[PSUBS-MAILIST] Hydraulic idea

Sun Nov 27 16:09:06 EST 2016

The problem with pneumatic actuators is that you can't meter the flow rate properly because air is compressible, and the moment you experience any change in ambient pressure, the force balance changes because of the exposed rods. Pneumatics will also be spongier than anything you could do with oil. You could get away with pneumatic actuators for applications which have the cylinders on or off (fully extended or fully retracted), but for an application requiring intermediate positioning, fluid power is the way to go.

On that note, one other possibility if you don't require a tremendous amount of force would be to do away with a central oil reservoir, and have a small piston reservoir supplying each action, where the piston is connected mechanically to an electric linear actuator. Then instead of valving you just stroke the electric actuators, and the hydraulics respond accordingly.

Sean

On November 27, 2016 12:50:54 PM MST, Alan James via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hank,
>in addition to my other negative comments; you aren't saving any air
>by doing it the way you are. You are making the system a little less 
>spongy if you do as Sean says & have a check valve in the system.
>But it would be way simpler to just have a pneumatic system. 
>With even 100psi you can get quite a strong holding power with a
>relatively
>small cylinder.
>   Also I get nervous hearing about home made pressure vessels like
>hard tanks etc. Dive tanks need regular testing & inspection as there
>have
>been numerous fatalities from them. But you could go for years without 
>checking a homemade tank, or on sell a potential bomb.
>Cheers Alan
>
>From: Alan James via Personal_Submersibles
><personal_submersibles at psubs.org>
>To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org> 
> Sent: Monday, November 28, 2016 8:16 AM
> Subject: Re: [PSUBS-MAILIST] Hydraulic idea
>   
>Hank,
>I may not be understanding it, but the concept seems fundamentally
>floored.
>Take one double acting hydraulic cylinder. You open a valve & let the
>air 
>pressure in your accumulating tank force the oil into the hydraulic
>cylinder,
>moving the piston along & forcing the hydraulic oil on the other side
>of the piston
>in to your low pressure tank. So you loose oil into this tank! How is
>it recovered again?
>Or do you have a large suppply of oil that goes from the high pressure
>to the low
>pressure tanks & when it has all gone from one side to the other you
>are done.
>With the cylinder moving the other way it would also be lost to the low
>pressure tank.
>   With the WD40 I was under the impression you were going to use it
>inside the hull.
>Maybe I got it wrong. If you are using a motor for a short time maybe
>WD40, but we
>did conclude that it can be detrimental to any plastic parts & o-rings
>over time also
>there were the other aspects covered recently in Brian's post.
>Cheers Alan
>
>From: hank pronk via Personal_Submersibles
><personal_submersibles at psubs.org>
>To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org> 
> Sent: Monday, November 28, 2016 1:47 AM
> Subject: Re: [PSUBS-MAILIST] Hydraulic idea
>  
>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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