Vibration Absorbers

Chapter 1: What is the principle of operation of vibration absorbers?

This section will explore vibration absorbers, including their terminology and how they function.

What is a Vibration Absorber?

A vibration absorber functions as a one-degree-of-freedom (DOF) spring-mass arrangement that addresses vibrations in systems subjected to harmonic forces, such as engines, motors, and conveyors. Its main goal is to reduce or eliminate these vibrations to ensure a smoother and safer operation. Usually, this device is mounted on the vibrating system and works by creating "anti-resonance" to counteract the vibrations. An example demonstrating how a vibration absorber handles these oscillations is provided below.

Nomenclature of Vibration Absorbers

Vibration absorbers are widely used in various machines and mechanical systems, making it crucial to grasp their terminology. This information is explained in the following sections.

Vibration in Vibration Absorbers

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.

What is Resonance?

Resonance refers to the phenomenon where the amplitude of vibrations increases when the frequency of an externally applied force matches the system's natural frequency. This effect can be either beneficial or problematic, depending on the context. In construction, resonance is often unfavorable because it can lead to structural damage, such as cracks or collapses in bridges, buildings, and walls due to amplified vibrations. Conversely, in the field of music, resonance is advantageous and is utilized to enhance sound production in musical instruments.

Measuring Amplitude

Amplitude refers to the highest point achieved by a displaced object or point within a system. For objects that are vibrating or displaced, this measurement is taken from the system's equilibrium position. In terms of sound waves, amplitude is determined by assessing the volume or loudness of the sound produced.

Energy Absorption

Energy absorption is defined as the process involved when the magnitudes of the strength of random motions (vibrations) are reduced.

Natural Frequency

The natural frequency is the number of vibration cycles completed by a body or object in a given period of time when it vibrates freely.

What Shock is

Shock refers to the disturbance or displacement of a body from its equilibrium position caused by a sudden application of force.

Vibration Control

Vibration control involves employing various devices like isolators, dampers, pads, and mounts to capture the kinetic energy generated by vibrating elements, thereby stopping it from transferring to nearby surfaces.

How Vibration Absorbers Work

Typically, vibration absorbers are employed in machines and systems that run at a steady speed or experience continuous exciting frequencies. The setup consists of the main system and the absorber, as illustrated in the diagram below.

Vibration Absorber System Set Up

The configuration for installing vibration absorbers can be arranged in the following manner:

The Primary System (Payload)

In certain systems, the primary system is referred to as the payload. This is the component that undergoes the undesired frequencies that need to be managed or mitigated. Examples include motors, engines, bridges, and machinery. Its inherent elasticity enables it to revert to its original state after being stretched or deformed.

The Absorber

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.

Vibration Absorber Energy Absorption

The vibrations generated by equipment represent a type of energy—specifically, kinetic energy—that dissipates through random oscillations. Rubber is effective at absorbing significant amounts of kinetic energy due to its elasticity. Its durability and temperature resistance make it suitable for high-temperature environments like engines. Rubber's ability to bond with various metals, along with its high shear modulus and nonlinear stiffness, makes it a preferred material. Common natural and synthetic rubbers used in applications requiring heat and chemical resistance include silicone, butyl, fluoroelastomer, neoprene, and EPDM.

Vibration Absorber Hydraulic Shock Systems

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.

Vibration Absorber Design

Effective design of vibration absorbers involves several considerations to ensure they perform their intended function. The design approach depends on the complexity of the system. In some cases, combining both springs and rubber may be the most effective solution, while in other scenarios, a straightforward rubber mount might be sufficient.

Design of Rubber Mounts

Rubber mounts are designed to dampen vibrations in systems by placing them between components, such as metals. They help reduce wear and tear, which decreases maintenance costs. These mounts are available in various shapes, sizes, and materials to suit different applications.

Rubber Mount Deformation

Rubber mounts can deform under the load from machinery, and without proper design considerations, they may experience rapid deformation. Finite element analysis is sometimes necessary to ensure effective vibrational control before installation. Shock loading plays a crucial role in evaluating rubber mount deformation, as the extent of deformation correlates with the applied load. Shock loads can be particularly damaging, affecting general vibrations. If a device or mount can withstand these shock loads, it is more likely to endure longer under mechanical vibrations.

Fabrication of Rubber Isolators

Rubber isolators are frequently used for vibration absorption and are typically constructed using a combination of springs and rubber. Both components contribute to vibration dampening, and their integration enhances performance. These isolators can be manufactured using various methods, including molding and extrusion techniques. To improve the strength and performance of the isolators, additives such as sulfur, urethane, peroxides, metallic oxides, and acetoxy silane may be incorporated, depending on the specific application and manufacturer requirements.

