Why does the hose need to be connected? I thought another member just said exactly the opposite..
Could you explain me? I am trying to understand how the hydraulics work.
Most, if not all, Kubota tractors use what's called an open-center hydraulic system. Large equipment, including Kubota excavators, I think, use a different type system. The reason open-center systems are used is they are simpler and less expensive for small systems. There are some downsides, but they are manageable.
Here's a diagram of the entire
L35 hydraulic system, with the backhoe installed. Pump 2 and the stuff on the right side are for the steering and transmission, so we'll ignore them. Pump 3 supplies the loader, backhoe and 3ph. The pumps are positive-displacement which means they try to supply a given flow rate, proportional to engine speed, regardless of the load or resistance to flow from their discharge. There are mechanical limits to the pump's strength, so a relief valve is used to limit the discharge pressure by relieving flow back to the tank. However, whenever the relief valve is discharging, power is wasted and heat is generated in the fluid.
So, in an open-center system the flow is allowed to flow unchecked through the lines and valves back to the tank. There is some power used and heat generated by the pressure drop in the hoses and valves, but it is minor. (This is the stated purpose of diverter valve 10. When shifted for backhoe use, it allows the fluid to flow directly back to the tank, bypassing the 3ph control valve. I think it also serves a safety purpose by preventing the operator from moving the 3 pt arms inadvertently while operating the backhoe.)
The valves are sort of diverters. When a valve is shifted, it diverts the flow to the desired cylinder. The flow from the cylinder is directed to the tank (for plain directional valves like on the backhoe, the loader valves both have special features). If a valve is held until the cylinder reaches the end of its travel, the fluid has nowhere to go and the relief valve opens to prevent overpressure.
As to your question regarding the backhoe loop, the red arrow actually shows the flow bypassing the bh valve. When the bh is installed, that U would not be there and the flow would go through the bh valve. There are spring-loaded valves in each connector that prevent flow when the connectors are un-coupled, so you can see when that hose on your tractor is left dis-connected, the loop is broken and your relief valve will have to operate continuously.
