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This article will take an in-depth look at vibration absorbers.
The article will bring more understanding on topics such as:
This chapter will discuss vibration absorbers, their nomenclature, and the principle of operation.
A vibration absorber is a single degree of freedom (DOF) spring-mass system that eliminates or reduces the vibration of a harmonically excited system such as the rotation of engines, motors, generators, belts, etc. to create a comfortable and safe environment. A vibration absorber is normally attached to the vibrating body and is designed to produce “anti-resonance” to the system. An example of a vibration absorber used to neutralize the oscillations created by the vibrating forces is shown below.
Vibration absorbers are common in many machines and mechanical systems, and it is important to understand the nomenclature of vibration absorbers. This is detailed below.
Vibration can be defined as a mechanical phenomenon that is experienced when oscillations occur about an equilibrium point. Vibrations can be desirable; like in the construction industry, they can be used to mix concrete in a concrete vibrator. Vibrations can have negative effects in some cases, especially when they are producing unwanted sounds and in other instances, they tend to waste energy. In electric motors, engines, and many mechanical operations vibrations are not wanted. When the produced vibrations are not wanted, absorbers are implemented to eliminate, reduce or remove the vibrations.
Resonance is defined as the creation of increased amplitude that occurs when the natural frequency of a system is in tune with the frequency of the periodically applied force. Again, this phenomenon can be desirable or undesirable depending on where it occurs. In the construction industry, resonance is mostly undesirable as it causes structures such as bridges, walls, buildings to crack or fall due to the increased shaking or vibrating resulting from increased amplitudes. In the music industry, resonance has positive results, and it is used to produce sound in musical instruments.
Amplitude is the maximum value reached by a displaced object or point in a body. In a vibrating body or other displacement, this value is measured from the equilibrium of the body. In a sound wave, the amplitude value is obtained by measuring the loudness of the sound.
Energy absorption is defined as the process involved when the magnitudes of the strength of random motions (vibrations) are reduced.
In a given unit of time, the number of cycles that a body or object vibrates freely is referred to as the natural frequency.
Shock is the instability or the shifting of a body from its equilibrium point when a force is suddenly applied to it.
Vibration control uses different devices such as isolators, dampers, pads, and mounts to absorb the kinetic energy released by vibrating bodies to prevent the energy from reaching adjacent surfaces.
In most cases, vibration absorbers are used with machines and systems that operate at constant speed or with systems that constantly have exciting frequencies. The system is made up of the primary system and the absorber, as shown on the diagram below.
The system setup of vibration absorbers can be set up as follows:
The primary system is also known as the payload in some systems. It is the system that experiences the unwanted frequencies which ought to be controlled or neutralized. This could be a motor, engine, bridge, machine tool, etc. Its long chains allow it to recover when stretched or deformed by returning to the original state.
The absorber is also known as the support because it helps the primary system by eliminating or reducing the vibration encountered in the system. This involves components like rubber mounts, springs, pads, etc. The absorber could be hydraulic, where the vibrations are absorbed by using a hydraulic system.
The vibrations produced from equipment are technically another form of energy (Kinetic energy) that gets dissipated in the form of random oscillation. Rubber has characteristics that allow it to absorb large amounts of kinetic energy, such as its elasticity. Rubber is resilient and resistant to temperature, making it a good fit in high-temperature conditions like engines. It can bond with most metals, has a high shear modulus and nonlinear stiffness characteristics which make it one of the most sought-after materials. Silicone, butyl, fluoroelastomer, neoprene, and EPDM are some of the most common natural and synthetic rubber materials used for an application that requires heat and chemical resistance characteristics.
A hydraulic system comprises a hydraulic cylinder, piston rod, and hydraulic fluid. The diagram below illustrates the components of a simple hydraulic system. The vibrations impact or put a force onto the piston rod, which in turn will force the fluid inside the cylinder to move. The liquid inside the cylinder will convert the kinetic energy from the vibrations into heat. Below is an image showing the different parts of a hydraulic shock absorber.
When designing vibration absorbers, there are certain things to consider for the design to be effective for the desired job. The design varies with the complexity of the system. In some instances, it is best to use both the spring and the rubber, whereas in some a simple rubber mount works effectively.
Rubber mounts are a piece of materials designed to absorb vibrations in vibrating systems by mounting them between materials such as metals. They prevent wear and tear of materials thereby reducing maintenance costs. Rubber mounts come in different shapes, sizes and material types depending on the application.
Rubber mounts deform when put under load from the machines and in some cases if there are no design considerations made on these mounts, they might end up deformation very quickly. There is a need for finite element analysis in some cases, which requires critical vibrational control before installing them. Shock loading is an important factor in analyzing rubber mount deformation because the amount of deformation is related to the load onto the rubber mounts. Shock loads are more detrimental and destroy general vibrations and, in most cases, if the device or mount can survive these loads it is likely to stay longer under mechanical vibrations.
