A hydraulic seal is any of a number of soft non-metallic rings used to prevent leakage of fluid in hydraulic and pneumatic machinery. They're most often made from polymeric materials such as rubber, PTFE, or polyurethane.
A single hydraulic system will usually make use of several different hydraulic seals, each designed to prevent leakage at a different point of pressure within the system. One basic hydraulic cylinder system may make use of hydraulic cylinder seals, lip seals, hydraulic piston seals, wear rings, scrapers, and more to function effectively. This is often referred to as a seal kit.
Quick links to Hydraulic Seals Information
The History of Hydraulic Seals
The creation and development of the hydraulic seal mirrors the lengthy development of the hydraulic system, though modern hydraulic seals only appeared as polymers before they became better understood and utilized.
The earliest hydraulic system, invented by Joseph Bramah in England during the Industrial Revolution, made use of leather seals to achieve the same results as modern hydraulic seals made of polymeric materials.
One might point to the later invention of the standard O-ring by J.O. Lundberg in Sweden in 1896 as the rise of more modern seals, though the biggest changes in hydraulic seals came with the rapid development of polymers alongside the rest of chemistry during and after World War I.
Benefits of Hydraulic Seals
For the vast majority of hydraulic applications, hydraulic seals are a non-optional component; it simply isn't practical or reasonable to manufacture and use systems which do not use a number of hydraulic seals. To do so would require a combination of highly precise machining, unusual materials, low stress on components, and specific fluids—and even then, such a system would fail relatively quickly.
Polymeric seals make for an easy, inexpensive, replaceable, effective barrier to fluids. There's simply no reason any machine making use of hydraulics should not be making liberal use of them anywhere a barrier for fluids is necessary.
Design of Hydraulic Seals
Hydraulic seals are produced in most cases like any other mechanical seal or component made of polymers. Common manufacturing techniques include extrusion, injection molding, transfer molding, and pressure molding.
Hydraulic Seal Materials
Hydraulic seal manufacturers generally fabricate seals from rubber or metal, and in some cases leather or felt. Some of the rubber materials used to create seals are nitrile, silicone, natural rubber, butyl and styrene butadiene. Metals used to make the washers in bonded seals include stainless steel, aluminum, brass, bronze and carbon steel, all of which are often galvanized or plated for extra strength and protection against oxidation. Most elastomeric seals are made via extrusion, while Teflon seals are sintered in an oven from powder form, since Teflon cannot melt. Those that are bonded to metal washers use a bonding chemical to create a permanent and strong bond between the rubber material and metal. Hydraulic seal manufacturers create many products that are application specific and specially designed to prevent leaks under very particular circumstances.
Factors of Hydraulic Seals
Design and engineering of hydraulic seals needs to take into account a huge number of factors, including but not limited to:
- System dynamics
- Expected lifespan
- Mating materials
- Environmental hazards
These factors together will suggest different types of hydraulic seal, different materials, different production methods, etc. The complexity of design makes it crucial that you work with a manufacturer familiar with your specific usage needs if at all possible.
Furthermore, it's important to note that a single system will encompass a variety of designs, which will share some design factors due to environment or application but vary wildly in stress factors and geometry.
Types of Hydraulic Seals
There are two main types of hydraulic seals: static, which seals within a confined space like a gasket, and dynamic, which is exposed to movement on its inner diameter, usually around a shaft of some kind.
The distinctive types of sealing positions and directions that are used for hydraulic and pneumatic seals are axial seals, rod or internal seals, external or symmetric seals and piston seals. The sealing direction is important for hydraulic seals and pneumatic seals in mechanisms that use axial movement, such as cylinders and pistons. Because the action may be single or double, a seal can be used for one or two directions.
Dynamic Hydraulic Seals
Dynamic hydraulic seals, also called shaft seals, include rod seals and piston seals, which are hydraulic cylinder seals that keep pressurized liquids contained within the cylinder and contaminants and air from getting inside. These seals are made with a lip that provides superior sealing while being exposed to linear movement.
Lip seals are made of many different elastomeric materials, including polymers, plastics and rubbers, and they are effective in providing a seal in revolving machinery and equipment. These are called rotary seals.
Rubber seals, particularly Teflon seals, are generally the most widely used because of their strength, durability and resistance to chemicals, moisture and high temperatures.
Bonded seals are composed of an elastomeric seal that is adhered to a metal washer for high pressure applications.
Many hydraulic systems use oil as hydraulic fluid. They use oil seals (or metric oil seals outside of the United States) that are made of resistant materials like Teflon, silicone, polyacrylate or a fluoroelastomer that can handle continuous exposure to oil. Without sealing devices, hydraulic systems would completely fail and cause the machinery they power to become unusable. While they are reliable and last for a long time, regular maintenance checks ensure the seals’ extended performance.
The rod seal is radial and is press fitted into the housing bore.
