TechArchive

Original article date: October 1998

Vibration and shock can cause as many problems for sensitive instruments and other extreme environments such as heat and pressure. So it is often vital to invest in vibration control equipment.

Vibration even when undetectable to human senses can upset the accuracy of sensitive instruments. The more damaging vibration tends to be at low frequency and is sometimes hardest to detect. Problems perhaps involving precision work or electronics are often attributed to other causes when the real culprit is ground-borne vibration even from far afield.

Fabreeka’s Precision Aire Table (PAT) comprises what is essentially a substitute support frame and floats the instrument on a cushion of compressed air. More specifically it is mounted on a series of pressurised bellow-like diaphragms which isolate the unit from direct ground contact.

Apart of safeguarding accuracy its use in factory applications can also improve on-line efficiency. It enables instruments such as the coordinate measuring machine shown in Fig 1 to be sited where they are needed at the point of production where shock vibration can be most severe.

Vibration can be deceptive and merely suppressing it is not the complete answer. Eliminating it – a different matter from dampening it down – is based on diagnosing its frequency and being able to neutralise it – in effect tuning it out. Called natural frequency different concretes and their underlying bedrock each transmit their own distinctive signature. Ignore it or worse misjudge it and the resultant tuning fork effect may even magnify the problem. An initial vibration survey is crucial and determines the right vibration to guild into the stand.

In efficiency terms measured by the reduction in transmitted vibration the PAT can achieve close to 100% even in ground-shaking conditions where instruments can still record micron differences with unwavering accuracy.

Questions About Vibration

What does it mean to isolate vibration?

Isolation refers to a reduction in transmitted vibratory forces or amplitude of motion.

How is isolation achieved?

It is achieved by placing an isolator (elastic element) between the unit vibrating and its support. This allows the inertia of the unit to oppose and thereby reduce the vibratory motion transmitted to the support.

What characteristics must an isolator have?

It must be (and remain ) elastic for the life of the installation. It must have the capacity to support the static weight of the unit as well as the unbalanced dynamic force. It must have a natural frequency lower than the offending unit’s disturbing frequency.

How do we determine the natural frequency of an isolator?

It is determined by a mathematical relationship depending on dynamic spring rate and the static weight of the isolated unit. Isolator manufacturers have this information readily available in their publications.

What natural frequency should an isolator have?

It depends on the desired percent reduction in transmitted vibration referred to as transmissibility and is governed by the ratio of disturbing frequency to isolator natural frequency. The larger this ratio the greater the reduction. Isolation begins at a ratio of 1.414.

How does this frequency ratio produce isolation?

It causes the isolated unit to be “out-of-phase” with the vibrating force thereby opposing it by virtue of its inertia. The greater the ratio the closer it gets to a 180deg “out-of-phase” condition. (Zero transmissibility)

Are there other ways to control or reduce vibration?

Yes there are:

• You can correct the cause of the problem namely the unbalanced condition that exists by dynamically balancing the machines rotating or reciprocating parts. However this may not be practical to do or cannot be done.
• The speed of the unit could be reduced to lower the disturbing force since the unbalanced centrifugal force is directly proportional to the square of the speed of the machine. Many times this cannot be done because the machine must run at the higher speed.
• If the option exists you can select to use a rotary rather than a reciprocating unit since a rotary unit generates less unbalanced force than a reciprocating unit.
• A relocation of the offending machine may be a solution if the vibration is not harmful to the machine itself but is a problem in the surrounding area.
• In the case of a sensitive piece of equipment being disturbed from an outside vibrating source you can modify the response of the equipment through structural changes for example by adding mass or increasing stiffness.

Does the use of an isolator change the disturbing frequency of the unit?

No it does not. It simply allows the inertia of the unit to oppose the dynamic unbalanced force or motion of the unit when the frequency ratio of the machine and “the-machine-on-its-isolators” exceeds 1.414.

What is natural frequency?

All physical systems have a natural frequency which is determined by the systems’ spring rate and weight. The natural frequency of a system is its response after being displaced from its equilibrium (static) position. It responds by vibrating freely until it dissipates all of the kinetic energy it received and stored (potential energy) due to its elastic nature. This free vibration is its natural response and consequently is referred to as its “natural frequency”. It is this natural response that opposes the unbalanced dynamic force generated by the equipment. The larger the difference in the frequency ratio i.e. disturbing-to-isolator the greater the reduction in force transmission to the machine support.

How does dissipation of energy by the responding system occur?

Damping is a term used to describe the physical phenomenon of converting the energy of motion (kinetic) to another form namely heat. This is accomplished through frictional resistance. The isolator itself may exhibit damping in the form of internal friction or hysteresis. If the isolator material has very low inherent damping then another physical element is added to the isolator system to give the damping required.

What are the benefits of having damping in an isolator system?

In a “free vibration” system it reduces the time required for the system to come to rest. In a “forced vibration” system it reduces the maximum force developed and transmitted at resonance.

What is resonance?

Resonance is an exact frequency match between the disturbing frequency and the isolator natural frequency. They are “in-tune” with each other. This produces a perfect “in-phase” frequency relationship that causes maximum amplification of transmitted vibration. Obviously this condition must be avoided. However it cannot be avoided entirely since an isolated unit will pass through resonance as it “runs-up-to” and “coasts-down-from” its normal running speed. If the unit “runs-up-to” and “down-from” speed quickly the machine’s inertia acts to minimise amplification. However if the unit passes slowly through resonance damping must be provided.

• Fabreeka International
• 01274 531333

October 1998