Oil Recirculation in closed loop system

[ponytug]

Ken and MR: I don't know if this helps, but...

As I see it, the video is correct, although a bit pedantic for my tastes. His simulation of the volume changes due to weight are exaggerated to make the point. For all practical purposes hydraulic oil does not compress, and steel cylinders do not bow outwards. Technically, yes.

On to MR's diagram and the specific case of PTs, at least those with dual steering cylinders; imagine trying to turn. As the diagram illustrates, the two cylinders are connect crossways. If you look at the flow carefully, I think that you will see that when you are trying to steer that oil flows into both cylinders and it flows out of both cylinders to the tank during the operation, but in an "X" fashion, i.e. FR&LR are tied together, and FR&LF are tied, so that to steer, one pair drains to tank, while one pair gets applied oil at high pressure. In detail:

For the steering, the cylinders are tied front one side to back side on theater, i.e. the left push is tied to the right pull and vice versa. To steer, you just swap where you are applying the power. i.e. High pressure in left front half, is tied to high pressure into right rear that combined will cause the left cylinder to contract and the right cylinder to lengthen. (At the same time, oil flows out of the left back half and the front right half to tank.)​

Hopefully, this isn't as clear as mud.

All the best,

Peter

 

[MossRoad]

What is the aux PTO pump.

My steering pump feeds the steering, FEL, and aux circuit for a cyl on front attachments.

My PTO pump is independent, and only iused for the PTO.

On our PT425, the variable volume pump drives the tractor. There is a two section PTO pump, one half of which is the main PTO that drives the mowers, etc... the other half of which is the AUX PTO pump that supplies the steering, FEL and AUX PTO. The AUX PTO is the smaller connectors that operate the quick attach circuit and the lever is by the operator's knee.

 

[MossRoad]

Peter, what you are saying is what I am saying. I am trying to understand the other description given to see if my understanding is wrong.

Ken

If it was one cylinder you'd be correct. However, the dual cylinders in the steering circuit aren't hooked up like the video you posted. Go back and look at your hydraulic schematic. The hoses are cross-fed between the two steering cylinders. In essence, there are two steering circuits. Left and Right. When left has pressure, the right just sloshes the fluid from the top of one cylinder to the bottom of the other. When right has pressure, the left does the sloshing. (its not actual sloshing of course, but just movement under less pressure than the other direction). That's why the fluid in your left and right circuits never leaves the circuits. It just travels back and forth between the cylinders. Only a very, very small amount is exchanged near the steering valve. And I mean a very small amount.... only any amount that could leak by the piston in one of the cylinders would force fluid out of the left or right circuit and would then be made up.

 

[MossRoad]

Ken and MR: I don't know if this helps, but...

As I see it, the video is correct, although a bit pedantic for my tastes. His simulation of the volume changes due to weight are exaggerated to make the point. For all practical purposes hydraulic oil does not compress, and steel cylinders do not bow outwards. Technically, yes.

On to MR's diagram and the specific case of PTs, at least those with dual steering cylinders; imagine trying to turn. As the diagram illustrates, the two cylinders are connect crossways. If you look at the flow carefully, I think that you will see that when you are trying to steer that oil flows into both cylinders and it flows out of both cylinders to the tank during the operation, but in an "X" fashion, i.e. FR&LR are tied together, and FR&LF are tied, so that to steer, one pair drains to tank, while one pair gets applied oil at high pressure. In detail:

For the steering, the cylinders are tied front one side to back side on theater, i.e. the left push is tied to the right pull and vice versa. To steer, you just swap where you are applying the power. i.e. High pressure in left front half, is tied to high pressure into right rear that combined will cause the left cylinder to contract and the right cylinder to lengthen. (At the same time, oil flows out of the left back half and the front right half to tank.)​

Hopefully, this isn't as clear as mud.

All the best,

Peter

I concur.... (like I'm some kind of authority! hahahhahaaa).

 

[SpringHollow]

Moss, In your description, what provides the driving force? It has to be the pump. That means there has to be flow back to the pump or to the pump via the tank. If it just goes back to the pump, then you can have a basically closed circuit with little exchange except due to leakage/makeup. But the flow can not be from one cylinder into another cylinder directly because you would have a lower pressure trying to outfight a higher pressure. As an aside, you would actually get some mixing though even with a 200 psi differential which I find amazing - we have pressurized systems that are O2 free that will get contaminated with minute traces of O2 from the air when there is the most minuscule of a leak. These systems are so sensitive to O2 that we can tell when it happens.

Ken

 

[MossRoad]

Moss, In your description, what provides the driving force? It has to be the pump. That means there has to be flow back to the pump or to the pump via the tank. If it just goes back to the pump, then you can have a basically closed circuit with little exchange except due to leakage/makeup. But the flow can not be from one cylinder into another cylinder directly because you would have a lower pressure trying to outfight a higher pressure. As an aside, you would actually get some mixing though even with a 200 psi differential which I find amazing - we have pressurized systems that are O2 free that will get contaminated with minute traces of O2 from the air when there is the most minuscule of a leak. These systems are so sensitive to O2 that we can tell when it happens.

Ken

The steering pump is always pumping. It never stops. It can't stop. So, when you are not moving the steering wheel, or operating the FEL or aux PTO, the full flow of the pump goes straight through the steering valve and back to the tank.

If you turn the steering wheel, the pressure becomes available to the steering rams. That doesn't mean much (if any) of the flow is going to the rams at all, just the pressure.

 

[MossRoad]

Take a look at this circuit. It is two rams for a lift circuit. In this example, the tops of both cylinders are tied together, as are the bottoms. If you push the rams out, the fluid fills the bottoms of both cylinders and forces the fluid out of the tops of both cylinders. Most of the fluid exiting the tops will go back to the tank (except what stays in the hoses as it reaches the end of travel). The reverse happens when you lower the cylinders. Fluid is exchanged in this example, and I think this is what you are thinking of.

 

[MossRoad]

So that is not a dual ram steering circuit. Its a dual ram lifting circuit.

 

[SpringHollow]

I am definitely thinking in terms of the FEL circuit when I am thinking about this. But it seems to me that the only difference in the steering circuit is that opposite ends of the cylinders are tied together instead of the same ends. You have movement of the cylinder rams which means you must have flow, not just pressure. I really can not see how it could be any other way.

Ken

 

[J_J]

The steering pump is always pumping. It never stops. It can't stop. So, when you are not moving the steering wheel, or operating the FEL or aux PTO, the full flow of the pump goes straight through the steering valve and back to the tank.

If you turn the steering wheel, the pressure becomes available to the steering rams. That doesn't mean much (if any) of the flow is going to the rams at all, just the pressure.

I don't agree.

The steering is available at all times, and has priority.

The FEL get the PB fluid from the steering valve.

The fluid is flowing through the steering valve through the FEL, then to tank.

You always have volume in GPM, the hyd components develop the pressure.