A few facts about the invention of the Unistress Piston Ring Expander.
For the reliable and economic operation of diesel engines it is
of paramount importance That the piston rings fit closely to the cylinder
wall along their entire circumference. The manufacturers therefor take great
care in machining the piston rings to such a shape that the rings, when
installed, will match the cylinder bore correctly.
It would be an advantage if the machining could also take into
account the deformation given to the rings during assembly in slipping
them over the piston top. This is, however, not possible since the magnitude
of the deformation is an uncertain factor owing to the methods employed in opening the rings.
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The figure shows the stress distribution in a ring opened with a
Unistress Piston Ring Expander (UPRE). The UPRE subject the ring to four
forces as indicated and therefore to a uniform moment throughout, except
at the ends where it decreases rectilinearly towards zero. Constant moment
means constant stress which again means the lowest stress possible for a
desired opening of the ring. Each UPRE is provided with a stop, limiting
the deformation of the ring to the minimum required to slip it over the
piston diameter. As a result one may count on a UPRE always being used once
it has been procured. The UPRE are delivered adjusted and ready for use
for any type of piston ring and diameter upward of 150 mm.
A UPRE has the advantage of making change of the piston rings much
easier. Even the biggest rings can easily be changed.
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