[PSUBS-MAILIST] Hydraulic idea

Sun Nov 27 11:36:02 EST 2016

That would work but there is no real benefit, I may as well just use the pump, flow rate is negligible since the manipulator should operate slowly and the piston size is small due to the lack of power  requirement.   Hank 

    On Sunday, November 27, 2016 9:28 AM, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

  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 <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 <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   

_______________________________________________
Personal_Submersibles mailing list
Personal_Submersibles at psubs.org
http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20161127/a50be15f/attachment-0001.html>