The article contains everything you need to know about polyurethane bushings and their use.
You will learn:
A polyurethane bushing is a friction reducing component that is placed between moving and stationary components as a replacement for lubricants. The use of polyurethane for the production of bushings is due to its resilience, strength, and its ability to withstand stress and friction. Polyurethane bushings are long lasting and durable bushings that out last plastic and rubber bushings.
Polyurethane is a polymer produced by combining diisocyanates, an organic compound consisting of two isocyanate groups, and polyols. The term polyurethane is in reference to several types of polyurethane each of which has a different molecular structure. One of the distinct differences between the types of polyurethane is softness and hardness, which is formulated to fit the needs of a specific application.
Although polyurethane and rubber perform similar functions, there are several differences between the two materials. Polyurethane is known for its strength, durability, longevity, and toughness in the face of stressful conditions. Additionally, unlike rubber, polyurethane does not degrade over time.
Polyurethane bushings are produced using different methods, including casting, extrusion, and injection molding. The choice of method is dependent on the type of polyurethane material, the type of bushing being produced, and the quantity, which can vary from a few hundred to thousands. In all cases, polyurethane is compressed and shaped to the desired configuration for the application.
With the injection molding process, molten polyurethane material is forced, under pressure, into steel molds where it is allowed to cool and solidify. It is a mass production method that has a clamping unit and injection unit with the mold placed between the two units.
The clamping unit has to be able to withstand the forces that are applied to the mold during the injection process. The polyurethane material is fed into the injection unit through a hopper above the screw mechanism of the injection unit. As the screw turns, the polyurethane is melted and forced by the screw into the mold that is held in place by the clamping unit.
The three types of clamping units are:
The injection unit is made up of the hopper, heaters, screw, and motor. Polyurethane pellets or powder are poured into the hopper set on top of the injection molding unit. The hopper opens into the screw that moves the material to the die and increases the pressure. As the screw moves the polyurethane material to the die, it is heated by heating units set along the length of the screw shaft. The screw is turned by a motor at the opposite end of the screw shaft from the mold.
The mold is in the shape of the polyurethane bushing and made of steel. It has a sprue that connects it to the injection unit through which the molten polyurethane is forced into it. The mold is precision crafted down to the finest detail to ensure the proper tolerance and dimensions of the bushing.
Molds normally have multiple cavities in order to produce multiple polyurethane bushings at one time. In the case of multiple cavity molds, the sprue that brings in the molten polyurethane is attached to runners that connect to the various cavities. In order to ensure that there aren't any flaws or distortions in the collection of bushings, all of the runners are the same distance from the sprue.
After the polyurethane material is injected into the mold cavities, it is held under pressure. The length of the holding time varies depending on the type of polyurethane and the complexity of the bushing. This aspect of the process is critical since it ensures that the formed bushing sets correctly. During the cooling process, the screw withdraws to remove its pressure to allow for proper cooling. At the end of the cooling process, the clamps release and the part is ejected.
The casting of polyurethane is similar to the injection molding process without the use of the clamping and injection units. It involves the use of a mold in the shape of the bushing and is a less expensive process that is normally used for short or medium production runs. The casting process begins with a mold.
The process for casting polyurethane bushings begins with the mixing of the various compounds to form the type of polyurethane used to produce a bushing. It is formed by reacting a polyol with a diisocyanate in the presence of a catalyst and additives. The different types of diisocyanates and polyols determine the kind of polyurethane. The characteristics of polyurethane makes it possible to mold a wide variety of shapes and configurations.
Polyurethane casting molds are made from aluminum, silicone, or steel. The types of casting molds are closed molds that have two halves that close over the injected polyurethane and open molds, where the polyurethane compound is poured into the mold and consists of a single section. In both cases, heated liquified polyurethane is injected into a mold that has the shape and features of a bushing.
After the molds have been injected with polyurethane, they are moved into ovens where they are exposed to high temperatures to harden the polyurethane and solidify it. This aspect of the process has to be closely monitored to ensure the quality and dynamics of the bushings.
Compression molding is a high temperature process that compresses heated measured polyurethane, referred to as a charge, into a bushing mold. The molds for the process are produced by machining, die casting, and 3D printing. The process for compression molding is similar to injection molding in that it injects the charge into the mold. Unlike injection molding, compression molding does not include the injection and clamping units.
