This article takes an in depth look at rubber bushings and their use.
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A rubber bushing is a form of vibration isolator that is placed between two parts to limit the motion between them and absorb, mollify, and buffer the energy produced by their interaction. They are very versatile tools that can be shaped, configured, and formed to fit any type of application, with the most common bushing being cylindrical with a hole in the middle.
A vital part of a rubber bushing function is to keep connected pieces from moving and vibrating, which is necessary to eliminate wear and damage. Regardless of their small size and rubber structure, rubber bushings are a resilient tool capable of withstanding extreme stress and radical deformation.
Rubber bushings are made from synthetic rubber and natural rubber. The type of rubber determines how the bushing can be used since there are differences between the characteristics of natural rubber compared to synthetic rubber. Several types of rubber bushings are part of automobile manufacturing, which was their original use. They serve as a pivotal point due to their elastic and attenuation characteristics.
Rubber bushings are needed for several types of equipment and machinery due to their stability and strength. Although they are an essential part of vehicles for their vibration suppression properties, other forms of machinery also use them for their nullifying characteristics in order to protect equipment from wear.
Types of rubber bushings vary in accordance with the hardness value of the rubber required to meet the needs of an application. Additionally, they vary in size to perfectly meet volume and static stiffness values.
While small and having a simple design, rubber bushings are an essential part of high stress applications. All bushings have a hollow interior used to separate metal connections and are made of soft absorbent materials. Aside from reducing vibrations, they are used as a method for sound and noise suppression.
Natural rubber bushings have high tensile strength and tear and fatigue resistance and are used for static or dynamic applications where noise and vibration suppression are necessary. The wide use of rubber bushings is due to their ability to withstand multiple impacts and still be able to offer optimal functioning. They have outstanding resilience with exceptional rebound elasticity as well as flexibility at low temperatures.
Regardless of natural rubber bushing’s positive characteristics, they should not be used for high temperature applications or be exposed to the ozone, oil, and hydrocarbon solvents. Operating temperatures should not exceed 155o F, and cylindrical style mounts cannot be used for tension applications.
Conical rubber bushings are made from silicone rubber with a lower compression set. The silicone material is able to withstand radically high temperatures and is compatible with oil and grease. Conical bushings are also known as rubber cone bushings and are suitable for compression and shearing loads with a fail safe capacity and high load capacity. They are widely used in automotive and marine applications.
Leaf spring bushings are used on the front eye of automotive springs and can be encased in steel or be all rubber. They provide a cushion for all the springs on a vehicle with ones on the front encased in steel while in the rear they are all rubber. Leaf spring equalizer bushings support the ends of leaf springs and allow them to articulate. They have a center bushing that helps cushion shock on the vehicle's frame.
A flanged bushing is a type of sleeve bushing with one end flanged and extending laterally outward. They are made of styrene butadiene rubber (SBR) or ethylene propylene diene monomer rubber (EPDM). The design of flanged bushings makes it possible for them to support axial loads with the flanged part of a flanged bushing acting like a washer.
Rubber sleeve bushings can serve as an insulating lining and provide good abrasion protection. They protect parts from vibrations, corrosion, and shock. The different forms of rubber sleeve bushings include shaft sleeves, suspension system sleeves, bearing sleeves, and sway bar sheaths. Aside from being shockproof, sleeve rubber bushings are breakage proof serving as rubber dampers and bumpers.
Control arm rubber bushings prevent the control arm of a vehicle from resting on the frame. Their essential function is to prevent metal to metal contact, which can cause wear. As with all rubber bushings, control arm rubber bushings also work as noise suppressors and dampen vibrations.
The first evidence of a control arm bushing failing is loss of control in steering. When they fail, they fall apart and allow the control arm to rest on the frame. Once a control arm rubber bushing fails, things get rapidly worse.
Rubber metal bonded bushings are a very common type of bushing where rubber and metal are bonded together to serve as vibration and noise suppressors. There are different forms of bonded rubber bushings with metal down the center of the bushing and rubber compressed on the interior and exterior by metal.
Center Bonded – Center bonded rubber bushings have a cylindrical metal that is surrounded and fully bonded with rubber on the outside.
