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Use of Radial Lip Seal Technology In The Engineering Industry

Posted by philgnorris on August 16, 2011
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Phil The Main Editor

In a previous post in April, I talked about What’s In Hydraulic Seals?.  This time, I thought I would regale you with a few stories about the use of sealing technology in the engineering industry.  Sealing technology has moved on a-pace since the age of steam. Going hand-in-hand with the development of really clever designs of seals for every size and complexity of applications, the use of esoteric materials continues to grow to fulfil these applications.

Take radial lip seals.  In case any of you don’t know what these are, they are the seals that retain oil and grease within large shaft bearings or rotating machinery such that that can contain up to 2 bar pressure differential.   This is pretty remarkable considering that the seal is typically one retaining radial lip of an elastomer material pressing against the rotating shaft or machinery surface that is slipping past at 15 m/s but could be up to 30m/s.

A typical Radial Lip Seal from James Walker

Hydraulic seal manufacturers  now make radial lip seals in all sizes.  I believe the record is 11 metres diameter (for a tunnel boring machine or TBM).  For anything above say 2.2 metres diameter, moulding a precision-made radial lip seal in one piece becomes a bit impractical both on economic and practical grounds.  To meet this large-scale need, continuous moldings or extrusions are manufactured. Naturally these continuous mouldings need to be jointed to make a ring.

By using a specially tooled manufacturer’s jig and typically a high-performance epoxy adhesive, these rings can be jointed on site.  Site jointing opens up the possibility not only for OEM fitting, but also for retrofitting in place.  Dismantling to expose a shaft end over which to place a ring is rarely feasible and many tunnel boring machine contractor will attest.  They want that drive bearing radial lip seal replaced in hours otherwise the TBM may become jammed by geological redistribution of rock strata and thus can become entombed.  No pressure then to get it turning and cutting again!

There are many types of radial lip seal to suit the different duties, some are designed to work within a machined radial groove and some not retained and sit within a radial recess.    Yet others work with a retaining ring that is fixed into position after the radial lip seal is set in place.

Tunnel Boring Machine Uses Radial Lip Seals

For such arduous duties such as the tunnel boring machine, lubrication can be a problem in such an aggressive environment.  One solution that I have seen, has two radial lip seals set into a groove and a lantern ring set in between them.  This lantern ring is bored through and connected to a pressure lubrication system.  The increased security of a two ring system that is positively pressure lubricated certainly increases the operational life of this vital peripheral bearing.

Radial lip seals are manufactured in a number of modern elastomer materials to suit the temperature and peripheral speed of the shaft or rotating machinery.  Such material have been specially developed or modified in some way to suit hydraulic seal applications. Pressure resistance is of lesser importance in this respect as the detailed design shape features of radial lip seals  take care of the relatively low pressures involved (>2 bar).

Contact with the rotating part is more-often than not is helped by use of a metal peripheral spring moulding into the seal section. Temperature is of critical importance for radial lip seal design and the selection of the material to be used.

An unfortunate property of most modern elastomers is that if an elastomeric radial seal in tension is subjected to frictional heat, it tightens on the shaft to generate higher friction, wear and more heat. This counter-intuitive property is known in science and the seal manufacture industry as the Gough‑Joule Effect.  They ways to overcome this are to reduce the temperature rise or as at least one manufacturer has done, modified the shape so that the critical seal element is in compression rather than tension.

Typical elastomers in use on these seals include Nitrile (NBR) which is suitable for use with water and the majority of oils and greases.  The maximum lip (peripheral) speed varies between 12 and 15 m/s.   One of the latest developments is Hydrogenated Nitrile (HNBR).  This  reformed Nitrile material has an especially low coefficient of fiction for improved running at higher speeds and greater abrasion resistance for longer life.

Other materials that are used on more specialised applications include fluoroelastomer (FKM) for high working temperature applications for self‑retaining seals in open housings and  where electrical insulation is necessary,  polytetrafluoroethylene (PTFE) for those aggressive chemical  environments.

As you can imagine, rotating body misalignments have to be kept in check even with the use of radial lip seals.  To meet these problems, ingenious designs include floating cartridge rings that follow the misalignment whilst retaining the lip pressure on the shaft within operating parameters.  Other solutions include specialised long lip sections and modified lip sections. So whatever the application for large lip seals, be it retrofitting bow thruster seals on Queen Mary II cruise liner or new 1,800mm diameter slew ring seals on elevated wind turbines, there will be a radial lip seal design for the application.

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