Glossary term
Operating Deflection Shape
Forced-response shape of a structure or machine at a specific operating frequency, speed, load or time condition.
Definition
conceptAn operating deflection shape is the measured or animated forced-response shape of a structure or machine at a specific operating frequency, speed, load, phase reference or time condition.
An operating deflection shape, often abbreviated ODS, shows how measurement points move relative to one another during actual forced response. Unlike a mode shape, it can contain contributions from several modes, forcing patterns, boundary conditions and operating loads. ODS is used in vibration diagnostics, troubleshooting and validation because it shows what the structure is doing under the condition being measured.
An operating deflection shape (ODS) is the shape a structure or machine appears to take while it is responding under a specific operating condition. It may be measured at one frequency line, one shaft order, one phase reference, one load state or one instant in time.
For measurement point i, an ODS at frequency f can be represented by a complex response:
The set of responses across measurement points forms the operating shape:
The shape is a forced response. It depends on excitation, frequency, damping, boundary conditions, sensor locations, operating load and phase reference.
Engineering Role
ODS is useful when engineers need to see how a machine, structure, pipework system, floor, panel, bracket, hull or support actually moves during operation. It is common in rotating machinery diagnostics, run-up testing, noise and vibration troubleshooting, structural modification studies, shaker tests, acoustic response investigations and field validation.
An ODS can reveal:
- a flexible support rocking at 1x shaft speed;
- a floor or frame amplifying a machine order;
- a panel moving in a forced bending pattern;
- a pipe span responding to pump or flow excitation;
- a local bracket motion hidden by a global vibration measurement;
- a boundary-condition or support-stiffness problem;
- a response shape that differs from the expected finite-element or modal-test shape.
ODS is especially useful when combined with order tracking, waterfall spectra, tachometer phase, FRFs, known mode shapes and operating data. It helps answer “what is moving together?” before the engineer decides why it is moving.
Worked Example: Normalize an Operating Shape
A machine base is measured at three points during steady operation. A tachometer confirms that the dominant response is the 1x order at:
The measured displacement amplitudes and phases, referenced to the once-per-revolution mark, are:
| Point | Displacement amplitude | Phase |
|---|---|---|
| A | 18\ \text{micrometres} | 0^\circ |
| B | 42\ \text{micrometres} | -35^\circ |
| C | 30\ \text{micrometres} | -80^\circ |
Normalize the amplitudes by the largest measured amplitude:
Point A:
Point B:
Point C:
The normalized ODS can be reported as:
Engineering comment: point B has the largest operating displacement at this condition, and point C lags point A by 80^\circ. The phase differences matter. If the points were assumed to move in phase, the animated shape would be misleading.
If the displacement amplitudes are peak values, the peak velocity at point B is:
or:
The sinusoidal RMS velocity is:
Engineering comment: the ODS shows the response pattern, while the velocity calculation connects the largest point to a vibration severity quantity. The result is valid only for the measured operating condition, frequency, sensor directions and phase reference.
Distinction from Related Terms
Operating deflection shape is not a mode shape. A mode shape is a natural deformation pattern of the structure. An ODS is the forced response under a particular excitation and may combine several modes.
Operating deflection shape is not operational modal analysis. OMA estimates modal properties from output-only data. ODS visualizes measured operating response and does not by itself separate natural modes from forcing patterns.
Operating deflection shape is not an FRF. An FRF is a response/input ratio with measured or defined excitation. ODS can be built from response data alone or from response at selected FRF frequencies, but it is a shape display, not the full response function.
Operating deflection shape is not order tracking. Order tracking identifies response by shaft order. An ODS can be extracted at a tracked order, but it adds spatial amplitude and phase across measurement points.
Operating deflection shape is not finite-element mode animation. It may be compared with finite-element modes or operating response predictions, but it is measured or reconstructed from operating data.
Validation and Common Mistakes
A defensible ODS states sensor locations, coordinate directions, calibration, amplitude unit, phase reference, operating speed or frequency, load state, filtering, averaging, response quantity, normalization, animation scaling and uncertainty.
Common mistakes include:
- calling an operating response shape a mode shape without modal identification evidence;
- animating magnitude-only data without phase;
- mixing sensor directions or coordinate conventions;
- comparing ODS measurements taken at different speeds, loads or boundary conditions;
- ignoring sensor mass loading, mounting stiffness, cable motion or weak signal-to-noise ratio;
- interpreting a high-response point as the forcing source without phase, FRF or operating evidence;
- assuming an ODS at one frequency represents the structure over the full operating range.