INDICATORS, STEAM-ENGINE. The Tabor _Indicator is shown in Fig. 1. The special peculiarity of the Tabor indicator lies in the means employed to communicate a straight-line movement to the pencil.
A stationary plate containing a curved slot is firmly se cured in an upright position to the cover of the steam cylinder. This slot serves as- a guide and controls the motion of the pencil-bar. The side of the pencil-bar carries a roller which turns on a pin, and this is fitted so as to roll freely from end to end of the slot with little lost motion. The curve of the slot is so adjusted, and the pin attached to such a point, that the end of the pencil-bar which carries the pencil moves up and down in a straight line, when the roller is moved from one end of the slot to the other. The curve of the slot just com pensates the tendency of the peneil-point to move in a circular arc, and a straight-line motion results. The outside of the curve is nearly a true circle, with a radius of 1 in. The pencil mechanism is carried by the cover of the outside cylinder, and consists of three pieces —the pencil-bar, the back-link, and the piston-rod link.
The two links are parallel with each other in every position they may assume. The lower pivots of these links and the pencil-point are always in the same straight line. If an imaginary link he supposed to connect the two in such a manner as to be parallel with the pencil-bar, the combination would form an exact pantograph. The slot and roller serve the purpose of this imaginary link. The springs are of the duplex type, being made of two spiral coils of wire. They are so mounted that the points of connection of the two coils lie on opposite sides of the fitting; this equalizes the side strain on the spring, and keeps the piston central in the cylinder.
The Crosby Indicator is shown in Fig. 2. The movement of the piston of the indicator is transmitted to the pencil by a simple parallel motion which gives it a movement in a straight line at right angles to the atmospheric line. The movement of the piston is multiplied
to give a diagram of convenientsize, and at the same time to have the movement of the spring so slight that the pencil will immediately respond to any change of pressure in the cylinder. The spring is of unique and ingenious design, being made of a single piece of steel wire, wound from the middle into a double coil, the ends of which are screwed into a head D with four radial wings having spirally drilled holes to receive and hold them securely in place. .:1djustment is made by screwing the spring in or out of the head until it is of the right strength, when it is securely fastened.
The Batchelder Adjustable Spring Indicator is shown in Fig. 3. The special features of this instrument consist in the T-shaped hollow ease, adjustable flat spring, positive parallel motion, and stop-motion for paper drum. The cylinder is separate from the case proper. The flat steel spring works in the horizontal body of the ease, one end being rigidly secured and the other attached to the connecting-rod between the piston and peneil-lever. The change of spring is made by removing the screw that connects it to nu, piston-rod, and the one which holds it in the ease. Connection is made with the piston with a ball-and-socket joint. The scales are marked on the face of the ease, the upper one being for low pressure and the other for high pressure. The parallel motion is secured by confining the end of the Itenci1-lever in a small roller which runs in the vertical shit. The height of the atmospheric line is adjustable by means of a swivel in the connecting-rod near the pencil-lever. The movement of the paper drum is controlled by the cone-shaped spring, which is adjustable to any tension according to speed.