Principles of naval engineering - Historic Naval Ships Association

Chapter 7-PRINCIPLES OF MEASUREMENTThe instruments give continuous readings andmake very rapid— almost instantaneous— adjustmentsto changes in rotational speed.STROBOSCOPIC TACHOMETERS61.16XFigure 7-36.— Resonance tachometer mountedon rotating machine.vibrates at a different frequency. The reeds arefastened in a row, in order of frequency; the rowis mounted with the reeds at right angles to theback of the instrument. The unattached ends ofthe reeds extend through a horizontal slit in theface of the instrument; the scale is stampedalong the slit. When the instrument is solidlyattached to the foundation or casing of a rotatingmachine, the reeds which are nearest in frequencyto the rpm of the machine begin tovibrate. In figure 7-36, notice that five reedsare out of line with the rest of the reeds; thesefive are vibrating noticeably more than theothers, and the one in the middle is vibratingmore than the other four. To read the rpm ofthe machine, then, it is only necessary to readthe scale marking underneath the reed that isvibrating the most— that is, the one which ismost out of line with the others in the horizontalslit.Resonance tachometers are particularly usefulfor measuring high rotational speeds such asthose that occur in turbines, generators, andforced draft blowers. They are also particularlyuseful in applications where it is difficult orimpossible to get at the moving ends of shafts.A stroboscopic tachometer is a device whichallows rotating, reciprocating, or vibrating machineryto be viewed intermittently, under flashinglight, in such a way that the movement ofthe machinery appears to be slowed, stopped,or reversed. Because the illumination is intermittent,rather than steady, the eye receives aseries of views rather than one continuous view.When the speed of the flashing light coincideswith the speed of the moving machinery,the machinery appears to be motionless. Thiseffect occurs because the moving object is seeneach time at the same point in its cycle of movement.K the flashing rate is decreased slightly,the machinery appears to be moving slowly inthe true direction of movement; if the flashingrate is increased slightly, the machinery appearsto be moving slowly in the reverse direction.To measure the speed of a machine,therefore, it is only necessary to find the rateof intermittent illumination at which the machineryappears to be motionless. To observethe operating machinery in slow motion, it isnecessary to adjust the stroboscope until themachinery appears to be moving at the desiredspeed.The stroboscopic tachometer furnished forshipboard use is a small, portable instrument.It is calibrated so that the speed can be readdirectly from the control dial. The flashing rateis determined by a self-contained electronicpulse generator which can be adjusted, by meansof the direct-reading dial, to any value between600 and 14,400 rpm. The relationship betweenrotational speed and flashing rate may be illustratedby an example. If an electric fan is operatingat a rate of 1800 rpm, it will appear to bemotionless when it is viewed through a stroboscopictachometer which is flashing at therate of 1800 times per minute.Because the stroboscopic tachometer isnever used in direct contact with moving machinery,it is particularly useful for measuringthe speed or observing the operation of machinerywhich is run by a relatively small powerinput. It is also very useful for measuring thespeed of machinery which is installed in relativelyinaccessible places.149