Chapter 2: What is damping in vibration absorbers?

This section will explore the concept of damping, including the different forms it can take. Additionally, it will cover the various methods used for vibration damping.

Types of Damping

Damping involves minimizing or removing undesired oscillations or vibrations from a system. This can be achieved using methods such as overdamping, underdamping, and critical damping.

Light Damping

Light damping is characterized by oscillations that decay slowly over time rather than decreasing in a linear or exponential manner until they reach a minimal amplitude. An example of light damping can be seen in a child playing on a swing or a pendulum. Over time, the swing's amplitude diminishes gradually until it eventually comes to a stop. This effect is illustrated in the displacement-time graph provided below.

Heavy Damping

Heavy damping results in oscillations taking a longer period to settle back to the equilibrium position. An example of a heavily damped system is a door that gradually closes rather than slamming shut, which helps avoid potential damage. The graph below demonstrates the gradual return to equilibrium observed in heavily damped systems.

Critically Damped System

In a critically damped system, the oscillator returns to its equilibrium position as swiftly as possible without oscillating. A typical example is a car's suspension system navigating over uneven terrain; if the system is critically damped, it will stabilize and return to its normal position quickly, minimizing the time spent in motion after hitting a bump.

The following graph demonstrates the behavior of a critically damped system over time.

Types of Vibration Damping

Vibration damping involves absorbing kinetic energy from various sources of vibrations, including noise, mechanical oscillations, and alternating currents, to decrease the overall energy generated by a system.

There are several methods of vibration damping, including:

Unconstrained Damping

This damping method involves placing a cushioning pad between moving or vibrating parts, such as metal plates. The pad adjusts to the movement of these parts and absorbs the vibrations, which are then slowly dissipated as heat. This simple yet effective technique helps in protecting materials and preventing damage.

Constrained Damping

Constrained damping utilizes viscoelastic materials like Sorbothane to manage vibrations within systems. It operates similarly to unconstrained damping, where the material is positioned between components. However, constrained damping is employed in scenarios requiring more precise vibration control and greater attention to detail.

Tuned Viscoelastic Damping

This method of vibration damping employs targeted control techniques to manage vibrations more precisely. Sorbothane is commonly used in this approach to address specific wavelength ranges that could cause damage. Tuned viscoelastic damping is frequently highlighted as a highly effective means of managing vibrations within specific frequency ranges.

Vibration Isolation

Vibration isolation involves separating machinery from the primary source of vibrations. When vibrations are excessively strong, it is often necessary to isolate systems from these high levels of vibrations to prevent potential damage and reduce noise. To achieve this separation, vibration isolators are employed. These devices are positioned between the vibration source and the components that need protection.

Classification of Vibration Isolation

Vibration isolation is categorized into the following types:

Passive Vibration Isolation

Passive vibration isolation systems typically consist of springs, masses, and damping materials. Mechanical springs and rubber pads are common examples of components used in this passive control method. The combination of spring and mass establishes a natural frequency, which is influenced by damping. During vibrations, energy is transferred at this natural frequency, and damping helps to minimize the amplification of these vibrations at the natural frequency.

Active Vibration Isolation

Active vibration isolation systems, similar to many other active control systems, incorporate electronic components such as circuits, feedback mechanisms, controllers, sensors, actuators, and springs. Typically, these systems utilize processor-based actuators for precise control. Vibrations are detected and transmitted to the control system, which processes the input and sends corrective signals to an electromagnetic actuator. This setup allows for greater accuracy compared to passive isolation and prevents resonance issues. The CAD model of an active vibration isolation system is illustrated in the image below.

Considerations for installing vibration isolators include:

• Size and weight of equipment to be isolated
• Machine movement must be taken into consideration.
• Nature of the vibrations
• The vibration isolator must be selected in accordance with the operating environment.
• What are the operation pressures?
• Is it going to be an indoor or an outdoor isolator?
• Vibration isolators must be kept away from sensitive materials (Machine location)
• Location of isolators – where exactly are the isolators going to be placed in the system
• The isolators should be of proper size – they should not be too big or too small, otherwise, they might not be effective.
• Adjustment- the spring used must have the ability to freely travel and not be compressed excessively.

Vibration isolators are widely used in industries to protect equipment such as:

• Compressors, pumps, and heavy loads
• Vehicles and aviation systems and equipment
• In aerospace and electron microscopes
• Machinery and instruments

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Chapter 3: What are the types of vibration absorbers?

