Operating Deflection Shape, ODS

A Study of a Bad Foundation
By Allen Plymon

Due to the recent upturn in sales, a wire manufacturing facility needed to increase its production of metal cased conductors. In order to increase the production, the operating speed of the metal forming machines needed to be increased from 900 RPM to 1,200 RPM. However, when operated at 1,200 RPM, the forming machines experienced abnormal vibration. Vibration analysis was performed and levels approaching 1 in/sec 0-PK were recorded at the new fundamental (operating) frequency of the machine, 1200 RPM (20 Hz).  These elevated levels are generally due to either mass unbalance or the unit operating at or near a natural frequency.

The easy solution to mass unbalance correction was to attempt to balance the unit in place. These efforts were successful at the original operating speed of 900 RPM; however, at 1,200 RPM, the vibration levels would return to unacceptable levels – thus eliminating mass unbalance as the root cause. The plant vibration group elected to utilize Operating Deflection Shape analysis of the metal forming machines.

The following image is a photograph of the unit:

Operating Deflection Shape (ODS) is measured with a machine functioning under its normal operational condition.  This type of analysis measures the machine’s response at a specific time or frequency.  Both amplitude and phase information are collected at various locations on the structure and utilizing a special software program, the vibrating “shape” or response of the machine can be animated.  These animations reveal “how” the machine is moving during normal operation.  Note that this is not necessarily a resonant response of the machine, but its operational response.  The forces within the machine are responsible for the motion, or shape of motion measured with this analytical tool.  For example, the unbalance response of any rotating system will produce a response or driving force at 1x RPM.  Misalignment and looseness generally produce synchronous multiples of running speed (2x RPM, 3x RPM…).  Machine information such as operating speed(s), belt and sheave information, blade, vane or gear tooth counts and past structural modifications to the original design are all important for precise diagnoses.

The data collection points and directions are defined based on experience with machinery and how beam, plate and shell structures will bend, twist and deform under vibration forces.  The main concern is collecting enough measurement points to accurately “visualize” the motion.  Another concern is properly measuring across bolted interfaces such that relative motion of the joints, if present can be seen.

Based on these concepts, the following model was constructed where acquired data is assigned and eventually animated:

     

 

 

Just under 200 points were acquired in order to provide the required resolution of the ODS animation.  Data was collected in the X, Y and Z planes on Channel 1 (reference point) and Channels 2, 3 and 4 at each of the remaining 199 points in the model.  The data was acquired and then stored in the analysis software on the PC.  This information was processed through an extensive matrix where orientation polarity was established consistent with field measurements.  Frequency Response Function (FRF) data was calculated and is defined as the measure of magnitude and phase of the output (Channels 2,3 and 4) as a function of frequency in comparison with the input (Channel 1-Reference).

Finally, this FRF matrix is imported into the animation software and points matched for animation.  With the resulting animation, a thorough study can be performed of the movement of the machine.

To summarize what was expected to be seen, is the frame flexing, which would point to a possible structural resonance and the need to shut down the machine and perform additional impact testing for further analysis.  Some engineering work had already been performed on how to modify the frame structure, based on this belief.

The following movie file links represent multiple views of the machines’ Operation Deflection Shape while operating at 1,000 RPM with a full coil installed.  Please note that the extended dark gray region surrounding the structure represents the main production floor and the rectangular center portion (lighter gray) is where the poured concrete base is inserted.  In addition, the yellow coil guard (entry) is also provided to orient the view of the machine.

3D2 View

There are definitely areas of interest based upon the resultant animations.  These are as follows:

  1. The most pronounced movement is found to be the poured concrete base on which the Machine is mounted relative to the main floor.  As a result, the Machine itself is moving excessively due to its mounting onto this concrete base.
  2. The movement is primarily radial and moves similar to a cantilever with motion at its lowest at the bottom of the machine and levels increasing to the maximum deflection toward the top of the machine.
  3. Motion indicates either a poor attachment of the inertia base or an inadequate sized inertia base for the machine.
  4. The main floor exhibits very low vibration relative to the excessive movement of the machine and machine base.
  5. The frame, bottom machine mounting plate, and inertia base connections as a unit is all in phase and reveal no disconnects or appreciable changes in amplitudes.  This verifies no issues related to the base integrity or soft foot condition.

Further testing revealed a natural frequency at 1,200 CPM (desired operating speed) and that the structural steel was not the source of the elevated vibration.   The concrete pad on which the unit is mounted was rocking on the soil beneath, indicating poor soil compaction.  Upon reviewing the base schematic, it was determined to be insufficient to properly support the forces present on the machine.

Thanks to Operating Deflection Shape analysis capabilities, the solution to the problem was very evident.