An excellent brake for very large cranes is Matthew's hydraulic brake, in which water is passed from end to end of cylinders fitted with reciprocating pistons, cooling jackets being provided.
In the brake shown, the cone I is pressed against a corresponding recess in the ratchetwheel J, which latter turns loosely in the casing and is provided with a pawl not shown in the figure; this pawl allows freedom of motion when the load is being raised.
To find the distance s through which a brake, exerting the friction F, must rub in order to stop a machine having the total actual energy E at the moment when the brake begins to act, reduce, by the principles of 96, the various efforts and other resistances of the machine which act at the same time with the friction of the brake to the rubbing surface of the brake, and let R be their resultantpositive if resistance, negative if effort preponderates.
In one of these a copper disk, called the brake disk, revolves, and in the other a copper armature disk.
In addition to the brakes on the lifting gear of cranes it is found necessary, especially in quickrunning electric cranes, to provide a brake on the subsidiary motions, and also devices to stop the motor at the end of the lift or travel, so as to prevent over-running.
The defects of this method are that the tops are liable to split in the brake and the butts to remain foul.
Per hour in 13 seconds, the rate at which work would have to be absorbed by the brake blocks would represent 237 H.P. This is lost in heat produced by the friction between the brake blocks and the wheels, though in some systems of electric driving some of the energy stored in the train may be returned to the central station during retardation.
See also DYNAMOMETER for illustrations of the use of what are essentially friction-straps of different forms for the measurement of the brake horse-power of an engine or motor.
The external forces holding the brake from turning are W, distant R from the axis, and the reaction, W 1 say, of the lever against the fixed pin P, distant R1 from the axis.
The torque T at any instant of steady running is therefore I WR Lord Kelvin patented a brake in 1858 (fig.