CHAPTER LXV TRACTION* Q. What is meant by traction? A. Two things; one the amount of grip or drawing power that a locomotive or other similar engine has, and the other the percentage or proportion of the amount of drawing power that is wasted or absorbed by friction between the rolling body and the road upon which it is drawn, and also the amount of axle friction of the vehicle being drawn, where such vehicle has wheels. Q. Where is traction the greatest? A. Of course upon up grades the traction is greater than upon levels; and upon down grades it is less. Upon grades the friction of the axles remains the same as upon levels, but going down grade there may be very little retardation between the vehicle and the road. Upon curves there is more retardation or waste traction than upon straight tracks or, as the engineer of permanent way calls them, "tangents." Q. What name is given to the percentage or proportion of force required upon railways and other different roads? A. The traction co-efficient. Q. How great is it? A. It runs about as follows under ordinary average conditions: Upon railroads in good condition, with well lubricated axles, 4 pounds per ton of load; upon railroads under ordinary, but not very good conditions, 8; upon a very smooth pavement, 12; on ordinary street pavements in good condition, 20; on street pavements and turnpikes, 30; on turnpikes new laid with coarse gravel and broken stone, 50; on common roads in bad condition, 150; on en* See chapter on "Starting." tirely loose ground or sand, 560. Of course where the load stalls in mud the traction is 2,240 pounds per gross ton. Q. What increases this adhesion or traction in the case of a self-propelled vehicle, as a locomotive or an automobile? A. Weight on the drivers. Other things being equal, the greater the weight thereon the greater the tractive. effort. Q. Why not make all the wheels of a locomotive drivers and thus have all the engine weight utilized in giving traction, instead of having one-fourth to one-third of it: on the trucks? A. That would necessarily lengthen the rigid wheel base. While it would do for slow speeds on straight. roads it would not do at all on curves, by reason of the long wheel base, or for high speeds, on account of the greater tendency of large wheels to leave the track. Q. Can a locomotive have too much cylinder power? A. Yes; it may have cylinder power in excess of tractive power, and thus slip its wheels instead of driving the whole machine ahead. Q. How can tractive force be best measured? A. By a traction dynamometer: an instrument applied between the motor and the train and by or through which it is hauled; the compression or expansion of a spring therein or the amount of pressure exerted by a piston in a cylinder of oil as registered on a gage, showing the force passing through it. Q. How much pull can an ordinary engine exert? A. According to the speed; say from 2,500 pounds at sixty miles an hour up to 12,000 at ten miles.. Q. What are principal causes of slipping? A. Half-wet rails; over-hard tires or over-soft rails; too little weight on drivers; sometimes too short wheelbase or too light springs. Q. What precaution should be taken as regards the condition of the rails in making an up grade? A. Care should be taken that there are no drops of water or of oil on the rails to lessen the traction. Q. How should sand be used? A. Very sparingly; only enough to give the drivers a grip, without unduly covering the rails with a sand crust so as to increase the resistance. Q. What kind of sand is necessary? A. Clean, dry, and sharp. Q. How should it be used? A. Sparingly; that is, sprinkled on. Q. What is the evil effect of thick sanding? A. (1) Stalling on up grades; (2) sanding of axle boxes, rod brasses, and guides. Q. What is the evil effect of sanding one rail more than the other? A. Unequal traction, hence risk of breaking crank-pins and side rods. Q. What is the relation between driver diameter and tractive power? A. Other things being the same, small drivers have the best pulling power, but the weight on drivers and the cylinder diameter and stroke have also influence. It makes a difference (1) where the main rod takes hold, (2) how much of a pull or push it can give, and (3) how much of this pull can be utilized without slipping. Q. About how many pounds pull should it take to move an ordinary train of 500 tons on a level track? A. From 3,000 to 4,500 pounds, according to the wheeldiameter, journal-diameter, character of track, kind and quantity of lubricant supplied, etc., say, 3,750 pounds for average conditions. Q. If the entire weight of an engine could be put on two wheels instead of on six or ten, without injuring the track, could she be started as well? A. Yes. Q. How may the starting and hauling power of noncompound engines be figured? A. By multiplying the square of the cylinder diameter in inches by the mean effective pressure in pounds per square inch and the stroke in inches, and dividing by the actual wheel diameter at the rail in inches. This gives the gross power at the rail in pounds, not allowing for engine friction. Q. At what piston positions has the engine the weakest starting power? A. At latest cut-off. Q. Figure up the tractive power of a 20 x 24-inch non-compound engine with 60-inch drivers and mean effective pressure of 170 pounds? A. 314.16 2 24 × 2 × 170 3.1416 X 60 27,200 pounds. Q. At what may we take the mean effective pressure for starting? A. At 85 per cent of the boiler pressure. Q. What is the formula for the gross tractive power of a two-cylinder compound, when the mean effective pressure is known? A. d2 Ps ; where d is the diameter of the L. P. cyl2 D inder, P the mean effective pressure therein in pounds per square inch, s the stroke, and D the driver diameter (all measurements in inches). Q. Express this formula in words instead of letters. self, by the mean effective pressure in that cylinder and by the stroke and divide the product by double the driver diameter (all measurements being in inches) to get the tractive power in pounds. Q. Suppose there are no indicator diagrams? A. Call the M. E. P. 70 per cent of the boiler pressure. Q. How much should be deducted for internal friction? A. Say 7 per cent. Q. How may the tractive power of a four-cylinder compound be calculated? A. Roughly, by considering the M. E. P. in both cylinders at 45 per cent of the boiler pressure, then dividing 45 by the cylinder-area ratio; and multiplying 0.45, plus the quotient just found, by the boiler pressure, gives the mean effective pressure in the L. P. cylinders. Then the formula for gross tractive power is d2 Ps D the letters having the same meaning as in the preceding answer. Q. Express this formula in words instead of in letters? A. Multiply the diameter of the L. P. cylinder by itself, by the mean effective pressure in pounds per square inch in that cylinder and by the stroke, and divide by the driver diameter (all measurements being in inches) to get the tractive power in pounds. Q. What is the formula for calculating the tractive effort of a Vauclain four-cylinder compound when working as a simple engine? where T is the tractive effort, C the diameter in inches of the H. P. cylinder; c the diameter in inches of the |