Engineering reminiscences contributed to "Power" and "American machinist"

by Charles T. Porter

When the stone-dressing machine was started a difficulty presented itself. The governor was in constant motion a short distance up and down, causing the engine to oscillate, running alternately too fast and too slow. There was nothing that should have caused this action, so far as I could observe. The load on the engine was constant. However the work done on the stone may have varied, the work of the engine was to lift the hammers, and these, being lifted successively, presented a uniform resistance. The oscillation was not very great, as nearly as I can remember about 12 per cent. of the speed; which would give to each hammer a variation of thirty-six blows per minute. This, however, produced a waving surface on the stone. The more rapid the blow, the stronger it was and the deeper the cut. These waves were slight, only about ¹⁄₅₀ of an inch variation in depth, but yet it was not possible for our rubbing-machine to grind them off without great loss of time. So we had to employ three or four stone-cutters to chisel off these ridges, which were about 4 inches apart.

It was evident that this oscillation must be stopped. I tried to remedy it by changing the pressure of the steam, and then by changing the pulleys so as to run the engine faster, the speed of the governor, however, necessarily remaining the same. But these had no effect. Having exhausted my own stock of ignorance on the subject, I applied to professional experts for more, and I got it. Three persons, who I supposed ought to know, and who probably did know, all that was then known on the subject, gave me the same advice. It was that I should get a larger engine and a great deal larger fly-wheel. This advice did not seem to me reasonable. I knew that the engine was large enough, because while the governor was in the lowest position, in which it did not open the throttle entirely by any means, the machine ran too fast. They then told me I must have a heavier fly-wheel at any rate, and they explained to me that the fly-wheel performed two offices—one to carry the crank over its dead centers with an approximately uniform motion, and the other to give the governor time to act. I replied that the engine passed its dead centers with absolute uniformity then, as nearly as I could see, and as was shown by the surface of the stone, and consequently for that purpose the fly-wheel I had must be sufficient. The oscillations were regular, occupying about 30 revolutions of the machine, or 6 seconds of time, and had no connection with the dead centers, and I did not see why the governor should require any time to act. They told me that all governors required time to act, of course.

I then examined the governor more critically, and made up my mind that its action was hindered by friction in the driving-joints at the top of the spindle. These joints were about 4 inches apart, on opposite sides of the spindle, and were of a character in which the force transmitted through them to drive the balls produced a pinch between the broad faces of the joints. The governor could not act until by change of its speed it had accumulated force enough to overcome this pinch, and then it moved too far. Again I applied to my authorities for some way of getting rid of this friction. They told me that was easy enough. All I had to do was to put a yoke on the governor spindle, through which the governor arms were threaded and by which the driving pressure was applied close to the balls. So for the first time I took their advice and had a yoke put on the governor. I could not discover that this helped the matter at all. The improvement was too trifling to be noticed. I also saw clearly enough why this was so. The pressure applied was lighter than that applied through the joints, but it was also applied at a correspondingly increased distance from the axis, so that the effect in retarding the action of the governor was substantially the same.

I saw that if I got any relief I must find a way to it myself. So I began studying the subject of governors. My engineering library at that time consisted of Haswell’s Engineers’ Pocket Book. What little book-knowledge I had respecting mechanics I had learned from Haswell. I turned to Haswell and read what he had to say about governors. I learned that they were conical pendulums and made half as many revolutions in a minute as the vibrations of a pendulum whose length was equal to the height of the cone, the base of which was the plane in which the center of oscillation of the balls and arms revolved, and its apex the point of intersection of the axes of the arms, if produced upward, and that their revolutions varied inversely as the square root of the height of this cone. I did not see that this got me out of my difficulty at all. I then referred to the subject of centrifugal force, with which I had made some acquaintance before, and I read this champion mind-muddler: “All bodies moving around a center or fixed point have a tendency to fly off in a straight line. This is termed centrifugal force.” This did not help me any more, nor interest me much at that time.

But I read further that the centrifugal force of a body revolving in any given circle varies as the square of the speed. “Thus a body making 10 revolutions per minute will exert four times as much centrifugal force as will be exerted by the same body making 5 revolutions per minute.” The governor on my engine was making 50 revolutions per minute, and in thinking the matter over it occurred to me that if the governor could be run as fast as my machine, namely, at 300 revolutions per minute, the centrifugal force of one pound would be as great as that exerted by 36 pounds at 50 revolutions per minute. I cried, “Eureka! I have found it.” One-pound balls in place of 36-pound balls would be easily driven. I told my experts of the great find that I had made, and they laughed at me. They told me I ought to know that the momentum of the balls increased in the same ratio with their centrifugal force, MV² being the expression common to both, so, in the same circle, while the centrifugal force of the balls at 300 revolutions per minute would be 36 times greater than at 50 revolutions, it would require also 36 times the force to drive them, and that I would gain nothing by my proposed change, but instead I would have to rotate also the weight that I would need to use to hold the small balls down, and the last case would be worse than the first. This staggered me, and I pondered awhile what I should do.

I had a friend living near by on Fourteenth Street, west of Seventh Avenue—a Mr. Thompson, a mathematician and the author of a series of mathematical books then largely used. So I called upon him and stated my trouble and asked his advice. He illuminated the subject to me as follows: “You seem to be a persevering young man; keep hard at it and you will solve the difficulty by and by.”

In my despair I just had before me this one thought: The friction must be cured at any rate. After a time I thought that if I made a long joint at the top embracing the center of gyration of the counterpoise, so that the pressure required to drive the balls and counterpoise would be applied at some distance from the axis of the spindle and for that reason would be much lighter, and also would be normal to the surface of the joint-pin instead of being a pinch between opposite faces, the difficulty would be cured, as the force to overcome the friction would be exerted at the ends of levers 50 or 100 times the radius of the pin. I felt so sure of this that I risked making a governor with a single joint at the apex of the cone, as originally employed by Watt, thus making the governor more sensitive, as the height of the cone would not be changed at both ends, still fortunately holding to my little balls and high speed, though I cannot tell why. The joint at the top I made 6 inches in length.

When this governor was started, the trouble absolutely vanished. The engine ran with perfect uniformity while the load was constant. I use the adjective “perfect” advisedly, for the governor slide was as motionless on the spindle as if it were screwed tight, and the governor proved to be the most sensitive possible index of the variations of speed. When the belt was thrown off to the loose pulley the engine ran idle. The counterpoise then rose promptly but gently to its fixed highest position, and stood there motionless until the belt was thrown on and the hammers were started, when it moved as gently but promptly down to its lower position and stood there again motionless so long as the hammers were running. We could not detect by the eye the variation in speed that caused this action of the governor. The heaviest load on the engine, however, was dragging rapidly the two tables loaded with stone. This caused the governor to settle still further, but always the motion of the engine seemed to be the same so far as I could detect. The surface produced on the stone left nothing to be desired. The machine cut true planes, free from any windage, and the surfaces were left so smooth that the rubbing-machine had but little to do, and kept up with the cutting-machine very easily. The governor fascinated everybody who witnessed its operation.

The First Porter Governor.