Tow/Bypass Valve Limitations PT-425 & Perhaps Others

[MossRoad]

And my final comment was my own personal sense of dark humor. Sorry if you got offended guys...

Carl

I was not offended. I just figured since you deleted your original comment, you probably didn't want it in the quote, so I **** over it.

 

[ponytug]

Dear MR,
After reading the Sauer-Danfoss document, where they stress that their concern is the evacuation of the hydraulic lines, I began to wonder if what they were really worried about was depleting the oil in the pump, and causing wear. Your manual below makes it sound as if they are trying to flood the return line to ensure that there is always oil in the pumping system, even if the wheels are pumping in reverse, sucking oil out of the tank.

Given the PT design, what is the risk? Is it that the tram pump is sufficiently blocked that it will prevent oil from entering/leaving the system?

Otherwise, where does the oil go, such that it iisn't made up? For those PT owners with case drains, would they function in reverse to relieve vacuum?

Thoughts?

All the best,

Peter

Just be glad you don't own one of these.... according to my owneerr's manual, you have to add four gallons of Hytran Fluid to the transmission case, tow it to where you need it, then drain out that four gallons. You also have to put the High-Low Range Selector Control into neutral and you cannot exceed 20 MPH. YIKES, if that thing ever got up to 20 I wouldn't want to be anywhere near it.

 

[SnowRidge]

Given the PT design, what is the risk? Is it that the tram pump is sufficiently blocked that it will prevent oil from entering/leaving the system?

Otherwise, where does the oil go, such that it iisn't made up?

The charge pump isn't operating, so no oil will enter the system, but if I understand correctly, some will be forced out by the motors operating as pumps.

 

[J_J]

The charge pump isn't operating, so no oil will enter the system, but if I understand correctly, some will be forced out by the motors operating as pumps.

I am thinking that the hydraulic system on the PT's are closed loop systems. If they lose fluid, the charge pump makes up the loss. The charge pump is only pumping when it needs to. No, the case drains only pass fluid back to the tank. I am also thinking that the pump is designed to run in a certain direction, and when you tow it, you might be forcing fluid back through the pump in the wrong direction. Jamming things up, breaking things, etc.

I have a similar situation on my hydrostatic Wheel Horse, they say to not push or tow. Somebody did push it down some ramps to get it off, and it is now jammed up. There is supposed to be a release valve on there somewhere, but I have not found it.

 

[SnowRidge]

I am thinking that the hydraulic system on the PT's are closed loop systems. If they lose fluid, the charge pump makes up the loss. The charge pump is only pumping when it needs to. No, the case drains only pass fluid back to the tank. I am also thinking that the pump is designed to run in a certain direction, and when you tow it, you might be forcing fluid back through the pump in the wrong direction. Jamming things up, breaking things, etc.

I have a similar situation on my hydrostatic Wheel Horse, they say to not push or tow. Somebody did push it down some ramps to get it off, and it is now jammed up. There is supposed to be a release valve on there somewhere, but I have not found it.

They are closed loop. The charge pump doesn't work if the engine isn't running, which is likely to be the case if the bypass valve has to be opened. I know that I have only opened it when I had a dead engine or the battery out, and I needed to move the machine across the shop.

 

[ponytug]

I guess my question is, if it leaks out, where does it go?

On motors with case drains, it would get returned to the sump, or sucked from the sump.

Is the consensus that with the bypass enabled, oil will leak out of the wheel motor circuit?

If so, where does it leak to?

If it doesn't leak out, what is the issue?

Wheel motors are designed to take pressure both ways, so they won't 'know' whether they are being pumped, or pumping. The main motor is bypassed and not moving, so where is the wear?

What am I missing?

Confused,

Peter

The charge pump isn't operating, so no oil will enter the system, but if I understand correctly, some will be forced out by the motors operating as pumps.

 

[SnowRidge]

I guess my question is, if it leaks out, where does it go?

On motors with case drains, it would get returned to the sump, or sucked from the sump.

Is the consensus that with the bypass enabled, oil will leak out of the wheel motor circuit?

If so, where does it leak to?

If it doesn't leak out, what is the issue?

Wheel motors are designed to take pressure both ways, so they won't 'know' whether they are being pumped, or pumping. The main motor is bypassed and not moving, so where is the wear?

What am I missing?

Confused,

Peter

The wheel motors drive the tram pump in reverse, pressurizing it in a way that wasn't intended, which can apparently damage it. The fluid eventually goes out tram pump's return line due to internal leakage. The wheel motors lose their oil bath, which may damage them, and the tram pump loses its charge, which means, at minimum, it will have to be bled prior to starting the engine.

 

[J_J]

I think this document will explain things about the bypass valve. You can read the whole thing, but pay attention to pages 11/12

http://hydraulics.eaton.com/products/pdfs/03-205.pdf

 

[J_J]

Another bit of data you should find useful.

Temperature of the hydraulic fluid should not exceed 180ºF (82ºC) anywhere in the system. At higher temperatures, the fluid may start to deteriorate, and the viscosity gets too low for reliable lubrication of pump and motor components. As long as the fluid remains clean, stays below 180ºF at all times, and contains no water (less than 150 ppm), there is no need to ever replace the fluid.

 

[Rivco]

I think this document will explain things about the bypass valve. You can read the whole thing, but pay attention to pages 11/12

http://hydraulics.eaton.com/products/pdfs/03-205.pdf

JJ,
Thanks for that link!!. it takes away the mystery of PowerTracs so called name for the pump. (Tram Pump). I've always called it a variable displacement piston pump.... same as the manufacturers, but PowerTrac likes to come up with their own little names. It's just like the Aux. Hydraulic circuit
( real name )... PowerTrac calls it an Aux PTO. circuit... last I knew...a aux. PTO circuit supplied power to an attachment via a drive shaft or a Hydraulic motor.. not a hydraulic cylinder in the case of the PowerTrac. It would be nice if PowerTrac stuck with Industry standards when naming their parts & components.