From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Guide on using oscilloscopes and pickups for measuring material smoothness, noise levels, magnetic fields, and vibrations.
| Identifier | ExFiles\Box 138\1\ scan0299 | |
| Date | 30th June 1938 | |
| NO. 7. SMOOTHNESS OF MATERIALS: An indication of the relative smoothness of materials is often desirable in comparing polished surfaces, gauges, paper, finishes, etc. The No. 156 Vibration Pickup is best suited to this type of work. The Neobeam Oscilloscope should be set to highest sensitivity for this work. In making these tests, the point of the Pickup is drawn across the surface at the rate of 1 ft. per second under its own weight only. The best results are obtained with the prod inclined at an angle of 45°. Flexible materials such as paper should have a glass plate backing. Outputs on some common materials are as follows: Sulphite Bond Paper . . . . . . . . .6 to .8 volts Enameled Paper (70 lb.) . . . . . . .4 to .5 volts Coated Cardboard. . . . . . . . . . .2 to .3 volts Brass, unpolished . . . . . . . . . . . .08 volts Brass, polished chrome. . . . . . . . . .03 volts Soft pine wood . . . . . . . . . . . .05 to .8 volts Glass Plate . . . . . . . . . . . . . . . .02 volts NO. 8. NOISE LEVELS: The No. 151 Neobeam Oscilloscope, microphone combination form an excellent means of comparing the relative energies of sound levels and furnish a means of checking overall noise from machinery in production testing, as indicated in example No. 1 on Electric Motors. No attempt has been made to convert our results into decibels, but using the No. 152 Microphone with the Neobeam Oscilloscope set at maximum sensitivity, a comfortable sound level is reached at 1 volt output. At 4 volts, the sound level is uncomfortable. While the ear is most sensitive to frequencies around 2,000 C.P.S., the physiological perception is not greatly affected by frequency. The set-up and a No. 154 Speaker, connected to the Neobeam output, can be used as the basis for a very interesting experiment to demonstrate how high frequencies become audible to the human ear. Feeding the output from an audio oscillator to the Neobeam Oscilloscope, as the frequency is gradually increased, it gradually fades out of hearing, while the meter continues to read the same value. The speaker response falls off in volume at about 6500 cycles, but still has good audibility up to the limit of hearing. Sounds below the limit of convenient audibility can be picked up by the No. 156 Pickup and compared with the No. 154 Speaker. Examples are watch movements, clock escapements, etc. NO. 9. ISOCHROMETER METHOD OF TUNING: The Neobeam Oscilloscope Models 150 or 151 may be used as the basic unit in the isochrometer method of tuning or matching two sources of audio frequency. This method consists of a means of measuring the duration of beats formed when two frequencies approach unison, and, while the principle is not new, the method employed is effective. In order to illustrate a typical case, we will review the procedure as applied to tuning forks; it could be any audio oscillator or vibration source. Usually forks are tuned by ear, the difference in frequency and the amount of adjustment being determined by the number of beats. However, it is difficult to perceive beats of 1 second or more duration, and the beats do not indicate whether the non-standard is higher or lower in pitch than the standard. By using the Neobeam Oscilloscope, microphone, meter combination, beats of relatively long duration, 10 seconds and more, can be observed. The beat pattern can be viewed either in the Neobeam Oscilloscope or observed by the rise and fall of the voltmeter needle. In production work, the latter is the simpler expedient. The operation is as follows: As the two sound sources come within 4 to 5 cycles of unison, each beat is indicated by a "kick" in the voltmeter needle. As the frequencies become closer, the beats become of longer duration, and the accuracy can be carried to very close values. The Neobeam is also used to determine whether the non-standard is higher or lower than the standard. To do this, the Neobeam Oscilloscope sweep is synchronized to the frequency of the standard so the pattern of the wave form is stationary. This is possible with the mechanical sweep of the Neobeam Oscilloscope, but is not possible with an electrical sweep which tends to "lock in" when close to the input frequency. With the standard frequency synchronized, the non-standard fork is held to the microphone. If its frequency is low, the pattern will drift to the left--high, it will drift to the right. While our tests have been confined to the audio spectrum, there seems to be no apparent reason why the same principles cannot be applied to higher or lower frequencies. The Isochrometer method of tuning permits relatively inexperienced non-technical operators to perform accurate tuning with resultant savings. The method seems suited to all classes of tuning, such as standards, chimes, reeds, organ pipes, and audio oscillators. NO. 10. BALANCING MAGNETIC FIELDS: An interesting application of the Neobeam Oscilloscope lies in the balancing of magnetic fields of small shaded pole motors, such as self-starting electric clock motors. In this type of motor, the small field coil is usually shifted on the yoke of the magnet frame to the point of the balance thus assuring synchronism. To measure the balance of the magnetic field, two non-magnetic pickup coils are placed near identical and opposite positions of the magnet. The design of the coils is not important, as the Neobeam Oscilloscope is sensitive enough to pickup a very low "EMF," but the small pancake type coil, as used in 75 to 100 millihenry radio chokes, serves very well. In connecting the pickup coils, care must be taken that they buck each other, otherwise the null point cannot be obtained when the fields are in balance. After the balancing potentiometer between the coils has been set, it is not adjusted further. The motor coil is next moved to the position of minimum modulation in the Oscilloscope at which point perfect balance is indicated. The output voltmeter, as used on other tests may also be used to indicate balance and may prove more convenient to read in production