Shore hardness in hydraulic seals: what the numbers mean and how to choose

Shore hardness determines the resistance of a seal's material against permanent indentation. In the case of hydraulic seals, it directly affects the extrusion resistance as well as the level of friction as well as sealing efficiency under high pressure. The majority of hydraulic seals are between Shore A 60 and Shore A 90. Selecting the incorrect hardness for your pressure and clearance requirements is among the leading reasons for premature seal failure.

Why is it that hardness isn't just an attribute of the material?

When engineers design a hydraulic seal, the main focus is usually on the materials of the type—the most common are NBR, FKM, PTFE, and polyurethane. Hardness is usually viewed as an underline, a number found in the datasheet, which is seldom challenged.

This is a mistake.

Shore hardness isn't just an indication of the material. It's a functional requirement that explains the way a seal performs under pressure and also how it reacts to gaps in clearance and how long it lasts in a highly dynamic setting. If you don't follow the correct procedure, you'll be faced with extrusion failures at high pressures or a lot of heat and friction at lower pressures. Both outcomes are unacceptable in industrial hydraulics.

What shore hardness actually measures?

A Shore durometer test, as standardized as ASTM D2240, evaluates the material's resistance to indentation using the use of a spring-loaded probe. The probe is placed on the surface under a predetermined weight, and the extent of the penetration after a predetermined time is converted to an amount of hardness measured in a range of zero to 100.

Two scales are important to hydraulic seals:

Shore A is the standard that is used to measure elastomers—rubber-based compounds like NBR, FKM, EPDM, and polyurethane. Shore A 0 would be an almost liquid material. Shore A 100 would be more or less rigid. The range of hydraulic seals is within Shore A 55 and Shore A 95.

Shore D is a term used to describe more difficult materials, such as flexible plastics and PTFE compounds, as well as hard backup rings. If you're speculating on PTFE seal elements or hard anti-extrusion rings, you'll have to deal with using Shore D values, typically within the 50-80 range.

The two scales are overlapping in the vicinity of each other around the Shore D 20 / Shore A 90 border; however, they're not interchangeable. The material that is specified as Shore A 90 is not the same stiffness as Shore D 90.

The hardness–pressure relationship

This is the main engineering decision-making process in the seal determination of the seal's hardness.

The soft seal (Shore A 60-70) conforms easily to mating surfaces. They are able to fill minor surface imperfections and require less seating force and create less friction in the case of dynamic applications. But their softness comes with a price in high pressure: the softer elastomers can flow cold into the space between the piston or rod and the bore of the housing. This is referred to as "extrusion," and it can cause damage to the seal.

The hard seals (Shore A 80-90+) resist extrusion due to the fact that the material has a higher structural strength and is resistant to being pulled through the gaps. They maintain their shape even when pressure is sustained and are more suited for high-speed dynamic applications in which heat buildup from friction is important. This means less flexibility and greater breakout friction, in particular static or slow-cycle applications.

This is a useful decision matrix:

If pressures are greater than 400 bar, the hardness of the seal isn't always sufficient. Backup rings that prevent the expansion of the seal—usually PTFE or Nylon—are placed with the seals that are used for primary sealing to close the gap mechanically.

Temperature complicates the picture

The hardness of a seal's value on a datasheet can be measured at an ambient temperature of 23°C. But hydraulic systems operate across wide temperature ranges, and hardness is temperature-dependent.

Elastomers shrink significantly when heated. An NBR seal rated as Shore A 75 at ambient temperature can behave similarly to a Shore A 60 compound at 100 degrees Celsius operating temperature. For high-temperature systems, heating exchangers, injection molding machines, or steel milling equipment, this softening of the thermal surface alters the effective hardness to a range that is not able to resist extrusion under working pressure.

Cold temperatures can have opposite effects. At sub-zero conditions, elastomers stiffen. A material that is able to seal at ambient temperatures may become too rigid when temperatures drop below -30°C, thereby becoming less flexible and creating leak pathways.

This is the reason FKM (Viton) can usually be utilized for Shore A 75-80 for high-temperature applications in which NBR in Shore A 70 would soften and expand. The greater base hardness offers the necessary safety margins once thermal effects are used.

Dynamic vs. Static applications: Different hardness logic

The dynamic seals, piston seals, rod seals, and seals for rotary are always in movement. The hardness of the seals affects the heat generated and friction as well as wear rates. The more rigid seals produce more heat and friction but are able to withstand extrusion and wear faster when pressure is high. The soft seals are more quiet and are quieter, but they could not withstand abrasive working cycles under high pressure.

For static seals, such as sealing flanges or face seals and O-rings that are fixed in grooves, the dynamic friction aspect is eliminated. Static seals are based on initial compression to form an elastomeric contact. The softer compounds are typically chosen for static seals because they provide effective sealing at lower clamp loads and are able to withstand variations in surface finish more effectively.

This is why a 70-70 Shore A NBR O-ring being used as an in-place face seal could not be the best choice if that same compound is required to function in the role of a rod seal within that same process. The geometry and the function determine the requirement for hardness and not only that of the material's family.

Polyurethane: The special case

Polyurethane hydraulic seals function in a different region of the spectrum of hardness than other elastomers. They are worthy of special examination.

The majority of hydraulic polyurethane seals are made in the range of Shore A 90-98, which is significantly more durable than standard NBR and FKM compounds. The high degree of hardness isn't an impediment; it's deliberate. Polyurethane with this hardness provides extraordinary resistance to extrusion, exceptional resistance to abrasion, and a robust mechanical tear resistance. These characteristics make polyurethane the most popular choice for heavy-duty sealing of rods and pistons on construction equipment and for other applications that require contaminated fluids as well as extreme side loading on pistons.

The price is lower flexibility at low temperatures and more friction than soft elastic elastomers. Seals made of polyurethane are designed for extreme wear and pressure conditions but not for ultra-low friction as well as cryogenic ones.

How to specify hardness without getting it wrong?

A variety of inputs need to be evaluated in conjunction:

The pressure of the working area is the main driver. Look up the seal's pressure-hardness chart for a specific design. The majority of manufacturers provide extrusion limits in relation to the hardness and the clearance gap and can be more accurate than the general rules of thumb.

Radial clearance can be used to increase or decrease the risk of pressure. A clear clearance allows a soft compound to withstand pressures that are higher. A worn bore with a higher clearance will require greater hardness or the use of backup rings.

Operating temperature ranges must be adjusted to meet the specification of ambient hardness. If your fluid's temperature is high at 90 degrees Celsius, choose an appropriate compound that will meet your requirement for extrusion resistance at 90°C and not 23°C.

Fluid compatibility is a factor that affects the materials you can choose from. Not every hardness is available in every material. NBR is readily available throughout Shore A 60 to 90. FKM options are less obtainable over Shore A 85. Make sure that the hardness you require is available in the chemical compound your fluid needs.

Surface finish and tolerances for machining contribute to conformity requirements. Surfaces that are rough or out-of-round benefit from soft substances that can cover surface imperfections. Precision-ground surfaces can take on harder seals and not be at risk of leaks.

Shore hardness can be a definition that you do not accept by default. When you select a hydraulic seal, make sure that the quality of the seal is suitable for the operation pressure, clearance requirements, and temperature range and not just to ensure that your material selection you select is correct. The distinction of Shore A 70 and Shore A 85 in an NBR rod seal could be an enormous difference in a service life of 5,000 hours and a failure of the extrusion within the initial week of service.

Check the hardness of the seal against the conditions Check the tables for pressure clearance from the manufacturer and take into account thermal effects. This distinction separates seals that work from those that are separated at the bench.