Operating Deflection Shape (ODS) Characterization


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Example operating deflection shape (ODS) animation.

Click to view example operating deflection shape (ODS) animation.

MSI performs Operating Deflection Shape (ODS) characterization to measure a mechanical system’s vibratory response to operational excitation.  For rotating machinery in particular, this helps to diagnose problems ranging from imbalance and misalignment, to looseness, soft foot, resonance, and more.

To measure the operating deflection shape, the system must be run at steady state conditions for an extended period of time, typically one to several hours depending on the complexity and size of the system.  Depending on the type of machinery involved, this often means constant speed, power, flow, pressure, etc.

One accelerometer is placed at a location of known high vibration to serve as the “reference” accelerometer.  Triaxial accelerometers–typically up to four at a time–are then placed at strategic locations around the system while the signals are recorded for approximately one minute.

ODS Measurement Hardware

Hardware used to measure the operating deflection shape, including a laptop, data acquisition system, a single axis reference accelerometer, and multiple triaxial accelerometers.

After each recording, each triaxial accelerometer is moved to a new location, while the reference accelerometer remains in place.  A new recording is captured and the process is repeated until a comprehensive mapping of the system has been created.  Often, measurements are created at tens or even hundreds of locations, with the process expedited by using multiple triaxial accelerometers to reduce the number of recordings.

Magnitude spectra and relative phase spectra (relative to the phase of the reference accelerometer) are computed for the entire dataset.  The number of spectra is equal to the number of recordings × the number of triaxial accelerometers × 3 directions per accelerometer.  These spectra are mapped onto a CAD model of the system and animated at frequencies of interest to characterize the operating deflection shape.  For rotating systems, the motion of the shaft can be included using data collected from proximity probes.

Imbalance and misalignment can be identified by animating the CAD model using spectra data at 1X and 2X the running speed.

ODS animation of a machine train with an unbalanced rotor.

Click to view ODS animation of a machine train with an unbalanced rotor.

 

ODS animation of a machine train with a parallel misalignment between elements.

Click to view ODS animation of a machine train with a parallel misalignment between elements.

 

ODS animation of a machine train with an angular misalignment between elements.

Click to view ODS animation of a machine train with an angular misalignment between elements.

 

Looseness and soft foot can be similarly diagnosed by focusing on measurement locations on either side of a bolted connected, or where the machine meets the foundation.

ODS animation showing looseness between a pump and its pedestal.

Click to view ODS animation showing looseness between a pump and its pedestal.

Resonance can be identified by animating the CAD model at the running speed and comparing the motion to known mode shapes.

Characterizing the ODS of mechanical systems and rotating machinery is an important tool used to troubleshoot vibration problems.  Many examples of its value can be found by exploring our case studies.  If you’re interested in learning more about this testing method, or wish to discuss your needs with an engineer, please contact us.