Glitch – Sources of Glitch – Glitch Reduction – Protection of Probe Areas
Glitch – Sources of Glitch – Glitch Reduction – Protection of Probe Areas
The term “GLITCH” is Bently Nevada Corporation’s nomenclature for describing all forms of vibration measurement error from an observed shaft surface. This does not include other possible noise and signal error sources, such as electrical line noise, monitor problems, or problems associated with casing mounted transducers. Casing mounted transducers, such as velocity coils or accelerometers are also subject to signal error sources in the form of cross axis vibrations, improper mountings, line noise as well as several others particular to each type of transducer; however, glitch deals with observed shaft surface anomalies only. In order to clarify the sources of these vibration measurement errors, sometimes referred to as runout, the following discussion shall focus on the two categories of “GLITCH”, mechanical and electrical.
Sources of Glitch
Mechanically Induced Runout
A shaft surface which has been improperly machined (egg shaped or nonconcentric) will yield a sinusoidal displacement signal with a frequency coincident with the rotational speed of the rotating element. A second condition on a rotating element which will yield the same apparent dynamic motion condition is manifest when the rotating element has been physically bent or bowed.
Sources of improperly machined surfaces can usually be traced to a worn or defective set of bearings on the machine used for final machining or grinding or a worn-out set of lathe centers on a lathe. Bows in a rotating element are typically introduced due to improper handling of the rotor during its manufacturing cycle. This may be the result of a sudden or jarring load applied or due to long-term storage of the element with improper supports for the rotor. In the case of the latter, an improperly supported rotor may introduce a permanent sag or bow due to gravity forces.
Bows introduced into a rotor from asymmetrical heating or improper cooling are generally referred to as thermal bows, and can usually be removed by placing the rotor on turning gear, and or slowly starting and heat soaking the machine at low speeds.
Surface Irregularities or Imperfections
The presence of surface imperfections or irregularities will yield a runout condition as observed by the proximity transducer. The surface imperfections discussed herein are in the form of scratches, dents, burrs, etc.
In general, surface irregularities are created due to improper handling of the rotor during the manufacturing cycle. Care should be taken to protect the shaft surface to be used for dynamic motion measurements. In essence, these surface areas should be given the same protective measures used to protect a bearing journal surface. Crane lifts should be made with cables attached to shaft areas away from the probe measurement surfaces. Support fixtures for storage of rotors should not introduce surface scratches, dents, etc.
Occasionally, surface irregularities are introduced via a machine cutting tool. If the tool is dull or the feed is too rapid, some tool chatter may occur which can introduce small ripples in the shaft surface.
Electrically Induced Runout
In general, proximity transducers will operate satisfactorily in the presence of magnetic field, as long as the field is uniform or symmetrical and not localized to a particular location on the rotor. If a particular area or zone of the shaft surface is highly magnetic and the remaining surface is non-magnetic or at a much lower value, an electrical runout condition will be manifest. This is due to the resultant change in sensitivity on the shaft surface to the applied field from the proximity transducers.
Residual magnetism runout problems are seldom encountered. However, various physical inspection techniques employed during the manufacturing cycle, such as the use of magnetic chucks, can introduce residual magnetism problems. The most common inspection technique, where residual magnetism may be a byproduct, is a Magnetic Particle inspection (Magnaflux®) to check for cracks on castings, or weldments, or after other manufacturing cycles. The magnetic field introduced to the rotor for this inspection should be neutralized after the inspection program is completed. This is done with the Magnaflux machine and involves continuously reversing the polarity and passing a current through the rotor at continuously decreasing amperes. If done properly, this procedure should neutralize the magnetic properties of the rotor. In some cases a proper polarity reversal is not performed, and residual magnetism is produced.
17-4 pH steel nearly always presents an electrical runout problem. Some form of material replacement (shrink a collar, overspray a material) is normally required to eliminate glitch.
Other pH steels, such as 15-5 pH, seems less prone to glitch, but any pH steel may cause difficulties.
The scale factor yielded from a proximity transducer is dependent upon several variables. One variable involves the specific metal or metallurgy it is to observe. Typically, steel alloys for shaft materials contain a variety of alloying agents. In general, the final metallurgical composition of these alloys is a homogenous mixture. On some rotating elements microscopic segregation of the steel alloys may occur. Since the proximity transducer responds with different voltage outputs depending upon specific metals, the lack of a homogenous metallurgical composition around the circumference of a shaft may give rise to varying electrical outputs.
Residual Stress Concentrations
During the normal manufacturing cycle of rotors, a variety of machining and surface treatment processes can introduce small amounts of localized stress concentrations. Although these stress areas do not adversely affect the mechanical properties of the rotor, they may give rise to an electrical runout from the proximity transducers. Since one of the variables affecting voltage output from the transducer is the resistivity of the observed shaft surface, any deviation of the resistance around the circumference of the shaft (due to varying stress levels) will produce a voltage change.
