PURPOSE OF THE INDICATOR.

In the course of these pages, reference has several times been made to the indicator. This instrument is of great service to the steam engineer in helping him to determine with accuracy questions of steam distribution, that, without its aid, would remain in the realms of speculation. Its diagram presents a reliable record of what is going on inside the cylinders of a steam engine; and master mechanics who desire to run their engines economically, can not afford to dispense with the accurate information imparted by the indicator respecting individual locomotives.

DESCRIPTION OF INSTRUMENT.

The indicator consists essentially of a small steam cylinder, whose under side is connected by pipes to the main cylinder of the engine under inspection. Inside the indicator cylinder is a nicely fitting piston, whose upward movement is resisted by a spring of known strength. The piston-rod passes up through the top of the indicator cylinder; and its extremity is connected with mechanism for operating a pencil, and marking on a card a diagram whose lines coincide with the movement of the indicator piston.

OPERATION OF THE INDICATOR.

Fig.30 is a perspective view of the Tabor indicator, an instrument well adapted for application to high-speed engines, such as the locomotive. In Fig.31 the indicator is shown in section. The construction of the instrument can be well understood from a careful examination of these figures. In the sectional figure, the piston is seen with its rod encircled by the spring passing up through the top of the cylinder, and connecting with the lever which operates the marking-pencil. By ingeniously designed mechanism, the lever is made to move the pencil in a perfectly straight vertical line. The card to be marked is fastened in the paper drum attached to the indicator. This drum receives a circular motion from a cord which is operated by the cross-head, and this connection is so arranged that the drum will begin to move round just as the main piston begins its stroke. The circular motion of the drum is continued till the piston reaches the end of its stroke, when the drum reverses its movement, and returns to the exact point from whence it started. Now the indicator cylinder being in communication with the main cylinder, when the latter begins to take steam, the pressure will be applied to the indicator piston, which is pushed upward, at the same time transmitting its movement to the pencil. The indicator piston will rise and fall in accordance with the steam-pressure in the cylinder; and, the circular movement of the drum coinciding with the cross-head movement, the pencil will describe a diagram which represents the pressure inside the main cylinder at the various points of the stroke.

LINES OF THE DIAGRAM.

Fig.32 represents an indicator diagram, such as might be taken from a locomotive with well-constructed valve-motion cutting off steam at about one-third of the stroke. In the figure, the line IJ represents the line that would be drawn by the pencil when no steam is admitted to the indicator piston; so it is called the atmospheric line. This is the neutral line of the diagram representing the position of the pencil when both sides of the indicator piston are exposed to the atmosphere: hence it is the base line from which pressures either above or below are measured. The line AB is the admission line, so called because its beginning A represents the point where the valve begins admitting steam to the cylinder: BC is the steam-line, usually considered as beginning at the point of positive change in direction of the admission line. Admission of steam does not cease till the point D is reached, but wire-drawing begins at C. The curve DE is the expansion line, and is traced while the valve is closed, and previous to the opening of the exhaust; the pressure falling by expansion. At E, release begins, and the exhaust line extends to F; from F to G is the line of counter-pressure, drawn during the return stroke of the piston and while the exhaust is open; and G to A is the compression line, drawn during the return stroke after the exhaust-valve has closed.

DATA NECESSARY FOR ANALYZING THE DIAGRAM.

In proceeding to ascertain from the diagram calculations of the work done by the engine, it will be remembered that vertical measurements represent pressures in the cylinder, and horizontal measurements positions of the piston. To understand the language of the diagram properly, it is necessary that several things having reference to permanent or accidental conditions be known. Those absolutely necessary are, the scale of the indicator spring, the diameter of the engine cylinder and stroke of the piston, the strokes made per minute, and the boiler pressure. It is also desirable to know the dimensions of steam-pipes, steam-ports, and exhaust nozzles.

ADVANTAGES OF INDICATING LOCOMOTIVES.

The purposes for which the indicator can be advantageously applied to locomotives, are to show the condition of the valves and pistons, and to prove whether or not the steam is used properly within the cylinders. By its use the amount of power developed by the cylinders can be computed.

Were the indicator used daily on locomotives, much waste of coal now going on through steam being lost by leaky valves and pistons would be avoided. The graphic tale of the diagram would force out of practice the ordinary habit of running engines with the steam throttled, and indubitable proofs of ruinous back pressure would make contracted nozzles intolerable.

There is not room in a work of this kind to give the necessary information for those who wish to learn the operating of the indicator and the analysis of the diagram. Engineers who are ambitious to enter upon this line of study, should secure one of the standard books treating the subject.