Curtis, W.F.

English

MIT Libraries Dome

3 9080 02993 0630V393.R467NAVY DEPARTMENTDAVID TAYLOR MODEL BASINWASHINGTON, D. C.ALTERNATING TORQUE TESTS ON A TURBINE SHAFTU.S. DESTROYER MOFFETTbyW.F. CurtisU NeEf I EDReport R-85-- --- --\HLOF TECHNolJUN 17 1976''tR ENGINEFR'MN LMay 1943THE DAVID TAYLOR MODEL BASINRear Admiral H.S. Howard, USNDIRECTORCaptain H.E. Saunders, USNTECHNICAL DIRECTORCommander W.P. Roop, USNSTRUCTURAL MECHANICSK.E. Schoenherr, Dr.Eng.HEAD NAVAL ARCHITECTD.F. Windenburg, Ph.D.HEAD PHYSICISTPERSONNELThe tests were conducted at sea by Lt. Comdr. A.G. Mumma, USN, W.F. Curtis,and L.E. Wedding, of the David Taylor Model Basin Staff. Lt. D.C. Campbell, USNR,and V.E. Benjamin, also from the David Taylor Model Basin, assisted in installingthe gages. Preparations for the sea trials were made with the help of the Navy Yard,Charleston. The report was written by W.F. Curtis.ALTERNATING TORQUE TESTS ON A TURBINE SHAFTU.S. DESTROYER MOFFETTABSTRACTThe alternating component of the torque present in the port low-pressureturbine shaft of the USS MOFFETT was measured by electrical resistance strain gages.The primary purpose of the measurement was to determine whether torsional oscilla-tions of dangerous magnitude existed, and if so, whether or not the oscillations wereexcited by the propeller and which of the several possible natural frequencies of thesystem was associated with the maximum amplitude of oscillation. It was found thattorsional oscillations were present but were low in magnitude, that these oscilla-tions were excited chiefly by imperfections in the reduction gears, and that maximumamplitude occurred at the lowest, i.e., first order, natural frequency.INTRODUCTIONThe reduction gears of the USS MOFFETT (DD362) and of other vessels of thatclass were found to be wearing much faster than was reasonably to be expected. Thegear manufacturers suggested that this difficulty might be due to torsional oscilla-tions excited by the three-bladed propeller, particularly at the second-order tor-sional critical speed, which they estimated should occur at about 240 RPM.By ascertaining the RPM at which high stresses occurred and the frequencyof alternations it was possible to determine whether oscillations at the second-ordertorsional critical speed were responsible for the wearing of the gears. Therefore,in accordance with instructions from the Bureau of Ships (1)* the alternating compon-ents of torsional stress present in the port low-pressure turbine shaft of the USSMOFFETT were measured during post-repair trials conducted on 28 October 1942.TEST APPARATUSAs the turbine shaft did not present enough exposed length for attachingeven the very small metalectric gages, it was decided to place these gages on theshaft inside the low-pressure pinion, through which it extended as a quill shaft,keyed to the pinion at its outer end. Advantage was taken of a time when the low-pressure pinion was being renewed to install the gages on the pinion quill shaft.Steains were measured by electrical resistance or metalectric gages cement-ed to this shaft, substantially as described in an earlier Taylor Model Basin report(2). Slight departures from the instrumentation and procedure described in that re-port were adopted on account of special features of this test.Experience had shown that carbon, metallized graphite, or pure metal brush-es on bronze slip rings provided uncertain contact and became too hot when the shafts* Numbers in parentheses indicate references on page 9 of this report.1101 11111M IINNIW1111 10  10Figure 1 - Schematic Diagram showing Locations of Gage Leads and Slip RingsThe metalectric gages were cemented to the quill shaft about 7 inches to the left of the key.were turning at speeds as high as those of the low-pressure turbine shaft on theMOFFETT. For these tests, therefore, stainless steel rings dipping into mercury-filled wells were employed to make electrical contact from the recording apparatus tothe strain-gage bridge mounted on the shaft. The general arrangement is shown inFigure 1.Special gages were made up of Number 40 Advance* wire secured to thin paperwith bakelite cement. The