[PSUBS-MAILIST] Hydraulic idea

Sun Nov 27 14:29:50 EST 2016

We have done an accumulator with Carbon fibre cylinder used in the
pneumatics industry which is designed for O'rings on pistons.  6" ID x 10 "
long and a spring providing 3-7 psi (empty to full) compensating pressure.
It has a 6" stroke and the piston rod has a magnet in the end and a magnetic
switch to tell when it is nearly empty.  Hugh 

From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org]
On Behalf Of Sean T. Stevenson via Personal_Submersibles
Sent: Monday, 28 November 2016 5:28 AM
To: Personal Submersibles General Discussion
Subject: Re: [PSUBS-MAILIST] Hydraulic idea

The commercial accumulators are heavy because they are designed for high
service pressures.  If you know for a fact that your vessels will only ever
see e.g. 1100 psi, you can get away with something lighter.  The concept is
exactly the same.  Pistons require tight tolerances and surface finishes -
the bladder is the easier solution.  Since the bladder will never see any
delta-P across it (disregarding it's own elasticity), it doesn't need to be
that substantial - even an inner tube might work.  Just something to allow
you to completely purge all air out of the oil volume, and which is
materially compatible with the oil, and which will maximally expand within
the pressure vessel.

As an alternative to your constant pressure accumulator, you could go with a
sealed accumulator and a small hydraulic pump which is capable of high
pressures but not necessarily high flow rates.  For example, empty the
accumulator of oil, and precharge the bladder to 1000 psi.  This ensures
that just before going completely empty, it would still be supplying fluid
at 1000 psi.  If you do this, you can use a pressure gauge on the gas side
of the accumulator to monitor the oil level, so you pump oil in until the
gauge reads 3000 psi, and you then know the vessel is 2/3 full of oil.  You
can recharge the accumulator over time at a much lesser flow rate than is
actually demanded by the manipulator, because the accumulator supplies the
high flow necessary for operation.

Sean

On 2016-11-27 08:44, hank pronk via Personal_Submersibles wrote:

Sean,

I agree completely except the operating pressure is  not as high as you
suggest.  I know for a fact Gamma ran at 1,100 psi from the maintenance
records.  You do make a point that I missed.  Without a bladder, I could
leak oil into the water when relieving the pressure while surfacing.  I was
considering an accumulator either  a piston or rubber bladder type, but they
are very heavy.   I would like to figure out how to use this idea with a
water pressure tank with bladder for the light weight.  

Maybe this is not the way to go, but an interesting brain exercise.   

I do have a small hyd  pump that is pretty light and makes 1,100 ps,i it is
just such a pain in the butt. Mind you with external batteries and external
pump it will not be as bad.

On Sunday, November 27, 2016 7:38 AM, Sean T. Stevenson via
Personal_Submersibles  <mailto:personal_submersibles at psubs.org>
<personal_submersibles at psubs.org> wrote:

Hank, what sort of total oil volume are you considering?  The immediate
problem I see is the loss of pressure in your HP supply, given the necessary
operating pressure of the hydraulic system.  Regulating a cylinder at e.g.
4000 psi down to 2000 for drive pressure is still not going to last very
long, and you also have the problem of pressure increase in the receive tank
if it is not vented reducing your total available delta-P (i.e. you have a
drop in pressure on the drive side combined with an increase in pressure on
the receive side - it may not take long to become ineffective).  If that
isn't an issue for you, then there's no reason it wouldn't work.  You are
essentially powering your system from a precharged accumulator, and draining
into another one at lesser charge pressure.  I would incorporate a check
valve between the supply accumulator and the control valve, so that pressure
pulses are not transmitted upstream and no sponginess is evident.  You might
also want to take a concept from commercial gas charged accumulator designs
and incorporate some sort of bladder inside each pressure vessel to
physically separate the oil from the charge gas.  If you insist on using air
(I still prefer nitrogen), this would prevent the mixing of high pressure
oxygen with oil, and also allow you to vent the receive tank to the water
without worrying about oil contamination in either direction.  It also makes
the system insensitive to vessel attitude.  Then your pumbing becomes simple
- one tank is charged and one is vented, and when all the fluid is moved to
the receive side, you just switch the direction.

Such a system gets its energy input from the compressor that charges the HP
air or nitrogen source, versus having an on-board hydraulic pump drawing
energy from the batteries.  The advantage of the former is that the energy
input is decoupled from the vessel and need not be carried, but you only
achieve constant power for as long as you can maintain constant precharge
pressure, which will be a function of the pressure setting and the
respective volumes of both the hydraulic system and the HP gas source.  The
advantage of the latter is the ability to run continuously without having to
switch directions, and that energy storage in batteries is probably more
efficent than energy storage in compressed gas.

Sean

On 2016-11-27 05:47, hank pronk via Personal_Submersibles wrote:

Alan,

Think of it as a hydraulic system, not an air system.  The arm is identical
in every way except it is powered by air pushing the oil instead of a pump.
It would not be spongy because the cylinders are full of oil.  The air never
leaves the pressurized oil tank.    When the  valve is activated, the oil
moves as if it had a pump.  Instead of the oil returning to the pump
reservoir, it is sent to a holding tank that is a HP bottle.  The air in the
receiving tank compresses as the oil flows into it.  Nothing is vented, it
can not vent because I need to maintain a balance between the two tanks for
buoyancy.  As the oil leaves the pressure tank, the tank gets lighter.  At
the same time the receiving tank gets heavier, so they are balanced.  

