Re: bomb recharge procedure???

[Orin Eman <orin@WOLFENET.com>]

> >I imagine that Audi had also given that approach some thought, but one
> >reason it's not done probably has to do with the difficulty of engineering
> >a small valve for recharging at 2000 psi. While that pressure is routinely
> >available and handled well by gas cylinders and their valves, the valves
> >are pretty beefy hunks of brass. Something along the lines of a Schraeder
> >valve would be pretty ambitious for that pressure.

> There is a line of Schrader-type valves that handle these pressures, and
> more.  All of the hydraulic system accumulators on the CNC machines where I
> work are recharged via this sort of valve.  Some are charged to over
> 4000psi.  The valves don't look like anything special, just like an
> all-metal tire-filler...the charging rig uses a thread-type clamp you screw
> onto the valve, then a separate valve depresses the core for pressure
> checking or system pressure tuning...BTW, these accumulators are a *lot*
> bigger than an Audi bomb...

Can't say I'm surprised.  There is already a valve inside the bomb that
can handle that pressure, albeit hydraulic oil, not N2.  And the
whole hydraulic system up to the servo runs at that pressure,
so controlling 2000psi isn't that difficult!

As for the diaphragm, in use, it mostly has equal pressure on both sides.
It's only when it isn't installed that there is a big pressure difference
across it (2000psi one side, 14.7 or so on the other!), probably accounting
for the limited shelf life.

(In use, it spends most of its time with the engine off, therefore,
with healthy check valves and servo, there will be a pool of hydralic
oil the other side at the same pressure as the N2.  Over time, we
assume that some of the N2 diffuses across the diaphragm, hence
causing the eventual demise.  When the car is driven, there will
be a pressure difference across it when the brakes are used,
but this will be a lot less of a difference than seeing
atmospheric pressure on the other side... especially since the
PS pump is busy trying to keep the pressure up!) 

Orin.