Restriction in open and closed center systems
- Thread starter AKKAMAAN
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I do not understand what you mean with a "valve like that"??
=)lease tell what your "idea" is.....
Most of the cheap valves do not have alot of the features of the one you are showing. Such as load checks. The typical valve I see being recommended for most projects would be a simple 4 way, 3 position valve. Put a plug in the B port and flow is regulated by position of spool. Maybe I am taking the whole wrong approach of this thread.
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You only need load check for valves that are hooked up to gravitational or inertial loads .... log splitter valves deal with frictional loads and wont need load check valve...
My point is to show that additional flow control valves need different properties, depending on type of system....
I am improvising here, off my main topic....will be back later, with the restrictors on open and closed center system....
I see people finding valves at a bargain deal they cant resist, some are too small and some are too big....most of the time it will work out, but not perfect....
So let's figure out what we want from sizing our open center control valve properly....
What valve properties do we want from a, lets say a FEL control valve?? (There is especially one property that we tend to forget about, and we try to solve the problem with additional flow control)
What are the pro's and con's with an over sized control valve??
What are the pro's and con's with an under sized control valve??
(do not worry about cost/prizing....lol)
I was kind of wondering if this was not the subject.
Is the valve in your example with the restricters controlled only by spool movement? Is there stops?
I guess the first I seen of where controls were to go, I was not thinking in terms of a block diagram of the valve it self.
I am looking forward to out come of thread.:thumbsup:
Oh yes....my restrictors are just symbols for the real spool....but basically the do would do the same job even if they were "real" restrictors... a spool valve is, together with the cores in the spool valve housing a "set" of variable restictors that are synched to move together....you could build the function of a spool valve with a set of variable restrictors....it would be hard to operate them synchronized but it would still do the same job...
This will show how these restrictors work in the real "valve symbol"...
Still, "my" valve properties is
#1 PROPORTIONALITY, that spool movement have a pressure impact from start of the movement. If valve is "too big", it wont start building pressure until the end of the spool stroke, and we have then lost all the fine metering ability....valve have more or less become an "on/off" valve....for example lets say the spool travel is 1/2" from neutral to end. If we say we can manually set spool to every 1/32" of the spool travel, we can say we can build 16 different pressure/flows, if valve is over sized we might only be able to use just a few of these 16, at the very end of spool travel...
#2 LOW RESISTANCE. Low flow resistance, especially in neutral, will reduce the power losses and excessive heat in the system....
To get both of these two, you control valve have to "FIT" the flow.....
Got your point.:thumbsup:
How about the valves lets say slightly undersized going to react upstream or down stream? During load or no load? A slightly under sized valve in continuous flow, will it also restrict flow with no load?
Some of these I do have a good idea on, but not 100%. Since you are covering these ideas thought now is the time to ask!
How about the valves lets say slightly undersized going to react upstream or down stream? During load or no load? A slightly under sized valve in continuous flow, will it also restrict flow with no load?
Some of these I do have a good idea on, but not 100%. Since you are covering these ideas thought now is the time to ask!
very good follow up question....
I am sure you can figure that one...think about the opposite con...valve is already pressurized in neutral...that means if we open workport, cylinder will start travel immidiately, and we have lost the feathering abilifty....we also risk the PRV will start bypassing way before center is fully closed
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wdchyd
Elite Member
Now that you bring it up, I have something to get off my chest....
For quite some time lots of people here give the advice to others about using DCV's with the same ratings in GPM's as the pump in use......
My understanding with DCV's is the rating in GPM's is MAX!!!......I.E. using a pump with a 10 gpm max output at max rpm's will only produce 10 gpm's at the outlet of the pump right?? But hooked up to a cyl with a disp ratio of let's say 2:1 and retracting , will give out potentially 20 gpm's in waste flow coming out of the base end port.....All that flow goes thru the core of the valve....Right??
IMHO using a DCV matched up to max flow from your pump without considering max return flow is way too much pressure drop thru the DCV.....making more pressure/heat and wear and tear on your valuable hyd components......
My point is that we wouldn't buy a car or truck based on the max rating on the speedometer and/or payload rating......how long would our new car/truck last if we ran it at max speed with a full load....????
Another good follow up question!
I want to point out that I, so far haven't given any advice about matching flow ratings.....I have stayed at pointing out pro's and con's
Pump ratings are mis leading the way they are pronounced....10 GPM rating do not tell us directly AT WHAT RPM....you have to DIG to get that info.....I prefere to classify pumps by DISPLACEMENT and MAX RPM
But hooked up to a cyl with a disp ratio of let's say 2:1 and retracting , will give out potentially 20 gpm's in waste flow coming out of the base end port.....All that flow goes thru the core of the valve....Right??
That absolutely true!! There are, very rare though, some older DCV's valve bodies with different rating (different sized cores) through the A port and the B port. Usually 2:1.....Today this is solved with different spool options (more "expensive" DCV';s)
You gotta negotiate with your self here......preferring a fully proportional valve, or a valve with less back pressure on retract return flow....
This problem is about back pressure on return, and I do not think std DCV's will give you any info about this, there are to many different cylinder ratios available....
I'll look up some different valves and see what I can find
Most DCV's have info about pressure drop through ports at different flows....see this example from a Prince loader valve, page 2
In a closed center system, your free flow in neutral, will not be an issue....P to T...