Vibration and Shock Isolation
Vibration and Shock Isolation
DEICON, Inc. was invited to make a presentation on shock and vibration isolation at the global superyacht forum in Amsterdam. The following is an excerpt of the talk.
There exist a number of different isolation schemes, e.g., single mounting, double mounting, active isolation, semi-active isolation, air mounting, etc., with different pros and cons. Either one of these schemes can use rubber, air, or even metal (steel) as the isolation medium. Each one of these schemes and media have their own pros and cons. Shock and vibration isolation schemes are required to:
- reduce the propagation of base vibration to the isolated object (machinery),
- abate the transmission of vibration energy of machinery to the hull, and
- lower the impact of shock from the hull to isolated object or vice-versa.
By varying the two main attributes of an isolator, i.e., its stiffness and damping, as well as the mass of the isolated machine, one can either emphasize the achievement of one of above mentioned isolation requirements at the expense of others, or optimize the achievement of all the requirements with moderate levels of effectiveness. The latter is the commonly used approach by designers. The following web page provides a good overview of ‘vibration isolation’ http://www.deicon.com/vibration-isolation .
The metric for measuring the performance of an isolation scheme is what is known as ‘transmissibility’ which is a measure of how much of the vibration force is being transmitted from the isolated machine to the base (hull of a yacht) at various frequencies. ‘Transmissibility’ is also the measure of how much of the motion of the base (support structure, e.g., the hull in a watercraft) will be transmitted to the machine. The goal is lowering the transmissibility at the frequencies where the vibration energy lies, as much as possible, without causing the machine to experience excessive motion.
As indicated in the vibration isolation page (http://www.deicon.com/vibration-isolation ), softer isolators with negligible damping will have the lowest transmissibility at off-resonant frequencies. Their excessive transmissibility at resonance is normally addressed by having the resonant frequencies well below the vibration excitation frequencies. Thus, the softer and the more underdamped the mount, the higher is its vibration isolation erformance. Unfortunately, the improved isolation using soft and highly underdamped isolators is achieved at the expense of excessive low-frequency motion of the isolated machine in response to shock disturbances; we all have seen how a Diesel engine experiences excessive undesirable motion during start up and shut down, straining all the plumbing and wiring connections to the engine. On the other hand, stiffer mounts with high damping are good in tightly holding the isolated machine and thus avoid excessive motion, but they transmit most of the vibration to the support structure.
No one passive solution quite satisfies all the requirements of an ideal isolation system. The common practice used by isolation system designer has been centered around a compromise design which satisfies all the requirements to some degree, but not to the highest possible degree. A number of enhancements to the plain passive isolation have been proposed over the years but again none satisfies all the requirements of an ideal isolation. For example double (two-stage) mounting, while effective at high frequencies, has no better low-frequency isolation effectiveness than a single mounting system. Shock isolation of double mounting is also inferior to that of single mounting. In addition, double mounting imposes unfavorable weight and space penalties.