Rubber isolators are common in the absorption of vibrations. They are made out of springs and rubber. Both the spring and the rubber can absorb vibrations, and, in this case, they are combined. There are ways of manufacturing or fabricating this isolator, such as the molding and extrusion techniques used with the press. In most cases, additives such as sulfur, urethane, peroxides, metallic oxides, acetoxy silane can be used to enhance strength in the isolators based on the application and manufacturer specifications.
This chapter will discuss damping and its various types. It will also discuss vibration damping and its types.
Damping is the process of eliminating or reducing unwanted oscillations or vibratory movements from a system. This can be done through critical damping, heavy damping, and light damping.
In light damping, the oscillations are not reduced linearly by exponentially until they reach a minimum value. An example of light damping is a child using a swing or a pendulum. As time progresses, the swing reduces in amplitude until it ceases to swing. This phenomenon is best described below with a displacement time graph as shown below.
In heavy damping, the oscillations take longer to return to the equilibrium position. Examples of heavily damped systems include doors where they return to the closing position over some time. This prevents damage that happens when the door is shut faster. Below is a graph illustrating how heavily damped systems slowly return to equilibrium over time.
In a critically damped system, the oscillator is displaced from the equilibrium,quickly returning to the equilibrium position. A common example is of a car experiencing bump roads, if the car is critically damped, the dampers will make the car return to the equilibrium position in a short period.
The graph below illustrates how a critically damped system operates with time.
Vibration damping refers to the absorption of kinetic energy from vibrations such as noise, mechanical oscillations, alternating currents, etc. in order to reduce the total amount of energy being produced by a system.
Types of vibration damping include:
In this type of vibration damping, a pad is created and placed between the parts that are moving or causing vibrations, such as metal plates. The pad sits between the moving parts such that it conforms to the motion of the parts. As it interacts with the parts during movements, it absorbs the vibrations that are involved, which are later released slowly as bits of heat energy. This damping method is one of the simplest that can be used to damp vibrations and is very effective for material protection and damage prevention.
In constrained damping, a viscoelastic material such as Sorbothane can be used to control vibrations in the systems. It works similarly as unconstrained damping where the material is placed between the parts. The main difference is that constrained damping is used where closer attention and vibration control are required.
In this type of vibration damping, more specific control measures are used to control the vibrations. Sorbothane is also used in this type of damping. It is used to eliminate ranges of certain wavelengths in a system that may create damage. The tuned viscoelastic damping is often referred to as the most effective way of putting vibrations to a certain range of frequencies.
Vibration isolation is the process involved when machinery is separated from the main source of vibration. Sometimes the vibrations created are too high such that it is best to separate a system from such high quantities of vibrations that have the potential of great damage and create noisy environments. To separate from such vibration, devices known as vibration isolators are used. Vibration isolators are placed between the source of vibrations and the other material to be protected.
The classifications of vibration isolation include:
Most passive vibration isolation systems comprise spring, mass, and damping material. Devices such as mechanical springs and rubber pads are used in this technique of passive control. Spring and mass are used in such a way that they create a natural frequency and in this case the damping will affect the natural frequency. During vibration, there is the transfer of energy at the natural frequency produced and the natural frequency amplification is reduced by damping.
Just like many active systems, active vibration isolation comprises electronic components like circuits, feedback configurations, controllers, sensors, actuator, and spring. Usually, a processor-based actuation system is used for active isolation. Vibrations created are transferred to the control system, which is usually fed into an electromagnetic actuator and based on the controller program, the vibration will be neutralized. There is more accuracy in this isolation type than passive vibration isolation and resonance is not experienced. The image below shows a CAD model of an active vibration isolation control.
Factors to consider when installing vibration isolators include:
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Vibration isolators are widely used in industries to protect equipment such as:
This chapter will discuss the various groups of vibration absorbers and the common types of vibration absorbers.
Below are the common types of vibration absorbers commonly used.
An air spring is a sect of air confined within a fabric and rubber container usually shaped like a bellow. They were designed to provide comfortability in cars such that when moving at fast speeds the body lowers and when traveling off roads the body of the vehicle is lifted. Air springs are effective and give a favorable stroke ratio to compressed height, especially in comparison to air cylinders. They can be used in many vibration isolation applications because of their flexibility and can be used in a variety of actuation media. These media include water, air, nitrogen, etc.
