PineRidge's original question related to not having a physical lock feature for his NH TC40D. The attached hydraulic diagram should clear up this issue for our specific model of tractor, but there are no guarantees any other tractor is the same. My guess is that there are several with similar designs, but I'm the kind of person who wants to see the actual schematic and make my own personal decisions about my comfort level with the design.
In the attached diagram, the hydraulic pump and filter are shown on the left and immediately after the high pressure fluid leaves the pump, it goes to a relief valve. Therefore, the pumps outlet pressure is always limited by this relief valve (150 bar, 2135 lb) on our Boomers.
The fluid continues into the HPL control valve that provides position control to our 3PH. Inside the HPL valve, there is no pressure relief valve. Any pressure feedback from the lift cylinder assembly (ref no. 7 & 8) is felt by the HPL valve components. The pump relief valve will only be in the circuit while the 3PH is actively lifting a load. When the position control nulls out the control handle input, the lift cylinder becomes isolated from the pump circuit because the unloading valve opens and directs input fluid back to the reservoir.
There is, however, a pressure relief that is always in the system. That is a high pressure relief valve that is an integral part of the hydralic lift cylinder. That valve relieves pressure at 3650 psi (250 bar). It's function is to relieve extreme shock loads from the lift cylinder.
When the flow valve (ref no. 8) is fully closed, the shock pressure is relieved from the HPL valve and is only felt by the lift cylinder. The flow valve effectively isolates the entire HPL valve circuit from the shock. If there is damage, it will be limited to the lift cylinder only. Therefore it is my contention that this valve could be useful for locking the 3PH position because ultimate protection is still supplied by the high pressure relief valve and the HPL valve is completely protected. /forums/images/graemlins/cool.gif