Measurement of Glitch
The observation of mechanical runout at a rotor speed below which dynamic vibration is eliminated (typically less than 10% of the rotor operating speed) on an oscilloscope will yield a sinusoidal dynamic waveform for non-concentric surfaces or shaft bows. Surface irregularities or imperfections will appear on an oscilloscope as sharp voltage spikes superimposed on the dynamic wave form.
Nonconcentric surfaces or shaft bows may also be measured by a dial indicator mounted in the probe area of the shaft which in turn is mounted at the bearing journals, in vee blocks, or roller bearings. The circumference of the probe area is marked off in 36 positions (10 degree intervals) with zero in line axially with the thrust collar keyway. Dial gauge readings are recorded at each indicated position. Two sets of readings are taken, approximately half an inch apart axially, one on each side of the probe position centerline. The two sets of readings are averaged to give a record of mechanical runout. American Petroleum Institute (API) Standard 670 recommends that “the combined total electrical and mechanical runout does not exceed 25 percent of the maximum allowed peak to peak vibration amplitude or 0.25 mil (6 micrometers), whichever is greater.” The shaft surface finish should be from 16 to 32 micro inches (0.4 to 0.8 micrometers) root mean square, also per API 670.
The observation of a residual magnetism runout condition on an oscilloscope can yield a sinusoidal motion indication. However, the sine wave will be distorted and to some extent tending toward a square wave. A final check for residual magnetism embraces the use of a small handheld field strength indicator manufactured by Magnaflux Corp. Holding this meter at the shaft surface and hand turning the rotor will confirm the presence or absence of magnetic fields of less than 2 gauss with variations less than 1 gauss.
An oscilloscope observation of metallurgical segregation will typically indicate a somewhat sinusoidal waveform with high voltage, high frequency spikes superimposed on the waveform.
Observation of residual stress concentrations on an oscilloscope will yield a sinusoidal waveform with high voltage, high frequency spikes superimposed on the waveform.
It should be noted that the oscilloscope waveform in all the above cases may also be very irregular, depending on the amount of other shaft surface anomalies.
If desired, the electrical runout can be determined by subtraction of mechanical runout from the total runout.
Various methods of reducing glitch are available and have been successfully used by firms. It is not possible to define which method is best because each can achieve the desired result. However, it is possible to narrow the choice of methods when they are considered on a cost and time basis.
It is also notable that proper material selection, heat treating, and allow control can have a large effect on the runout condition of a rotor. If rotors are to be replaced or rebuilt, it is far more cost-effective to detect and correct glitch at the earliest stages of machine assembly.
- (a) Degaussing– Residual magnetism in a shaft, caused as a result of magnetic particle crack detection or by working in a magnetic field, can produce very serious electrical runout. It is therefore prudent to measure residual magnetism in the probe area of every shaft before attempting glitch removal. A localized residual magnetism of field strength 5 gauss on a rolling shaft can give an electrical runout in the order of 0.5 mil. Thus any shaft which exhibits residual magnetism in excess of 2.0 gauss, or variations greater than 1 gauss, should be degaussed. This is generally not a complete glitch removal process on its own, but it does help to ensure that glitch readings do not change as a result of a shaft losing residual magnetism in service.
- (b) Diamond Burnishing – This API recommended method has a high success rate of reducing glitch to within acceptable limits. The probe area is rolled under a diamond burnishing tool to work a shaft surface to a uniform finish. In effect, this procedure produces an even work-hardened surface which requires no additional treatment. This method is undoubtedly the easiest with very little skill required.
- (c) Further Machining – Should a probe area be outside acceptable limits, the target area can be reground, and should be degaussed following grinding. This can be a hit and miss method that could lead to even more unacceptable results.
- (d) Polishing or Stoning – Similar process to (c) but not quite so drastic. However, the problem of making matters worse still exists.
- (e) Sleeving – Shrinking a sleeve onto the shaft has been used, but it is rather an expensive way of producing results as unpredictable as (c) and (d) above. As with both of those methods, further treatment may well be necessary.
- (f) Plasma Spray Finishing – Although not recommended by API, metal coating has been used effectively as a solution for stubborn runout problems. Aluminum – Nickel flame spray coatings are currently being used very successfully in industry to reduce glitch problems, often to less than 1/2 mil pp.The Metco® process consists of machining a 1-1/2 inch wide, 0.060 deep grove around the shaft. The groove is then grit blasted, and coated with a bond coat of Metco 447 to about 0.010 inch thick. The remainder of the grove is then flame sprayed filled with Metco 52C aluminum silicon composite and machined flat with the surface of the shaft. Since the electrical properties are different than 4140 steel, a specially calibrated Proximitor® must be used at these locations.
