Ropes were used as belts for the transmission of power before belts became common. Because of the low coefficient of friction between the rope and the pulleys, multiple loops were usually used, either as a single rope passing several times around the pulleys, or multiple ropes on the same pulleys. The advantage of multiple ropes is that if one fails, the others will continue to transmit power; however, it is difficult to get equal tension without a separate tensioner for each rope. A single rope will distribute the tension evenly, but the tensioner will also have to take the rope leaving the last groove on one pulley, pass it back over the other loops of the rope, and place it on the first groove of the other pulley, to keep the rope from "walking" off the end of the pulleys.
The two pulleys were usually groved to increase the grip of the rope, and of as large a diameter as reasonable to minimize the bending (and wear) of the rope. Rope drives work best when the two shafts are at the same elevation - if one is above the other, the weight of the rope will reduce the effective tension on the lower pulley. For similar reasons, the top span of rope should be under tension, and not the bottom.
I have, in fact, used a rope as a belt on my table saw. It worked, though it slipped easily. To use a rope for power transmission, you need to splice the ends together to form a loop. I used a thick "short splice" which caused a great deal of vibration and noise, and would have worn the rope out quickly if I had not eventually found a V-belt lying in the road on the way to work, and used that instead.
The following is a description of making a "long splice", which is no thicker than the rope, and is therefore better for power transmission. It is taken from "Kent's Mechanical Engineer's Pocket Book", 2nd Edition, 1896, p. 341-343.
The splice in a transmission rope is not only the weakest part of the rope but is the first part to fail when the rope is worn out. If the rope is larger at the splice, the projecting part will wear on the pulleys and the rope fail from the cutting off of the strands. The following directions are given for splicing a 4-strand rope.
The engravings show each successive operation in splicing a 1-3/4 inch manila rope. Each engraving was made from a full-size specimen.
Tie a piece of twine, 9 and 10, around the rope to be spliced, about six feet from each end. Ten unlay the strands of each end back to the twine.
Butt the ropes together and twist each corresponding pair of strands loosely, to keep them from being tangled, as shown in Figure 78.
The twine 10 is now cut, and the strand 8 unlayed and strand 7 carefully laid in its place for a distance of four and a half feet from the junction.
The strand 6 is next unlaid about one and a half feet and strand 5 laid in its place.
The ends of the cores are now cut off so the just meet.
Unlay strand 1 four and a half feet, laying strand 2 in its place.
Unlay strand 3 one and a half feet, laying in strand 4.
Cut all strands off to a length of about twenty inches, for convenience in manipulation.
The rope now assumes the form shown in Fig. 79 with the meeting points of the strands three feet apart.
Each pair of strands is successively subjected to the following operation:
From the point of meeting of the strands 8 and 7, unlay each one three turns; split both the strand 8 and the strand 7 in halves as far back as they are now unlayed and "whip" the end of each half strand with a small piece of twine.
The half othe strand 7 is now laid in three turns and the half of 8 also lain in three turns. The half strand now meet and are tied in a simple knot, 11, Fig. 80, making the rope at this point its original size.
The rope is now opened with a marlin spike and the half strand of 7 worked around the half strand of 8 by passing the end of the half strand 7 through the rope, as shown by the engraving, drawn taut and again worked in. This half strand 13 is now split, and the half strand 7 drawn through the opening thus made, and then tucked under the two adjacent strands, as shown in Fig. 81.
The other half of strand 8 is now wound around the other half strand 7 in the same manner. After eqach pair of strands has been treated in this manner, the ends are cut of at 12, leaving them about four inches long. After a few days' wear they will draw into the body of the rope or wear off, so that the locality of the splice can scarecely be detected.
- Kent's Mechanical Engineer's Pocket Book, 1896
© 2005 W. E. Johns