The article contains everything you need to know about polyurethane bushings and their use.
You will learn:
A polyurethane bushing serves as a friction-reducing element, positioned between moving parts and stationary surfaces as an alternative to traditional lubricants. The choice of polyurethane for these bushings is due to its impressive durability, robustness, and resistance to stress and friction. These bushings are more enduring compared to those made from plastic or rubber.
Polyurethane is a type of polymer created through the reaction between diisocyanates—organic compounds with two isocyanate groups—and polyols. The term "polyurethane" refers to various formulations of the polymer, each with unique molecular structures. These variations in softness and hardness allow polyurethane to be tailored for specific applications.
While both polyurethane and rubber are used to fulfill similar roles, they exhibit key differences. Polyurethane stands out for its superior strength, longevity, and resistance to harsh conditions. Unlike rubber, polyurethane does not deteriorate over time.
Various manufacturing techniques are employed to produce polyurethane bushings, such as casting, extrusion, and injection molding. The selection of a particular method depends on factors like the type of polyurethane used, the specific bushing design, and the production volume, which can range from several hundred to thousands of units. Regardless of the method, the polyurethane is shaped and compressed to meet the specifications required for its intended use.
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 must endure the stresses exerted on the mold during the injection molding process. Polyurethane is introduced into the injection unit via a hopper located above the screw mechanism. As the screw rotates, it melts the polyurethane and pushes it into the mold, which is secured by the clamping unit.
There are three main types of clamping units:
The injection unit comprises several components: the hopper, heaters, screw, and motor. Polyurethane pellets or powder are introduced into the hopper positioned above the injection molding assembly. The hopper feeds material into the screw, which advances the material toward the die while increasing pressure. Along the length of the screw shaft, heating elements warm the polyurethane as it moves toward the die. The motor, located at the end of the screw shaft opposite the mold, drives the rotation of the screw.
The mold is designed in the shape of the polyurethane bushing and constructed from steel. It includes a sprue that links it to the injection unit, allowing molten polyurethane to be injected into the mold. The mold is meticulously engineered to achieve precise tolerances and dimensions for each bushing.
Typically, molds are equipped with multiple cavities to produce several polyurethane bushings simultaneously. For such multi-cavity molds, the sprue is connected to a series of runners that distribute the molten polyurethane evenly to each cavity. To avoid any defects or inconsistencies in the finished bushings, the runners are uniformly spaced from the sprue.
Once the polyurethane is injected into the mold cavities, it is maintained under pressure. The duration of this holding period can differ based on the type of polyurethane used and the complexity of the bushing. This step is crucial for ensuring that the bushing solidifies properly. During cooling, the screw retracts to relieve pressure and facilitate adequate cooling. When cooling is complete, the clamping unit releases, and the part is ejected from the mold.
Polyurethane casting resembles the injection molding process but omits the clamping and injection units. Instead, it uses a mold shaped like the bushing and is a more cost-effective method typically employed for short to medium production runs. The process begins with preparing a mold for the casting operation.
The casting process for polyurethane bushings starts with mixing the necessary compounds to create the specific type of polyurethane required. This involves reacting a polyol with a diisocyanate, along with catalysts and additives. The particular types of diisocyanates and polyols used influence the properties of the resulting polyurethane. Thanks to its versatile characteristics, polyurethane can be molded into a diverse range 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.
Creating the mold involves using a pattern that can be made through various techniques. The crucial aspect of the mold is its capacity to endure the pressures and temperatures involved in the compression molding process. Die casting is a widely favored method for mold production because of the durability of the metals employed. Additionally, contemporary production methods frequently utilize computerized numerical control (CNC) machining and 3D printing to fabricate molds.
Proper preparation and measurement of the charge are essential for ensuring the quality of the molded part. If the charge is excessive, extra material may escape from the mold, resulting in flash that must be trimmed off. This part of the process requires careful monitoring and control. After the charge is correctly prepared, it is then placed into the mold.
The top, movable section of the mold is positioned over the lower, stationary half. The mold is then heated and pressurized to shape the charge into the bushing form. As the pressure builds, the polyurethane charge cures and conforms to the mold's design.
