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

by Charles T. Porter

John F. Allen

I was then ignorant of his state of mind on that subject, or of what had produced it. I learned these afterwards, and will state them here. In one of our interviews, in reply to my question as to what had led him to make this invention, he told me it was his experience when he was engineer of the propeller “Curlew,” a freight-boat running on Long Island Sound, between New York and Providence, which had a Corliss engine. He became impressed with what he thought to be a serious defect in the liberating system. The governor did not control the point of cut-off, but the point of release; this point being at the beginning of the closing movement of the valve, while the cut-off took place near the end of that movement. When the engine was worked up to nearly its capacity, as was the case in a ship, the port was opened wide, and quite an appreciable time elapsed between the release and the cut-off. During this interval the piston advanced considerably, and if the engine ran fast enough it might get to the very end of the stroke before the cut-off took place. He said that in smooth water they had no trouble, but in the open ocean, going around Point Judith, it was always rough, and sometimes in stormy weather the screw would be thrown quite out of the water, and the engine, having no fly-wheel, would race most furiously. The faster it ran the further the steam would follow, and was pumped out of the boiler very rapidly. Springs were employed to accelerate the closing movement of the valves, but in these cases they seemed to be of little use, and were continually breaking. He saw that this difficulty could be avoided only by a positive motion gear which would enable the governor to control the point of cut-off itself; and, accordingly, he set himself to work to devise such a system. We know now that this judgment, formed from observations made under very exceptional conditions, was not well founded. The difficulty in question does not practically exist in engines having fly-wheels and the present improved liberating gear, and running at moderate speeds; but the experience naturally made a deep impression upon Mr. Allen’s mind, and led to the invention of the positive motion system.

This he did not tell me at the time, so that I was at a loss to understand his reluctance to admit what was really the great value of his invention. However, I told him I would be willing to attempt its introduction, provided he would allow me to apply it at once to a high-speed engine; that being a field into which the liberating system could not enter. We had quite an argument on this point. I told him his invention interested me only because it would enable two or three times the power to be obtained from a given engine without additional stress on any part, the fly-wheel to be reduced in size, and the means for getting up the speed of machinery to be largely dispensed with. I represented to him also that a high-speed engine ought to be more economical and to give a more nearly uniform motion.

He finally agreed to my condition, and I took him directly to the office of Mr. Richards and engaged him to make an analysis and drawing of Mr. Allen’s system under his direction, and soon afterwards gave him an order for the plans for an experimental engine, 6×15 inches, to make 160 revolutions per minute.

As the diagram of the link motion was at first drawn, the center of the trunnions vibrated in an arc which terminated at points on the line connecting the center of the engine shaft with the ends of the rocker arms, and which in the [diagram] on page 48 is named “radius of link.”

I determined to work out this link motion myself on a large scale. For this purpose I drew a diagram in which the throw of the eccentric was 4 inches, and the distance from the center of the shaft to that of the trunnions of the link in their mid-position was 12 inches. I made a three-point beam compass. Two of these points were secured permanently on the beam, 12 inches apart. As one of these points traversed the path of the center of the eccentric, the other could be made to traverse the arc of vibration of the trunnions of the link.

I divided the former into 40 equal divisions measured from its dead points, making needle-holes in the circle, in which the taper compass-points would center themselves accurately. The paper was firm and the points of division were fixed with extreme care; and they lasted through all my experiments. I then set out 20 corresponding divisions in the arc of vibration of the center of the trunnions. These showed distinctly the modification of the motion at the opposite ends of this vibration as already described.

The third point was adjustable on a hinged beam which could be secured in any position. I drew two arcs representing the lead lines of the link, or the lines on which the link would stand when the eccentric was on its dead points. The third point was now secured on its beam at any point on one of the lead lines, when the other points stood, one on the dead point of the eccentric and the other at the end of the trunnion vibration.

The apparatus was now ready for use, the corresponding points on the circle and the arc being numbered alike. By setting the first two points in any corresponding holes, the third point would show the corresponding position of that point of the link at which it was set. I thus set out the movements of six different points of the link, the highest being 12 inches above the trunnions. These represented the movements of the valves of the engine when the block was at these points in the link. The apparatus being firm, it worked with entire precision. To my surprise, it showed much the larger valve opening at the crank end of the cylinder, where the movement of the piston was slowest. That would not do; we wanted just the reverse.