vibration analysis2

LESSON

3 VIBRATION AND NOISE MEASUREMENT

LECTURE

SUB - OBJECTIVE

At the end of the lesson the Trainee will be able to demonstrate an understanding of Vibration and Noise Measurement.

1.0 INTRODUCTION

Electronic instruments for measuring machinery noise and vibration are generally classified as METERS, MONITORS or ANALYZERS. The vibration Meter, Fig. 13-3-1, is a small portable instrument used to take periodic vibration checks on rotating machinery to determine the “overall” vibration level. The vibration/sound level meter shown in Fig. 13-3-2 may be used to take or noise level readings.

A vibration monitor, Fig. 13-3-3 is similar to a vibration meter but is permanently or semi-permanently installed to provide continuous protection from excessive machinery vibration. Vibration monitors normally incorporate alarm relays in conjunction with preset vibration levels to warn when vibration has increased beyond a given level. Some monitors may actually trigger the shutdown sequence if vibration becomes critical.

The analyzer, Fig. 13-3-4, includes a tunable filter for separating the individual frequencies of complex noise and vibration. A stroboscopic light which is triggered in synchronism with the vibration of noise is also included and is a valuable aid in analyzing machinery problems and for dynamic balancing.

SPECIFIC COURSE FOR ENGINEERS MECHANICAL MAINTENANCE MODULE 13VIBRATION ANALYSIS & CORRECTION LESSON 3 PAGE 1

Fig. 13-3-1. A vibration meter is used to take periodic checks of machine vibration to detect developing trouble.

Fig. 13-3-2. This vibration/sound level meter is used to measure both noise and vibration.

MECHANICAL MAINTENANCE MODULE 13 SPECIFIC COURSE FOR ENGINEERSLESSON 3 PAGE 2 VIBRATION ANALYSIS & CORRECTION

Fig. 13-3-3. Vibration monitors are permanently installed to provide continuous 24 hour protection of critical machinery.

Fig. 13-3-4. A vibration analyzer has all the provisions needed to diagnose machinery problems and perform in-place dynamic balancing.


TRANSDUCERS (PICKUPS)

Regardless of which type of instrument is used to measure the vibration or noise (meter, monitor or analyzer), the heart of the measurement system is the pickup or Transducer. A transducer is simply a sensing device which converts one form of energy into another form of energy. A vibration pickup converts mechanical vibration into an electrical signal. A microphone converts pressure oscillations into an electrical signal.

Because there are a number of transducers available, the following paragraphs are presented to familiarize you with the general operation and use of the pickups most commonly used. These will include the seismic-velocity type pickup, accelerometer and non-contact (proximity) pickups for measuring vibration; and the microphone for measuring noise.

THE SEISMIC VELOCITY VIBRATION PICKUP

Fig. 13-3-5 is a schematic of the seismic velocity pickup showing the principal parts. The Mechanalysis Model 544 and Model 544M are seismic velocity type vibration pickups. The system consists of a coil of fine wire supported by springs with low stiffness. A permanent magnet is firmly attached to the case of the pickup and provides a strong magnetic field around the suspended coil.

When the case of the velocity pickup is attached to or held against a vibrating part, the permanent magnet (being firmly attached to the case) follows the motion of the vibration. The coil of wire (conductor), supported by springs with low stiffness, remains stationary in space. Under these conditions, the relative motion between the magnetic field and coiled conductor is the same as the motion of the part relative to a fixed point in space; and the voltage generated by the pickup is directly proportional to this relative motion. The faster this motion, the larger the voltage. In other words, the voltage output of the pickup is proportional to the velocity of the vibration. As the velocity of the vibrating part changes, the voltage generated changes proportionately. Hence the name – VELOCITY PICKUP.

The voltage output of a velocity pickup is normally expressed in milli-volts per inch per second. This is also referred to as the sensitivity of the vibration pickup. For example, the models 544 and 544 M pickups have a sensitivity of approximately 1080 millivolts peak per inch per second. This means that for each inch per second peak velocity, the pickup generates 1080 milli-volts peak. This information on pickup sensitivity may be used to check the accuracy of your Mechanalysis instrument using a standard signal generator and voltmeter. Further details are available on request.

LOW FREQUENCY VIBRATION MEASUREMENT

The sensitivity of the velocity pickup is only ocnstant over a specified frequency range. At low frequencies of vibration the sensitivity actually decreases because at the lower frequencies the pickup coil is no longer stationary with respect tot he magnet. This decrease in pickup sensitivity begins at a frequency of approximately 600 CPM; below 600 CPM pickup output drops exponentially. The significance of this fact is that amplitude readings taken at frequencies below approximately 600 CPM, using the standard velocity pickup, are not true readings. The amplitude meter of your instrument will indicate a value less than the actual amplitude of vibration being measured.


Even though the sensitivity falls off at low frequencies, the standard velocity pickup is still quite usable. This drop in sensitivity will have little or no effect on the use of your instrument for balancing or for detecting increases in machinery vibration as required in a preventive maintenance program. However, for those who must take readings for comparison with specific criteria, the drop-off in sensitivity is reasonably predictable, and accurate data can be obtained by using the correction factor chart, Fig. 13-3-6. All readings applied to the chart must be filtered amplitude readings obtained using the filter of your vibration analyzer instrument. Overall or filter out readings such as those obtained with the vibration meter should not be applied to the chart.

To use the chart, simply note the frequency of the vibration on the horizontal scale at the bottom of the chart. From this point, move upward to intersect the curve, and cross over to the vertical axis on the left side and read the value of the correction factor from the scale. Next, simply multiply the observed amplitude meter reading times, the correction factor to obtain the true amplitude of vibration.

MOUNTING THE PICKUP

Like most vibration pickups, the Mechanalysis velocity pickup is sensitive only to vibration occurring in the direction in which it is pointed In addition, the pickup may be placed in any position without affecting its operation or accuracy.

There are several methods which can be used to apply the vibration pickup as illustrated in Fig. 13-3-7. Each of the methods shown will hold the pickup in place without distorting the actual vibration, but only over a limited frequency range. Fig. 13-3-8 lists the highest recommended frequency for each mounting method. Vibration readings taken at higher frequencies may be subject to error.

The fact to consider when mounting the vibration pickup is that any object (bracket, probe, etc.) used between the pickup and measured surface will have spring-like qualities and thus will tend to amplify or distort the true vibration at certain frequencies. Of course, whenever this distortion occurs, errors in measurement result.

STUD MOUNTING

By far, the best and most reliable mounting technique is to fasten the pickup directly to the measured surface with a threaded stud. The mounting surface should be flat, with the entire face of the pickup in contact as shown in Fig. 13-3-8A. Avoid those mounting conditions shown in Figs. 13-3-8B and 13-3-8C.

Stud length is an important factor to consider. The stud should not be so long as to bottom in the end cap of the pickup as illustrated in Fig. 13-3-8D. Bottom the stud in the end cap of the pickup may not only damage the pickup by forcing the stud through the sealed cap, but will also reduce the usable frequency range of the mounting.


HAND HELD WITHOUT A PROBE

The hand held pickup without probe is quite satisfactory for most periodic vibration checks and analysis applications. The pickup should be held against a reasonably flat surface. When applying the pickup to curved or irregular surfaces, keep in mind that the pickup measures vibration only in the direction parallel to the pickup axis, and any unsteadiness of the hand which allows the direction of the axis to vary may result in unsteady vibration readings.

Use only enough pressure to keep the pickup from chattering or “walking” on the surface. When the vibration pickup is hand held, a tingling sensation in the hand indicates the presence of the a high frequency vibration and the need for more pressure.

HAND HELD WITH STANDARD 9” PROBE

The standard 9” (23 cm) probe provided with Mechanalysis Meters and Analyzers is a convenience device for reaching out of the way points for measurement and to assist in locating the pickup at a specific point on the machine. The probe may be used for the majority of periodic check and analysis applications; however, caution should be exercised where vibration frequencies are above 16,000 CPM.

VISE GRIP PLIERS

The vise grip pliers require a firm mounting, preferably with the jaws in contact with the mounting surface along their entire length. Actually, vise grips are items of convenience for use during in place balancing, and are not recommended for vibration analysis purposes because of their very limited frequency range.

MAGNETIC PICKUP HOLDER

The magnetic pickup holder should be mounted on a reasonably flat, smooth, clean surface. Dirt or grease between the magnet and the mounting surface reduces the holding power of the magnet and consequently the maximum usable frequency. If the magnetic surface is not flush with the mounting surface, the holder may rock at some unpredictable low frequency, producing erroneous readings. When properly applied, the magnetic holder produces good results as a vibration pickup holder.

