Vibration analysis to detect early failure of bearings

Dr. Steve Lacey, engineering manager, Schaeffler (UK) Ltd examines the key factors behind the failure of modern bearing systems and how to monitor these effects:

London -- Vibration produced by rolling bearings can be complex and can result from geometrical imperfections during manufacture, defects on rolling surfaces or geometrical errors in associated components. Noise and vibration is becoming more critical in all types of equipment since it is often seen as synonymous with quality and often used for predictive maintenance.

Rolling contact bearings are used in most types of rotating machinery, whose reliable operation depends on the type of bearing selected and the precision of associated components such as shafts, housings and spacers.

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, bearing fatigue life, which is related to sub-surface stresses, is not the limiting factor and accounts for less than 3% of failures in service.

Many bearings fail prematurely because of contamination, poor lubrication, misalignment, temperature extremes, poor fitting, unbalance and misalignment. All these factors lead to an increase in bearing vibration and so condition monitoring is used to detect degrading bearings before they catastrophically fail, resulting in downtime or significant damage to other parts of the machine.

Rolling element bearings are often used in noise-sensitive applications, such as household appliances and electric motors. Bearing vibration is therefore becoming increasingly important from both an environmental consideration and because it is synonymous with quality.

It is now generally accepted that quiet running is synonymous with the form and finish of the rolling contact surfaces. As a result, bearing manufacturers have developed vibration tests as an effective method for measuring quality. A common approach is to mount the bearing on a quiet running spindle and measure the radial velocity at a point on the bearing’s outer ring in three frequency bands, 50-300, 300-1,800 and 1,800-10,000Hz. The bearing must meet RMS velocity limits in all three frequency bands.

Vibration monitoring is now a well-accepted part of many planned maintenance regimes and relies on the characteristic vibration signatures that rolling bearings exhibit as the rolling surfaces degrade. However, in most situations, bearing vibration cannot be measured directly and so the signature is modified by the machine structure. This is further complicated by vibration from other equipment on the machine, such as motors, gears, belts, hydraulics and structural resonance. This makes interpreting vibration data difficult other than by a trained specialist, and can in some cases lead to a mis-diagnosis, causing unnecessary machine downtime.

Sources Of Vibration

Rolling contact bearings represent a complex vibration system whose parts (rolling elements, inner raceway, outer raceway and cage) interact to generate complex vibration signatures.

Although rolling bearings are manufactured using precision machine tools and strict quality controls, they inevitably have degrees of imperfection and generate vibration as the surfaces interact, through a combination of rolling and sliding.

Although the amplitudes of surface imperfections are now in the order of nanometers, significant vibrations can still be produced in the entire audible frequency range (20Hz-20kHz). The level of vibration will depend upon many factors, including the energy of the impact, the point at which the vibration is measured and the bearing’s construction.

Variable Compliance

Under radial and misaligning loads, bearing vibration is an inherent feature of rolling bearings, even if the bearing is geometrically perfect and is not therefore indicative of poor quality. This type of vibration is often referred to as ‘variable compliance’ and occurs because the external load is supported by a discrete number of rolling elements whose position with respect to the line of action of the load continually changes with time.

Geometrical Imperfections

Because of the very nature of the manufacturing processes used to produce bearing components, geometrical imperfections will always be present to varying degrees depending on the accuracy class of the bearing. For axially loaded ball bearings operating under moderate speeds, the form and surface finish of the critical rolling surfaces are generally the largest source of noise and vibration. Controlling component ‘waviness’ and surface finish during manufacture is therefore critical, since it may not only have a significant effect on vibration but also may affect bearing life.

Surface Roughness

Surface roughness is a significant source of vibration when its level is high compared with the lubricant film thickness generated between the rolling element-raceway contacts. Under this condition, surface asperities can break through the lubricant film and interact with the opposing surface, resulting in metal-to-metal contact. The resulting vibration consists of a random sequence of small impulses, which excite all the natural modes of the bearing and supporting structure.

Waviness

For longer wavelength surface features, peak curvatures are low compared with that of the

Hertzian contacts and rolling motion is continuous with the rolling elements following the surface contours. The relationship between surface geometry and vibration level is complex and is dependent upon the bearing and contact geometry, as well as load and speed. Waviness can produce vibration at frequencies up to 300 times rotational speed but is usually predominant at frequencies below 60 times rotational speed.

For typical bearing surfaces, poor correlation of parallel surface heights profiles only exists at shorter wavelengths. Even with modern precision machining technology, waviness cannot be eliminated completely and an element of waviness will always exist albeit at relatively low levels.

Discrete Defects

Whereas surface roughness and waviness result directly from the bearing component manufacturing processes, discrete defects refer to damage of the rolling surfaces due to assembly, contamination, operation, mounting and poor maintenance. These defects can be very small and difficult to detect but can have a significant impact on vibration-critical equipment or can result in reduced bearing life.

