Ripping with Hyd Top Link

[Glenn9643]

</font><font color="blue" class="small">( That is correct but you need a double piloted valve that has a cracking pressure of 500 psi or you will not get the desired effect. )</font>

CCI,

Please explain why 500 psi cracking pressure is needed. I'm a little slow in this area and it doesn't immediately sound rational. Thanks.

 

[HayDR]

If you have a cracking pressure of 5 PSI then if you have a shock load of normal use it will move the position of the cylinder. As tractors age the hydraulic systems have an increasing amount of drift, what was once perfect in the show room is not anymore.

 

[Glenn9643]

It is my understanding that the load on the cylinder, whether it is a normal working load or a shock load, is contained within the cylinder and it's seals when the "lock valve" or "double piloted check-valve" is used. The cracking pressure is the initlial pressure from the host machine's system required to open the check-valves and allow fluid to flow for adjustments. I'm supposing that by "shock load" you are referring to pressures created by encountering a sudden load or obstacle, such as might be created by dragging the box blade over a stump or driving the loader bucket into a pile of dirt? I still don't have a proper understanding of why 500 psi is any better than 25psi cracking pressure.

 

[Henro]

I am still learning too...

That particular pilot check valve had a cracking pressure of 25 psi.

What does this mean? I would guess it means that you need 25 psi on the incoming lines before it would let the incoming fluid flow into the cylinder.

What is the relationship between the pressure within the cylinder and this cracking pressure? Is there a relationship?

I am trying to learn so if this question is as basic as it may be...please bear with me on this.

I would like to have a top and tilt system on my tractor and have some ideas about what I need, but still am in the learning process... /forums/images/graemlins/blush.gif

 

[JerryG]

The cracking pressure is like a buffer that has to be over come before the cylinder will move.

 

[HayDR]

An easier way to understand the pilot operated check valve is the amount of force needed to move the cylinder must exceed the cracking pressure to move the cylinder. If under normal use required pressure of 100 psi is needed to keep the cylinder in the desired position and the cracking pressure is only 25 PSI then the pilot operated check valve will be as if there is no valve because the normal operating pressure has exceeded the cracking pressure of the valve. It is common to see tractors to have bleed by pressure of 100-300 PSI so to counter this bleed by one will need to exceed that pressure to keep the cylinder locked. Most tractors have working pressure of 2000+ PSI so the tractor will have adequate pressure to open the piloted check valve to position the cylinder in the desired position.
Just slapping a valve in a hydraulic system can have negative effects cause a restriction of flow and causing heat build up. It is not rocket science but you can flat cause some expensive damage in a short period of time if you add something non-compatible with your tractor. Not every dealer can answer questions because most people answering the phone are sales people and may not have had training in technical areas. You might have to ask for the service manager because he is trained in dealing with problems and finding functional answers for customer problems.

 

[Glenn9643]

Thanks for your explanation. I assume (there's that word again!) that bleed-by pressure would be present on both hoses to the cylinder. Apparently the cracking pressure applied doesn't have to be "directional" or from one hose in order to open the valve and allow movement then.

 

[HayDR]

The bleed-by pressure usually will be on one side or if in both sides it will be greater on one side. Say you have a FEL and you are doing something that does not require use of the FEL and suddenly you realize the FEL is blocking your view. Likely you have some bleed-by in its control valve that has very slowly moved the FEL. Many of the new & larger FEL have piloted operated check valves built into the cylinders for safety reasons.

 

[Glenn9643]

Again, thanks for your response. I've done some more research (dangerous) and found that several of the valves offered indicate a "cracking pressure" at varying lbs-pressure that is achieved by a spring loaded mechanism designed to prevent valve opening under no-load conditions. This "cracking pressure" seems to vary from 6 lb to 60 lb and perhaps more. When under load these valves are rated as "pilot-ratio" 4:1, 3:1, etc. Using the 4:1 ratio as an example if the cylinder was under a 2000 psi load it would require 1/4 of that, or 500 psi to open the valve and allow adjustment. This makes more sense now, but I may still be lost in the wilderness. But I feel better.

 

[Henro]

CCI,

Are you sure what you are saying is correct for pilot operated check valves?

