Equipment for predicting bearing failure
Spike energy detection What is spike energy?
Normal vibration analysers which measure displacement, velocity and acceleration over a frequency range of 10Hz to 1 kHz have now become available with an additional readout of “spike energy”. This is defined as the ultrasonic microsecond-range pulses caused by impacts between bearing elements which have microscopic flaws.
Special circuits have been designed to detect this pulse amplitude, the rate of occurrence of the pulses and the amplitude of the high frequency broad band vibratory energy associated with bearing defects. These three parameters — pulse amplitude, pulse rate and high frequency random vibratory energy— are electronically combined in the single quantity g-SE. This is recognised as a measurement of bearing condition and has the units of acceleration but in the ultrasonic frequency range.
Early spike energy meters
An example of an early spike energy meter is shown in Figure 15.15. The meter had a cable input from a transducer with hand-held probe or a more permanent magnetic pick-up, this being applied to the bearing housing with a light, steady pressure so that it did not chatter. To establish a programme for checking the condition of anti-friction bearings, a comparison
Figure 15.15 Early spike energy meter
Technique was used. Thus the g-SE levels of similar machines were measured and those which diverged from the average were identified. A close watch was kept on any such bearings as being a source of potential trouble. The method led to the quick establishment of criteria for determining whether a bearing was good or bad.
It should be noted that g-SE is dependent on rotational speed rev/min. A doubling in speed would result in the spike energy measurement doubling for the same bearing condition. From a vibration severity standpoint it should, however, be remembered that low speed bearings can usually tolerate more damage than high speed bearings since the former will tend to deteriorate more slowly. With single machines, measurements had to be taken periodically and any trends noted.
An unchanged level of g-SE over a period of time would indicate a good bearing, but any significant upward trend would signal imminent failure. General experience over the range of 600 to 3600 rev/min and using the 9 inch hand-held probe shown, g-SE value of over 0.5 usually indicated a defective bearing. This value was used with caution as it might have been dependent on bearing type and mounting.
Apart from bearings, there are other sources of spike energy. Incipient gear defects, rubbing of seals or guards are all possible causes. Where these elements are close to the bearings,
Values shown are for measurements taken on or close to bearing housing with standard
9 inch probe
10 HP motor (3600 rev/min): ‘no load severe bearing 1 ‘ defects
20 HP motor (1750 rev/min)
‘ under load detective bearing
1.5 HP pump (1750 rev/min): under load sand in new bearing
150 HP motor (1175 rev/min) under load good bearing T~
1.5 HP pump (1750 r/min) funder load good bearing
S 8 8!
Additional readings should be taken to avoid misinterpreting the data.
The meter was best used with spike energy severity charts, as shown in Figure 15.16. These led to the establishment of g-SE severity criteria for a given machine and its bearings. No specific severity levels such as smooth, good etc. were given, since they were dependent on the machine, its bearing type(s), speed and loads. Some case histories have nevertheless been plotted to indicate the resultant range.
Present day spike energy meters
These are now produced by Rockwell Automation Ltd who offers products for the fan site engineer. Figure 15.17 shows these instruments, remarkable for their reduced size when compared with Figure 15.15.
One is a small lightweight portable data collector/analyser that monitors the condition of equipment found in many process industries. It is easy to use and has features normally associated with bulky real time analysers. It also uses the latest advances in analogue and digital electronics and screen technology to provide speedy and accurate and both unlimited and reliable data collection.
Another is a Windows-based, 2-channel data collector and signal analyser. It enables easy condition monitoring of equipment including vibration information. Bearing assessment is also available which integrates with other information systems and software.
Figure 15.17 Data collector and data collector/analyser Courtesy of Rockwell Automation Ltd
Shock pulse measurements Theory
This method detects development of a mechanical shockwave caused by the impact between two bodies. As an example, consider a ball dropping onto a bar as shown in Figure 15.18.
At the moment of impact, or initial phase 1, no detectable deformation of the material has yet taken place. An infinitely large particle acceleration therefore results, its magnitude being solely dependent on the impact velocity v. The result is unaffected by the sizes of the two bodies or by any mechanical vibration present. Two compression waves are set up, that in the bar propagating ultrasonically in all directions, whilst the other travels through the ball. The magnitude of the wavefront is an indirect measure of the impact velocity v.
During the second phase of impact 2, the ball and bar surfaces deform, the energy deflecting the bar and setting up vibrations. This is the motion detected during normal vibration analysis.
It must be emphasized that the shock pulse method is concerned solely with phase 1 by detecting and measuring the magnitude of a mechanical impact from the resultant compression wavefront. A piezoelectric accelerometer is used, which is
Figure 15.18 Illustration of shock pulse mechanism during impact
Not influenced by background vibration or noise. This transducer is tuned mechanically and electrically to have a resonance of 32 kHz. The compression wavefront or shock pulse sets up a dampened oscillation in the transducer at its resonant frequency. This also is shown in Figure 15.18 as the dampened transient electrical output caused by the impact. The peak amplitude of this oscillation (A) is therefore directly proportional to the impact velocity v.
As the transient is well defined, and decays at a constant rate, it is possible to filter out electronically all the normal vibration signals. The measurement and analysis of its maximum value is the basis for determining the condition of rolling element bearings.
Testing anti-friction bearings
As previously stated, the running surface of a bearing will always have a degree of roughness, from microscopic flaws or indentations which will increase as it approaches failure. When the bearing rotates these surface irregularities or fatigue craters will cause mechanical impacts between the rolling elements and thus become a shock pulse generator. The magnitude of the shock pulses is dependent on the surface condition and the peripheral velocity of the bearing (ocrev/min x size). As the shock pulses increase with age it is possible to follow the progress of a bearing’s condition from installation, through the various stages of deterioration to ultimate failure.
