You are correct
TMGT
Elite Member
Looking at the workshop manual page that was posted, I would start by checking the charge pressure for the HST, looks the relief for it might be sticking. The charge pressure should be fairly low, 1-200psi, but is run off a system that is capable of much more.
It's also a possibility that there is an issue in the HST that could be causing the excessive pressure, but the relief should prevent overpressure of the system.
I'm not sure where the test port is for it, but it will be listed in the WSM.
It's also a possibility that there is an issue in the HST that could be causing the excessive pressure, but the relief should prevent overpressure of the system.
I'm not sure where the test port is for it, but it will be listed in the WSM.
rScotty
Super Member
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The high pressure side of a hydraulic ciruit doesn't always operate at high pressure. It only develops high pressure when it encounters a load....like moving a cylinder or a plugged filter. Otherwise, if there is no load then the fluid just circulates at very little pressure.
And some systems - like the loader - can move an unloaded bucket with a few hundred pounds and then automatically respond to a big load by allowing local pressure to climb up to 2000 to 3000 psi. Think of psi as being proportional to force. The hydraulic system automatically creates enough psi to move the load. The top psi is set by the relief valve.
Unlike Front end Loaders, Power Steering systems don't develop very high pressures. All PS has to do is turn the wheels. So although power steering is also in the "hydraulic side of the circuit, it operates at lower pressures. The power steering relief valve will often set a limit of 150 to 300 psi instead of the 2000 to 3000 psi for the FEL.
After leaving the power steering circuit, a portion of the fluid is still under a few hundred pounds of pressure and is diverted to the HST system. That is called the "charge pressure", and is a poor choice of a name. It comes about because larger tractors will have an additional small pump called the "charge pressure hydraulic pump" specifically to feed the HST. Larger Kubotas all have that. Your BX doesn't have that extra charge pressure pump. Instead, it just uses a little bit of the extra flow from the power steering. But unfortunately, that flow is called by the same name...it is still called "charge pressure".
But regardless of how that slightly pressurized charge flow gets to the HST, it should have only just enough pressure to drive fluid through the special filtration media in the HST filter so that ultra cleaned oil goes forward to the HST. The HST requires the cleanest oil possible. That is why the HST filter is the only hydraulic filter that operates under a slight positive pressure - usually the same pressure range as the power steering.
As you say, all other hydraulic filters are suction types and operate under negative pressure pulling flow under suction from the tank to the primary hydraulic pump.
I can't recall ever seeing a tractor design that allowed filtration on the return to tank side of a hydraulic system. I guess it would work if it had a super sized safety relief valve, but it is much simpler to just filter on the suction side to the pump. Then there is no complexity or hazard.
Yes, your whole problem could be a series of defective filters. You should keep them for your mechanic to inspect. He will want to. But it is more likely that the filters are ok, and there is some other form of an obstruction between filter and HST. The most common obstruction is caused by long term use of water-contaminated hydraulic fluid, but it could be anything that blocks the flow.
rScotty
I will relay your message. The pressure relief valve has been tested and we even replaced it without adding shims, but seems that is after the filter and didn't stop the pressure. Next step is to see if there is a stuck valve.
TMGT
Elite Member
The charge lines pressure relief or the main loader/3pt relief?
See item 3 on the diagram Scotty provided earlier.
View attachment 3631324
Last edited:
oldnslo
Super Member
How did you determine that excessive flow is going to the filter?
If and I repeat if you are talking about the filter that screws onto the front of the transaxle that is filtering the priority flow that goes to steering first and then into the HST as “charge” flow to keep a positive pressure on the HST and replace leakage flow from the HST circuit. If this is seeing excessive flow then loader would operate slowly or possibly not at all especially at lower engine RPM.
Loader circuit should be totally separate circuit running at totally separate pressures.
Yes it is one common pump feeding both steering and implement but the priority valve separates them.
There is a system relief before the priority valve. Then Steering will have a relief valve, and that then feeds the filter, after the filter is the “charge” circuit relief. Everything between the filter and HST is suspect on what could cause the filter to fail. If it runs 10+ hours and then fails points towards something changes suddenly increasing this pressure.
Have you noticed any significant speed change on loader movement or steering suddenly being faster?
TMGT
Elite Member
And don't forget the pto circuit after the steering but before the filter and hst charge circuit, each with their own pressure relief valve!
KWentling
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rScotty
Super Member
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Yes, important that there are relief valves everywhere. Each hydraulic circuit has at least one. On schematic diagrams, relief valves are the little box with a squiggly tail and a dotted line connecting one side with another. Their main function is to open up and return any overpressure back to the tank. That tank return path is sometimes shown and sometimes just assumed.
Relief valves are not always shown unless the view is expanded. For example, look below. We know that the steering priority valve must have its own relief valve, because every circuit does. But although we see the steering priority valve, its relief valve s not shown in the overall diagram. To find it we must dig deeper into the WSM. The same goes for the relief valves in every circuit. There are many. In fact, the HST has several internal ones. All are in the WSM if you look deep enough.
Relief valves are a simple one-way valve set to open at a specified pressure. Each one has a different operating pressure that is set by shims or an adjustment screw. Relief valves are deliberately easy to access, disassemble, clean, adjust and re-use. WHEN THEY OPEN THEY TEND TO MAKE A LOUD & CONTINUOUS SHRIEKING SOUND. Like any alarm, that is deliberate. Relief valvesare also considered "fail-safe" because in normal operation They cannot fail to open when their operating pressure is reached.
IF A RELIEF VALVE IS NOT OPENING, CHECK FIRST TO SEE IF THE PORT LEADING TO THE RELIEF VALVE IS PLUGGED. (ZERO PRESSURE ON GAUGE) . IT IS COMMON FOR A PHYSICAL OBSTRUCTION TO BE PREVENTING FLOW TO THE RELIEF VALVE. COMMON CULPRITS ARE GLOBS OF DEBRIS. DIG THEM OUT AND FLUSH.
rScotty
