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).
Instructions and applications for the Neobeam Oscilloscope and associated vibration pickup equipment for analysis.
| Identifier | ExFiles\Box 138\1\ scan0300 | |
| Date | 8th June 1938 guessed | |
| in the event the pickup should be used with other equipment than the Neobeam. The equation of the curve is given and is useful in determining intermediate outputs within the linear range of the curve. NO. 2. PICKUP VOLTAGE - SENSITIVITY DIAL CURVES: In these curves the "Sensitivity" dial of the Neobeam is plotted against the peak voltage which will produce a constant output of .75 volts on the A.C. rectifier type output voltmeter. In order to cover the range of voltages delivered by the pickups in practical work, approximate voltage ranges are selected by the "Sensitivity Multiplier" and the intermediate voltages are calibrated against the dial reading on each range. There is, therefore, a separate curve for each range. [Graph Title: SUNDT ENGINEERING CO. CHICAGO, ILL.] [Graph Subtitle: NEOBEAM OSCILLOSCOPE MODEL 156 OR 151, PICKUP VOLTAGE VS.{J. Vickers} SENSITIVITY DIAL CURVE] NO. 3. FREQUENCY CORRECTION CURVE: Since the efficiency of an audio frequency output transformer falls off rapidly below 40 or 50 cycles, a correction factor must be applied at low frequencies to compensate for the lower output voltage. This curve gives the amount by which the apparent results must be divided in order to equal linear response at these low frequencies. No correction is required for the higher frequencies encountered in practical work, the limit of which is about 2,000 cycles. [Graph Title: SUNDT ENGINEERING CO. CHICAGO, ILL.] [Graph Subtitle: NEOBEAM OSCILLOSCOPE MODEL 156 OR 151, VIBRATION AMPLITUDE CORRECTION FACTOR VS.{J. Vickers} FREQUENCY, A CURVE OF THE FACTOR BY WHICH THE APPARENT AMPLITUDE OF LOW FREQUENCIES MUST BE DIVIDED AT THE VARIOUS SENSITIVITY DIAL DATA TO CORRECT FOR VOLTAGE DROP ACROSS OUTPUT TRANSFORMER DUE TO LOW FREQUENCY.] DETERMINATION OF FREQUENCY, AMPLITUDE, AND VELOCITY: Frequency determinations are made with the Neobeam Oscilloscope and are possible because of the 0-1000 calibrated mechanical sweep which is expressed in R.P.M. of the revolving mirror. After the "Sensitivity Multiplier" and "Sensitivity" dial have been set to produce an output on the voltmeter of about .75 volts, the "Sweep" dial is adjusted to the point where the sweep synchronizes with the frequency and the pattern will appear stationary. Frequency is then determined by the formula F = R.P.M. / L where F = frequency; R.P.M. = Sweep speed and "L"= inches between cycle peaks. The screen calibrated in 1/4" spaces facilitates measurements of L. A very simple and practical way to read frequency directly is to adjust the sweep until the cycle peaks are 1/2" apart. Frequency in cycles per second is then read directly on the "Sweep" dial. At 1" apart the dial will read double the frequency - 1/4" half the frequency, etc. AMPLITUDE OF VIBRATION IS DETERMINED AS FOLLOWS: Set the input controls to obtain .75 volts on the voltmeter. Read the voltage input being applied from the pick-up by interpolating the "Sensitivity" dial reading on the curve "Voltage - Sensitivity." Next, turn to the "Pickup Output" curve and read the output at .001" overall motion at the frequency as determined in the preceding paragraph. Divide this figure into the input voltage as determined above and the answer is the amplitude in thousandths of an inch. EXAMPLE: By interpolating on the "Voltage-Sensitivity" curve we find the input voltage is 4 volts. We read the output as shown in the Oscilloscope to be 400. Referring to the "Output" curve on the 156 Pickup, we read the output at 400 cycles for .001" amplitude is 2.83 volts. Dividing 4 by 2.83 = 1.41 or .0014" overall vibration amplitude. VIBRATION VELOCITY: The vibration velocity is the product of the frequency and amplitude and is a measure of the rate the vibrating body moves, usually in R.M.S. thousands of an inch per second. It is a factor not often used in practical work but may be used where energy considerations are involved. In machinery, as a general rule, the higher the frequency the smaller the amplitude, the vibration velocity tending to remain constant. Knowing the frequency and amplitude, the velocity can easily be calculated. ACCURACY, COST OF CURVES, ETC. In order to maintain accuracy without holding expensive tolerances in the component parts and production a Volts-Sensitivity Dial curve is furnished with each Neobeam Oscilloscope intended for vibration work. In effect it provides a high sensitivity vacuum tube voltmeter. The Pickup Output Curve and Frequency Correction Curves are standard and are supplied no charge. The Voltage vs.{J. Vickers} Sensitivity Dial calibration curve complete with a clear cellulose acetate (noninflammable) binder is $3.00. All calibrations are based on sine wave or simple harmonic motions and with reasonable care in handling the curvelet within 1% may be obtained. In practice this margin is often extended to 10% but this will enclose enough. Complex, mixed, and peaked wave forms will introduce inaccuracies that are very difficult to compensate for and comparative averages are usually all that can be expected with such vibrations. Most vibrations, particularly in machinery, are fairly sinusoidal, though this may not be true when the vibrating mass is high compared to the weight. It is also true that the pickups may be applied with uniform pressure -- preferably only their own weight. In the analysis of the wave form of vibration the frequency squared output characteristics of a crystal pickup must be taken into consideration. Thus a 5% harmonic component of 200 C.P.S. will be expressed by an electrical wave of 100 C.P.S. with a 20% component. This is an advantage, due to the fact that almost all high frequency vibrations are of low amplitude and would be difficult to measure if the output were independent of frequency. For a very thorough and complete analysis of vibration, a means should be provided for measuring the relative energies of the