Hydraulic Seals: Construction, Types, Applications, and Benefits

04 Nov.,2022


high pressure hydraulic seals

Hydraulic Seals

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This article will take an in-depth look at hydraulic seals.

The article will bring more detail on topics such as:

  • Principle of Hydraulic Seals
  • Types of Hydraulic Seals
  • Applications, Benefits, and Common Failures of Hydraulic Seals
  • And Much More…

Chapter 1: Principle of Hydraulic Seals

This chapter will discuss what hydraulic seals are, their construction, and factors to consider when choosing a seal.

What are Hydraulic Seals?

Hydraulic seals are a form of gasket-like rings that are used to fill gaps between hydraulic cylinder components. Many different components are found in hydraulic cylinders, some of which get in contact with the fluid. Hydraulic seals are used to prevent fluid from seeping around these components. Hydraulic seals are made to suit the components of a hydraulic cylinder while providing a leak-proof seal.

O-rings and hydraulic seals are similar to each other. However, hydraulic seals have a groove in the lip that enables components to be fit in. To produce a leak-proof seal, the groove will move over the component. The component will not leak even if the hydraulic cylinder's piston moves and the fluid pressure rises. The hydraulic seal will keep the fluid contained in the correct container, preventing leakage around the component.

The Construction of Hydraulic Seals

In the construction of hydraulic seals, the manufacturing process and the materials to be used in the manufacturing should be considered.

Manufacturing Process

During the manufacturing process, the hydraulic seals are made on a Computerized Numerical Control (CNC) lathe machine. They can be programmed with both standard and custom seal profiles. The CNC lathe works to cut the seal's shapes from the specified material using the profile's digitized data.

Hydraulic Seal Materials

Hydraulic seals come in a wide range of materials. Hydraulic seals are commonly made of rubber. Rubber hydraulic seals are malleable, long-lasting, and crack-resistant. Polyurethane hydraulic seals are also available and are the most common material used in hydraulic seals. Many of the features of polyurethane hydraulic seals are similar to those of rubber seals, however they are generally more durable and resistant to wear.

Polytetrafluoroethylene (PTFE) is a type of hydraulic seal that isn't as well-known as rubber or polyurethane (PTFE). PTFE is a flexible, durable, and temperature-resistant synthetic polymer. Specific operating parameters or constraints imposed by fluid type, pressure, fluid chemical compatibility, or temperature determine the material type.

Considerations When Choosing a Seal

The various considerations when choosing a seal include:

Shaft Speed

The maximum shaft speed is determined by the shaft finish, runout, housing bore and shaft concentricity, kind of fluid being sealed, and oil seal material.

Temperature Range

The temperature range of the mechanism in which the seal is installed must not be higher than the seal elastomer's temperature range.

Seals and Pressure

Most conventional oil seals are only intended to resist extremely low pressures (about 8 psi or less). If there is or will be increased internal pressure, pressure relief is required.

Shaft Hardness

Shafts with a Rockwell (RC) hardness of 30 or higher might expect a longer seal life. The hardness should be increased to RC 60 when subjected to abrasive pollution.

Shaft Surface Finish

The best sealing results come from having the best shaft surface treatments. The spiral lead and the direction of the finish tool marks have an impact on the sealing efficiency. Polished or ground shafts with concentric (no spiral lead) finish marks produce the best sealing results. If spiral finish leads are required, they should point toward the fluid as the shaft rotates.

Concentricity of Seals

Seal life is reduced when the bore and shaft centers are misaligned because the wear is localized on one side of the sealing lip.

Shaft and Bore Tolerances

When shaft and bore tolerances are near, the best seal performance is attained. Shaft eccentricity, end play, and vibration are some of the other issues to consider.

Amount of Runout

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.

Seal Lubricants

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.

Chapter 2: Types of Hydraulic Seals

The various types of hydraulic seals include:

Hydraulic Cylinder Seals

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 prevent fluid passage, seal material must adhere to imperfections in metal surfaces. The seal must expand or compress swiftly to follow dimensional variations in order to adjust to clearance gap size changes. Finally, the seal must have enough modulus and hardness to cope with shear stress caused by system pressure in order to avoid being extruded into gaps.

Fluid containment within fluid power systems and components while excluding contaminants is required for successful sealing. The sort of seal to use is determined by the surfaces that come into touch with it. The surface might be static or dynamic, meaning it can move or not. When there is no relative motion between mating surfaces, static seals are used. Dynamic seals, on the other hand, are the polar opposite. When there is mobility between surfaces, they are utilized. This can be in the form of oscillating or reciprocating motions.

Lip Seal

Depending on the use, the lip design on any given seal will differ. Many seals have what are known as "lips" as part of their design. This is true of radial, rotary, and linear shaft seals as well.