During the compression molding process, several factors are carefully controlled and monitored to ensure the quality of the polyurethane bushing. These include the temperature, pressure, and duration of compression. Preheating the charge can enhance its viscosity before it is introduced into the mold. The required pressure varies based on the polyurethane type, with denser materials needing higher pressures. Additionally, the duration of compression depends on both the type of polyurethane and the bushing design.
The compression molding process typically operates at 350°F (176°C), with mold pressures ranging from 100 psi to as high as 2000 psi. This compressive force ensures that the charge material is evenly distributed throughout the mold, achieving the desired shape of the polyurethane bushing.
Curing is essential for hardening the molded part and may involve cooling the mold or applying hardening agents and catalysts. The duration of curing depends on the specific type of bushing and the polyurethane used. Typically, curing occurs over 8 to 16 hours in an oven maintained at temperatures exceeding 200°F (93.3°C).
Once curing is complete, the bushing is removed from the mold either automatically or manually. For small production runs, manual ejection is often used, while automatic ejection involves a mechanism that pulls or pushes the bushing out of the mold. Although not frequently done, a release agent may be applied to the mold’s surface to prevent the bushing from sticking during removal. The ease of ejection depends on the complexity of the bushing design.
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.
Altering the types of polyols and diisocyanates results in different types of polyurethane, each with unique properties. These variations allow polyurethane to be utilized across a broad range of applications. It’s important to note that polyurethane is distinct from rubber or plastic; it is a rigid, firm, yet flexible material that stands out as one of the most versatile polymer materials available.
Polyurethane generally falls into two categories: polyester and polyether, both of which are elastomers known for their abrasion resistance and elastic qualities. The primary distinction between these types lies in their thermal properties. Polyester polyurethane can endure high temperatures for prolonged periods, while polyether polyurethane is better suited to handle a wide range of temperature fluctuations.
Polyester polyurethane offers greater strength and rigidity compared to polyether polyurethane, exhibiting higher tensile strength. Among the two types, polyester is generally the more cost-effective option but is prone to degradation in humid, damp, or wet conditions. Polyester polyurethane contains an ester component, which is a compound formed from the reaction between an acid and alcohol.
The coefficient of friction (COF) refers to the friction between two surfaces in relative motion, which can either benefit or hinder an application. Low friction is advantageous for sliding interactions, a property that makes polyester polyurethane well-suited for bushing production. Additionally, the hardness of polyester polyurethane affects its COF; harder polyester polyurethane typically has a lower COF than its softer counterparts. Generally, the kinetic COF for polyester polyurethane ranges from 0.2 to 2.5.
One of the key attributes of polyester polyurethane is its exceptional abrasion resistance. It outperforms many plastics, metals, and rubbers in terms of durability. This property, known as sliding resistance, is a significant advantage of polyester polyurethane.
Polyester polyurethane exhibits strong heat resistance, particularly at elevated temperatures over extended periods. This characteristic makes it highly resistant to heat aging.
Polyurethane can be customized to achieve a wide range of hardness, from very soft with A scale hardness to extremely hard with D scale hardness. For polyurethane bushings, the hardness typically ranges from 80A to 95A, extending up to as hard as 75D on the Rockwell scale. The combination of tensile strength, durability, and hardness makes polyurethane an ideal material for producing bushings.
Polyester polyurethane has excellent energy absorption properties, making it suitable for shock absorption and vibration dampening. It is commonly used in situations where steel springs are impractical due to issues like corrosion, vibration, or magnetism.
The impressive tensile strength of polyester polyurethane contributes to its resistance to cutting and tearing, significantly boosting its durability.
The structural integrity and strength of polyester polyurethane make it highly resistant to damage from fuels, oils, and solvents. This resistance makes it well-suited for use in industrial and manufacturing environments.
Polycaprolactone polyurethane is a distinctive type of polyester polyurethane known for its UV resistance and superior tensile and tear strength compared to polyether polyurethanes. In contrast to polyester polyurethane, polycaprolactone polyurethane offers enhanced performance at both low and high temperatures, as well as better hydrolytic stability. It is primarily used as an additive in resins to enhance their toughness, flexibility, compression set, and tear strength. Additionally, PCL is a biodegradable polymer often utilized in 3D printing applications.
