Newsgroups: comp.robotics
From: National@hyperbar.demon.co.uk (National Hyperbaric Center)
Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!news2.near.net!news.mathworks.com!europa.eng.gtefsd.com!howland.reston.ans.net!pipex!demon!hyperbar.demon.co.uk!National
Subject: Help: ROV Sub shaft seals..!!!!
Organization: Myorganisation
Reply-To: National@hyperbar.demon.co.uk
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Date: Mon, 17 Oct 1994 17:44:56 +0000
Message-ID: <782415896snz@hyperbar.demon.co.uk>
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In answer to your question about how to seal ROV shaft against water ingress.

There are a number of philosophies involved. When using hydraulic motors as 
is done on the large work class ROV systems, simple lip seals are used and
the complete ROV hydraulic system is held at a presure typically 15 psi
above the ambient presure by units called compensators. A compensator is simply
a piston full of oil and acted on by a spring. As leaks occur the oil leaks out 
and the spring pushes down the piston maintaining the presure above ambient
untill the piston is empty. Wise designers put alarms on their compensators
to tell the pilot when to recover the ROV and fill up the compensation system
Because there is never more than 15 psi across the seal in this method leaks
are minimised and the sealing method kept simple.

When using electric motors the same technique can be employed but if you are
using brushed motors the carbon detritus from the wearing brushes will 
eventually make the oil conductive and there will be a big bang. This was
the case with many of the early eyeball ROV. The problem has now largley
been overcome by the use of brushless DC rare earth electric motors which
of course do not have the brush problem they are however expensive and difficult
to control needing special electronics.

If you do not want to oil fill and compensate your electric motors there are 
other options. I have heard that something called a "crane seal" is very
good but I know nothing about them, maybe others can advise. The other option
is to use two O rings around the shaft held in a machined plastic holder 
and to oil compensate the space between the two o rings as the o rings wear
oil will leak past them and give you an indication that it is time to do
some preventive maintainance, my earlier comment about alams applies.

Hope this is of some help.

Jan Paterson			Views expressed are my own etc.



-- 
National Hyperbaric Center
