Dumb Question on hydraulic pressure

[k0ua]

Yeah, as the fluid thins out with high temps, everything surely leaks more. The pump itself, the selective control valves, even possibly the piston seals itself. Makes sense to me.

 

[Tx Jim]

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.

Ditto I agree 100% pressure at relief valve will be the same at engine idle or WOT

 

[MHarryE]

Direct acting (low cost) relief valve is a spring loaded poppet. With more flow going through the relief at higher flow rates, the spring needs to be compressed more to allow the poppet to open enough to bypass the flow - consequently higher pressure. As someone earlier in this thread Kubota specified in their WSM to check pressure at max rpm. Direct acting reliefs with which I have worked have a curve - cracking pressure where the first begin to bypass flow and a line showing pressure vs flow after cracking pressure.

Indirect reliefs can be designed to dump flow at a specified pressure and maintain that pressure as flow increases.

Closed center systems (more expensive) generally are pressure and flow compensated piston pumps. Once they reach peak pressure the pump will destroke maintaining the design pressure down to zero flow.

 

[the old grind]

...Indirect reliefs can be designed to dump flow at a specified pressure and maintain that pressure as flow increases.

Yes, this is a simple system setup vs the de-stroking pumps that we see on tractors. The valve is able to bypass full gpm and is called an "unloader". It's quite common on pressure washers and other 'closed center', usually static systems.

I'm sure we see such variety of circumstance because we all have different amounts of line length/dia/friction, different fluids/wts and valve spool clearances, and are comparing tractors with different hours on the clock at different temps. :laughing:

btw, I had this s__ in college so long ago that 'data' only had one 'T' and the 'processy' hadn't been developed yet.