Seal material selection
Maybe it's just me but for some reason choosing the most appropriate seal material for an application used to make me uncomfortable. Bamboozled by the plethora of the various product catalogues (for this was before the Internet age) the decision would often be left to the last possible moment, with all the attendant issues it created. I'm sure I wasn't alone but back then small leaks were often tolerated and, with engine accessibility far easier, could be readily repaired or replaced. In modern, cramped engine compartments though, when even seeing where a leak is coming from may be impossible, an oil or water leak is much more of a drama.
It was only when I came across the acronym 'T-A-M-P' that my confidence rose, and while it certainly didn't solve the problems, the structured approach it offered ensured that they were tackled in a clear and logical way. The letters of course stand for Temperature, Application, Media and Performance, and may be somewhat dated today what with all the complex solutions available, but as a starting point it should surely begin to focus the issues.
The first parameter to consider is that of the maximum temperature to be experienced. Tackling this aspect in the first instance will quickly eliminate many of the possible candidate materials, especially if temperatures above 130 C are to be experienced.
However, when doing so, account must be taken of other sources of heat, for instance friction in a rotating lip seal, which could add as much as 20-30 C to the bulk metal temperature in the region. The close proximity of a turbocharger turbine housing might also need some thinking about. In some environments it might be wise also to consider the minimum temperature likely to be experienced, and the chance of that happening. Fluoroelastomers like FFKM for example perform exceptionally well at high temperatures. Unfortunately their low-temperature performance may be lacking when less expensive compounds could do a better job.
Having ruled out a number of elastomer technologies, attention can now be focused on the application and any particular characteristics required. Rotating lip seals may need a certain amount of damping if the lip is to remain in contact with the (slight) elliptical orbit of the crank surface. Hoses, water oil or turbo air may need to have some flex resistance. On the other hand, an O-ring or moulded seal might need to deform significantly if it is to fill the groove designed for it, and in such cases an elastomer with a slightly lower Shore number may be required.
The fluid to be sealed may at first appear simple, but when one considers that gasoline can consist of 400 or so different hydrocarbon compounds, in addition to any number of aggressive (to elastomers) additives, it may not be quite so easy. Add to that the move towards bio-components such as ethanol or butanol, and the problem begins to escalate.
Lubricants also have their concerns. Mineral oils, for instance, may be very easily absorbed into some elastomers, causing them to swell, while other components (PAO) in oil can have quite the opposite effect. Some oil suppliers are beginning to include more ester technology into their blends, so this may also need to be considered.
In some versions of the acronym, some might include P for pressure. In automotive applications pressure is perhaps of less concern, whereas performance - and in particular, things like shaft surface speed - are of more importance. In the engine world, 'high pressure' generally means high temperature and the use of a mechanical joint. For all others some form of elastomer technology would seem to rule. Nevertheless, these days, with the ever-increasing complexity of materials and fillers, designs and coatings, at this point you'd do well to consult the experts; at least you will have a clearer idea of what is required.
And who knows, rather just specifying the most expensive material, using this approach may be more suitable and save you some money as well.
Fig. 1 - Oil pump seal
Fig. 2 - Simple oil seals
Written by John Coxon