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Theoretical Relationships

Conformability is defined as the ability of a piston ring to conform to a deformed cylinder bore. A long process of calculation is required to derive the mathematically exact solution [4] from theory. In practice, however, a simplified equation may be used derived from the theory of the closed (uncut) ring [5] and the ring with constant radial pressure [6].

The conformability opposite the ring gap (see also Fig. 5) in a cylinder with an "i"th order radial deformation ui under which the ring is still light tight at a contact pressure p = 0 is calculated as:

ui = radial deformation of the cylinder by the "i"th order from its nominal radius
k = piston ring parameter (equation 10)

Since with increasing i the conformability decreases by approximately the 4th power, it follows that high order cylinder distortions are particularly critical for the functioning of piston rings.

It should be noted that the simplified theory only indicates the conformability opposite the ring gap and not the local conformability around the ring periphery.
In the case of self-conforming piston rings the conformability decreases progressively from the region opposite the ring gap towards the gap. (Fig. 5)

Conformability is improved by the gas pressure pz acting behind the piston ring:

Spring backed rings have a very uniform conformability around the whole periphery.

Accordingly, for a spring loaded ring:

Fig. 5: Conformability


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