Shock pulses generated by a typical bearing will increase by a factor of up to 1000 times from when it is new to when it is replaced. To simplify the readout of such a large range, figures in decibels (dB) are used. It should be remembered that the decibel is by definition a ratio on a logarithmic scale. Apart from noise, it can and is used for a number of other purposes e. g. acceleration values. In the present case the intensity of the shock pulses generated by the bearing is measured in dBsv (decibel shock value) and the scale thus compressed to 60 dB sv, i. e.
Readings expressed in dB sv refer to the total or absolute value of the shock pulses.
Empirical testing has shown, as expected, that even a new, properly installed and properly lubricated bearing will generate shock pulses. This initial value or dBi is primarily dependent on rotational speed rev/min and bore diameter mm (see Figure 15.19).
As the bearing ages and deteriorates the dBsv total shock pulse value increases. This increase is defined as its dBN or normalized value i. e. dBN = dBsv — dBj.
Figure 15.20 shows the relationship between bearing condition and percentage bearing life.
Less than 20
20 to 35
Table 15.9 Bearing operating zones
Figure 15.19 Initial value of — Relationship with bore and speed
By experience the dBN scale has been divided into three zones as shown in Table 15.9.
Periodic measurements should be taken and, in the early days were plotted on the chart shown in Figure 15.21. Decisions can then be made as to when bearings should be changed.
It is worthy of note that over the years, since the author bought his first shock pulse meter, with the increasing miniaturization, many of the functions and calculations are now performed within the instrument itself in the most recent versions. However this explanation of the earlier versions is given as it most readily describes the theory and workings of shock pulse.
An early shock pulse meter
Figure 15.20 Relationship between bearing condition and percentage life
The early portable meter was hand-held and battery-powered as shown in Figure 15.22. Before any readings were taken the bore diameter mm and speed rev/min were dialled into the meter by aligning their values on the respective scales. The dBj of the bearing was then automatically subtracted from the trans-
Figure 15.22 Early shock pulse meter
Figure 15.21 Chart for plotting shock pulse dB measurements
Ducer output which measured dBsv. This additional amount was, of course, the dBN and a direct indicator of bearing condition.
The transducer signals were compared within the meter to a manually set threshold level, which could be adjusted by rotating the large outer dial relative to the large black stationary arrow. Starting with a dial setting of 0 dBN1 a continuous tone, generated by the instrument, was heard from the built in speaker and external earphones. As the dial was turned to higher scale values, a point would be located where the tone was intermittent. This dBN reading was defined as the bearing’s carpet value dBc. By continuing to turn the scale to higher readings, the tones became more and more intermittent, until they finally disappeared. This value of dBN was defined as dBM maximum or peak, and indicated the bearing condition.
During bearing operation, not only peak shocks appeared, but a number of differing amplitudes and rates of occurrence. The relationship between shock amplitude read on the dBN scale and rate or number per unit time gave the amplitude distribution of the bearing shocks. Again the distribution was assessed by listening to the built in speaker on the meter or the external earphones.
Figure 15.23 is an evaluation flow chart where every individual shock pulse measured at the meter was represented by a vertical line whose height corresponded to the shock amplitude dBN. Bearing condition, installation, fit, alignment and lubrication were all assessed by measurements of maximum and carpet values. Additional comments in explanation of some of the items in the flowchart are:
A) Good bearing, properly installed, properly lubricated
In a good bearing, the shocks are mainly caused by the rolling contact on normal surface roughness, which means that there will be a low shock noise carpet and random shocks with slightly higher value. The carpet value should be under 10dBN and the peak value under 20dBN.
B) Damaged bearing
When the bearing raceways or rolling elements are damaged, high peak amplitude shocks will appear.
Through coincidence between different damages in different running surfaces, these shocks will appear randomly. Often, the carpet value will be below 20dBN. However if the bearing is badly damaged, the overall surface roughness will increase and so will the carpet value. Usually howeverthere is a large difference between the peak and carpet values.
C) Improper installation or lack of lubricant
These are operating condition problems. If the bearing is improperly installed (out-of-round or pinched housing, too tight or loose a fit) the internal load in the bearing will increase locally and thereby the shocks caused by the rolling motion will also increase even if the bearing is not yet damaged on its running surfaces. It is characteristic of this type of problem that the peak and carpet values are relatively close together.
A bearing running with insufficient lubricant has a shock pattern similar to an improperly installed bearing. The lack of lubricant will increase the carpet value. Lack of lubricant will normally only appear in greased bearings. Therefore, greasing the bearing is recommended when an increase in carpet value is noticed. The carpet value should decrease after lubrication.
D) Mechanical rubbing
Mechanical rubbing near the bearing between a rotating and stationary part (for example, rubbing between the bearing seal and shaft) will cause rhythmic shock bursts at a certain dBN level. They are easy to identify because of their repetitive nature.
E) Machine cycle load shocks
If a bearing is exposed to a cyclic shock load, a measurable shock signal may appear in the bearing. These shocks will appear with a rhythm related to the machine working cycle and are therefore simple to determine and isolate. They will be very repetitive but the peak and carpet values of the bearing can usually. be determined.
Pinion damage in a gear box can also generate a shock pattern similar to the above load shocks. These shocks will appear with a rhythm related to the speed of the shaft involved. Moreover, it is typical for pinion damage to generate the same repetitive shock pattern on all the bearings involved.
Present day shock pulse meters
These too have changed considerably from the early meters.
One of the meters is produced by SPM Instruments AB and is a portable, multi-functional instrument for bearing and lubrication condition monitoring, vibration analysis. It includes corrective maintenance features such as balancing and alignment, see Figure 15.24.
Figure 15.24 Portable machine condition analyser Courtesy of SPM Instrument AB
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