Hydralic Pressure?

[nhman]

Now that I've owned a tractor for a few months, I'm interested in just how hydralic pressure does all its magic? There's lots of smart folks on this web site. Can anyone tell me the principles behind how my loader can lift all that weight?

nhman

 

[MChalkley]

It sure seems like magic when you put a little pressure on that small lever and all that work just happens, doesn't it? /w3tcompact/icons/smile.gif

My guess is that you'll get different answers to this question, but I'd say the magic is best summed up as: PSI - Pounds per Square Inch. If you think about the pressure that the pump creates, say 2500 psi, that means that 1 square inch of it can lift 2500 pounds of weight. (I'm over-simplifying here, because flow is involved too, but everybody always accuses me of over-complicating things, so this time I'll try it the other way.)

Now, pretend a cylinder is 3 inches square (I know the ones on your tractor aren't square, but it's easier to calculate the surface area of a square cylinder. I'm over-simplifying, remember? /w3tcompact/icons/smile.gif) That's 3" x 3", or 9", of surface area on the piston, with 2500 pounds of pressure on each inch. A magical amount of stuff can get done when you've got that much potential force at your command.

 

[kubmech]

The hydraulic pump puts out hydraulic "flow" (measured in gallons per minute/GPM). On most tractors this pump is comprised of two gears (gear pump) that pulls the fluid from the reservoir (transmission housing on most modern tractors)
and pumps it to the loader control valve, with the control valve in the neutral position the flow continues back to the reservoir. This is what is known as an "open center" hydraulic system. If the flow was not permitted to "go back to tank" the pump would continue to provide flow, and pressure would build (keep in mind the pump is not providing the pressure, the resistence to flow is providing the pressure) until something lets loose, usually the pump would explode at this point or a hyd. line would burst. So that's why we are constantly returning fluid to tank in an open center sytem.

So now we have a supply of hyd. fluid flow available at the loader control valve. To route this flow to the cylinders a spool valve is moved through the path of this flow and "redirects" it to lets say the lift cylinders. The cylinders fill with fluid (keep in mind a fluid has a very high resistence to compression)and begins to move the piston inside the cylinder
which extends the cylinder till the piston reaches the end of the cylinder, at the same time lifting the loader frame. Now when the cylinder reaches the end of its stroke we are back to the problem of keeping the fluid moving back to tank. This is where the relief valve comes in. The relief valve is basically a poppet valve that opens at a predetermined pressure to release the fluid back to tank by a spring. The spring tension is usually set by using shims or a threaded rod and nut. This relief valve is what sets the "power" of the loader.

If your cylinder pistons are let's say 3 inch pistons. Find the area of the piston. Radius 1.5 x 1.5 = 2.25 x 3.14 (pi)=7.065.
Now let's say your relief valve is set to open at 2500 psi.
The area of the piston that the psi is pressing against is 7.065
inches so 2500 psi x 7.065"=17662.5 "lbs divided by 12 will give you foot lbs of 1471.9, Multiply that by the number of cylinders in this case 2, and you have cylinder lift capacity of
2943.8 lbs. Subtract the weight of the loader assy itself and you will have the total lift capacity of the loader/ Well basically, not exactly that simple do to the lever action of the loader frame and position of the cylinder (moment and arm thing) but that's the basic idea. (I knew I should'nt have left that part out)

PSI is a measurement of weight or force and GPM is a measurement of how much how fast.

So PSI is created by the reief valve which equates to "POWER"

The pump creates flow/GPM which equtes to "SPEED"

 

[mbjacobs]

Check out this web page for a good description of how hydraulics work.

http://www.howstuffworks.com/hydraulic.htm

 

[BKColeman]

Excellent explanation Kubmech! /w3tcompact/icons/king.gif I really enjoyed your post.

-Brad

 

[daffyduk]

I enjoyed YOUR post Mark. And I understood IT.

Gene

 

[MChalkley]

Gene - That's gotta be a first. I've had folks say they didn't like a post because they didn't understand it, liked one even though they didn't understand it, or didn't like one even though (or maybe because) they did understand it, but I don't ever remember someone liking a post and understanding it... /w3tcompact/icons/laugh.gif (I'll send you the 10 bucks tomorrow... /w3tcompact/icons/wink.gif)