Fig. 29
From the principle just given, it is plain that, if p o, [Fig. 28], represents a brake lever with the reach rod attached at q, a smaller pull will be exerted on the brake band if the lever is moved to the position b o than would be exerted if a lever were moved through the same angle from b o to d o. The movement from p o to b o is a convenient and easy one for the engineer to make, while the movement from b o to d o is inconvenient. To overcome the inconvenience and still to obtain the advantage of this latter movement, the differential lever shown in [Fig. 29] is used. By means of an arm placed on the lever, the point of attaching the reach rod is at l instead of p; hence, when the handle r b is moved to the position s b, the point l moves to m, thus securing a greater and gradually increasing pull with the easier movement of the handle.
A differential lever may be advantageously used in connection with any band or post brake and on a drum running in either direction. Such levers are considered by many preferable to the differential brake.
48. Power Brakes.—Large drums and heavily loaded drums cannot be controlled by hand-power brakes, and in such a case some other form of power, such as steam, compressed air, or water, must be used.
Fig. 30
[Fig. 30] shows, in outline, how such power is applied. The movements of the hand lever A, instead of being directly communicated to the lever operating the brake, merely control the valve v connected with the cylinder a. By means of this valve, steam, compressed air, or water is admitted to either end of the cylinder and this moves the piston in the direction necessary to apply or release the brake. There are a number of varieties of such power brakes, differing in structural details, but the action of all is essentially the same. With steam or air power, the brake would be applied with its full force almost instantaneously, thus subjecting the various parts of the mechanism to very severe and objectionable strains, unless the valves were modified so as to regulate the admission of the steam or air. One method of controlling this action is the use of a valve that requires a long travel to give it a full opening. Such a valve can be opened a little, so as to allow the steam to leak through and thereby increase the pressure in the cylinder gradually. As the motion is difficult to regulate, a better method is by means of a floating valve, described in Hoisting, Part 1.
49. Crank Brake.—In addition to the brake applied to the drum and intended for use mainly in emergencies, many hoisting engines are also fitted with a strap brake applied to the crank-disk. In some states, crank-brakes are required by law. In order to give a large bearing surface, the crank-disk is made very large.
HOISTING
(PART 4)
Serial 851D Edition 1