How Much Pressure Is In Hydraulic Lines ?

[TheKubotaKing]

Hi everyone, and good morning.
I've been wondering about this for some time... when you look at load a tiny BX tractor or any tractor for that matter can lift, how much pressure /PSI is on those hydraulic lines ? I'll be honest and say I am no expert in how hydraulics work, and perhaps all that pressure is in the hydraulic cylinder itself, so lines are not really under so much pressure.... ( ? ) From a few videos on YouTube, it seems the amount of pressure that is created is 10x or more, by compressing that hydraulic fluid.... but where is all of that really happening ? is it in the cylinder itself ?

Hopefully a good question !

 

[BigG]

Look up Pascal's Principle and it will explain the idea behind hydraulics for you.

 

[Henro]

Look up Pascal's Principle and it will explain the idea behind hydraulics for you.

In addition to this, since the tractor uses an open loop system, the pressure is very low until you shift one of the valves, and cause something to move. Then the pressure will increase, to a maximum that is set by the pressure relief valve, depending on the work being done.

On a BX I think the max pressure is about 1800 or 1850 PSI. Or in that range anyway.

If you do not want to study hydraulics, this may answer your question...

 

[D2Cat]

Just figure there's 2000 pounds of pressure. Never put any part of your body in the path of hyd fluid shooting out of a leaking fitting/hose. It can penetrate your skin and cause you great harm.

 

[dirtydeed]

From a few videos on YouTube, it seems the amount of pressure that is created is 10x or more, by compressing that hydraulic fluid.... but where is all of that really happening ? is it in the cylinder itself ?

The hydraulic fluid is pressurized...not compressed. You can't compress a liquid (in practical terms).

 

[freewheel3]

The pressure is generated by the hydraulic pump. All lines and cylinders that are under pressure will have the same pressure inside them. The amount of pressure is determined by where the relief valve is set on your particular tractor. It can vary usually between 1200 and 2500 psi depending on the tractor and engine RPM.

 

[xrocketengineer]

Great replies. I would only add that liquids unlike gases are not (theoretically) compressible that is why a pressure change anywhere in the system is reflected everywhere else. That is the reason when you use a hand pump to pump up a tire you can see the pressure slowly going up as the air is compressed. However, if your tire, hose and pump were filled 100% with water, you would see the pressure go up with barely a fraction of the pump motion.
That is also the reason why you do not want any air in your car's hydraulic brakes. Whatever pressure you generate as you press the brake pedal, you want it to be at your brakes immediately. If you have air, the brake fluid has to compress the air bubbles to nothing first before the full pressure is applied to the brakes. That is the "spongy" brake pedal feel.

 

[85Hokie]

Hi everyone, and good morning.
I've been wondering about this for some time... when you look at load a tiny BX tractor or any tractor for that matter can lift, how much pressure /PSI is on those hydraulic lines ? I'll be honest and say I am no expert in how hydraulics work, and perhaps all that pressure is in the hydraulic cylinder itself, so lines are not really under so much pressure.... ( ? ) From a few videos on YouTube, it seems the amount of pressure that is created is 10x or more, by compressing that hydraulic fluid.... but where is all of that really happening ? is it in the cylinder itself ?

Hopefully a good question !

AS mentioned - 1750 ish psi is more or less stock....... 1850 is what specs call for and some of us juice it up to 2000 psi. As others have said, PSI will hurt/kill you even at 10 psi!!!

Any question is a good question when knowledge is obtained. Most of the information about orange tractors I have learned here listening to those who know much more than I do. No question is dumb until you ask it the second time!

 

[bucktail]

Most of the newer compact and subcompacts have the relief valve set to pop around 2000 psi. I've been told that my L1500 pops at 1500m. As a practical matter hydraulic fluid doesn't compress. It can be compressed but that results in exciting states of matter.

 

[torch]

From a few videos on YouTube, it seems the amount of pressure that is created is 10x or more, by compressing that hydraulic fluid.... but where is all of that really happening ? is it in the cylinder itself ?

I wonder if you are confused by the difference between pressure and force?

The pressure in the line and the cylinder will be the same. But the force that the cylinder can apply on a load can be tens of times that pressure.

For the sake of argument, let us suppose that the pressure is 2,000 psi. If the face of the piston in the cylinder is one square inch, then the cylinder can apply 2,000 lbs of force on the load. However, if the face of the piston is ten square inches, then the cylinder can apply 20,000 lbs of force on the load.

The flip side is that the 10 sq in. cylinder will move ten times slower than the 1 sq. in. cylinder, because it requires 10 times the volume of fluid per inch of extension. Nothing is free.

Also, note that the cylinder has the most surface area available when extending. The area of the rod itself subtracts from the available surface area on the retraction side. Therefore cylinders exert greater force in extension than in retraction.

 

[Henro]

I wonder if you are confused by the difference between pressure and force?

The pressure in the line and the cylinder will be the same. But the force that the cylinder can apply on a load can be tens of times that pressure.

For the sake of argument, let us suppose that the pressure is 2,000 psi. If the face of the piston in the cylinder is one square inch, then the cylinder can apply 2,000 lbs of force on the load. However, if the face of the piston is ten square inches, then the cylinder can apply 20,000 lbs of force on the load.

The flip side is that the 10 sq in. cylinder will move ten times slower than the 1 sq. in. cylinder, because it requires 10 times the volume of fluid per inch of extension. Nothing is free.

