Closed loop system

[J_J]

I remember reading a post about a closed loop system the other day, and started thinking, is the variable speed pump circuit on the PT, a closed loop system. If this is true, then we are not filtering a large a mount of oil. Consider this, in a closed loop system, the oil is pumped through the wheel motors, and back to pump, not to tank. Any excess oil or leakage by the wheel motors, is fed back to the tank. Any loss is made up by the charge pump which keeps the oil at around 320 psi. The question is, how much oil are we filtering? It would seem that the hydraulic filter is on the input to the variable speed pump via the charge pump. I don't believe the PTO pump fluid is filtered. I don't believe the steering or lift circuit is filtered. I don't have a schematic of the hydraulic system, so I am thinking through the system with my brain, and most times I come up with a logical conclusion, but on this subject I am not that positive. Can someone check out their hydraulic schematic and enlighten me if I am wrong.

 

[MossRoad]

J_J,
I'm pretty sure on my unit, if I read the schematic correctly, oil travels through the filter, then to the variable volume pump, then back to the tank when the directional pedal is not pushed. If you push the pedal in either direction, some fluid goes to the wheel motors, then back to the tank.

There is a second pickup for the main PTO and aux PTO/steering pump.

What is interesting is, in the winter, if I start the tractor up and do not move it, the oil heats up on its own, just from circulating through the pumps. How much of that heat is due to the variable volume pump and how much is due to the two other pumps is not known. But since the main PTO is rated for 8gpm at 3600 RPM and the aux PTO/steering pump is a little less, I'd say most of the heat generated in the oil at high idle is probably due to those pumps and not the variable volume pump.

 

[J_J]

J_J,
I'm pretty sure on my unit, if I read the schematic correctly, oil travels through the filter, then to the variable volume pump, then back to the tank when the directional pedal is not pushed. If you push the pedal in either direction, some fluid goes to the wheel motors, then back to the tank.

There is a second pickup for the main PTO and aux PTO/steering pump.

What is interesting is, in the winter, if I start the tractor up and do not move it, the oil heats up on its own, just from circulating through the pumps. How much of that heat is due to the variable volume pump and how much is due to the two other pumps is not known. But since the main PTO is rated for 8gpm at 3600 RPM and the aux PTO/steering pump is a little less, I'd say most of the heat generated in the oil at high idle is probably due to those pumps and not the variable volume pump.

If that is true, then we don't have a true closed loop system. I would say that at idle, the squash plates are turning but doing a whole lot of nothing. at idle, I believe very little fluid is moving through the VSP, and there is not much oil being filtered. So in theory, in we were to set up for backhoe/fronthoe work, and work for about 4 hours, how much of that oil would be filtered. My thoughts would be very little to none. I think you are right about the PTO circuit. It is pushing the hydraulic oil at the given rate, through the PTO valve and back to tank, and would eventually heat up the oil. The same goes for the steering circuit.

If anyone has a copy of the hydraulic schematic for an early 90's PT-1445, I would appreciate a copy. The people at Power Trac don't even have one, so they say.

 

[MossRoad]

If that is true, then we don't have a true closed loop system. I would say that at idle, the squash plates are turning but doing a whole lot of nothing. at idle, I believe very little fluid is moving through the VSP, and there is not much oil being filtered. So in theory, in we were to set up for backhoe/fronthoe work, and work for about 4 hours, how much of that oil would be filtered. My thoughts would be very little to none. I think you are right about the PTO circuit. It is pushing the hydraulic oil at the given rate, through the PTO valve and back to tank, and would eventually heat up the oil. The same goes for the steering circuit.

If anyone has a copy of the hydraulic schematic for an early 90's PT-1445, I would appreciate a copy. The people at Power Trac don't even have one, so they say.

Yes, I agree that during stationary work, not much oil is going to get filtered.

As for your schematic, make your own. You are the only one with a unit with the Ford gas engine and I think the only one with a 1445 from the 90's. I would start with a big piece of paper, draw a square on each corner to represent the wheel motors and a few rectangles to represent the pumps. And a smiley face for the filter! Then start tracing hoses by hand. I'm going to have to diagram my backhoe plans pretty soon and am not looking forward to it. But having your own schematic will be most helpful come troubleshooting time.

Do you think we should be filtering the input to the PTO pumps? Could a filter handle the flow rates?

 

[J_J]

I think each circuit should have it's own filter. They make a high pressure filters for circuits like our PTO.circuit. The way it works now, is that any contaminates in the system is either sent back to tank or through the hydraulic motor, which is not good. The only circuit that is filtered before the oil is used is the charge oil. If I had the money, I would put flow gages, and pressure gages on each circuit. That VSP pump is one of the most expensive pieces on the PT, at about $1500 to $2200 per unit. My VSP is leaking at the area where the PTO pump is mounted. I believe it is the seals on the VSP. Could be easy to fix, or be hard to fix. Either way, I have to jack up the engine assembly to take off the PTO pump, and maybe pull the back plate off the VSP to change out the seals.

 

[BobRip]

Looking at my schematic, I believe that the makeup pump has excess capacity which goes through the hydraulic fluid cooler and then back to the tank. If you are not moving, you would actually have more fluid going through the filter, the cooler, and back into the tank.

 

[Bob999]

I remember reading a post about a closed loop system the other day, and started thinking, is the variable speed pump circuit on the PT, a closed loop system. If this is true, then we are not filtering a large a mount of oil. Consider this, in a closed loop system, the oil is pumped through the wheel motors, and back to pump, not to tank. Any excess oil or leakage by the wheel motors, is fed back to the tank. Any loss is made up by the charge pump which keeps the oil at around 320 psi. The question is, how much oil are we filtering? It would seem that the hydraulic filter is on the input to the variable speed pump via the charge pump. I don't believe the PTO pump fluid is filtered. I don't believe the steering or lift circuit is filtered. I don't have a schematic of the hydraulic system, so I am thinking through the system with my brain, and most times I come up with a logical conclusion, but on this subject I am not that positive. Can someone check out their hydraulic schematic and enlighten me if I am wrong.

JJ, My understanding is that the system is not closed loop for two reasons:
1. there is a "charge pump" internal to the variable displacement pump that is continuously providing relatively low pressure oil to the variable displacement pump and this flow returns to the reservoir.
2. There is leakage at the wheel motors and, at least on the 1845, there are case drain lines at each of the motors that return the leaked or bypassed oil back to the reservoir.

However, even after noting the above, I agree with you that it would be desirable to have additional filtration. Ideally every circuit would have a filter on the return to the reservoir. (I say return line because that is normal practice to avoid the cost of filters capable of withstanding high pressure. Suction line filters, such as the one on the PT variable displacement pump supply, are typically only used with variable displacement pumps or other applications that are particularly sensitive to particulate contamination. The problem with suction line filters is that if the begin to load up cavitation can be induced in the pump and cause very expensive damage. The concern about cavitation is what drives the very frequent filter replacement recommendation for the PT suction line filter.) Practically, for a machine where the PTO is used regularly, having a return line filter on the PTO would probably achieve more than a 90% solution.

I have thought about trying to add a return line filter to the PTO circuit but there is a real space problem. I can't find space in the engine compartment and I don't think it is acceptable to have a quick connect fitting or any other restrictive fitting between the filter and the reservoir so mounting the filter at the front of the machine probably wouldn't work.

 

[NJBill]

J J
This is for a 2004 PT 422
Hope it helps.

 

[KMA]

This is for a 2004 PT 422

And 425 if I read the schematic right.

 

[J_J]

J J
This is for a 2004 PT 422
Hope it helps.

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.

 

[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.