This section will explore the different categories of vibration absorbers and highlight the most commonly used types.

Common Types of Vibration Absorbers

The following are some of the most frequently utilized types of vibration absorbers.

Air Springs

An air spring consists of air enclosed within a fabric and rubber envelope, often shaped like a bellows. Designed to enhance vehicle comfort, air springs adjust the vehicle’s height, lowering it at high speeds and raising it when driving on uneven terrain. They offer an effective stroke-to-compressed-height ratio, especially compared to air cylinders. Due to their adaptability, air springs are suitable for various vibration isolation applications and can work with different actuation mediums such as water, air, and nitrogen.

Bushings

Bushings are vibration isolators designed to cushion the interface between two components, effectively absorbing vibrations at the contact surfaces. These inserts come in various metals, including stainless steel, brass, copper, bronze, and plated options. Typically, the steel used is case-hardened and may feature end serrations for improved grip in torsional applications. The outer bushing often has a centerless ground diameter and is press-fitted into the receiving bore. The choice of elastomer for the bushing is based on the environmental conditions of the application, such as exposure to oils, ozone, and extreme temperatures.

The diagram below illustrates the principle of vibration absorption using bushings.

Cam Follower

A cam follower is a mechanical component that tracks the motion of a camshaft. It helps minimize vibrations caused by high-speed cam movement and aids in identifying any misalignments that might occur on the camshaft. Additionally, cam followers play a role in reducing vehicle skidding.

Dashpots

Dashpots are vibration-damping devices that utilize fluid to generate resistance. Commonly found in small devices and instruments, they are extensively employed in manufacturing industries for precision applications.

Helical Isolators

A helical isolator is a type of multidirectional wire rope assembly engineered to absorb both shock and vibration. These isolators are often utilized to protect sensitive or fragile materials during transport, especially in challenging weather or environmental conditions.

Rubber Pads

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

Shock absorbers are components designed to absorb vibrations and impacts, primarily found in vehicles.

Groups of Vibration Absorbers

Vibration absorbers are classified into two primary categories: passive and active absorbers.

Active Vibration Absorber

Active vibration absorbers operate using electrical power and consist of electronic components such as switches, sensors, actuators, and controllers. These devices detect the frequencies and forces impacting the system and, based on their programming, respond by absorbing or eliminating unwanted noise.

Passive Vibration Absorbers

Passive vibration absorbers operate without active components like switches, electrical circuits, or sensors. They typically consist of rubber mounts and mechanical springs designed to absorb energy. Common types of passive absorbers made from rubber include base isolators, pads, and elastomers. These materials, such as dense cork foam, are frequently used to isolate vibrations from machinery, home appliances, and vehicles. They are particularly effective for managing medium to high-frequency noise and vibrations in heavy machinery. Passive absorbers generally have a natural frequency range between 3 and 40 Hertz.

Chapter 4: What are the applications of vibration absorbers?

Vibration absorbers are highly useful in various mechanical systems, helping to prevent damage and reduce wear and tear on materials. They are generally cost-effective, making it advisable to use them in equipment subjected to vibrations and shaking. Below is a list of some common applications for vibration absorbers.

Machine Tools

A machine tool, such as a milling machine, lathe, grinder, or drilling machine, operates using power to shape, drill, or finish materials. These tools generate significant vibrations due to their rotating and moving components, including spindles, gears, motors, and accelerometers. Without adequate vibration absorption, these machines could potentially cause accidents or damage. The diagram below illustrates the parts of a lathe machine that are subject to high levels of vibration.

Overhead Power Transmission Lines

Overhead power transmission lines are electric cables suspended from poles or towers. These lines can experience vibrations due to various factors, including weather conditions like wind, storms, and rain, as well as the movement of electrical charge. The vibrations may sometimes be audible, resulting from the discharge of energy when the electric field strength on the conductors exceeds the breakdown strength of the surrounding air.

Engine Crankshaft Torsional Vibration

Torsional vibration in engines occurs due to the combined effect of angular oscillations, primarily resulting from components like the gearbox, crankshaft, and propeller shaft. To counteract these vibrations and ensure smoother operation, vibration absorbers such as engine mounts, rubber bumpers, and transmission mounts are employed. Without these absorbers, engines would be significantly less comfortable and practical to use.

Rubber Bumpers

Rubber bumpers, also referred to as rubber mounts, are designed to absorb vibrations from machinery and equipment. They protect equipment by damping vibrations and typically come in various shapes, with the most common being circular with a central hole. Some bumpers feature adhesive on one side for easy attachment to surfaces. These bumpers are widely used in applications such as between metal surfaces on doors and between floors and equipment. The selection of a rubber bumper depends on factors like environmental conditions, material type, equipment weight, and durability. For environments with moisture, silicon bumpers are recommended due to their water resistance, while nitrile bumpers are ideal for oily environments as they resist chemical reactions.

