The Role of Vibration Monitoring in Predictive Maintenance

Phil Black - PII Editor 20/06/2011

316 7 minutes read

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Dr. S. J. Lacey, Engineering Manager Schaeffler (UK) Ltd

Introduction
As greater demands are being placed on existing assets, either in terms of higher output or increased efficiency, the need to understand when things are starting to go wrong is becoming more important. Also, as plant and equipment becomes more complex and automated the need to have a properly structured and funded maintenance strategy is critical.

There is also a need to understand the operation of your own equipment such that improvements in plant output and efficiency can be realised. In today’s increasingly competitive world all of these issues are of key importance and can only be achieved through a properly structured and financed maintenance strategy that meets the business’ needs.
Vibration Monitoring
Vibration monitoring is probably the most widely used predictive maintenance technique and with few exceptions can be applied to a wide variety of rotating equipment.

Machine vibration comes from many sources e.g. bearings, gears, imbalance etc and even small amplitudes can have a severe effect on the overall machine vibration depending on the transfer function, damping and resonances. Each source of vibration will have its own characteristic frequencies which can manifest itself as a discrete frequency or as a sum and/or difference frequency.
At low speeds it is still possible to use vibration but a greater degree of care and experience is required and other techniques such as measuring shaft displacement or Acoustic Emission (AE) may yield more meaningful results although the former is not always easy to apply. Also, while AE may detect a change in condition it has limited diagnostic capability.

Rolling Element Bearings
Rolling contact bearings are used in almost every type of rotating machinery whose successful and reliable operation is very dependant on the type of bearing selected as well as the precision of all associated components i.e. shaft, housing, spacers, nuts etc. Bearing engineers generally use fatigue as the normal failure mode on the assumption that the bearings are properly installed, operated and maintained. Today, because of improvements in manufacturing technology and materials, generally bearing fatigue life, which is related to sub surface stresses, is not the limiting factor and probably accounts for less than 3% of failures in service.

Unfortunately though, many bearings fail prematurely in service because of contamination, poor lubrication, misalignment, temperature extremes, poor fitting/fits, unbalance and misalignment. All these factors lead to an increase in bearing vibration and condition monitoring has been used for many years to detect degrading bearings before they catastrophically fail with the associated costs of downtime or significant damage to other parts of the machine.

Rolling element bearings are often used in noise sensitive applications e.g. household appliances, electric motors which often use small to medium size bearings. Bearing vibration is therefore becoming increasingly important from both an environmental consideration and because it is synonymous with quality.

Vibration monitoring has now become a well accepted part of many Predictive Maintenance regimes and relies on the well known characteristic vibration signatures which rolling bearings exhibit as the rolling surfaces degrade. However, in most situations bearing vibration cannot be measured directly and so the bearing vibration signature is modified by the machine structure and this situation is further complicated by vibration from other equipment on the machine i.e. electric motors, gears, belts, hydraulics, structural resonances etc (pic condition monitoring3 to go here)

This often makes the interpretation of vibration data difficult other than by a trained specialist and can in some situations lead to a misdiagnosis resulting in unnecessary machine downtime and costs.

Bearing Characteristic Frequencies
Although the fundamental frequencies generated by rolling bearings are related to relatively simple formulas, they cover a wide frequency range and can interact to give very complex signals. This is often further complicated by the presence of other sources of mechanical, structural or electromechanical vibration on the equipment.

Since most vibration frequencies are proportional to speed, it is important when comparing vibration signatures that data is obtained at identical speeds. Speed changes, will cause shifts in the frequency spectrum causing inaccuracies in both the amplitude and frequency measurement. In variable speed equipment sometimes spectral orders may be used where all the frequencies are normalised relative to the fundamental rotational speed. This is generally called “order normalisation” where the fundamental frequency of rotation is called the first order.

Analysis of bearing vibration signals is usually complex and the frequencies generated will add and subtract and are almost always present in bearing vibration spectra. This is particularly true where multiple defects are present. However, depending upon the dynamic range of the equipment, background noise levels and other sources of vibration, bearing frequencies can be difficult to detect in the early stages of a defect.

However, over the years a number of diagnostic algorithms have been developed to detect bearing faults by measuring the vibration signatures on the bearing housing. Usually these methods take advantage of both the characteristic frequencies and the “ringing frequencies” (i.e. natural frequencies) of the bearing.

By measuring the frequencies generated by a bearing it is often possible not only to identify a problem but to also identify the cause.