terminals consisted of Number 28 copper wire soldered tothe gage wire; it was found necessary to place an extra insulating thickness of paperunder the soldered joint to avoid short-circuiting to the shaft. The gages were ce-mented to the shaft with bakelite cement. The appropriate electrical connectionswere made with Number 22 wire insulated with acetate braid, and the whole shaft waswrapped with cotton tape over the gages. The cotton tape was painted with glyptolvarnish and the entire assembly was then baked at 175 degrees fahrenheit. This pro-vided excellent electrical insulation and mechanical protection for the gages andthe connecting wires.The shaft diameter at the gage location was 6 3/4 inches and the maximumdiameter over the installation was 7 inches ± 1/64 inch, leaving a total diametralclearance of 7/64 inch inside the pinion.A Dumont Type 208 Oscilloscope and a moving-film camera were used in placeof the usual string oscillograph for recording the data.* Advance is a trade name for one of the types of resistance wire commonly used for these gages.It contains approximately 60 per cent copper and 40 per cent nickel.YYII I __ _____.__ __ ____._ __1111111TEST PROCEDUREVisual observations of torsional stresses were started while the USS MOFFETTwas moored and the propeller shaft was being jacked over preparatory to getting under-way. Visual observations were continued while the ship was underway proceeding tosea. These maneuvers occupied approximately five hours; propeller shaft speeds dur-ing this interval ranged from 80 to 140 RPM. Some records were taken near the end ofthis period. All shafts were then stopped for a short time to obtain zero readings.When the ship was again underway records were taken at appropriate intervalswhile the speed was gradually increased to the maximum obtainable, i.e., 338 RPM.Course and speed were maintained constant while each record was being taken. Speedwas maintained at approximately 338 RPM for two hours. At the end of this period,the shafts were again stopped for a zero check, and further records were taken whilethe ship was slowing down.TEST RESULTSThe oscillograms shown in Figures 2 to 6 are typical of those obtained inthese tests. The results are given in Tables 1 and 2, and are represented graphicallyin Figures 7, 8, and 9. In most cases, the time taken for 6 cycles of the most obvi-ous oscillatory component is slightly more than the time for 1 propeller revolution.The low-pressure turbine shaft makes 5.72 revolutions per revolution of the propellershaft. The periodicity apparent onthe oscillograms is very close to this TABLE 1value; agreement is not quite exact, Results of Alternating Torque Testson Port Low-Pressure Turbine Shaftprobably owing to the difficulty ofexactly locating points of equivalentphase on different cycles.All oscillograms suitablefor the purpose were analyzed with aMader harmonic analyzer. In mostcases, two different periods wereconsidered in the analysis of eachoscillogram.It will be noted from Table2 that the results of the two analysesare in general quite similar; this isbecause the analyzer is unable to dis-tinguish accurately between two alter-nations of nearly equal periods. Thedata obtained from analyzing the"maximum-to-maximum" period were chos-en for plotting in Figures 7 and 8, asTotal TotalAlternating AlternatingPropeller Torque, Stress,Entry RPM lb-ft. lb/in2 .Peak Single Peak SingleAmplitude Amplitude1 50 7050 14602 55 15,550 32203 60 15,300 31704 82 9163 19005 90 3412 7106 100 3191 6607 110 6211 12858 120 10,670 22209 130 10,880 225010 140 2371 48011 160 1510 31012 200 1060 22013 220 1510 31014 230* 1520 31515 24016 250* Observable torsional oscillations did not occurat speeds in excess of 230 RPM.---------- - IIIIIYY  IFigure 2 - Oscillogram of Torque on Port Low-Pressure Turbine ShaftPropeller RPM 60The trace of the propeller revolution marker is at the bottom of the filmstrip. The pinion makes 5.72 revolutions for one revolution of the propeller.Figure 3 - Oscillogram of Torque on Port Low-Pressure Turbine ShaftPropeller RPM 82The propeller revolution marker was not functioning. The period was estimated from the film speed.Figure 4 - Oscillogram of Torque on Port Low-Pressure Turbine ShaftPropeller RPM 130As gages were supplied with an alternating "carrier" frequency current, torsional oscillationsare represented by variations in the width of the central band in all these oscillograms.Individual oscillations of the carrier current are not resolved by the recording process used.