Hydraulic Logic; linking of two inputs to one output?

[CWAnthony]

Hi all,

I have a question relating to hydraulic logic, I think that's the right way of phrasing it.

I am planning to connect hydraulic cylinders in a fixed volume system based below. At the moment I am considering it as an ideal system, fluid not compressible, not considering temp changes etc.

Here is a diagram of the system:

Cylinders one and two are connected to front and rear axles of a car respectively. I am considering the car to have an uneven F:R weight distribution, hence unequal forces (120:100N). The fluid from both cylinders will be displaced into cylinder 3 when the static weight of the car is fed through pistons 1 and 2. I need to know the force that will act at piston 3 with a specified area, so I can add a spring into cylinder 3 (bounce spring) and specify it's rate to calculate the strokes of pistons 1 and 2.The problem I have is this:

If I specify the areas of pistons 1 and 2 to be the same (A1), surely with different forces there would be different pressures if the cylinders were not connected?
But as they are connected, I know the pressure must be equal, but what effect would this have on the system, and how would I calculate it?
I estimate that the forces must equalise somehow, but I don't know how I would calculate it, or if this is even correct.

I understand the basics of hydraulic leverage in a one input cylinder-one output cylinder system, but am struggling to apply that logic to this system. I plan to model this mathematically using SimHydraulics but I've never used Simulink before so it will take some time before I can try that.

If anyone could share some wisdom I would be most gratfeul.

 

[MikeHalloran]

It won't work like you're thinking it will.
One of the axle cylinders will collapse completely, and extend the other.
The problem is analogous to synchronizing extension of multiple hydraulic cylinders; there is no mechanism within the hydraulic system to force equal strokes, so you end up with flow dividers, or servovalves linked electronically, or sturdy mechanical linkages.




Mike Halloran
Pembroke Pines, FL, USA

 

[chicopee]

My thought: P1 will prevail,ie, its fluid will flow into the receiving until such time P1 equal to P2, then the receiving cylinder will sustain a combined flow from both source cylinders.

 

[CWAnthony]

Thanks for the replies folks, had a chat with a mate about this and we came to a similar conclusion. There is basically a resultant force of 20N, so the left axle will raise until it hits the top of the cylinder, whilst the right one drops (as you both said).

Here is another diagram: -

http://i104.photobucket.com/albums/m194/chris_anthony_/HydraulicsPitch.jpg

This time there are two circuits, so two different pressures can be held. I take it as there is more force from the left cylinder, the middle piston moves to the right until the spring reacts the resultant force of 20N, then the system is in equilibrium?

Which leads me onto my final one, can anyone tell me what would happen here? It's a merge of the two: -

http://i104.photobucket.com/albums/m194/chris_anthony_/HydraulicsPitchHeave2.jpg

Thanks again

 

[Compositepro]

You are seriously on the wrong track. Google one of Mike's suggestions "hydraulic flow divider". Computer controlled servo valves are another option. Your approach will only work for one exact set of conditions which will never be the case in reality. There are many resources on the web. Some of the more useful ones are the product catalogs of the hydraulic component manufacturers. This site is not for basic education.

 

[CWAnthony]

Oops, I posted the wrong image -

http://i104.photobucket.com/albums/m194/chris_anthony_/HydraulicsPitchHeave.jpg

As I said before, I can't find any information on calculating forces and movements with two inputs and one output. All systems I've found on the internet consider a hydraulic pumps with reservoirs providing pressure, this system is of a fixed volume and won't have a pump, the only pressure comes from movement of both pistons.

If you consider my theoretical questions too basic then please don't feel compelled to reply. If not, would you mind explaining this comment more clearly: "Your approach will only work for one exact set of conditions". Why wouldn't it work for other conditions?

Mike said "there is no mechanism within the hydraulic system to force equal strokes", I don't want equal strokes of each piston, I want a way of controlling the displacement of each when 1) the forces are unequal and 2) when forces are equal.