CAPSTAN. (Cabestan, Fr.; Spille, Germ.) A machine whereon the cable is wound successively in weighing the anchor of a vessel. It is a species of wheel and axle; the axle being vertical, and pierced with holes near its top for the insertion of the ends of horizontal levers, called handspikes, which represent the wheel. These are turned by the force of men moving in a circle. The power applied to the lever is to the resistance to be overcome, (the weight of the anchor, for example,) when the forces are in equilibrio, as the radius of the cylinder round which the cable is coiled is to the circumference described by the power.
It is manifest that the radius of the axle must be augmented in this computation by half the diameter of the cable, which is supposed to lie always one coil thick upon it. The force of a man, thus applied, has been commonly estimated as equal to the traction of 27 pounds hanging over a pulley.
Friction being so variable a quantity in capstans, renders the exact calculation of its mechanical effect somewhat uncertain.
A stout man, stationed near the bottom of the axle, holds fast the loose part of the cable, which has already made two or three turns; and, being aided by its friction upon the wood, he both prevents it from slipping backwards, and uncoils each turn as it is progressively made.
Mr. Hindmarsh, master mariner of Newcastle, obtained a patent, in February, 1827, for a contrivance to enable a capstan or windlass to be occasionally worked with increased mechanical advantage. With this view, he placed toothed wheel-work, partly in the drum-head of the capstan, and partly in the upper part of the barrel, upon which the cable is coiled and uncoiled in successive portions.
The drum-head, and also the barrel, turn loosely upon a central spindle, independent of each other, and are connected together either by the toothed geer, or by bolts. On raising or withdrawing the connecting pinion from the toothed wheels, and then locking the drum-head and barrel together, the capstan works with a power equal only to that exerted by the men at the capstan-bars, as an ordinary capstan; but on lowering the pinion into geer with the wheel-work, and withdrawing the bolts which locked the drum-head to the barrel, the power exerted by the men becomes increased in proportion to the diameter and numbers of teeth in the wheels and pinions.
[Fig. 255.] is the external appearance of this capstan. [Fig. 256.] a horizontal view of the toothed geer at the top of the barrel. The barrel, with the whelps a a, turns loosely upon a verticle spindle fixed into the deck of the vessel. The drum-head b also turns loosely upon the same spindle. The circular frame c c, in [fig. 256.], in which the axes of the toothed wheels d d d are mounted, is fixed to the central spindle. The rim e e e, with internal teeth, is made fast to the top of the barrel; and the pinion f, which slides upon the spindle, is connected to the drum-head.
When it is intended to work the capstan with ordinary power, the pinion f is raised up into the recess of the drum-head, by means of a screw g, [fig. 255.], which throws it out of geer with the toothed wheels, and it is then locked up by a pin z: the bolts h h are now introduced, for the purpose of fastening the drum-head and barrel together, when it becomes an ordinary capstan.