993

THEORY ANI) PRACTICE OF ENGINEERING.

Hook II.

Fig. 1494. eckart’s power capstan.

l>e multiplied by the number, which is equal to the weight multiplied into the radius ofthe barrel.

Pou-er Capstan has a compound barrel, consisting of two cylinders of different radii; therope is fixed at the end of the largest, and after passing over the pulley, which is attachedby the hook to the load to be moved,it is coiled round the other cylinder,and fastened at its upper end. 13ymeans of the bar at the two ends, thebarrel is urged about its axis, so thatthe rope may coil round the largecylinder, whilst it unwinds itself fromthe smaller. Supposing the diameterof the one to be 21 inches, and theother 20 inches, it will be evident that Aone turn will gather 63 inches in onecylinder, whilst 60 inches will beuncoiled from the other; the quantityof wound rope exceeds the quantityunwound by 3 inches, the half of whichspace will be the distance through•which the pulley moves by one turnof the bar. I3y a common capstanof the same dimensions the length ofrope coiled round the barrel by onerevolution of the bar would be 60inches, and the space described by thepulley or load half that amount.

The power of such a machine isequal to the velocity of the impelled point, divided by the velocity of the working point ;or to the velocity of the power divided by the velocity of the weight.

The lever, then, on the two capstans of similar diameters will exert a power as 1^ to 30,

30

that is, inversely as the velocity of their weights, and the power of one will be - j = 20, orequivalent to twenty fold tackle of pulleys.

To double the power of the machine it is only necessary to cover the small cylinderwith a thickness of a quarter of an inch, making the difference between the radii of eachhalf what it was in the first instance ; the powerof this capstan increasing as the difference betweenthe radii of the cylinder is diminished. Such acapstan is often used as a windlass for raisingweights, and a winch applied instead of a bar toturn it; it lias the advantage of not requiring aratchet-wheel and catch to stop it, as the twoparts of the rope pull on contrary sides of thebarrel; the rope which coils round the largebarrel acts with u largei le>er and with greaterforce than the other, but not sufficiently to over-come the friction of the gudgeons, so that theweight remains wherever it has arrived.

When the anchor of a ship is weighed, thecable itself would be too large to bend round thecapstan ; it is therefore connected to an endlessrope called a messenger, which, as it coils roundthe lower part of the capstan, quits the upper, sothat its place becomes lower and lower, till ithas no longer room on the capstan; and it isnecessary to force it up from time to time, whichis called surging the messenger. This is doneby beating it; to add to the facility of the oper-ation, the capstan is made conical, which enablesthe rope by its tension, when guided by a pulley,to bring the messenger into its proper place.

In the common windlass the power is applied by means of a winch, which is a rectangularlever ; in other cases no wheel is attached to the axle, but it is pierced with holes directedtowards its centre, in which long levers are introduced, and a continuous action producedby several men working together, so that while some are changing the lever from one holeto the other, the rest are turning the windlass.

Fig. 1495.