Lip seals, in addition to acting as a dam or a barrier, also function as a pump. They are commonly employed with rotary, reciprocating, and oscillating shafts. The basic functions of a lip seal (also known as a radial or rotary seal) are to keep lubricants in, contaminants out, pressure contained, and fluids separated.

Radial Seals vs Rotary Seals

Wiper scraper seals include lips as an intrinsic component of their design, hence they are technically lip seals. It's worth mentioning, however, that a design known as a lip seal does not have a standard. To add to the confusion, radial seals (another type of seal that is frequently referred to as lip seals) are also referred to as rotary seals in the United Kingdom. Though theoretically valid, radial is more commonly used in the United States since the seal is radially electrified (usually with a spring) and is smaller than the shaft passing through it.

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.

Purpose of Lip Seals

A lip seal's main function is to keep impurities out while keeping lubricants in. Lip seals work by sustaining friction by their very nature. They can be employed in a wide range of applications, from slow-moving machinery to high-speed spinning and at temperatures ranging from below zero to over 500 degrees Fahrenheit.

A lip seal must maintain good contact with its rotating counterpart to succeed. This will be influenced by the seal's correct selection, installation, and maintenance. New lip seals often leak almost immediately after they've been installed. This is usually due to shoddy installation. Other seals leak at first, but once the seal material has seated to the shaft, they stop leaking.

Lip Seal Design

A simple lip seal used to consist of a leather strap on a wheel axle. A number of factors influence the performance of today’s lip seals. There are non-spring and spring-loaded seals, as well as varied contacting patterns. A non-spring seal is typically less expensive and can retain viscous materials such as oil at low shaft speeds. Conveyors, vehicle wheels, and lubricated components are examples of common applications. Spring-loaded seals are commonly found on a wide range of equipment and are generally utilized with oils.

Mechanical Seal

Mechanical seals are leak control components applied on rotating equipment such as pumps and mixers to keep liquids and gas leakage into the ecosystem.

A mechanical seal is made up of two main parts. To achieve a seal, one component remains stationary while the other rotates against it. Mechanical seals come in a variety of shapes and sizes, ranging from simple single-spring designs to much more complicated cartridge seals. The pressure, temperature, rotational speed, and product to be sealed determine the design, layout, and materials of construction.

Mechanical Seal Design

The design typically has 7 components: stationary component, which is usually known as the seat, stationary component sealing member, rotating component, rotating component sealing member, spring, gland plate, and clamp ring.

There are four main sealing locations on a mechanical seal. The primary seal is between the rotating and stationary faces. The gasket is between the stationary member and the stuffing box face. The secondary seal is between the rotating member and shaft or shaft sleeve and may be an o-ring. The gland plate and stuffing box seal is usually a gasket or an o-ring.

Sealing Points

The sealing point between the spinning and stationary components must be considered (faces). This fundamental seal is the foundation of a mechanical seal design and is responsible for its functionality. The spinning and stationary components are forced against each other by spring force, in most cases.

Both components' mating faces are carefully machined (lapped) to be exceedingly flat (usually to within 2 light-bands, which is an optical method of measuring flatness). This flatness reduces leakage to the point that it is practically non-existent. In truth, there is some leakage between these sides, although it is minor and appears as a mist for the time being. The initial face pressure is usually provided by spring compression. When the seal is at rest, the spring(s) retain this pressure, preventing leakage between the faces.

Fluid Film

Face friction and the resulting heat would cause the mechanical seal faces to wear out (and the seal to fail) if they rotated against each other without any type of lubrication. As a result, lubrication is necessary, which is provided by the product medium for ease of use. This is known as the fluid film, and keeping it stable is critical if the seal is to offer satisfactory and consistent service.

Metric Oil Seal

Grease, fluid, and dirt seals are all terms used to describe oil seals. In mechanical equipment, these seals cover gaps between stationary and moving components. Oil seals are used to keep lubricants from escaping. They also keep pollutants out of the machinery. This is especially critical under harsh conditions. They also keep different mediums like lubricating oil and water from combining.

Seals for new types of machinery can be custom-made to match the bearings. They safeguard all types of precision, precisely-fitted ball bearings, sleeve bearings, and roller bearings in almost every sort of machine, including vehicles. The oil seal's structural integrity is provided by an interior metal ring that acts as an inner skeleton.

The outer skin of the seal is constructed of a more flexible substance, such as nitrile rubber, or different materials depending on the seal's physical environment. The lip of the seal is supported by a spring, which prevents the lubricant from leaking. Contaminants from the outside are kept out by the lip construction.

The outer skin layer can be constructed of silicone where the loads are light. To defend against high temperatures, it can be built of fluoroelastomer (or Viton) (more than 120 degrees Celsius). Poly Acrylate or Polytetra-FluroEthylene can also be used for the skin.