Fully Bonded – A fully bonded rubber bushing is encased in metal with metal covering its exterior and interior. It consists of two concentric metal sleeves securely attached to a rubber cylinder. Fully bonded rubber bushings are designed for small torsion, conical, and axial movements for radial loads. They eliminate high frequencies through the elastic deflection of the rubber when external couples act on the metal cylinders.
Cylindrical bushings are known as rubber bobbins and are used for active and passive vibration applications. They are made of stainless steel and natural rubber, making them ideal for shear and compression vibrations and noises.
Rubber bumper stops are cylindrical rubber mounts that are used as anti vibration stops for HVAC systems, engines, aerospace, motors, fans, and electronics. They have female threading on one end only, which allows them to be screwed into applications to protect them from striking or bumping and be able to support static loads.
A reducer bushing is a flexible rubber bushing that connects cast iron, plastic, steel, or copper piping for non-pressure, gravity flow applications. They are easy to install and form a perfectly tight seal. Reducer rubber bushings are used on fittings and come in sizes of 1.5 inches up to 4 inches.
All types of structures deflect vertically over time, which causes damage and repairs. Damage from structural deflection can be prevented with the pre-installation of step bushings that create a friction free gap for friction free motion. They allow a structure to deflect but leave studs and all other connections secure. Step bushings come in sizes to fit any type of screw and bracket.
Heavy duty bushings have extra rubber to produce a more solid form, with several layers of rubber added. The purpose of the additional material is to prevent the bushing from distorting or being ripped. Due to their solid form, heavy duty bushings can be noisier but provide superb protection and quality performance.
Several different types of rubber are used to produce rubber bushings with natural latex rubber being one of them. The type of rubber chosen to manufacture bushings depends on how they are going to be used since natural rubber cannot be used in applications that have oil, chemicals, or extreme heat. Most rubber bushings are made of synthetic rubbers and contain some form of plastic. They have the same feel and properties as rubber but are more durable to fit the needs of a variety of conditions.
Natural rubber is made from the latex or sap of the Hevea brasiliensis tree and is a natural polymer. The sap is a mixture of protein and rubber particles with rubber being 33% of the mixture. The sap collected from the trees is filtered and diluted with water and treated with acid to cause the rubber to coagulate.
The texture and feel of natural rubber are soft and elastic, which gives it a great deal of tear resistance and tensile strength. Although natural rubber has strong characteristics, its strength does not last long. It ages quickly, cracks, and comes apart.
SBR is the oldest and most widely used type of synthetic rubber due to its excellent mechanical and elastic properties. It is an ideal material for the manufacture of rubber bushings since it is capable of withstanding the wear and stress demanded of bushings.
The contents of SBR include butadiene mixed with 10% to 35% styrene, the combination of which is what gives SBR its wear and aging resistance. The properties of SBR make it possible to use it for a wide array of industrial applications. Aside from its tensile strength, unlike natural rubber, SBR is crack resistant and friction wear resistant.
On the downside, SBR is not as soft and flexible as natural rubber, which is one of the reasons that part of its mixture includes natural rubber.
NBR also begins with butadiene as its foundation with the addition of acrylonitrile and is referred to as nitrile. It tolerates contact with oils, fuels, and grease but is not resistant to aging, like natural rubber, or weathering.
Silicone rubber is a combination of carbon, hydrogen, oxygen, and silicon, which comes from silica. The backbone of silicone is polydimethylsiloxane, the bond of which varies to produce different types of silicone. Silicone rubber has high temperature resistance and excellent flexibility at a wide range of temperatures.
The basic properties of silicone rubber make it adaptable to a wide range of applications. It is resistant to weather stress, including ozone and UV ray exposure, which makes it possible to use it for outdoor applications. For bushings, silicone rubber stabilizes joints, reduces vibrations, and significantly reduces noise.
EPDM is made from the polymerization of propene, ethene, and diene. It is an excellent synthetic rubber that is resilient when exposed to sunlight and stressful weather conditions. EPDM ages very well and is resistant to acids and bases as well as salt water.
The rubber for EPDM bushings is prestressed to increase its durability and strength. Materials made from EPDM can withstand torsional movement and axial and radial loads. Common applications for EPDM are bushings for the auto industry and construction projects. It is a common bushing used for freezer seals due to its ability to withstand the effects of any temperature.
