In the previous chapter, the history and performance of simple machine elements such as, the lever, inclined plane, wedge, screw, and the pulley, and the elementary theories of friction and lubrication were discussed. This chapter continues in the same vein with the history and performance of slightly more advanced devices. Ropedrives, brakes, gearing, cams, and bearings are surveyed. These devices, simple though they are, constitute the essential elements that are used in the construction of almost all pre-industrial machinery and a large proportion of modern equipment. Their efficient use rests, largely, on friction or lubrication, and their useful life depends on wear rates. Thus, the theories of hydrodynamic lubrication and wear are introduced.
The process of snubbing described in the previous chapter, uses friction to amplify the tension in a rope wrapped around a driven drum. Power transmission by ropes or belts probably grew out of snubbing and is first seen in the fifteenth century. The earliest illustration of a continuous rope drive is usually considered to be in a Hussite manuscript of about 1485, see Gnudi, in Ramelli (1588, 579). Although pulleys, winches, and capstans were in common use by this time, continuous rope-drives were rare. Leonardo illustrates such a drive in his sketch of a spindle flyer, Fig. 2.1, where a large diameter, hand-cranked pulley drives the spindle shaft so it rotates and reciprocates axially. In this way, a thread is distributed over a bobbin as spinning proceeds; it is a forerunner of Arkwright's spinning machine of 1775. The reciprocating motion, incidentally, is generated by a pair of lantern gears on the left, which are rotated by an intermittent drive from a crown gear having teeth on only half of its periphery.