HST Problem Solved?

[jerkyboy]

You should not have the port block (yes, that is the correct term) "resurfaced", just not a good idea. Just looking at the pictures, it looks as if the charge pressure for the system was low. But I admit that it is strange to see that type of damage on 1 side only. On the BX, the charge pressure is created by the main hydro pump (if I remember correctly) and a regulator valve the splits oil off to steering, pto, and hst. I would check the charge pressure after reassembling.

I checked the charge pressure before I took it apart and it was 100 psi, which is exactly what the WSM says it should be. Although, I didn't check it with the tractor under any substantial load. I did it with the tractor sitting still and the throttle maxed out....per the WSM. The regulator valve you're talking about is the flow priority valve, which I'm thinking of changing too.

 

[Yandaddy1110]

I had purchased my tractor with the hst already bad, so I didnt know what caused it to burn up. After rebuilding it, I used it all winter no problem. When the weather warmed up, the hst started to fail again one day while doing some loader work.
The cause was a plugged up radiator which heated the fluid till cavitation occurred and smoked the hst. Fortunate for me , the hst was fine, I shut down the tractor in time and clean out the radiator which was almost plugged solid with dirt and dust.

 

[MattJ]

When you get it back together check the charge pressure when driving up a steep hill or doing something that puts a load on the HST. On mine the charge looked fine at a stand still and at full throttle, but start to go up a hill and it dropped like a rock to below 20 psi. With the new flow priority it held fine at my spec of a bit above 50 psi even under the heaviest of loads. May want to check before you buy as the priority valve is around $160.

Also, you did the right move with replacing the whole HST. Its also cheaper as the pump assemblies you showed are around $700 each.

Good luck

Matt

 

[Yandaddy1110]

When you get it back together check the charge pressure when driving up a steep hill or doing something that puts a load on the HST. On mine the charge looked fine at a stand still and at full throttle, but start to go up a hill and it dropped like a rock to below 20 psi. With the new flow priority it held fine at my spec of a bit above 50 psi even under the heaviest of loads. May want to check before you buy as the priority valve is around $160.

Also, you did the right move with replacing the whole HST. Its also cheaper as the pump assemblies you showed are around $700 each.

Good luck

Matt

Those pump assemblies are less that $200 if if you are willing to investigate,search and stay clear from the Kubota dealer for them.

 

[oldnslo]

Not sure what the slippers on the pistons are. I did a pretty thorough look over of the internals and that is all that I noticed but I'll check again later this week.

Pistons are what actual pump the fluid in the cylinder bores. In looking at the exploded parts I see that these do not have slippers on them but probably just have a rounded head. Look very similar to what is used in Hydrogear units. Was this head scored or pitted in any way?

The prior (original) owner did use it every winter for snow blowing. He also had it dealer serviced (I have all the receipts) during his ownership. The tractor does sit in an unheated garage, but mine is very well insulated and does not typically go below 30 degrees.

I use it mostly for mowing and I have the Kubota leaf system, which I use extensively in the fall. The problem started happening when I was moving crushed stone (6 ton) and leveling out a small area with the FEL.

If for some reason the charge flow / pressure is limited you would get more chance of scoring when doing FEL work due to higher loads on the HST Vs mowing lawn. The higher the load/pressure in the HST the greater the leakage flow that the charge system must make up.

I have never seen an actual hydraulic schematic for the BX. Basic diagram yes but no detailed schematic.

The questions I have are:
Where is the relief valve for the power steering?
Where does it vent to? Hopefully into the charge pump loop.

Where does any leakage in hydro steering go?

On most priority valves the valve will fail in the condition it provides flow to the priority leg first and then the secondary functions like 3PH, FEL, etc. Have never had one of the Bota ones apart so not sure how they are made.

 

[crazyal]

Looks like you are getting some help on this. I would have to say that if it's not dirty fluid then it's a lack of fluid that cause the problem. Along with what others have said I would also check the temp of the fluid after it's been used hard after you get it fixed. Maybe even check all the oil lines and passages to make sure nothing is restricted.

 

[MHarryE]

Normally I would think of wear in this area as contamination but yours looks more like lack of lubrication. Charge pressure under load has already been discussed - you should monitor that. As for temperature, your hydro works best from 10 cSt to 16 cSt. UDT is 9.3 cSt at 100 degrees Celcius. Once the oil at the working faces gets hotter than that wear goes up at a very high rate. Check your system to make sure oil cooling is doing its job, and if you can, monitor your temp periodically when you are really working it. I use a I-R thermometer that is accurate enough for monitoring something like this.

 

[MattJ]

The relief for the power steering is in the steering column and it vents to the charge for the HST. When I took apart my flow priority valve and really looked at it I noticed in the design if the spring fatigued it would at high pressure under load block the HST and give flow to the implements, exactly what its not supposed to do.

When I drove it up the hill the charge would drop and my loader pressure was at full relief around 1850psi and raised the bucket quickly.

 

[oldnslo]

The relief for the power steering is in the steering column and it vents to the charge for the HST. When I took apart my flow priority valve and really looked at it I noticed in the design if the spring fatigued it would at high pressure under load block the HST and give flow to the implements, exactly what its not supposed to do.

When I drove it up the hill the charge would drop and my loader pressure was at full relief around 1850psi and raised the bucket quickly.

Matt,
thanks for the info. Kind of sounds like a potential OH Sh*t waiting to happen. So based on this there is the possibility of loosing power steering and charge which I would think would be a big design oooops.

 

[jerkyboy]

All done with my HST repair. Replaced the unit with a new one and tested it this weekend. Moved 11 yards or topsoil with the FEL and mowed my lawn; new HST is working flawlessly. I did like Matt J and installed an oil pressure gauge to monitor my charge pressure, which stayed consistent during the latest workout.

If anyone is attempting a similar repair, I would highly recommend taking a lot of pics and being very patient.....it's totally doable for the average DIY'er.

FYI, here's a couple pictures of my gauge install.