[PSUBS-MAILIST] Hydraulic idea

Sun Nov 27 14:40:48 EST 2016

Such an accumulator sounds like it might be ideal for oil filled volume pressure compensation (i.e. 3-7 psi above ambient) if mounted external to the vessel.  Do you have a link? Unfortunately, 3-7 psi isn't much help in a hydraulic power application.

Sean

On November 27, 2016 12:29:50 PM MST, Hugh Fulton via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>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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