[FAQ] Vibration Isolators

How do I select the correct size isolator?

There are several good tutorials by isolator manufacturers, that describe how to select the correct size of isolator for you particular application. Look under the links for vibration isolator manufacturers.

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There are so many to choose from, which sort is the best??

Once you have been through the selection charts and calculations to decide the required spring rate for your application, you have to select a type of isolator. There are many different styles to choose from, and they can perform all the same function.

HOWEVER, there is one little detail that the isolator manufacturers never tell you. Many of the fancy designed isolators require that they are setup with a great deal of precision, so that they will operate correctly.

I have see hundreds of isolators that have been incorrectly installed, which resulted in the housing of the isolator hitting something, and short circuiting the vibration isolation.

Many isolators have built-in seismic restraints that cause the isolation to bottom-out when there are large excursions during seismic events. In my experience, this type of isolator has the worst rate of correct installation. These isolators are also poorly designed in terms of maintenance, because they are difficult to inspect to ensure that they are functioning correctly.

The best type of isolator that I have seen, in terms of reliable installation and for ease of inspection, is the plain vanilla big fat spring. You can get them in sizes for just about any load rating.

The seismic restraints that can be use with a big spring look like a tag, with one end welded to the machine, and the other end is bolted to the ground. It is important that you can clearly see the operation of the seismic restraint, to ensure that it is not fouling on anything.

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Well that is not really for me to say. However, be wary of companies that make claims like “the world’s best vibration isolators.” After all, how much technology can going into a spring? There are a couple of companies that make special isolators for optical instruments. Hence, they are quite different from the companies that make the run of the mill vibration isolator for industrial equipment.

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I want vibration isolation, but I don’t want my equipment to move

This is the fundamental problem with vibration isolation. In order to get significant vibration isolation, the resonance frequency of the system has to be low, which means that the springs are usually soft and hence the equipment will move easily.

If you have rotating machinery that vibrated mostly at a single frequency, you have to make sure the run speed is greater that sqrt(2) time the resonance frequency of the unit mounted on the springs.

If you are concerned about the displacement at the resonance frequency of the system, your alternative is to use something with higher damping properties, rather than metal springs that have very little damping. You can use neoprene pads or cork in some cases. It all depends on the environment that it is used. One common application is for pumps that start and stop frequently, as the shaft on the pump increases speed and passes through the resonance, the vibration is transmitted into the building and annoys the occupants. This frequently occurs on the penthouse level where the execs get annoyed with the mechanical equipment installed on the floor above! The alternative is to use neoprene pads that have greater damping, but usually not isolate as well. It is all a trade off game.

The other thing to be very wary of is the attachment of piping to machinery and then hard mounting the piping to the walls, which effectively short circuits your expensive vibration isolation package. Piping should be sprung mounted or use flexible connectors.

If you are designing new equipment or an installation of equipment that has vibration requirements, I cannot stress enough, get professional help *at the beginning* of the project, not at the end. It sounds like an obvious thing to do, but I am constantly staggered at how often this simple advice is ignored. Often vibration and noise concerns are a system level problem, which means that the installation of vibration isolation will affect another component. Often at the end of the project, all the design decisions have been made and hence there are few options for vibration isolation, and hence the isolation that can be achieved is severely limited.

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