CHAPTER XXXV. SHAFTING AND BELTING FOR TRANSMISSION OF POWER. In a majority of cases, and from various causes, the operation of machinery by direct connection to the engine shaft is inadvisable. The engine requires to be exactly proportioned to the speed and duties of the one machine, which unfits it for any other duties. Therefore, the use of shafting to transmit power is unavoidable, and as it is a factor in deciding on the form of motor to be employed, it is here dealt with. Heavy Shafting.-A very common error, and one that causes much waste of power, is the use of shafting that is unnecessarily heavy. It will probably astonish a great many mechanics to tell them it will require twice as much power to revolve a 4-inch shaft a given number of times per minute as it will a 2-inch shaft, even though the shaft be hollow and weigh no more than the 2-inch shaft. And it will probably surprise them still more to tell them that in the transmission of power a 4-inch shaft is eight times as strong as a 2-inch. It is true, nevertheless, all other things being equal. The means of ascertaining the proper strength of an iron shaft is the following formula : The diameter should The force applied in pounds x the length of lever or crank applying it in inches 1,700 In the case of a wheel applying the power, the length of the lever is obviously the half diameter of the wheel. In the case of a mild-steel shaft, the diameter may be reduced, safely, ten per cent. Revolutions per Minute. DIAMETER OF IRON SHAFTING PROPER FOR TRANSMITTING VARIOUS POWERS. EFFECTIVE HORSE-POWER REQUIRED TO BE TRANSMITTED. | 20 30 40 50 60 70 | 80 | 90 10 100 Belting. In driving machinery by belting, a ready rule is 70 square feet of belt surface per second = 1 horse-power. So that as the diameter of most engines' fly-wheels is stated with the price in manufacturers' lists, together with the revolutions, it is easy to take out the width of belt required to be driven off the fly-wheel. Approximately The width of single belting, say thick S = 1,100 × the effective horse-power. The velocity of belt in ft. per min. A capitally arranged practical table, for which I am indebted to Mr. Charles L. Hett, A. M. I. C. E., is the following: Double Belting.—As a greater tension can be put upon double belts, the power transmitted by a given width is naturally greater, and a safe rule to assume is one-half more power than a single belt. Ropes. For driving machinery by hemp ropes, the circumference of the driving pulley must not be less than 30 times the circumference of the rope; a good proportion is 100 times. (The circumference of any diameter = 3.141 x the diameter.) The velocity of the rope should be from 3,000 minimum to 6,000 maximum lineal feet per minute. For small powers the ropes should be 44 inches circumference. For large mill-driving the ropes should be 54 to 61⁄2 inches circumference. Weight of hemp ropes The square of the circumference × .04 lbs. per lineal foot. = Some ropes have run for over 10 years, but the average life of ropes is from 3 to 5 years. The V = velocity of ropes in lineal feet per minute. circumfer 4,000 x the indicated horse-power Vx the number of ropes This being found out, add one extra rope as a spare, to allow for changing and repairs. |