Protection of Probe Areas
The removal of glitch by one or a combination of the treatments discussed above is essential for customer acceptance of levels of recorded vibration. Having treated the probe areas, it is essential to protect these areas to prevent corrosion damage, scratching and indiscriminate “cleaning up”. During production and testing, this protection is afforded by the care taken by skilled craftsmen who appreciate the sensitive nature of this surface.
Between acceptance testing and startup much less care can be exercised. There have been many field reports of probe areas being damaged by rust, scratches or dents. Cleaning up the damage by stoning or polishing may give the desired appearance, but the original glitch recordings will have been completely altered and undoubtedly taken beyond acceptable limits. Glitch removal then has to be carried out in the field by selective micropeening, and considering the adverse conditions, the results are invariably inferior to those which can be achieved in the shop. It is therefore recommended that after glitch treatment and recordings are taken in the shop, the probe areas be given a coating of non-metallic epoxy resin which can remain in position for the life of the machine. This coating will not affect probe readings but will protect the probe area from corrosion and all minor mechanical damage.
Obviously, shaft treatment to remove the source of runout is the most desirable procedure. If this is done, there is no reason to have to “account for” runout in subsequent vibration signals.
However, the occasional shaft material or forging may not respond well to the standard shaft treatment methods. The shaft may have a “permanent” bow, or it may be impossible or impractical to treat the shaft surface before a time when vibration data is required on a given machine. If it is impractical to treat the shaft surface or remove the shaft bow, an electronic method may be used. The following is an explanation and discussion of this application.
Vector Nulling – Digital Vector Filter
Also called slow roll compensation, this system is an integral part of the Digital Vector Filter. It provides a means for nulling a slow roll vector. It should be noted that the nulling operation is a true vector subtraction (phase and amplitude) and not merely a voltage suppression circuit. The nulling circuit operates on the filtered vibration waveform (the vector information in the DVF). Since the filter employed in the DVF-3 is tuned to the rotational (1X rpm) frequency of the rotor, the nulling circuit eliminates that portion of shaft runout which is coincident with the rotational frequency. Typically, it is used to eliminate a 1X component of runout such as a bow in the shaft or a nonconcentric (egg-shaped) shaft condition at the probe measurement plane. All higher orders of runout (non 1X components, such as scratches, metallurgical irregularities, etc.) are eliminated through the filter action of the DVF-3.
Once the initial slow-roll vector has been nulled, it is automatically subtracted from all future dynamic signals. This system provides the means for properly examining the mechanical response and impedance of a system, definition of the balance resonance’s (critical speeds), and amplification factors, over the operating speed range. Vector nulling also allows for the compensation of the residual unbalance vector after a balance resonance, and for observation of a higher balance resonance response.
It is possible, and even probable on larger machinery, that nominal axial position changes and differential expansion up to running speed will cause a vibration probe to observe a “new” lateral location on the shaft. When considering the overall runout pattern, this “new” shaft location may be significantly different than the overall pattern observed with the machine at slow-roll.
Vector nulling does not, however, deal with the overall runout pattern; because of the filter in the system, only the 1X runout vector is considered. The once-per-turn runout vector is not likely to change from slow-roll to operating speed and temperature. In this regard, vector nulling offers a distinct advantage over any other type of digital runout compensation. Vector nulling also offers the capability of nulling the residual vector of shaft motion after passing through a resonant speed region to observe the action of the next higher resonance when Bode plots (amplitude vs. rpm and phase vs. rpm) are made.
The above considerations lead to the following conclusions and recommendations by Bently Nevada:
- 1. Glitch often can be controlled at its source (the shaft) to a level acceptable for monitoring purposes and in most cases to levels usable for machine acceptance testing and diagnostic purposes. Every reasonable attempt should be made to correct the runout problem at its source.
- 2. The use of electronic runout compensation for continuous machine monitoring should be avoided except in rare cases (e.g. a damaged shaft that cannot be corrected until the next turnaround). Bently Nevada does not recommend the use of electronic runout compensation in a vibration monitor system wired for automatic machine shutdown.
- 3. Nulling – compensation for an initial 1X vector – can be accomplished with the Digital Vector Filter 3.
- 4. When reproducing vibration data from magnetic tape, special care should be taken to ensure proper synchronization of the signal. Most tape recorders provide a function whereby one channel can be dedicated as a synchronizing signal for tape flutter compensation.
- 5. When runout compensation is used, it should be used as a “last resort”. In all cases, both the original transducer signal and the compensated signal must be available for observation on external instruments.