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

by Charles T. Porter

I decided, first, to increase the multiplication of the piston motion, by means of the lever, from three times to four times, thus reducing by one quarter the movement of the piston required to give the same vertical movement to the pencil, and, second, to increase the cylinder area from one quarter to one half of a square inch. The latter was necessary in order to afford sufficient room for springs of proper size, and correct reliable strength in their connections.

The first problem that presented itself was how to produce cylinders of the exact diameter required, .7979 of an inch, and to make an error in this dimension impossible. This problem I solved in the following manner: At my request Elliott Brothers obtained from the Whitworth Company a hardened steel mandrel about 20 inches in length, ground parallel to this exact size and certified by them. Brass tubes of slightly larger size and carefully cleaned were drawn down on this mandrel. These when pressed off presented a perfect surface and needed only to be sawed up in lengths of about 2 inches for each cylinder. Through the whole history of the manufacture that removed all trouble or concern on this account.

The pistons were made as light as possible, and were turned to a gauge that permitted them to leak a little. The windage was not sufficient to affect their accuracy; a thickness of silk paper on one side would hold the pistons tight; but they had a frictionless action, and the cover of the spring case having two holes opening to the atmosphere, there could be no pressure above the piston except that of the atmosphere.

SPRING-TESTING INSTRUMENT.
USED IN THE MANUFACTURE OF THE RICHARDS INDICATOR.
Designed by Charles T. Porter.
LONGITUDINAL SECTION.
SCALE, HALF SIZE.

END VIEW

The second problem was to insure the accuracy of the springs. This was more serious than the first one. The brass heads of the springs were provided with three wings instead of two, which mine had. The spring, after being coiled and tempered, was brazed into the grooves in the first two wings, and the third wing was hammered firmly to it. This prevented the stress on the spring from reaching the brazed joints, and these heads never worked loose. One head was made fast at once; the other was left free to be screwed backward or forward until the proper length of the spring was found. To insure freedom from friction, I determined to adjust and test the springs in the open air, quite apart from the instrument. For this purpose I had a stout cast-iron plate made, with a bracket cast on it, in which the slides were held in a vertical groove, and bolted this plate on the bench, where it was carefully leveled. The surface of the plate had been planed, a small hole drilled through it at the proper point, and a corresponding hole was bored through the bench. A seating for the scales also was planed in the bracket, normal to the surface of the block. The spring to be tested, in its heads as above described, was set on the block, and a rod which was a sliding fit in the hole was put up through the bench, block, and spring. This rod had a head at the lower end, and was threaded at the upper end. Under the bench a sealed weight, equal to one half the extreme pressure on the square inch to be indicated by the spring, was placed on the rod.

Between the spring and the scale I employed a lever, representing that used in the indicator, but differing from it in two respects. It was of twice its length, for greater convenience of observation, and it was a lever of the first order, so that the weight acting downward should represent the steam pressure in the indicator acting upward.

The weight was carried by a steel nut screwed on the end of the rod and resting on the upper head of the spring to be tested. This nut carried above it a hardened stirrup, with a sharp inner edge, which intersected the axis of the rod, produced. A delicate steel lever was pivoted to turn about a point at one fifth of the distance from the axis of the rod to the farther side of the scale seat. The upper edge of this lever was a straight line intersecting the axis of its trunnions. The short arm of the lever passed through the stirrup, in which it slid as the spring was compressed, while the long arm swung upward in front of the scale. The latter was graduated on its farther side, and the reading was taken at the point of intersection of the upper edge of the lever with this edge of the scale.

The free head on the spring was turned until the reading showed it to be a trifle too strong. It was then secured, and afterwards brought to the exact strength required by running it rapidly in a lathe and rubbing its surface over its entire length with fine emery cloth. This reduced the strength of each coil equally. This was a delicate operation, requiring great care to reduce the strength enough and not too much. A great many springs had to be made, several being generally required, often a full set of ten, with each indicator. This testing apparatus was convenient and reliable, and the workmen became very expert in its use.