Why not check valves in FEL?

[kennyd]

I can see how a check valve would disable float, but since "float" only applies (at least on tractor FEL's) to the loader arms and not the dump/curl cylinder, I suppose a DPOCV on just the dump/curl cylinder would be ok?

Sure...

 

[MtnViewRanch]

Would the fast dump cycle still work if there were DPOCV's on the curl cylinders? :confused3:I know that something abnormal goes on with the hydraulic fluid for the fast dump cycle to work. Would that abnormality still work with DPOCV's?

 

[picker77]

Darned if I know, didn't know anything "abnormal" went on in the curl/dump circuit.

Dealer called this afternoon, the Dual Rear Remote kit came in today, and I'm delivering the tractor to him Monday for the install. Getting another step closer to actually having top and tilt on this thing.

 

[AKKAMAAN]

Not quite AKKAMAANI. I operated large digger trucks and large bucket trucks for many years when I was a Lineman for the provincial electrical utility. They had pilot operated check valves on ALL the hydraulic cylinders. It was required to do a daily check on them to ensure they were working as designed. It was a simple test, just put pressure on a cylinder (both sides of the piston on certain cylinders), shut the pump off, and operate the hand controls. If there was movement in the cylinder, it failed and if there was cylinder creep, that was a failure too (which may or may not have been related to the pilot operated valve).
Your statement "Pilot check valves will not protect load from falling when a pressure line breaks" is absolutely wrong!

Totally agree that the Pilot check valve will hold load if valve is not leaking. Thank you for sharing the test procedure.

How about if we imagine this scenario....you operate a crane with pilot check valves. To lift the crane you pressurize the blind/capped side of cylinder. Then the pressurized hose from the directional valve breaks. The pilot check valve will "catch" the falling boom, and stop the flow. So far so good. Now the crane is hanging there on a pressurized lift cylinder (blind/capped side). Now without thinking, engine on, pump running, you operate the directional control valve to lower the crane (which is a stupid move but part of the human factor)....you pressurize the pilot line and that will open the check valve....there is no "throttle" in the directional valve to reduce return flow now when hose is broken.....will the boom fall down now??? slow or fast??

That is exactly why they are used and they are mounted on the cylinder to ensure there is minimum opportunity for anything to fail (think less parts).

Agree they have to be mounted on the cylinder.

Having a pressure line between them and the cylinder defeats their purpose.

Will still prevent from drifting....

Your statement "pilot check valves only works both ways on a double action cylinder if piston seal is 100% sealed." is only partially true, they have to be mounted directly on the cylinder.

I was not clear enough here....there are no 100% leak free piston seals, I think we all agree on that....The pilot check valves on the capped side and the rod side will sooner or later both be exposed to the higher pressure from "the pressure intensification effect"....a 4" cylinder with a 2" rod will boost (4 times) the initial load pressure....If load pressure is 3000psi on the capped side, pressure will become 12000psi when a few drops of oil have leaked over to the rod side....this have to be considered in the design of the system

...larger diameter rods will reduce pressure intensification....a vented rod side of cylinder will eliminate pressure intensification, but will allow drift through the piston seal.....

A load hanging on the rod side of cylinder, will drift all the the way down, or to the point there is not enough pressure to create a piston seal leak....if capped side void (volume of the rod) not get filled with oil or air, vacuum is created....(minus 15psi at the most)

After reading your post I sure agree that a pilot check valve have safety benefits if we do not include the "human operator error factor", but system will require adequate design....

Problem with pilot check valves in continuous operated motion implements like a FEL, timber crane etc, is that it creates counter pressure and power loss through pressure drop....and that will be negative on hydraulic response time too....Pilot check valves fits better on non motional implements, to prevent drift, like a basket crane , support legs etc, while a flow fuse is preferred on motional implements like an excavator boom

 

[MtnViewRanch]

Darned if I know, didn't know anything "abnormal" went on in the curl/dump circuit.

Dealer called this afternoon, the Dual Rear Remote kit came in today, and I'm delivering the tractor to him Monday for the install. Getting another step closer to actually having top and tilt on this thing.

I believe abnormal only in the fast dump position. The far right position on a lot of loaders. I do not actually know, only guessing. :ashamed:

Picker, you keep thinking about that side link.

 

[kennyd]

I believe abnormal only in the fast dump position. The far right position on a lot of loaders. I do not actually know, only guessing. :ashamed:

Picker, you keep thinking about that side link.

The "abnormal" is called regen or the regenerative function of the valve. here is a great post on it: 3 position valve-vs-4 position valve - TractorByNet.com

Also, a search of the term "regen" will turn up hours of reading


As to how it effects a DPOCV equipped cylinder?...I will have to think about that one.

