Wear factors

SAB wear

When evaluating ring wear, it should be noted that bore wear is measured in diameter whereas ring wear is measured at the cut (si) and is thus expressed in developed length.

For a cylinder diameter wear of 0.05 mm, the cut gap will increase by 0.05 mm x n = 0.157 mm. Furthermore, it is considered that for removal of the same volume of material on the cylinder and the ring, the ring wear will be 40 times greater than the cylinder wear. For a cylinder bore wear of 0.06 mm, the cut gap will increase by 2.40 mm.

With the precautions indicated above taken into consideration for the evaluation of wear, this wear could be due to an inadequate choice of ring or inappropriate operating conditions.

Reminder:
 to guarantee the ring fulfils its required function correctly, in addition to selection of the appropriate dimensions and form, attention must be paid to ensure it retains its radial freedom of movement after having been fitted in the piston groove.

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Some causes of premature ring wear

 the specific pressure of the ring on the cylinder bore is too high;
– radial thickness al too high (distortion during assembly, tangential force too high….);
– radial thickness too low (excessive sensitivity of the ring to the additional pressure of the fluid in the groove base);
– cross-sectional form inappropriate (M, L. B and variants IF or IW);
– excessive exterior chamfer which reduces the height range;
– leakage at cut (si) or via surface gaps (roughness inappropriate) allowing a high fluid pressure in the base of the groove.
– insufficient lubrication: bad oil film quality at the top of the cylinder close to the combustion chamber, oil fluidity too high due to high temperature, insufficient oil film due to excessive specific pressure.

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Mechanical strain

– inappropriate cylinder bore material,
– incorrect cylinder form,
– cylinder roughness non-conform;
– cut (si) gap too narrow resulting in a buttressing of the ends;
– parallelism and alignment of grooves non-conform resulting in the rings gripping due to the lateral play;
– groove depth insufficient or groove base radius too high causing the ring to grip;
– scaling of the groove base causing the ring to grip…

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