How many valves/spools can be put in series before they need to be oversized?
VERY GOOD question!! Seems like you have your hydraulic "nerves" plumbed right...:thumbsup:
This is the negative part of Constant Flow, CF, systems (Open Center, OC), the more PB implements we add (in series) , the more flow resistance we create, which results in more heat added to the system. A Constant Pressure, CP, system (Closed Center, CC) will not react that way when we add more implement valves, because we add, which in parallel. A regular CP-CC system uses a variable displacement pump which adds a substantial cost to this system. Now there is a variant of CF-OC, the CF-CC system also, that uses a low cost fixed displacement pump and a dump valve. The implement valves need to be connected to dump valve, via a signal line or load sensing sensing line, but can be added unlimited in parallel. We can call it "poor mans CC system", and it has gained a lot of popularity on the hydraulic marketplace.
In the 60's, some tractors had a manual valve to dump (by pass the CV) flow to tank, that reduced a lot of heat when road travelling....
Most of the heat in a CF-OC system is created in two different ways..
#1 Road travelling on high rpm's=high flow (implement valves in neutral)
#2 Throttling or feathering implements (diverting) at high pressure and consequently high rpm's=high flow
Constant high rpm is a great way to call for heat problems, so when running implements, use a variable engine speed on demand of flow....
Every time we make a
design change in a CF-OC system, we get a change in how system manages
HEAT. These CF-OC system are basically
not designed to carry a lot of extra implement valves. A tractor that comes standard and "clean", are not designed to carry more than one added valve for a FEL. The system is designed for a certain flow, valve resistance and cooling capacity. If we add several valves, we create more heat and we will get problems to control heat.
THATS why I think your question is SO good!!
It is easy to make a "science" of your question, but harder to give an adequate SIMPLE answer.... especially when input data is lacking necessary details....
My tractor hydraulics are speced at 7.8gpm.
This is a very typical way to describe hydraulic capacity, but it actually don't tell you much....Is this pump flow, valve rating or what??? at what rpm's??
In order to control heat (heat status quo), YES, it is good if next valve is slightly over sized, everything added down stream would need to be over sized in order to control heat. But oversizing a valve to much will negatively affect the operational properties on the valve (proportionality and feathering). So you have to negotiate between heat and operation.
I do not think a 10 GPM valve (slightly over sized) will make a noticeable operational difference for you, neither a 8 GPM valve would make a major difference about the heat.
But it all comes back to what specific conditions you are operating under....constant or intermittent, stationary or a lot of 'wheel' travel....