A rough surface finish is required on the shaft on which the oil seal is placed. The shaft should also be toughened to prevent grooves from forming on the shaft as a result of the spring's pressure on the seal. The area where the seal is installed must also be ground to avoid grooves that cause the oil seal's lip to wear out faster than normal.

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.

The oil seals are rotary shaft seals that serve to close the gap between moving components and stationary components and prevent the escape of lubricant and thus stopping the contamination through the clearance. Oils seals are found in different types that suit different applications and surroundings and are constructed from different materials.

The medium used with the oil seal and the application determines the right material and type of seal. The standard oil seal has an inner skeleton of a metal ring, which brings structural stability to the oil seal. The outer part of the oil seal is made from metal or rubber, depending on the seal’s application and requirements.

The spring on the lip of the oil seal offers support to the lip and prevents the leakage of the lubricant to the outside and also prevents the entering of contaminants. When a dust lip is present, it protects the sealing lip while blocking dirt and dust from entering the system, ensuring a long life time of the seal.

Dust lips are found on the seal’s inner diameter. An oil seal with a dust lip is also known as a double lip oil seal. There is also a garter spring on oil seals, which is a coiled spring connected at either end to create a circular shape. It serves to maintain the radial force, which is exerted by the sealing lip around the surface of the shaft.

How Oil Seals Work

The radial oil seals which are the most common, work by creating a thin layer of oil between the shaft and the rubber sealing lip which results in the lifting of the sealing lip clear of the shaft by the oil. This thin layer of oil creates a barrier and prevents oil from leaking past the sealing lip. Because of this reason, rubber oil seals are not a perfect match for dry running applications or high pressure.

Types of Oil Seals

The most commonly used types of oil seals are metal and rubber cased oil seals.

Metal Cased Oil Seals

These types of oil seals are utilized when installed to a housing bore that is made from the same material. This permits equal expansion and contraction of the materials during operation, preventing the occurrence of leakage. Metal-cased seals are more cost effective than rubber seals, typically.

Rubber Cased Oil Seals

These types of oil seals are the most common oil seal execution, utilized when a metal-cased seal has the potential of failure, for instance due to thermal expansion. Oil seals with a rubber case do not rust compared to oil seals with metal cases. Moreover, rubber cased oil seals are capable of sealing a slightly damaged housing much better as compared to oil seals with a metal case. In pressures and temperatures that are high, fast expanding rubber is capable of providing a tight fit and more stable sealability.

The type R is the most commonly used type. This type consists of a carbon steel insert and a rubber outside diameter. The rubber offers a sealing capability that is very good, even when the housing is not totally in tolerance. The sealing lip with spring brings interference on the shaft for effective sealing. Consequently, this allows for press-fitting in the housing, with interference on the rubber that is sufficient to bring static sealing.

The sealing element is made from a high performance nitrile rubber. This together with a garter made from galvanized steel of high quality brings the oil seal an optimum life. It is necessary that the contact area of the sealing lip on the sleeve or shaft does not have any traces of machine lay, in order to prevent leakages caused by the hydrodynamic pumping effect.

Lip Designs Available for Oil Lips

The oil seals can have a lip design which can be:

Single Lip

This design makes use of a garter spring and generally seals against internal media in applications that involve lower pressure. The single lip is not recommended to be used in applications in which the environment contains contaminants or dirt.

Double Lip

This design is also made use of a garter spring and consists of a primary sealing lip that seals against internal media in lower pressure applications.

Rod Seals

These are the most critical seals on any type of fluid power equipment that prevent fluid leakage from within the cylinder to the outside.

The primary materials used to make rod seals are PTFE blends and polyurethane.

Function of a Rod Seal

When there is leakage through the rod seal, the performance of the equipment can be reduced and also in extreme cases, environmental issues can be caused. The rod seal and the wiper must be paired perfectly for the best sealing performance. When the pairing is done between an aggressive rod seal and an aggressive wiper, the microscopically thin film of the remaining oil on the rod in surface imperfections that are microscopic during the return stroke, can be scraped off by the wiper. This will result in system leakage.

There is a wide range of hydraulic rod seals available for both single-acting systems and double-acting systems. Rod seals also help contribute to the prevention of environmental contamination by means of a wiper seal.

Rotary Seals

Rotary seals are seals used to protect applications with a rotating shaft in wet environments. The rotary seal keeps the lubricant (grease, oil, or water) inside the application and prevents the ingress of dirt and water. These sealing systems serve to protect the critical parts of pumps, ships and tidal turbines against fluids.

Rotary seals include the most commonly known seals which are: rotary shaft seals, radial oil seals; double-acting O-ring energized polytetrafluoroethylene rotary seals for bore and shaft, radial and axial lip seals, rubber V-rings, mechanical face seals, and many more.

