Date:         Wed, 11 Jul 2012 16:10:22 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: power steering
Comments: To: Pat <psdooley@VERIZON.NET>
In-Reply-To:  <0M7000JAXGNE1995@vms173019.mailsrvcs.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 03:26 PM 7/11/2012, Pat wrote:
>Thinking about it, other hydraulic systems do the same thing.
>The fuel pump has the fuel pressure regulator to limit it's flow and build
>some pressure.  I'm not sure if the FP would cavitate without the regulator,
>but its something to think about.

I suspect "cavitate" is being used rather loosely here since it's
something that happens on the suction side of things, not the outlet side.

Both fuel pump and P/S pump are "positive displacement" pumps -
within their pressure capacity their pumping speed is primarily
related to drive rpm, not outlet pressure.  If you removed the fuel
pressure regulator you'd simply have about one liter per minute of
fuel flowing from the tank around and back into the tank with very
little pressure in the system.  The injectors wouldn't produce
anything.  With the regulator, the injectors have ~36 psi across them
to drive fuel through them at a constant rate when they're open (but
the pump is still pumping a liter per minute through the
circuit).  If you ran the fuel pump without a return line, it would
be heavily overloaded and might stall, but it would put I believe
over 100 psi on the line.

The P/S system operates at pressures up around 1700 psi max at idle
speed (pump pressure test, 5 seconds max, Bentley 48.7).  The
pressure is less when there's no steering effort but I don't know if
that means 20 psi or 200 or 700.  If the pressure is fairly low an
outlet restrictor could perhaps keep the vanes quiet in their slots
as they pass the outlet, or could dampen pressure pulses that could
generate noise in the system.  A hose instead of pipe up to the front
might have some similar effect of dampening pulsations.

Yrs,
d