OCR

Normal or controlled braking is effected in two steps. After receiving a signal to stop, a braking force is applied which is normally adequate to stop a fully loaded conveyor. The time for a complete stop is adjustable, and is presently set at six seconds. Then, after an adjust- able time period (eight seconds from the time the stop signal is received), a final and larger force is applied. The mechanical linkage of the brake includes a holding cylinder. This is a fail-safe feature to provide braking force in the event of a loss ox air pressure. This cylinder holds a set of weights in an "up" position until there is a loss of pressure. When pressure is lost, the weights fail and they are heavy enough to apply a force equal to the final force applied during a normal stop. For additional safety the brake wheel is keyed directly to the drive pulley shaft. In the initial phase of the conveyor operation, 1000 LTPH and with two drive units installed, one of the mechanical brakes has sufficient capacity to stop a fully loaded conveyor. In the final phase, or at a rate of 2000 LTPH, any two of the three brakes will have a combined capacity to safely stop a fully loaded conveyor. Belt Tension: Belt tension is maintained through a cable winch which has a self-locking worm gear and a spur gear driven by a 12-hp motor. The operation of the take-up is controlled by a load cell located in the take-up carriage. The output of this load cell is amplified and transmitted to a meter calibrated in tons, which provides operating personnel a continuous indica- tion of belt tension. The take-up winch is automatically operated to correct conditions of slip between belt and drive pulley. The device which detects slip is a combination of three small tacho-generators . One each of these units is attached to the driving pulleys, and the third to a driven pulley. Any difference in the output of these three generators signals that a condition of slip exists. This signal starts the take-up winch, which will continue to operate until slip is eliminated or the tension reaches, a preset maximum limit. If this maximum limit is reached, the conveyor will continue to operate for about three minutes; however the preceding conveyors are stopped immediately.