Hydraulic Seals, Gaskets and O-rings

Hydraulic Seals

Sealants, Seals, Gaskets and O-rings are compulsories item for hydraulic components and systems. Without these, high pressure systems are impossible. So selecting the right material is an art because most applications are compromises, such as resilience versus stiffness, softness versus hardness, low temperature versus high temperature and performance versus cost.

Nitrile Rubber Seals

Nitrile (also known as Buna N) seals are widely used in hydraulic industry because of excellent resistance to oil, a good practical range of working temperatures (-40 degrees C to as high as 135 degrees C), and low cost. Oil on the surface of nitrile rubber even helps prevent oxidation and aging, which is a problem when nitriles are exposed to air and sunlight.

Performance in fluid power system is generally excellent. For example, Parker Seal Company ran a series of brutal tests to determine how much fluid pressure a common nitrile O-ring can withstand without extruding into the gap. To make the tests more meaningful, they held the temperature at 71 degrees C and cycled the pressure 100,000 times at each value.

The hardness for elastomers is measured by shore durometer. The most typical nitrile rubber seals have hardness ranges of 70, 80 and 90 shore.

Urethane Resists Abrasion

Polyurethane elastomers, commonly called urethanes, bridge the gap between synthetic rubbers and plastics. They have a unique combination of hardness, resilience, and load-bearing capacity plus exceptional abrasion resistance. The temperature rating is moderate: 93 degrees C. The mixes are proprietary and result in a wide range of available properties. The material can be formulated for extrusion, moulding, casting or machining. Hardness can be as soft as a pencil eraser or as hard as a bowling ball. Urethane can significantly outwear most rubbers and plastics and even some metals. Also, the coefficient of friction is fairly low and gets even lower as hardness is increased.  Urethane seals enjoy wide application as rod scrapers and other dynamic seals in high pressure hydraulic hose, which further attests to its strength and its ability to take abuse.

Viton Fluoroelastomer

Dupont supplies this particular material to the seal manufacturers. It is fairly expensive but is recognized for certain performance advantages that make it economical in numerous applications. It even replaces low-cost seals where reliability outweighs initial cost.

The normal maximum operating temperature is 200 degrees C to 315 degrees C for intermittent service, and some hardness and resilience are retained at that high level. Even if the high operating temperature isn’t needed, the hot tear strength of Viton makes it easy to strip from a mould without damage, enabling intricate shapes to be formed. Dynamic properties and low-temperature performance are also good.

Teflon Flouroplastic

Teflon (Dupont) and Halon and some similar flouroplastics from other companies have two key properties that make them interesting for sealing. They work at temperatures from cryogenic to 315 degrees C, and they are self-lubricating. The prime disadvantages are creep (cold flow) and limited techniques for forming the parts. The creep is taken care of by designing the seal to either trap the plastic flow and thus contain it at the seal area or to add a metal or elastomer spring to maintain sealing force despite the creep. Teflon seal blending and manufacture is an art similar to powder metallurgy.

Dual-Material Seals

Grover/Universal Seal solves conflicting requirements of resilience on the one hand and hardness on the other with uni-ring, a dual-durometer one piece sealing ring that does both. The sealing surface of the seal is hard (97 durometer) urethane; the backup is softer (45 durometer) urethane. There are also many other forms of combination, for example inserts a nitrile rubber O-ring within a urethane U-cup to achieve a spring-loaded rod and piston sal. The O-ring assures sealing at low fluid pressure, and the sealing force increases as pressure rises.

O-Rings

O-rings are another wonder piece of man-made art, humble looking and yet do wonder for many things. An O-ring is a one piece moulded elastomeric seal with circular cross-section that seals by distortion of its resilient elastic compound. Dimensions of O-rings are given in ANSI/SAE AS568A. , Aerospace size standard for O-rings. It has both imperial (inches) and metric (mm) sizes. The standard ring sizes have been assigned identifying dash number that in conjunction with the compound (ring material), completely specifies the ring. Although the ring sizes are standardized, however ANSI/SAE AS568A does not cover the compounds used  in making the O-ring. In this case, manufacturers will use different designations to identify various ring compounds. For Example, 230-8307 represents a standard O-ring of size 230 (2.484” ID x 0.139” width, or thickness of O-ring) made with compound 8307, a general-purpose nitrile compound.

O-Rings Materials

Thousands of O-ring compounds have been formulated for specific applications. The shore A durometer is the standard instrument used for measuring the hardness of elastomeric compounds. The softest O-rings are 50 and 60 shore A and stretch more easily, exhibit lower breakout friction, seal better on rough surfaces, and need less clamping pressure than harder rings, For a given squeeze, the higher the durometer hardness of a ring, the greater is the associated friction. This is because a greater compressive force is exerted by hard rings than soft rings.

O-rings Installations

When properly installed in a groove, an O-ring is normally slightly deformed so that the naturally round cross-section is squeezed diametrically out of round prior to the application of pressure. This compression ensures that under static conditions, the ring is in contact with the inner and outer walls enclosing it, with the resiliency of the rubber providing a zero-pressure seal.

When pressure is applied, it tends to force the O-ring across the groove, causing the ring to further deform and flow up to the fluid passage and seal it against leakage. As additional pressure is applied, the O-ring deforms into a ‘D’ shape. If the clearance gap between the sealing surface and the groove corners is too large or if the pressure exceeds the deformation limits of the O-ring material (compound) , the O-ring will extrude into the clearance gap, reducing the effective life of the seal.

For very low-pressure static applications, the effectiveness of the seal can be improved by using a softer durometer compound or by the additional squeeze may reduce the rings dynamic sealing ability, increase friction, and shorten ring life.

As you can see here at Hydraproducts we really know our seals! If you’d like some professional advice on the design or build of any Hydraulic sytems, power packs or units and the detail of what would be best for your needs then just come on over to the main website, HydraProducts.co.uk and drop us a line from there. Or pop a comment in below and we’ll do our best to get back to you as soon as possible.