Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERING:,5r«2i^;^S4^ra;-,-- -FIXED-MOVABLECOLLARDRIVESHAFT2.66XFigure 7-33.— Main parts of centrifugaltachometer.lower collar is free to move up and down theshaft. A spring which fits over the drive shaftconnects the upper collar and the lower collar.As the drive shaft begins to rotate, the flyballsspin around with it. Centrifugal force tends topull the flyballs away from the center, thusraising the lower collar and compressing thespring. The lower collar is connected to thepointer, and the upward movement of the collarcauses the pointer to move to a higher rpm readingon the dial. The centrifugal tachometer registersrpm of a rotating shaft as long as it is incontact with the shaft. For this reason it is calleda constant-reading tachometer.CHRONOMETRIC TACHOMETERSA chronomotric tachometer (fig. 7-35) is acombination of a watch and a revolution counterwhich measures the average number of revolutionsper minute of a rotating shaft. The deviceis not a constant-reading instrument; the outerdrive shaft runs free when the instrument isapplied to a rotating shaft until a starting buttonis depressed to start the timing element. Afterthe drive shaft has been disengaged from therotating shaft, the pointer remains in position onthe dial until it is returned to the zero positionby the operation of a reset button (which may bethe same as the starting button.)RESONANCE TACHOMETERSA resonance tachometer (fig. 7-36) consistsof a number of steel reeds, each one of which2.66XFigure 7-34.— Dial of centrifugal tachometer.2.66XFigure 7-35.— Chronometric tachometer.148