[PSUBS-MAILIST] Hydraulic idea

Sun Nov 27 09:38:21 EST 2016

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 <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
> <personal_submersibles at psubs.org>
> *To:* Personal Submersibles General Discussion
> <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 <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 <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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