0.3 Simple machine elements

Complex machines are made up of moving parts such as levers, gears, cams, cranks, springs, belts, andwheels. Machines deliver a certain type of movement to a desired location from an input force applied somewhere else. Somemachines simply convert one type of motion to another type, such as rotary to linear. While there is a seemingly endless varietyof machines, they are all based upon simple machine elements. The elements discussed here include inclined planes, levers, wheels and axles,pulleys, and screws.

It is important to remember that all machines are limited in their efficiency by friction. No machine is 100 percent efficientin its efforts, so the mechanical advantage gained will require additional energy to accomplish the task. For more information on friction, see this module

The inclined plane

An inclined plane decreases the force required to raise an object a given height byincreasing the distance over which that force must be applied, see . Imagine lifting something twice your weight to a 4 foot high shelf. Now imagine rolling the same mass up a gentlysloping surface. The latter would be much easier. Inclined planes are commonly put to use in cutting devices and oftentwo inclined planes are put back to back to form a wedge. In a wedge, forward movement is converted into a parting movementacting perpendicular to the face of the blade. A zipper is simply a combination of two lower wedges for closing and an upper wedge for opening, as shown in .

The screw

The screw is basically an inclined plane (see ) wrapped around a cylinder. In an inclined plane, a linear force in thehorizontal plane is converted to a vertical "lifting" force. With a screw, a rotary force in the horizontal plane isconverted to a vertical "lifting" force.

When a wood screw is turned, the threads of the screw push up on the wood. A reaction force from the woodpushes back down on the screw threads and in this way the screw moves down even though the force of turning the screw isin the horizontal plane. Screws are known for high friction, which is why they are used to hold things together. A worm gear is sometimes used in machines, but they also have high friction that can waste considerable power.

Levers

A lever has three points of interest: the fulcrum, the load, and the effort applied to the lever. The fulcrum is the point around which the lever pivots rotationally. The load is what we wish to manipulate with the lever, and the load is described by its position relative to the fulcrum, and the force (magnitude and direction) it exerts at that point.The effort is also a force that has a magnitude and a direction, and a position with respect to the fulcrum. A leveris used to change the direction of movement, and to trade the magnitude of the effort for the distance over whichthe effort is applied.