Fig. 20
36. The Block Brake.—The block brake, [Fig. 20], consists of one or more wooden blocks or shoes b attached to a lever having a fulcrum at d, and connected by a rod to the lever c. Block brakes are objected to mainly because they throw a great load on the journals of the drum when they are applied; they cannot be relied on when there is a heavy load on the drum, and they require the application of great force to the lever c for a given braking power. They are, however, cheap and easily applied to a drum, and the shoe is readily replaced when worn.
37. The Post Brake.—The post brake, [Fig. 21], is composed practically of two block brakes applied at two places on the drum diametrically opposite each other, thus equalizing the pressure on the journals. The blocks are generally somewhat longer than in the block brake, or about one-quarter of the circumference of the drum on each side. In [Fig. 21], a is the drum; b are wooden brake blocks; c are the posts which in the brake shown are of massive, built-up, steel construction; d are the fulcrums on the plates e, which plates are adjustable by means of the nuts f; by means of these nuts, the fulcrums may be brought closer together as the wooden blocks b wear away; g is a tension rod generally furnished with a turnbuckle to adjust its length as the wooden blocks wear away. Power is applied at the end of the bent lever h, as shown by the arrow.
Fig. 21
The stops i are adjusted so that the blocks b on each side are equally distant from the drum when the brake is off. The fulcrums d should be some distance below the drum and brake ring, for if they are too near the drum it will be difficult to swing the lower end of the wooden blocks far enough to clear the drum.
Fig. 22
38. Improved Post Brake.—In order to have an equal clearance at top and bottom, and to have a more powerful leverage than in the ordinary post brake, the posts may be made movable at both top and bottom, [Fig. 22]. The tops of the posts a a′ are moved, as in [Fig. 21], by the tension rod b and the lever c, the latter being connected by rod d to lever e. This lever is pivoted at f and motion is transmitted to the fulcrums j by the link g, the lever h, and the tension rod i. The back post a is supported by the uprights k, which are pivoted at l and swing backwards and forwards like a parallel ruler. The front post a′ is supported by the single upright m, pivoted at n. The setscrews o regulate the motion of the bottom of the posts so as to give equal clearance to the bottom and top of the posts.
An objection to both the block and the post brake is the fact that, if the drum surface to which the brake is applied is not perfectly round, the resistance of the brake will not be uniform when applied while the drum is in motion.