Polyether polyurethane is well-suited for high-stress applications due to its durability and ability to endure extreme temperatures, both high and low. Unlike polyester polyurethanes, polyether polyurethanes offer superior resistance to moisture, humidity, and microorganisms.
Polyether polyurethane exhibits outstanding resistance to impingement abrasion, thanks to the high rebound capability of its ether components. Unlike scraping abrasion, which involves frictional wear, impingement abrasion occurs when particles hit the polyurethane surface at high angles. This characteristic enables polyether polyurethane bushings to endure challenging conditions effectively.
Polyether polyurethanes are capable of withstanding extreme temperature fluctuations, from very high to very low. Unlike polyester polyurethanes, polyether polyurethanes maintain their integrity and flexibility without cracking when exposed to such temperature variations.
Both types of polyurethanes can be produced with a wide range of hardness levels, from as soft as a pillow to as hard as a bowling ball. On the Shore hardness scale, polyether polyurethane typically ranges from 55 Shore A to 75 Shore D, which encompasses the usual hardness levels found in polyurethane bushings.
Polyether polyurethane exhibits excellent rebound resilience, meaning it efficiently returns energy after deformation. When it is compressed or deformed, it loses minimal energy in the process. Additionally, the energy required for recovery from deformation is also minimal. Polyether polyurethane stands out for its remarkable ability to retain and recover the energy used during deformation, demonstrating exceptional resilience.
An important property of polyether polyurethane is its hydrolytic stability, which refers to its resistance to water-induced degradation of its molecular structure. Given that polyurethane bushings are often exposed to various environmental conditions that include moisture, this stability is crucial. The high hydrolytic stability of polyether polyurethane is attributed to its ether linkages, which contribute to its exceptional durability against water-related effects.
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Polyurethane's flexibility allows it to be molded into a wide range of products, parts, and components. It is a versatile material that can be shaped and formed at room temperature. Its popularity stems from its ability to be chemically engineered to exhibit various unique properties, making it highly adaptable to different applications.
Polyurethane bushings come in a diverse range of shapes and sizes, with custom options available for specialized applications. Their popularity is increasing as new technologies and devices emerge that require the durability and reliability of a high-performance material. Bushings serve to minimize friction between moving and stationary components and can replace traditional lubricants due to their durability and low maintenance needs.
Polyurethane bushings are preferred over rubber, plastic, and steel alternatives because of their superior load-bearing capacity and exceptional compression resistance, which contributes to their longer lifespan. These bushings are available in various forms, from soft sponge-like to extremely tough, and can be customized in different colors, shapes, sizes, and dimensions. They offer resistance to environmental factors such as heat and moisture, as well as to solvents, chemicals, and wear.
Suspension bushings were among the first applications of polyurethane bushings due to their effectiveness in minimizing friction and absorbing shocks, ensuring a smoother ride over rough terrain. They help regulate the movement of steering joints to reduce sway during turns. These bushings are typically found in control arms, ball joints, tie rods, sway bars, shock absorbers, and strut mounts.
Polyurethane bushings combine the strength and durability of materials like wood and plastic with the flexibility and resilience of rubber. Their standout feature is their exceptional durability, which makes them particularly suitable for automotive use. Suspension bushings are subjected to continuous stress and deformation, and their hardness enhances road feel and vehicle control for drivers.
Flange bushings, sometimes referred to as flange bearings, do not actually contain bearings. Instead, they serve as a protective interface between components, absorbing and redirecting energy through the bushing. Featuring a shoulder on one end, they are often used to facilitate the mounting and alignment of bearings, simplifying the installation process. Unlike standard sleeve bushings, flange polyurethane bushings are equipped with this additional shoulder. The key dimensions of a flange bushing include the inner diameter (ID), outer diameter (OD), overall length (OAL), flange outer diameter (FLOD), and flange thickness (FLTH). These measurements are crucial for manufacturers during the production of flange bushings.
Polyurethane motor mount bushings are engineered to minimize the transfer of noise, vibrations, and movement that can lead to wear on engine parts. They serve as aftermarket replacements for factory-installed rubber mounts, which tend to deteriorate over time. These bushings offer flexibility and anti-vibration properties. Unlike rubber mounts, polyurethane motor mount bushings are resistant to oil, coolants, and grease, and they can endure rapid temperature changes.
