Vibration Isolatio

Vibration Isolation Terminology

Important Vibration Isolation Terminology

Understanding a few terms used in the vibration isolation industry will help you sort through all the noise when you are comparing one vibration isolation solution to another.

Instead of regurgitating a lengthy list of terms and definitions from an engineering textbook, we have chosen just a few terms to discuss. We believe these are the issues that are important to you when addressing your vibration challenges.

What percentage of vibrations are cutoff by an isolation bearing or device? Transmissibility curves can help answer this question.

How fast will the isolated equipment settle when it is set in motion? In other words, what is the isolation bearing's settling time?

A transmissibility curve plots the percentage of vibrations that pass through - - or are transmitted through - - a particular isolation bearing or device over a range of particular frequencies. For instance, if more than 100% of input vibrations are passed through the isolation bearing, then vibrations are amplified; but if less than 100% of the input vibrations are passed through, then the bearing isolates.

In order to generate a transmissibility curve, measurements are taken at two points: one point to determine the vibration input and at another point to determine the vibration output. These two measurements will be taken on opposite sides of the "plane of isolation". The plane of isolation is defined by the location of the isolation bearing. For instance, if one were interested in the transmissibility curve for a table top Vibration Isolation Platform used to isolate an optical microscope, the input measurement would be taken on the table top or floor, and the output measurement would be taken on top of the Vibration Isolation Platform upon which the microscope sits.

The output measurements are divided by the input measurements for a series of input vibration frequencies and with these points, the plot is created.

These two measurements could be taken and plotted for vibration displacement, acceleration or velocity - - whichever criteria one were interested in.

Transmissibility curves are used extensively in the vibration control industry and provide information useful to decision makers comparing one isolation product to another. The following information can be obtained by looking at transmissibility curves and should be used to compare one isolation product to another:

The isolation bearing's natural frequency
This is the vibration frequency at which the isolation bearing resonates and will amplify input vibrations the most. In general, the lower a bearing's natural frequency, the more effective it will isolate at every frequency which is 1.41 times higher than the bearing's natural frequency.

The isolation bearing's damping characteristics
This is proportional to the dissipation of vibration energy. Only damping reduces amplification at resonance, and is desirable so long as the bearing is not over-damped. At high frequencies, damping works against isolation

Isolation quality
The percentage of vibrations that are not transmitted through the bearing and therefore do not cross the plane of isolation. The steeper the curb drops to the right of the natural frequency on the transmissibility curve, the more effective the isolation bearing is.

Is That Transmissibility Curve for the Horizontal or Vertical Vibrations?
This is a good question and one that is often overlooked. If a transmissibility curve does not designate whether it is a plot of the bearings response to vertical or horizontal vibrations, then the practice is that it is for vertical vibrations only.

At Vistek, we recognize that the two components are different and should each be addressed. For instance, in an instrument such as an optical microscope, horizontal vibrations will shake the image left to right and vertical vibrations will cause the image to go in and out of focus. The image becomes jumpy or blurry accordingly. For this reason, we will always provide separate transmissibility curves for our bearings response to each component.

The following curves are for provided as an example and are for our series 300 and 3000 Vibration Isolation Platforms. The data for the air system is for a tabletop air platform. For specific information on a particular product manufactured by a company that sells air isolation technology based products, we recommend that you contact them directly.

Isolation Efficiency

Vistek VIPAir Platform
@ 5Hz
Vertical60%25 to 50%
Horizontal98%0 to 30%
@ 10Hz
Vertical85%60 to 90%
Horizontal99%30 to 80%
If a piece of equipment is isolated but not damped, then in theory it would oscillate forever on top of the isolation bearings once set in motion and then left to settle on its own. (An object in motion will stay in motion until an outside force acts on it. Newton's second Law of Motion). The isolated equipment could be set in motion by manual operation, inadvertent bumping, or motion induced vibrations originating above the plane of isolation, for instance from a moving stage. All isolation systems possess some minimal level of damping and therefore the isolated equipment will eventually settle. However, long settling times can be disruptive to a process and, at the least, annoying.

Settling time is the number of cycles it takes for an isolated object to come to rest once it is set in motion. Settling time is shortened by added damping.

Attenuation is analogous to damping. Attenuation is the measure of consecutive amplified peak reduction in real time observable in a vibration displacement time history. The higher a bearing's attenuation is, the faster the settling time. For instance, 100% damping prevents any vibration. A bearing with 35% to 100% damping causes the motion above the plane of isolation to die out within the first cycle. Damping of 18% to 32% causes the motion to die out within the second cycle; damping of 14% to 18% causes motion to die out within the third cycle, and so on. The effect is linear. A bearing with only 1% damping level will go through 47 cycles before it dies out.