Vibration Engineering – ‘How it works’
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In analysing the vibration of a machine, which is a more or less complex mechanical system, it is useful to consider the sources of vibration energy and the paths in the machine that this energy takes. Energy always moves, or flows, from the source of the vibration to the energy absorber where it is converted into heat. In some cases, this may be a very short path, but in other situations, the energy may travel relatively long distances before being absorbed.
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The most important terms in vibration engineering are frequency f, amplitude A and damping factor D. Natural frequency as well as resonance are also important figures when considering isolation systems.
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Frequency f is a measure of the number of complete vibration cycles per second.
Structure-borne noise are vibrations that are conveyed in a solid body. Low frequencies are generally what we call mechanical vibrations.
The amplitude is the vibration wave range around its equilibrium. It determines the severity of vibration and is usually expressed in terms of acceleration or displacement.
Damping D designates the measure of amplitude reduction of the vibration of a freely oscillating spring-mass system through friction. Damping refers to the conversion of energy to heat.
Natural Frequency (fn)
When a material or a suspended body is excited, it will vibrate freely with a ‘natural frequency’ until it is allowed to come to rest.
Natural frequency of a body is the frequency with which the body freely oscillates around its equilibrium without external influence. Each body has its own natural frequency, which however can only be calculated in the simplest of cases.
Vibration isolation
A rigid body has six degrees of freedom. 3 Translation + 3 Rotation. The most general motion of a free rigid body is a translation plus a rotation about.
These frequencies can be ‘coupled’, which means by exciting the body in one direction it is possible that all other modes of vibration can be excited, which is not ideal. By positioning the AV mountings on the principle axis, decoupling of the frequencies can be achieved.
If the stiffness increases, the natural frequency also increases, and if the mass increases, the natural frequency decreases. If the system has damping, which all physical systems do, its natural frequency is a little lower, and depends on the amount of damping.
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Vibration isolation is a commonly used technique for reducing or suppressing unwanted vibrations in structures and machines. With this technique, the device or system of interest is isolated from the source of vibration through insertion of a resilient member or isolator.
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Calculating the Vibration Isolation
Natural Frequency
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Resonance
Resonance occurs when a material oscillates at a high amplitude at a specific frequency. When resonance occurs, the resulting vibration levels can be very high and can cause rapid damage. The Vibration Solution Limited Products can help prevent costly breakdowns.
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Harmonic Order
Harmonics are unwanted higher frequencies which are superimposed on the fundamental waveform creating a distorted wave pattern. Harmonics are AC voltages and currents with frequencies that are integer multiples of the fundamental frequency. On a 60-Hz system, this could include 2nd order harmonics (120 Hz), 3rd order harmonics (180 Hz), 4th order harmonics (240 Hz), and so on. Normally, only odd-order.
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Isolator
In order to control the vibration of any machine or equipment from affecting the surroundings, a vibration isolator is used as a cost-effective solution. Vibration isolation is achieved by using special mounts designed to absorb the vibrations or movements caused by the machinery or equipment.
Damper
A damper is a device that helps in dissipating excess energy from a system. It can shock absorber, diode or suspension. In shock absorber, it is a mechanical device to dissipate kinetic energy.
The effect of damping is to increase dynamic stiffness which reduces the frequency ratio and in turn reduces the isolation efficiency of the mounting. That’s why the majority of AV mountings are manufactured in natural rubber which has a low static (Ks) to dynamic (Kd) stiffness ratio.
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However, some damping is useful to control excessive movements when the equipment passes through its resonance frequency during start-up & shut down.
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Rubber - ‘What is it’
Rubber materials have been well known as a vibration isolator to dissipate vibration energy. It has been in use for thousands of years, during which time it has been produced in numerous variations with distinct characteristics that make them suitable for different applications. Most existing works on the vibration isolator, especially the mathematical models only consider the performance of the vibration isolator due to the static force.
Rubber is an excellent insulator.
· Highly Elastic
· Self-extinguishing
· Impermeable to gas
· Resistant to Fuels, Oils, Acids and other hazardous substances
· Can be moulded into any shape or form
· Electrically insulating
· Provides isolation from Vibration, Noise and Shock
· Withstand temperatures from -40°C to +300°C
· Resistant to attack from Ozone and Weathering
· Available in many different colors
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Rubber Hardness
Rubber shore hardness is the measure of the resistance a rubber material (like Silicone or EPDM) has to indentation, or quite simply, the hardness of the rubber. The hardness of Rubber is specified using either the ‘Shore A’ or ‘Shore D’. The A scale is for softer materials such as rubber, while the D scale is for harder, less elastic materials.
AntiVibration mountings are available from soft 30sh rubber upto 75sh hard rubber. The hardness of a mounting is directly related to a mounting's stiffness.
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Stiffness
Stiffness is a measure of the force required to deflect a mounting by a given deflection, and is commonly measured in Kg/mm. Rubber is an incompressible material, much like a fluid, therefore the ‘Free Area’ of an AV mountings rubber section, known as the shape factor, has a considerable influence on the mountings stiffness, in addition to the rubber hardness. The stiffness of a mounting is directly related to its natural frequency.
Creep
Creep is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. Creep in service is usually affected by changing conditions of loading and temperature. The creep mechanism is often different between metals, plastics, rubber, concrete However, most of the Creep deformation will take place within the first 48 hours of the load being applied. It is also accelerated with increased temperature.
Compression or ‘Permanent’ set
Compression set is the amount of permanent deformation that occurs when a material is compressed to a specific deformation, for a specified time, at a specific temperature. Compression set testing measures the ability of rubber to return to its original thickness after prolonged compressive stresses at a given temperature and deflection. ... As a rubber material is compressed over time, it loses its ability to return to its original thickness.
Dynamic Properties
When rubber is stretched the stiffness gradually decreases and it becomes relatively easy to stretch as the chains uncoil; once this has happened the rubber is much stiffer. Under repeated cyclic compression of rubber, hysteresis will dissipate some of the energy by converting it into heat. Hysteresis is measured by the difference between the input energy and the energy returned, i.e. the energy loss. Low hysteresis rubber, such as natural rubber provides Low Damping and High Resilience which gives excellent vibration isolation properties. The stress–strain graph for rubber (below) shows that the behaviour as a load is removed is not the same as that when the load is being increased. This is called hysteresis and the curves are said to make a hysteresis loop.
Energy absorbed when a material is being stretched and the energy that is released when the force is removed. Rubber absorbs more energy during loading than it releases in unloading. The difference is represented by the area of the hysteresis loop, shown shaded in the stress–strain graph.
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Rubber Fatigue
Fatigue is a failure mechanism that involves the cracking of materials and structural components due to cyclic (or fluctuating) stress. While applied stresses may be tensile, compressive or torsional, crack initiation and propagation are due to the tensile component. Several types of rubber can crystallize when exposed to external stretching,1, 2, 3, 4 such as natural rubber (NR), polychloroprene rubber and polyisoprene rubber (IR). Strain-induced crystallization (SIC) behaviour endows these rubbers with excellent mechanical properties and good resistance to crack growth.
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To assist you in selecting the correct isolator we have listed the isolation efficiency that should be used under normal conditions of operation. the isolation efficiency at any given deflection and disturbing frequencies can be obtained by using the simple graph above.
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