The axial seal has the function to axially seal an adjoining housing or other mechanical component. When purchasing hydraulic seals, there are some important factors to consider, such as the diameter of the housing bore and the inner or outer seal, the dimension of the outer shaft and the axial or radial cross section needed for the application. Also, it is important to consider certain operating parameters like the working temperature, vacuum rating and highest working speed and pressure.
Balanced seals are seals that are designed to reduce the effects of hydraulic pressure in the seal compartment.
Bellow seals use a formed or welded bellows to supply a secondary sealing and spring loading.
Bidirectional Pressure Seals
Bidirectional pressure seals uniformly seal in response to pressure from both directions. Bidirectional pressure seals are also known as "double balanced seals" or "reversible balanced seals."
Isolators are a type of bearing seal used in place of oil seals to supply greater reliability.
Mechanical seals prohibit fluid leakage around the rotating shaft in a state of extreme pressure, temperature and shaft speed. Typical uses for mechanical seals are for agitators, pumps, mixers and cryogenic seals.
O-ring seals are sealing rings that have a circular cross section that is either solid or hollow.
Shaft seals are tools that help to prevent the contamination of hydraulic shafts. They are also used to help retain fluid pressure in hydraulic systems.
Teflon seals are seals that have been made from Teflon® or are coated with Teflon®.
Stainless steel seals are often galvanized or plated for extra strength and protection against oxidation.
Hydraulic Seals Applications
To achieve a similar blockage of fluid without the use of seals is often unfeasible or completely impossible, due to the requisite precision and rapid wear of the relevant components. A hydraulic seal can achieve far more consistent results, last longer than a precision-machined system without seals, and can easily and inexpensively be replaced should it fail.
Various types of seals are used in applications that have constantly moving equipment, such as the rotating or reciprocating shafts and cylinders that are an essential part of hydraulic and pneumatic systems. Devices that use seals include hydraulic rams, presses and mixers, brake devices, valves, cleanrooms, conveyors and test equipment. The most common place to find these seals, however, is in a hydraulic cylinder. Hydraulic cylinders use water, oil or other pressurized fluids to facilitate the movement of heavy machinery. The seals are located in the piston, on the cylinder head or on the rod shaft. They are used to prevent the hydraulic fluid from flowing across the piston, from leaking past the interface between the rod and the head and to keep it from leaking to the outside of the cylinder. They also provide a degree of lubrication and decrease metal-to-metal contact and friction. Wear rings are another type of sealing device, as well as rod wipers. They are used to prevent contaminants from entering the cylinder when the extended rod retracts back.
Hydraulic seals can be found in any industry which makes use of hydraulic systems. Outside of highly specific edge cases, any system which needs to separate or block fluids in a reciprocating motion application will make use of hydraulic seals. This encompasses any and all industries which make use of hydraulic and pneumatic machinery. This of course includes but is not limited to fields such as:
- Agriculture. Used in various farming equipment.
- Aerospace. Various applications including landing gear.
- Automobile. Brakes, hydraulic pumps, motors, lifts, gasket o-rings, etc.
- Construction. Machines and equipment such as excavators, jackhammers, backhoes, and more.
- Entertainment. A common component in amusement park rides and similar entertainment.
- Fitness. Used to generate resistance on a variety of personal fitness systems in the gym.
- Heavy industry. Necessary in mining, excavation, oil extraction, and numerous other heavy industries.
- Manufacturing. Any number of manufacturing tools, packaging systems, etc. rely on hydraulics or pneumatic seals.
- Printing. A key component in printing presses and other printing equipment.
Installation of Hydraulic Seals
Installation of hydraulic seals varies widely across different types of seal and different systems. In most cases, it will be more useful to refer to the instructions for use and installation of the hydraulic system at hand for specific information on installing a given seal.
That said, always inspect seals carefully before installation, and only remove them from proper storage shortly before installation. This is especially important for seals in high-stress systems or with unusual properties.
Hydraulic Seal Standards and Specifications
Hydraulic seals of different types are governed by various standards and regulations at international, national, and local levels. Make sure you confirm all relevant compliance issues for general use of hydraulic seals in your application, as well as identifying any relevant standards based on industry, application, environment, etc.
For most common applications, your chosen manufacturer should be familiar with the necessities of compliance, safety, etc.
Things to Consider When Purchasing Hydraulic Seals
Where you'll be using your hydraulic seals matters just as much as what you're using them on.
Make sure to consider the difference in maintenance requirements between different hydraulic seal solutions.
Upgrades or Expansion
If you have plans for upgrades or expansion of operations in the future, this may impact the best solution for hydraulic seals today.
Precision and repeatability over months or years of operation should be a factor in deciding what hydraulic seals to select for your project.
Not all hydraulic seal options scale well, so order size now and in the future may strongly shape your ideal solution.
