Rough and Tumble Engineering

by James H. Maggard

LEAD

What is lead? Lead is space or opening of port on steam end of cylinder, when engine is on dead center. (Dead center is the two points of disc or crank wheel at which the crank pin is in direct line with piston and at which no amount of steam will start the engine.) Different makes of engines differ to such an extent that it is impossible to give any rule or any definite amount of lead for an engine. For instance, an engine with a port six inches long and one-half inch wide would require much less lead than one with a port four inches long and one inch wide. Suppose I should say one-sixteenth of an inch was the proper lead. In one engine you would have an opening one-sixteenth of an inch wide and six inches long and in the other you would have one-sixteenth of an inch wide and four inches long; so you can readily see that it is impossible to give the amount of lead for an engine without knowing the piston area, length of port, speed, etc. Lead allows live steam to enter the cylinder just ahead of the piston at the point of finishing the stroke, and forms a "cushion," and enables the engine to pass the center without a jar. Too much lead is a source of weakness to an engine, as it allows the steam to enter the cylinder too soon and forms a back pressure and tends to prevent the engine from passing the center. It will, therefore, make your engine bump, and make it very difficult to hold the packing in stuffing box.

Insufficient lead will not allow enough steam to enter the cylinder ahead of piston to afford cushion enough to stop the inertia, and the result will be that your engine will pound on the wrist pin. You most likely have concluded by this time that "lead" is no small factor in the smooth running of an engine, and you, as a matter of course, will want to know how you are to obtain the proper lead. Well don't worry yourself. Your engine is not going to have too much lead today and not enough tomorrow. If your engine was properly set up in the first place the lead will be all right, and continue to afford the proper lead as long as the valve has not been disturbed from its original position; and this brings us to the most important duty of an engineer as far as the engine is concerned, viz: Setting the Valve.

SETTING A VALVE.

The proper and accurate setting of a valve on a steam engine is one of the most important duties that you will have to perform, as it requires a nicety of calculation and a mechanical accuracy. And when we remember also, that this is another one of the things for which no uniform rule can be adopted, owing to the many circumstances which go to make an engine so different under different conditions, we find it very difficult to give you the light on this part of your duty which we would wish to. We, however, hope to make it so clear to you that by the aid of the engine before you, you can readily understand the conditions and principles which control the valve in the particular engine which you may have under your management.

The power and economy of an engine depends largely on the accurate operation of its valve. It is, therefore, necessary that you know how to reset it, should it become necessary to do so.

An authority says, "Bring your engine to a dead center and then adjust your valve to the proper lead." This is all right as far as it goes, but how are you to find the dead center. I know that it is a common custom in the field to bring the engine to a center by the use of the eye. You may have a good eye, but it is not good enough to depend on for the accurate setting of a valve.

HOW TO FIND THE DEAD CENTER

First, provide yourself with a "tram." This you can do by taking a 1/4 inch iron rod, about 18 inches long, and bend about two inches of one end to a sharp angle. Then sharpen both ends to a nice sharp point. Now, fasten securely a block of hard wood somewhere near the face of the fly wheel, so that when the straight end of your tram is placed at a definite point in the block the other, or hook end, will reach the crown of fly wheel.

Be certain that the block cannot move from its place, and be careful to place the tram at exactly the same point on the block at each time you bring the tram into use. You are now ready to proceed to find the dead center, and in doing this remember to turn the fly wheel always in the same direction. Now, turn your engine over till it nears one of the centers, but not quite to it. You will then, by the aid of a straight-edge make a clear and distinct mark across the guides and cross head. Now, go around to the fly wheel and place the straight end of the tram at same point on the block, and with the hook end make a mark across the crown or center of face of fly wheel; now turn your engine past the center and on to the point at which the line on cross head is exactly in line with the lines on guides. Now, place your tram in the same place as before, and make another mark across the crown of fly wheel. By the use of dividers find the exact center between the two marks made on fly wheel; mark this point with a center punch. Now, bring the fly wheel to the point at which the tram, when placed at its proper place on block, the hook end, or point, will touch this punch mark, and you will have one of the exact dead centers.