This article will take an in-depth look at hydraulic seals.
The article will bring more detail on topics such as:
This chapter will cover hydraulic seals, including their design, construction, and the key considerations for selecting the appropriate seal.
Hydraulic seals are specialized gasket-like rings designed to fill the gaps between components in hydraulic cylinders. They play a crucial role in preventing fluid leakage around these components. These seals are crafted to fit the specific parts of a hydraulic cylinder, ensuring a reliable, leak-proof seal.
O-rings and hydraulic seals share similarities, but hydraulic seals feature a groove in the lip that allows components to fit securely. This groove ensures a leak-proof seal as it moves over the component. Even with piston movement and increasing fluid pressure in the hydraulic cylinder, the seal effectively prevents leakage by keeping the fluid contained within the appropriate chamber.
When constructing hydraulic seals, it's important to consider both the manufacturing process and the materials used in their production.
In the manufacturing process, hydraulic seals are produced using a Computerized Numerical Control (CNC) lathe machine. This machine can be programmed to create both standard and custom seal profiles. The CNC lathe then cuts the seal shapes from the chosen material based on the digitized data of the profile.
Hydraulic seals are available in various materials. Rubber is a common choice for hydraulic seals due to its flexibility, durability, and resistance to cracking. Polyurethane is another popular material for hydraulic seals, known for its superior durability and wear resistance compared to rubber seals, while offering similar functional properties.
Polytetrafluoroethylene (PTFE) is a less common material for hydraulic seals compared to rubber or polyurethane. PTFE is known for its flexibility, durability, and resistance to high temperatures. The choice of material often depends on specific operating conditions such as the type of fluid, pressure levels, chemical compatibility, and temperature requirements.
The various considerations when choosing a seal are:
The maximum speed of the shaft depends on factors such as the shaft's finish, runout, housing bore, concentricity, type of fluid being sealed, and the material of the oil seal.
The mechanism's temperature range where the seal is installed should not exceed the temperature range of the seal's elastomer.
Typical oil seals are designed to handle very low pressures (around 8 psi or less). For situations with increased internal pressure, a pressure relief mechanism is necessary.
Shafts with a Rockwell hardness of 30 or above can expect a longer seal life. For environments with abrasive contaminants, increase hardness to RC 60.
Optimal sealing performance is achieved through superior shaft surface treatments. The effectiveness of the seal is influenced by the spiral lead and the orientation of the finishing tool marks. For the highest sealing efficiency, shafts should be polished or ground with concentric finish marks (no spiral lead). If a spiral finish is necessary, ensure the spirals are oriented in the direction of the fluid flow as the shaft turns.
When the bore and shaft centers are not aligned, seal longevity decreases due to wear concentrated on one side of the sealing lip.
Optimal seal performance is achieved when shaft and bore tolerances are closely matched. Additionally, factors such as shaft eccentricity, end play, and vibration should be taken into account.
The amount of runout must be maintained to a bare minimum. Bearing wobbling or shaft whip are the most common causes of the center of rotation movement. This difficulty is exacerbated when it is combined with misalignment. Flexible couplings, contrary to popular assumption and usual practice, cannot adjust or compensate for misalignment.
When seals are regularly lubricated with oil that has the suitable viscosity for the application and is compatible with the seal lip elastomer material, they perform significantly better and last much longer. The possibility of seal incompatibility, especially with specific additives and synthetic lubricants, should not be overlooked.
The various types of hydraulic seals are:
Hydraulic cylinder seals seal openings between the various parts of the hydraulic cylinder. Their design makes them retain hydraulic fluids, keep solid or liquid impurities out, and maintain hydraulic pressure. These tasks require a wide range of seal designs and performance-boosting features.
To ensure fluid containment, the seal material must conform to any irregularities on the metal surfaces. The seal needs to adjust quickly by expanding or compressing to accommodate changes in clearance gap size. Additionally, the seal should possess adequate modulus and hardness to withstand shear stress from system pressure, preventing extrusion into gaps.
Effective sealing in fluid power systems requires not only containing the fluid but also keeping contaminants out. The choice of seal depends on the nature of the surfaces it interacts with. Static seals are used when there is no movement between the surfaces, while dynamic seals are necessary when there is relative motion, such as oscillating or reciprocating actions.
