DYNAMOMETER Another method for finding the power required to operate machinery is by using a dynamometer, which is a power measuring instrument. The accuracy in determining the power is in proportion to the kind and condition of the measuring instrument. The Prony Brake is one of the most simple and familiar examples of the dynamometer. Let A, Fig. 57, represent a pulley 126.04 inches diameter that makes 200 R. P. M., then TX126.04×200 12 A Fig. 57. = 6600 F. P. M. If a pull of 5 pounds is made Р at P the work is 6600X5=33,000 ft.-lbs. per minute or 1 H. P. To record the amount of pull at P is the office of the dynamometer, as the diameter and R. P. M. of pulley can be readily obtained. Fig. 58 shows the general principle of the prony brake and the way the apparatus is commonly constructed. Shoes a and b can be clamped to pulley with bolts c, c; when the pulley is revolved in the direction of the arrow, the tendency is for the entire brake and lever to rotate in the same direction, which is prevented by weights P in the scale pan (the weight W is to counterbalance the weight of lever arm A when pulley is at rest). When the pulley revolves at its normal R. P. M., sufficient weight P is put in the pan to balance the lever between pins d, d, which are placed to prevent lever from revolving. The power absorbed by the brake shoes a, b, is equal to the amount of work which is accomplished in ft.-lbs. per minute by the revolving shaft. This work in ft.-lbs.=P×N× 2′′ × L The brake must be well lubricated to prevent seizing. Example. An engine shaft makes 150 R. P. M. What is the H. P. developed when a weight of 10 lbs. is just balanced at the end of an 8 ft. lever attached to a pair of brake shoes as in Fig. 58? 1. An engine shaft revolving at 74 R. P. M. will support a weight of 2,000 lbs. at the end of a 10 ft. lever. What is the H. P. of the engine? 2. A pulley on a motor shaft that revolves 750 R. P. M. just balances a weight of 25 lbs. on end of a 5′ 3′′ lever. What H. P. is the motor developing? 3. A two cylinder gasolene motor has a fly wheel making 1,000 R. P. M. When a 51 ft. lever arm balances 25 lbs., what is the H. P. of the motor? 4. What length of arm will be required to balance a 25 lb. weight on a shaft making 150 R. P. M. doing 2 H. P. of work, when the brake shoes are clamped to the fly wheel pulley? 5. Find the weight required to balance the brake shoe lever 5′ 3′′ long on a pulley at 200 R. P. M. and transmitting 10 H. P. ENGINE FLY WHEELS Fly wheels are used to regulate the motion in machinery by storing energy during increasing velocity and giving it out during decreasing velocity. There is no power gained in the use of a fly wheel, in fact, power is used in overcoming friction in bearings when the shaft is loaded with the extra weight of the wheel. Fly wheels are usually made of cast iron and the safe velocity is one mile per minute 5280 88 feet per second. 60 or W = weight of rim in pounds. R= mean rad. of rim in feet. N=R. P. M. of wheel. V = velocity in feet per second = 2πRN÷60. The rotating parts of machinery must be well balanced on their axes on account of the centrifugal force which would cause wear on the bearing and vibration to the machinery. Example. What strain will be put on the bearing of a fly wheel shaft with an unbalanced weight of 5 lbs. on the rim of fly wheel which is 10 ft. dia. making 100 R. P. M.? The tensile strain (S) on the cross section of rim is found by dividing the force by 27, then S=WXRX N2×.00005427. The maximum tension per square inch for cast iron, allowing factor of safety of 10, is 1,000 pounds, corresponding to a velocity of 6,085 F. P. M. so that one mile per minute is within the safe limit. The diameter of a fly wheel is found from formula 1. When R-7 ft., find the strain in a 15 ft. cast-iron fly wheel rim that is 1 in. thick and 16 in. face, running 6,000 F. P. M. 2. What extra strain will be put on the rim in problem 1 if a weight of 10 lbs. is placed at some point on the rim? 3. What strain will be found in a cast-iron fly wheel 4 ft. dia. when the cross section outline of rim is 4 in. square running at 6,000 F. P. M.? 4. What is the strain at a speed of 5,000 F. P. M. of a 4 ft. dia. cast-iron fly wheel with a cross section outline 4 in. dia.? 5. If R=8 ft., what is the tensile strain in a cast-iron fly wheel 16 ft. dia. at 74 R. P. M. with a rim which is 11⁄2 in. thick and 26 in. wide? 6. What is the tensile strain in a cast-iron fly wheel, 4 ft. dia. at 400 R. P. M. with a cross section outline of rim 4 in. square? |