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57.—Second Pulley System. The only system of pulleys frequently employed for hoisting is what is called the second system, where there are two blocks of pulleys, one attached to the weight, and the other to the beam; and where the same rope passes round all (fig. 57). The mechanical advantage in this case is simply equal to the number of strings supporting the [...] [more...] |
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58.—Screw Lever Pointer. 139. A combination of levers is sometimes used, but more often for the purpose of magnifying small motions than for exerting great force; that is, for increasing the factor s in the product work at the expense of the factor F. In fig. 58 the motion of the screw is magnified, the pointer describing a considerable arc for one turn of the screw. (p. 138) [more...] |
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59.—Screw Press. A lever and an inclined plane may be combined together into a screw-press, the incluned plane being coiled up into a spiral or screw-thread (fig. 59). For every complete revolution of the lever, the weight is raised a distance equal to that between the spires of the screw-threads; hence the mechanical advantage of such a press is the circumference [...] [more...] |
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61.—Wheel and axle comined with a screw. A wheel and axle may be combined with a screw, as shewn in the contrivance of fig. 61. When the handle is turned, the screw-thread on its axle sends the cog-wheel forward one tooth for every revolution. Such [...] but which merely rotates in ordinary bearings, is called an ‘endless’ screw. (pp. 139, 140) [more...] |