The design of the seal lip varies according to its intended application. Most seals feature what are referred to as "lips" in their construction, including those used in radial, rotary, and linear shaft seals.
Lip seals not only serve as barriers or dams but also act as pumps. They are frequently used with rotary, reciprocating, and oscillating shafts. The primary roles of a lip seal, also known as a radial or rotary seal, include retaining lubricants, keeping contaminants out, maintaining pressure, and separating fluids.
Wiper scraper seals inherently include lips in their design, classifying them as lip seals. However, the term "lip seal" lacks a standardized definition. Additionally, radial seals, often called lip seals, are referred to as rotary seals in the UK. Although "rotary" is technically accurate, "radial" is more commonly used in the US because these seals are radially energized (typically with a spring) and are smaller than the shaft they encase.
The more popular European name for this type of seal – rotary shaft seal – is selected since it is most usually utilized where a rotating shaft passes through it. Rotary seals, which are similar but slightly different, are also utilized in linear applications like motorcycle fork stanchion sliders. These necessitate a whole distinct lip design, leading to a misunderstanding of the entire rotational terminology. These misunderstandings can lead to application issues.
The primary role of a lip seal is to prevent contaminants from entering while retaining lubricants. By design, lip seals generate friction, making them suitable for various applications, from slow-moving machinery to high-speed rotations, and across a broad temperature range from below zero to over 500 degrees Fahrenheit.
For a lip seal to be effective, it must maintain proper contact with its rotating counterpart, which depends on correct selection, installation, and maintenance. New lip seals might leak initially due to improper installation, but some may cease leaking once the material conforms to the shaft.
Historically, a basic lip seal was a leather strap fitted around a wheel axle. Modern lip seals are influenced by various factors and come in different types, including non-spring and spring-loaded versions, as well as various contact patterns. Non-spring seals are generally more affordable and effective at retaining viscous substances like oil at lower shaft speeds. They are commonly used in applications such as conveyors, vehicle wheels, and lubricated components. On the other hand, spring-loaded seals are widely used across various equipment and are typically employed with oils.
Mechanical seals are used in rotating equipment like pumps and mixers to prevent leaks of liquids and gases into the environment.
A mechanical seal consists of two primary components: one stationary and one rotating against it to form the seal. These seals vary widely in shape and complexity, from basic single-spring designs to intricate cartridge models. The design, materials, and construction depend on factors like pressure, temperature, rotational speed, and the type of product being sealed.
A typical mechanical seal consists of seven key components: the stationary component (often referred to as the seat), the stationary component sealing member, the rotating component, the rotating component sealing member, the spring, the gland plate, and the clamp ring.
Mechanical seals feature four primary sealing points. The primary seal is located between the rotating and stationary faces. The gasket seals between the stationary member and the stuffing box face. The secondary seal, which may be an o-ring, is positioned between the rotating member and the shaft or shaft sleeve. Lastly, the seal between the gland plate and the stuffing box is typically achieved with a gasket or o-ring.
The key sealing point between the rotating and stationary components is crucial to the mechanical seal's design and function. Typically, spring force presses the rotating and stationary faces together.
These mating faces are meticulously machined (lapped) to achieve extreme flatness, often within 2 light-bands (an optical measurement standard). This precision minimizes leakage to almost negligible levels. However, some minimal leakage may still occur and manifest as a mist. Spring compression usually provides the initial face pressure, and when the seal is idle, the springs maintain this pressure to prevent leakage.
Without lubrication, the friction between the seal faces and the resulting heat would lead to wear and eventual seal failure. Therefore, lubrication is essential, typically provided by the product medium. This lubrication forms a fluid film, which is crucial for maintaining the seal’s performance and ensuring reliable and consistent operation.
Oil seals, also referred to as grease, fluid, and dirt seals, are designed to cover gaps between stationary and moving parts in mechanical equipment. They serve to prevent lubricants from leaking out and to keep contaminants from entering the machinery, which is particularly important in challenging environments. Additionally, oil seals help to prevent the mixing of different fluids, such as lubricating oil and water.
Custom seals can be tailored for new machinery to fit specific bearings, providing protection for various types of precision bearings such as ball, sleeve, and roller bearings in nearly all types of equipment, including vehicles. The seal's structure is reinforced by an internal metal ring that acts as a supportive core.
