. The steam-engine and other heat-motors . Fig Indicator—Cress-section. undue leakage by the piston. The transverse web near its centersupports a central socket, which projects both upward and down-ward; the upper part is threaded inside to receive the lower endof the piston-rod; the upper edge of this socket is formed to fitnicely into a circular channel in the under side of the shoulder of thepiston-rod when they are properly connected. It has a longitudinalslot which permits the straight portion of the wire at the bottom ofthe spring, with its bead, to drop to a concave bearing i
. The steam-engine and other heat-motors . Fig Indicator—Cress-section. undue leakage by the piston. The transverse web near its centersupports a central socket, which projects both upward and down-ward; the upper part is threaded inside to receive the lower endof the piston-rod; the upper edge of this socket is formed to fitnicely into a circular channel in the under side of the shoulder of thepiston-rod when they are properly connected. It has a longitudinalslot which permits the straight portion of the wire at the bottom ofthe spring, with its bead, to drop to a concave bearing in the upperend of the piston-screw, 9, which is closely threaded into the lower STEAM-ENGINE INDICATOR AND ITS CALIBRATION. 49> part of the socket; the head of this screw is hexagonal and may be turned with the hollow wrench which accompanies the indicator.,r The above paragraph gives some idea of the care used in the design of the piston. The moving parts of an indicator should. Fig. 27.—Crosby Indicator—Outside Spring. be theoretically without weight (on account of inertia stresses),and should be frictionless. With the advent of superheated steamand the use of outside springs, this firm changed the shape andsize of the indicator-piston. For, they say (Fig. 27), the otherand more important difference lies in the shape and size of thepiston. This piston is 1 square inch in area and is in formthe central zone of a sphere, thus affording great active force witha very light pencil mechanism. In other words, this piston servesas a universal joint to take care of the torsional strains of thespring when it operates the pencil mechanism of the indicator. 50 THE STEAM-ENGINE AND OTHER HEAT-MOTORS. The Piston-rod, 10, is of steel and is made hollow for lower end is threaded to screw into the upper socket of thepiston. Above the threaded portion is a shoulder having in itsunder side a circular channel formed to receive the upper edge ofthe socket when
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