Acceleration Enveloping to Detect Bearing Damage
Introduction
In any manufacturing or processing plant, machine breakdown has significant consequences. Productivity and profitability are usually affected and there is also a health and safety risk. Surprisingly, however, the cause of the breakdown is rarely identified. Instead, the bearings that were damaged and failed are often simply replaced, in the hope that they may last longer next time.
However, bearings do not fail without reason. There is always an explanation for why they do, and it is usually for one of a number of reasons: the machine is running unbalanced, misaligned or at a critical speed; a bearing has not been fitted correctly; over or under lubrication has occurred; or the wrong lubricant has been used.
Vibration Analysis
When problems such as these occur in industrial equipment, sensitive accelerometers are used to detect and analyse the vibrations. This technique is known as vibration analysis and it can identify bearing failure in the very early stages, when there is a microscopic defect on the raceway for example.
This allows issues to be detected prior to machine breakdown and action can be taken proactively to minimise damage to assets. There is one important problem to note about this technique though: the identifying signal is usually drowned out in all the other noise produced by the machine.
What Is Acceleration Enveloping
It is vital to catch bearing defects as early as possible, to stop them developing into more serious problems, and one way of achieving this is to use a signal processing technique called acceleration enveloping.
It works by progressively filtering out unwanted parts of the vibration spectrum until the signal of a bearing defect can clearly be seen.
Acceleration enveloping is most commonly used in roller bearing systems but can also be applied in areas such as electric motors and gearboxes. It is a key factor in the success of condition based maintenance programmes.
Without acceleration enveloping, maintenance teams and engineers will first learn of damage and failure when overall vibration increases, lubricants are contaminated and temperatures rise. By the time this occurs, the remaining usable life of the failing machine elements could be short and the damage might be more extensive than if faults had been detected earlier.
In worst case scenarios, bearings may fail and the machine could break down before operators notice or resolve problems.
How Acceleration Enveloping Works
Acceleration enveloping is a two stage process.
The first step is to apply a band pass filter to the mix of low and high frequencies of a defective bearing’s unfiltered waveform. This isolates only the frequencies in which the signal of interest is hiding.
The filtered output will identify repeating high frequency signals. These are the impacts from the rolling elements hitting the defect of the rotating bearing.
The second step in the process is to pass the filtered output through an enveloper, which rectifies the waveform by inverting the negative part to positive, and extracts the repetition rate of the energy bursts.
This envelope is now used as a true vibration signal, helping it to stand out from background noise.
The envelope helps to contain regularly spaced signals such as a single defect on a raceway, while other causes of noise, such as shaft rub, are random and will not produce evenly spaced peaks.
It is important to note that some experience is required to ensure the steps in this process are completed correctly, particularly when selecting the correct high and low pass frequencies.
Acceleration Enveloping in Action
Acceleration enveloping is most commonly used in roller bearing systems but can also be applied in areas such as electric motors and gearboxes.
It has repeatedly proven to be a key factor in the success of condition based maintenance programmes. While it is predominantly used with signals in the acceleration spectrum, it can also be used to improve other measurements such as shock pulse.
Application Example: Acceleration Enveloping in Wind Turbines
The average wind turbine has around eight thousand separate components. Of these, a large number are associated with the drivetrain, which has been recognised as the major cause of extended downtime.
Wear in gearboxes and bearings in particular is known to cause problems and can lead to expensive repairs. Regular vibration monitoring can prevent these issues occurring.
The complexity of a typical wind turbine presents a challenge for vibration monitoring. Components such as the main turbine, gearbox and generator produce unique vibration signatures with different amplitudes and frequencies, which can be difficult to isolate and may be masked by noise from surrounding systems.
This is where acceleration enveloping plays a crucial role.
Sensor Selection and Installation for Wind Applications
To be effective, accelerometers should be fitted to all key rotating parts in a turbine. These include the main bearings, planetary, intermediate and high speed gear stages, the generator, and ideally the nacelle traverse and axial movements.
Accelerometers should be selected depending on the frequency of enveloping signals. On wind turbine drivetrains, particularly the generator output shaft, rotational speeds can be relatively slow and may require special purpose low frequency AC accelerometers with sensitivities between one hundred millivolts per g and five hundred millivolts per g.
Each accelerometer must be mounted securely on a clean and solid base, as close to the component being monitored as possible. Standard M8 mountings are normally used.
Data should be collected regularly and consistently to enable changes in operating conditions or trends over time to be identified at the earliest possible stage.
Data can be collected on site using hand held data collectors with software capable of automatically calculating acceleration enveloping, or transmitted to a remote monitoring centre for subsequent analysis.
Interpreting Acceleration Enveloping Data
Once the signal has been filtered, the information can be collected from the accelerometer using a data collector and reviewed by a specialist.
Based on the data, decisions can be made on whether maintenance work is required immediately, can be planned as part of routine schedules, or if no action is required at that time.
Limitations of Acceleration Enveloping
Despite acceleration enveloping appearing to be the definitive answer to detecting bearing failure, there are situations where it may not be the best option.
The technique detects faults involving repetitive metal to metal interactions. Anything that masks this, such as gaskets or dampers, may place a machine outside the scope of acceleration enveloping.
Key Factors for Success
If an application is suitable for acceleration enveloping, several factors help ensure best possible results.
Accelerometers must be selected carefully in the correct frequency range to suit the specific application. An experienced provider can assist in selecting between AC and four to twenty milliamp accelerometers.
Accelerometers must be mounted correctly, close to the component being monitored and on a flat, clean surface to guarantee consistent results. Poor mounting reduces reliability and can make collected data redundant.
Once installed and calibrated, data readings should be taken at regular intervals to allow accurate trend analysis. This enables a steadily deteriorating condition to be identified and addressed before bearing damage worsens.
It is important to understand that vibration data does not provide a simple yes or no answer and requires experience to interpret correctly. In some cases, amplitude may reduce over time even though deterioration continues.
Using Acceleration Enveloping as Part of a Wider Strategy
The benefits of acceleration enveloping are clear, but it should not be relied upon as a standalone technique.
For the most reliable results, acceleration enveloping should be implemented as part of a wider monitoring and analysis regime. Combined with the guidance in this document, it can help engineers safeguard machine health, performance and productivity while preventing breakdowns and lost output.
