Newbie question about hydraulics

jeffgreef · Aug 1, 2009

Please explain-

What is a selective control valve, and what is power beyond?

thanks

JG

J_J · Aug 2, 2009

POWER BEYOND (HIGH PRESSURE CARRYOVER): A sleeve attachment which permits the oil flow from one valve (when in neutral) to be used by another valve downstream. Hence, a 3-spool valve cou1d be connected to a 2-spool valve to create a 5-spool valve, The first valve takes priority and thrust have a separate outlet port to return oil from an activator back to the reservoir.

http://www.tractorbynet.com/forums/...4-additional-hydraulic-scvs-power-beyond.html

jeffgreef · Aug 2, 2009

thanks JJ

I see you have answered a lot of hydraulics questions- thanks for your time. I just want to understand what the choices are as I shop for a tractor. Here's a couple more questions while your beer is chilling in the fridge.

Evidently the spool is usually set up so that priority is given to certain circuits. Can you give an example- for instance, the priority goes to what implement for what reason? Why does one get priority? What advantage is this?

Second, I see that some tractors have two pumps rather than one, and that's supposed to be an advantage. Can you give an example of how this helps when you are working- one pump works half the FEL while the other works the other half or what? Again, where is the advantage?

Concrete examples usually work their way into my brain easiest, if you know what I mean. (I learn by experience cause it usually hurts...)

But, as well, your explanation of the principle involved helps too.

thanks

JG

J_J · Aug 2, 2009

JG,

Priority valve. A priority valve, is essentially a flow-control valve that supplies fluid at a set flow rate to the primary circuit, thus functioning as a pressure-compensated flow-control valve. Flow in excess of that required by the primary circuit bypasses to a secondary circuit at a pressure somewhat below that of the primary circuit. Should inlet or load pressure (or both) vary, the primary circuit has priority over the secondary.

The word priority essentially describes it self by the fact that something has to come first, such as steering circuits, over any thing else on the machine.

For instance, On my Power-Trac, my steering circuit has it's own pump, but also including in the steering is my lift and tilt for the FEL. Whatever hyd fluid is left over from the steering is passed on to the lift and tilt. If I am not turning, all the fluid goes to the FEL. If I steer a little, some fluid is used for steering and the remainder goes to the FEL. You should always want steering control.

Valve priority. If you have two valves connected through the PB port. you could say the first valve has priority, the second valve only gets fluid when all spools on first valve are in neutral.

One , two, or three pumps, means that you can handle multiple things going on without decreasing work being preformed. Since I have three pumps, I can use all three to their full potential. I can steer, run the hydraulic PTO, and use the hydro-stat pump to run my hydraulic motors. To do all this, the engine running all three pumps, has to be large enough to accomplish the task. If the one pump is able to handle all the task required, then you only need one pump. Now, say you want to put a BH on the tractor, the hyd pump may not be large enough to run it, and then you could add another pump to the PTO shaft, with a small reservoir, and get the job done.

What you should do when looking for a tractor, is to state the work you hope to do, and the guys that live, breathe , and love their tractors can help you make a logical decision. There is a lot of tractor stuff out there. Things like farm size, implements you desire, HP wanted, gear or hydro-stat, FEL, tires, brand name, grey market, US made, foreign made.
etc.

http://www.hydraulic-control.com/PRIORITY-VALVE.php