This article is to help you get a better understanding of how the hydraulic system works on Kubota B and BX tractors. There are other systems and more complex systems out there so this doesn’t apply to all tractors. Also this article does not cover the details of how the hydraulic system works in the hydrostatic portion of the transmission.
These tractors use whats called an open center hydraulic system. This means when every valve is centered or stationary (not lifting 3 point hitch, lifting bucket, etc) then hydraulic fluid from the pump is just flowing effortlessly through the system. Once you open a valve to raise your 3 point hitch or to raise or lower your loader, then fluid is diverted to the appropriate cylinder.
All valves in the system must be plumbed in a series. You cannot put a Tee fitting in a pressure line. Must go in 1 valve, out that valve into the next one and so on. Most modern valves will have 3 system lines (not including the cylinder lines). One is the “IN” which is a pressure line coming from the pressure source (hydraulic pump or from the valve before it), one is “PB” or power beyond which is a line feeding pressure back out to the next valve and 1 is the “OUT” which is a low pressure return line dumping back into the transmission housing. You can put a tee in the low pressure return line if needed. Power beyond is an extra layer of security. Some older valves did not have power beyond and just have 2 lines “IN” and “OUT” but in those the “OUT” would be the high pressure line feeding the next valve.
There are 2 pressure readings for these tractors. 1 is the actual hydraulic pump pressure, the second is the attachment pressure. Attachment pressure is what psi the attachment is designed to run at. In some cases you can have different attachement pressures for each attachment whereas in the BX tractors you only have 1 attachment pressure for everything attached. This pressure is set with a relief valve. The relief valve is a spring and poppet designed to open at a specific pressure. So you may have 3000 psi from the hydraulic pump, but your attachment is only designed to run at 2000 psi. Thats where a relief valve comes in and opens at 2000 psi thus limiting the pressure to the attachment. B2400 for example has a relief valve for the 3 point cylinder but also has a relief valve for the loader. All BX models have 1 relief valve in the transmission that sets the pressure for both 3 point hitch and loader.
Models equipped with power steering there is also whats called a priority valve. This valve sets priority of hydraulic flow to the steering and all other valves as secondary.
Now you have a better understanding of how the system works. If you have any questions feel free to use the contact us page. Thank you