BELTS AND SHAFTING BELT FASTENINGS THE best fastening for a belt is the cement splice. It is far beyond any form of lacing, belt hooks, riveting, or any other method of joining together the ends of a belt. The cement joint is easily applied to leather and to rubber belts, but to make a good cement splice in a canvas belt requires more time and apparatus than is usually at hand. Good glue makes a fine cement for leather belts, and fish glue is less affected by moisture than the other. Many of the liquid glues are fish glue treated with acid so as not to gelatinize when cold. A little bichromate of potash added to ordinary hot glue just before it is used will render it insoluble in water. There are many styles of belt hooks in use, some of the more common kind being shown in Figs. 1, 2, 3 and 4. Fig. 2 is practically a double rivet, Fig. 3 a malleable iron fastening, although similar hooks have been made of pressed steel, and Fig. 4 is the Blake stud, which has the advantage of not weakening the belt but makes a hump on the outside where the ends turn up. Fig. 5 is the Bristol hook of stamped steel which is driven in the points turned over on the other side. Fig. 6 is the Jackson belt lacing and is applied by a hand machine which screws a spiral wire across the ends of a belt. These are then flattened and a rawhide pin or a heavy soft cord used as a hinge joint between them. These joints are probably equal to 90 per cent. of the belt strength. Lacing Belts Belts fastened by lacing are weakened according to the amount of material punched out in making the holes to receive the lacing. It is preferable to lace with a small lacing put many times through small holes. Such a joint is stronger than a few pieces of wide lacing through a number of large holes. Figs. 7 and 9 illustrate two forms of belt lacing, the latter being far preferable to the other. The lacing shown by Fig. 7 is in a double leather belt 5 inches wide. The width makes no difference as the strength is figured in percentage of the total width. There are four holes in this piece of belt, each hole inch in diameter. The aggregate width thus cut out of the belt is 4 X inch I inches. Then 1.5 5 0.30, or 30 per cent. of the belt has been cut away nearly one third of the total strength. = 12 = = In Fig. 9 a different method is followed. Instead of there being a few large holes, there are more smaller ones one fourth more, in fact. There are five holes, each inch in diameter, making a total of 1 inch, or 0.9375 ÷ 5 18 per cent., leaving 81 per cent. of the total belt strength, against 70 per cent. in the belt with large holes. A first-class double leather belt will tear in two under a strain of about 500 pounds to each lace hole, the strain being applied in the holes by means of lacings. = The belt shown by Fig. 9 has 81 per cent. of 1.875 square inches of section, 1.525 square inches left after cutting out the five holes. This amount is good for 3000 X 1.875 = 5625 pounds breaking strain, and as the lacing will tear out under 2500 pounds, it will be seen that we cannot afford to use lacings if the full power of the leather is to be utilized. This, under a factor of safety of 5, would be 1125 pounds to the square inch, or 1125 X 1.525 = 1715 pounds = working strain for the belt, or 1715 ÷ 5 343.5 pounds to each lace. This, too, is too much, as it is less than a factor of safety of 2. The belt to carry 40 pounds working tension to the inch of width must also carry about 40 pounds standing tension, making a strain of 80 pounds to the inch, or 80 X 5 400 pounds. This is a better showing, and gives a factor of safety of 2500 ÷ 400 61. Still, we are wasting a belt of 5625 pounds ultimate strength in order to get from it 400 pounds working strain. This means a factor of safety of over 14 in the body of the belt but of only 6 at the lacing, which shows the advantage of a cement splice. = Fig. 10 shows a method sometimes used to relieve the lace-holes of some of the strain. Double rows of holes are punched as at a b, and the lacing distributed among them. As far as helping the Distance in Feet, from Reel to Point of Support strength of the belt is concerned, this does nothing, for all the stress put upon the belt by the lacing at c must be carried by the belt section at a; therefore this way of punching holes does not increase the section strength. Neither does staggering the holes as shown at d and e. The form of hole-punching shown at a b c is desirable for another reason. It distributes the lacing very nicely and does not make such a lump to thump when it passes over the pulleys. ALINING SHAFTING BY A STEEL WIRE A STEEL wire is often used for alining shafting by stretching it parallel with the direction of the shaft and measuring from the shaft to the wire in a horizontal direction. This steel wire can also be used for leveling or alining in a direction at right angles to the other, 280 SAGS OF A STEEL ALINING WIRE FOR SHAFTING Distance in Feet, from Reel to Point of Measurement IO 20 30 40 50 60 70 80 90 100 110 120 130 140 270 260 250 240 230 220 210 64 160 150 140 130 32 64 32 7 16 64 cocol co 16 32 6 Sag of the Wire in Inches Distance in Feet, from Reel to Point of Support| by making vertical measurements, if it is stretched under established conditions and if the sags at the points of measurement are known. The accompanying table gives the sags in inches from a truly level line passing through the points of support of the wire, at successive points beginning 10 feet from the reel and spaced 10 feet apart for a No. 17 Birmingham gage high grade piano wire, stretched with a weight of 60 pounds, wound on a reel of a minimum diameter of three and one-half inches and for total distances between the reel and point of support of the wire varying by increments of 10 feet from 40 to 280 feet. Thus a wire of any convenient length, of the kind indicated, can be selected, so long as this length is a multiple of 10 feet and between the limits specified, and the table gives the sags from a truly level line at points 10 feet apart for its entire length when it is stretched under the conditions designated. These sags 280 270 260 250 240 230 220 210 200 190 180 170 SAGS OF A STEEL ALINING WIRE FOR SHAFTING Distance in Feet, from Reel to Point of Measurement 150 160 170 180 190 200 210 220 230 240 250 260 270 being known, direct measurements can be made to level or aline a shaft by vertical measurements. The method was originally developed for alining the propeller shafts of vessels, but it is equally serviceable for semi-flexible shafting, as factory line shafts. SPEEDS OF PULLEYS AND GEARS THE fact that the circumference of a pulley or gear is always 3.1416 or 3 times the diameter makes it easy to figure speeds by considering only the diameter of both driver and driven pulleys. Belting from one 6-inch pulley to another gives the same speed to both; but if the driving pulley is 16 inches and the driven pulley only 4 inches it is clear that the small pulley will turn 4 times for every turn of the large pulley. If this is reversed and the small pulley is the driver, the large pulley will only make one turn for every four of the small pulley. The same rule applies to gears if the pitch diameter and not the outside diameter is taken. The following rules have been arranged for convenience in finding any desired information about pulley or gear speeds. Multiply Diam. of Driving Pulley by its Speed and divide by Diam. of Driven Pulley. Multiply Diam. of Driving Pulley Speed of Driving Pulley Diam. of Driven Pulley by its Speed and Diam. of Driving Pulley Speed of Driven Pulley Diam. of Driving Pulley Speed of Driven Pulley Divide by Speed of Driven Pulley. Multiply Diam. of Driven Pulley by Diam. of Driven Pulley Speed of Driving Pulley its Speed and Di Diam. of Driven Pulley Speed of Driven Pulley Diam. of Driving Pulley vide by Diam. of Driving Pulley. Multiply Diam. of Driven Pulley by its Speed and Divide by Speed of Driving Pulley. These rules apply equally well to a number of pulleys belts together or to a train of gears if all the driving and all the driven pulley diameters and speeds are grouped together. |