Vibration and modal analysis that keeps resonance away
Find the natural frequencies and mode shapes of your structure, see how it responds to the excitation it meets in service, and move resonances clear of running speeds before they cause noise, wear or cracking.
Every structure has natural frequencies at which it vibrates freely. If a motor, fan, pump, gear mesh or road input excites one of them, small forces produce large motion: brackets crack, fasteners loosen, sensors read noise and covers rattle. The fix is cheap on screen and expensive in service, because by then the cause is often a mounting arrangement repeated in every unit built.
More about vibration and modal analysis
Modal analysis finds the natural frequencies and the shape the structure takes at each one. That shows which modes sit near your excitation frequencies and which features control them. For a simple spring and mass, f = (1/2π)√(k/m), so frequency rises with stiffness and falls with mass; in a real structure, the mode shape shows where added stiffness or removed mass has the greatest effect. Harmonic response analysis then applies a sinusoidal load across a frequency range and predicts the amplitude of displacement, acceleration and stress, including the peak at resonance, which depends heavily on damping.
F-CAD®’s design engineers in Glasgow carry out the studies, and each one ends with a specific change, such as a rib, a gusset, a moved support or a stiffer mount, with its new frequencies predicted.
Typical work: motor and pump mounts, fan housings and guards, equipment skids, sensor and camera brackets, vehicle-mounted equipment, pipework supports and machine frames.
What a vibration study answers
What are the natural frequencies?
The lowest modes of the part or assembly as installed, with the frequency and mode shape of each.
Are any close to our excitation frequencies?
Natural frequencies set against running speed, blade-pass, gear-mesh and electrical frequencies, with the separation at each.
How large is the response at resonance?
Displacement, acceleration and stress amplitude across a frequency sweep, for the damping in your structure.
Which feature controls the problem mode?
The mode shape shows where the structure moves most and where it bends, which is where stiffening or mass changes work best.
Which modes will the excitation actually drive?
The direction and location of each mode’s motion shows which ones a given input will excite and which can be set aside.
Will the fix move the problem elsewhere?
Frequencies of the revised design across the whole range, so a stiffer bracket does not shift one resonance onto another excitation.
What we analyse
Modal analysis
Natural frequencies and mode shapes for parts and assemblies, with supports and attached masses represented as installed.
Harmonic response
Steady-state response to sinusoidal forces or support motion over a frequency range, with damping applied.
Resonance avoidance
Natural frequencies compared with every excitation source in the operating range, at the margin your standard or customer specifies.
Stiffening and tuning
Ribs, gussets, section changes, support positions and mass changes compared on how far they move the critical modes.
Mounting and isolation
Flexible mounts and isolators represented by their stiffness, to place the mounted frequency well below the excitation and cut transmitted vibration.
Vibration stresses
Stress amplitudes from the harmonic response, ready to feed a fatigue assessment of the vibrating part.
Before you start
What to send us
- CAD of the structure, including mounts and significant attached masses
- How it is supported or bolted down in service
- Excitation sources: running speeds, numbers of blades or teeth, electrical frequencies
- Excitation amplitude, if known, for harmonic response
- Damping data, or the type of construction so a typical value can be used
- Any frequency targets or separation margins you need to meet
How far should natural frequencies be from the excitation?
That depends on the damping and on the code or customer requirement, so we work to the margin you specify. Where none is given, we will agree a sensible target for your type of equipment with you.
Do flexible mounts make vibration better or worse?
Either. An isolator only reduces transmitted vibration when the excitation frequency is above about √2, roughly 1.4, times the mounted natural frequency. Below that it amplifies the motion, so mount stiffness has to be chosen with care.
Why does damping matter so much?
At resonance, damping alone limits the response. With 2 % of critical damping, the peak response to a sinusoidal force is about 25 times the static deflection; with 5 %, about 10 times.
Is a modal analysis enough on its own?
Often, yes. If every natural frequency sits well clear of the excitation, there is little more to learn. Harmonic response is worth adding when a mode cannot be moved and you need to know whether the resulting amplitude is acceptable.
Send us the part that shakes
