Introduction to Simple Machines
How Mechanical Devices are Used to Transmit Forces
Humans have been building mechanical systems for thousands of years. Over time, several basic elements were created, and are now referred to as "simple machines". Simple machines are the very basic building blocks of mechanical operation, and provide the user with a form of mechanical advantage. Simple machines make working and living easier, and have allowed for continual human development throughout history.
Scientists in the Renaissance era classified 6 different simple machines:
- the lever
- the wheel and axle
- the pulley
- the inclined plane
- the wedge
- and the screw.
These 6 elements form the basis for complex machinery even today. The 6 machines can be divided into 2 categories based on their operation: machines that require an equilibrium of torque, and those that require an equilibrium of vector forces.
Machines Requiring Equilibrium of Torque
There are 2 simple machines that require equilibrium of torque: the wheel and axle and the pulley.
- The Wheel: If a weight is attached to a rope which is wrapped around the axle, and a second rope is wrapped around the rim of the wheel, when pulling down on the rope around the rim, the mechanical advantage is defined as the ratio of the rim diameter (D) to the axle diameter (d).
- The Pulley: A pulley can be used to change the direction of an input force, or to achieve mechanical advantage. Pulleys are often used with belt systems, such as automobiles to either input a force onto the belt, or to be turned by a moving belt. Multiple pulleys can be used in a single system to achieve mechanical advantage. A single, fixed pulley provides a mechanical advantage of 1. A single moveable pulley provides a mechanical advantage of 2. More complex compound pulleys can provide substantial mechanical advantage, which depends on the equilibrium of forces through each of the vertical rope sections in the system, and whether pulleys are fixed or moveable.
Machines Requiring Equilibrium of Vector Forces
There are 4 simple machines that require equilibrium of vector forces: the inclined plane, the lever, the wedge, and the screw.
- The Inclined Plane: An inclined plane, or ramp, allows the user to raise objects from one height to another by pushing them up a sloped surface. The force required to push the object depends on the weight of the object, the friction between the object and the ramp, and the angle of the ramp.
- The Lever: A lever is a rigid object that is used in conjunction with a pivot point to lift objects. The mechanical advantage is defined as the ratio of the distance from the pivot to the input force divided by the distance from the pivot to the lifting point.
- The Wedge: A wedge could be considered an inclined plane, but in this case it is the wedge that is moved. The mechanical advantage of a wedge is the ratio of its length to its thickest width. A wedge can be used to push two objects apart, or to lift an object.
- The Screw: A screw combines the advantages of the inclined plane with a rotational input. The mechanical advantage is based on the circumference of the screw divided by the pitch of the screw threads, assuming no additional mechanical advantage from an input mechanism, such as a screwdriver.
Simple machines can be found in complex machines and everyday objects. A bicycle contains several simple machines, including pulleys, wheels and axles, and levers. Most tools in a toolbox provide some form of mechanical advantage and use the principles of simple machines.