The appeal of polyurethane motor mount bushings lies in their extended lifespan, which is typically three to four times longer than that of rubber or plastic mounts. Due to their superior durability and strength, these bushings do not crack, dry out, or fracture. They are preferred over metal bushings because they do not damage surrounding components and do not generate vibrations or noise.
The design of polyurethane motor mount bushings is tailored to fit various vehicle makes and models, as engine bays and frames differ significantly. Consequently, manufacturers offer a range of motor mounts, including custom options, to accommodate different vehicles. An essential aspect of producing polyurethane motor mount bushings is rigorous testing to ensure their durability and performance.
Hydraulic cylinder polyurethane bushings function as wear-resistant liners designed to minimize friction between pins and connecting parts. These bushings are preferred over standard bushings because of their self-lubricating capabilities. They feature a metal base coated with polyurethane, which ensures ongoing lubrication throughout their use.
Split polyurethane bushings are designed to minimize friction in heavy machinery and feature a longitudinal split that simplifies installation. This split provides both flexibility and a secure fit. Additionally, the slit allows for easy shaft replacement without the need for retooling, making them ideal for applications where exact boring is not required.
Clench polyurethane bushings are similar to split polyurethane bushings but include interlocking tabs along the split edges. These tabs are specifically designed to eliminate gaps in the slit, ensuring a more secure and gap-free fit.
Sleeve polyurethane bushings are among the most basic types of bushings, featuring a cylindrical shape akin to a small tube. They are designed to handle radial loads and can be paired with thrust washers to accommodate heavier loads. Like other bushings, they reduce friction between moving parts and help absorb vibrations. Engineered for durability, sleeve bushings are suited for high-temperature environments and are compact in design.
DOM sleeve polyurethane bushings combine a polyurethane sleeve with metal cylinders, which can be crafted from materials such as bronze, steel, or stainless steel. The polyurethane sleeve offers lubrication, while the metal cylinders enhance the bushing's stability. These bushings are frequently used as motor mounts and suspension links. Available in standard sizes, they are also often custom manufactured to fit a range of vehicles and heavy-duty machinery.
When selecting materials for bushings, customers can choose between rubber and polyurethane, each offering distinct advantages. The decision largely depends on the bushing's intended use and performance requirements. Several factors influence this choice, and various types of plastics may also be considered as alternatives.
There is often confusion surrounding terms like polyurethane rubber or polyurethane plastic, but these do not accurately describe polyurethane. The key advantage of polyurethane lies in its unique properties, which set it apart from other materials.
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, an elastomer known for its elasticity, durability, toughness, and resilience, is available in several types including styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), silicone rubber, and ethylene propylene diene monomer (EPDM). Each type of synthetic rubber has specific properties that make it suitable for different applications.
Rubber is favored in bushing production primarily due to its cost-effectiveness, which contributes to its widespread use in the automotive industry. However, rubber does have a downside: it tends to degrade under repeated stretching, deformation, and stress.
Polyurethane stands out as a superior bushing material compared to rubber primarily due to its exceptional durability. Its strength and toughness enable polyurethane to outlast rubber by five to ten times. Typically, when rubber bushings reach the end of their service life, they are replaced with polyurethane bushings.
Polyurethane is particularly well-suited for applications involving constant stress thanks to its carbamate linkages. It combines the elastic qualities of rubber with the rigidity of plastic, making it versatile for similar uses as rubber and plastics. Moreover, polyurethane bushings can be produced in a wide range of colors, whereas rubber bushings generally come in only a few color options.
Originally, polyurethane bushings were developed for use in military equipment and racing vehicles. They remained exclusive to these applications until the 1930s when they began to be used more broadly. By the 1990s, polyurethane bushings quickly became a popular alternative to rubber bushings. Today, they are widely recognized as the benchmark for high-quality bushings. Polyurethane is a polymer created by reacting diisocyanates with polyols. The term "polyurethane" refers to a range of materials that can be engineered and combined in various ways to produce diverse properties. The distinctive molecular structure of polyurethane provides it with exceptional resilience, strength, durability, and longevity.
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