文档介绍：Machines for Lifting
The strongest athletes can lift close to 455kg just off the ground , but few ordinary people can lift more than about 68kg . Early man soon developed machines for lifting large stones and tree trunks .A simple device is a single pulley wheel arranged as a hoist .But a rope around one pulley merely changes the direction of the pull and friction in the pulley`s bearing in fact makes this simple machine less efficient than a straight pull .If the rope is wound around a wheel or cylinder to form a windlass ,a mechanical advantage is gained ,by means of which a man can easily lift more than his own weight . A small windlass can be driven by a hand crank ---first used in the ninth century ---and many early cranes and hoists used this principle.
Screws and Pulleys
Many basic inventions such as the screw and the pulley , cannot be credited to any one man .The Greeks were probably using screws by about 400 BC and by the time of Archimedes the screw certainly had various applications .Archimedes himself invented a type of pump consisting of a long helix in an upward-sloping tube; by turning a handle at the upper end the operator could ‘screw’ water from the lower end of aspiralto its upper end until it flowed out of the Nile Valley. In Roman and medieval times,screw presses were used for crushing olives and grapes.
In the thirteenth century the French monk Villard de Honneecourt made a machine that used ascrew for lifting instead of pressing downward .Today known as the screw jack,the device has many applications from lifting s car to change a wheel to jacking up whole buildings while a story is slipped underneath.
Pulleys were also known also known to the ancient Greeks who used them to lower a religious the time of Christ,Roman engineers were desiging and making multiple pulley blocks for lifting and making multiple pulley blocks for lifting heavy 200t Egyptian granite obelisk,similar to the so called Cleopatra`s needles now sta