Hydrostat damage from towing

[Mike McKown]

I have been hearing about hydrostat damage from towing a tractor with/without the valves released. The same people that warn me of impending damage have never told me what can/will get damaged.

I've towed many of them. Never seem to have damaged anything. Someone please elighten me what can/will be damaged by towing and how that can happen.

[Sam Mac]

Mike

I also have not seen one damaged but looking at the design my guess is that the slippers on the end of the pistons and the valve plate in the motor section of the pump would suffer from a lack of lubrication as the motor rotates while towing. I don't see this being a problem for short distances at SLOW speed as is recommended in the operators manual.

[J-Mech]

Mike,

Sam is correct. The main failure is on the motor side of the hydro on the slipper pads, and rotor plates just as Sam said.

Lets just talk about the motor side of the hydro, and not the pump for this example. The motor's pistons are not spring loaded. When the machine is running and the hydro is engaged, high pressure oil is sent to the piston. The high pressure causes the piston to extend and drive down the ramp of the swashplate, thus rotating the output shaft. As the piston bottoms the ramp, the block keeps rotating and the piston (one of several) is filled with high pressure oil again. Now, the piston is hollow, and there is a small feed hole in the end that the slipper pad is connected. This small hole feeds high pressure oil to the slipper head. The oil actually lifts the slipper just slightly off the swashplate and it literally slides on a film of oil. The slipper does not actually contact the swashplate. Now, when the machine is off, and no oil is present, as the cylinder block rotates, the pistons are pushed into their bore. no oil is present to lube them, and no pressure to push them out. They collapse into the bore after running shortly with no lube. When the machine is started again, the pistons somewhat quickly, are shoved back down to the swashplate, and you hope that they land flat and not with the slipper cocked to one side. If it doesn't land straight...... well, it might just break the head off. Also, the rotor plate has no lube on it during towing. Now this is a high tolerance fit of two metals that, lets just say, don't play well together without oil. The cylinder block is spring loaded with a pretty hefty spring to keep the rotor and valve plate together when they are under pressure. As a fail safe, the block can lift off the rotor plate and dump pressure if the limit of the motor is exceeded. So, to sum up you have several tight tolerance and dissimilar metals running with no lube, that can really work against each other when in a towing situation.

As always pics are helpful!

Here is a pic of the hole in the slipper pads. As you can see, there is no other way for the oil to escape except to "lift" the slipper pad up and keep it floating on an oil cushion.

IMG_20141126_151439.jpg


Here is a pic of the spring that holds the piston block against the valve plate.

IMG_20141126_151459.jpg


And finally, you can see the back of the piston block and the valve plate. As you can imagine, without oil, the steel block and the brass plate don't like to rub together.

IMG_20141126_151518.jpg


There have been pics uploaded to this site of internal failures due to towing. I don't remember who it was or when, but I'll look and see if I can find them later. Gotta run now, hope this explanation helped!

[darkminion_17]

I am not sure if this was from towing or abuse from the owner,as J mech mentioned the slippers ride on a cushion of oil ,I have taken apart about a dozen,seen a lot with scratches on the slippers and swash plate,some with no scratches at all.

[Mike McKown]

Do you think this is the really risky part are do you think all you described holds equal weight?

"....no oil is present to lube them, and no pressure to push them out. They collapse into the bore after running shortly with no lube. When the machine is started again, the pistons somewhat quickly, are shoved back down to the swashplate, and you hope that they land flat and not with the slipper cocked to one side. If it doesn't land straight...... well, it might just break the head off."

I hadn't given this part of it any thought.

[J-Mech]

Well, you have to remember that there is a significant gear reduction after they hydro, and then again at the ring and pinion. So, when you tow it, that piston block is moving a pretty fast pace. Even at slow speeds. Whether the bulk of the damage is done during the towing, or after the start-up I do not know. I have not ever had one fail..... on a Cub Cadet. But I've worked on lots of hydrostat set ups on other machines (mainly combines) and I can tell you for sure that you can damage them when you tow them. As a note, a combine is a little different as you can put the trans in neutral. As long as you don't forget... FWIW, I've also towed my tractors pretty far distances before with no issues, I just went very slow. I really don't think as long as you use your head you will hurt it. It's when you snag a cub with a truck or a 4 wheeler and go towing it 15 MPH that you damage them.

Very nice damage pic Lew. The way that one grenaded I'd say it was being towed at a pretty good rate of speed. Even if it the hydro failed while it was in use at full speed on a road, I'd be kinda surprised if it would punch a hole in the case, but I could be wrong.

[Mike McKown]

Okay, thanks.

Seems like if the shift lever was in neutral when towed, the swash plates are parallel with one another and there is no piston pumping going on? Still, the pistons wouldn't get any oil pressure to lube them on the swash plates.

[Sam Mac]

Mike

The swash plate in the motor section is always at an angle. The pump swash plate is the only one that pivots.

[Mike McKown]

Originally Posted by Sam Mac

Mike

The swash plate in the motor section is always at an angle. The pump swash plate is the only one that pivots.

Duh! Yeah, that's right.

[rwairforce]

Wow!
Thanks for the pictures and education guys. I hope to never need to get inside of one of these, just not sure I could keep track of everything's proper place.
Excellent pictures and help.

Happy Thanksgiving!