Oil Recirculation in closed loop system

[ponytug]

Dear MR,

I hate to disagree, but to move a cylinder, there has to be a volume change. When you steer, you adding fluid to both the left and the right cylinder (opposite ends). That causes the cylinders plural to move. That movement dumps an identical volume to the tank. That volume is ~ 2x30=60cu inches per lock to lock cycle. I don't know if the analogy works, but it's like climbing on a teeter totter. You need a pressure (weight) difference to get movement. That movement does displace oil to the tank. As you correctly point out, in the steering circuit the cylinder ends are flipped, but just like the lift circuit, oil goes into one end and out the other. It is just that in the steering circuit, instead of having both bottoms filled, we are filling the left front and the right rear, while draining the left rear and the right front. (or vice versa).

Maybe this won't be quite so mud clear, but, feel free to point out the error in my thinking so that I can understand hydraulic systems better.

All the best,

Peter

Yes, you lose a little fluid back to the tank when steering, but it has to be an extremely small amount. Going full left and then full right moves most of the fluid between the cylinders, not back to the tank. Look at the schematic. If fluid leaves the bottom of one of the steering cylinders it has to be made up in the top of the opposite steering cylinder. Very little fluid will pass through the post of the TEE back to the steering valve. It has to go through the cross of the TEE to fill the other cylinder.

I'm saying the steering valve is pressurizing one side of the steering circuit while relieving pressure on the other, not flowing tons of GPM, just tons of pressure.

:confused3:

 

[J_J]

My simple answer is that expended fluid goes to tank from the valve and gets new fluid every time the cyl is activated.

I have stated how to check it, and that is using some hyd fluorescent dye in the cul to see oif it evere gets to tank.

The steering is not a closed loop system.

Cyl expended fluid is not normally reused, but can be used in certain circumstance.

There are two different volumes used when you steer. the base end holds a certain amount of fluid, and the rod end holds less.

The fluid does not bounce back and forth in the cyl circuit.

Can you not agree that if the base end is empty or the cyl is retracted, as the valve opens to extend the cyl, the pump is pushing new fluid in the hose plus the hose fluid.

 

[ponytug]

Dear JJ,

I agree with your points- just trying different words to see if we can all come to agreement.

All the best, Peter

 

[J_J]

The only way for the fluid to stay in the cyl and move back and forth is with a hose size and length that could contain the same volume of fluid as in the cyl.

Say you had one quart of fluid in the base end of a cyl. What would be the length of 3/8 in hose to keep fluid from going to tank. Other wise , the length of hose from cyl to valve that could contain one qt of fluid.

The volume of fluid in a 3/8 hose is .006 gal per ft.

A 30 ft hose would only contain about .18 gal.

One qt = 1/4 gal.

So every time you fully extended the cyl, you would dump some old fluid and add new when extended again

So the answer is that the fluid is always changing.

 

[J_J]

Most closed loop systems do leak about a pint to quart an hour so the whole system is filtered just slower, and trash will circulate, which is why high pressure loop filtering is required anytime the closed loop is opened. The steering cylinders will not leak unless the seals are bad and they will keep circulating the same fluid, and the fluid gets bad after a while...

Not true.

Read my other post.

You are always adding new fluid from valve to cyl.

You would need about 55 ft of 3/8 hose from the valve to the cyl to keep using the same fluid.

 

[Marks481]

Maybe the issue is that you don't get good flushing of the fluid in the cylinder (it essentially looks like a dead-end section of pipe... that moves back and forth). So wear products tend to build up in the fluid inside the cylinder; some of the the fluid expelled from the cylinder as it strokes isn't washed all the way back to the tank, but stays in the hose, to be flushed back into the piston when it strokes the other way,

 

[J_J]

A small amount of fluid may be left in cyl, but the new fluid will mix with the old and keep mixing as you use the cyl.

I would suggest the discoloring of the fluid is due to contamination by water, causing acids , which could make new compounds.

 

[woodlandfarms]

So I am still here, have been following this with great interest. But I am not seeing it the way you guys are so help me a bit cause I am, well, no scientist.

So oil goes to the steering control under pressure. If not assigned to a pipe it returns to a tank (pipe being the hose to a steering cylinder).

So now I turn the wheel, and oil is assigned down a hose, pressurizing that hose, and in turn pressurizing the hydraulic cylinder.

Now the Hose to the Cylinder is long (my magical hose chart says 37" for each hose) and the Cylinder is actually quite small. I guess in the end the hose has a quite small diameter so maybe this is where the logic fails me....

Anyway, cause the hose is so long, and the oil in the cylinder has no place to go, how is oil recirculating. Maybe there is some mix happening in the hose at the head as it is pressurized but I don't see how fresh oil is going to make it all the way to the cylinder. Fresh oil is only going to go so far down the hose until the Limit of Travel on the cylinder is reached.

Now my whole theory could be bunk because the hose holds X amount of oil and the cylinder requires Y amount so indeed the hose is being purged to the point of getting fresh oil from the steering control.

Anyway, poke holes folks. Got lots to learn before I die.

 

[J_J]

You can rest assured that a hose that short from cyl to valve is going to get a fresh supply of fluid each time you cycle the cyl.

The old fluid from the cyl has gone to tank after one cycle.

Hypothetically if you only use micro movements of the steering, the fluid might not exchange for a while, but I can bet that you do go full rotation sometimes.

I don't know if your hyd system is close to mine, but if the volume of fluid in the cyl is more than the volume in hose to the valve, it is going to add and exchange be it a little at a time if you.

It is not a closed loop system, and nobody would design it that way with a logical mind.

Say the cyl at full retract has 1 oz of fluid in the vase end, and say the hose to the cyl has 6 oz of fluid in the hose, then to fill a 32 oz cyl, you get to add 25 oz of new fluid from the valve which has new fluid from the pump.

Don't know more that I can say.

I would say that the dark fluid in your cyl was contaminated.

2 in bore
1 in shaft

16 in stroke

Volume extended .2175 gal
---------retracted .1631 gal

Your 37 in hose contains about .0176 gal

Can you not see what will happen?

Your cyl contains way more than the hose, so it is gong to exchange/add each time.

If your steering cyl is 3 in bore, the volume will be greater

Cylinder Calculator: Hydraulic cylinder valume capacity

 

[woodlandfarms]

I knew you would have the math...

Thanks as always. Going to pull oil out of the tank again and double check. Maybe it was just me thinking the oil looked too dark