Bushings are a type of vibration isolators that provide an interface between two parts such that they absorb the vibrations between the surfaces. The bushing inserts are available in numerous metals such as stainless steel, brass, copper, bronze, or plated metal. The steel used is typically case hardened and can be applied on torsional bushings for gripping using end serrations. The outer bushing can have an outside diameter that is centerless. It is typically ground to size and press fit to fit into the receiving bore. The elastomer selection will be determined by the application’s environmental factors, such as exposure to oils, ozone exposure, and extreme cold or heat.
Below is a diagram showing the bushings principle of absorbing vibrations.
Bushings from Bushings Inc.A cam follower is a mechanical device designed to follow the motion of the camshaft. They also assist in reducing the skidding of vehicles. When the cam moves at high speeds the cam follower reduces the vibrations produced and helps detect misalignments that may occur on the camshaft.
Dashpots are vibration absorption materials that use fluid to create resistance. They are common in small devices and instruments. In manufacturing industries, they are widely used in precision processes.
A Helical isolator is a multidirectional wire rope structure designed to absorb shock and vibration. They are commonly used to transport sensitive and delicate materials or property in harsh weather/climatic conditions.
Rubber pads are also known as anti-vibration pads or vibration isolators are rubber materials that are used to absorb vibrations in materials by placing them between materials or moving parts. In some instances, these pads are made of ribbed patterns so that they avoid slippage of materials on their surfaces. Rubber pads can be grouped into many types based on the pattern formed and the common types include the KHL. KH, KHS, RHS, etc.
Shock absorbers are devices used to absorb vibrations and jolts mainly and are mainly used in vehicles.
Vibration absorbers are grouped into two main types: passive and active vibration absorbers.
Active vibration absorbers use electric power to operate. These are usually electronic components such as switches, sensors, actuators, controllers that can detect the frequencies and forces that will be acting on the system. They will in response absorb or eliminate the noise or unwanted noise based on how they were programmed to operate.
These types of absorbers do not have active parts like switches, electrical circuits, sensors for them to operate. They are usually made of rubber mounts and mechanized springs, which are used for the absorption of energy. Passive absorbers which are made from rubber include base isolators, pads, and elastomers. Rubber pads or elastomers are most common as well as dense and cork foam. They are used to separate vibration from machines, home equipment, and vehicles. They are mainly used when there is a need for medium to high-frequency noise and vibration to be isolated from heavy machinery. Different pads are used in these passive vibration absorbers, and the typical natural frequency range is from 3 to 40 Hertz.
Vibration absorbers come in handy and are beneficial to many mechanical systems because they prevent damage and, in most cases, minimize wear and tear on materials. Most of these absorbers are relatively cheap, so it is advised to implement vibration absorbers on equipment that encounters vibrations and shaking. Below is a list of some of the common applications of vibration absorbers.
A machine tool is a machine that uses power for its operation, such as a milling machine, lathe, grinder, drilling machine, etc. to shape, drill, or finish material. These machines experience a lot of vibrations that result from the rotating and moving parts inside them, such as spindles, gears, motors, accelerometers, etc. Without proper absorption of these vibrations, the machine tool may cause accidents or damage. The diagram below shows parts of a lathe machine that experience high vibrations.
Overhead power transmission lines refer to electric lines which are suspended by poles or towers. These lines experience vibrations usually caused by a change in weather such as wind, storms, rain and also caused by the movement of electric charge. This sometimes can be heard as a sound and this sound comes from the discharging of energy as a result of the electric field strength on the conductors being stronger than the breakdown strength of the air that surrounds the conductor.
Torsional vibration in engines is an example of shaking or vibration that results from the superposition of angular oscillations and it mainly comes from the propulsion of the engine’s parts such as the gearbox, crankshaft, propeller shaft, etc. On machines that experience torsional vibration, absorbers assist in stabilizing and neutralizing the system and without them, many engines would have been uncomfortable and unfriendly to use. This is achieved by the use of vibration absorbers such as engine mounts, rubber bumpers, and transmission mounts.
Rubber bumpers, also known as rubber mounts, are materials that are made of rubber to absorb vibrations from machines or equipment that vibrate. They are designed to protect equipment and although they come in many shapes most of them have a circular shape with a small hole at the center. Other bumpers are made with adhesive on the other side so that when one wants to use them, they will press the side with adhesive on the machinery to attach them to that position. They are commonly used on doors usually between metals and also between the floor and the equipment or machinery. The choice of which rubber bumper to use depends on many factors such as environment to be used, type of material, the weight of equipment, durability. Some of them are made of silicon, whereas others are made of nitrile. If the environment has moisture, it's recommended to use silicon as it is resistant to water. On the other hand, nitrile bumpers are handy when used in an oily environment because they do not react with it.