The outer layer of the seal is made from a flexible material like nitrile rubber or other substances based on the operating conditions. A spring supports the seal's lip to prevent lubricant leakage, while the lip design helps keep contaminants out.
For light load applications, the outer skin may be made of silicone. To withstand high temperatures (above 120 degrees Celsius), fluoroelastomer (Viton) is used. Alternatives such as Poly Acrylate or Polytetrafluoroethylene (PTFE) can also be employed.
The shaft where the oil seal is mounted must have a rough surface finish. It should be toughened to avoid grooves that could form under the pressure of the seal's spring. Additionally, the installation area must be ground to prevent grooves that could accelerate wear on the seal’s lip.
To prevent leakage, oil seals have a flexible lip that brushes against the rotating shaft or housing. The lip is kept in contact with the shaft by the spring. Dynamic seals with a rotor or rotating member and a stator or stationary member are known as bearing isolator oil seals. The rotor rotates in tandem with the shaft. Bearing isolators with a "labyrinth" construction are used in some oil shafts. Others use O-rings that are less complicated.
Oil seals are rotary shaft seals designed to close the gap between moving and stationary components, preventing lubricant escape and stopping contamination through the clearance. They come in various types and materials to suit different applications and environments.
The appropriate material and type of oil seal depend on the medium used and the application. Typically, a standard oil seal features a metal ring as its inner skeleton for structural stability, while the outer part may be made of metal or rubber based on the specific requirements.
The spring on the oil seal’s lip supports the lip, preventing lubricant leakage and blocking contaminants. When a dust lip is present, it protects the sealing lip from dirt and dust, extending the seal’s lifespan.
Dust lips are located on the inner diameter of the seal, and a seal with a dust lip is known as a double lip oil seal. Additionally, a garter spring, a coiled spring shaped into a circle, is often used to maintain radial force exerted by the sealing lip around the shaft surface.
Radial oil seals, the most common type, function by forming a thin film of oil between the shaft and the rubber sealing lip. This oil layer lifts the sealing lip slightly away from the shaft, creating a barrier that prevents oil leakage past the lip. As a result, rubber oil seals are not ideal for dry running applications or high-pressure environments.
The most frequently used types of oil seals are those with metal and rubber casings.
Metal-cased oil seals are used when installed in a housing bore made from the same material, allowing for equal expansion and contraction during operation and thereby preventing leakage. Typically, metal-cased seals are more cost-effective compared to rubber seals.
Rubber-cased oil seals are often used when metal-cased seals might fail, such as due to thermal expansion. Unlike metal-cased seals, rubber-cased seals do not rust and can better accommodate slightly damaged housings. Rubber expands quickly under high pressures and temperatures, providing a tight and stable seal.
The Type R seal is one of the most commonly used types. It features a carbon steel insert with a rubber outer diameter, offering excellent sealing even with a slightly out-of-tolerance housing. The sealing lip, reinforced by a spring, ensures effective shaft sealing and allows for press-fitting in the housing with sufficient interference for static sealing.
Made from high-performance nitrile rubber and featuring a high-quality galvanized steel garter, this seal is designed for optimal durability. To prevent leakage caused by hydrodynamic pumping, it is crucial that the contact area of the sealing lip on the shaft or sleeve is free from any machine lay traces.
Oil seals can feature various lip designs, including:
This design incorporates a garter spring and is typically used for sealing against internal media in lower pressure applications. Single lip seals are not recommended for environments with contaminants or dirt.
Similar to the single lip design, the double lip seal also uses a garter spring but includes a primary sealing lip that addresses internal media in lower pressure situations. It provides enhanced sealing capability compared to the single lip design.
Rod seals are essential components in fluid power equipment, crucial for preventing fluid leakage from inside the cylinder to the external environment.
Rod seals are primarily made from PTFE blends and polyurethane.
Leakage through a rod seal can decrease equipment performance and potentially lead to environmental issues. Optimal sealing performance requires perfect pairing of the rod seal with the wiper. If an aggressive rod seal is matched with an aggressive wiper, the wiper may scrape off the thin film of oil left on the rod during the return stroke, leading to system leakage.
Various hydraulic rod seals are available for both single-acting and double-acting systems. Rod seals also play a role in preventing environmental contamination when combined with a wiper seal.
