O-Ring Sizes for Hydraulic Seals

10 July 2018

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An O-ring is a loop of elastomer with a round cross-section designed to be seated in a groove/gland and compressed during assembly, creating a seal between two or more parts. O-rings cover a wide range of applications and are used in all industries, for both the maintenance of older products and for new product designs. The relatively low-production costs and ease of installation and use have made the O-ring the most widely used seal. Practically all liquid and gaseous media can be sealed using one of the many elastomeric O-rings available.

ApplicationsFigure 1: O-rings are particularly well-suited for hydraulics, both for maintenance and new applications. Source: Seals-ShopFigure 1: O-rings are particularly well-suited for hydraulics, both for maintenance and new applications. Source: Seals-Shop

O-rings are particularly well-suited for hydraulics, both for maintenance and new applications. They seal over a wide range of pressures, temperatures and tolerances, require very little space and are lightweight and sometimes reusable. They come in many sizes and materials, often specialized for particular applications. O-rings are particularly important for hydraulic systems since they seal connections by being forced into a channel and held in place by hydraulic pressure. The O-ring is used to prevent leakage and prevent outside contaminants from penetrating the hydraulic system. Following is a breakdown of some specifications, materials and features of O-rings for hydraulics.


There are several essential specifications for selecting the correct O-rings in hydraulic systems. The inside diameter, outside diameter and cross section define the dimensions of an O-ring. The inside and outside diameters are the distances from the center to the O-ring’s inside and outside surface respectively. The cross section is the difference between the outer and inner diameters. O-ring inside diameters and outside diameters can vary significantly, from a few millimeters to a meter or more. Be sure to leave some space when specifying O-ring inside diameter to account for swelling and possible thermal expansion.

Avoid stretching the O-ring more than five percent, if possible, as stretching can cause the cross section of the O-ring to flatten. O-ring cross sections tend to vary less, from a few millimeters to tens of millimeters. When selecting an O-ring for your hydraulic application, it is very important to know the depth of the gland so that the O-ring selected has the appropriate cross section to avoid extrusion and tearing and to ensure a proper seal.


O-rings come in a variety of materials, including nitrile rubber (NBR), ethylene propylene diene rubber (EPDM), fluoroelastomers (FKM) such as Viton®, silicone rubber (VMQ) and others. NBR has excellent resistance to hydraulic fluids and can be used in the temperature range of -35°C to +120°C. NBR is a copolymer of butadiene and acrylonitrile that becomes more resistant to hydraulic fluids as its nitrile component increases, but it also becomes less flexible. Although NBR does not possess good resistance to ozone, sunlight or weather, it has excellent resistance to compression set, tear and abrasion. O-rings made of NBR generally are an excellent choice for hydraulic applications.


EPDM O-rings have excellent resistance to heat, water, steam, ozone and sunlight. EPDM O-rings are particularly useful when sealing phosphate-ester hydraulic fluids and may be appropriate for some applications in place of NBR. FKM (Viton®) O-rings have very high resistance to high temperatures, greases, fuels, silicone oils and many other materials and are well-suited for aerospace applications. Increasing fluorine content in FKM increases chemical resistance while low-temperature characteristics regress. FKM can be used with non-flammable hydraulic fluids and may be an appropriate choice for hydraulic applications under certain circumstances. Finally, VMQ has good ozone and weather resistance as well as good cold flexibility and high heat resistance, though it is susceptible to superheated steam. VMQ O-rings have relatively low tensile and tear strength and are susceptible to wear. Although excellent for many applications, VMQ is not usually used for hydraulic O-rings.

Figure 2: O-rings have very high resistance to high temperatures, greases, fuels, silicone oils and many other materials. Source: Seals-ShopFigure 2: O-rings have very high resistance to high temperatures, greases, fuels, silicone oils and many other materials. Source: Seals-ShopAnother set of important O-ring specifications are the maximum pressure and the operating temperature range.

Generally, maximum pressure tends to be expressed in megapascals (MPa) and operating temperature range is expressed in degrees Celsius. Depending on the O-ring material, maximum pressure can be as high as 25 MPa and operating temperature ranges can vary from -40°C to +200°C. It is very important when selecting an O-ring that the maximum pressure and operating temperature range encompass the extremes of the O-ring application. O-ring chemical resistance, flexibility, tear resistance, abrasion resistance, weather resistance and other properties can be compromised if an O-ring is exposed to conditions beyond its maximum pressure or operating temperature range.

Quad Rings

Some applications involving pressures greater than 5 MPa may be ideal for a backup ring. When internal pressure becomes too high, an O-ring may extrude and quickly lose material through wear. Seal failure usually follows fairly quickly afterward. A backup ring is a ring of hard, extrusion resistant material that fits between the O-ring and extrusion gap, thus preventing O-ring extrusion. This can be a cost-effective alternative to more durable yet more expensive materials. An alternative to O-rings that can also be used with backup rings is the X-ring, or quad-ring. Quad-rings have two sealing areas per side as opposed to the one for O-rings. As a consequence, quad-rings require less deformation to maintain an effective seal. Less deformation means less friction and wear, which gives quad-rings a longer seal life compared to O-rings. Quad-rings come in a variety of materials and sizes and can be an alternative to O-rings for hydraulic system sealing.


The specifications of inner diameter, outer diameter, cross section, material, maximum pressure and operating temperature range are critical to identifying the correct O-ring for your hydraulic application. Backup rings can help prevent extrusion in high-pressure applications while quad-rings offer a longer-life alternative to O-rings for particular applications. Although O-rings are usually a very small and inexpensive part of your hydraulic system, they can be critical to its performance and are worth researching and understanding so that the correct O-ring (or quad-ring) is selected and installed. Putting in the time to understand the O-ring that you need now may buy you years of operation for your hydraulic system. If you are unsure of the article number of your seal, product finders or some online-shops (such as quite often have the option to search by the aforementioned criteria when applying filters during product selection.

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Re: O-Ring Sizes for Hydraulic Seals
2018-Jul-11 3:09 PM

no mention of durometer????

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