does open center take constant HP?

   / does open center take constant HP? #1  

ericm979

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Santa Cruz Mountains CA, Southern OR
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Branson 3725H Deere 5105
In an open center system, does running the pump take a constant amount of power whether there's working being done by a cylinder or not? Or does the power requirement on the pump go up when the hydraulic system is doing work? Or is it somewhere in between? (that's my guess)

The reason I'm asking is that I have a pto chipper that's got it's own hydraulic system to operate the feed rollers. It's about 3 gpm at about 2000 psi which is about 3.5hp. I was thinking that I could disconnect the chippers' pump and run the feed rollers off the tractor's hydraulic system. Would that free up any HP which could add to the power turning the chipper?
 
   / does open center take constant HP? #2  
Open center absorbs only line loss power until a valve switches flow to do something like extend a cylinder. At that point all flow from the pump increases to the pressure needed to extend the cylinder. When the cylinder reaches its end pressure will increase to relief pressure until the valve is switched to the neutral position. At that point pressure will drop back to line loss. Depending on your need you can use the tractor hydraulics to drive the system but remember if you have a 10 gpm pump and you are using only 3.5 gpm to drive the rollers, all 10 gpm will be delivered by the pump at 2,000 psi with the remainder being lost as heat. Of course you can throttle back to meter the flow and reduce any heat loss but watch hydraulic temps.
 
   / does open center take constant HP? #3  
Great question and a great answer, I love this forum!

A couple of other things to consider...

Do the two systems use the same hydraulic fluid? Do you want to to mix the chipper system with your tractor hydraulics?

That hp is all coming from the same place... So All you'd be saving is the efficiency of not converting mechanical to hyd 2x.
 
   / does open center take constant HP? #4  
In an open center system, does running the pump take a constant amount of power whether there's working being done by a cylinder or not? Or does the power requirement on the pump go up when the hydraulic system is doing work? Or is it somewhere in between? (that's my guess)

The reason I'm asking is that I have a pto chipper that's got it's own hydraulic system to operate the feed rollers. It's about 3 gpm at about 2000 psi which is about 3.5hp. I was thinking that I could disconnect the chippers' pump and run the feed rollers off the tractor's hydraulic system. Would that free up any HP which could add to the power turning the chipper?
Regarding an online hydraulic calculation site, a pump running unused, consumes about 5% of the power that it's capable to deliver at full load.
In your case, it would be 0.17 hp.
 
   / does open center take constant HP? #5  
Open center absorbs only line loss power until a valve switches flow to do something like extend a cylinder. At that point all flow from the pump increases to the pressure needed to extend the cylinder. When the cylinder reaches its end pressure will increase to relief pressure until the valve is switched to the neutral position. At that point pressure will drop back to line loss. Depending on your need you can use the tractor hydraulics to drive the system but remember if you have a 10 gpm pump and you are using only 3.5 gpm to drive the rollers, all 10 gpm will be delivered by the pump at 2,000 psi with the remainder being lost as heat. Of course you can throttle back to meter the flow and reduce any heat loss but watch hydraulic temps.

Very, very good answer that's easily understandable.

Hope you don't mind me adding this comment.

Open center systems do not create pressure until function requires it. So they take a miniscule amount of HP to run. They also generate very little heat until load is applied.
 
   / does open center take constant HP? #6  
In an open center system, does running the pump take a constant amount of power whether there's working being done by a cylinder or not? Or does the power requirement on the pump go up when the hydraulic system is doing work? Or is it somewhere in between? (that's my guess)

The reason I'm asking is that I have a pto chipper that's got it's own hydraulic system to operate the feed rollers. It's about 3 gpm at about 2000 psi which is about 3.5hp. I was thinking that I could disconnect the chippers' pump and run the feed rollers off the tractor's hydraulic system. Would that free up any HP which could add to the power turning the chipper?
making the feed rollers rotate about 3 times their normal speed don't sound good at all!!.. you'll probably stall the engine, as it was not designed to chip wood that quickly!!..
 
   / does open center take constant HP?
  • Thread Starter
#7  
It has a flow control valve. But yea, I hadn't realized that the 10 gpm would spin the motors faster.

I don't plan to do this, just trying to increase my understanding. Thanks everyone.
 
   / does open center take constant HP? #8  
Very, very good answer that's easily understandable.

Hope you don't mind me adding this comment.

Open center systems do not create pressure until function requires it. So they take a miniscule amount of HP to run. They also generate very little heat until load is applied.

I think his horsepower savings would be very little. From what I have seen in a typical open center system with the resistance of the hoses and the piping, something like 100 psi or less is developed in the system from this resistance. The fluid flowing along the hoses and piping and thru open valves develops this resistance which causes the pressure to come up. I am sure this varies from tractor to tractor with different sizes of hoses etc.
 
   / does open center take constant HP? #9  
I think his horsepower savings would be very little. From what I have seen in a typical open center system with the resistance of the hoses and the piping, something like 100 psi or less is developed in the system from this resistance. The fluid flowing along the hoses and piping and thru open valves develops this resistance which causes the pressure to come up. I am sure this varies from tractor to tractor with different sizes of hoses etc.

I agree James.

Excellent additional information as well.

Open center hydraulics are sometimes hard to understand for beginners.
 
   / does open center take constant HP? #10  
I agree James.

Excellent additional information as well.

Open center hydraulics are sometimes hard to understand for beginners.

Most people have no understanding of what pressure and flow and how they relate to each other even in a garden hose. :)

Which by the way is the exact same thing as voltage and current in an electrical circuit.
 
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