Dumb Question on hydraulic pressure

[Dave M7040]

My newer Kubota's WSM says to check relief valve at max rpm's.

I conclude from that internal leakage at low RPM's may limit the pressure reached. YMMv

Dave M7040

 

[npalen]

I think that there is a bit of "flywheel" or inertia effect when running the engine at a high idle trying to overcome a load. An example would be when pulling a fence post with the FEL. Low idle might not get the job done where high idle and repeated attempts may. Perhaps the higher volume at high idle provides more of a shock load before the relief valve has a chance to do its thing but then I'm just speculating here.

 

[ovrszd]

Here is the real thing to remember. The hydraulic pump in your tractor doesn't make any pressure at all, either at idle or full RPM. Hydraulic pumps make flow, NOT pressure. The only pressure you could measure from a hydraulic pump would be the slight back"pressure" from the resistance to the open center flow of the fluid in the piping and hoses. This might be less than 100 psi.

So what "makes the pressure"? Pressure is made when the flow encounters a resistance. When the flow is directed by the selective control valve to the end of a hydraulic piston then the pressure starts to spike. if the piston moves, the pressure stabilises as the piston continues against the load.

As the stoke of the piston ends, and the idiot operator keeps on applying flow to the end of the piston, the pressure rises even higher and higher. If the relief valve pressure is reached, the relief valve opens and dumps much of the flow back to the tank. If for some reason the relief valve did not open the pump or the hoses or something would "grenade".

It doesn't matter a bit whether the pump is at idle or at full RPM. The top pressure in the system if the pumps flow is restricted is the same. The top pressure is limited by the relief valve. There is a tremendous difference in the flow rate but not the pressure.

Some have noted that to lift a maximum load some tractors will need a higher RPM. This is because of "leaks" in the system have to be overcome by more flow so that pressure can be developed by the restriction of a cylinder.

Again, pumps make no pressure, they just make flow.

Here is a little experiment you can do at home without any danger. On your water hose, after you turn off the water at the hose bib, place your thumb over the end of the hose. Now just barely crack the water valve (water bib) so that the flow rate would just be a drip.

Now find yourself a comfortable chair and have a seat. Keep that thumb firmly over the end of the hose. You will start to feel an ever increasing pressure from the water on your thumb. This pressure will rise until the maximum pressure that your water system is capable of is reached. This is often 60 psi or more.

You will have difficulty keeping your thumb on the end of the hose. It takes some time to do this as the flow rate is very low, but it will build against the obstruction of your thumb to the full system pressure.
This is the equivalent of your tractor at idle. Yes it takes more time and the hydraulics are more "sluggish", but the pressure has the potential to rise to full system pressure.

James, I think you were speaking in such a quiet voice that they didn't hear you.......

 

[k0ua]

James, I think you were speaking in such a quiet voice that they didn't hear you.......

I think some people don't like to read long explanations. Maybe most people. ..

 

[ovrszd]

I think some people don't like to read long explanations. Maybe most people. ..

But knowing you I realize there's value in there, just gotta focus!!!!

 

[Leejohn]

No one has said anything about HP needed to reach max psi at idle. As the pump is getting to max psi and engine at idle the HP is real low and that is why you have to speed up to get the engine HP up a little. Some tractors can't get max psi at idle without killing the engine because of lack of HP.

 

[ovrszd]

No one has said anything about HP needed to reach max psi at idle. As the pump is getting to max psi and engine at idle the HP is real low and that is why you have to speed up to get the engine HP up a little. Some tractors can't get max psi at idle without killing the engine because of lack of HP.

Although I've never witnessed that, I guess it's possible.

 

[k0ua]

No one has said anything about HP needed to reach max psi at idle. As the pump is getting to max psi and engine at idle the HP is real low and that is why you have to speed up to get the engine HP up a little. Some tractors can't get max psi at idle without killing the engine because of lack of HP.

That sounds reasonable to me, as the pressure rises to near the relief valve opening, it makes sense that the pump which is mechanically connected to the engine would put a heavier and heavier load on the engine as the pressure rises. While I haven't seen it kill my engine, before the relief opens, maybe it could kill some. I know when you go into relief, my engine does drop RPM.

 

[ovrszd]

I guess it's possible on a small 2 series JD. I've never had an FEL on anything that small.

 

[IceStationZebra]

James wrote a very god explanation, the only two things I would add. First is that sometimes flow forces come into play and you will get slightly different relief pressure readings at different flows (i.e. engine RPM). An ideal relief valve wouldnt do this obviously, but the actaul effect depends on how the valve is made and plumbed to the system. And second, the amount of leakage is going to increase with higher temps. ISZ