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.