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

by Charles T. Porter

I called Mr. Allen in and showed him the defect. After considering it a few minutes, he said he thought it would be corrected by lowering the trunnions, so that their arc of vibration would coincide with the line of centers at its middle point, instead of terminating on it. This was done, and the result was most successful. The lead was now earlier and the opening wider at the back end of the cylinder, as the greater velocity of the piston at that point required, and the cut-offs on the opposite strokes more equal. The link has always been set in this way, as shown in the [diagram].

From this description of the link motion, it will be seen that the correct vertical adjustment of the trunnions of the link was an important matter. To enable this adjustment to be made with precision, and to be corrected, if from wear of the shaft-bearings or other cause this became necessary, I secured the pin on which these trunnions were pivoted to the side of the engine bed in the manner shown in the following [figure]. To hold the wedge securely, the surface of the bed below was reduced, so that the wedge was seized by the flange. The correct position of this pin was determined by the motions given to the valves.

VERTICAL ADJUSTMENT
OF SUSTAINING PIN
FOR TRUNNIONS
OF THE ALLEN LINK

I now took a more prominent part myself in steam-engine design. I had got an idea from Mr. Sparks that took full possession of my mind. This was the exceedingly unmechanical nature of the single or overhanging crank. The engines of the “New York,” built by Caird & Co., of Greenock, were among the first of the direct inverted-cylinder engines applied to screw propulsion. They were then known as the steam-hammer engines, their leading feature being taken from Mr. Nasmyth’s invention. I am not sure but Caird & Co. were the first to make this application. The forward engine had a single crank. The vital defect of this construction became especially apparent in these vertical engines of large power. The stress on the cap bolts during the upward strokes and the deflection of the shaft alternately in opposite directions over the pillow-block as a fulcrum were very serious. Mr. Sparks told me that on his very first voyage he had a great deal of trouble with this forward bearing, and it caused him continual anxiety. He got into such a state of worry and apprehension that as soon as he reached New York he wrote to the firm: “For God’s sake, never make another pair of engines without giving a double crank to the forward engine.” The reply he got was, to mind his own business: they employed him to run their engines; they would attend to the designing of them. He told me not long after that he had the satisfaction of seeing every ship they built except his own disabled, either by a broken shaft or broken pillow-block bolts. He attributed the escape of the “New York” from a like disaster to his own extreme care. They did, however, adopt his suggestion on all future vessels, and, moreover, added a forward crank and pillow-block to the engines already built. This they evidently found themselves compelled to do. I saw this addition afterwards on the “Bremen,” sister ship to the “New York.” The added pillow-block was supported by a heavy casting bolted to the forward end of the bedplate.

I went everywhere visiting engines at work and in process of construction, to observe this particular feature of the overhanging crank, which was universal in horizontal engines. In this class of engines, running slowly, its defective nature was not productive of serious consequences, because no stress was exerted on the cap bolts and the shaft was made larger in proportion to the power of the engine, as it had to carry the fly-wheel. But I was astonished to see the extent to which the overhang of the single crank was allowed. Builders seemed to be perfectly regardless of its unmechanical nature. First, the crank-pin was made with a length of bearing surface equal to about twice its diameter; then a stout collar was formed on the pin between its bearing surface and the crank. The latter was made thick and a long hub was formed on the back of it. I was told that the long hub was necessary in order to give a proper depth of eye to receive the shaft. This being turned down smaller than the journal, so that the crank might be forced on up to a shoulder, the eye needed to be deep or the crank would not be held securely. Finally, the journal boxes were made with flanges on the ends, sometimes projecting a couple of inches. Altogether, the transverse distance from the center line of the engine to the solid support of the shaft in the pillow-block was about twice what it needed to be. I also saw in some cases the eccentric placed between the crank and the pillow-block. Fifteen years later I saw a large engine sent from Belgium to our 1876 Exhibition which was made in this manner.

I determined at once that such a construction would not do for high-speed engines, and proceeded to change every one of these features. The single crank could not be avoided, but its overhang could be much reduced.

OLD AND NEW CRANKS
AND JOURNAL BOXES.

THE CRANKS ARE SHOWN IN
THE VERTICAL POSITION.