Bearing Characteristic Frequencies

Although the fundamental frequencies generated by rolling bearings are related to relatively simple formulae, 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.

The bearing equations assume that there is no sliding and that the rolling elements roll over the raceway surfaces. But in practice, this is rarely the case and due to a number of factors the rolling elements undergo a combination of rolling and sliding.

Raceway Defect

A discrete defect on the inner raceway will generate a series of high energy pulses at a rate equal to the ball pass frequency relative to the inner raceway. Because the inner ring is rotating, the defect will enter and leave the load zone causing a variation in the rolling element-raceway contact force, hence deflections. While in the load zone the amplitudes of the pulses will be highest but then reduce as the defect leaves the load zone resulting in a signal, which is amplitude-modulated at inner ring rotational frequency.

Rolling Element Defect

Defects on the rolling elements can generate a frequency at twice ball spin frequency and harmonics and the fundamental train frequency. Twice the rolling element spin frequency can be generated when the defect strikes both raceways, but sometimes the frequency may not be this high because the ball is not always in the load zone when the defect strikes and energy is lost as the signal passes through other structural interfaces as it strikes the inner raceway. Also, when a defect on a ball is orientated in the axial direction it will not always contact the inner and outer raceway and therefore may be difficult to detect.

The bearing cage tends to rotate at typically 0.4 times inner ring speed, has a low mass and therefore, unless there is a defect from the manufacturing process, is generally not visible.

Unlike raceway defects, cage failures do not usually excite specific ringing frequencies and this limits the effectiveness of the envelope spectrum. In the case of cage failure, the signature is likely to have random bursts of vibration as the balls slide and the cage starts to wear or deform and a wide band of frequencies is likely to occur.

Contamination is a common source of bearing deterioration and premature failure and is due to the ingress of foreign particles, either as a result of poor handling or during operation. The magnitude of the vibration caused by contamination will vary and in the early stages may be difficult to detect, but this depends on the type and nature of the contaminant. Contamination can cause wear and damage to the rolling contact surfaces.

Vibration measurement can be characterised into three areas – detection, diagnosis and prognosis.

Detection uses the most basic form of vibration measurement, where overall vibration is measured on a broadband basis, for example in a range, 10–1,000Hz or 10-10,000Hz. In machines where there is little vibration other than from the bearings, the ‘spikiness’ of the vibration signal indicated by the Crest Factor (peak/RMS) may imply incipient defects, whereas the high energy level given by the RMS level may indicate severe defects.

Other than to the experienced operator, this type of measurement gives limited information but can be useful when used for trending, where an increasing vibration level is an indicator of a deteriorating machine condition. Trend analysis involves plotting the vibration level as a function of time and using this to predict when the machine must be taken out of service for repair. Another way of using the measurement is to compare the levels with published vibration criteria for different types of equipment.

Although broadband vibration measurements provide a good starting point for fault detection, it has limited diagnostic capability and although a fault can be identified, it may not give a reliable indication of where the fault is, i.e. bearing deterioration, unbalance, misalignment.

Where an improved diagnostic capability is required, frequency analysis is normally used, which gives a much earlier indication of the development of a fault and also the source of the fault.

Having detected and diagnosed a fault, the prognosis is much more difficult and often relies on the continued monitoring of the fault to determine a suitable time when the equipment can be taken out of service or relies on known experience with similar problems.

Overall vibration level is the simplest way of measuring vibration and usually consists of measuring the RMS (Root Mean Square) vibration of the bearing housing or some other point on the machine with the transducer located as close to the bearing as possible.

This technique involves measuring the vibration over a wide frequency range e.g. 10-1,000Hz or 10-10,000Hz. Measurements can be trended over time and compared with known levels of vibration or alarm levels can be set to indicate a change in the machine condition.

Alternatively, measurements can be compared with general standards. Although this method represents a quick and low cost method of vibration monitoring, it is less sensitive to incipient defects. Also, it is easily influenced by other sources of vibration e.g. unbalance, misalignment, looseness, electromagnetic vibration, etc.

Frequency spectrum analysis plays an important role in detecting and diagnosing machine faults. In the time domain the individual contributions e.g. unbalance, gears, etc. to the overall machine vibration are difficult to identify. In the frequency domain they become much easier to identify and so can be easily related to individual sources of vibration. A fault developing in a bearing will show up as increasing vibration at a characteristic frequency making detection possible at a much earlier stage than with overall vibration.

When a bearing starts to deteriorate, the resulting time signal often exhibits characteristic features, which can be used to detect a fault. Also, bearing condition can rapidly progress from a very small defect to complete failure in a relatively short period of time; so early detection requires sensitivity to very small changes in the vibration signature.