Chapter 7-PRINCIPLES OF MEASUREMENT 



61.16X 

 Figure 7-36.— Resonance tachometer mounted 

 on rotating machine. 



vibrates at a different frequency. The reeds are 

 fastened in a row, in order of frequency; the row 

 is mounted with the reeds at right angles to the 

 back of the instrument. The unattached ends of 

 the reeds extend through a horizontal slit in the 

 face of the instrument; the scale is stamped 

 along the slit. When the instrument is solidly 

 attached to the foundation or casing of a rotating 

 machine, the reeds which are nearest in fre- 

 quency to the rpm of the machine begin to 

 vibrate. In figure 7-36, notice that five reeds 

 are out of line with the rest of the reeds; these 

 five are vibrating noticeably more than the 

 others, and the one in the middle is vibrating 

 more than the other four. To read the rpm of 

 the machine, then, it is only necessary to read 

 the scale marking underneath the reed that is 

 vibrating the most— that is, the one which is 

 most out of line with the others in the horizontal 

 slit. 



Resonance tachometers are particularly use- 

 ful for measuring high rotational speeds such as 

 those that occur in turbines, generators, and 

 forced draft blowers. They are also particularly 

 useful in applications where it is difficult or 

 impossible to get at the moving ends of shafts. 



The instruments give continuous readings and 

 make very rapid— almost instantaneous— adjust- 

 ments to changes in rotational speed. 



STROBOSCOPIC TACHOMETERS 



A stroboscopic tachometer is a device which 

 allows rotating, reciprocating, or vibrating ma- 

 chinery to be viewed intermittently, under flash- 

 ing light, in such a way that the movement of 

 the machinery appears to be slowed, stopped, 

 or reversed. Because the illumination is inter- 

 mittent, rather than steady, the eye receives a 

 series of views rather than one continuous view. 



When the speed of the flashing light coin- 

 cides with the speed of the moving machinery, 

 the machinery appears to be motionless. This 

 effect occurs because the moving object is seen 

 each time at the same point in its cycle of move- 

 ment. K the flashing rate is decreased slightly, 

 the machinery appears to be moving slowly in 

 the true direction of movement; if the flashing 

 rate is increased slightly, the machinery ap- 

 pears to be moving slowly in the reverse direc- 

 tion. To measure the speed of a machine, 

 therefore, it is only necessary to find the rate 

 of intermittent illumination at which the ma- 

 chinery appears to be motionless. To observe 

 the operating machinery in slow motion, it is 

 necessary to adjust the stroboscope until the 

 machinery appears to be moving at the desired 

 speed. 



The stroboscopic tachometer furnished for 

 shipboard use is a small, portable instrument. 

 It is calibrated so that the speed can be read 

 directly from the control dial. The flashing rate 

 is determined by a self-contained electronic 

 pulse generator which can be adjusted, by means 

 of the direct-reading dial, to any value between 

 600 and 14,400 rpm. The relationship between 

 rotational speed and flashing rate may be illus- 

 trated by an example. If an electric fan is oper- 

 ating at a rate of 1800 rpm, it will appear to be 

 motionless when it is viewed through a stro- 

 boscopic tachometer which is flashing at the 

 rate of 1800 times per minute. 



Because the stroboscopic tachometer is 

 never used in direct contact with moving ma- 

 chinery, it is particularly useful for measuring 

 the speed or observing the operation of machin- 

 ery which is run by a relatively small power 

 input. It is also very useful for measuring the 

 speed of machinery which is installed in rela- 

 tively inaccessible places. 



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