Re: [PSUBS-MAILIST] Sub with arm

["D. Blake" <dblake@bright.net>]

Okay Dan, I'll take your word for it.  As I mentioned before, I'm just
learning hydraulics.
So without actually having the components here in front of me, I have to get
a mental image of what is taking place in order to reach an understanding.
I based my comments on the literature I have recently read that included
graphic representations of a simple closed system with differing sized
pistons.  My understanding at this point is that regardless the size of the
slave it will only extend/retract as much as the master cylinder will allow.
Perhaps if the slave were extended/pulled in some way beyond the capability
of the master, a vacuum would occur and throw the whole system out of
whack....
Is that what you are referring to?
Anyway yeah, two duplicate cylinders with mechanical advantage arrived at
via mounting = no way to go wrong.

TTYL,
BigDave

-----Original Message-----
From: Dan H. <machine@epix.net>
To: personal_submersibles@psubs.org <personal_submersibles@psubs.org>
Date: Sunday, May 13, 2001 10:14 AM
Subject: Re: [PSUBS-MAILIST] Sub with arm


>Dave,  with out a doubt you are 100% correct with your reference to
Pascal's
>law.  But!
>
>As I wrote in my posting, it's the ratio of rod to piston diameter that
must be
>equal in a closed loop system.  If I understand the original problem It was
to
>direct couple two cylinders together. Two cylinders, two hoses, with no
>reservoir to act as an expansion tank.  What ever comes out of the master
>cylinder has to go directly into the slave cylinder.  Also, the returning
oil
>form the other end of the doubling acting slave cylinder has to have a
place to
>go.  That place in back into the other end of the master cylinder.  The
inverse
>happens when the master cylinder is moved in the opposite direction.
>
>You may use a larger cylinder and a smaller cylinder to gain a mechanical
>advantage but the cylinder to rod RATIO must allow for the proportional
amount
>of returning oil to have a place to go, other wise you create a system that
is
>either sucking a vacuum in one side or locked solid with oil on the other
side.
>The volume or rod going in on one cylinder must equal the volume of rod
going
>out on the other for what ever the travel is made.  With out having to
design
>one cylinder with the exact dimensions to go with one you have, it's easier
to
>just use two identical cylinders and position the mountings to provide a
>mechanical advantage.  If the design won't allow this, there is no choice
but to
>do the math and design a pair of cylinders of different size taking into
the
>fact that they must work in harmony.  i.e. Custom design for one of them.
>
>Dan H.  in Pa.
>
>"D. Blake" wrote:
>
>> Well, according to 'Pascal's' law, "In a closed system filled with a
fluid,
>> the pressure will be the same in every part of that system."  The amount
of
>> force psi applied by a 1-inch actuater will be equally transferred to a
>> 1-inch slave in a closed circuit.  The amount of travel will also be the
>> same.
>> Notice here we refer to lbs. per square inch.  Both actuater, and slave
have
>> the same per square inch of piston surface area.
>> According to 'Pascal's' law, if we increase the slave from 1-inch to
4-inch
>> surface area, we in turn multiply the advantage psi x4, but decrease the
>> travel x4.  Thank of it as a lever.  A 4-inch travel at 2-lb psi force
>> applied by a 1-inch actuater to a 4-inch slave will result in the 4-inch
>> slave having 8-psi force, but with only 1-inch travel.
>> Garage floor jacks reject the opposing force thru a check valve that
allows
>> fluid to flow in one direction only, and so you have to release the valve
to
>> lower the jack.
>> The manipulator arm Stan is suggesting has no such check valve to hinder
the
>> flow of fluid to or from(although he could add one to the gripper part of
>> his design), and so the only limiting factor would be the ratio in terms
of
>> amount of travel vs. desired force in psi.
>>
>> TTYL,
>> BigDave