Peter Cowan/Lexi Cameron wrote:

Steve Gagnon thanks for your comments on oils. Perhaps you would

elaborate

further on some points that I've wondered about.

Don't Ford and Gm suggest different transmission oils? Is there really a
difference?

What exactly is hydraulic oil compared to, say straight, mineral oil?

Many aircraft use a red tinted brake fluid while some use (I believe)
mineral oil. What is the diference?

My old tracter use TDH fluid. Is it untinted transmision fluid?

What the world needs is ONE oil for all applications!
Thanks,
Peter.

I'll butt in here for a moment with my 2 bits worth.  Most all machine oils
have a mineral oil base ('cept of course the synthetics).  The major
difference between various mineral oils are just the viscosities.  Then the
manufacturer adds the additive packages, and that is where all the
differences occur.  Two major classes exist: Engine oils and Turbine oils.

Engine oil additives contain detergents and emulsifiers to keep soot and
other combustion products in solution while the emulsifiers keep water
suspended so that is doesn't separate out and get sucked into the oil pump
intake of a just-starting engine and wipe out the bearings.  This is why
your engine oil turns dark and milky with time:  more and more soot and
emulsified water.  Another additive may be used to control oil pH due to
acidic combustion products (like sulphuric acid from some high sulphur
diesel fuels).  This additive can be particularly troublesome if you use an
oil with it and *don't* have acidic oil.  Basically, you make soap in the
engine.  Kinda breaks down the lubricity and clogs passages.

The other major category are turbine oils.  The additives in these enhance
water separation (de-emulsifiers) and promotes separation/settling out of
wear particles.  These are used in "clean" machines like steam turbines that
have moisture separators/dryers and centrifuges/filters  installed to
continuously clean the oil.  The oils last for years without changing out.

In an ideal situation the PSRU lubrication would be isolated from the
engine, and would run a turbine oil.  But clearly that ain't gonna happen
since it would require a separate pump, filter and possibly a drier.  Much
too complicated.  Just change the engine oil frequently and supply the PSRU
with clean, filtered oil.

Brake fluids can be a whole 'nuther animal altogether (but not necessarily).
Rather than mineral oil, some hydraulic oils (specifically those used in
high pressure applications or where fire is a concern) use a phosphate
ester.  This stuff looks like oil and feels like oil, but is *very*
different.  It is non-toxic and absolutely non-flammable (good to have when
a brake locks up and starts smokin'), but it softens and/or dissolves most
common plastics and rubbers (like the soles on my work shoes!), and really
does a number on most paints (not epoxy based paint thought).  You must make
sure you are using Viton or one of a few other specialty compounds for
O-rings and other soft seals in these systems.  But it is great insurance
against fire, and I believe this is what most automotive brake fluid systems
use.  Makes sense to me that aircraft brake systems should try to use the
same.

Oh yes.  One other drawback of the phosphate ester oils: They are
hygro-scopic (will absorb moisture out of the air), and the resulting
moisture in the oil will eventually cause it to break down.  I haven't seen
this to be a problem with brake fluids (which really don't get a whole lot
of duty anyhow), but it is a big factor in industrial hydraulic systems that
have a heavy duty cycle.

Doesn't look very likely that we'll have a single all-purpose oil any time
soon!
Tom Kendall

 
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