The ramp, wedge and screw are simple machines belonging to the inclined plane family. Inclined planes makes it easier to move an object. Lifting an object vertically requires a great amount of force. Using a ramp requires less force, but also requires the object to be moved a greater distance.
The two families of simple machines are the inclined plane and the lever. The lever family includes the lever, the wheel and axle, and the pulley. Levers move and inclined planes remain stationary.
Ramps are the most basic type of inclined plane. This group also includes the wedge and screw. The wedge is two inclined planes placed back to back. Wedges provide several advantages. They provide a grip for things to be lifted and can be used to hold things in place. Conversely they may also be used to push things apart. Doorstops and chisels are both examples of wedges.
When an inclined plane is wrapped around a cylinder, it is called a screw. The inclined plane is the threads of the screw. Threads climb up the central cylinder in the same way an inclined plane angles up from the ground. Screws are used to move or raise liquids or solids and to fasten or press objects together.