Engine Mounts

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.

Transmission Mount

Similar to engine mounts, transmission mounts are designed to support and secure a vehicle's transmission system. They typically feature a metal plate for durability and a rubber bushing to absorb vibrations produced by the transmission during operation. The image below displays a basic transmission mount commonly used in vehicles.

Silent-Lign Pillow Blocks

Pillow blocks consist of an oil-resistant neoprene flexing medium combined with a continuously lubricated sleeve bearing, all housed in a welded steel mounting bracket that is stamped and coated with durable zinc trivalent plating. These pillow blocks can be used in various applications to:

• Minimize or stop vibrations
• Self-lubricate constantly
• Eliminate noise transmission
• Self-align
• Low profile clearances

Vibro-leveler Mounts

This shear mount made of rubber has a dual function: it minimizes noise and vibration transmission and supports precise machine leveling. Vibro-leveler mounts include:

• Easy and quick installation
• Designed with a fail safe
• Environmentally friendly
• Protect machinery from vibration and shock
• Minimize fatigue related to vibration and noise

Vibro-levelers feature a straightforward design and are generally built to be durable. They include a stud fixed to the inner cylinder and are insulated with rubber that is mechanically bonded.

Footbridge Structures

Footbridges often have slender designs, making them susceptible to damage from wind or vibrations caused by pedestrians. To prevent issues such as shaking or resonance-induced damage, these structures typically incorporate vibration absorbers to mitigate such effects.

Engine Pumps and Generators

Machines with rotating shafts, such as pumps and generators, generate vibrations that can impact other components or the surfaces they rest on, such as tables, floors, or bases. To address and mitigate these potential damages, vibration absorbers are installed to reduce or eliminate these disruptive effects.

Considerations when Selecting a Vibration Absorber

Several factors should be considered when choosing a vibration absorber, including:

Size and Weight of Absorbers

The size of the machine affects the vibration absorption requirements. Larger machines may need multiple mounts to handle vibrations effectively, while smaller machinery might only require a single mount to manage all vibrations. It is important to consider the weight of the equipment as a key factor before sourcing vibration absorbers.

Vibration Environment

Understanding the installation environment for a vibration absorber is crucial, as different materials respond differently to various conditions. For instance, absorbers designed for laboratory settings differ from those used in industrial applications. Some absorbers, intended for indoor use, may deteriorate when exposed to UV rays, compromising their effectiveness. Many rubber-based products, such as rubber pads and bumpers, are not resistant to chemicals and can be prone to damage.

Nature of Vibration

Understanding the nature of vibrations is essential. Key aspects include analyzing the frequency, amplitude, and direction of the vibrations. To measure these parameters accurately, tools like accelerometers are used, and it's recommended to consult a specialist for precise data. The amplitude's magnitude will indicate whether vibration isolation is needed. Additionally, knowing the vibration’s direction helps in positioning absorbers correctly. Incorrectly identifying the direction may result in ineffective vibration control, even if appropriate mounts are utilized.

Absorber Cost and Maintenance

Similar to other mechanical systems, vibration absorbers will need maintenance over time due to wear and tear. Additionally, expenses related to shipping, site surveys, and installation should also be factored into the overall cost.

Absorber Upkeep

Considerations should address questions such as: What will the maintenance costs be for the system with the chosen absorber? How straightforward is the replacement process for this absorber? Are there trained technicians available to service and maintain both the absorber and the overall system?

System Flexibility

This considers how the system will handle modifications if needed. An optimal system should offer enough flexibility to accommodate changes in absorber installation. However, if the absorber is designed perfectly, flexibility may not be a major concern.

Contact Surface of the Absorber

Determining which services or parts will interact is crucial for choosing the correct absorber for specific vibrations. Without detailed information about these contact points, there's a risk of causing damage to the system. Additionally, understanding the nature of these contact surfaces can help predict the lifespan and effectiveness of the absorber.

Lifespan of the Absorber

The lifespan of absorbers is influenced by a variety of factors, as outlined earlier. This is closely tied to cost considerations because typically, absorbers of higher quality are more durable and have a longer service life, assuming the system is well-designed.

Load on the System

It's crucial to understand the load that will be placed on the absorber before making a purchase. Heavier loads require more robust absorbers, while lighter loads can be managed with less substantial ones.

Conclusion

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.

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