The creation of the mold involves the use of a pattern that can be produced using various methods. The key to the mold is its ability to withstand the forces required for the compression molding process and the heat that is required. Die casting is the most popular method for producing molds due to the durability of the metals used to produce the molds. As with many modern production methods, computerized numerical control (CNC) machining and 3D printing are commonly used to produce molds.
The charge has to be carefully prepared and measured to ensure the quality of the molded part. If the charge is too large, excess material will creep out of the mold and create flash that will need to be removed. This aspect of the process has to be carefully monitored and controlled. Once the charge is properly prepared, it is placed in the mold.
The top, movable half of the mold is placed over the lower fixed half of the mold. The mold is heated and pressurized to force the charge to take the shape of the bushing. As the pressure increases, the polyurethane charge is cured into the desired shape of the bushing.
Factors that are controlled and monitored during the compression process to ensure the quality of the polyurethane bushing are the amount of heat, pressure, and length of compression. Heating of the charge can take place prior to it being placed in the mold to increase its viscosity. The amount of pressure is dependent on the type of polyurethane with denser materials requiring greater pressure. The length of time the charge is placed under pressure varies according to the type of polyurethane and the type of bushing.
The compression molding process is completed at 350o F (176o C) with typical mold pressures at 100 psi, which can go as high as 2000 psi. The compressive force distributes the material of the charge into every part of the polyurethane bushing mold.
The purpose of curing is to harden the compressed part and can involve lowering the temperature of the mold or the use of hardening agents and catalysts. Curing times vary according to the type of bushing being molded and the type of polyurethane. In most cases, curing takes place over 8 to 16 hours in an oven that is over 200°F (93.3°C).
At the completion of curing, the bushing is removed from the mold by being ejected automatically or manually with manual ejection used for small production runs. Automatic ejection uses a mechanism to remove the bushing from the mold by pulling or pushing it out. Although it is not common, a release agent may be applied to the surface of the mold to keep the bushing from sticking to the mold during ejection. The complexity of a bushing determines how easy it is to eject it.
Casting, injection molding, and compression molding are three common methods for the production of polyurethane bushings but are not the only methods. How they are produced varies between manufacturers and can include vacuum molding and forming by machining. Each of the various methods has their advantages and are used in accordance with the requirements of the bushing being produced.
The changing of the polyols and diisocyanates produces different types of polyurethane with different properties. Variations in polyurethane make it possible to use it in a variety of applications. What needs to be noted is that polyurethane is not a form of rubber or plastic but is a rigid, firm, flexible material with properties that have made it one of the most widely used of polymer materials.
The two general types of polyurethane are polyester and polyether, which are elastomers with elastic properties that are resistant to abrasion. A basic difference between the polyurethanes is their reaction to temperatures. Polyester polyurethane can withstand high temperatures for extended periods of time while polyether polyurethane resists the effects of a wide range of temperature changes.
Polyester polyurethane is stronger and more rigid than polyether polyurethane with higher tensile strength. Of the two polyurethanes, polyester is the least expensive and can deteriorate in humid, damp, and wet environments. The ester part of polyester polyurethane is a class of compound made from an acid reacted with alcohol.
The COF is in regard to two bodies that slide against each other, which help or hurt an application. Low friction is desirable when sliding occurs and is a characteristic of polyester polyurethane that makes it ideal for the production of bushings. Additionally, the hardness of polyester polyurethane further influences its COF with harder polyester polyurethane having a lower COF compared to softer polyester polyurethane. In most cases, the kinetic COF of polyester polyurethane is between 0.2 and 2.5.
Abrasion resistance is one of the more notable properties of polyester polyurethane. It is capable of outwearing plastics, metals, and rubber. The abrasion resistance of polyester polyurethane is referred to as sliding resistance.
The heat resistance of polyester polyurethane is specific to high temperatures for long periods of time, which makes it resistant to heat aging.
Polyurethane can be formulated with hardness ranging from very soft A scale hardness to very hard D scale hardness. The durometer reading for polyurethane bushings is from a hardness of 80A to 95A up to rock hard 75D. The tensile strength, durability, and hardness of polyurethane is the reason that it is used to produce bushings.
Polyester polyurethane is able to absorb energy and is used for shock absorption and vibration dampening. It is often used in applications where corrosion, vibration, and magnetism prevent the use of steel springs.
The high tensile strength of polyester polyurethane makes it resistant to cutting and tearing, a property that enhances its durability.
The structure and strength of polyester polyurethane is highly resistant to the attack of fuels, oil, and solvents, which makes it ideal for industrial and manufacturing conditions.
