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Posted for "Robert Overmars" <robert.overmars@tiscali.it>:
Salutti tuti
Appended below are photos of our leaking hydraulic cylinder(s) and you can
clearly see that the od of the piston's o-ring is perfectly flush with the
piston od. In fact the piston with the o-ring installed is an almost perfect
slip fit into the cylinder without any preload on the o-ring. When I measured
the o-ring od when installed on the piston it measured 0.001 less in diameter
than the id of the cylinder...and predictably the cylinder leaks internally.
The cylinder id measures 1.750" or 44.45 mm. The bottom of the piston o-ring
groove measures 37.77mm, meaning the depth of the piston o-ring groove is
44.45-37.77=6.68/2= 3.34mm. Consulting "The Seal Man's O-Ring Handbook", for a
nominal 1/8" o-ring as used on the cylinders the groove width should be 4.8mm
(it measures 4.78mm) and the groove depth should be 3.10mm so it appears the
piston o-ring groove depth is cut too deep by 0.24mm which results in there
being no o-ring preload (ie, compression). For example compare the piston's
o-ring on the right with the cylinder's o-ring on the left and you can clearly
see the cylinder o-ring od is well and truely above it's groove and results in
good o-ring preload and sealing when the cylinder end cap is installed.
Actually this o-ring groove depth is 2.77mm so it is fractionally on the small
side...so much for design consistency.
What brought about this discovery was that one hydraulic cylinder was leaking
internally, audibly so, when I pressurised them both with air to take the
weight of the gear legs and tyres for measuring the gear door clearances. One
leaked and the other did not and the reason for that is also interesting... it
appears not all o-rings are equal, eg a nominal 1/8" o-ring has an actual
width of 0.139", + - 0.005" and when I measured the o-rings' widths sure
enough one was at the lower limit of it's tolerance, 0.134", and the other
was fractionally greater and that's the one that did not leak (much, as it
slowly let down overnight).
Having pulled off one cylinder because it was leaking I decided to check the
other and uninstalled, disassembled and checked its piston o-ring groove id
measuring 37.77mm as well, so after measuring two such consistently made
samples I rather suspect that your LIV hyd cylinders will probably measure the
same. But your hydraulic pressure doesn't bleed down?.... perhaps your o-rings
are on spec or on the high side of the spec. However if the o-ring groove
depth is oversize then I suspect that wear over time will erode the o-ring
that eventually it will start bypassing oil. I know of one LIV that had
unexplained loss of hydraulic pressure such that the pump had to kick in,
eventually over time, every few minutes in flight to sustain pressure.
Interesting....but what to do about it...I consulted The Seal Man's
engineering dept and they replied the best thing to do to make new pistons
with correct o-ring grooves. Alternatively....try oversized o-rings ....but
but but... So not liking but but buts I'm throwing this open to the LML
experts and Joe & Co.
over to you...
ciao Roberto d'Italia.
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