Welcome to the module entitled Hydraulic Seals - Basics.
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In this lesson, you will be provided with information about the characteristics of hydraulic seals, as well as the various types of hydraulic seals. You will also have an opportunity to see how these types of seals work and what the main considerations are when choosing a hydraulic seal for a particular application.
Types of Elastomers
Seals are devices for closing a gap or making a joint leak-tight. Seals also prevent fluid from leaking out of the system. Lastly the seal also makes it possible to transfer forces within the system. These are the main characteristics of a seal.
Trelleborg Sealing Solutions has developed innovative sealing systems for demanding industry applications with a focus on leak-free performance and long service life.
When talking about hydraulic seals, we think about an entire system of sealing elements for Rod and Piston. The interaction between individual sealing elements needs to be guaranteed in order to create a reliable sealing system.
Click on the tabs to learn more about the functionality of rod and piston sealing systems.
Piston Sealing System
The piston sealing system consists of a piston seal and a number of wear rings.
The piston seal seals the dynamic surface between the piston and the bore. Piston seals are normally double acting, which means that they are symmetrical and can seal pressure from both sides. The wear rings center the piston, guide the reciprocating movement and protect the seal and metal parts from damage.
Rod Sealing System
The rod sealing solution prevents leakage of hydraulic fluid from the system. With the help of the scraper it also prevents dirt and pollution from entering the hydraulic system.
The seal that will face the whole system pressure is called the primary seal. The secondary seal will only face a smaller pressure and can be made from a softer material to add sealability. Rod seals are normally single acting which means that they are optimized to seal pressure from one side
A hydraulic sealing system consists of four parts - piston seals, rod seals, scrapers, and Wear Rings. The four components of a hydraulic sealing system have very specific functions.
Click on the images for detailed information about each component.
Rod Seal
Rod seals are external seals, used to avoid leakage of hydraulic media out of the system. This sealing system consists of two rod seals. Rod seals are normally single-acting seals, which mean that they are asymmetrical and optimized to seal pressure from only one side.
When you use two rod seals it is possible to optimize the performance for each seal. The primary seal is facing the internal pressure, and by using a harder seal material like PTFE you will combine extrusion resistance and low friction. The secondary seal will only face a smaller part of the system pressure and here you can use a softer seal material like polyurethane to increase seal ability.
Piston Seal
In this example you will see a typical piston seal that is double-acting. Double-acting means that the seal is symmetric and designed to seal pressure from both sides, which makes it possible to extend and retract the cylinder.
Scraper
Scrapers, or wipers, work within a sealing system to keep dirt or other contamination out of the system.
Scrapers can have single or double-acting functions. A single-acting scraper has only one function, to keep contamination out of the hydraulic system by scraping or wiping the rod. The double-acting scraper also adds
a sealing function towards the oil side. Scrapers are made from different types of material, such as rubber, polyurethane or PTFE. The broad variety of materials and designs makes this component optimal for different
types of scraping.
Wear Ring
Wear rings, also known as guide rings are used together with both rod and piston seals to guide the components and avoid metal-to-metal contact.
By absorbing load from the system, the wear ring protects the seals and hardware components. The dimension
of the wear ring can be calculated based on side load and temperature in the system. Trelleborg Sealing Solutions
are using the name Slydring® as a brand name for wear rings.
As you can easily imagine, different types of hydraulic sealing systems are used in many applications. Seals are vital components since they ensure the function and smooth operation in hydraulic cylinders and prevent the ingress of abrasive media (e.g. off-highway vehicles, construction equipment, backhoes). They prevent environmental pollution and soil contamination (in the case of agricultural vehicles). They comply with safety requirements, ensure “low friction”, reduce abrasion, and ensure long service life for critical components.
Requirements in a hydraulic system are something that needs to be fulfilled and should not be compromised. Both hydraulic systems and seals have requirements that should be considered. In the coming slides we will speak about pressure, hardware and general system related requirements that will help you in the selection process, so you can find the best sealing solution for your application and needs. Ask yourself: Can you assign the correct seal to your requirements?
Pressure is one of the limits of the seal. In Europe and Asia, MPa or Bar is commonly used. In the US or UK psi (pound force per square inch) is used. Once you know the pressure limit in your system you can start to think about a seal.
Different equipment has different pressure ranges and therefore also different sealing systems. You can see some examples of pressure and applications on the screen.
The expectations for a hydraulic system lifetime or service life are normally quantified as you can see in the example, and this can be a requirement for the product. But how can you get your system to live up to your expectations?
The answer to the question is that the hardware is one of the most crucial aspects to consider. The better the conditions at the hardware level, the better the seal will perform.
In this slide you will learn more about installation dimensions based on information in our Hydraulic Seals catalog. Click on the circles to get more information.
Serial No.:
We will use a 200 millimeter piston seal named Turcon® Glyd Ring® T with the part number PT0402000 and 15 MPa pressure as an example to find the installation dimensions. The Series PT04 refers to the cross section of the seal and will also lead us to the row in the table where the information for the PT04 series is.
Bore diameter:
The bore DN = 200 should have H9 tolerance. 200 will be found in the range between 133-329.9 mm which makes it a standard application seal.
Groove diameter:
The groove diameter is calculated by using the Bore diameter DN (200mm) - 21= 179 mm with h9 tolerance.
