[PSUBS-MAILIST] Hydraulics

["Dan H." <machine@epix.net>]

Hello Carsten,

I'm not an expert but I have designed several cylinders and worked with
many hydraulic applications.

If your cylinder has only a simple O-ring seal, it will seal both ways,
either pressure inside the cylinder or outside.  Only the cheaper
cylinders are made like this though.  As you probably already know, a
simple O-ring seal isn't the best cylinder seal and won't last as long
as a harder "lip seal" with a soft core material.  Most, and almost all,

lip seals are made for pressure from one direction.  You can design a
cylinder for your application using two seals but place them so that
they won't lock pressure between them.

As for a hydraulic motor, most motors have a high pressure seal within
the motor and a low pressure shaft seal where the shaft exits the
motor.  The space between them is usually connected back to the tank
through a case drain line.  If there is no case drain line on your
motor, it's because your motor contains a "internal case drain" with a
valve that allows the seeping oil to exit through which ever motor line
is at return pressure at the time.   I doubt that any motor has a
external shaft seal that can withstand over 120 PSI external pressure.

You will probably have to add a shaft seal placed so it will hold back
the pressure of the sea water.  I believe that any off the shelf motor
used in this application will surely let water into your hydraulics at
depth.  Contact the motor manufacture.  I'm sure they will be able to
tell you what pressure their seals will be able to handle.

Good luck,  Dan H.