J_J got a minute?

[retiredmgn]

I'm attempting to get a better grip on hydraulics. A post you made on a dfkrug thread stated
"You have the cracking pressure which starts to open the relief, and then you have the full relief
pressure. The cracking pressure will be higher. It should be relieving fully while you are adjusting."
Reading a site online I came across,
"The pressure at which a valve first begins to pass flow is the cracking pressure of a valve. The
pressure at which a valve passes its full-rated capacity is the full-flow pressure of a valve.
Because of spring rate, a full-flow pressure is higher than a cracking pressure."
Logically, it seems to me cracking pressure would be less than full relief due to less spring compression.
What am I missing here?

 

[glastron23]

retiredmgn you are correct, cracking pressure is lower than max system set pressure, you do set the PR at max flow.
when setting the system pressure you can get a little oil flow from the tank side of a PR at 1/2 to 3/4 of the system pressures..

 

[oldnslo]

Some of this depends on the style of relief valve being used.

Direct operated reliefs usually have a higher pressure rise from crack to full flow than pilot operated valves. Reason being is that on pilot operated valves the pilot section only moves a very small amount and the main section has a softer or lower rate spring resulting in lower pressure rise.

In almost all cases crack pressure will be lower than full flow pressure. Some special design relief valves are made so the pressure will actually drop some at full flow but i would be surprised to find this on most Ag equipment.

 

[retiredmgn]

That was going to be my next question. If my fel has a rated lift capacity of 1700 lbs, would that be the the cracking or max flow setting of the PR?

 

[J_J]

retiredmgn,

My statement was wrong, but it still seems like the initial opening , cracking pressure, of the relief would take a certain high pressure to open, and it does, but the differential is small.

If relief is 2000 psi, a 1998 psi might would crack the valve, maybe a couple of drops a minute, and if the load produces pressure above the relief setting, the relief valve goes to full relief.

There is a difference in the kinds of relief valve , spring or direct acting, pilot operated, ball or poppet.

Here is an excerpt:

[ Cracking pressure and pressure override - The pressure at which a relief valve first opens to allow fluid to flow through is known as cracking pressure. When the valve is bypassing its full rated flow, it is in a state of full-flow pressure. The difference between full-flow and cracking pressure is sometimes known as pressure differential, also known as pressure override.]

Setting the relief valve on your first valve in series , usually the FEL relief valve, will determine system pressure.

You would set your relief valve when fully relieving to about 50 to 100 psi below your pump max rating, and not worry about the cracking pressure. Just max out your cyl to achieve full relief.

My initial thoughts on this was that it must take a larger force to raise the poppet off the seat, and then less pressure to keep it open.

 

[CJONE]

With a direct acting relief it will "crack" or open at a set pressure. Then as you force more oil past it it will take more pressure to open more because you are compressing the spring more. A pilot operating relief is more consistant but also more expensive. Direct acting reliefs have a certain max GPM flow, once you exceed that the pressure will rise considerably. CJ

 

[retiredmgn]

"Setting the relief valve on your first valve in series , usually the FEL relief valve, will determine system pressure."

Do I have to buy the service manual to see a diagram of the hydraulic system for my 3038? Are such things posted somplace on the web?

"You would set your relief valve when fully relieving to about 50 to 100 psi below your pump max rating, and not worry about the cracking pressure. Just max out your cyl to achieve full relief. "

How does one achieve full relief? Wouldn't that require attempting to pick up something heavier than the rated lift capacity of my loader and holding the joystick back steady?

 

[CJONE]

"Setting the relief valve on your first valve in series , usually the FEL relief valve, will determine system pressure."

Do I have to buy the service manual to see a diagram of the hydraulic system for my 3038? Are such things posted somplace on the web?
someone should have it
"You would set your relief valve when fully relieving to about 50 to 100 psi below your pump max rating, and not worry about the cracking pressure. Just max out your cyl to achieve full relief. "
I would set the relief to the lowest part of you operating system. If the pump is rated at 3k but the cylinders are 2500 then you should set it to 2500.
How does one achieve full relief? Wouldn't that require attempting to pick up something heavier than the rated lift capacity of my loader and holding the joystick back steady?

A cylinder at max stroke or retracted all the way and the eng rpm at max when the machine is warm and the stick back in the up, retract or whatever to deadhead the cyl. CJ

 

[J_J]

"Setting the relief valve on your first valve in series , usually the FEL relief valve, will determine system pressure."

Do I have to buy the service manual to see a diagram of the hydraulic system for my 3038? Are such things posted somplace on the web?

"You would set your relief valve when fully relieving to about 50 to 100 psi below your pump max rating, and not worry about the cracking pressure. Just max out your cyl to achieve full relief. "

How does one achieve full relief? Wouldn't that require attempting to pick up something heavier than the rated lift capacity of my loader and holding the joystick back steady?

Anything that restricts the hyd flow will cause the valve relief to activate, such as a load above the rated load for the tractor, a cyl at full extend or retract, or a hyd motor at stall pressure, or a deadhead situation like an uncoupled QD .