Choosing a Good Hydraulic Seal Manufacturer
There are a few traits that any good manufacturer of hydraulic seals should have. These are essentially universal traits for any type of manufacturer. A company with these traits is respectable and on-the-level, but may not be the best choice.
First and foremost, a good manufacturer will be transparent in its dealings with you.
Guarantees and Warranties
Closely related to transparency is the matter of guarantees and/or warranties on products and services. A hydraulic seal manufacturer that isn't willing to put its promises in writing doesn't trust its ability to deliver what it promises.
Good referrals and testimonials from relevant sources should be a given when hiring any type of manufacturer, barring unusual circumstances.
Certifications and Compliance
A good manufacturer will be in good standing with any and all relevant certification and regulatory bodies at the national, local, and potentially international level.
Experience in the Industry
While a fresh-faced manufacturer isn't necessarily a bad manufacturer, by definition it can't be considered a good one either. Working with an untested organization is a gamble you'll probably want to avoid.
Choosing the Right Hydraulic Seal Manufacturer
The right manufacturer is in fact a separate concept from a good one; a manufacturer may be a sterling example of hydraulic seal manufacturers, but still not suitable for your needs. To make sure you find the right manufacturer as well as a good one, look for these traits in particular:
You'll want a manufacturer as familiar with your specific application as possible. Matching your expectations for the product, logistics, communications, follow-ups, etc. will be far easier for a hydraulic seal manufacturer that's worked successfully with similar businesses. If you need a rotary shaft seal, you don't want a manufacturer that never works with shaft seals.
Hydraulic Seal Scale
If a company works primarily with companies requiring a much larger or much smaller volume of hydraulic seals than yours, it's probably not going to be compatible with your needs and expectations. You may be too small, and thus become a low priority—if your business is accepted at all—or too large, and thus face major issues with a congested supply line. When judging scale, also consider the future; do you anticipate a growing or shrinking volume of orders in the future?
Compatible logistics can save a world of headache when dealing with a manufacturer dealing with components of this type.
Versatility of Seals
If you're looking to optimize your use of hydraulic seals over time, you'll want to screen manufacturers for matching versatility. If they can't adjust to changes as you need them, then you're always going to be lagging behind your optimal configuration. Of course, this is less important if you're simply looking for a supplier of a consistent fixed product.
The way you communicate with your business partners should be something you consider as you screen manufacturers for your needs.
Proper Care for Hydraulic Seals
Hydraulic seals should be a fairly high priority for checkups by your maintenance team.
Seals not currently being used need to be adequately stored, lest they deform, corrode, etc.
Taking the time to choose the right hydraulic seal for the job from moment one is the best thing you can do for performance and long-term reliability. A well-matched seal will perform better, wear less, and be far less likely to cause problems over time.
Observation and Analysis
Careful analysis of your system's performance over time is important to optimizing performance with hydraulic seals. In most cases, your best option for improving performance will be to replace your current seal with a more effective seal—something you can only do if you notice that it is a limitation on performance.
Depending on the application and the type of seal you're using, preventative maintenance measures may be one the most critical aspects of maintaining optimized performance.
Hydraulic Seals Accessories
It's difficult to pin down specific accessories for use with hydraulic seals, due to the extremely wide range of applications, industries, and types to consider. For general purpose accessories, you only have a few options to consider, such as stainless steel storage containers. Otherwise, you'll want to consider accessories in the context of the hydraulic system at hand, not the seals you're using.
Hydraulic Seal Terms
- The surface loss of a material caused by friction.
- Abrasion Resistance
- A material’s resistance to the surface loss caused by frictional forces.
- A state in which two surfaces are bonded together through interfacial forces.
- Axial Movement
- Motion along an axis or parallel to the center line of a shaft.
- A component made from corrugated plastic or rubber around a shaft that has the ability to stretch.
- Bill of Material
- A list of all materials needed to manufacture a certain product.
- A change of appearance in rubber, resulting from a solid or liquid material moving towards the surface. Bloom resembles surface dust.
- The tendency of a material to crack under deformation.
- A material's property of being able to conduct/transmit electricity.
- Cross Section
- The view of an o-ring when cut at a right angle to the axis, which shows its interior construction.
- Used in a hydraulic or pneumatic cylinder to exclude or scrape the rod clean.
- A material's ability to move in a mold during the molding process.
- The resistance to motion when two surfaces are in contact.
- Occurs when a material's surface reacts with oxygen, usually causing a change in the appearance or texture of the surface.
- A conical rubber component pressed into a hole or tube to finalize a seal.
- The measure of small holes in a material.
- A material's ability of returning to its original state after deformation.
- The tendency of a material to contract while cooling.
- A ring with flat sides to provide specific dimensional spacing between two components.
- The resistance of a material to flow under stress.
- Round, flat rings used as spacers, gaskets or slip devices under the head of a bolt.