There are principally two known approaches to work on vibration problems; either a signal based assessment or a system based response analysis; most of in-operation based vibration analysis are inferred by understanding the "output only" data which is the vibration signature of the equipment in consideration; here, every participating sub-system of the equipment adds its frequency and amplitudes to make the output data as a complex combination of multiple sources which needs to be de-coded by using suitable techniques of frequency analysis to determine the sources based on the kinematics and other operational details of the equipment.
Most vibration-based consulting to investigate reported problems are one-off in nature, the longer the equipment is in service, lower are the chances of obtaining machine details and their design drawings or engineering data. This said, the testing and investigation heavily relies on the experience and expertise of the person conducting the task; even so, there could be many sources of vibrations and their modulations that fall into the grey area of unknowns to effectively understand the root cause of the problem.
System response tests such as modal hammer approach help identify natural frequencies that are likely to relate the exhibited vibration responses to resonance conditions; however, the global natural frequencies may not explicitly depict the sub-system frequencies due to multiple factors, thus limiting the linkage of vibration behaviour to their probable sources.
order tracking outputs on a software display
Rotating machinery and more specifics like gearboxes, engines and such other complex equipment need a good mix of both the above approaches to understand and infer their vibration behaviour, with a caveat that the obtained data may still be insufficient to reveal the overall dynamic response of the equipment.
While FFT based vibration analysis is vastly used for stationary vibrations, it fits earnestly low into analysing rotating equipment of varying speeds and slew rates. The inherent fixed sampling approach of Fourier method also suffers from leakages, spectral voids and even fails to capture fast changing events in low inertia, high speed rotating equipment. The 2D data depiction on the frequency axis keeps varying with no appropriation to comprehend the data effectively.
Order tracking on the other hand is the technique of using an external sampling synchronised with the rotating equipment that helps overcome many of the hindrance of FFT methodology
Order tracking setup phylosophy
An order is defined as a vibration frequency normalized by shaft rotational speed, represented as,
Order=Rotational Frequency / Vibration Frequency
For example:
- Shaft speed = 3000 RPM
- Rotational frequency = 3000 / 60 = 50 Hz, which is the 1st order
even with the change in RPM, the rotational frequency may shift, but the order remains unchanged.
Since sampling in order tracking is "adaptive", meaning that the sampling frequency is directly defined by the reference rotating member to which a tachometer is configured to track the RPM. Given this arrangement of acquiring vibration data in synchronisation with RPM tracking, a whole lot of good things happen in the way the data gets understood and interpreted.
Firstly and most importantly, the whole limitations of fixed frequency sampling and their influence on data quality, leakage and inferior representation of vibrations responses due to non-stationary operating conditions are completely overcome by the order tracking approach. The other benefits are of able to infer the entire behaviour of the rotating system as a function or order and not as that of frequencies.
High slew rates, RPM variations, kinematics of complex configurations are all covered near correctly with the order tracking approach to move towards root causes of better credibility. From a single rotating system to a series of connected drive and driven rotating elements can all be assessed with lot more ease and credibility by this method.
NV Dynamics uses some of the industry proven tools for order tracking and analysis in a variety of applications, more aptly in engine testing, gearbox analysis and automotive evaluations. Our qualified hardware to acquire multi-location data along with laser RPM tracking devices combined reliable Test.lab based order processing suite makes a great combo offering some of the best assessments and root cause findings.