Choosing the appropriate O-ring material for hydraulic fluid is vital to prevent seal failure in applications and systems where leak prevention and contamination control are essential.
Each material type has its own set of unique properties and will present numerous advantages when used in the right environment.
At SSP, we provide a vast O-ring range covering various material types, all boasting high quality. If you have any questions or would like any further information on which type of O-ring would be most suitable for your application, speak to a member of our team today.
O-rings for Hydraulics
O-rings come in various shapes, sizes, thicknesses, and material types. All are designed to meet specific application requirements, and all are designed to prevent leaks and contamination.
Cost-effective and easy to install and maintain, O-rings sit within the space between two mating surfaces, which, when compressed, form a tight seal – if compatible.
This makes choosing the right o-ring critical to ensure applications operate smoothly and at optimum performance levels.
High-pressure hydraulic O-rings provide a tight seal in varying temperature, pressure, and tolerance systems.
They’re also lightweight, require little space, and can be reusable depending on the material choice.
Within hydraulic systems, o-rings are positioned and held in place by the hydraulic pressure. However, it’s important to note that when choosing the most suitable o-ring for hydraulic applications, consider the measurement of the inside and outside diameter of the o-ring, including the cross-section, to account for swelling and thermal expansion.
It’s also important to choose the most appropriate cross-section to avoid extrusion and tearing.
Best O-ring material for hydraulic fluid
| Fluid Type Material | Mineral Oil (general hydraulic) | Water-glycol | Phosphate ester (Skydrol) | Biodegradable HEPR |
| NBR (Nitrile) | Compatible (offers excellent resistance and has a wide temperature range) | Not compatible (causes NBR to swell, degrade, and lose sealing integrity) | Not compatible (causes severe swelling, o ring can become brittle, and degrade quickly) | Compatible (provides stability for long periods of time) |
| HNBR (Hydrogenated Nitrile) | Compatible (resists swelling and degradation, and provides high tensile strength) | Compatible (widely used in automotive coolants) | Not compatible (causes swelling and shrinking, and o ring can become brittle on contact) | Not compatible (chemically incompatible) |
| FKM (Viton) | Compatible (excellent compatibility with high temperature limits) | Conditional (standard FKM is suitable for basic glycol-water mixtures, but it is not recommended for high water content fire resistant hydraulic fluids) | Not compatible (causes swelling, softening, and degradation) | Conditional (outcome is dependant on the specific FKM material used) |
| EPDM | Not compatible (EPDM will swell, lose strength, and cause complete failure) | Compatible (industry standard) | Compatible (offers high chemical resistance and prevents swelling) | Not compatible (causes swelling and fast degradation) |
| FFKM | Compatible (wont swell or lose seal integrity, but is costly compared to other materials available) | Compatible (excellent chemical compatibility, no swelling or degradation) | Not compatible (causes swelling, softening, and degradation over time) | Compatible (provides high chemical resistance and temperature stability) |
Type of O-ring for hydraulic oil
It’s important to note the material temperature ranges for the types of O-ring materials available, noting their suitability for specific applications.
- NBR: -40°C to +120°C continuous. Mineral hydraulic oil, general purpose. Acrylonitrile content of around 30 percent is typical for hydraulic applications.
- HNBR: -30°C to +150°C. Higher pressure, improved heat, and ozone resistance. Common in modern high-pressure hydraulic systems.
- FKM (Viton): -20°C to +200°C. High temperature, biodiesel, and aggressive fuels. Specific grades for phosphate ester.
- EPDM: -50°C to +150°C. ONLY for phosphate ester or water-glycol fluids. NOT for mineral oil.
- FFKM: -20°C to +260°C. Highest chemical resistance of any elastomer. Typically, 5 to 10 times the cost of NBR.
Hydraulic O-ring material
NBR (Nitrile) O-rings are one of the most common material selections for O-rings, offering an all-around solid seal, with high levels of resistance to hydraulic fluids. NBR O-rings perform best in temperatures from -40°C to +120°C continuous service. The acrylonitrile (ACN) content determines the balance between oil resistance and low-temperature flexibility, with around 30 percent ACN being typical for hydraulic applications. Nitrile O-rings are predominantly found in oil, gas, and water applications due to their resistance to compression, tear, and abrasion. However, note that this material O-ring does not perform at optimal levels when faced with ozone or sunlight.
EPDM (Ethylene Propylene Diene Monomer) is excellent for water-based and phosphate ester hydraulic fluids, with a service range of -50°C to +150°C. Important to note, EPDM is incompatible with mineral hydraulic oils. Using EPDM hydraulic fluid in a mineral oil system causes the seal to swell and fail – see critical safety information below.
FFKM (perfluoro elastomer) O-rings provide the highest chemical resistance and operating temperature range of any elastomer, with service temperatures from -20°C to +260°C. They are typically specified for aggressive chemical environments where other elastomers fail. FFKM is significantly more expensive than NBR or FKM, often five to ten times the cost, so it is reserved for applications where the chemical environment makes it the only viable option. (Make sure to check out our high-temperature O-ring guidance online.)
