| Fig. 1 | Fig. 5 SCALE 40 265 REVS. PER MIN. 11¹⁄₄″ × 16″ PORTER-ALLEN. |
| Fig. 4 SCALE 40 265 REVS. PER MIN. 4.416 „ „ SEC. | |
| Fig. 2 | Fig. 3 |
Apparatus for Graphically Showing the Acceleration and Retardation of the Reciprocating Parts of an Engine.
Let the motion be in the direction from the crank. The crank now begins insensibly, by pulling through the spring e, to arrest the motion of the weight h. This pull will increase in intensity to the end of the stroke, when the weight is brought to rest, and the spring will become correspondingly elongated. Then, by a continuance of the same pull, the crank puts the cross-head and this free weight in motion in the reverse direction. This pull gradually relaxes, until at the mid-stroke it has ceased. The weight h has acquired its full velocity again; all stress is off the spring, and the spring and weight are back in the positions in the box d from which they started. This action is repeated during the opposite half of the revolution, but in the reverse direction, the pull being changed to a push, and the spring being compressed instead of elongated. Thus at every point the position of this free weight shows the amount of the accelerating or retarding force that is being exerted upon it at that point, elongating or compressing the spring.
This varying accelerating or retarding force is recorded as follows: A paper b, [Fig. 2], is stretched on the surface ff. This surface is the arc of a circle described about the center j, and is secured on the lath B, so that as this lath vibrates by the motion of the cross-head the different points in the length of the paper pass successively under the pencil. This is set in the end of the long arm a of the right-angled lever-arms 4 to 1 seen in [Fig. 2], which is actuated by the rod e passing centrally through the spring and secured in the head c. This pencil has thus imparted to it a transverse motion four times as great as the longitudinal motion of the weight h in the box d. The pencil is kept lifted from the paper (as permitted by the elasticity of the arm a) by the cord m. By letting the pencil down and turning the engine by hand, the neutral line x, [Fig. 2], is drawn. Then when the engine is running, on letting the pencil come in contact with the paper, the diagonal lines are drawn as shown on [Fig. 2].
Edwin F. Williams
If the rotation of the shaft were uniform and there were no lost motion in the shaft or connecting-rod, this diagonal line would repeat itself precisely, and would be a straight line modified by the angular vibration of the connecting-rod. On the other hand, these lost motions and the variations in the rotative speed must be exactly recorded, the latter being exhibited with a degree of accuracy not attainable by computation and plotting, and their correctness would be self-demonstrated. For this purpose this instrument must be found highly valuable, if it is really desired to have these variations revealed rather than concealed. [Fig. 5] represents the inertia diagram drawn by this instrument applied to a Porter-Allen engine running in the Boston Post Office at the speed of 265 revolutions per minute. [Fig. 4] shows the same diagram with the transverse motion of the pencil enlarged to correspond with the scale of the indicator, so exhibiting the force actually exerted on the crank-pin at every point, which is represented by the shaded area, and from which the rotative effect on the crank can be computed. The steam pressure absorbed at the commencement of the stroke by the inertia of these parts is represented by the blank area above the atmospheric line xx. This is not all imparted to the crank at the end on account of the compression.
I have myself had no experience in the use of this instrument, but I do not see why it might not be so made that the diagonal line or lines in [Fig. 4] would be drawn at once. The variations of motion would thus be shown much more accurately than they can be by the enlargement of these small indications. This would require the spring e to bear the same relation to the inertia of the weight h that the spring of the indicator bears to the steam pressure on its piston area. The steam diagram and the inertia diagram would then be drawn to the same scale. A separate instrument would be required for each scale. It would seem desirable that this instrument, which is not expensive, should be brought before the public in this practical shape.
The 16″×30″ engine exhibited at this fair of the American Institute was sold from the exhibition to the Arlington Mills, at Lawrence, Mass. For a reason that will appear later, I have always regarded this sale as the most important one that I ever made.