936

THEORY AND PRACTICE OF ENGINEERING.

Book II.

that they can be made to turn only one way,which is serviceable in the raising of weights ;the click enters a new notch whenever thepulley moves round, and prevents its receding.

M. Coulomb made several experiments onthe simple pulley, where the friction of the axisand the rigidity of the rope produced a jointresistance ; when lignum vita?, moved on iron,was either a fifth or seventh of the weight inall velocities, besides the rigidity of the rope,the mean was a sixth, or with a small weighta trifle more. For axes of iron on copper,where the velocity was small, not quite atwelfth, the friction being always a little lessthan that on plane surfaces : when the axis wasgreen oak, and the pulley was of lignum vit»,the friction with tallow was a twenty-sixth, andwithout it a seventeenth; with a pulley ofelm a thirty-third in the first state, and a

twentieth in the other: with the axis of box, and the pulley of lignum vita?, it was atwenty-third and fourteenth : with a pulley of elm, a twenty-ninth and twentieth; andwith an axis of iron, and a pulley of lignum vita; with tallow, it was a twentieth.

There are some which are contrived with aspring at top, which enables the click orratchet which holds the wheel to be lifted up,so that the pulley may turn in a contrarydirection.

In all these contrivances we must haveremembrance that the ropes are subject toconsiderable rigidity; and this is supposed tovary as the diameter, as the curvature, and asthe tension. Coulomb found the power of the jfdiameter expressing the rigidity to be generally ^

1 -7 or 1 -8, never less than 1 -4, and that a 1constant quantity must be supposed to beadded to the weight. Small wet ropes aremore flexible than dry; and when large, lessso. Ropes that are tarred are stiffer by aboutone-sixth, and in cold weather somewhat more ;and in general after a little rest, the stiffness of all ropes is increased.

In a rope of three strands each of two yarns, 12£ lines in circumference, 6 inches ofwhich weighed gros, or 125 grains English , being bent on a fixed axis 4 inches indiameter, required a constant force of the tenth of a pound French , and a fifty-fourth of theweight, to overcome the rigidity : the same rope tarred required the fifth of a pound and afiftieth of the weight. In a rope of five yarns, the circumference 20 lines, and the weight6^ gros, the rigidity was equal to half a pound, and a twenty-third of the weight; whentarred, the rope required one pound and one twentv-one parts of the weight to move it. With strands often yarns, a circumference of 28 lines, and a weight ofJ2{ gros, for 6 inches, the untarred rope showed arigidity of two pounds and a thirteenth of the weight,and the tarred rope of 2*3 pounds, and a tenth of theweight. These experiments were made on a rollerwhich was allowed to move in a horizontal plane,while the rope was coiled around it; it was thereforenecessary to make some allowance for the friction itwas liable to, which varied as its weight, and in-versely as its diameter.

It has been found convenient at times to mount onepulley in such a manner that it has an eccentric motion,which prevents the cord or rope from slipping so easilyfrom off the edge: the pivot upon which the wheel ismounted is placed a little out of the centre, by whichmeans the revolution is changed from that of the regularpulleys, and when the rope attached to it is pulled itpresses the eccentric against that which passes over theedge of the lower pulley, and prevents its slipping off.

Fig. 1278.

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Fig. 1279.