 

[wdchyd]

A couple of other reasons to not use DPOCV's and Counterbalance valves on tractors:

We already complain about how cyl seals don't last long enough.....imagine all the extra pressure spikes to extend and retract these cyls during normal HD usage........The tractor mfg's will have to spend more $ on building their cyls better......

How about this situation....In a twin cyl application......a DPOCV or counterbalance valve doesn't open exactly at the same pressure and you are trying to feather the control......If someone goofs around with any adjustments on these valves you end up with one severely bent rod (whether under load or not)........:thumbdown:

 

[picker77]

Man, I love this forum. I'm getting a free education here, and I'm dead serious about that. Thanks, guys!

 

[brokenot]

Problem with pilot check valves in continuous operated motion implements like a FEL, timber crane etc, is that it creates counter pressure and power loss through pressure drop....and that will be negative on hydraulic response time too....Pilot check valves fits better on non motional implements, to prevent drift, like a basket crane , support legs etc, while a flow fuse is preferred on motional implements like an excavator boom

It's pretty simple to design a system around the use of pilot checks/holding valves, and it is common practice on FELs of another "type" that do see constant in/out, up/down cycling during operation all day long.

Every cylinder on these machines for lift/lower, extend/retract, tilt, frame sway, etc. is designed and built in such a manner. Productive cycle times are very much a consideration with these machines. And nope, never seen one drop a boom when a hose ruptures during operation. Operator error is a huge consideration on machines like this, because many of the guys using them on jobsites are first-time operators. I guess what I'm saying there, is that we see plenty of hoses get damaged, by plenty of operators that aren't *thinking* their way through what controls they should/shouldn't touch next.

Manlifts like these are also designed the same way. No, you don't need rapid cycling with these machines, but it is a requirement on telehandlers like the SkyTrak above:

We already complain about how cyl seals don't last long enough.....imagine all the extra pressure spikes to extend and retract these cyls during normal HD usage........The tractor mfg's will have to spend more $ on building their cyls better......

The manufacturers do build these cylinders better. Far better. Pull one apart and you'll find 2X or 3X at least more, (and better), sealing rings on the pistons and rod-end gland nut or cap. Everything is beefier and higher-quality from one end to the other. Actually, everything to do with the hydraulics is that way.....since the entire machine is designed to be a hydraulic power unit. Many tractors are designed from the beginning to be tractors first, with hydraulic systems being sort of an "accessory".

If you compared the innards to a typical simple cylinder you'd find for sale at the local Tractor Supply, there's well.....no comparison at all. Simple cylinders like the ones you can pick up for $100-200 for basic hydraulic systems are very much built to a price point. It's pretty amazing they operate as long as they do before leaks or other issues develop....

 

[wdchyd]

It's pretty simple to design a system around the use of pilot checks/holding valves, and it is common practice on FELs of another "type" that do see constant in/out, up/down cycling during operation all day long.

Every cylinder on these machines for lift/lower, extend/retract, tilt, frame sway, etc. is designed and built in such a manner. Productive cycle times are very much a consideration with these machines. And nope, never seen one drop a boom when a hose ruptures during operation. Operator error is a huge consideration on machines like this, because many of the guys using them on jobsites are first-time operators. I guess what I'm saying there, is that we see plenty of hoses get damaged, by plenty of operators that aren't *thinking* their way through what controls they should/shouldn't touch next.

Manlifts like these are also designed the same way. No, you don't need rapid cycling with these machines, but it is a requirement on telehandlers like the SkyTrak above:

The manufacturers do build these cylinders better. Far better. Pull one apart and you'll find 2X or 3X at least more, (and better), sealing rings on the pistons and rod-end gland nut or cap. Everything is beefier and higher-quality from one end to the other. Actually, everything to do with the hydraulics is that way.....since the entire machine is designed to be a hydraulic power unit. Many tractors are designed from the beginning to be tractors first, with hydraulic systems being sort of an "accessory".

If you compared the innards to a typical simple cylinder you'd find for sale at the local Tractor Supply, there's well.....no comparison at all. Simple cylinders like the ones you can pick up for $100-200 for basic hydraulic systems are very much built to a price point. It's pretty amazing they operate as long as they do before leaks or other issues develop....

Well that's where I differ from you.....I see it all the time.....new tractor cylinders are not engineered the same as these pics you show of manlifts, cranes, and quality construction equipment......not the same internally.......

Just open up a Bobcat cyl and look at the piston seal.....it's .125 wide....They are a great machine but there cyls have always been chinsy....