[PSUBS-MAILIST] thruster compensation

Sat Feb 18 03:55:47 EST 2017

Hey guys - don't get too carried away by re-inventing the wheel - have a look at what you can get off shelf and modify. A number of years ago we used Kodak plastic concertina chemical bottles for compensators - image shows a couple enclosed but exposed to pressure to compensate RPV thrusters - they worked down to 2000-ft depth. We also used plastic Coca Cola containers for for RPV adjustable buoyancy systems - they worked also. As did wine cask bladders! 
Go to K-Mart - perhaps with some lateral thinking the answer may be sitting on a shelf -  I doubt you are trying to achieve James Cameron's depths.
Cheers
Keith Gordon

On 18/02/2017, at 3:25 PM, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

> Brian,
> you may prefer to build a "rolling diaphragm" compensator.
> http://www.tecnadyne.com/cms/images/products/pdf/Model%20PCX%20Brochure.pdf
> These compensators were used on James Cameron's sub. 
> Also google rolling diaphragm or volume compensator diaphragm if you
> aren't sure what I am on about.
> In the technadyne link, it looks like they clamp the diaphragm between the two
> halves of the cylinder.
> I remember pricing a compensator years back & it was a fortune; but could be easily &
> cheaply made.
> Alan
> 
> Sent from my iPad
> 
> On 18/02/2017, at 1:22 PM, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
>> Brian,
>> they use pvc for plumbing so that should be OK regarding
>> water absorption.  Another alternative would be to use some
>> acrylic tube; that would have a smoother finish & you could
>> see the oil level.
>>    I liked the paint pot idea I had, as they are very cheap & have
>> a screw on lid for filling as well as a fitting in the base.
>>    If you decide to make one; the commercial compensators I
>> have seen have a spring for giving the unit a 4-6 psi overpressure.
>> This would be a help because if there were any stiffness in the piston 
>> you would end up with less than ambient pressure. If you aren't creating
>> an overpressure you may as well just use a bladder. Having said that
>> a lot of people are happy without an overpressure. Karl Stanley just
>> uses a first stage regulator with the spring removed to give ambient
>> pressure air compensation, & lets the overpressure on the way up
>> force it's way past the thruster seal.
>> Alan
>> 
>> 
>> 
>> Sent from my iPad
>> 
>> On 18/02/2017, at 9:03 AM, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>> 
>>> Alan,        I was thinking it would be easy to make a compensator just using a pvc pipe and then machine a delron disk ( you wouldn't need the piston rod)  with a o ring , the disk could move freely inside the pvc pipe and the pvc pipe would give you the ability to glue fittings on the ends which would give you a threaded connection to attach a hose line to.
>>>  
>>> Brian
>>> 
>>> --- personal_submersibles at psubs.org wrote:
>>> 
>>> From: Alan James via Personal_Submersibles <personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
>>> Subject: Re: [PSUBS-MAILIST] thruster compensation
>>> Date: Fri, 17 Feb 2017 04:57:40 +0000 (UTC)
>>> 
>>> Brian,
>>> yes true, but if I can hack something that's already out there
>>> all the better. That's what Hank is doing with his pneumatic
>>> cylinder. The screw cap on the items I posted is an advantage for filling
>>> & retaining the spring, but would be a pain to make.
>>> Those professionally manufactured compensators cost a fortune.
>>> Alan
>>> 
>>> 
>>> 
>>> From: Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
>>> Sent: Friday, February 17, 2017 4:36 PM
>>> Subject: Re: [PSUBS-MAILIST] thruster compensation
>>> 
>>> Alan,   You could probably make a plexi glass unit like that, you could machine a aluminum disk for the piston.  You would be able to observe what is happening!
>>>  
>>>  
>>> Brian
>>> 
>>> --- personal_submersibles at psubs.org wrote:
>>> 
>>> From: Alan James via Personal_Submersibles <personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
>>> Subject: Re: [PSUBS-MAILIST] thruster compensation
>>> Date: Fri, 17 Feb 2017 02:34:31 +0000 (UTC)
>>> 
>>> Hank,
>>> why waste a good pneumatic cylinder when you can have
>>> one of these. (sauce syringe) All you need is a stainless spring to put inside it for
>>> overpressure, a compatible hose fitting, & maybe replace the piston
>>> o-ring. I have only seen them up to 100ml in volume & aren't sure whether
>>> that would be enough.
>>> Alan
>>> 
>>> 
>>> From: hank pronk via Personal_Submersibles <personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
>>> Sent: Thursday, February 16, 2017 1:25 PM
>>> Subject: Re: [PSUBS-MAILIST] thruster compensation
>>> 
>>> Alan,
>>> The port is open to the water and acts on the piston directly.
>>> Hank
>>> 
>>> 
>>> On Wednesday, February 15, 2017 3:44 PM, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>> 
>>> 
>>> Hank,
>>> in hindsight there is a problem in that when the external water pressure
>>> pushes on the rod & tries to make the piston move down the cylinder
>>> there is a vacuum created behind the cylinder that needs filling with oil.
>>> Not sure if that's the right terminology.
>>> If water can move in to that void through one of the ports, it would work. 
>>> Or the rod & piston were the same diameter. You may have already thought 
>>> through this. I encountered this when trying to compensate linear actuators.
>>> Alan
>>> 
>>> Sent from my iPad
>>> 
>>> On 16/02/2017, at 11:25 AM, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>> 
>>> Brian,
>>> 
>>> The piston is mid way.
>>> Alan,
>>> I was going to install an internal spring to create internal pressure but I only have one seal oriented to keep oil in.  I will try this first and see where it leads
>>> I have tried this in the past on Gamma's prop shaft and it did not work- I am sure it is because of the seal orientation.
>>> Hank
>>> 
>>> 
>>> On Wednesday, February 15, 2017 3:21 PM, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>> 
>>> 
>>> Hank,  where is the piston situated in the cylinder upon start up of operation ?   Half way ?
>>>  
>>> Brian
>>> 
>>> --- personal_submersibles at psubs.org wrote:
>>> 
>>> From: hank pronk via Personal_Submersibles <personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
>>> Subject: [PSUBS-MAILIST] thruster compensation
>>> Date: Wed, 15 Feb 2017 21:32:47 +0000 (UTC)
>>> 
>>> I have just installed my two additional motors on Gamma that are mounted to the manipulator assembly borrowed from Elementary 3,000.  To compensate them I used a single air cylinder 1.5 in bore with 4 in stroke as the compensation bladder.  The cylinder is filled with light hydraulic oil and the motors are filled with WD40.  There are hydraulic hoses connecting the cylinder to the two motors.  I reversed one seal in each motor so the one seal keeps water out and one seal keeps oil in.  There is a tiny bit of air in the system but that makes no difference because the cylinder piston has enough travel to compress the air.  I think oil enters compensated motors because the oil spinning creates pressure from centrifugal force and the seals are oriented to keep water out.  The oil can then escape, that is why I have turned one seal around.  I will be testing in a month or so, and we will see, I guess.
>>> Hank
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