Groove width:
The groove width is 8.1 for PT04 and has the tolerance +0.2.
Radius:
The radius in the bottom of the groove is 1.8 mm Radial clearance is.
Radial Clearance S Max:
Depending on the pressure. 15 MPa is less than 20 MPa and this gives 0.6 mm max.
40MPa:
If the pressure is bigger than 40 MPa you should consult your local contact at Trelleborg Sealing Solutions to make sure to follow the instructions regarding clearance.
Pressure> 40MPa:
If the pressure is bigger than 40 MPa you should consult your local contact at Trelleborg Sealing Solutions to make sure to follow the instructions regarding clearance.
During the construction and procurement process, it is essential to think about the seals and the system requirements.
Resistance against extrusion, Efficiency in terms of cost and performance, Low friction, Easy handling and assembly, Stability towards temperature and media, Compliance with industry standards and Reliability in terms of both performance and logistics.
Keeping these requirements in mind will guarantee optimum results and an efficient construction process. Point at the displayed terms to get additional information.
You know by now that sealing elements have a significant influence on the function and service life of hydraulic cylinders.
The main challenge is to match the requirements like pressure, hardware, expected lifetime and conditions of the hydraulic system to the correct choice of seals.
The more system information and requirements that are available, the easier it is for the TRELLEBORG SEALING SOLUTIONS contact to choose the correct seal for the application.
If you have any doubts regarding a sealing solution, always contact your local Trelleborg Sealing Solutions office.
Types of Elastomers
What is friction, where does it come from and what does it do? The next slides should help you to understand the basic principles of friction.
Try rubbing your hands together - what do you feel? When two surfaces are rubbed against each other, heat is created . This effect is called friction.
You’ll find that the faster you rub, the warmer your hands will get. If you increase the speed you will increase the temperature in the system due to friction.
The compression of a seal in the grove is called squeeze. The level of squeeze is a balance of friction and leak tightness. The aim is to squeeze the dynamic seal so it seals in a good way and compensates for the tolerances in the system, but not so much that it creates problems in terms of friction heat and material stress that will reduce the seal’s lifetime.
The previous slides mention speed and initial squeeze that generates friction heat. When you add the system pressure from the oil to this example, then it looks like the conditions in a hydraulic system. If the hydraulic system is run frequently with high speed and high pressure, the system will generate heat. Dynamic systems will always create heat due to friction, but you would like to have this under control. Therefore it is important to use low friction seals, proper hardware and sufficient cooling and circulation of the media to ensure the function of the sealing system.
In the coming slides we will talk about the lubricating media and the effects of the lubrication film and leakage.
In a hydraulic system there is normally an oil to transfer the force. The lubrication film that is created between the dynamic seal and the counter surface is important and helps keep friction and wear low in the system. Oils should be carefully selected together with an oil specialist to match the demands of the application.
In the case of stick-slip the dynamic seal does not automatically create the oil film between the seal and the sliding surface. The oil film is important for an even movement. The seal will create a jerking movement, and this phenomenon makes the seal “stick” and “slip”.
To avoid this problem, Trelleborg Sealing Solutions recommends the use of special low friction materials like PTFE.
All materials have their limits, and seal materials can fail when this limit is exceeded. But how sensitive is a seal to temperature?
Click on the buttons to see how different temperature conditions can affect the seals.
Hot
An old rule of thumb for elastomer sealing materials is that for each positive 10 degrees Celsius you add, you decrease the lifetime of the seal by 50%. If you exceed the maximum temperature the seal will be permanently damaged.
Cold
Low temperature can lead to leakage because the seal loses its elastic properties.
Low temperature should not damage the seal, and when temperature is back to the working range it will seal again.
The word extrusion means the tendency of a seal to flow into the clearance gap between the seal surfaces under pressure.
Extrusion is essentially influenced by pressure, and this can be a problem in some applications. Normally you can solve this problem by replacing the seal with something that is more pressure resistant, such as harder seal materials or corner-reinforced seals.
Decreasing the gap and pressure are other alternatives.
The lubrication film is important in a dynamic system and helps keep friction and wear low. Wear can occur on different parts of the system for one or more reasons.
The most important thing is to find the source of the wear and remove this from the system in order to regain a healthy system.
The reason for the wear can come from tough working conditions in the application but also from contaminants, bad guidance or wrong hardware design or quality.
Trelleborg Sealing Solutions has developed tools that help an engineer finding and selecting seals. All of these online tools are available via registration and login on the Trelleborg Sealing Solutions website.
One such tool is the Sealing Solutions Configurator. This configurator allows engineers to identify a proven sealing solution for their specific application.
You will find CAD download services that provide technical drawings as well as product catalogs and films specific to different industries and products.
Trelleborg Sealing Solutions constantly develops apps to make life easier for engineers. The apps are available for iOS applications and Android.
Please find information about our apps on our website.
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What are the main functions of wear rings?
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What is the main function of a scraper?
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Which statement does not fit as a description of a seal?
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What is important to make sure of in order to make the seal work optimally in terms of performance and lifetime?
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Which type of extreme temperature can permanently damage the seal material?
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Which seal is the primary seal in a rod sealing system that consists of two rod seals?