I am not worried about space, it is weight I am thinking about and
complexity.  Of coarse if you were doing construction with the arm it would
need to be electric.  But to grab one gold bar it can be air driven.  Air
drive saves battery power also.   Having said all that, it may have a
problem I have not expected, so speak up and save me some trouble anyone.  

 I don't see a problem with a motor submerged in WD40, my vertical thrusters
are full of it.   I could use something else as long as it has a low enough
viscosity for the motor to run.  

Hank

On Saturday, November 26, 2016 8:03 PM, Alan James via Personal_Submersibles
<mailto:personal_submersibles at psubs.org> <personal_submersibles at psubs.org>
wrote:

Hi Hank,
Wouldn't you have to exhaust the air as in a pneumatic system?
Also air would have to displace the volume of oil in the cylinder
as the piston moves in & out, so there would be marginal benefits
over a pneumatic system. It would also be spongy like a pneumatic 
cylinder because any bouncing force on the cylinder would compress the air
that is the source for the movement.
You were talking about saving space with this idea, but if compressed
air was a good form of power we would have pneumatic thrusters 
instead of batteries & electric motors..
I once did the maths on how much energy was stored in a dive tank;
can't remember the result, but there was at least 3 x more energy in
a battery of an equivalent size.
WD40 as hydraulic oil? It is flammable with a reasonably low flash point.
Also at that viscosity you would be more prone to leaks wouldn't you?
https://www.wd40company.com/files/pdf/wd_40tec16952473.pdf
Cheers Alan

  _____  

From: hank pronk via Personal_Submersibles
<mailto:personal_submersibles at psubs.org> <personal_submersibles at psubs.org>
To: Personal Submersibles General Discussion
<mailto:personal_submersibles at psubs.org> <personal_submersibles at psubs.org> 
Sent: Sunday, November 27, 2016 4:59 AM
Subject: Re: [PSUBS-MAILIST] Hydraulic idea

Sean,

The receiving tank will take the full depth pressure and be large enough  to
take all the oil without building up excess air pressure, no need to vent
off, since it is only receiving oil and displacing air.  I need a balance
with two tanks to maintain neutral buoyancy.

I am not worried about oxidation of the oil because the oil is not going
through a pump and the flow rate is so small.   I ordered air cylinders for
the manipulator with a 5\8 rod to reduce the back pressure.  Gamma's
manipulator operated at 1,100 psi when the sub was at 1,000 feed of depth.
The arm will loose power, but I don't expect that to be an issue, because
the oil tank will be powered from a separate bottle of air.  

I have to work with what I have to keep the cost in check, so I can modify
an open centre valve by blocking the final pressure port drain.  I also have
some HP tanks.  If it does not work out easily, I have a few electric pumps
I can use.  

If I go electric, I intend to submerge the motor pump unit in the oil
reservoir with a bladder top to compensate.  That means I will use WD40 as
hydraulic fluid.

On Saturday, November 26, 2016 6:25 AM, Sean T. Stevenson via
Personal_Submersibles  <mailto:personal_submersibles at psubs.org>
<personal_submersibles at psubs.org> wrote:

How do you intend to control the pressure on the receive tank? Just vent it
through a check valve to the water? You need to design carefully to prevent
contamination in either direction.

I wouldn't use air as the drive gas under high pressures, in order to
prevent oxidation of the oil. Charge with nitrogen if you intend to do this.

To generate the same drive capability as a hydraulic pump, you are talking
about very high precharge pressure - approaching the pressure at which HP
bottled gas is supplied, unless you can source e.g. 6000 psi nitrogen and
regulate it down to 2500-5000 depending on your manipulator requirements. In
any case, if your cylinders are single acting, or even dual acting with a
single rod, you have to contend with the force from the ambient water
pressure, so your receive tank pressure needs to be this at minimum, and
with a pressure reservoir source instead of a pump, that available delta-P
is further reduced the moment you demand any fluid from the system (i.e. HP
bottle pressure will drop), so your manipulator becomes weaker over time.

Just a few things to think about.

Sean

On November 25, 2016 5:25:47 AM MST, hank pronk via Personal_Submersibles
<mailto:personal_submersibles at psubs.org> <personal_submersibles at psubs.org>
wrote: 

Hi All,

I have an idea to replace the hydraulic pump for my new manipulator with a
air over hydraulic system.  It is quite simple, the hydraulic oil reservoir
is a hp tank that can be pressurized from a designated HP supply.  The oil
return goes to another HP tank  to receive the oil.  This eliminates the
pump completely and that is a dream.  The manipulator can go through 54
complete extensions and retractions, that is 54 complete movements of all
functions.  

After the oil is used up, the oil can be returned to the pressure tank by
reversing the air flow.

Hank

  _____  

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