Actual Mechanical Advantage and Efficiency
Because of the large area of sliding contact between the moving and stationary threads, screws typically have large frictional energy losses. Even well-lubricated jack screws have efficiencies of only 15% - 20%, the rest of the work applied in turning them is lost to friction. When friction is included, the mechanical advantage is no longer equal to the distance ratio but also depends on the screw's efficiency. From conservation of energy, the work Win done on the screw by the input force turning it is equal to the sum of the work done moving the load Wout, and the work dissipated as heat by friction Wfric in the screw
The efficiency η is a dimensionless number between 0 and 1 defined as the ratio of output work to input work
Work is defined as the force multiplied by the distance moved, so and and therefore
- or in terms of torque :
So the mechanical advantage of an actual screw is reduced from what it would be in an ideal, frictionless screw by the efficiency . Because of their low efficiency, in powered machinery screws are not often used as linkages to transfer large amounts of power (except in lead screws) but are more often used in positioners that operate intermittently.
Read more about this topic: Screw (simple Machine)
Famous quotes containing the words actual, mechanical, advantage and/or efficiency:
“What is rational is actual and what is actual is rational. On this conviction the plain man like the philosopher takes his stand, and from it philosophy starts in its study of the universe of mind as well as the universe of nature.”
—Georg Wilhelm Friedrich Hegel (1770–1831)
“There is only one evil, to deny life
As Rome denied Etruria
And mechanical America Montezuma still.”
—D.H. (David Herbert)
“Justice is simply the advantage of the stronger.”
—Thrasymachus 5th century B.C., Greek philosopher. The Presocratics, p. 259, ed. Philip Wheelwright, The Bobbs-Merrill Co., Inc. (1960)
“Nothing comes to pass in nature, which can be set down to a flaw therein; for nature is always the same and everywhere one and the same in her efficiency and power of action; that is, nature’s laws and ordinances whereby all things come to pass and change from one form to another, are everywhere and always; so that there should be one and the same method of understanding the nature of all things whatsoever, namely, through nature’s universal laws and rules.”
—Baruch (Benedict)