Also, note that the cylinder has the most surface area available when extending. The area of the rod itself subtracts from the available surface area on the retraction side. Therefore cylinders exert greater force in extension than in retraction.

Just to add something that we may have not emphasized, the hydraulic pump is rated in GPM or similar. NOT PSI.

The flow out of the pump goes through the open loop circuit, and really develops little pressure until it is met by a restriction. Generally the restriction is offered by a cylinder or cylinders doing some work, like on the loader, or whatever, when a control valve is shifted.

Once there is a restriction to flow, the pressure in the system up to that point increases to be equal to whatever load is being moved. Heavier material in the bucket, or on the pallet forks, the higher the pressure the system feels while the loader is moving.

Once the loader reaches its limit of travel, pressure rises to the point that the pressure relief valve pops open, and pressure is maintained at that level. If such a situation is maintained, the hydraulic oil is heated excessively, but rarely do we hold the loader or other implement at the full limit of travel, causing the PRV to stay open for much time.

Generally, the pressure in the system is related to the work being done. If you have so much weight in your loader bucket, for example, that you cannot lift it, the system pressure will immediately rise to the point of popping the PRV. Nothing moves much...

So the bottom line is pressure in the system is related to the work being done, up to the point where the PRV pops and nothing moves.

If the PRV were eliminated and replaced with a pipe plug, you would likely soon destroy your hydraulic pump. But who would do this? Nobody who understood basic hydraulic principles...

 

[TheKubotaKing]

Wow, I was not expecting so many answers. I just read through each of them, and maybe retained 1% of what I read, but after typing this, I'm going to go back and slowly read through them again. Amazing community here. Thank you for all the input.

So you may be wondering... why did he ask the question ? So I'm assuming all hydraulic lines are rated at a value that will greatly exceed the amount of force that the tractor can create. So I'm going to assume these hydraulic lines are manufactured to very strict guidelines etc. I just thought, what happens if one of those lines popped off one of the the attachments, or somehow popped off the main manifold where it connects to say the BX line of tractors. With such force in those hydraulic lines, how do they stay in-place ? and I'm guessing, nobody should be anywhere close to one of those lines, in case you hit something while driving through brush, and a sharp branch slices into one of those lines... etc.....

My question was really around the safety aspect, and I guess I should have said that in the beginning paragraph. As a newbie, it's a good question.... as I get older, I get wiser and much more paranoid about potential accidents. Anyway, amazing info here, I never thought this would be such a hot topic !

 

[PaulR]

I was told (by a guy who I think knows his stuff) these little tractors don't have really real hydraulic systems compared to larger machines? Something about the system or the pressure they operate at is significantly different from say, a full size dozer. Not sure if there's any truth to that or a hydraulics expert can lend more info?

 

[xrocketengineer]

Here is an interesting demonstration about how much pressure the tractor can put into the cylinders protected by a relief valve vs. the pressure overload caused by back driving the cylinders when the pressure is locked in between the cylinders and the control valve an there is no relief valve in between.

 

[torch]

I was told (by a guy who I think knows his stuff) these little tractors don't have really real hydraulic systems compared to larger machines?

Oh they are real hydraulic systems, rest assured! However, as mentioned by Henro, one significant specification is the flow rate. A little sub-compact might be able to supply 3 or 4 gallons per minuted, where a skid steer might be capable of 20 or 30, even though the pressures may be roughly similar.

And yes, large excavators usually run higher pressures -- perhaps 5,000 or 6,000 psi. Otherwise they would need much larger diameter cylinders to do the work. But then, the boom on one of those probably weighs more than a compact tractor!

 

[g9vi6185]

I posted this in another area as well as I wanted to make sure I was in the right area

I have a L3901 and have been trying to figure out how to check my hydraulic pressure. I have a LP 3rd function set up for a Grapple as well.
So I made a gauge with a 5000 psi liquid filled gauge attached to a Kubota female QD coupler. I have seen a few pics where guys attach the gauge to the "blue cap" (top right connecter of the four) loader male connects on the valve assembly by the loader handle. I get nothing on the gauge? I tried all the connections and still get nothing?
Do I have a bad gauge? Do I need a male fitting and put the gauge on the loader line instead of the valve assembly (saying it wrong?). Do I need a 3/4 pioneer coupler and hook to my 3rd function to test?
I see the disconnect line move, tighten up when I bump the right loader control.

Thanks

 

[Lil Foot]

Not familiar with your setup, but when I tested the pressure in my FEL & my backhoe, I used this tee-

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with the gauge mounted on it.

Lines were connected normally, except with the tee/gauge inline. No pressure until the stick was moved, then got readings while that particular function was operating.
Whatever you do, don't deadhead the pump.

 

[85Hokie]

are you moving the stick while checking? You must move the valve body (stick) to THAT location or port.
The gauge will read that you are dead heading that port and give you a reading- do not hold it there from more than a few seconds.

Without moving the stick - the fluid is simply moving through the system unrestricted.

 

[g9vi6185]

Yes moved it quickly in all directions. Bucket functioned as it should with the exception of the line disconnected. Nothing moved on the gauge

 

[g9vi6185]

Figured out my own problem. Took apart the gauge set up and found a small piece of thread dope plugged the hole... Stupid but easy fix.

Running little over 2500 with a .02 shim added if any one is interested.