There is a growing tendency toward the substitution of hemp and cotton ropes for belting and line shafting as a means of transmitting power in large factories and shops. The advantages claimed for the rope driving system are: 1. Economy; for a rope system is cheaper to install than either leather belting or shafting. 2. In the rope system there is less loss of power by slipping. 3. Flexibility; that is, the ease with which the power is transmitted to any distance and in any direction. In this country, a single rope is carried around the pulley as many times as is necessary to produce the required power, and the necessary tension is obtained by passing the rope around a tension pulley which is weighted to give the desired tension. The ropes used in rope transmission are either of hemp, manilla or cotton. Manilla ropes are mostly used in this country. They are of three strands hawser laid, and may be from inch to 2 inches in diameter. The weight of ordinary manilla or cotton rope is about .3 D2 pounds per foot of length, where D represents the diameter of the rope in inches. Letting w = the weight per foot of length, W = .3 D2. The breaking strength of the rope varies from 7,000 to 12,000 lb. per sq. in. of cross-section. The average value may be taken as 7,000 D2, when D is the diameter of rope. For a continuous transmission, it has been determined by experiment that the best results are obtained when the tension in the driving side of the rope is about th of the breaking strength. That is, Ꭲ, = tension in tight side = 7,000 D2 35 = 200 D2. The ropes run in V-shaped grooves, and the coefficient of friction is, of course, greater than on a smooth surface. The coefficient for grooves with sides at an angle of 45° may be taken at from .25 to .33. The horsepower that can be transmitted by a single rope running under favorable conditions is given by the formula in which H = horsepower transmitted. D= diameter of rope in inches. v = velocity of rope in feet per second. The maximum power is obtained at a speed of about 84 feet per second. For higher velocities, the centrifugal force becomes so great that the power is decreased, and when the speed reaches 145 feet per second, the centrifugal force just balances the tension so that no power at all is transmitted. Consequently, a rope should not run faster than about 5,000 feet per minute, and it is preferable on the score of durability to limit the velocity to 3,500 feet per minute. EXAMPLE.-A rope fly-wheel is 26 feet in diameter and makes 55 revolutions per minute. The wheel is grooved for 35 turns of 11⁄2 inch rope What horsepower may be transmitted? SOLUTION.-Velocity in feet per second = 30.16, the horsepower transmitted by one rope or turn. Then |