,, Natural Modes, finite element analyst, prototype engineer
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18.5kW Boost Blower Platform, Modal Analysis

Using a preliminary design from our client, MDA designed this platform to European specifications for use in Romania. Our design and analysis led to a 35% weight reduction over the preliminary design significantly decreasing material and construction costs. The platform will be mounted to an existing skid and supports two 18.5 [ kW ] boost blowers operating at 49.08 [ Hz ]. Modal and frequency response analyses were performed to determine structural response to the blowers. Modal analysis revealed seven modes in the 45-55 [ Hz ] frequency range. The figure below presents the natural mode at 49.07 [ Hz ] demonstrating the pipe and braces are strongest participants in the mode. For clarity, grating is not shown. The displaced mode shape is scaled 5x. The displacement magnitude has no meaning; it is relative. Read more on our Mechanical Design and Frequency Response pages.

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350HP Vacuum Pump Baseplate, Modal Analysis

MDA designed and structurally substantiated this platform mounted baseplate to support a 350 [ HP ] motor and vacuum pump operating at 11.9 [ Hz ]. Massless rigid beams connect nodal masses (located at the pump and motor centroids) representing the pump and motor weights to the baseplate. The natural frequencies of the pump/baseplate/platform system are calcuated to determine any structural modes near the pump operating frequency of 11.9 [ Hz ]. The figure below shows the mode shape at a frequency of 10.39 [ Hz ]. This is the only structurally significant mode close to the operating frequency. The displacement magnitude has no meaning; it is relative. Read more about this analysis on our Linear Static and Frequency Response pages.

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350HP Vacuum Pump Platform, Modal Analysis

MDA designed and structurally substantiated this platform used to mount three 350 [ HP ] motors and vacuum pumps operating at 11.9 [ Hz ]. The analysis included the baseplates and rotating equipment masses. Modal analysis was performed to determine any natural frequencies near the operating frequency. The figure below shows the mode shape at a frequency of 10.39 [ Hz ]. This is the only structurally significant mode near the operating frequency. The displacement magnitude has no meaning; it is relative. Read more about this analysis on our Linear Static and Frequency Response pages.

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Industrial Fan, Modal Analysis

MDA performed modal analysis on this industrial fan. The modal analysis provided the natural frequencies of the fan assembly. The figure below shows the mode shape at a frequency of 9.55 [ Hz ]. The displacement magnitude has no meaning; it is relative. The modes close to the operating frequency were then used in the frequency response analysis to determine true displacements and stresses. Read more about this analysis on our Frequency Response page.

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Hydraulic Oil Cooler, Modal Analysis

MDA performed modal analysis on this truck chassis mounted hydraulic oil cooler. Our client was having field issues with a panel retaining bolt loosening at the location indicated by the arrow in the figure below. They recreated the problem on a shaker table in the 30-40 [ Hz ] range. Our analysis confirmed large shear forces being transmitted between the two panels connected by the bolt at a frequency of 36 [ Hz ]. The figure below shows the mode at 36 [ Hz ]. The displacement magnitude has no meaning; it is relative. We recommended design changes to the panel to redistribute the shear forces in the assembly thereby decreasing the fastener load and validated this with revised modal and frequency response analyses.

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About Modal Analysis

All structures have particular frequencies at which they will vibrate with large amplitudes when subjected to relatively small dynamic loads. These frequencies are referred to as natural frequencies, and the shape which the structure takes up at each frequency is referred to as the mode shape. For a structure subjected to dynamic loads, it is important to know the natural frequencies in order to confirm the applied loads/environment will not excite any structural modes, but this is frequently overlooked. Modal analysis involves determining the natural frequencies and corresponding mode shapes of a structure. Modal analysis also reports displacements of the mode shape. However, these displacements are only relative displacements to assist in visualization of the mode shapes. The true displacements of the mode shape are dependent on the applied forces and the frequencies at which those forces are applied. A frequency response analysis must be performed to determine true structural response (stresses and displacements). When a structure is not of simple geometry, finite element analysis (FEA) is used to perform a modal analysis which determines the natural frequencies and mode shapes of the structure. As the first part of a dynamic analysis, the natural frequencies are compared to the frequencies of any excitation loads to ensure they are sufficiently separated. When performing this stage of a dynamic analysis on rotating equipment, it is important to consider that imperfections (e.g. misalignment of a shaft, or a bent shaft) can result in forces acting at frequencies which are a multiple of the rotational speed of the shaft. It is common for specifications for equipment which have significant dynamic loads to state that the equipment must have no structural modes within a specified frequency range.

MDA and Modal Analyses

MDA has performed dozens of modal analyses. A brief list is shown below:

  • Skid mounted generator sets
  • Skid mounted pumps
  • Industrial fans
  • Robotic components
  • Industrial coolers
  • Platforms, stacks, & derricks
  • Floors of buildings
  • Diesel truck chassis and engine components
  • Heavy machinery
  • Component parts
  • Complex assemblies

In some projects, modal analysis identifies the design has natural frequencies which are too close to potential excitation frequencies, and hence needs to be revised. In these instances, when requested, MDA works with our client to improve the design so it complies with requirements. Once the modal analysis has been performed, a frequency response analysis may be performed to determine the magnitudes of displacements and stresses. Read more on our Frequency Response page. Click here to view an example of an analysis report we provide.

Our FEA Capabilities
  • Linear
  • Static stress & deflection
  • Dynamic stress & deflection
  • Critical buckling load
  • Nonlinear
  • Thermal
  • Topology optimization
  • Size optimization
  • Geometric
  • Material
  • Contact
  • Postbuckling (Riks)
  • Mechanical event simulation
  • Dynamic stress & deflection
  • Dynamic
  • Modal analysis
  • Frequency response
  • Time response
  • Response spectrum
  • Random vibration
  • Transient stress
  • Explicit & implicit
  • Drop and direct impact events
  • Rotor dynamics
  • Shock and seismic
  • Power train vibration analysis
  • Fatigue life & durability