Rotary seals offer low-friction properties as well as excellent wear resistance. They help prevent corrosive moistures, abrasives, and other harmful contaminants from entering machinery. They also prevent mixing two different mediums such as water and lubricating oil. Rotary seals are made by vulcanizing an elastomer to a ring made of metal.

Rubber Seals

These are the perfect solution for the elimination of vibration, or drowning out of noise. Rubber seals include co-extruded pedestals with sponge rubber bulbs, ribbed profiles, lid seals, and triangle sections.

Rubber seals are easy to install and are utilized in the automotive, marine, industrial, and manufacturing industries.

Types of Rubber Seals Materials

The types of materials used in rubber seals include:

Neoprene Rubber

This is a synthetic rubber that is manufactured by the polymerization of chloroprene. It is also called polychloroprene and is highly versatile. It can be applied in various harsh environments over a broad range of industries because of its resistance to alkalis, acids, oil and grease, sun, ozone, and weathering. It is also excellently resistant to twisting and flexing. Therefore hydraulic rubber seals made from this type of rubber exhibit the same properties of this material.

Nitrile Material

This is the most commonly used elastomeric material for seals. It is excellently resistant to alkalis, acids, gasoline, petroleum based compounds, and hydraulic fluids. This type of rubber is ideal for producing hydraulic seals when water permeability, wear and tear from abrasion is a great concern.

Silicone Rubber

This is a high performance elastomer. It exhibits temperature properties that are excellently low and high, withstanding temperature ranges of -75 degrees Fahrenheit to +500 degrees Fahrenheit. It can also resist damage from oxygen, ozone, aging, water, and weathering.

Viton Material

This is a high performance rubber material with excellent properties of resistance to chemicals and high temperatures. Its tensile strength is high and its compression set is low. This type of rubber is suitable for use in temperature ranges of -15 degrees Fahrenheit to +400 degrees Fahrenheit, intermittent to +500 degrees Fahrenheit.

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Chapter 3: Applications, Benefits and Common Failures of Hydraulic Seals

This chapter will discuss the applications of hydraulic seals, their benefits, and common failures.

Applications of Hydraulic Seals

  • Power transmission
  • Oil refineries
  • Off-highway
  • Manufacturing
  • Automotive

Benefits of Hydraulic Seals

  • Hydraulic seals prevent dirt from entering any internal part of a mechanical system.
  • It can have a design that allows it to work in low and high pressure system operations.
  • Hydraulic seals prevent any leakage, thereby reducing any chances of repair or downtime.
  • Hydraulic seals are affordable and require simple maintenance.
  • They are also highly adaptable and durable.
  • Hydraulic seals are capable of resisting wear and tear as well as water and chemicals.
  • Hydraulic seals are capable of reducing blowouts on oil plants to a greater extent.
  • A hydraulic seal can be changed within a short time (a few minutes).
  • They are designed to offer long services.

Common Failures of Hydraulic Seals

The common failures of hydraulic seals include:

Hydraulic Seal Hardening

When exposed to high temperatures, hydraulic seals become hard. The cause of this is either high speed heat generation from stroking operations or high fluid operating temperatures. When seals harden, they lose their elasticity and crack, leading to the failure of the seal.

Seal Wear

Considerable damage can be caused on a seal by wear on the dynamic face of a seal lip due to excessive lateral load or insufficient lubrication.

Scarring in the Seal

The operating life of seals is dependent on the installation tools and processes. If there is an improper installation, cuts or dents may be caused in the dynamic lip of the seal. This will affect the hydraulic seal efficiency and introduce foreign elements into the hydraulic fluid.

Seal Fracture

Fracturing is a condition that results in breaking, bents, long cracks, and a complete breaking off of the seal’s dynamic side. This is because of excessive backpressure, high pressure shocks, or low-grade materials when the seal is made.

Improper Installation

As already mentioned, improper installation may create problems with hydraulic seals. The result of it may be uncleanliness, contamination, unsafe handling, and incorrect sizing of the chosen seal. The design must be done correctly before the building of the seal to ensure proper sealing.

Contamination in Hydraulic Seals

When external flotsam and jetsam are introduced into the hydraulic rod, this causes contamination. When particles like mud, dirt, powder, or other tiny elements attach to the piston, they cause the seal to become dirty. If the seal is dirty, its ability to hermetically prevent contaminants from the area of the piston is lost.

Chemical Erosion

When the seal material encounters a corrosive fluid, it will break down. This occurs when the improper seal material is selected for an application. Chemical attack by hydrolysis, oil additives, and/or oxidation reduction of seal elements can occur, when there is a use of non-compatible materials. This results in the loss of the interface of the seal lip, swelling, softening of the seal durometer, and/or shrinkage of the seal. Discoloration of the seal is a sign of chemical erosion.


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