Rotary seals are designed for applications with a rotating shaft in wet environments. They keep lubricants (such as grease, oil, or water) contained while preventing dirt and water ingress. These seals are crucial for protecting critical components in pumps, ships, and tidal turbines from fluid damage.
Rotary seals encompass a variety of types, including rotary shaft seals, radial oil seals, double-acting O-ring energized polytetrafluoroethylene seals for bore and shaft, radial and axial lip seals, rubber V-rings, mechanical face seals, and others.
These seals are known for their low-friction properties and excellent wear resistance. They effectively prevent corrosive moisture, abrasives, and other contaminants from entering machinery and also keep different mediums, such as water and lubricating oil, from mixing. Rotary seals are typically made by vulcanizing an elastomer onto a metal ring.
Rubber seals are ideal for reducing vibration and noise. They include various designs such as co-extruded pedestals with sponge rubber bulbs, ribbed profiles, lid seals, and triangular sections.
Rubber seals are straightforward to install and find applications in the automotive, marine, industrial, and manufacturing sectors.
Rubber seals can be made from various materials, including:
This synthetic rubber, known as chloroprene or polychloroprene, is created through polymerization. It is highly versatile and suitable for use in diverse, harsh conditions across many industries due to its resistance to alkalis, acids, oils, greases, sunlight, ozone, and weathering. Its excellent flexibility and resistance to twisting make it ideal for hydraulic rubber seals, which inherit these robust properties.
This elastomeric material is widely used for seals due to its excellent resistance to alkalis, acids, gasoline, petroleum-based compounds, and hydraulic fluids. It is particularly well-suited for hydraulic seals where water permeability and abrasion resistance are critical concerns.
This high-performance elastomer is known for its exceptional temperature stability, withstanding extremes from -75 degrees Fahrenheit to +500 degrees Fahrenheit. It also resists damage from oxygen, ozone, aging, water, and weathering.
This high-performance rubber material boasts excellent chemical resistance and can endure high temperatures. It has strong tensile strength and a low compression set, making it suitable for use in temperatures ranging from -15 degrees Fahrenheit to +400 degrees Fahrenheit, with intermittent exposure up to +500 degrees Fahrenheit.
This section will cover the applications of hydraulic seals, their advantages, and typical failure modes.
The common failures of hydraulic seals are:
Hydraulic seals can harden when subjected to high temperatures, which may result from either rapid heat generation during stroking operations or elevated fluid operating temperatures. As the seals lose their elasticity and develop cracks, they ultimately fail.
Significant damage to a seal can occur from wear on the dynamic face of the seal lip, often due to excessive lateral load or inadequate lubrication.
The lifespan of seals is significantly influenced by the tools and methods used during installation. Improper installation can cause cuts or dents on the dynamic lip of the seal, affecting its efficiency and potentially introducing contaminants into the hydraulic fluid.
Seal fractures involve breakage, bending, long cracks, or complete separation of the seal's dynamic side. This damage is often due to excessive backpressure, high-pressure shocks, or the use of substandard materials in seal manufacturing.
Improper installation can lead to various issues with hydraulic seals, including contamination, unsafe handling, and incorrect seal sizing. It is crucial to ensure that the design is correctly executed before constructing the seal to achieve proper sealing performance.
Contamination occurs when external debris, such as mud, dirt, or fine particles, enters the hydraulic rod. These particles can adhere to the piston and cause the seal to become dirty, compromising its ability to effectively keep contaminants away from the piston area.
Chemical erosion happens when seal material is exposed to corrosive fluids. This problem arises from using inappropriate seal materials for the specific application. Chemical attacks from hydrolysis, oil additives, or oxidation can lead to the degradation of the seal, including loss of the seal lip interface, swelling, softening, or shrinkage. Discoloration of the seal often indicates chemical erosion.
Hydraulic seals serve to prevent the leakage of fluid from within a system to the outside. There are different types of hydraulic seals, each offering its own unique properties. The properties of the hydraulic seals are dependent on the type of material as well as the design. Therefore when opting for a hydraulic seal, careful considerations must be made to ensure that a seal that perfectly suits a particular application is selected. The considerations that can be investigated prior to selecting any seals are fluid pressure range, temperature range, stroke speed, fluid type, hardware dimensions, and cylinder application.
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