FKM (Viton) can withstand temperatures exceeding +120 degrees Celsius and should be used when the chemical fuels include alcohol components, due to their high level of chemical resistance. However, It’s important to note that standard FKM is not compatible with phosphate ester, but specific grades (Viton GF, GLT) are.
HNBR is increasingly common in modern high-pressure hydraulic applications due to its ability to offer better resistance to heat and ozone compared to standard NBR. HNBR also has an operating temperature range of between -30 degrees Celsius and +150 degrees Celsius, making it suitable and compatible with mineral hydraulic oils, fuels, and refrigerants. Often specified for high-pressure hydraulics above the NBR continuous-service limit.
Phosphate ester (Skydrol) hydraulic fluids are used in aerospace and military applications, as they offer high levels of fire resistance, hence the safety preference for aircraft hydraulic systems. However, they do require EPDM seals or specific FKM grades such as Viton GF or GLT. It is critical to note that standard NBR and standard FKM are NOT compatible with phosphate esters. Critical safety consequence: Substituting NBR for EPDM in a Skydrol system causes rapid seal failure.
NBR vs FKM hydraulic
Understanding your application’s requirements, as well as knowing what the seal itself will be exposed to, is vital in choosing the right o-ring material.
In essence, nitrile vs Viton comparison:
NBR is the most cost-effective o-ring type available; FKM is the most suitable when temperatures of the application exceed +120 degrees Celsius.
NBR will offer higher tear and abrasion resistance in dynamic applications; however, for high heat, ozone resistance, and high chemical exposure, FKM O-rings will provide you with a more robust seal.
Why EPDM is NOT suitable for mineral oil hydraulics
This is an important point for hydraulic system designers and procurement engineers to heed:
- EPDM is fundamentally incompatible with mineral hydraulic oils.
- In contact with mineral oil, EPDM swells, degrades, and loses sealing ability.
- EPDM is appropriate ONLY for water-based or phosphate ester hydraulic fluids.
Choosing EPDM for mineral oil hydraulics is one of the most common material selection mistakes. Make sure you avoid falling into this trap.
Choosing the right O-ring material for hydraulic fluid applications
Provide content for a decision flowchart (Lloyd or designer will produce the visual):
- Step 1: What is your hydraulic fluid? Mineral oil → Step 2. Water-glycol → EPDM. Phosphate ester → EPDM (or specific FKM grade). Biodiesel or aggressive chemicals → FKM or FFKM.
- Step 2: What is your continuous operating temperature? Below 120°C: NBR. 120°C to 150°C: HNBR. Above 150°C: FKM or FFKM.
- Step 3: What is your operating pressure? Above 10 MPa (1500 psi): add a backup ring.
Backup o-rings
When operating pressure exceeds approximately 10 MPa (1500 psi), look to add a backup ring.
Backup rings help to prevent extrusion of the O-ring into the clearance gap between mating parts and avoid seal failure through wear and potential shredding.
Being aware of the pressure and operating temperature ranges that the O-ring will be subject to is essential in order to understand if the O-ring type can withstand the conditions it is being exposed to, and these are not beyond its limits.
O-ring installation and maintenance
O-ring maintenance should be carried out regularly to ensure the seal is working properly and there are no visible signs of cracks or swelling.
In your hydraulic o-ring kit, as well as various o-ring types, sizes, and styles, it’s important to also have hydraulic O-ring lubrication.
Lubrication of the O-ring should take place before installation, applying a thin layer of suitable lubricant around the O-ring before inserting it into place.
This helps to provide extra protection, reduce friction, and prevent early wear and tear, expanding the lifespan of the O-ring.
When carrying out general maintenance, it is advisable to clean all o-rings first before reapplying lubricant and re-inserting.
FAQ
Question: What is the best O-ring material for hydraulic oil?
Answer: Nitrile is the best material type for use in hydraulic oil applications, offering a reliable seal and high levels of resistance.
Question: Can I use EPDM in a mineral oil hydraulic system?
Answer: No, this is safety-critical. EPDM should NOT be used in mineral oil hydraulic systems as it will cause the material to swell and the seal to fail.
Question: What temperature range does NBR support in hydraulic service?
Answer: NBR O-rings perform best in temperatures from -40°C to +120°C continuous service.
Question: When should I specify HNBR instead of NBR?
Answer: You should specify HNBR instead of NBR in high-pressure hydraulic applications and applications that are exposed to more heat and ozone, as HNBR offers more resistance in these areas. Often specified for high-pressure hydraulics above the NBR continuous-service limit.
Question: What O-ring material is needed for Skydrol hydraulic fluid?
Answer: Skydrol hydraulic fluid requires EPDM o-ring material seals, or specific FKM grades such as Viton GF or GLT.
O-rings play a vital role in the performance of hydraulic systems, making choosing the material type critical.
If you’re unsure or have any questions, call us at 01535 274 776 or email your requirements to sales@specialistsealingproducts.co.uk.
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