For example, if you look at the online Prince catalog here at the RD-1400 pilot operated lock valve [I see the catalog page lists it as a pilot operated check valve] it states that pilot pressure on the lock valve will open the check valve on the other side of the cylinder. Also states that the cylinder will be locked in place when a directional control valve is in the neutral position. Unfortunately nothing is said about cracking pressure or the effect of shock loads... /forums/images/graemlins/frown.gif

There is also a physical drawing of this lock valve, which I did not look at until after starting to type this question.

Looking at that drawing, it looks to my eye like the "cracking pressure" most likely refers to the amount of pressure required to overcome the check valve spring when the check valve operates and allows fluid to pass in the operating direction. A shock load would simply push the ball of the checkvalve against its seat harder, adding to what the spring of the check valve is alread doing. So a shock load should not cause the check valve to operate. And cracking pressure would not play a part with respect to shock load.

From what I can see, looking at this drawing in the Prince catalog, when no pressure is supplied from the control valve, the balls in both check valves are held agains thier seats, and both sides of a double acting cylinder see a closed line. A shock load would just push one of the balls harder against its seat, and in an ideal world, the cylinder rod would not move the slightest. I guess in the real world, there could be a little bleed through depending upon how well the ball seats in the socket.

So in this particular Prince valve it would seem that cracking pressure would refer to the amount of pressure needed to push the ball against the spring and away from its seat, allowing the hydraulic fluid to flow INTO the cylinder, while at the same time it might refer to the amount of pressure required to shift the pilot spool to cause the spool to push the ball of the opposite side away from its seat, allowing hydraulic fluid to flow OUT OF the other side of the cylinder.

So I guess what I am saying is that in this particular valve, cracking pressure has no relationship to cylinder movement as the result of a shock load.

Is the operation of this locking valve different from what is used on your cylinders?

If anyone sees a flaw in my analysis of how this particular Prince pilot operated check valve works please point it out as I am still in the learning mode, driven by my desire to put a T&T on my tractor. /forums/images/graemlins/smile.gif

 

[Rowski]

Not sure about this if this is what CCI meant. A shock load in hydraulic system on the tractor side of the pilot valve, from the FEL or other remote operated device. The shock loads on the cylinder side of the pilot valve should not make the cylinder move. Kind of like a diode in an electrical system.

 

[Glenn9643]

On the Prince page you referenced, in the small print is tells you the pilot ratio is 4:1.
Here is a link to another mfgr indicating 4:1 pilot ratio with varying available crack pressures: http://www.normanequipment.com/downloads/Snap_Tite_catalog.pdf

Seems to me that the amount of pressure required to allow flow of fluid increases with the load on the cylinder, which would make sense.
This is another mfgr description that may help:
http://www.hydraforce.com/Directnl/Dir-pdf/4-250-1.pdf

 

[JerryG]

A regular pilot operated check valve such as the one on a HTL has nothing to do with shock load in the cylinder. The pilot pressure does have to be able to compensate for system variance such as surges. If another valve in the system is operated the system with have a surge of some amount. This surge or variance is one of the things that the pilot is adjusted for. In Mark’s example of 500 pounds of pilot pressure, when the valve is moved that operates the cylinder in question, the line pressure must build to 500 pounds of pressure before the pilot will open the check and let the fluid flow the move the cyl

 

[HayDR]

Jerry's explanation is the most concise. Guys, Don't make it too hard or we all will get a headache and we'll have to send for Nomad..LOL. Maybe JimMC can shed some light on the subject since he is the engineer and I'm just a barnyard engineer.

 

[JimMc]

Just got in from a family wedding, so I may not make much sense, but here goes.

The easy way to look at this is that until the pilot operated check valve sees "X"psi on the intake side of the cylinder, it does not open the exhaust side of the cylinder and allow fluid to exit that side. (It is important to block the exhaust side when attempting to hold a given position because with the intake only blocked, the cylinder can still move by pulling a vacuum (negative pressure) on the intake side. This is really a bad thing as contaminants can be pulled into the system and seals can be damaged by the generation of a great enough vacuum.) This means that there must be pressure from outside of the cylinder (which normally would only come from the operator opening a valve) before the cylinder can move. The "dual" part is important because in a 2 way cylinder either port can be the intake or exhaust, depending on which way you're trying to move the rod.

The whole idea is to isolate the hydraulic pressures inside the cylinder caused by forces trying to extend or retract the rod and only allow movement when the operator wants it. Clear as mud now - right? /forums/images/graemlins/laugh.gif