This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The crab and winch handle in its simplest form is a simple application of the mechanical principle of the wheel and axle, the handle representing the wheel and the crab or drum the axle. It is used for raising small loads of earth, etc., from deep excavations, and consists of two upright posts, a round wooden drum, through the centre of which an iron rod is passed, this being cranked at one end to form the winch handle, and two iron straps bent into shape to form bearings in which the iron rod can be made to revolve.
But few hoisting machines are so simple as to comprise only the winch handle and drum. The power gained would be totally insufficient for lifting heavy loads. In the single-purchase crab (Fig. 227) the first advance on this device occurs. The winch handle is not put on the same shaft as the drum, but on another lying parallel to it, and the two shafts are geared together by means of toothed wheels, a small one actuated by the winch handle directly actuating a large one on the same shaft as the drum.
The mechanical gain is proportional to the difference in diameter of the large and small toothed wheels, the small one being known as the "pinion" and the large one as the "wheel," on the mechanical principle of the wheel and pinion. A further development of the same appliance occurs in the double-purchase crab (Fig. 228). In this case the power is applied by the winch-handle to the drum through a series of either two or three gear wheels; i.e. the winch handle can be applied to either the first or second wheel of the series, the power being proportionally increased in each case.
A brake should be fitted to all crabs to facilitate lowering operations, and is usually in the form shown in the figures, namely, a flexible steel band passed round the periphery of a flat drum which is of larger diameter than the drum of the crab, but cast in one piece with it. This flat steel band is so arranged that, by depressing the hand-lever attached, it can be tightened on the drum, and by the friction so caused arrest its movement gradually or instantly, according to the amount of pressure exerted.
In use the crab is generally bolted down to a rough timber framework which extends some distance behind it. The framework is then loaded with bricks or cast-iron fire-bars, or some such weighty material, in order to give the crab the necessary stability. The rope (wire or manilla) from the pulley blocks is then passed two or three times round the drum, and on the winch being turned is either allowed to coil up on the drum or the free end past the drum is held by hand and coiled on the ground as it comes in. These crabs are made to lift (with the aid of three sheave pulley blocks) weights from 2 to 20 tons, the load up to which they may be safely used being marked on the side frame by the manufacturer.