The /pecial claims for this system, or method, are : That it is positive—no allowances for slip have therefore to he made; cheap—costing much less than leather belting or line shaft ing, if either the power to be transmitted or the distance between shafts is considerable; noise less—even at the highest economical speeds ; that it does not require rigidly exact alignment of shafts, and is therefore not sensitive to slight settling of buildings: and that it permits changes of direction at will, so that power may be readily carried to any part of the building or plant, and he subdivided in accordance with the requirements of the various machines to be operated. There are two methods of putting ropes on the pulleys: one, in which the ropes are single and spliced on, being made very taut at first and less so as the rope lengthens, stretching until it slips, when it is resplieed : the other method is to wind a single rope over the pulley as many turns as needed to obtain the necessary horse-power. and put. a tension pulley to give the necessary adhesion and also take up the wear. The essential parts of a continuous rope transmission are the sheaves, the rope. and the tension device. The sheaves, or grooved wheels, are of two forms: one used only for idlers, having a rounded groove, pref erably of radius but little greater than that of the rope employed : the other having the '-grooved rim required for driving sheaves. Numerous experiments have been made to determine the best angle for the sides of the grooves in a driving-sheave; and practice still leeks uniformity in this respect, but the most general practice at the present. time employs 45°. The bottom of the grooves should be round, and the sides, of .course. smooth or pot. fished, to prevent abrasion of the rope. In multiple grooved sheaves it is of vital importance that all the grooves be of exactly equal diameters and angle. If there be any inequality, the rope will travel in the groove of larger diameter at an increased speed, thus causing the several robes to pull against each other, and throwing the strain of the transmission on less than the whole number of ropes. Nothing has so militated against the general employment of rope driving in this country as the use of imperfect multiple grooved sheaves, those con structed of wood having proved specially faulty. The unequal density of wood permits un equal wear of grooves, and the sheave soon becomes of differentiol diameters. The rope gen erally employed in this country is monilla. Cotton is largely used in England for transmis sion work, but has not seemed to meet special favor here. Nonni transmission rope should be of long fiber, and be laid in tallow, to reduce the fiber friction caused by the bending of the strands in passing round the sheaves. Such rope tests about as below: The above table is based on tests of best long-fiber pure mnilla, made specially for trans mission purposes. The best practice employs in rope driving but 3 per cent of the ultimate strength, though as high as 6 per cent is figured when conditions are exceptionally favorable. A large margin of safety is required to provide against imperfect splicing.
The tension device—necessary where the continuous wrap system is employed—consists of a movable tension-carriage traveling in suitably constructed ways and carrying an idler sheave. the tension required by the traveling ropes being given by a suspended weight conven iently attached to the carriage. The rope having been wrapped round the driving and driven sheaves the proper number of times for the required driving force, the last strand on the slack side should pass over the tension-wheel (which is deflected to lead the two ends of the rope together), and should not become a direct driving strand until it has passed over the driven wheel. Before reaching the driven wheel this strand may have to pass over idlers or
over a groove in the driven wheel itself, but in such cases the groove receiving it should be loose, that the sag may be quickly taken up. As large an amount of the rope as possible should be under the direct influence of the tension-carriage. From 18 to 25 per cent is de sirable, though as low as 5 per cent has been found sufficient under certain conditions. 'Tae number of driving sheaves over which the rope passes enters into the problem as well as the length of the rope itself. 'Where the rope passes over four or five sheaves (as in transmitting power to several floors of a building) it is often desirable to employ more than one tension carriage. The best practice is to use one for every 1.200 ft. of rope. and put not less than 10 per cent of the rope under direct influence of the tension. In direct drives the number of feet of rope may be slightly increased.
The speed of a transmission rope should not exceed 5,000 ft. per minute, as from this point centrifugal force gains so rapidly on the power derived from the increased rope speed that at about 5,500 ft. per minute the power will begin decreasing in the same proportion as its pre vious rise. Taking 6', centrifugal force in lbs.: 0, gravity : IF. weight of rope per running foot ; S', speed of rope in ft. per second, the centrifugal force may be found as follows: 11' X - • The wear of rope increases in proportion to the increase of speed; consequently, a velocity of from 2,500 to 3,500 ft. per minute is most efficient and economical. (In the size of the sheaves employed depends very directly the life and efficiency of a rope transmission. The dhuneters should never be less than thirty times the diameter of the rope. and best results are obtained when the sheaves and idlers on the driving side are forty times, and those on the loose side thirty times, the rope diameter. With smaller sheaves the internal friction 'of the rope fibers is considerable, naturally increasing the wear, and the rope itself, through its stiff ness, can not hug the sheaves closely, thus increasing the loss of power due to centrifugal force. Idlers used merely to support a long horizontal span may, if not too far apart. lie as small as eighteen diameters without perceptibly injuring the rope. This exemption to the rule given above is based on practice, however, and is not theoretically correct. The coeffi cient of triet ion of it rope in a 45° grooved sheave has been considered as variable, but several tests recently made where the power transmitted was determined occurateiy by brake-test, owl, all conditions taken into consideration, showed this coefficient to vary only from .33 to Fig. 7 represents a rope drive recently constructed. The number of wraps of rope de pend on the power to be transmitted. arc laid in the sheaves of pulleys a and 1. The rope is led from the last sheave on driven pulley 1), to ttml over the and /, to the first sheave on engine pulley a. The idler" / is the tension-carriage. The best practice wraps on the rope so that the neighboring- ropes are half the length of the trope apart. This is accomplished by starting from the second sheave on o to second sheave on b. thence to fourth on a, etc.; from the last sheave on b 1-(1 the idlers and lack to first sheave on a, continuing to fill t he vacant sheaves to start ing-point, where as long splice is wade. Fig. 7 shows t he method of taking off power at an angle.