Does it always work?Tags : engine-structure
This month I have chosen not to write about ‘real’ content on the topic of gaskets and seals, but to focus more on the ‘how do we know it is right’ side of things.
When I read about developments of some kind in gaskets or seals, I sometimes also wonder if the developments really work and if they will keep working in all kinds of circumstances. And ‘circumstances’ can be read here in the broadest sense – do they function under all kinds of ambient conditions, for example, but also do they work with all their surrounding parts with all their deviations. In other words: how robust is the gasket or seal in its environment?
There all kinds of seals and gaskets, but in general you can differentiate between flat seals and form seals, both of which have their various different shapes and designs. Flat seals can for example be made out of materials like organic compounds and steel sheet, while form seals can consist of many materials, like EPDM, Viton and others, and can be shaped like a simple O-ring or as complex shape to seal more intricate components.
Given this broad range of applications, how can we be certain that these gaskets will remain intact throughout their design life. Most suppliers will say their product is the best there is, and most of the time they are directing the statement towards racing in particular by adding that the products are the most robust ones available, that they are made especially for racing purposes. I always ask myself what that means. To my mind, every component is designed to withstand the boundary conditions it has to meet. So, racing or not, robustness is just a matter of what was written in the technical requirement specification, nothing more, nothing less.
And let us look at the initial use of the part and not bother, for now, about durability over its lifetime. Why? Because part of the issue is related to processes, so it appears in very early life/use of the part.
For example, many tests during the development process are being done with early development components, which are often taken very good care of at the parts’ suppliers. These parts are so good that most of the specifications are being met, and are often made to be nominal parts – which of course they won’t be when they are getting more into their production phase. And it really does not matter that much whether we are talking high volumes (production race series engines, for example) or low volumes (prototype racing parts). Suppliers always have a different viewpoint from engine manufacturers: when the full specification range can be used, more parts can be approved, which brings the most money to the company. And that is what every company needs to survive and make a profit – and that is how it should be, otherwise no business would survive.
During development therefore, near-limit parts are not being tested regularly, simply because they are not available (it is really very difficult to produce near-limit parts).
So, depending on the engine manufacturer, near-limit testing will be done to a lesser or greater extent. Specific parts are often distorted in certain areas – local damage on gasket surfaces, roughness steps at the sealing areas, and so on – in order to predict possible deviations in later ‘production’ parts, and tested accordingly. Of course, these test specimens should not fully destroy the testing result, since testing time and equipment is too valuable for that. This type of testing is often scheduled as a smaller part of a broader test, in order to be as efficient as possible with test facilities.
To minimise the influence of gaskets and seals on the robustness of an engine, as far as possible gasket and seal types are being chosen that have little sensitivity to deviations in mating parts, which is why you can often see the O-ring type of form seals being used. These have the best track record when it comes to surface defects, due to the fact that their sealing performance depends on compression of the material. Often the material is being compressed to about 75% of its nominal thickness, on which small surface defects have very little influence.
Typical flat gaskets are ‘desensitised’ by adding so-called beads to them to create greater compression, to try to approach the behaviour of form seals. The most sensitive gaskets as regards surface defects are steel sheet flat types, and these are therefore tested near limit as the structural part of development. Also, the mating parts with these kinds of gaskets – often for cylinder heads and blocks – are checked in great detail for surface quality and defects. Only in this way can the design be made and kept robust, from the very first test sample to the later, more production-related components.
Written by Dieter van der Put