Damping Treatment
A damping treatment addresses the system response at resonance and is only part of the solution to most vibration issues. Often, making structural modifications to change the resonant characteristics is the first step, when possible, before a damping treatment is considered. The source of vibration is also commonly addressed to limit force input, either by vibration isolation or increasing stiffness to ground, or by using passive cancellation (tuned reaction mass) or active cancellation (such as servo controlled "self tuning" cancellation systems).
Time Domain Ringdown Comparison with and without Damping Treatment
Damping structural vibration involves creating an effective means of capturing and removing vibrational energy. Without expert knowledge this can be a frustrating experience when damping materials are patched on to the structure, under supporting feet, and laid over panels, and the problem does not seem to go away. What is missing is an understanding of how the structure is deforming and how to design the correct damping treatment. We have made the design and testing of damping treatments our specialty for over 30 years and we are still improving our repertoire of materials, devices, and methodologies.
FRF Comparison of Original System, with Constrained Layer Damping, and With Tuned Mass Damper (TMD)
FRF Comparison of Original System, with Constrained Layer Damping, and With Tuned Mass Damper (TMD)
To engineer a damping treatment we need to understand the structural dynamics, and that usually means performing Dynamic Testing and Modal Analysis Testing. This testing shows us how the structure is moving so we can evaluate what approach is best and where to attach a damping treatment. We cannot do this without having a good idea about the deformed shape the structure takes when the problematic vibration is present. Sometimes the shape is obvious and we will not need to do a full modal analysis. However, the dynamic testing is still necessary in order to quantify the level of improvement in response per unit of force applied over the frequency range of interest.
There are a number of potentially very effective passive and active damping treatments that each act in a different way to turn vibrational energy ultimately into heat. Each has very particular considerations. Sometimes we want to change the dynamics without adding damping. The nature of the vibration and/or acoustic noise problem and the deformations of the structure at the problem frequencies will dictate which damping treatment is the best approach. Some of the treatments we design are listed here:
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Viscoelastic damping
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Viscous damping
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Tuned mass damping
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Constrained layer damping
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Active damping
Elastomeric Damping - Elastomeric damping is perhaps most common. However, damping treatments using damped elastomer often fail, or are only partially effective. The reason for this is, in part, because the deformed shape and structural dynamics are not well understood, but also because of the very nature of these damped elastomers. The figure below is a diagram (nomogram) of the elastic and damping properties of a typical damped elastomer. The thing to note is that the damping changes with both disturbance frequency and temperature. Note that the peak of the hump on the damping curve lines up with the steepest slope of the elastic stiffness curve. This means that when you have optimal damping, the stiffness can change radically with temperature and disturbance frequency. This has profound consequences that can bite if we do not design well.
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How much damping do we need?
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How much stiffness do we need?
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How much change in damping and stiffness can we tolerate given the temperature and disturbance frequency changes?
The Nomogram for a Viscoelastic Material can be used to engineer a Damping Treatment knowing the dynamic characteristics of the Structure
When a steady load is applied to an elastomeric damping material we need to consider viscoelastic creep. This is the slow stretching of an elastomer under a steady load and is the non-linear part of the damped-elastomeric response. When we need precise positioning of a composite mechanical structure containing damped elastomer the elastomer will slowly "relax" as the static stress in the elastomer, that provides part of the restoring force, slowly decreases. This change in restoring force will cause the structure to shift. The effect is much like that of a traditional spring in series with a viscous dashpot. We can often engineer this expected shift to stay below allowable tolerances and still capture the benefits of the increased damping.
Viscoelastic Materials Creep under a constant load, similarly, Viscoelastic materials will Relax their resulting internal stress when held with a given amount of stretch. Knowing how to Engineer solutions with these damped materials requires experience and some theoretical and computational abilities
Viscous Damping has many advantages such as behaving linearly, having easily engineered damping values given shear or orifice geometry, no required loss of static (or low frequency) stiffness of the system when implemented well. The disadvantages are that such dampers are more work build in practice. You much contain the viscous fluid. Whereas with damped elastomers you can cut it out and stick it on, sandwich it between moving elements, injection mold it, or buy it in a myriad of forms and formulations for many applications. However, sometimes viscous damping is exactly what one needs.
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Tuned Mass Dampers (TMDs) are amazing devices when properly applied. They are also often designed improperly and misapplied. A tuned mass damper changes the structural dynamics in an engineered and predictable way to couple the damping of the TMD into the dynamic system. We have made many custom tuned dampers for a variety of situations from damping hundred pound optics used in the production of the laser glass for the NIF laser fusion project at Lawrence Livermore Labs, to tiny dampers for the optics of interferometers used for semiconductor inspection.
FRF Comparison of Original System, with Constrained Layer Damping, and With Tuned Mass Damper (TMD)
Each of these damping treatment types has its benefits and pitfalls. We have the experience and hard-earned skill set to model, design, and prototype the appropriate damping treatments as an important part of the solution to problems with system dynamics.
Tuned Reaction Masses (TRMs) are also amazing devices when properly applied. We recently designed tuned reaction masses to reduce the vibration response from a large vibration source located near the top of a 6 story building. This is what we mean by "passive cancellation". These devices are amazing. Getting the details right and knowing the structural dynamics of the existing system through careful testing are key to project success. We modeled the building to match experimental dynamic tests, including Modal Analysis and ODS Testing. We then created a Finite Element Model that we could use to evaluate various stiffening options, custom vibration isolation options for the vibrating multi-ton equipment, and tuned reaction masses (TRMs).
The animation below is a modal analysis representation of the top three floors of 6 story building supporting a multi-ton piece of process equipment located on the 5th floor (represented as the red elements with large exaggerated motions to make the floor system deformations visible, all levels were included in the modal). We performed strain gage testing at critical locations on many of the structural members based on our original ODS Testing. The stain data allowed a structural engineering team to insure that the building was no in danger of structural failure. The floor system vibrated enough, however, that workers were not comfortable in the building and this was a problem for the client. We came up with a suitable design specification for allowable floor vibration levels for worker comfort and safety and went to work engineering solution options for the structural team to meet those specifications.
Our FEA model of a 6 story building supporting a large vibration source was guides and tuned using our experimental testing, modal analysis, and dynamic analysis
We developed an FEA model for this 6 story building which was guided by our on-site experimental modal analysis, many dynamic stiffness tests, and ODS testing. We evaluated structural modifications that included major stiffening members to achieve the desired floor vibration levels. We evaluated custom vibration isolation options for mounting of the vibration source on the 5th floor. The most cost effective solution, however, was the use of tuned reaction masses (TRMs).
Our extensive Client List represents hundreds of projects, some involving days of work, others lasting several months, over the last 30+ years. Descriptions of many of our projects can be found in the left hand links under the various topics that describe our test, analysis, and design work. Our website is put together by our engineers.