work. A jig complete with coils, etc., will be made to order if it is suggested that a sample motor of the type be balanced accompany the request for quotation. NO. 11. MEASURING COMMUTATOR ROUGHNESS: Commutator roughness is checked accurately with the Neobeam Oscilloscope and No. 156 Vibration Pickup. In this test the Pickup is held against the commutator brush and production units compared to an accepted standard. Brush "bounce" when present is shown by the wave form in the Neobeam and two or three "bounces" are often observed at each commutator bar. The general level of roughness is best compared in the output meter. Perfect smoothness seems almost impossible to obtain but relative smoothness and degrees of brush "fit" are easily compared. For a given frequency the output in volts is directly proportional to the amplitude of vibration. For true harmonic or sine wave motion the voltage output varies as the square of the frequency or velocity and directly as the amplitude within the linear output range of the pickup. The voltage output is thus a direct measure of the energy of vibration. The output characteristics of the Model 156 are given in the accompanying curves. See below. Required load impedance 2 to 5 megohms. The weight of the unit complete with removable duralumin prod is 11 ounces, without the prod 6 ounces. The hardened steel point may be threaded directly on the case or the case may be mounted on a 1/4 - 28 threaded screw where permanent installation is required. This is especially desirable under severe vibration. 8 ft. of shielded cable is furnished. Care should be exercised when using inertia type pickups so that a uniform pressure is applied between tests, thus not changing the period of vibration due to excessive pressure. The lower limit of sensitivity with respect to amplitude is difficult to determine, being in the vicinity of a millionth of an inch--depending on frequency. The maximum will also depend on frequency and the practical value when held in the hand is about 1/16" at 60 cycles. A frequency-voltage output curve is furnished with each pickup. Price $17.50 net. MODEL 157--VIBRATION PICKUP (WAFER TYPE). The Model 157 Pickup, while designed primarily for use on musical instruments, may be applied to industrial applications where: (1) the frequencies are 5000 or less; (2) the amplitudes are too great to permit holding the No. 156 Pickup in contact; (3) the vibrating bodies are relatively light in weight. Though very small, it is of the inertia type. In practice the No. 157 Pickup is usually clamped or cemented to the vibrating surface with a rubber or balsem cement. The physical size is 1-1/2" x 3/4" x 5/16" thick. The weight of the unit alone is only ½ ounce. 15 ft. of shielded cable is furnished, the shield being always grounded. The load impedance should be between 3 to 10 megohms. Below a bismpth crystal no temperature effects are encountered but since the crystal is set in wax, temperatures above 120°F.{Mr Friese} should be avoided. The resonant frequency is 10,500 cycles per second and the voltage response curve is given below. Price $25.00 net. VIBRATION STUDY: General It is hardly within the scope of this literature to make a comprehensive discussion of the theoretical and mathematical factors involved in the study of vibration. Moreover, in practical work only the basic elements of such a study are even considered. In the great majority of practical cases measurement of vibration resolves itself to a comparison against units that have been found acceptable and arbitrary values are adopted as "Go" and "No Go" standards. The following outline is, therefore, written from a practical point of view to show how the instruments described above may be effectively applied in practical cases. The basic factors to consider in the study of vibration are; frequency, amplitude, and velocity. Frequency is given in cycles per second, amplitude in .001" overall motion and velocity, the product of frequency X amplitude, is the average rate, practically speaking (R.M.S. technically) the vibrating body moves in thousandths of an inch per second. MAKING VIBRATION MEASUREMENTS: In making vibration measurements with the Neobeam the output or amplifier load is held constant and the Input ("Sensitivity" dial) is calibrated against voltage. A calibration curve is supplied with each instrument. The frequency of vibration is determined by the Neobeam as explained below. To set up for vibration measurements proceed as follows: Connect the green lead of the Vibration Pickup to the "Input" terminal and the black shield lead to the post marked "GND" on the front of the Neobeam. In most cases an external ground to a water pipe, etc., is not necessary. Its necessity can be checked by connecting one side of the "60 cycle" output from the rear of the instrument to the "Input", the other side can be connected to the "Ground" on the rear by a short jumper wire. See instruction book accompanying the instrument. If no difference in wave form is observed with or without the external ground it is not necessary. All calibrations in the following instructions are based on the "Glow" set at "80." This allows a margin for overmodulation peaks, reduces the load on the amplifier somewhat, and improves the life of the Neobeam Neon Tube which is normally 1000 hours. Now turn the "Switch" to "on," snap on the meter switch and in a moment a straight, unmodulated band about 2" wide will appear across the Neobeam Screen. The meter should read zero with no input. Next, set the "Sensitivity" dial to "0" and the "Sensitivity Multiplier" to the highest value later returning it to the value found best after the Pick-up has been applied to the vibrating surface. The Voltmeter reading will be found to vary with the "Sensitivity" setting; this dial is calibrated with a constant output of .75 volts at which point approximately 100% modulation is obtained. The remaining directions are given under the discussion of the 3 calibration curves supplied with each instrument. These curves are as follows: NO. 1. VIBRATION PICKUP RESPONSE CURVE. This curve gives the voltage-frequency relationship at a constant amplitude of .001" and is supplied with each Pickup we sell. It is furnished as general information. | ||