PRINCIPLES OF NAVAL ENGINEERING 



FIXED- 



:,5r«2i^;^S4^ra;-,-- - 



MOVABLE 

 COLLAR 



DRIVE SHAFT 



2.66X 

 Figure 7-33.— Main parts of centrifugal 

 tachometer. 



lower collar is free to move up and down the 

 shaft. A spring which fits over the drive shaft 

 connects the upper collar and the lower collar. 

 As the drive shaft begins to rotate, the flyballs 

 spin around with it. Centrifugal force tends to 

 pull the flyballs away from the center, thus 

 raising the lower collar and compressing the 

 spring. The lower collar is connected to the 

 pointer, and the upward movement of the collar 

 causes the pointer to move to a higher rpm read- 

 ing on the dial. The centrifugal tachometer regis- 

 ters rpm of a rotating shaft as long as it is in 

 contact with the shaft. For this reason it is called 

 a constant-reading tachometer. 



CHRONOMETRIC TACHOMETERS 



A chronomotric tachometer (fig. 7-35) is a 

 combination of a watch and a revolution counter 

 which measures the average number of revolu- 

 tions per minute of a rotating shaft. The device 

 is not a constant-reading instrument; the outer 

 drive shaft runs free when the instrument is 

 applied to a rotating shaft until a starting button 

 is depressed to start the timing element. After 

 the drive shaft has been disengaged from the 

 rotating shaft, the pointer remains in position on 

 the dial until it is returned to the zero position 

 by the operation of a reset button (which may be 

 the same as the starting button.) 



RESONANCE TACHOMETERS 



A resonance tachometer (fig. 7-36) consists 

 of a number of steel reeds, each one of which 



2.66X 

 Figure 7-34.— Dial of centrifugal tachometer. 



2.66X 

 Figure 7-35.— Chronometric tachometer. 



148 



