The carbon-face seal

Monday, October 07, 2013

Tags :  engine-structure

In engine design sometimes parts cannot be made out of a single piece, which unfortunately means that seals and gaskets are necessary to keep the several fluids and air inside the engine, and keep dust, debris and water on the outside.

I say ‘unfortunately’ because every gasket or seal is a weak spot in the engine. Seals are always made from a material with lower mechanical properties then the base block or head material, and the two mating surfaces always carry some risk of defects (distortions, machining errors and so on), which can lead to leakages. On the other hand, it does not really make sense to talk about something we cannot prevent, so let us see which kinds of seals we could choose from. In this article we will look specifically at sealing rotating parts.

In an engine and gearbox there are a lot of rotating parts, from lower to very high rpm. Many of these do not need sealing since they are fully encapsulated in the housing. Some though, for example the crankshaft, require a connection to the outside world, in this case to transfer torque, so at the point where the axle goes through the housing, some sort of sealing ring is required. Typically a seal consists of some kind of sealing lip as the actual seal, and a carrier such as a metal ring that can be pressed into a machined chamber, or a flange that can be bolted to the flat block surface. In order to function properly, the sealing surface needs to be controlled very tightly regarding diameter and roughness. In motorsport it is mostly PTFE that are used, owing to their tolerance of high temperatures, chemical resistance and relatively low friction.

With the increasing revs of modern race engines, however, PTFE seals no longer have the best sealing capabilities, mainly because of the rotational speed and crankshaft vibrations, which can lead to leakages between the seal and the shaft itself. In addition, the frictional losses rise exponentially with rpm, and lip-type seals require a certain level of pre-tension (albeit very low) leading to initial friction.

This is one reason why carbon-face seals were introduced to seal crankshafts in housings that run in the higher rpm ranges. Before they were brought in for race engines they were already in use in turbochargers, for sealing turbine shafts running at very high revs.

A carbon-face seal exits from a retainer that incorporates a carbon ring and a feature (spring, O-ring, bellows) that presses the carbon ring outwards, sealing against the sealing surface of the rotating part, in this case the crankshaft flange. Carbon-face seals can withstand far higher speeds than sealing lips and show lower friction. But given their design principle, in general being nothing more than a carbon ring sealing to a rotating crankshaft surface, these seals are more sensitive to angular misalignments, something against which the lip of a traditional sealing ring is very robust.

The carbon-face seal is known to show a higher degree of wear earlier then a lip seal, however, but in a race engine this is not a major disadvantage, since the running hours are not very high compared with other applications for them. And thanks to their production techniques, materials and surface finish are of such a high quality level that surface wear can be minimised.

Written by Dieter van der Put

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