it moves across the material. A circular saw makes flat cuts in material. But some types of material (such as hardwood) require a bit more force than others to make them change shape or state. That's where power tools come into play.
Electric drills are useful tools for creating holes in materials.
Compound machines are simply machines composed of two or more basic machines. A wedge, an inclined plane (like a ramp), a screw, a pulley, a wheel and axle, and a lever are examples of simple machines. A jack is a machine that uses these three devices simultaneously to lift heavy objects. The term "compound" comes from the fact that many of these devices are used together to create something more complex than the simple machines alone.
Jack-up rigs are commonly used by oil companies to lift tanks off the ground so they can be repaired or replaced. The lifting mechanism consists of two jacks, one at each end of the tank, with ropes connecting them. As one jack lifts the tank, the other jack pushes it away from the first one. This process is repeated until the tank is raised enough to be removed from the hole it was lowered into.
Cranes are very large versions of jacks that use levers and weights to lift very heavy objects. They are usually built in pairs with cables connecting the two cranes so that when one crane lifts something, the other one pulls it back down to the ground. There are single-boom cranes which look like enlarged versions of conventional jacks -- except that the lifting arm is only attached to a single side of the boom.
A compound machine is made up of at least two basic machines. Compound machinery include bicycles, automobiles, scissors, and fishing rods with reels. Compound machines are less efficient but have a bigger mechanical advantage than simple machines. For example, a bicycle with a big wheel and low gears is a simple machine, while a unicycle with large spikes attached to its bottom bracket and three-point suspension is a compound machine.
In science class, we are often taught about the different types of machines including simple and compound. Scientists use these terms when they want to explain how things work. With this knowledge, you can now identify simple machines and compound machines in everyday life. For example, a lawnmower is a simple machine because it uses one motor to turn several wheels that move back and forth to cut the grass. A car is a complex machine because it uses many different motors to drive the wheels, roll down the windows, and turn on the air conditioner.
Simple machines were first developed by Archimedes around 250 B.C. He called his device "a lever used as a balance". It was based on the fact that if you lift more weight over a small distance, then the force acting on that weight will be large compared to its mass. Simple machines have been improved upon since their creation, but they still work well for some applications.