What’s In Hydraulic Seals

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I am a the kind of engineer that likes to pontificate about all things technical, even subjects that I know little about but interest me.   I must also be a sad person who loves to find out about quite

Phil The Editor

ordinary things like why doesn’t my mower start or why is this car service guy telling me something that is plainly a load of tosh?   What’s more he is insulting my intelligence!

So instead today, I thought I would surprise you by talking about a subject that would be a challenge for most people; hydraulic seals.   From the early beginnings in the early 18th century during the industrial revolution here in the UK where a hydraulic seal meant that oiled leather disk sandwiched between two iron plates in a beam engine piston, today, hydraulic seals are the life blood of all power electromechanical machinery.   The pioneers of power soon found that if you could contain the oil within the machine, it saves an awful lot of manual oiling the piston with an oil can and make mobile machinery, notably the steam locomotive, a practical proposition.

In the old days, sliding joints, pistons, and bearings were metal-to-metal, requiring constant lubrication and any seals were designed and built by the machinery manufacturer himself.   Nowadays, hydraulic seal manufacture has become a specialist area requiring a lot of research and development and using some very esoteric materials.   It is now a partnership between the machinery manufacturer, the specialist hydraulic seal manufacturer and the chemical company developing special materials to suit ever more extreme conditions of temperature, pressure, abrasiveness and very aggressive chemical environments.  The aim is ever more efficiency and that means less down time, longer service intervals and ever less leakage round/through the seal.

The modern hydraulic seal is normally a blend of materials and inert fillers and braces to assist it in retaining its shape without undue distortion and keeping its sealing properties.  These inert fillers and bracing inserts include bronze, spring steel, short-strand glass fibre and copper wire.

There are an enormous array of 30+  elastomers as hydraulic seal materials are called, some of which are well known to engineers, but others, which mean nothing as they are TLAs (two or three-letter acronyms to you).   So just for you, here is a quick run-down of what’s around.   Firstly, the standards people, ISO and AST, have standardised on these and for the most part, they are coincident in their respective standard ISO1629 and AST MD 1418.

Just so you have an idea what these all are, and what they can do, it’ll give you a brief rundown on the materials now in common use.

Nitrile is a synthetic rubber or copolymer and it is commonly used with petroleum based oils, it’s resilient in compression and with an operating temperature range of -40°C to +115°C. Next, we have Ethylene-Propylene (EPDM).  This is usually specified for phosphate ester hydraulic oil (Skydrol) but it is not suitable for petroleum oils.   Applications include steam, acetone, dilute acids and alkalis.  If you want a good static seal with good resistance to dry heat, ozone or UV light and a wide temperature range (-90°C to + 230° C), then Silicone (VMQ) is what you need.

If you need good tensile resilience then Urethane (AU, EU) is the copolymer you need.  It has a good temperature range – 55° C to + 95° C.   It is commonly used with petroleum based hydraulic oils and thus has wide automotive use. Flourocarbons (FKM) come in many acronymic guises.   DuPont, the chemical giant, provides a wide variety with a wide resistance to chemicals like alohol, aromatics and ozone.   They are also UV light resistant.  The latest in this range includes DuPont’s Kalrez Spectrum perfluoroelastomer which has an even wider temperature range and is resistant to over 1,800 chemicals now found in industry.

Finally. We come to another of DuPont’s elastomer range; that is Teflone (PTFE). Typically, PTFE seals, are combined with short glass fibre, bronze flashes, carbon, graphite, or a combination of these fillers.   It has high chemical inertness, high heat resistance, low temperature flexibility, low running friction, high slip characteristics and a maximum operating temperatures of + 125° C.

I told at the outset that I have some strange enthusiasms.   You are probably either turned off with the whole business of hydraulic seals or alternatively, you would like to know more.   In any event, you must agree that hydraulic seals do help to keep pollution from leaks to a minimum.   You may however, feel that the carbon price of producing these modern elastomers is not worth paying.

Let’s hear your view.