Screw

SCREW, (from the Dutch, scroeve,) one of the six mecha nical powers; chiefly used in pressing or squeezing bodies close, though sometimes also in raising weights.

The screw is a spiral thread, or grove, cut round a cylin der, and everywhere making the same angel with the length of it. So that, if the surface of the cylinder, with this spiral thread upon it, were unfolded and stretched into a plane, the spiral thread would form the section of an inclined plane, whose length would be to its height, as the circumference of the cylinder is to the distance between two threads of the screw ; as is evident by considering, that in making one round, the spiral rises along the cylinder the distance between the two threads.

Hence the threads of a screw may be traced upon the smooth surface of a cylinder thus : cut a sheet of paper into the form of a right-angled triangle, having its base to its height in the above proportion, viz., as the circumference of the cylinder of the screw is to the intended distance between two threads ; then wrap this paper triangle about the cylin der, and the hypothenuse of it will trace out the line of the spiral thread.

\Vhen the spiral thread is upon the outside of a cylinder, the screw is said to be a male one ; but if the thread be cut along the inner surface of a hollow cylinder, or a round per foration, it is said to be female; and this latter is also some times called the box or nut.

\Vhen motion is to be given, the male and female screw are necessarily conjoined ; that is, whenever the screw is to be used as a simple engine, or mechanical power. But when

joined with an axis in peritrochio, there is no occasion for a female ; but in that case it becomes part of a compound engine.

The screw cannot properly be called a simple machine, because it is never used without the application of a lever, or winch, to assist in turning it. The force of' a power applied to turn a screw round, is to the force with which it presses upwards or downwards, setting aside the friction, as time distance between two threads is to the circumference where the power is applied.

For the screw being only an inclined plane, or half wedge, whose height is the distance between two threads, and its base the said circumference ; and the force, in horizontal diree tion, being to that in the vertical one as the lines perpendi cular to them, viz., as the height of the plane or distance of the two threads, is to the base of the plane, or circumference, at the place where the power is applied ; therefore the power is to the pressure, as the distance of two threads is to that circumference.

Hence, when the screw is put in motion; then the power is to the weight which would keep it in equilibria, as the velo city of the latter is to that of the former. And hence their two momenta are equal, which are produced by multiplying each weight, or power, by its own velocity.
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