There are various methods used to make rubber products that include extrusion and molding. The type of process used depends on the type of rubber, the final use of the bushing, and the shape or configuration of the bushing. Although bushings consist of a rubber cylinder, each type of cylinder can take the shape designed for a specific and unique application with varying lengths and diameters.
Rubber bushings are sound and noise suppressing and vibration isolator tools that can be made into any shape and provide a strong and resilient interface between metal parts. They fit tightly such that they attach to the surface of both parts to reduce or eliminate vibrations.
Extrusion is a process where the rubber is forced through a die that has the shape and configuration of the bushings. The raw rubber is a soft unvulcanized compound that is fed into the extruder. Once the completed piece leaves the extruder, it is vulcanized and hardened.
The rubber used for the extrusion process is uncured or unvulcanized. In its uncured form, rubber can easily be deformed under mechanical stress and has polymer chains that have not been linked, which is the reason it is so pliable and formable. Unvulcanized rubber comes in different grades each of which is designed to meet the requirements of particular applications.
In the preparation of the rubber compound, the raw dry rubber is mixed with different compounding ingredients. Adding the different ingredients is referred to as compounding, which leads to the final rubber compound for the extrusion process. The mixture is combined to achieve the necessary properties for the desired rubber bushing.
The extrusion process begins with the feed hopper, which is the opening where the raw rubber compound is poured into the extruder. Hoppers have the shape of a large cone with a wide diameter on top that gets progressively smaller. The smallest end of the hopper with the smallest diameter is connected to the starting point of the extruder.
There are a wide variety of extruder hoppers that include ones with mixers, control monitors, and various feeding methods. The original, and still used form of hopper, is an open mouth, gravity hopper where the raw rubber compound is poured into the hopper to be fed to the extruder.
The initial method for extruding was a hot process developed in the 1950s. Over the years, the process has been perfected and improved to enable the use of other ways of performing extrusion. The reason for the choice of extrusion for the production of rubber bushings is its ability to manufacture high quality products with the highest output. The two forms of extrusion are hot feed extrusion (HFE) and cold feed extrusion (CFE).
The extrusion process is a continuous process that is capable of producing bushings with accurate tolerances. After the rubber compound is loaded into the hopper, it drops into the barrel of the extruder that contains a helical screw. As the screw turns, the rubber compound is moved along the barrel toward the die with the cross section of the shape and form of the rubber bushing.
The movement of the rubber compound along the length of the barrel causes pressure build up, which pushes the raw rubber material through the die. The die-shaped bushings leave the extruder as a long tube like material that is still soft and pliable but has the shape of the bushings.
Hot Feed Extrusion – The screw of a HFE extruder is larger and shorter. This design provides a consistent output and keeps the dwell time and temperature increase to a minimum. The HFE hopper has a feed roller with a spiral undercut liner that allows the rubber compound to enter easier.
The rubber that is fed into a HFE is pre heated and warmed using a two roll mixing mill. This step in the HFE process forces the rubber compound to reach the proper degree of viscosity and temperature to be able to flow the rubber through the die of the extruder smoothly.
The difficulties of hot feed extrusion have led to the development of cold feed rubber extrusion.
Cold Feed Extrusion – With cold feed rubber extrusion, the rubber compound is fed into the hopper at room temperature. Cold feed extrusion is also known as compression forming. The screw has decreasing pitch with increasing diameter and more flight depth at the feed end to increase feed output. The pressure in cold feed extrusion is higher to force the rubber compound through the die.
A special version of cold feed extrusion is a pin barrel that has 80 to 100 pins sticking out of the barrel of the extruder pointed toward the screw. The purpose of the pins is to improve the mixing process, which improves its homogeneity and the quality of the extruded pieces. The flexibility of pin barrel cold extruders allows them to extrude hard rubber compounds, which is not possible with other cold feed extrusion processes.
Rubber extruders are categorized by the diameter of the barrel and their length, which determines whether the extruder is a hot feed extruder or cold feed extruder with hot feed barrels being smaller and shorter while cold feed barrels are longer and larger.
When the rubber bushing leaves the extruder, it is still pliable and soft. For it to be useful to fit the needs of an application, it has to be vulcanized to give it tensile strength, low water absorption, resistance to oxidation, wear, and tearing from abrasions. Vulcanization is a chemical process where the rubber is heated with an agent, normally sulfur, and an activator, accelerator, and retarder. The three steps of vulcanization are induction, curing, and optimum.