Engine mounts are vibration absorbers that are used to hold the engine together. They are made of rubber because rubber absorbs impact and vibrations. They are put between metals to avoid wear and tear between them. Some engine mounts are filled with liquid inside to enhance strength and in case they wear out, the liquid will start to leak. If they start leaking, it means maintenance or replacement should be done to them.
In the same manner, the engine mount operates, the transmission mount is made to support and hold together the transmission of a car. It usually contains a metallic plate for strength to hold the transmission and a rubber bush for absorbing vibrations coming from transmission systems as it operates. The picture below shows a simple transmission mount that is used in vehicles.
These pillow blocks are a combination of an oil-resistant flexing medium of neoprene and a sleeve bearing that is constantly lubricated. This happens in a welded steel mounting bracket that is stamped and plated in a sturdy zinc trivalent. In various applications, the pillow block can:
This is a rubber in a shear mount with a dual purpose which includes suppression of noise and vibration transmission and facilitates machine precision leveling. The vibro-leveler mounts have:
The vibro-levelers have a simple design and are typically rugged in construction. They have a stud attached to the inner cylinder and insulated using a mechanically bonded rubber.
Most footbridges have slender designs meaning they are prone to damage caused by wind or human-induced vibrations and require a protective mechanism to avoid shaking or damage due to resonance. Vibration absorbers reduce these impacts in most of these footbridges.
Equipment that has rotating shafts such as pumps and generators produce vibrations that may affect the other parts of the machine or affect the base which they are placed on such as table, floors, or bases. Vibration absorbers are put in place in such equipment and reduce or eliminate these impacts that have the potential for damage.
There are various considerations when selecting a vibration absorber. These include:
Machines differ in size, if the size of the machine is big, it might require several mounts to cater for the vibrations, whereas in small machinery only a single mount might be able to absorb all the vibrations. The weight is considered and should be part of the primary information before approaching supplies.
The environment in which the absorber is going to be installed is important to understand and analyze as different materials of absorbers will react differently. For example, laboratory absorbers are different from industrial absorbers. Some absorbers are designed for indoor operations and using them while exposed to UV rays may damage their properties and reduce their efficiency. Most rubber products like rubber pads, rubber pads, rubber bumpers, etc. are not resistant chemicals and may be easily damaged.
The nature of vibration is also important to note. Vibration magnitude and size can be analyzed in terms of frequency, amplitude, and direction of the vibrations. Devices such as the accelerometer can be used to measure the frequency of vibration and it is advisable to hire a professional who can measure and produce proper quantities of these variables. The magnitude of the amplitude will determine whether isolation is required or not. Getting the vibration’s direction will help locate the exact positions where absorbers should be placed. If the direction is not properly detected, it might not be possible to eliminate the vibrations even if the correct mounts are used.
Like many other mechanical systems, the systems which contain absorbers will require maintenance at some point as these components are prone to wear and tear. Shipping, site survey, and installation are other costs that may be incurred and should be taken into consideration.
This should answer questions like how much are the system’s maintenance costs with a certain absorber going to be? How easy is it going to be to replace a certain absorber? Will there be trained personnel to service and maintain the absorber and system as a whole?
This addresses how the system is going to accommodate changes if necessary. An ideal system should be flexible enough to allow changes in the installation of the absorber. If the design of the absorber is perfect, then flexibility is not to be considered much.
What type of services are going to be in contact with each other? By knowing this, one can design and be able to choose the right absorber for particular vibrations. Choosing an absorber without having useful information about the contact surfaces information may result in damaging the systems. The duration and lifespan of the absorber can be estimated by knowing the contact surfaces.
A combination of factors determines the lifespan of the absorbers and others have been listed above. This is related to costs involved because in many cases the higher the quality of the absorber, the stronger it is and the longer the lifespan of the absorber a proper design of the system is done.
The load applied onto the absorber is important and needs to be known before purchasing an absorber. Big loads are supposed to be handled by stronger absorbers and vice versa.
A vibration absorber is a single degree of freedom (DOF) spring-mass system that eliminates or reduces the vibration of a harmonically excited system such as the rotation of engines, motors, generators, belts, etc. to create a comfortable and safe environment. In selecting a vibration absorber for a particular use, it is critical to understand the type and application of the different vibration absorbers.
This paper deals with the construction and tests of two types of vibration dampers, selected from a number of original proposals, designed to damp out the vibration in steam-turbine impulse blades. These dampers were tested using models constructed of such size and physical dimensions as to simulate the conditions of operation in the actual turbine blade. Test results show that if a properly designed damper is attached to a freely vibrating system having 26 times the mass of the damper, it will reduce an initial deflection of the vibrating system to 4 per cent of its value in 10 cycles.
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