Sometimes a machine will require a special pickup mounting bracket or adapter. Or, perhaps a longer probe is needed to reach out of the way places. Where the usable frequency range is in question, a quick comparison check will usually reveal any problems. Simply select an accessible point on the machine with vibration characteristics similar to those where measurement will be taken. Measure the vibration first with the pickup hand held without the probe, and then measure the vibration at the same point using the selected bracket or probe. Any difference in the two sets of readings will probably be due to the selected mounting.


Fig. 13-3-5. Basic construction of the seismic-velocity vibration pickup.


Fig. 13-3-6. When using the IRD Seismic Velocity Pickup at frequencies below 600 CPM, if required, more precise amplitude readings may be obtained by multiplying the observed reading by the multiplication factor obtained from

the chart above. Only Filter In readings should be applied to the chart.


Fig. 13-3-7. Common Pickup Mounting Techniques.

SHAFT STICK

Many times it is useful to know the actual vibration of a shaft to compare with bearing housing vibration. This is especially true of high speed machines such as turbines and centrifugal pumps and compressors where the machine housing and rigid bearings will often have very little vibration even though the rotor and shaft may be vibrating excessively within the clearances of the bearings.

The shaft stick, Fig. 13-3-10, when used with the velocity pickup provides a convenient means of measuring shaft vibration. The shaft stick is simply a hardwood, fish-tail shaped stick with a stud for attaching the pickup.


The fish-tail shape provides two points of contact needed to keep the shaft stick on the circumference of a rotating shaft, and also permits the use of the stick with almost any diameter shaft or roll. In addition, the end of the stick is tapered to provide a smaller contact area with the shaft in order to reduce friction and prevent chatter.

Shafts must be reasonably smooth, preferably smooth, preferably turned or ground. Applying the shaft stick to shafts with rust, dents, pits or an otherwise rough surface will produce questionable information and may also damage the stick. Tuning the analyzer filter to the rotating speed frequency will normally improve the results. Also, take care to avoid keys and keyways, set screws and lubrication holes.

Fig. 13-3-8. Recommended frequency limits for different pickup attachment methods.

Fig. 13-3-9. The entire face of the stud-mounted pickup should be in contact with the measured surface. Mounting the pi8ckup at an angle (B) or

overhanging the measured surface (C) may result in errors. Mounting with an overly long stud that bottoms in the pickup end cap (D) may not only affect

your readings but may also result in damage to the pickup.


Fig. 13-3-10. The shaft stick used with the pickup for measuring shaft vibration – a valuable tool for many analysis and balancing applications.

For shaft speeds in excess of 3600 RPM, avoid holding the stick against the shaft for any length of time. Heat build-up from friction may burn the stick or actually cause scoring of the shaft. Frequent applications of a medium weight lubricating oil is recommended to reduce friction and heat build-up on high-speed shafts. Shaft stick readings are not recommended at shaft speeds above 12,000 RPM.

Two hand are normally required to apply the shaft stick; one hand one the stick to prevent walking up and down the shaft and the other hand on the pickup to regulate pressure and control the angular position of the pickup. When applying the shaft stick, use only sufficient pressure to prevent the stick from chattering. A tingling sensation in the hand accompanied by a distinctive squeal or growl indicates the presence of chatter and the need for more pressure. Maintaining a constant pressure is important also. Studies how that varying the pressure significantly may affect the accuracy of readings. This is particularly important in balancing or wherever a comparison of shaft stick readings is important.

In the same way that the shaft stick senses shaft vibration, it also senses any out of roundness or eccentricity of the shaft. Normally the influence from shaft runout is quite small and not a cause for concern. Again, the use of the analyzer’s filter tuned to rotating speed frequency will usually improve the validity of your readings. On shafts, couplings, pulleys or wherever runout is suspected, a visual check with a dial indicator or micrometer is recommended.

Although there may be sources of error in shaft stick measurement from irregularities in shaft geometry, variations in applied pressure and angular position, the shaft stick is a very valuable tool for analysis and in place balancing.


SHAFT RIDER ACCESSORY

Although the shaft stick is quite suitable for periodic vibration checks, analysis and in place balancing, where it is desired to continually monitor absolute shaft vibration the IRD Mechanalysis Shaft Rider Accessory, Fig. 13-3-11, is used.

Fig. 13-3-11. IRD Mechanalysis Shaft Rider Accessory.


The shaft rider accessory is permanently installed in the machine bearing housing and consists of a spring loaded probe held firmly against the rotating shaft to accurately follow shaft motion. The probe is fitted with a long-wearing non-metallic tip which is installed within the bearing area to provide tip lubrication. A velocity pickup or accelerometer mounted to the shaft rider provides an electrical output proportional to the absolute shaft vibration (relative to a fixed point is space).

Shaft rider vibration pickups are normally installed on large rotating machines such as turbo-generators with rather massive rotors. On these and similar large machines where shaft surface speeds are moderate, a measure of absolute shaft vibration is preferred for monitoring machinery condition and for in place dynamic balancing. For high-speed machines with relatively light-weight rotors, a measure of shaft vibration relative to the bearing is generally preferred. These measurements of relative shaft vibration are obtained using non contact pickups which are described later in this chapter.

NOTE:

When using non-contact pickups, inaccuracies can result if special attention is not given to details regarding shaft material. Conditions such as chrome plating, variations in alloy, use of non-ferrous materials, etc., can result in considerable error in readings obtained with non contact pickups. As a result, the shaft rider type pickup is also used on many high speed pumps, compressors and similar machines to avoid these problems.

MAGNETIC INTERFERENCE

Measuring the vibration on large A.C. motors or alternators sometimes presents a problem due to the alternating magnetic fields inherent with this type of machinery. Such magnetic fields can induce a signal in the velocity pickup at a frequency equal to the frequency of the A.C. field . the amplitude reading which results from the induced signal is actually a false reading which may have nothing to do with the condition of the machine. Of course, the strength of the induced signal will depend on the strength of the magnetic field where the pickup is placed.

The presence and approximate influence of a magnetic field can be easily checked using your velocity pickup and analyzer instrument. Connect the vibration pickup to the analyzer just as you would for measuring machine vibration. Next, suspend the vibration pickup by its cable in the area where vibration readings are normally made. See Fig. 13-3-12. Hold the cable as steady as possible, but do not touch the machine with the pickup. To measure the amplitude of the magnetic field, carefully tune your analyzer’s filter to A.C. line frequency and note the amplitude reading. This is the signal caused by the magnetic field.

If you find that magnetic interference is a problem, it is suggested that the pickup be installed in a magnetic shield as illustrated in Fig. 13-3-13. The shield reduces the magnetic interference by approximately 100 to 1.

Two magnetic shield assemblies are available. One shield (IRD part # 10449) is available for use with the standard Model 544 velocity Pickup, and another shield (IRD Part # 10140) has an integral 4-hole mounting for use with the Model 544 M Velocity Pickups normally used with permanently installed vibration monitors.


A temporary solution to overcome the problems of magnetic interference is to use a long extension probe. The long probe simply locates the vibration pickup away from the magnetic field, thus reducing the interference level. The limitation of this technique is a reduction in the usable frequency range of the measurement system.

THE DIRECT PROD VELOCITY PICKUP

Many times it is necessary to measure the vibration of a small, light-weight part or structure. However, holding or attaching the standard velocity pickup to a small part may actually reduce the vibration. To overcome this problem, the direct prod vibration pickup, Fig. 13-3-14, may be used.

The direct prod pickup is quite similar in construction and operation to the seismic velocity pickup. However, the direct prod pickup includes a prod extending through the end-cap of the pickup and directly attached to the movable coil inside.

Toe measure vibration using the direct prod pickup, the prod of the pickup is brought into contact with the vibrating part while the case of the pickup is rigidly held or mounted as a fixed reference. The prod may be attached to the part using the threaded tip, or it may beheld in place by a special magnetic tip.

The normal practice is to mechanically mount the direct prod pickup however it can be hand-held if special care is taken. When hand holding the pickup it is usually necessary to tune the analyzer filter to the vibration frequency of interest.

This will prevent errors in your readings resulting from the natural movement of the hand. In addition, be sure that the probe and pickup coil are free to move within the limits of travel at all times.

When using the direct prod velocity pickup, only the weight of the prod and moving coil is added to the vibrating part. This makes the pickup especially useful on small, light-weight objects where the added weight of a seismic velocity pickup may affect the actual vibration.

In addition, since the pickup coil is mechanically moved, the output of the direct prod pickup is virtually unaffected at the low frequencies.

Thus, this pickup is well suited for measuring low frequency vibration and is often selected for use on balancing machines where parts may be balanced at speeds as low as 50 RPM with excellent results.


Fig. 13-3-12. The effect of magnetic interference can be easily determined by simply suspending the velocity pickup by its cable in the area where vibration

readings are normally made.

Fig. 13-3-13. The effects of magnetic interference are greatly reduced by installing the velocity pickup in a magnetic shield.


Fig. 13-3-14. The IRD Mechanalysis Model 546 Direct Prod Pickup is used for measurement of low frequency vibration and on small, light weight parts.

ACCELEROMETER PICKUP

Another transducer commonly used to measure vibration is the ACCELEROMETER, Fig. 13-3-15. An accelerometer is a self-generating device with an output proportional to vibration acceleration.

Since acceleration is a function of displacement and frequency squared, accelerometers are especially sensitive to vibration occurring at high frequencies. This makes the accelerometer particularly useful measuring and analyzing the vibration from gears or anti-friction bearings. Accelerometers are often permanently installed to continually monitor the vibration of gas turbines and other machines with very high rotating speeds.

The small size and light weight make the accelerometer well suited for applications where space is limited or where pickup weight is important. In addition, accelerometers are much less sensitive to stray magnetic fields than velocity pickups and are finding greater use for monitoring vibration on large A.C. motors and alternators.

BASIC OPERATION

In many respects an accelerometer pickup is similar in operation to the seismic velocity pickup as indicated by the diagram, Fig. 13-3-16. However, in the accelerometer, the coil of wire used in the velocity pickup has been replaced with a material that produces an electrical charge when it is compressed (i.e., whenever a force is applied). The greater the applied force, the greater the electrical charge generated. Such a material is said to be piezoelectric, and may be a natural or synthetic crystal or a ceramic material.

The output or sensitivity of an accelerometer is expressed in pico-coulombs per “g”. The “g”, of course, is the standard unit of acceleration defined as the acceleration produced by the force of gravity at the surface of the earth.


The output of accelerometers is usually small when compared to the normal output of velocity type pickups. For this reason, pre-amplification of the accelerometer output is usually required before a usable signal is obtained. The accelerometer pictured in Fig. 13-3-17 has its own amplifier built in. this has the advantage of eliminating many problems such as limited cable length or cable interchangeability. The accelerometer in Fig. 13-3-15 does not have a built-in amplifier, but has the advantage of higher operating temperature and somewhat smaller size and weight magnetic field which in turn produces a proportional reduction in the amplitude of the carrier signal.

When the distance between the pickup tip and the metal object (shaft) changes, carrier signal amplitude changes also. The instantaneous amplitude changes or modulations of the carrier signal are detected as an AC signal which is proportional to the peak to peak vibration displacement of the shaft.

Fig. 13-3-15. An accelerometer pickup provides a signal proportional to the vibration acceleration.


Fig. 13-3-16. Block diagram – Non-Contact Pickup operation.

Non-contact pickups are installed in the machine with the pickup tip in close proximity to the rotating shaft. The distance between the tip and shaft is referred to as the gap. Typical gap settings are .020”, 030”, .050”, .060” and .100”. However, the specific gap setting for a pickup will depend on the type of pickup, shaft material and system calibration.

To facilitate installing the non-contact pickup and to provide assurance that the pickup gap is properly adjusted at all times, the gap meter on the accessory instrument or monitor indicates pickup gap.

Gap readings can be taken when the shaft is rotating (mean gap) or when the shaft is not rotating (static gap). The gap meter provides the means for installing and adjusting the non-contact pickup.

This is essential where methods of physical gap measurement are not possible, such as when a machine is operating or where the pickup is installed with in the bearing of the machine with no access to the pickup tip.

NON-CONTACT PICKUP INSTALLATION

Non-contact pickups can be supplied in many configurations as shown in Fig. 13-3-17. Some pickups may be only ¾” long while others may be several inches long. Where some pickups have a threaded body for installation in tapped holes, others may have a solid body for permanent installation with set screws or epoxy cement. Some pickups have an integral waterproof, oil tight connector; others have molded Teflon cables.


To provide maximum protection from cable damage, some cables may have armored covers. Fig. 13-3-17 illustrates only a few of the many styles of non-contact pickups which are available to match individual installation requirements.

Non-contact pickups are normally installed in holes drilled and tapped in the machine case or bearing cap as shown in Fig. 13-3-18. IN some cases, the machinery manufacturer may have already made the necessary provisions for installing the non-contact pickups, in which case installation is a simple matter of inserting the pickups in the provided locations and adjusting for the proper gap.

For field installations where there are no provisions for mounting the pickups in the bearing, the pickup may be mounted in a rigid bracket or adapter as shown in Fig. 13-3-19. Whenever adapters are used to mount the pickup, care must be taken to insure that adapters are securely fastened to the machine. Adapters must be kept short and massive to minimize any vibration of the mounting.

Fig. 13-3-7. IRD Mechanalysis Non-Contact Pickups are available in a variety of styles to meet individual installation requirements.


Fig. 13-3-18. Non-Contact Pickups may be installed in holes drilled and tapped in the machine case or bearing cap. (note: The bearing illustrated here

has been cut away to reveal details of installation).

THE MICROPHONE

There are several types of microphones commercially available; however, they all use a diaphragm arrangement which serves to convert the air pressure oscillations (sound) into mechanical motion (vibration). The major difference in microphones is in the method used to concert the resulting diaphragm vibration into an electrical signal.

The microphone provided with most IRD Mechanalysis instruments is a piezoelectric (ceramic) microphone. See Fig. 13-3-20. In this microphone, the diaphragm is coupled to a ceramic element which generates an electrical charge when stressed by the vibration of the diaphragm. In this respect, the piezoelectric microphone functions in much the same way as the accelerometer used to measure machinery vibration. Piezoelectric microphones provide good sensitivity and a frequency response which is quite adequate for most industrial applications. In addition, they offer simplicity and durability. For these reasons, the piezoelectric microphone is provided as standard with IRD Mechanalysis noise measurement and analysis instruments.


Fig. 13-3-19. The Non-Contact Pickup may be mounted on a rigid bracket firmly attached to the bearing cap.


Fig. 13-3-20. A ceramic microphone is provided with most IRD Mechanalysis instruments

POSITIONING THE MICROPHONE

Unlike vibration pickups which sense vibration only in the direction they are pointed, the microphone provided with your IRD Mechanalysis instrument is Omni-directional. This means that the microphone is sensitive to noise coming at it from vitally any direction. Even though the microphone is sensitive to noise coming from all directions, some care must be exercised when positioning the microphone in order to obtain reliable data. For example, Chapter II discussed the presence of three possible sound fields, NEAR, FAR and REVERBEARANT, and the effect each has on the noise levels measured. Further more, you will recall from Chapter II that noise sources are usually directional in nature and may require more than just one measurement.

The positions selected for noise measurement often depend on the purpose of the measurement. For example, some government regulations require measurements at positions which will represent the location of the worker’s ear, since their purpose is to determine the amount of noise to which the worker is exposed.


Other standards require measurement at a number of fixed positions (e.g., 3 feet from the machine surface, on each side of the machine at the height of its centerline, and at the mid length of the machine). Such measurement are generally made by machinery manufacturers to permit estimation of the noise a machine will make when placed in different surroundings. Still other standards require measurement positions at which the sound pressure level is the highest in order to estimate whether the machine will be a community noise problem.

Where no measurement standards have been set, the following procedure is a good approach to measure machinery noise for compliance with government regulation standards and to correlate noise with the mechanical condition of the machine:

1. First, sketch the top view of the machine on your data sheet and divide the region around the machine into four quadrants at the coupling between the driver and driven units as shown in Fig. 13-3-21. One sound measurement will be made in each quadrant to help determine the “directional” characteristics of the noise source. Where appropriate, large reflecting surfaces adjacent to the machine such as walls or nearby machines can be noted on the sketch along with their approximate distances.

2. Check the machine for operating conditions such as temperature, pressure, flow, load, speed, etc.; and note these conditions on the data sheet. Deviations in these operating conditions may influence your noise level measurements. Therefore such variations, when noted, should also be recorded.

3. Position the microphone in quadrant # 1 by locating it approximately 5 feet (1.6 meters) above the floor and 3 feet (1 meter) from the machine surface. Maintain these dimensions and move the microphone along the machine within quadrant # 1 until the position of highest noise level is observed on your instrument.

4. Move the microphone around within an approximate one cubic foot volume at this position and note any variation in the measured noise level. If this variation is less than 3 dB, then this position is satisfactory. However, if the variations are greater than 3 dB, this indicates that the microphone may be in the NEAR FIELD of the machine. In such case the microphone should be moved further from the machine to a distance of, say, 5 feet (1.6 meters) and the position of highest noise level re-established. If the noise level still varies significantly with position, the microphone may still be in the near field, or it may be in the REVERBERANT FIELD. Where these variations in noise level persist, select a position for measurement and carefully mark its location for future measurements.

5. Check to be sure that the microphone is more than 3 feet (1 meter) from reflecting surfaces. You will recall that large flat structures such as walls cause the greatest reflections; however, other surfaces such as large adjacent machines can also cause reflections. When the noise level increases as the microphone is moved away from the noise source and toward a wall or other reflecting surface, this is a strong indication that the microphone is in the reverberant field.


6. Once microphone position has been established for quadrant # 1, it should be permanently located by a suitable mark on the floor and by dimensioning the position on your data sheet.

7. Repeat steps 3 through 6 above to establish suitable microphone positions for quadrants 2,3 and 4.

8. If there are operator stations near the machine, noise level measurements should also be taken at these locations. The microphone should be located at the approximate position of the operator’s ear as illustrated in Fig. 13-3-22. The machine operator should not be near the microphone when the measurement is taken.

Fig. 13-3-21. Microphone position is properly dimensioned, to permit relocation for comparable readings later.


Fig. 13-3-22. Sound measurements are also made at operator stations at the approximate position of the operator’s ear.

POSITIONING THE OF THE INSTRUMENT OPERATOR

When the instrument operator is near the microphone, as would be the case when he is holding the ……………………………………….to make a measurement, his presence can affect the measured results by as much as 6 dB. For this reason, it is worthwhile to follow procedures which minimize this effect. The operator should stand with one microphone extended well in front of him with the apparent noise source either to his right or left. The microphone should not left. The microphone should not be pointed directly toward the source. See Fig. 13-3-23.

Hand holding the microphone is generally satisfactory when overall sound measurements are being taken. However, when an analysis of the noise is to be made, the microphone should be held by a clamp and the instrument operator moved 5 or 10 feet away.


Fig. 13-3-23. The instrument operator should stand with the microphone held about 2 feet in front of him with the sound source at his left or right. The

microphone should point away from the operator, but not toward the source.

INSTRUMENT OPERATION

This section outlines the basic features and operation of the instruments used to measure and analyze noise and vibration and for dynamic balancing. The instruments illustrated here are typical meters and analyzers. For more detailed information, refer to the instruction and operation manual furnished with your specific instrument.

FIBRATION AND SOUND LEVEL METERS

The meters shown in Fig. 1. And 2 are used for measuring overall vibration and noise amplitudes. The Vibration Meter, Fig. 1, both displacement and velocity. The vibration/Sound Level Meter, Fig. 2, measures vibration and also provides a measure of noise levels in dB utilizing the standard A, B and C weighting networks.

Some regulations on hearing conservation require that a check of instrument accuracy be made before and/or after conducting a noise level survey. Such a check can be made quickly and easily in the field using an optional sound level calibrator such as the unit shown in Fig. 13-3-24. The sound level calibrator illustrated is battery powered and generates a precise sound pressure. Simply simply fitting the calibrator over the microphone and comparing the resulting amplitude meter reading to the pre-established level noted on the calibrator checks instrument operation.


Fig. 13-3-24. A sound level calibrator is used to periodically check instrument accuracy.


MEASURING MACHINERY VIBRATION

To measure machinery vibration, first connect one end of the pickup cable to the vibration pickup and connect the other end of the cable to the pickup input receptacle on the instrument.

Test the instrument batteries using the “TEST” provisions of the instrument. Batteries which read low should be replaced immediately.

To measure the vibration, we must place the pickup at various points on the machine where we can gain the most useful information. Often the position and location of the vibration pickup will be dictated by the physical shape of the machine. In general, however, ………………………………………..near as possible to the machine’s bearing because it is through the bearings that the vibration forces are being transmitted.

It should be remembered that the vibration pickup measures only the vibration occurring in the direction in which it is pointed. For this reasons, it is necessary to identify the position of the pickup for all vibration measurements. Vibration readings are normally taken with the pickup in the HORIZONTAL, VERTICAL or AXIAL direction as illustrated in Fig. 13-3-25. Vertical and horizontal readings are taken with the pickup perpendicular to the machine’s shaft centerline and are generally called radial vibration measurements. Axial vibration readings are those taken with the pickup parallel to the centerline of the shaft of the machine. In either case, the pickup should be placed on the bearing housing as near to the shaft as possible. Proceed to measure the vibration as follows:

1. Place the vibration pickup on the machine in the vertical, horizontal or axial direction. The pickup may be hand-held with or without the standard probe, or it may be attached by pliers or magnetic holder as illustrated previously in fig. 43. When hand holding the pickup, the pickup should be held firmly against the machine with just enough pressure to prevent it from chattering. Hold the pickup as steady as you can to insure accurate readings.

2. Select the DISPLACEMENT of the VELOCITY of the vibration for measurement using the DISPLACEMENT-VELOCITY SELECTOR. See Fig. 13-3-26.

3. Starting at the least sensitive full-scale range (100 on instruments with English readout and 3000 on instruments with Metric readout), turn the AMPLITUDE RANGE SELECTOR clockwise to each position until a reading is obtained in the upper two thirds of the AMPLITUDE METER.


Fig. 13-3-25. Vibration readings are normally taken in the HORIZONTAL, VERTICAL and AXIAL directions at the bearings of the machine.

Fig. 13-3-26. IRD Mechanalysis Model 306 Vibration Meter.


NOTE:

Don’t worry if you happen to turn the amplitude range selector too far such that the amplitude meter goes off scale. Overload protection is built into the meter circuits so that the meter will not be burned out or harmed if it goes off scale. Simply turn the amplitude range selector back until the meter reads in the upper two-thirds of the meter scale.

You will note that the amplitude meter has two scales: the top meter scale reads from 0 to 1 and the bottom scale from 0 to 3. The two scales on the meter are designed so that an up-scale reading of at least one-third full scale is possible for all vibration measurements. This minimizes inherent meter error.

The setting of the amplitude range selector determines which scale applies. If the selector is set to a range which begins with 1 such as 100, 10, 1, etc., use the top scale when reading the meter, placing the decimal point as indicated by the amplitude range selected.

For example, on the 100 setting, the full scale amplitude on the meter becomes 100 mils (microns) or 100 inches per second (milli meters per second). IN this case, a reading of .6 on the meter becomes 60 mils (microns) or 60 inches per second (millimeters per second). If the amplitude range is set to a number, which begins with 3 such as 30, 3, 3, etc., use the bottom scale of the meter, placing the decimal point in the same way.

Fig. 13-3-27A through 13-3-27-D are some typical amplitude meter readings.

Study each example carefully, noting how 1) determines the final reading obtained on the meter.

The position of the DISPLACEMENT-VELOCITY SELECTOR; and 2) the range selected by the AMPLITUDE RANGE SELECTOR, which determines whether the top or bottom meter scale is used, and the full-scale amplitude of the meter.

MEASURING MACHINERY NOISE

To measure machinery noise:

1. First install the microphone in the INPUT RECEPTACLE. Test the instrument batteries using the instrument’s TEST provisions. If local regulations require, check instrument accuracy using the sound level calibration described earlier.

2. Select the appropriate A, B, or c weighting network depending on the purpose of your measurement. The “A” weighting is normally used for measurements taken to comply with legislation on hearing conservation. The “C” weighting is used for analysis and for correlating machinery noise and condition.

3. Place the meter in position for the measurement with the microphone properly oriented. Details on positioning the microphone are outlined in this Chapter under “Positioning The Microphone” and “Position of the Instrument Operator”.


Fig. 13-3-27. Typical vibration amplitude meter readings.


4. Starting with the AMPLITUDE RANGE SELECTOR in the least sensitive range (130 dB) turn the selector clockwise to each range position until a reading within the 0 to + 10 dB scale is obtained. (This provides better meter accuracy than readings obtained between –5 to 0 on the scale).

To obtain the noise level reading, note the position of the AMPLITUDE RANGE SELECTOR (50, 60, 70, etc.) and add this value to the reading indicated on the meter. For example, in Fig. 13-3-28, the amplitude range selector is set to 80, and the meter registers 6. Therefore, the noise level is 80 + 6 or 86 dB.

When recording noise level measurements, it is important that the weighting network used (A, B or C) be identified for each measurement value. For example, in fig. 13-3-28, the 86 dB noise level was measured using the “C” weighting network. Therefore, this reading must be recorded as 86 dB ©.

Fig. 13-3-28. To determine the noise level, add the meter reading to the selected dB range. Be sure to identify the weighting

network selected.


THE VIBRATION ANALYZER

Vibration and sound level meters measure overall machine vibration and noise to provide an indication of the general condition of machinery. However, in order to pinpoint machinery problems we must be able to measure and compare all the defining characteristics of vibration and noise – amplitude, frequency and phase. This is the job of the ANALYZER. In addition, most machinery vibration is some what complex, consisting of many frequencies all occurring at the same time. This is true of machinery noise too. Therefore, when the vibration or noise is complex, the analyzer must be able to separate one frequency from another so that each one can be measured.

Illustrated in Fig. 13-3-29 is a typical analyzer together with the standard accessories provided. Although analyzers may differ in general appearance, the basic features and operation described in the following paragraphs will apply in most cases.

Fig.13-3-29. IRD Mechanalysis Vibration Analyzer/Dynamic Balancer with accessories.

THE VIBRATION PICKUP

The standard VIBRATION PICKUP, provided with the analyzer is the same pickup used with the vibration/sound level meters. The pickup is connected to the analyzer by the PICKUP CABLE supplied. Longer pickup cables up to 750° feet in length may be used if necessary.

Many pickup input receptacles as shown in Fig. 13-3-30. This permits two pickups to be connected to the instrument at the same time to facilitate analysis and balancing.


Of course, the analyzer will only read the vibration from the one pickup at a time. The pickup receptacle connected tot he instrument is determined by the PICKUP SELECTOR SWITCH located on the front panel of the analyzer, Fig. 13-3-70.

Other vibration pickups such as non-contact and accelerometer pickups may be used with the analyzer using available accessories. Or, the analyzer may be connected directly to a permanently installed vibration monitor instrument which uses either velocity, non-contact or accelerometer pickups. All IRD Mechanalysis monitors feature a special ANALYZER JACK for this purpose. See fig. 13-3-32.

For analysis of machinery noise, the vibration/sound level meter may be connected directly to the analyzer as shown in Fig. 13-3-33. The standard pickup cable is used for this purpose.

Fig. 13-3-30. Two pickups can be connected to the analyzer to speed analysis and balancing.


Fig. 13-3-31. Pickup selector switch.

Fig. 13-3-32. Vibration analyzer connected to the ANALYZER JACK located on the front panel of all IRD Mechanalysis vibration Monitors.


Fig. 13-3-33. To analyze machinery noise, simply connect the vibration/sound level meter to the analyzer.

THE AMPLITUDE METER

The amplitude of vibration is read from the analyzer’s AMPLITUDE METER, 13-3-34. This meter is identical to the amplitude meter used on the vibration/ sound level meters, and uses the same 0 to 1 top scale and 0 to 3 bottom scale. Here also, the full scale range of the meter and the scale to use (top or bottom) are determined by the AMPLITUDE RANGE SELECTOR.

Displacement or velocity may be selected for measurement with the DISPLACEMENT – VELOCITY SELECTOR.

The amplitude meter on the analyzer pictured in Fig. 13-3-34 also has a dB scale. This scale is used when the analyzer is combined with the vibration/sound level meter for noise analysis.

Fig. 13-3-34. Vibration analyzer/dynamic balancer functions and controls.


THE FREQUENCY METER

The frequency of the vibration or noise being measured is indicated on the analyzer’s FREQUENCY METER, Fig. 13-3-34. Vibration and noise frequencies are read directly from the meter in cycles per minute (CPM).

The frequency rang eof the meter is determined by the FREQUENCY RANGE SELECTOR. Simply set the selector to the frequency range that gives the highest meter reading without having the meter read off scale. Like the amplitude meter, the frequency meter has built-in overload protection to the meter will not be burned out or harmed if you happen to go off scale.

NOTE:

The frequency meter will not function for extremely low amplitude readings. Normally the amplitude meter must read at least 10% up-scale before the frequency meter will function. Therefore, adjust your amplitude range selector for a reading preferably in the upper two-thirds of the scale before selecting the appropriate frequency range for an up-scale frequency meter reading.

In Filter Out the meter reads the frequency of a single strong predominant vibration when there are essentially no high frequency components present, or if present, are of a low amplitude level. If high frequency components are present, the frequency meter will tend to read them, especially in the velocity mode; in the displacement mode, high frequencies are attenuated and the predominant lower frequency is more likely to be indicated. Fig. 13-3-35A through 13-3-355D are sample frequency meter readings. Study each example carefully and note how the final frequency reading is determined, based on the frequency range selected and the steadiness of the meter.

THE TUNABLE FILTER

Because a machine may have more than one vibration or noise frequency present and, since we must rely on frequency measurements to pinpoint specific troubles in a machine.

We need some way to block out all but one frequency so we can see which parts are in trouble and require correction.

This is the job for the TUNABLE FILTER. The tunable filter is much like the tuner in your radio. You use it to reject the stations you don’t want while receiving the one you do want.

To properly use the tunable filter, it is important to understand some of the characteristics of filters. First, it is important to recognize that the tunable filter does not reject all noise or vibration frequencies except the one to which it is tuned. The filter is actually tuned to a narrow band of frequencies and accepts those frequencies within the band while increasingly rejecting those frequencies outside the band.

A typical filter response curve is illustrated in Fig. 13-3-36. Note that the filter bandwidth is defined by the upper and lower cut-off frequencies.


The cut off frequencies above and below the tuned center frequency, where the response to a signal is approximately 30% less than maximum response. In other words, at the cut-off frequency a signal would be reduced by 30% of its real amplitude. Beyond the cut-off frequencies the signal will be reduced by considerably more.

The bandwidth of a filter is usually expressed as the percentage between the filter’s turned center frequency and the upper and lower cut-off frequencies. For example, the bandwidth of the filter in Fig. 13-3-36 is 5%, which means that the filter bandwidth extends 5% above (+) and 5% below (-) the tuned frequency. Such a filter tuned to a frequency of 1000 CPM would, therefore, have a bandwidth extending from 950 CPM to 1050 CPM or a bandwidth of 100 CPM. (5% of 1000 = 50). IN addition, this same filter tuned to a frequency of 10,000 CPM would have a bandwidth from 9,500 to 10,500 CPM or a bandwidth of 1000 CPM. As you can see, the effective bandwidth of a filter in CPM depends on the tuned frequency.

Understanding filter characteristics is important in this respect. Suppose we tune our 5% filter to a vibration frequency at 1000 CPM, and its amplitude is 0.5 in/sec. In addition, suppose that there is also a 1.0 in/sec vibration present at a frequency of 950 CPM, where the rejection of our filter is 30%. The net result will be two vibration signals coming through the filter at the same time; 0.5 in/sec at 1000 CPM and 0.7 in/sec at 950 CPM. Of course, the frequency of the stronger vibration (950 CPM) will be indicated on the frequency meter even though the filter is set on 1000 CPM. Such action through the filter would appear to be undesirable. However, as long as the strongest source of noise or vibration can be detected, we can pinpoint the defect and solve the problem.

Most IRD Mechanalysis analyzers provide two bandwidth filters; one designated BROAD and one SHARP. The BROAD filter has a 5% bandwidth, and the SHARP filter a 2 – ½% bandwidth. The filter desired is selected and put into operation by the FILTER SELECTOR, Fig. 13-3-34. The BROAD filter is usually selected for rapid scanning of the frequency ranges to quickly determine the vibration or noise frequencies present. The SHARP filter provides better rejection and selectivity and is used when a particular noise or vibration frequency is studied.

TUNING THE FILTER TO A KNOWN FREQUENCY

Tuning the filter to a known frequency.

Many times we would like to tune the filter to a noise or vibration frequency we know (or suspect) is present. For example, with the motor/fan assembly in fig. 13-3-37, it would be safe to assume that some vibration is present at motor RPM (1800 CPM) and fan RPM (2200 CPM). To tune the filter to 1800 CPM.

1. Turn the FILTER SELECTOR to the SHARP position.

2. Set the FREQUENCY RANGE SELECTOR to the range which includes 1800 CPM. For the analyzer pictured in fig. 13-3-34, this range is 500 – 5K (5K = 5000).

3. Turn the FILTER TUNING KNOB, Fig. 13-3-34, until the TUNING DIAL indicates the desired frequency. This is only an approximate setting at this point.


4. Select either displacement or velocity for measurement using the DISPLACEMENT – VELOCITY SELECTOR.

5. Turn the AMPLITUDE RANGE SELECTOR to a range which gives an amplitude meter reading preferably in the upper 2/3rds of the meter scale.

6. Rock the filter tuning knob by turning it slowly back and forth while observing the amplitude metre. IN this fashion, make minor adjustments to obtain the maximum or peak reading on the amplitude meter.

This techniques of tuning the filter to obtain the peak amplitude is very similar to tuning your radio to a particular station to get the clearest reception. With the filter properly adjusted, the frequency meter should read steady at the tuned frequency.

TUNING THE FILTER TO FIND UNKNOWN FREQUENCIES

When machinery noise and vibration are complex, i.e., consisting of two or more frequencies, the analyzer’s tunable filter must be used to determine which frequencies are present. It is in this regard that the frequency meter is of great value.

When seeking unknown noise and vibration frequencies, the analyzer’s frequency range is scanned with the tunable filter.

When there is no particular vibration or noise present at the frequency to which the filter is tuned, the frequency meter pointer will move up and down the meter scale in a random manner.

However, as soon as the filter approaches a particular vibration or sound frequency, the frequency meter pointer will lock on to that frequency and cease its wandering over the scale.

This is the first indication that a particular frequency is present. Fine adjustment of the filter-tuning dial is then made to obtain a peak reading on the analyzer amplitude meter.

This method of tuning the filter to discover the various frequencies of noise or vibration is outlined in more detail in the following step-by-step procedure:


ABOVE LEFT: The frequency range selector is set on the 50 – 500 CPM range. Therefore, the indicated meter reading is; 420 CPM.

ABOVE RIGHT: The frequency range selector is set on the 500 – 5K CPM range. Therefore, the indicated meter reading is 1800CPM.

BELOW LEFT: The frequency range selector is set on the 5K – 50 CPM RANGE. Therefore, the indicated meter reading is 8000 CPM.

BELOW RIGHT: The frequency meter indication is unsteady, indicating that there is no vibration present at the frequency to which the filter is tuned. In the “Filter Out” mode this would indicate that there is no predominate vibration frequency.

Fig. 13-3-35. Sample frequency meter readings.


Fig. 13-3-36. Typical filter response.

Fig. 13-3-37. With this motor/fan unit, some vibration will likely occur at motor RPM (1800) and fan RPM (2200)


1. Turn the FILTER SELECTOR to the BORAD position. The BROAD position is selected because it allows us to scan the frequency ranges quickly. Scanning with the SHARP filter would take more time and is usually reserved for analysis of very complex noise or vibration.

2. Turn the FREQUENCY RANGE SELECTOR to the lowest frequency range position. For the analyzer in Fig. 13-3-34, this range is 50-500 CPM. (for finding noise frequencies, you may ignore those frequencies below 600 CPM as these are below the audible range).

3. Turn the Filter DIAL to the beginning of the frequency range.

4. Select either displacement, velocity or noise level (dB) for measurement. For scanning vibration frequencies, displacement is recommended below 600 CPM. Above 600 CPM, the velocity mode may be used. For finding noise frequencies use the “C” weighting network.

5. Turn the AMPLITUDE RANGE SELECTOR to obtain an upscale amplitude meter reading – preferably in the upper 2/3rds of the scale if possible. Remember that a reading of at least 10% upscale on the amplitude meter is needed before the frequency meter will function.

6. With your analyzer set up as described thus far, the frequency meter pointer will most likely be doing one of three things; 1) moving randomly back and forth over the meter scale, 2) reading off-scale, or 3) reading zero. Therefore, slowly turn the filter tuning dial through the frequency range while observing the frequency meter. Remember, the frequency meter is your guide! Also, check the amplitude meter often to be sure it is reading upscale and on-scale at all times.

Continue to tune the filter dial in this fashion until the frequency meter stops moving back and forth over the meter scale and locks on to a particular frequency.

7. Once the frequency meter locks on, this means that your filter is approaching a particular frequency. Note the reading on the frequency meter, and slowly turn the turning the dial reading is the same as the reading on your frequency meter. This is only an approximate setting at this point.

8. Turn the FILTER SELECTOR to the SHARP position.

9. Slowly rock the tuning dial back and forth to obtain the peak reading on the amplitude meter.

Once the filter has been adjusted for the maximum or peak amplitude reading, this means that the filter has been properly tuned to the particular frequency. To fin d the other frequencies of vibration or noise, switch the FILTER SELECTOR back to the BROAD position and continue to scan until the frequency meter locks on to the next frequency. Proceed to scan through each frequency range to discover all vibration or noise frequencies present.


THE STROBE LIGHT

The stroboscopic light of STROB LIGHT furnished with your analyzer is a high-intensity light which flashes on and off in synchronism with the vibration or noise frequency being measured. The light is connected to the analyzer by the STROB CABLE supplied.

To illustrate the operation of the strobe light, suppose the 1800 RPM motor in fig. 13-3-38 is out of balance. This will cause a vibration at a frequency of 1800 CPM or once for each revolution of the shaft, and the strobe light will be fired at the same rate. As the motor pulley rotates, the strobe light flashes each time the part reaches a certain position.

For the motor in Fig. 13-3-38, the strobe light flashes when the reference mark on the pulley reaches a position of 2: 00 o’clock. The motor pulley is illuminated briefly at that time. During the rest of the pulley’s rotation, the light is off and then the pulley is again illuminated when the mark is at 2:00 o’clock. This action makes the eye believe that the pulley is standing still.

The use of the strobe light to pinpoint the source of vibration or noise is very important. You will recall from chapter II that the frequency of vibration and noise is usually equal to or a multiple of the rotating speed of the pat at fault. Since the strobe light flash rate is determined by the frequency of noise or vibration, the part causing the problem will often appear to stand still under the light.

The motor pulley in Fig. 13-3-38 is standing still with one mark visible under the strobe light because the vibration frequency is 1 x RPM. If the vibration or noise frequency occurs at 2 X RPM, say due to mechanical looseness, you may see the pulley standing still with the reference mark in two positions as illustrated in Fig. 13-3-39.

Fig. 13-3-40 shows the reference mark frozen in three positions which could occur if the frequency was 3 x RPM.

Sometimes the rotor will not appear to stand still under the strobe light. The image may be erratic like the one in Fig. 13-3-41. This occurs when the vibration or noise is unsteady or complex. Or, perhaps the vibration or noise is coming from another source. If this occurs, the tunable filter should be used.


Fig. 13-3-38. The strobe light flashes in synchronism with the vibration. The reference mark is illuminated briefly each revolution when it reaches the 2:00

o’clock position to make the pulley appear to stand still.

Fig. 13-3-39. Vibration at 2 x RPM may show two reference marks under the strobe light.


Fig. 13-3-40. Reference mark “frozen” in three positions.

Fig. 13-3-41. An erratic strobe image usually means the noise or vibration is unsteady, complex or, perhaps coming from another source.


MAXIMUM STROBE FLASH

RATE

The maximum flash rate of the strobe light is normally limited in order to conserve bulb life. Some analyzers have a maximum strobe flash rate of 15,000 flashes per minute while others are limited to 5000 flashes per minute. For noise and vibration frequencies above the maximum flash rate, the analyzer maximum flash rate, the analyzer will automatically cause the strobe light to flash at sub-multiple rates of the frequency; i.e., ½ 1/3, ¼ , etc.

The importance of this fact can be illustrated by the following example: Assume that the maximum flash rate of your strobe light is 5000 flashes per minute, and the rotating speed (RPM) of a machine is 4600 RPM. If you measure the vibration at 2 x RPM or 9200 CPM, the strobe light must flash at a sub-multiple or 4600 flashes per minute in this case. Since this flash rate is equal to rotating speed, you would see only one mark under the strobe light even though the frequency being measured is two times RPM. Two marks will be seen only when the vibration at 2 x RPM is at or below the maximum flash rate of the strobe light.

Two important considerations can be drawn from this example:

1. First, the relationship of a noise or vibration frequency to machine RPM (i.e., whether the frequency measured is 1,2,3 etc., times RPM) should not be determined solely on the number of reference marks which appear under the strobe light.

2. Secondly, where the RPM of the machine is unknown, a frequency which causes one mark to stand still should not automatically be taken as the rotating speed. In the example above, one reference mark appeared even though the frequency being measured was 2 x RPM. For this machine, only one reference mark would be observed for frequencies of 1,2,3,4 and higher multiples of rotating speed. In addition, only one reference mark would appear for frequencies at ½, 1/3, ¼ or lower multiples of RPM.

A procedure for using the strobe light to determine the rotating speed of a machine is outlined later in this chapter under the heading “The Internal Oscillator”.

USING THE STROBE LIGHT TO CONFIRM FREQUENCY METER READINGS

The analyzer’s frequency meter will normally provide accurate frequency indications within 2% of the full scale meter range. However, the resolution and accuracy of the frequency meter is such that it is impossible to determine the frequency to an exact cycle. As a result, the question often arises; Is the indicated frequency exactly the same as the rotating speed (or multiple of rotating speed) of some part of the machine?

This question is easily answered using the strobe light. Since the strobe light and frequency meter are, for all practical purposes, triggered by the same vibration or noise signal, if the frequency and machine RPM (or multiple) are the same, the strobe light will make the rotor appear to stand still with one or more reference marks visible.


However, if the frequency being measured is not exactly the same as shaft speed or some multiple of shaft speed, then the shaft will not appear to stand still under the strobe light. For example, if the noise or vibration is a actually coming from another part of the machine or perhaps from a nearby machine, the strobe image may appear erratic like that in Fig. 13-3-41; or the reference mark may appear to rotate slowly.

USING THE STROBE LIGHT TO MEASURE PHASE

Another common use of the strobe light is to measure the phase of vibration. Phase measurements are often essential in vibration analysis to diagnose specific machine problems. In addition phase measurements are particularly use full for balancing rotating parts. The position of the reference mark changes when the position of the unbalance is changed. The position of the reference mark can be used then to determine the correct place for making weight corrections.

The use of phase measurements for analysis is discussed in Chapter IV, and dynamic balancing using phase measurements is outlined in detail Chapter V

The first step in using the strobe light to measure phase is to establish a common reference to which all phase measurements will be made. Normally, a reference mark is put on one end of the shaft which can be viewed under the strobe light. A reference mark can be made with chalk or paint, or an existing key or key way can be used. In some cases where the machine cannot be shut down and where no key or key is visible, a distinguishing blemish, nick, rust spot or grease spot on the shaft may be used.

A common practice is to view the end of the shaft as an imaginary clock face, in which case the phase of the vibration measured in Fig. 13-3-38 would be 2:00 o’clock. The clock face reference system is most commonly used when phase is observed for general comparison purposes.

When it is desirable to measure phase very accurately (such as for balancing) an angular (o° to 360°) phase reference system like that shown in Fig. 13-3-42 shows a phase measurement of 75°.

When taking phase measurements, certain precautions should be taken to insure accurate, reliable data:

1. First the direction of the pickup axis, together with the reference mark on the end of the rotating shaft and the superimposed clock face or angular reference, establish the fixed reference for taking comparative phase readings. Thus, pickup direction should not be changed from one reading to another. If it is necessary to change the direction of the pickup, this change must be noted so that the phase readings can be corrected accordingly for comparison.

To demonstrate the importance of pickup direction, measure the phase with the vibration pickup in the horizontal direction on a bearing as shown in Fig. 13-3-43. Next, move the pickup to the horizontal direction on the opposite side of the bearing so that the pickup axis is so that the pickup axis is reversed 180° from your original reading. You will note that the 180° shift in pickup direction produces a corresponding 180° shift in the observed phase reading.


Even minor changes in pickup direction will produce a corresponding phase change. For example, when hand holding the vibration pickup, any unsteadiness which allows the angle of the pickup to change will result in unsteady phase readings. This is especially important to remember when phase readings are taken for balancing purposes where accurate phase readings are essential for good results.

2. When using the analyzer’s tunable filter for taking amplitude and phase readings, it is essential that the filter be properly adjusted for each reading. Although a slight mis-adjustment of the filter may not appreciably change the amplitude reading, the phase reading may be changed by several degrees.

3. When taking comparative phase readings, avoid switching from one parameter of amplitude measurement to another. For example, phase readings taken when measuring displacement will differ by exactly 90° from those taken in velocity.

Fig. 13-3-42. When accurate phase readings are needed, such as for balancing, an angular phase reference is normally used.


THE INTERNAL OSCILLATOR

Standard IRD Mechanalysis Analyzers are provided with an adjustable INTERNAL OSCILLATOR. The internal oscillator is a separate means of flashing the strobe light, and is completely independent of the vibration or noise source.

The internal oscillator is put into operation by switching the FILTER SELECTOR to the “OSC” position. See Fig. 13-3-34.

The rate at which the internal oscillation fires the strobe light is adjustable over the entire frequency range of the analyzer; however, the strobe light flash rate will automatically be sub-divided above the strobe’s maximum flash rate just as it is for vibration or noise frequencies.

The oscillator is adjusted by the FILTER TUNING KNOB, and the oscillator frequency will be indicated on the frequency meter. When the internal oscillator is in operation, the amplitude meter will read zero.

The internal oscillator used in conjunction with the strobe light offers three important uses:

1. Perform slow motion studies.

2. Facilitate filter tuning.

3. Determine the rotating speed (RPM) of a part

When parts vibrate rapidly back and forth, the eye has difficulty trying to follow the motion. Normally, the inability of the eye to follow the motion of a vibrating object causes the part to appear blurred.

Slow motion studies with the oscillator and strobe light provide a means of observing dynamic conditions that could not be observed by other means except perhaps by costly high speed motion pictures.

Using the internal oscillator and strobe light for slow motion studies is easy. Simply adjust the flash rate of the strobe light to a rate slightly slower or faster than the frequency of motion of the part.

This will make the part appear to move slowly. For example, if a shaft is rotating at 1800 RPM and we adjust the internal oscillator to a frequency of 1780 CPM, the shaft will appear to rotate at 20 RPM under the strobe light (1800 – 1780 = 20).

Slowing down the motion this way allows us to observe the relative motion of parts and other conditions which may be detrimental to machine operation. For example, in one instance a high level of vibration was detected on a direct driven motor/fan unit shortly after start-up.

Slow motion observation of the coupling disclosed that the coupling was turning slightly back and forth on the fan shaft.

Visual inspection after the unit was shut down revealed that an undersized key had been used to install the coupling on the fan shaft. Replacing the key with one of the proper size eliminated the movement of the coupling and the high vibration.


TUNING THE FILTER WITH THE INTERNAL OSCILLATOR

If the rotating speed of a machine is known, the oscillator can be used to quickly and accurately tune the filter to rotating speed and multiples of rotating speed.

For example, suppose we wanted to tune the filter to a frequency equal to the rotating speed (RPM) of a motor where the nameplate on the motor gave its speed as, say, 1725 RPM. To do this, we would adjust the internal oscillator to approximately 1725 and make any minor adjustments until the motor shaft appeared to stand still under the strobe light. Adjusting the internal oscillator adjusts the tunable filter at the same time.

Therefore, when we switch the filter to either the BROAD or SHARP position, it is automatically tuned to the rotating speed frequency of the motor. This is a quicker and easier way to tune the filter than making fine adjustment to obtain the peak amplitude.

USING THE INTERNAL OSCILLATOR TO DETERMINE MACHINE RPM

If the rotating speed (RPM) of a machine is unknown, the internal oscillator and strobe light can be used to quickly and easily determine what the speed is. To do this, adjust the internal oscillator initially to a flash rate of 5000 flashes per minute. Now, slowly reduce the flash rate while observing a reference mark on the rotating part with the strobe light. Continue to reduce the flash rate until you reach a point where a single reference mark appears to stand still. Note this flash rate from the frequency meter. We call this first reading R1.

After recording R1, continue to slowly reduce the flash rate until, again, you see a single reference mark frozen under the strobe light. Note this flash rate from the frequency meter and call it R2.

Once the values for R1 and R2 have been found, the rotating speed of the part is calculated using the formula:

This technique for finding the rotating speed of a part will work for any two adjacent flash rates which cause the part to stand still with one reference mark under the strobe light. However, it is important not to miss a flash rate where a single mark appears, or your calculated RPM will be incorrect.

D.C. RECORDER RECEPTACLE

Most IRD Mechanalysis analyzers, monitors and meters include provisions for connecting an optional D.C. chart recorder.

See Fig. 13-3-44. The D.C. voltage output available from the instrument is proportional to the AMPLITUDE METER reading and by connecting a record vibration or noise amplitude can be recorded over a long period of time.


Recordings of amplitude versus time can be helpful in many ways. D.C. recorders are often used with permanently installed vibration monitors. By continually recording vibration amplitude levels, it is possible to determine whether machinery problem has developed suddenly or gradually over a period of time. Such information may be an important factor when making the decision to shut down an important machine.

Vibration and noise amplitude recordings can be a valuable aid in diagnosing certain machinery problems.

For example, one paper manufacturer was experiencing repeated bearing failures in the dryer section of a paper machine for no apparent reason. A vibration analyzer was brought in, however, the amplitude readings.taken at the time were very low, indicating that no significant mechanical problems existed.

After exhausting all realistic possibilities, the decision was made to connect a D.C. recorder to the analyzer and record the vibration amplitude for a few days to see if anything could be learned.

The next day, an examination of the chart recording revealed several brief periods of very high amplitude. Since the starting time of the recording as well as the chart speed were known, it was an easy matter to establish the approximate time of day that each severe vibration occurred. With this information, an investigation was undertaken to determine what might have occurred at these times to cause the high vibration.

It was learned that the periods of high vibration corresponded precisely with the schedule of trains running on a nearby track. Stiffening the dryer structure then eliminated excessive vibration in the paper dryer section caused by the passing trains. This eliminated the repeated bearing failures.

Time history recordings of noise levels may be necessary to establish compliance with legislation dealing with hearing conservation. Although hearing conservation laws normally specify the permissible exposure time for constant levels of noise, consideration is also given to those situations where the noise level is not constant.

Where the level varies, it is necessary to record the noise amplitude during a normal working day to determine the levels encountered and the duration of each level. This information is then used in a formula to calculate the equivalent exposure. Such recordings serve as valuable documentation of a hearing conservation program and are then kept on file as evidence of compliance.


Fig. 13-3-44. A.D.C. strip chart recorder, shown here being used to record noise levels, can also be connected directly to the analyzer for recording

vibration amplitude over long periods of time.

OSCILLOSCOPE RECEPTACLE

The OSCILLOSCOPE RECEPTACLE found on all IRD Mechanalysis analyzers provides an A.C. signal which is an exact reproduction of the mechanical vibration or sound pressure. By connecting an oscilloscope4 to the receptacle, the vibration or noise waveform can be observed. Virtually any general purpose oscilloscope can be used for this purpose;. Consult the scope manufacturer’s instruction manual for details on the operation of your particular oscilloscope.

Viewing the vibration or sound pressure waveform on an oscilloscope can provide much useful information. Many machinery troubles can be identified by the characteristic vibration waveforms they produce. For example, normal conditions of unbalance and misalignment will generate a common sine-wave as illustrated in Fig. 13-3-45A. However, these problems accompanied by mechanical looseness often produce a waveform like that illustrated in fig. 13-3-45B. The waveform in Fig. 13-3-45C is the result of oil whirl. Fig. 13-3-45D is characteristic of a faulty antifriction bearing.


Another application for the oscilloscope is measuring impact or transient noise and vibration. Measuring short-klived noise or vibration is often very difficult using the standard analyzer or vibration/sound level meter because built-in damping represents the meter from responding to the true peak values.

Since the oscilloscope does not have this built in damping, it will respond instantly and is often used with the analyzer or vibration/;sound level meter for this purpose.

The oscilloscope is also a valuable aid for evaluating data obtained from non-contact pickups. Earlier, it was mentioned that scratches on a shaft will sometimes cause misleading vibration amplitude and frequency readings. Connecting an oscilloscope to the instrument will quickly reveal the presence of scratches by the spike-like signals they generate on the waveform.

The oscilloscope receptacle on your vibration analyzer can be used for other applications. For example, when analyzing machinery noise (or vibration) a set of head phones can be connected to the scope receptacle as shown in Fig. 13-3-46.

This allows the analyst to actually hear the various noise or vibration frequencies which can be extremely helpful when tuning the analyzer filter.

Permanent records of machinery vibration or noise can be taken from the oscilloscope receptacle. Extremely high speed chart recorders such as a visi-corder or oscillograph can be used to provide hard copy data such as that shown in Fig. 13-3-47. Recordings of this type are usually taken to study noise or vibration which is undergoing rapid change such as during start-up or coast down of a machine.

Vibration and noise data from the oscilloscope receptacle can also be recorded on magnetic tape. In this way, the data can be gathered quickly and easily for detailed study and analysis at a later time.

Many companies use this technique for gathering analysis data from machines in remote locations. The tapes are then sent to a central location and played back through automatic or real-time analysis equipment for conversion to hard copy records. This data is then studied and evaluated by trained analysts to identify any apparent mechanical problems.

Thus far we have discussed only a few of the many possible applications for the oscilloscope receptacle found on your IRD Mechanalysis instruments.

Other applications include the use of an oscilloscope for remote phase measurements and observing the actual motion of a rotor shaft within its bearings utilizing non-contact pickups. These and other applications are covered in detail in the IRD Mechanalysis Advanced Training Program.


Fig. 13-3-45. Connecting an oscilloscope to the oscilloscope jack on the analyzer to view the time function waveform may reveal valuable analysis

information.

Fig. 13-3-46. Headphones connected to the analyzer oscilloscope jack allow the analyst to hear the vibration or sound peaks as he tunes the filter.


Fig. 13-3-47. An oscillograph connected to the analyzer oscilloscope jack provides high speed recording of the vibration or noise waveform.

REVIEW

The another has all the necessary features for measuring and analyzing machinery vibration or noise. The features and operation of the analyzer are covered in some detail in this chapter and in your instrument instruction manual.

Of course, reading about the analyzer is one thing, but there is no substitute for first hand experience. Therefore, if you have not already done so, set up your analyzer and become as thoroughly familiar with it as you can. Hook it up. Measure vibration or noise amplitude and frequency. BE SURE YOU UNDERSTAND HOW TO READ THE METERS CORRECTLY. Use the tunable filter, the strobe light and internal oscillator. Using a small electric motor or a device similar to the one illustrated in Fig. 13-3- , we suggest you conduct the following exercises.

1. Find the rotating speed (s) of the machine using the internal oscillator and strobe light.

2. Measure the overall filter out vibration one of the bearings. Observe and record the following:

One. vibration displacement.Two. Vibration velocityThree. Vibration acceleration (where appropriate)Four. Frequency meter indications for displacement, velocity and

acceleration measurements.Five. Observations with the strobe light for displacement, velocity and

acceleration measurements. View each rotating part.


3. Turn the motor off, and measure the background vibration amplitude and frequency in terms of displacement, velocity and acceleration.

4. With the motor operating and with the pickup on the same bearing as for (2) and (3) above, tune the filter to the rotating speed of each component using the internal oscillator. Record the vibration amplitude for each frequency.

5. Slowly scan the filter through each frequency range and note each frequency of vibration discovered. Fine-tune the filter to each frequency found to obtain the peak amplitude. Note and record each amplitude and frequency found. Also, record your observations with the strobe light for each frequency.

For analyzers equipped for noise measurement and analysis, the following exercises are suggested:

1. With the machine operating and the analyzer filter in the CUT position, locate the microphone approximately 2 to 3 feet from the machine. Observe and record the following:

One. Noise amplitude – dB (A) and dB(C).Two. Frequency meter reading for dB(A) and dB(C)Three. Observations with the strobe light for dB(A) and dB(C), viewing each

rotating member.

2. Turn the motor off and again observe and record the amplitude and frequency meter indications for both the “A” and “C” weighting networks.

3. Turn the motor on again and switch your instrument to the “C” weighting network. Beginning at a low frequency of 600 CPM, slowly scan the filter through each frequency range and note each nose frequency discovered. Fine tune the filter to each frequency found to obtain the peak amplitude. Note and record each amplitude and frequency found. Also record your observations with the strobe light for each frequency.

4. With the motor operating, move the microphone to within a few inches of the machine. Move the microphone along the length of the machine at this close range, and note any variations in the measured amplitude. Move the microphone along the length of the machine at a distance of 2 to 3 feet and, again, note any variations I the measured amplitude.


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