In the torsion dynamometers just described the measurement covers the average angular displacement for a given load with fair accuracy but it is obvious that where reciprocating engines are used, and even with steam turbines, the torque undergoes various changes in a com plete revolution; this is especially so when the shaft carries a propeller, in which case the com plicated fluctuations cause irregular oscillations in the angle through which the shaft is twisted. To determine these variations in twist through out a revolution various modifications of the torsion meter have been employed, by which instantaneous readings are obtained throughout a revolution. Several types have been arranged to record the twist graphically. Among these the Denny-Edgecombe torsion indicator merits attention. This instrument, Fig. 10, consists of a cast-iron sleeve made in two sections and parted to the shaft. If this were measured directly, however, the result would be subject to serious error, for there always is some longi tudinal movement of the shaft itself due to thrust or expansion. This is eliminated by means of a turned base or dummy flange cast upon the sleeve. The traveler flange is made to fit accurately against this base flange for setting in position, and is then drawn along the tube and secured to the wire rope at a con venient distance from the base flange. The variation of distance between these two flanges will thus be an exact measure of the variation of torsion of the shaft.
An indicator is applied to these two flanges to further magnify the movement and make it easily readable. If it be desired to obtain a
permanent diagram, a recorder may be attached to the front of the indicator. This recorder has drums which are driven by means of small double-worm gears from the spindle of the wheel which runs on the base flange. Change gears, actuated by a small lever, are provided, so that the drums can be kept revolving in the same direction whether the ship is going eaheads or °astern?) A diagram paper is thus made to move with the drums, upon which pens attached to the fixed scale and to the movable rack, mark respectively a zero line and a torsion line. In addition to this a clock is fitted with an elec trical arrangement for marking half seconds and another pen is provided which can be con nected electrically with a make-and-break fitting on the revolution counter of the engine ; a dia gram of simultaneous revolutions per minute and torsion is thus obtained.
Another form of the Denny-Edgecombe tor sion meter, especially useful for marine service with turbine or reciprocating engines, operates by transferring the movement electrically to an ammeter which may be fitted up in the engine room or in any part of the ship. Duplicate ammeters may also be fitted so that it is pos sible to read torsioin simultaneously in the engine room, the chief engineer's cabin and captain's cabin. The instrument is a robust electromechanical combination which is as simple to read as a pressure gauge. By pressing a button the torsion of the shaft may be read immediately and the corresponding shaft horse power obtained from tables or charts.