Specific pressure

Measurement of the specific pressure is still only carried out in laboratory studies.Specific pressure The ring sealing capacity is defined by the appropriateness of the specific ring pressure on the cylinder and by the correct ring wear resistance. Measurement of the specific pressure is still only carried out in laboratory studies.

One also uses the tare measurement to evaluate this pressure, i.e. the tangential force (Ft) or the diametric force (Fd)Fd = Ft x 2.21 for E = 100,000 N/mm2. This value is an indication only as the distribution of the tangential force is not uniform over the peripheral area or over the ring height. On the peripheral area it is not the same if measured at 15°, 90° or 180° from the cut. As for the axial height, the presence of chamfers, interior lips or asymmetric cross sections (type M) result in significant variations in pressure.

In the case of rings with a nominal diameter greater than or equal to 40 mm, the Ft value is measured.

Below 40 mm, the ISO standard recommends measurement of the opening gap. The tangential force of the ring (Ft) is based on several parameters:

– Material: This element defines the elasticity module (E). Standard untreated grey cast iron (FGL F55) has a module of E= 100,000 N/mm2. Spheroidal graphite cast iron (FGS F58) has a module of E = 160,000 N/mm2.

– Dimensions: Ft varies with diameter dl and the chamfer or form options for the ring peripheral surface (bevelled, rounded, chamfer KA forms,…). The tangential force varies proportionally to the cube of the radial thickness al. This also explains why some engine manufacturers choose an interior lip so that a ring R IW acts in the same way as a ring M, under the sole dynamic load of the specific pressure. Finally, the opening gap (m) in the ring also has a considerable influence. In general, the opening gap m is equal to 12 to 15% of the nominal diameter.

– Plating: the possible use of chromium plating on the ring results in a 10% drop in tangential force.

During operation, the specific pressure P is only very rarely checked. Indeed, when in use, the ring is subjected to an additional pressure on it inside surface due to the pressure applied by the fluid compressed at the base of the groove due to leakage at the cut or due to lateral gaps. It is therefore considered that the effective pressure is often higher than the nominal pressure.