Avoiding vibration-induced issues
24 August 2015
Neil Parkinson discusses the use and benefits of operating deflection shape analysis to interpret machine vibration issues.
Sustained, excessive vibration in many structures, including machinery, tools, pipes and support systems, can have catastrophic results. If left uncorrected it can cause failure of the unit and, as a direct result, losses in terms of productivity/downtime and unplanned maintenance or replacement costs.
Even before a unit fails, vibration can reduce machine reliability and generate environmental hazards – it may also render the machinery, and therefore the company or organisation that operates it, non-compliant with legal and regulatory requirements, for example by causing a hazard to employees' health and safety.
For complex systems with many degrees of freedom and with potentially multiple mode shapes and frequencies, it can be difficult for the structural vibration analyst to fully understand the nature of the vibration, making it difficult to determine the optimum corrective actions.
One tool available to assist in the interpretation of such complex vibration problems is a measurement-based technique which enables the actual in-service vibration modes to be individually visualised and animated. The technique is known as Operating Deflection Shape (ODS) analysis.
Identifying vibration patterns
ODS analysis clearly identifies the vibration patterns occurring under real life conditions, and so machinery and structural supports can be adjusted and optimised in light of those findings. When used as part of a proactive maintenance schedule. ODS analysis can help organisations to avoid the threats posed by vibration-induced machinery problems, and to make more efficient decisions around investment and product life cycles. It provides a way of visualising the vibration pattern occurring in a structure. Vibration can result from the structure's own operating forces, or from forces external to it. In ODS analysis, vibration is measured at various points and in various directions within the structure – these are referred to as degrees of freedom (DOFs).
Specialised software is used to convert the measurements into appropriate formats, such as animated graphic representations of the structure being analysed. These animations can show how and where the vibration is causing problems, and to what degree. Typically, the structural responses will be greatest at a number of specific natural or forced frequencies where, at each frequency, the structural deflection forms a particular mode shape, illustrated by relative amplitudes and phases at each position on the structure. In reality, all structural modes are happening at the same time. However, by animating each different frequency component of vibration separately, it is easier for the specialist to interpret very complex data.
The data can be used to identify a number of key factors:
•Which excitation frequency produces the greatest amplitude of vibration.
•Which positions of the structure show the largest displacement amplitudes.
•Are there any positions of looseness of fixings or any structural supports which appear to be too flexible?
•Where would be the best positions to try to re-inforce the structure to resist the observed directions and amplitudes of vibration?
There are four main types of ODS analysis:
Time ODS – used to investigate vibration as a function of time, this method can visualise deflection shapes generated by both transient and stationary signals.
Running orders ODS – used to evaluate vibration patterns at specific orders as a function of rotational speed or frequency.
Running modes ODS – evaluate vibration at given frequencies as a function of rotational speed or frequency.
Frequency/spectral ODS – used to examine vibration patterns at specified frequencies in stationary conditions.
Any item subject to ODS analysis has an almost infinite number of DOFs that could be measured. In practice, however, the number that are sampled during testing will vary according to the size of the structure, the applications and context involved. Typically, for pipework vibration assessments, vibration measurements would be taken at the following positions:
•Mid points of long spans.
•All support points.
•Small bore connections.
In some ways ODS analysis resembles modal analysis, which is concerned with the natural frequencies and mode shapes of multiple DOF systems. However, the main differences with ODS are that modal analysis will identify all modal responses of the system which could ‘potentially’ be excited. It will identify only those modal frequencies which are being excited at that time; and ODS will also identify any other vibration mode shapes caused by forced excitation i.e. not occurring at a structural natural frequency.
Causes of the consequent excessive vibration could include forces generated by the item itself; inadequate support systems; unbalances; and external loads.
In ODS analysis, the degree of vibration in an item is measured and clearly visualised using specialist software, making the decisions connected with maintenance, adjustment and optimisation much better informed and more likely to succeed than may otherwise have been the case.
ODS can provide a powerful tool that permits the clear and detailed visualisation of vibration. It can be used to pinpoint problems in a range of machinery, tools and structures, particularly those involving periodic excitation, and in a number of industrial and engineering settings. What is more, ODS analysis can be carried out while the plant is operational, so there is no downtime – a particular advantage in manufacturing environments.
Neil Parkinson is technical director at AVT Reliability.
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