Bearing Defects
Rolling contact bearings represents a complex vibration system whose components i.e. rolling elements, inner raceway, outer raceway and cage interact to generate complex vibration signatures. Although rolling bearings are manufactured using high precision machine tools and under strict cleanliness and quality controls, like any other manufactured part they will have degrees of imperfection and generate vibration as the surfaces interact through a combination of rolling and sliding.

Nowadays, although the amplitudes of surface imperfections are in the order of nanometers, vibrations can still be produced in the entire audible frequency range (20Hz-20kHz).

Variable Compliance
This occurs under radial or misaligning loads. It is an inherent feature of rolling bearings and it is completely independent of quality. Variable compliance vibration is heavily dependent on the number of rolling elements supporting the externally applied loads; the greater the number of loaded rolling elements, the less the vibration.

Bearing Speed Ratio
The bearing speed ratio (ball pass frequency divided by the shaft rotational frequency) is a function of the bearing loads and clearances and can therefore give some indication of the bearing operating performance. When abnormal or unsatisfactory lubrication conditions are encountered, or when skidding occurs, the bearing speed ratio will deviate from the normal or predicted values.

Electric Motor Example
An example of a vibration spectra measured axially on the drive end (DE) of a 250kW electric motor is shown in Figure 1.

The nominal rotational speed was 3000rev/min and the rotor was supported by two type 6217 C4 (85mm bore) radial ball bearings, grease lubricated. The vibration spectra are dominated by vibration at both harmonics and sub harmonics of rotor speed (49.7Hz). The spectrum 0-1kHz shows a number of harmonics and sub harmonics of rotor speed with no bearing characteristic frequencies being evident.
In the 0-5kHz spectrum there is a dominant discrete peak at 1141.8Hz which neither corresponds with a harmonic of rotor speed i.e. 1141.8/49.98 = 22.84 nor with any of the bearing generated frequencies. Either side of 1141.8Hz peak are sidebands spaced at rotor speed (49.98Hz) i.e. the 1141.8Hz frequency is amplitude modulated at rotor speed.

This is shown more clearly in Figure 2(a), which shows that from 0-650ms the signal is amplitude modulated at 20.2ms which, within the measurement accuracy, corresponds to 49.98Hz, rotor speed. Expanding the time scale from 500-600ms, Figure 2(b), shows that the time between peaks is 0.87ms i.e. 13.051ms divided by 15 cycles which corresponds to a carrier frequency of approximately 1149Hz. Within the measurement accuracy of 0.0796ms this corresponds to the frequency of 1141.8Hz (0.876ms) shown in Figure 1.

Dividing 1141.8Hz by the rotational speed of 49.98Hz gives 22.85 which is not close enough for the frequency to be a harmonic of rotational speed. One of the extensional vibration modes of the outer ring was estimated to be 1158Hz which is very close to the measured value of 1141.8Hz. One possible explanation is that the discrete peak at 1141.8Hz is an excited natural frequency of the outer ring.

(a) Vibration acceleration 0-650ms

(b) Vibration acceleration 500-600ms
Figure 2. Time signals of vibration acceleration measured axially on DE of 250kW electric motor.

The dominance of vibration at rotor speed and the absence of any frequencies related to the rolling bearings suggest that the bearings have experienced severe damage to the rolling contact surfaces, so much so that this has resulted in an increase in radial internal clearance allowing significant radial movement of the rotor.
The envelope spectrum, Figure 3, shows a dominance of peaks related to rotor speed with no evidence of any bearing characteristic frequencies.

Figure 3. Envelope spectrum of vibration acceleration measured axially on DE of 250kW electric motor.

When the bearings were removed from the motor and examined the NDE bearing had a ball running path offset from the centre of the raceway towards the shoulder, Figure 4.

Figure 4. Photograph of type 6217 inner ring showing running path offset from centre of raceway
The drive end bearing had significant damage all around both raceways and the rolling elements shows signs of severe distress. However, it was clear from the NDE bearing that the cause of the failure was too tight a fit between the outer ring and housing. This resulted in the bearing failing to slide in the housing and compensate for axial thermal expansion of the rotor, hence a high axial load.
During a “run up” test prior to installing in the plant, in the frequency range 0-1kHz the RMS vibration level of the motor was 0.304g and 0.335g before and after fitting the new bearings respectively.

Schaeffler (UK) Ltd
Sutton Coldfield
West Midlands

Can be contacted on:

Tel: 0121 313 5890
Fax: 0121 351 7579
E-mail: info.uk@schaeffler.com
Web: www.schaeffler.co.uk