- -- I I - - -- e3 -~--- I I C_I- - ~- - - - - -- -Figure 5 - Oscillogram of Torque on Port Low-Pressure Turbine ShaftPropeller RPM 230Figure 6 - Oscillogram of Torque on Port Low-Pressure Turbine ShaftPropeller RPM 250-rTABLE 2Harmonic Analysis of Torsional Oscillations on Port Low-Pressure Turbine ShaftThe magnitude of each component is in pound-feet of alternating torque.The results given in Columns 1, 2, 4, 6, and 8 were derived by analyzing a period taken frommaximum to maximum on the oscillogram. This period has been assumed equal to the period ofrotation of the turbine shaft, which it closely approximates. The results given in columns3, 5, 7, and 9 were derived by using a period equal to one propeller revolution, as indicatedby a contactor on the propeller shaft.Column 1 2 3 4 5 6 7 8 9Frequency of Component in 0.958 2.875 3 3.84 4 5.75 6 11.5 12cycles per propeller shaft rev.Cycles per turbine shaft rev. 1/6 1/2 0.521 2/3 0.695 1 1.042 2 2.085Time of Observation Prop.RPMDuring speed run 50 .4 1970 t t 1820 t 1650After speed run 55 o .? 1990 8640 8120During speed run 60 1230 5810 5580 7130 7300Before speed run 82 5440 3000 t 540 t 480 t 560 tDuring speed run 90 ** 300 ** 300 ** 1320 ** 950During speed run 100 930 330 550 260 1350 1280 2490 1660During speed run 110 210 660 4100 720 3670 900 4890During speed run 120 -4 2780 1670 750 4520 7060 7700 4200During speed run 130 ' 1280 1800 1360 1100 6790 7500 2700Before speed run 140 l 300 320 790During speed run 160 140 860 410During speed run 200 790* 790 610* 610During speed run 220 520* 520 550* 550During speed run 230 1310 270* The period determined by inspection of the vibration record is the same as the period deter-mined by the propeller revolution marker.- These oscillograms showed no apparent periodicities.t The propeller revolution marker did not function.they were considered more reliable than the data from the other analyses. On theo-retical grounds, it would seem that components of less than turbine shaft frequencyshould be related to the propeller shaft frequency rather than to the turbine shaftfrequency; exciting forces arising from the propeller or from irregularities in themain reduction gear are the most probable sources of such components. However, theoscillogram for 230 RPM clearly shows a major component of one-half the turbine shaftfrequency, not three times propeller shaft frequency.With respect to the data represented by the rest of the curves marked "one-half turbine shaft frequency" and "two-thirds turbine shaft frequency" there is areasonable doubt as to whether the chief exciting forces might not be respectivelyUIIMR_S16000E4 14000 .0_ En 12000 4-- 10000' BC 08000o oo 00 40 80 120 160 200 2 40Propeller RPMFigure 7 - Total Alternating Torqueon Port Low-Pressure Pinion Shafton Port Low-Pressure Pinion Shaftthree times propeller shaft frequency andfour times propeller shaft frequency, inspite of the fact that they were obtainedfrom analysis based on the frequenciesgiven. In any event, the component ofthree times propeller shaft frequency,which would be the chief component due topropeller excitation, must have been ofrelatively small magnitude. As far asthe higher frequency components are con-0 120 160 200 240Propeller RPMFigure 8 - Harmonic Analysis of Alter-nating Torque Components of Turbine ShaftFrequency and 1/2 Turbine Shaft FrequencyThe latter component might be chiefly due to anexciting force of three times propeller shaftfrequency rather than one-half the turbine shaftfrequency, these two frequencies being so nearlyequal that analysis failed to distinguish betweenthem. The frequency of the chief component ofthe torsional oscillations due to a three-bladedpropeller is three times the propeller shaft fre-quency. It is therefore