When the brake is applied, the friction between the brake strap and the circumference of the brake wheel produces a great strain on the pedestal c, which must be securely anchored.
The front ends of the straps are worked into eyes, as shown at f, and by these eyes and suitable pins passing through them the ends are fastened to the brake lever g. This lever is supported on and rotates about a pin h, so that when the braking force is applied at i, in the direction of the arrow, the brake lever rotates, pulling down on strap a and up on strap b; and, if the straps are held firmly at the back end, the more force that is applied at i the tighter will the drum be gripped by a and b.
The ends of the straps should be brought in as close to the drum as is practicable, both front and back, so as to give the greatest amount of contact between the drum and the straps and to get the best effect from the force applied. The springs j are used with straps that are not stiff enough to clear the drum when the brake is released.
42. The rotation of the drum may assist or retard the action of the lever in applying the drop brake. For instance, if, in [Fig. 23], the drum revolves in the direction indicated by the arrow, the pull of the drum at the brake strap is in the same direction as the pull of the lever when applying the brake and the action of the lever is then assisted by the motion of the drum. On the other hand, if the drum is revolving in the opposite direction, it opposes the action of the lever and a greater force must be applied to the lever to overcome this opposing pull of the drum. Hence, in the case of strap brakes, if possible, that end should be anchored which will cause the revolution of the drum to assist the lever in applying the brake and throw the strain on the anchor bolt instead of on the lever.
Fig. 25
43. If a brake is required to work with the drum running in either direction, there are several ways of bringing the strain due to the load on the anchorage in whichever way the drum runs. One of the simplest of these is shown in [Fig. 25], where a is a drum with a strap brake b embracing nearly the entire circumference; c is a lever bar that is attached to the ends of the brake strap by pins d and e, which work in the slots f in the iron anchor plates g. One anchor plate is on each side of the lever, and both are bolted to the foundation. If the band is kept of the proper length, then, no matter which way the drum is turning, the pull of the drum will come on the anchorage, and the pull on the lever need be only sufficient to take up the slack end of the band. To illustrate: If the drum is turning in the direction indicated by the arrow, the pin e holding the lower end of the band will be on the bottom of its slot and the pin d will be free in its slot and engaged in tightening the slack end of the band through the motion of the lever c. Were the drum running the other way, the pin d connected with the upper half of the band would move to the upper end of its slot and take the main load, while the pin e at the lower end of the band would only have to take up the slack. The outer, or long, end of the lever moves downwards in all cases to tighten the band. Provision must be made to lift the band clear of the drum when slack, but no anchorage other than at g should be attempted.
Fig. 26
44. The Differential Brake.—The differential brake has both ends of the brake strap attached to short lever arms operated by the brake lever, but these arms are of different lengths and are so arranged that as the longer arm tightens the brake strap the shorter arm yields and loosens the strap. The tightening, however, is more than the loosening or yielding and, as a result, the brake band is tightened about the brake wheel. The form of the lever arm is immaterial so long as the differential principle is retained, that is, that the shorter arm yields when the longer pulls, when the brake is thrown into action. This principle is illustrated in [Fig. 26]. In this brake, no provision is made for anchoring either end of the brake strap, but the entire load is thrown on the lever arms a and b. These lever arms are connected with the arm c, which revolves on the same shaft d and is operated by the reach rod e. The revolution of the drum is thus resisted by the shaft d.