Vibration Isolation Theory
Vibration is a physical movement based on simple harmonic motion where the frequency of an oscillation is constant. A tunning fork is an example of a physical item which when struck will vibrate at a constant frequency but with a decreasing, or decaying amplitude as the volume decreases with time. If a disturbing force is also present, then the amplitude of such a vibration can be alterred or maintained at a constant level - a steady state vibration. In some instances such as vibratory screening plant, vibratory feeders etc, vibration is a desired effect within the plant although isolation is usually required to prevent this from being passed through to a supporting structure.
Although vibration can have a positive use, it can also give rise to fatigue failure within rigid structures. The use of vibration isolation mounts overcomes this issue by allowing flexible movement and reducing the stress placed on the supporting structure.
This describes the level of vibration passing through (or transmitted through) an isolation system. In some applications the level of isolation required may be defined by the architect. For example, the air handling system above a hospital operating theatre might have a required transmissibility figure of 0.01. This is a ratio of the force being imparted onto a mounting (input force) to the force passing through it (transmitted force) into a supporting structure. % Isolation = (1 - T) x 100. If T = 1 then there is no isolation, T = 0 represents perfect isolation (no transmission).
Resonance occurs when the natural frequancy of a system is acted upon by a forcing frequency with a similar frequency or multiples of this. A child's swing is a good example of a resonant frequency. To maintain a uniform swing rate (frequency), it is not necessary to push each time the swing returns to its high point. A uniform frequency can be maintained by providing a small push every 2 or three cycles. The frequency is dependant only on the length of the ropes or chains which make up the swing. The longer the ropes, the lower is the frequancy and vice versa. Clearly if the forcing frequency is in between these integer multiples, then the uniform cycle will be disturbed.
Damping refers to any system or process which causes the oscillation amplitude within a vibrating system to decay. This could be inherent in the system such as in the child's swing example where, without any force input, the amplitude of the swing will reduce due only to the damping of the movement by the surrounding air. In a car suspension system, the damping of the road springs is provided by an oil filled cylinder with a small hole in the piston through which the oil must pass to permit movement. Otherwise referred to as dampers. Based on the principles of energy conservation, the energy required to force oil through a hole or deform a rubber element is converted to thermal energy. This is why it is usually best to avoid highly damped materials as excessive cycling can give rise to excessive temperatures which could reduce the product life at best.
In terms of vibration isolation, the best results are usually achieved with very little damping as in the case of steel springs which have very low damping. The transmissibility graph above shows that although low damping systems have better isolation beyond a fixed point on the graph, they also increase transmission below that point. A system with zero damping would in theory have an infinitely high resonance peak. The theoretical maximum transmissibility is 1/(2 x damping ratio)
Vibration Isolation - common solutions to practical problems are suggested here:
- Electricity Generator Sets (within an office building, hotel or vessel) - Spring Based AV Mounts
- Electricity Generator Sets (within an industrial environment) - Rubber based AV Mounts
- Engine mountings - typically rubber/metal based.
- Pumps and motor driven devices - typically rubber/metal mounts.
- Marine diesel engine in a sub-marine - wire rope isolators (Vibrostop Cavoflex)
- Vibratory plant - a wide choice is available including rubber/metal, spring and reinforced rubber springs.
- High precision laser systems - high deflection springs or air based mounts (Vibrostop Pneumofix)
Considerations When Selecting a Vibration Isolator:
Before selecting an isolator, it is first advisable to check that this is necessary. An external vibrating plant which is far removed from occupied premises may not need to be isolated at all. A second consideration is to check that the vibration is not due to an unintended out of balance which could be due to wear in a rotating shaft for example.
At the other end of the spectrum, vibrating plant which is in close proximity to occupied premises, particularly hotels or hospitals will require the best possible isolation, typically in excess of 95% and often better than 98% isolation.
The simplest and thus the most cost effective solution is often a rubber based mount fitted directly between the vibrating equipment and a mounting surface. As shown earlier, the level of isolation is directly related to the static deflection within the AV mounting. With rubber based AV Mounts the static deflection is typically limited to approximately 10% of the free height of the rubber material in the AV Mount. (although some items such as our SW-R range are optimised at 15% deflection). Clearly the actual deformation of the mounting is also related to the physical size of the AV mount. Very small mounts will have a relatively small static deflection and so will be unsuitable for isolation from low frequency vibration. Larger rubber based mounts can have deflections of 15mm or more and so can provide very high levels of isolation for many typical elecctric motor based applications. Reference to our isolation nomogram will help to clarify this.
Millitary Vibration Tests
For the most demanding of applications equipment can be tested through a wide range of frequencies and in all directions to expose any weakness in an isolation solution. In the typical example below it can be seen that at the 3 peak input values*, very good isolation is provided.
The types of tests cover the full range of frequencies likely to be encountered (the Wideband Random Spectrum) and also specific critical frequencies (*the Harmonic Swept Narrowbands). Typical input and output results are shown in the graph below for a successful isolation system. Clearly there is a resonant frequency (approx 6.5Hz) in this system which is identified by the tests.
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We specialise in all aspects of vibration and shock isolation using different rubber based materials, metals, springs, wire cable etc.
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