certain that the propeller-excited torsional oscillations do not exceed thevalues given on the curve marked "1/2 turbineshaft frequency."cerned, inspection of the oscillograms clearly shows that they are respectively tur-bine shaft frequency and twice turbine shaft frequency, and not six times and twelvetimes propeller shaft frequency.During the period preceding the speed runs, visual observations on the os-cillograph indicated very strong torsional oscillations with a period approximatelyequal to the time of one propeller shaft rotation. This effect was at a maximumwhile the shaft was being jacked over prior to getting underway; it gradually de-creased as time went on and as speed was gradually increased. The wave form shownin Figure 3 on page 4, which was recorded prior to starting the speed run, is typ-ical of wave forms observed during this interval and is quite different from wavenillilllilllli  1 11 , I4,IIIIi I9000ooo forms obtained later. This oscillogramis a replica of the one from which entryS8000oo 4 of Table 1 and point A of Figure 7< were obtained. It will be seen that thew7000 total alternating torque is noticeablyUhigher than would be expected from the.. 6000trend of the other determinations. Ascan be seen from Table 2, the discrep-000 2 times ancy chiefly affects the component ofTurbine Shafto Frequency approximately propeller shaft frequency.o4000CUnfortunately, the propeller shaft con-0 itactor did not record on this oscillo-0 I gram so that it is not certain whetherS2000I the period was 1 propeller revolution orSTurbine Shoft 6 turbine revolutions.I Frequencyo Frequency The 140 RPM measurement, entryS 4 10 of Table 1 and point B of Figure 7,was also taken prior to the speed run0. 40 80 120 160 200 240Propeller RPM but did not show this anomaly.The enhanced low-frequency com-Figure 9 - Harmonic Analysis of Alternat- ponent did not recur when speed was, againing Torque Components of Twice TurbineShaft Frequency and Two-Thirds Turbine reduced after the test. The 55 RPM mea-Shaft Frequency surement, entry 2 of Table 1 and point Cof Figure 7, was taken after the speedrun had been completed and agrees quite well with the other data.Observable torsional oscillations did not occur at speeds in excess of 230RPM. This is illustrated by the oscillogram of Figure 6 on page 5, which was takenat maximum usable sensitivity at 250 RPM.CONCLUSIONS1. Torsional oscillations are present in the low-pressure turbine shaft of theUSS MOFFETT, particularly at low speeds. The maximum peak single amplitude of alter-nating torque measured was 15,550 pound-feet, corresponding to an alternating stressof 3220 pounds per square inch. This occurred at 55 propeller revolutions per minute.It probably represents excitation of the first-order torsional natural frequency byirregularities in the pinion occurring once per pinion revolution.2. The exciting forces responsible for this oscillation originated in the re-duction gear, as shown by the frequency of oscillation.3. Propeller-excited torsional oscillations, if present, were of very smallamplitude, as shown in Figure 8 on page 7.hill||MIIIlllHIIYIilII lbellY ikIIIYI 31 .4. The highest amplitude of torsional oscillation at speeds of 140 propellerRPM and above was 1520 pound-feet, corresponding to a stress of 315 pounds per squareinch. This amplitude occurred at 230 RPM. This is doubtless the second-order tor-sional oscillation. Its amplitude is obviously much less than that of the oscilla-tion occurring at 55 RPM. Amplitudes above 230 RPM were much below this value.REFERENCES(1) BuShips letter C-DD362/S41-1(457) of 5 October 1942, to Director,David W. Taylor Model Basin.(2) "Electronic Methods of Observation at the David W. Taylor Model Basin,Part 2 - Measurements of Steady and Alternating Stresses in Rotating Shafts," TMB Re-port R-96, January 1942.543 98I rrm0630_ __I I~

Alternating Torque Tests on a Turbine Shaft - U.S. Destroyer MOFFETT

http://dome.mit.edu/bitstream/1721.3/51178/1/DTMB_1943_R085.pdf

Alternating Torque Tests on a Turbine Shaft - U.S. Destroyer MOFFETT

Abstract

Similar works

Full text

Available Versions

MIT Libraries Dome