Polycaprolactone polyurethane is a special form of polyester polyurethane with UV resistance and greater tensile and tear strength than polyether polyurethanes. Unlike polyester polyurethane, polycaprolactone polyurethane has better low and high temperature performance and hydrolytic stability. PCL is mostly used as an additive for resins to improve their toughness, flexibility, compression set, and tear strength. It is a biodegradable polymer that is used in 3D printing.
Polyether polyurethane is used for applications where there is high stress. It is a durable polyurethane capable of withstanding very high to very low temperatures. Unlike polyester polyurethanes, polyether polyurethanes are resistant to moisture, humidity, and microorganisms.
Polyether polyurethane has excellent impingement abrasion resistance due to ether’s high rebound capacity. Unlike scraping abrasion, impingement abrasion refers to particles striking the polyurethane surface at a high angle, which makes polyether polyurethane bushings capable of withstanding harsh conditions.
Polyether polyurethanes are able to withstand radical changes in temperature from very high to very low. The advantage of polyether polyurethane over polyester polyurethane is its ability to adapt to temperature changes without cracking.
The hardness of both polyurethanes is the same. They can be manufactured with the hardness of a pillow or that of a bowling ball. On the shore hardness scale, the hardness of polyether polyurethane varies between 55 shore A up to 75 shore D, which are the typical hardness of polyurethane bushings.
Polyether polyurethane has a high rebound or resilience of return energy. When it is deformed, it loses the energy that is necessary to achieve deformation. The process of recovering from deformation, also, requires energy. Polyether polyurethane has exceptional resilience when recovering from deformation and retains all of the energy it used during deformation.
One of the important properties of polyether polyurethane is its hydrolytic stability, the resistance to the effects of water on its molecular structure. Polyurethane bushings are exposed to a wide array of environmental conditions, most of which have some form of moisture content. The hydrolytic stability of polyether polyurethane is due to its ether linkage, which gives it an exceptionally high stability.
| Mechanical Properties of Polyester and Polyether Polyurethanes | ||
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| Resilience and Rebound |
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The flexibility of polyurethane makes it possible to shape and form it into any type of product, part, or component. It is an easy to form compound that can be manufactured at room temperature. Much of its popularity is due to the ability to change its chemical structure to achieve different and unique properties.
Polyurethane bushings are available in a wide variety of shapes and sizes with custom bushings for special and unique applications. Their use is constantly growing as new devices are being introduced that require the support, security, and strength of a resilient and reliable material. The function of bushings is to reduce the friction between moving and stationary parts and to perform as a replacement for lubricants due to their wear resistance and low maintenance.
The wide use of polyurethane bushings over rubber, plastic, and steel bushings is due to their high load capacity and high compression properties, which makes polyurethane bushings capable of lasting longer. The types of polyurethane bushings range from sponge soft to carbon steel strong in any color, shape, size, or dimensions. They are resistant to environmental effects such as heat and moisture and the effects of solvents, chemicals, and abrasion.
Suspension bushings were the initial type of polyurethane bushings due to their ability to reduce friction and absorb shock to provide a smooth ride when driving over uneven surfaces. They control the degree of movement of the steering joints to prevent sway when turning. Suspension bushings are located in the control arms, ball joints, tie rods, sway bars, shock absorbers, and strut mounts.
Polyurethane bushings have the strength and resilience of wood and plastic with the flexibility and elasticity of rubber. The main characteristic of polyurethane, which makes it ideal for use in automobiles, is its durability. Suspension bushings are constantly being exposed to stress and stretching. Their hardness gives drivers a better feel for the road and increased control over a vehicle.
Flange bushings are also known as flange bearings but do not contain bearings and act as a protective interface between parts to dampen their energy, which is redirected through the bushing. They have a shoulder on one end and can be used to assist in mounting and aligning bearings to make bearing installation easier. Flange polyurethane bushings are a form of sleeve bushing with the main difference from standard sleeve bushings being their shoulder. The five parts of a flange bushing include the inner diameter (ID), outer diameter (OD), length (OAL), flange outer diameter (FLOD), and flange thickness (FLTH). These dimensions assist manufacturers in the production of flange bushings.
Polyurethane motor mount bushings are designed to prevent the transfer of noise, vibrations, and movement that can cause wear on engine components. They are used as aftermarket replacements for factory installed rubber mounts that degrade over time and are flexible and antivibration. Unlike rubber mounts, polyurethane motor mount bushings are resistant to the effects of oil, coolants, and grease and can withstand rapid changes in temperature.