Induction – Induction is where the crosslinking process begins at temperatures between 180 °F or 82 °C and 230 °F or 110 °C.
Curing – In the curing stage, the chemical chains of the rubber are linked together, which gives the rubber its firmness and resilience.
Optimum – The optimum stage is a continuation of the curing stage and gives the rubber its elasticity. If this stage is extended too long, the rubber will continue to harden and have increased elasticity but lose its tensile strength.
In the extruding process, there are two methods for completing curing, which are in line and off line curing. Since extrusion is a continuous and ongoing process, both methods are designed to fit the process.
In Line – After the rubber is forced through the die, it enters a curing tunnel that raises the rubber to the vulcanizing temperature. The length of the tunnel determines the length of time of the curing process. When the cured rubber leaves the tunnel, it enters a cooling trough where it is cleansed and cooled.
There are four methods used for the curing tunnel, which are high velocity hot air, microwave ovens, hot liquid, or a bed of glass beads.
Off Line – In off line curing, autoclaves are used to expose the rubber to high pressure steam to bring the rubber to the appropriate vulcanization temperature and held there by a chemical reaction. Prior to entering the autoclave, the rubber is forced over a mandrel.
Rubber bushings are widely used for their ability to reduce shock and vibrations. In automobile construction, they improve safety and comfort as well as minimize noise and sounds. There are many types of rubber bushings that are used in equipment and machinery to reduce damage and increase the useful life of the equipment.
The initial use for rubber bushings was in automobiles as a method for reducing noise. They were used as rubber vibration isolating mounts that drastically reduced vibrations and noise in cars. From that beginning, the use of rubber bushings has expanded into other devices where metal surfaces meet.
Rubber bushings can be found in all types of machinery and can be designed to easily fit into applications that are extremely large or very small. This versatility has made them a major design factor for tools, production machinery, airplanes, railroads, and other complex devices. They can usually be found in conditions where there is the potential of vibrations and noise.
A major advantage of rubber bushings is their ability to absorb shock. This is one of the reasons that led to their use in automobiles. Though this is still a prominent part of their use in automobile manufacturing, this characteristic has made them viable for other applications that involve impact.
Vibrations in any mechanism can be damaging since they involve two pieces or surfaces striking each other. This constant pounding can lead to severe damage to components and breakdowns. Rubber bushings serve as a cushion and stabilizer between surfaces to control and prevent vibrations.
One of the essential design elements in the auto industry is sound suppression and control. Every manufacturer emphasizes that their product provides a quiet ride. The key component in achieving that goal is strategically placed rubber bushings that limit, control, and suppress noises from a car.
Part of rubber bushings functions in eliminating vibrations is their ability to restrict the torsional and conical movement of surfaces. They reduce the energy from moving components. Rubber bushings for this function are pressed into a housing. In some cases, this involves the use of cylinder bushings with inner and outer metal cylinders.
Rubber bushings have a very simple structure. Regardless of their design, which makes them capable of being adapted for a wide variety of vibration control and noise suppression applications. Made of synthetic or natural rubbers, rubber bushings are being used in commercial, residential, and industrial processes due to their strength, versatility, and resilience.
In a car's suspension system, rubber bushings dampen vibrations and assist in controlling the vehicle. They are used to protect wire lines, pipes, and tubes. The implementation of bushings are crucial to ensure a smooth and stable ride.
Rubber bushings are soft and pliable materials that are used in skateboards to aid in tilting and turning the board. They are connected to the axle of the board to help in smooth rotation and spin. The three common types of rubber bushings used on skateboards are double barrel, cone barrel, and cone shaped.
Shock rubber bushings are designed to buffer and isolate the energy of vibrations. The structure of shock rubber bushings allows a limited amount of movement that radically reduces noise. They are mainly used in automobiles and skateboards but can be found in other applications where shock absorption is necessary.
A tank bushing is used to seal a connection when a pipe passes through the wall of a tank. They are held in place by friction and are less rigid than bulkhead fittings. They come in different dimensions to fit the needs of the tank and are installed on the top above or below the waterline.
Rubber bushings are used in fans and blowers as vibration suppressors. They are designed to take a vertical load in shear and compression form to isolate vibrations. Fan rubber bushings come in shapes and configurations to fit the design of the type of fan.
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