harmonic components of the frequencies observed. This calls for a tuned circuit, either electrical or mechanical, which will select a narrow band of frequencies which may then be analyzed. Such filters are difficult and expensive to build and can only be applied to vibrations of a continuous character. These measurements are tedious to make and are hardly practical outside of the laboratory. A very useful application for the No. 151 or No. 156 Vibration Pickup, is as a vibration Go - No Go gauge. In these applications the Neobeam is set to an accepted standard and variations in output used as limits. For instance; in a production line of motors a certain "Sensitivity" setting for .75 volts output could be set as standard; variations between .5 and 1 volt could then be considered as limits. A speaker (154) may be used to render audible and compare vibrations of very low intensity, but the meter is always preferable to eliminate the human equation. When using the Neobeam for vibration inspection work, it is preferable, but not absolutely necessary, to check daily against a standard. The number of applications to which the Neobeam Oscilloscope and vibrations auxiliaries may be applied are great. A partial list follows: MACHINERY PRODUCTION TESTS: Ball bearings, crankshafts, cams, gear trains, motors and generators, lathes and milling machines, grinders, refrigerators, and air conditioning equipment. NOISE AND VIBRATION DETECTION: Automobile bodies, airplane wings, nodes of vibration of sheet metal housings and frames, structural members, pipe lines, resonance point of resonators and boxes, knocks in reciprocating and rotating machines. Miscellaneous tests include tests for determining the relative smoothness of surfaces, detection of fractures in elongated rods and tubes, efficiency in sound deadening of various materials. Typical tests are as follows: NO. 1 ELECTRIC MOTOR TESTING: In making this and similar tests, it is important that the motor is set on a sponge rubber mat or similar absorbent to avoid vibration from the bench being transmitted to the motor. The points of greatest vibration, planes of vibration, and the axis of vibration can be determined quite accurately. A very quiet motor 1,800 R.P.M., hardly audible in overall noise, will yield readings of .4 to 1 volt at maximum "Sensitivity." Another practical test on Electric Motors is overall noise test. To do this; the motor must be enclosed in a soundproof box, together with a No. 152 Crystal Microphone and the output compared to an approved standard. Equipment required: Nos. 151, 156, 152, if noise level is desired. NO. 2. BALL BEARING TESTS: A very practical production test. The bearings are placed on a well balanced, rigidly supported shaft, the end bearing of which may be removed. The steel prod point is screwed directly into the casing of the No. 156, and the weight of the pick-up alone is applied to the outer race of the bearing. The Neobeam Oscilloscope set-up is the same as in No. 1. Not many ball bearings at 1800 R.P.M. will be better than .5 volts at maximum "Sensitivity." Equipment required: Nos. 151, 156. NO. 3. GEAR TRAINS: Vibration studies of gear trains are useful in: Comparing relative vibration levels of different trains; locating which of several gears is noisy; effects of different type of lubricants on vibration level and frequency, and the effect of adjustments. Gear trains often vibrate at surprisingly high frequencies and amplitudes. The wave form is usually very complex, being formed from frequencies ranging from 50 to 1,000 cycles in most cases. Usually a noisy gear may be detected by a predominant "beat" which can be traced to one revolution. These beats can usually be detected on the meter if on the order of 10 cycles per second or less; if above, they will be seen in the Neobeam Oscilloscope. Equipment required: Nos. 151, 156 Pickup. NO. 4. CAM ACTIONS: In high speed machinery employing cam actions, a speed may be attained near the natural period of the cam and its actuating mechanism. There is thus a "bouncing" action created which usually can be detected with the Vibration Pickup. Equipment required: Nos. 151, 156 Pickup. NO. 5. FRACTURED RODS: With reasonable care, cracks and fractures in longitudinal rods can be detected with a fair degree of accuracy with a vibration pickup. The amount of research we have done along this line at this time does not permit dogmatic conclusions but the following seems to be true; to get good indications, the rod must be at least 1/4 the cross section of the rod; if possible, the resonant frequency of the rod should be applied. Otherwise the effect of frequency change is not pronounced; intense vibrations give a better indication than low level vibrations; vibration intensity and point of pickup must be the same on comparative tests to give consistent results. For best results, a constant frequency vibrator must be used as a vibration source. Equipment required: Nos. 151, 156 Pick-up Vibrator. NO. 6. SOUND INSULATION: Due to the diversity of this field, only general recommendations can be made here. Knowing the specific application, a good deal of work, however, requires a knowledge of the relative sound insulating qualities of different materials and in some cases, their effectiveness as sound baffling materials. The procedure, in most cases, is relatively simple, requiring only tests before and after applying the insulating materials to obtain comparative results. The effect of frequency and resonance should not be overlooked, since materials that are effective at high frequencies are often inefficient at frequencies below 100 C.P.S. In some tests a microphone may be more suitable than a vibration pickup, but in most cases Model No. 155 or No. 157 are most suitable. In practically all cases the meter is the best indication of vibration level. The Sensitivity Multiplier of the Neobeam Oscilloscope in this type of work is practically always set at maximum sensitivity and for very low level work a preamplifier may be required. Prices on preamplifiers suited to any particular requirement will be furnished on request. | ||