39. The Strap Brake.—A strap brake consists of a wrought-iron band or strap that partly encircles the drum and is connected at its free ends to levers with which the band may be tightened on the brake wheel and the drum thus firmly held. The iron or steel band either lies directly against the wooden lagging of the drum or on wooden blocks bolted to the drum; or else it has bolted to it a lining of wooden blocks that bear on the drum when the band is tightened.
The most efficient forms of strap brakes are those in which the strap or straps are in contact with 270° or more of the circumference of the drum. The greater the arc of contact, the more securely is the drum held by the brake. A single strap is sometimes used, but this is only satisfactory with small drums, say 8 feet or less in diameter; on large drums two straps are generally used, each extending half way around the drum. The levers for transmitting the power from the hand lever or treadle to the brake strap are variously arranged. In some cases, the force is multiplied by several short levers; in others, by one long lever. The treadle or foot-lever, however, has been replaced almost entirely by the hand lever.
Fig. 23
40. The simplest form of strap brake, [Fig. 23], consists of a single strap a, with one end anchored at b and the free end attached to the brake lever c. This brake acts on the same principle as the block brake and is open to the objection that it brings an undue load on the journals, but it is more efficient and holds the drum more firmly under a heavy load than a block brake.
Fig. 24
Block brakes are usually run dry, but in band brakes and post brakes with ample surfaces and proper leverage the wood may be occasionally slightly oiled with black oil, which greatly adds to the durability of the blocks without unduly lessening the power of the brake.
41. A two-strap brake is shown in [Fig. 24]. One end of each strap a, b is fastened to the pedestal c by either of the methods shown in [Fig. 24 (a), (b), and (c)]. In the method shown in [Fig. 24 (a) and (b)], the forgings d, d′, drawn out to the form of bolts, are riveted to the ends of the straps and passed through a casting c that is secured to the foundation. The object in giving one bolt to one strap and two bolts to the other strap is to allow the straps to pass each other and yet have their lines of action intersect. The bolts are fastened to c by four nuts on each bolt, i. e., two principal nuts and two locknuts. This gives a means of adjustment in the length of the strap to take up the wear.
A second method of securing or anchoring the back ends of the straps is shown at (c). In this case, a wrought-iron angular piece is riveted to each strap, and these pieces are passed over the bolt e that takes the place of the casting of the former arrangement. Nuts are used, as shown, to adjust the straps for wear. The bolt should be short and stiff, so as to be well able to stand up to its work when the drum is moving or tending to move in the direction shown by the arrow.