Tow/Bypass Valve Limitations PT-425 & Perhaps Others

[SnowRidge]

While researching the Sauer Danfoss series 15 tram pumps, I came across this document. Among other things, it discusses limits to the use of the tram pump's tow/bypass valve, which I don't believe Power Trac states explicitly.

Short summary, tow fast or far and you will damage the pump. Fast and far are more than very slowy, and more than a few feet. In fact, Sauer Danfoss eschews the term "tow valve" completely.

Scary stuff. If it breaks down in the field, you can't just tow it home. You have to winch or push it onto a trailer in order to recover it.

 

[SnowRidge]

OK, I know jack about hydraulics but I think you are misreading this. Yes, you should not open the bypass valve with the engine RUNNING. That is the no-no. If you are towing from a breakdown your pump is not turning... Just the wheel motors are pushing oil around and that oil has to go somewhere and to alleviate damage to the pump you open the bypass valve...

*** EDITED By MossRoad.

Carl

Well, I don't know what to make of the last comment, so I won't remark on it.

Did you actually read the PDF? It contains a lot of info, but the following two paragraphs are of note.

"The intent of this valve is to allow a vehicle using a closed loop propel circuit to be moved a short distance at slow speeds. Typically, this translates to loading a vehicle onto a trailer or getting a malfunctioning machine away from the work area or out of a traffic lane."

"Do not operate the bypass in bypass mode at more than 10 ~ 15% of rated motor speed and do not exceed a duration of five minutes."

From the data I have on hand, that works out to around 450 feet max at a speed of about 1 MPH.

The reason for all this caution seems to be the loss of hydraulic fluid in the closed loop. Since the engine is not running, the charge pump is not turning, and the fluid is not being replenished.

 

[dsb5610]

SnowRidge,

I have the same concern with my Toolcat. Not so much if it breaks down in the field but when I run between places on the road. The Toolcat manual states it can be towed no more than 25 feet while not exceeding 2mph. Basically you can pull it off the side of the road or load it on a trailer.

dsb

 

[ldabe]

Seems like I remember someone on here making a portable dolly to put the PT on and move it or push it up onto a trailer.

I think the PT had to be jacked up to get the dolly under it.

I can't remember what kind of wheels were on the dolly, or who made it.

 

[MossRoad]

I read the document that you linked to and I think you have the correct grasp of what it is saying.

"...the following
rule of thumb for flow rate and duration has been used
successfully to quantify the limitations during a bypass
mode.
Do not operate the bypass in bypass mode at
more than 10 ~ 15% of rated motor speed and do
not exceed a duration of five minutes."

Somewhere along the line of my PT425 ownership I was told the bypass valve was for moving it a short distance only and never tow the tractor at more than a crawl.

 

[J_J]

I read the document that you linked to and I think you have the correct grasp of what it is saying.

"...the following
rule of thumb for flow rate and duration has been used
successfully to quantify the limitations during a bypass
mode.
Do not operate the bypass in bypass mode at
more than 10 ~ 15% of rated motor speed and do
not exceed a duration of five minutes."

Somewhere along the line of my PT425 ownership I was told the bypass valve was for moving it a short distance only and never tow the tractor at more than a crawl.

This is true on most hydrostatic transmission. Even on the hydro zero turn mowers, tow slow and short distance. Even on automatic transmissions, they tell the tow truck to tow no more than 45 mph, you see them hauling ask at 60 or 70 mph, and then you are thinking, I hope they don't tow my car that fast. I think the reason to tow slow is that you are using the wheel motors as a hydro pump, and if towed to fast, you may build up to much pressure and blow the pump, or whatever.

 

[MossRoad]

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.

 

[J_J]

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.

Hey David, is that the one you got rid off. Looks like a tough old tractor in it's time.

 

[woodlandfarms]

Snowridge.

I read through the document once and completely misread it. Made a comment, posted it, went back and re-read the document and saw you were right on and then deleted my post. Guess not quick enough cause you saw it...

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

Carl

 

[MossRoad]

Hey David, is that the one you got rid off. Looks like a tough old tractor in it's time.

Yeah, that's the one I sold to the IH salvage yard and am using the proceeds for my backhoe project.

 

[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.