The popularity of polyurethane motor mount bushings is their longevity, which is three to four times longer than rubber or plastic mounts. Polyurethane motor mount bushings do not crack, dry out, or fracture due to their exceptional durability and strength. They are used in place of metal bushings because they do not mar or damage surrounding components and do not produce vibrations or noise.
The design of polyurethane motor mount bushings varies in accordance with the make and model of the vehicle where they are used since frames and engine bay space varies. It is for this reason that manufacturers have several types of motor mounts, including custom designed ones, to meet the needs of multiple vehicles. A critical factor in regard to the manufacture of polyurethane motor mount bushings is the testing of their viability to ensure their durability and performance.
Hydraulic cylinder polyurethane bushings are used as wear resistant liners to reduce friction between pins and connecting components. They are used in place of regular bushings due to their self-lubricating property. Hydraulic cylinder polyurethane bushings have a metal base with a polyurethane coating that provides continuous lubrication.
Split polyurethane bushings reduce friction in heavy equipment and have a split along the length of the bushing that makes them easy to install. The split in polyurethane split bushings gives them flexibility and provides a strong hold. The slit makes it possible to change shafts without retooling in applications that do not demand precise boring.
The construction of a clench polyurethane bushing is the same as a split polyurethane bushing with the addition of interlocking tabs along the edge of the split. The shape of the tabs is designed to prevent gaps in the slit.
Sleeve polyurethane bushings are the simplest form of bushing. They have a cylindrical shape that is similar to a very small tube. Sleeve polyurethane bushings are used for radial loads and can be used with thrust washers for heavy loads. As with all forms of bushings, they reduce friction between components and absorb vibrations. They are designed for heavy loads, high temperatures, and have a compact design.
DOM sleeve polyurethane bushings are a combination of a polyurethane sleeve bushing and metal cylinders with the metal cylinders made of a variety of metals including bronze, steel, and stainless steel. The polyurethane sleeve provides lubrication while the metal cylinder or cylinders give the bushing extra stability. The common use of DOM sleeve polyurethane bushings is as motor mounts and suspension links. They come in standard sizes and are commonly custom made for various vehicles and heavy-duty equipment.
In the production of bushings, customers have a choice between rubber and polyurethane since each material has benefits and positive properties. The choice of material is very much dependent on the purpose of the bushing and its performance. There are several factors to consider when making the selection between the two materials with various plastics also being a possible choice.
There has been some confusion regarding rubber and polyurethane with names such as polyurethane rubber or polyurethane plastic, none of which apply to polyurethane. The main distinction of polyurethane is the many positive properties it provides compared to any other type of material.
Rubber bushings have been used for years in its synthetic and natural form. It is valued for its flexibility, shock resistance, and durability. Rubber is capable of absorbing shock and compressing under a load due to it being a soft material that gives under pressure. It can be damaged by heat, oil, and chemicals and does not wear well. When exposed to certain environmental conditions, rubber can be significantly damaged.
Synthetic rubber is an elastomer that is known for its elasticity, durability, toughness, and resilience. It is produced in several forms including styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), silicone rubber, and ethylene propylene diene monomer (EPDM). The various forms of synthetic rubbers have properties and characteristics that make them suitable for specific applications.
The attraction of rubber in the production of bushings is its cost, which is the reason that it is so widely used in automobile manufacturing. An unfortunate aspect of rubber is its degradation in the face of repeated stretching, deformation, and stress.
The basic characteristic of polyurethane, that makes it a better bushing material than rubber, is its durability. The strength and toughness of polyurethane makes it possible for it to last five to ten times longer than rubber. When rubber bushings reach the end of their life cycle, they are normally replaced with polyurethane bushings.
Polyurethane is ideal for applications where there is constant stress due to its carbamate links. It has the elastic properties of rubber with the rigidity of plastic, which makes it usable in the same applications as rubber and plastics. Additionally, polyurethane bushings can be manufactured in several colors while the variations in the color of rubber bushings is limited to a few colors.
The original purpose of polyurethane bushings was as components for military equipment and race cars. They were exclusively used for those applications until the 1930s when they became available for other applications. During the 1990s, polyurethane bushings gained rapid acceptance as the replacement for rubber bushings. Since then, they have become the widely accepted standard for the high quality bushings. Polyurethane compounds are polymers made by combining diisocyanates and polyols. The term polyurethane is a descriptor for a group of materials that can be engineered and combined to produce a variety of materials. The unique molecular structure of polyurethane makes it resilient, strong, durable, and long lasting.
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