Typical Hydraulic Pump Operating Pressure???

[Henro]

I know maximum operating hydraulic system pressures are nearly always specified, but that is not my question. I am curious what the normal minimum system presssure might be.

Most small tractors have open center hydraulic systems. By definition, the control valves pass the hydraulic fluid back to the tank with minimum restriction to flow.

[Just to be clear, I was trying to say that each valve in the open-center loop allows the fluid to flow through it and on to the next component in the loop, and that the fluid eventually finds its way back to the tank, after passing through all components that are part of the loop (normally control valves, tubing and hoses). ]

But there is some resistance to flow, in the tubing, hoses and control valve passages between the pump and the tank.

As the result of this resistance, pressure is dropped across the loop and a minimum pressure is felt at the point between the pump and the first control valve in the loop, which may be the loader valve, when all control valves are centered and the engine is running.

Has anyone measured the pressure coming out of the pump or at the input to the first control valve in the loop? How high might we expect the minimum pressure to be? In other words, what is an expected range of pressures likely to be felt by the tractors hydraulic pump? Top would be PRV setting. How about the bottom. I am certain it is not zero. Could it be 1,000 psi? More? Less?

I know in an ideal world the loop would have no flow resistance and no pressure would be developed in the loop until a valve was shifted and work was asked of the system. But we all know this is not an ideal world.

Grateful for any input.

By the way, Junkman and MadRef are responisble for this question! /forums/images/graemlins/grin.gif

 

[SPIKER]

Hey Henro:

this is a good question but it is nearly impossable to answer. reason is that every system is going to be different. as you stated the valves hoses and pumps as well as fluid types will change the actual pressure on the outlet of the pump.

anyhow a gauge installed at the first valve would let you know what is dow stream of the pump as far as normal system resistance /pressure.

MarkM

 

[_RaT_]

Bill, I am not sure I understand the question. The static pressure will be the same at the pump and at the valve body. Not until you use hydraulic fluid would this show up and even at that, it would depend on the device being operated, the greater the devices gpm requirement, the greater the pressure drop/loss. Unless your operating a high demand device like a hydraulic auger motor, it's unlikely that you will be able to tell much difference since as soon as a hydraulic cylinder for example encounters resitance to push or pull, the flow will start to slow some and eventually become a static load either by bottoming out or by the inability to move any further as would be the case when picking up an object the loader can not lift.
I have a gauge on my tractor that shows the pressure when I use the bucket dump. It reads very low pressure when I am dumping until it encounters resistance or reachs the end of the cylinder. In that case, I now have a static load and can read my maximum pump pressure since there is no flow. It also lets me know when I am exceeding the hose, tubing working limits when I back drag. If I have the bucket tilted (dumped) considerably, it creates a leverage on the cylinder that can sppike the hydraulic system to many times beyond the working pressure. Eventually, something will give, a hose, a steel line or a cylinder. I monitor mine pretty carefully when back dragging especially in rough material.

 

[Henro]

Rat and Spiker,

The reason for the question was that I used to think that a tractor's hydraulic pump would not develop much in the way of pressure unless it was called upon to do some work, like pushing something with a hydraulic cylinder.

Junkman's thread that I referred to above indicates that this is not the case. That there is significant pressure developed by the pump, when it simply pushes fluid through the open center loop and back to the tank.

I was just wondering if anyone had put a gage on the pressure line coming to the loader valve and measured what that pressure is on their tractor, when the pump is doing nothing more than circulating fluid.

Like Spiker says, it would vary from tractor to tractor, and even between identical tractors that have different hydraulic stuff installed.

Apparently Junkman has enough pressure developing at the feed side of his loader valve, that he has been able to hook up a closed center control valve between that point and the return to tank hose, and is able to operate a grapple with it without problems.

Until I read this, I never expected that there would be enough pressure there to do this, and so I am wondering if anyone has actually measured pressure at this point on a real world tractor, providing a measurement confirming that really could work (I am not doubting that it does work for Junkman, I am doubting my understanding of what hardware he has in place).

Apparently there is a pretty good amount of pressure developed (at least on a Kubota BX22) or my understanding of what Junkman has installed on his tractor is not correct.

I used to think the range of hydraulic pressure produced by the pump could be anywhere from near zero to system PRV setting. Now I realize it is never close to zero, but I don't have any feel for what the minimum might be...just never thought it would be more than maybe 100 psi...now it seems it might be...

 

[Junkman]

</font><font color="blue" class="small">( By the way, Junkman and MadRef are responisble for this question! /forums/images/graemlins/grin.gif )</font>

You can blame me all you want...... I have been chased away by worse people for less reasons!!! /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif This, to me, is like a radio.... I don't have to know how it works, but only that it does work. When all of you sort this out, send me a post card and let me know. In the meantime, I have to get out my tools to start assembling my new toy....../forums/images/graemlins/crazy.gif /forums/images/graemlins/smirk.gif Junk....

 

[WhiteRock]

I don't think that the no demand pressure should be higher that 1-5% of the manufacturers's PRV setting.

There a formulas for computing orafice/hose sizes based on flow. I did not see the nominal pressure constant specified that the formulas are shooting for. But it is out there if you want to research it.

The only way I see this different is if the hydraulic system has a single pump system that runs the power steering and hydraulic devices. In this case there is a flow diverter making sure the power steering gets it share. There may always be a slight demand on the power steering circuit. This pressure will show up in front of the flow diverter.

On a two pump system, I would expect the hydraulic device side pressure to be real close to 0 with no demand. If not I would say some one has been messing with it and put in a restriction.

A test for Junkman's scenario would be to watch his pressure when turns the steering wheel. A change would say that his pressure is coming from the power steering circuit.

Edited:
Probably should have read the other thread first. Looks like SPIKER described the scenario, and better. I may delete this post.

 

[RickB]

I stayed out of the other thread because I think the use of a closed center valve on a BX is inappropriate, but don't know for sure. To answer your question, a pure open center system without any priority flow divider should develop less than 200 PSI of backpressure at no demand. Or somewhere close to it.

 

[rambler]

Not been on in a while, I will have to go look for the other thread....

Back pressure would likely be 25- at most 200 lbs, depending on how cold/ hot the oil is & how many spools you have hooked up to it. If it were 1000#, you would have a smoking tractor in an hour or so, that would be way way too much....

--->Paul

 

[rambler]

Wow that other thread got long - only looked at the first page.

Is not Junkman powering his closed center valve with oil that is _after_ the regular open center valve and trapped in the lifting cylinders? Is he not actually using the weight of the loader to 'pump' oil through his closed center valve? This is why he is limited to 1500# pressure?

Got to go, but will have to look at the other pages later.

--->Paul

 

[Henro]

Guys, thanks for the feedback!

In that OTHER /forums/images/graemlins/blush.gif /forums/images/graemlins/smile.gif thread, JerryG reported having seen 100~200 PSI as system pressure with all valves opened (centered).

So I guess it is a safe assumption to say that a normal range would probably be 20 to 200 PSI, depending on the tractor and how many valves and hoses have been added to the open center loop.

If I ever get around to it, I am going to buy a gage and measure what I have on my tractor just for grins...but that won't happen for a while... /forums/images/graemlins/smile.gif

Thanks again!