Theoretically, the winch would have to work at arate of 12 horsepower to raise the anchor in 2 minutes.Of course, you’ve left out all friction in this problem, sothe winch motor would actually have to be larger than12 hp.You raise planes from the hangar deck to the flightdeck of a carrier on an elevator. Some place along theline, an engineer had to figure out how powerful themotor had to be to raise the elevator. It’s not too toughwhen you know how. Allow a weight of 10 tons for theelevator and 5 tons for the plane. Suppose that you wantto raise the elevator and plane 25 feet in 10 seconds andthat the overall efficiency of the elevator mechanism is70 percent. With that information you can figure whatthe delivery horsepower of the motor must be. Set upthe formulas:Substitute the known values in their proper places,and you have:So, you need 136.4 horsepower if the engine has 100percent overall efficiency. You want to use 70 percentefficiency, so you use the formula:This is the rate at which the engine must be able towork. To be on the safe side, you’d probably select a200-horsepower auxiliary to do the job.FIGURING THE HORSEPOWERRATING OF A MOTORYou have probably seen the horsepower rating plateson electric motors. You may use several methods todetermine this rating. One way to find the rating of aFigure 8-3.-A prony brake.motor or a steam or gas engine is with the use of theprony brake. Figure 8-3 shows you the prony brakesetup. A pulley wheel is attached to the shaft of themotor and a leather belt is held firmly against thepulley. Attached to the two ends of the belts are springscales. When the motor is standing still, each scalereads the same— 15 points. When the pulley turns in aclockwise direction, the friction between the belt andthe pulley makes the belt try to move with the pulley.Therefore, the pull on scale A will be greater than15 pounds, and the pull on scale B will be less than15 pounds.Suppose that scale A reads 25 pounds and scale Breads 5 pounds. That tells you the drag, or theforce against which the motor is working, is25 – 5 = 20 pounds. In this case the normal speed ofthe motor is 1,800 revolutions per minute (rpm) and thediameter of the pulley is 1 foot.You can find the number of revolutions by holdingthe revolution counter (fig. 8-3, C) against the end of theshaft for 1 minute. This counter will record the numberof turns the shaft makes per minute. The distance(D) that any point on the pulley travels in 1 minute is8-3

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