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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- Apr 21, 2001
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- Rural mountains - Colorado
- Tractor
- Kubota M59, JD530, JD310SG. Restoring Yanmar YM165D
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
