Closed loop system

[BobRip]

Maybe you could describe it as a semiclosed loop. I don't know the capacity of the charge pump, but I bet it's enough to keep the hydraulic cooler working and the system cool. Probably several gallons a minute. The pressure is low so it would not take a lot of horsepower to run this.

 

[NJBill]

J J
These will complete the system.
They all return to the tank-eventually.

 

[MossRoad]

Thanks for the schematic. After carefully looking at the schematic, I still can not see that the fluid ever go to the tank except for the output of the charge pump. Follow the circuit around the loop, and see what you come up with. From my understanding, the pump is pushing or pulling fluid back and forth using the charge pump to make up any loss through motor leakage, or case drain. In theory, the most effective cooling would be to run the fluid from the motors through a cooler before it went to tank. I do not see an output from the motors or pump that goes to tank except as noted above. Some days I think I have it nailed down, and other days, it is just relative theory. Closed loop to me means that fluid just recirculates through the system. Open loop to me means that fluid is sucked up by the pump, and sent to hydraulic motors or something doing work, and then back to tank.

When I look at that diagram, I see two circuits:

One circuit comes off the pump, goes through the cooler, then back to the tank.

The second circuit is for the wheel motors.

When you are not moving and the variable volume pump is turning, as I understand it, all the fluid is being pumped through the cooler and none to the wheel motors. As you step on the directional valve, fluid is directed to the wheel motors and less is going through the cooler.

Or does the vairable volume pump not take in any fluid from the tank until you step on the pedal?

How does a variable volume pump really work, anyway? I've got the swash plate concept down pretty good, but does it take in fluid constantly like the PTO pumps and just spit out the unused portion back to the cooler and tank?

 

[J_J]

David, In a closed loop system, fluid from the pump goes to the hydraulic motors and then back to the input of the pump.
A charge pump is required to keep the system full of fluid during normal operation. A charge pump has an output of at least 20% of the transmission pump output. In practice, the charge pump not only keeps the loop full of fluid, it pressurizes the loop to between 110 and 360 psi. Once the loop is charged to the setting of the relief valve, the flow from the charge pump passes over the relief valve, through the case of the pump , and back to tank. If the VSP were a 10 gal pump, there would be about 2 gal going through the cooler and back to tank under little pressure.
At rest,or neutral, very little pressure is generated, and very little fluid is pumped.
Some times, motor case drain will go back to pump case and then to tank.
I think the main purpose of the closed loop system is to keep the circuit clean. Any contaminants would come from metal particles, or rubber seals in the pump/motor circuit. Any additional fluid would come through the filter, and into the charge pump.
Therefore, I believe the hydraulic system in the PT is a closed loop hydraulic system.
That's my story, and I am sticking to it.

 

[MossRoad]

So it is only sucking make-up fluid through the filter, not the full flow that it is(or could be) circulating through the wheel motors?

 

[RegL]

So it is only sucking make-up fluid through the filter, not the full flow that it is(or could be) circulating through the wheel motors?

That kind of makes sense if you consider that the oil that stays within the closed loop should be clean, so why restrict it with a filter.

 

[MossRoad]

So it is only sucking make-up fluid through the filter, not the full flow that it is(or could be) circulating through the wheel motors?

And only the bypass oil through the variable volume pump gets cooled?
And the PTO circuits run continuosly at full flow?
Wouldn't adding a filter to the PTO circuit make sense, then?
And also adding a cooler to the PTO circuit as well?

YIKES!!! Too many questions... must... eat... lunch................

 

[BobRip]

Would an outboard filter that is installed on the PTO when you are not using it make sense. It could filter large volumes of fluid and since it is running anyway, why not? It would use the quick connects.

 

[J_J]

I believe each circuit should have a filter. If not on the suction side, then on the high pressure side. The filter should be after the Quick Connect and before the hydraulic motor, when using attachments. When not using attachments, the PTO circuit is in bypass mode, and a return line filter would filter at 8 to 12 gal per minute, or 480 gph, and in an 8 hr day, 3840 gal minimum. The high pressure filter price range is between $59 to $150.

I already have the parts for an external device to filter the hydraulic oil in the tank using a 115v AC motor driven pump, and two filters, a 10 micron, and a 1 micron. You can switch between the two when ever.