Dumb Question on hydraulic pressure

[Dave Pee]

This is my JD compact tractor.

Trying to lift heavy loads, like an engine, hanging from the grapple in front.

If I increase engine speed, it does not seem to lift more weight (as opposed to running at idle speed).

Does the hrdro pump deliver more pressure to the lift arms as engine RPM increases? It appears that the hydraulic pressure is a constant value.

TIA
Dave

 

[the old grind]

You won't need high rpm to reach full pressure, but would gain more flow AT pressure if you were using it. Adjusting hydraulic pressure upward (a few hundred psi is typical) is a simple 'upgrade'. Gauge, hose and fitting <$100 to see what your system puts out.

You surely want to rig as close to the loader's pins as possible, but don't expect huge gains.

One trick I learned while grubbing saplings or grappling rocks is to mix a bit of 'uncurl' while raising the arms to maintain height, then using a stronger curl (if you have that capacity) to gain more. Can't do it with all loaders, but my Shibaura-built CUTs and JD controls on big Green allow it with 'open center' because the bucket spools have regen capability.

I raised and cribbed-up incremental steps while doing this, but it's kind of an "old guy has alla time in the World" method.

 

[CincyFlyer]

You are correct, the pressure is limited, and most tractors will deliver max pressure at idle, though not max oil flow. So your loader or grapple will operate more quickly at higher RPM, but won't handle any more weight (assuming no small leaks that more flow will disguise).

 

[MossRoad]

The pump is rated at gallons per minute and a pressure at a given RPM. It may get to the point that increaseing RPM will only increase the speed of the lift(more gallons per minute), but not increase how much pressure it can develop (PSI). It's the pressure that does the work. The GPM is the speed of the work.

Make sense?

 

[oldnslo]

Depending on the style of relief valve used there could be minimal pressure difference from idle to full throttle. On system with a fixed displacement pump the relief should be sized to handle full flow with minimal pressure increase. I.e the amount that can lift will change very little.

 

[Industrial Toys]

What has been said is true. However, I have seen machines not lift at idle and then lift with a little extra RPM. Don't know why? Maybe a worn or leaky system.

 

[the old grind]

'Idle rpm' won't get you there, ever. IME, max psi is usually reached near 'peak torque rpm', and that's so often between 1500 & 2000.

If you want to 'calibrate' for a certain capacity, just rig up and operate the lift spool while creeping rpm up until it budges and, say, note that value before and after shimming the pressure relief valve.

Recognize that factory pressure settings (say 2250 vs 2450) may be watered down so that standard loader option capacities don't 'overload' the tractors. An example might be a FEL that's available on models of fairly different length, width, and weight.

Ford offered the same 7108 FEL on 1520 and 1920, with 6" different width and wheelbase and 600 lb difference in bare weight, but should the lift capacity be the same and still be 'safe'? I do plan to shim two of my CUTs and have the gauge setup (Surplus Center) to do the checks. (I like to live on the edge)

 

[CincyFlyer]

'Idle rpm' won't get you there, ever. IME, max psi is usually reached near 'peak torque rpm', and that's so often between 1500 & 2000.

If you want to 'calibrate' for a certain capacity, just rig up and operate the lift spool while creeping rpm up until it budges and, say, note that value before and after shimming the pressure relief valve.

Recognize that factory pressure settings (say 2250 vs 2450) may be watered down so that standard loader option capacities don't 'overload' the tractors. An example might be a FEL that's available on models of fairly different length, width, and weight.

Ford offered the same 7108 FEL on 1520 and 1920, with 6" different width and wheelbase and 600 lb difference in bare weight, but should the lift capacity be the same and still be 'safe'? I do plan to shim two of my CUTs and have the gauge setup (Surplus Center) to do the checks. (I like to live on the edge)

Mine will "hit" the relief valve at idle, so would the other tractor which I have experience with. We're talking about small tractor open-center hydraulic systems in this case.

 

[k0ua]

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.

 

[the old grind]

Mine will "hit" the relief valve at idle, so would the other tractor which I have experience with. We're talking about small tractor open-center hydraulic systems in this case.

So am I, so I'm pretty sure we're on the same page. I'm referring to Ford 1520 (1.3L, 23 hp), NH T-1520 (1.7L, 35 hp), and Case/IH DX 26 (1.1L, 26 hp) from past and current experience, only one of them bought new.

I've had or have 7308 and TL-110, twins to 7108 with comparable OC pumps valves and OEM 60" buckets (T110 manual lists/compares all three). None of these will raise much over 800 lbs, or I wouldn't be struggling to lift stuff with whacky shortcuts.

Anything smaller I would call a SCUT (the Case/IH Kinda is). As for my 'FrankenDeere', no prob lifting the rears, 8' two-row disc, and 450lb rock with FEL at idle but that's with a 12 gpm 2800 PSI system (also OC, btw) and 2 1/4" cylinders. Maybe that's an orange among our apples.

My CUTs will/would lift more at 2200 rpm than at 1800 but 'max' by there, and IMO that's typical of the type/size. :confused3: