Hydraulics questions (a bit long)

Don If I can remember my basic hydraulic classes of so long ago, there are two types of hyd. systems. Open center and closed center types. The difference centers around the type of pump used..The closed center type only flows then there is some demand, and the open center flows constantly, broadly speaking.

To design a hyd attachment would start with the type of pump you have, if any, and what type of system ie open or closed center. Give us some more info--what type of tractor,power steering, brakes etc. Are you retrofitting an older 9N type or is this something a more modern/larger?

DE

I'd also like some good links or recomended texts on Hydraulics as I'm starting to 'play' with them. First project will be a press. I have a powerpack (AC 5Hp) already but I don't want to 'screw it up' I have found and understand the basic theories, but could anyone suggest a good place to get a glossary or explanaition of terms like spools, 2 3 4 way valves etc.

I understand the Army (US) had a publication, but since most of those sites were secured last year you cannot download the docs anymore.

Any reference texts (basics) you recommend?

Most of what I know was found in the Hydraulics 101 section of

Sorry Don, can't answer your questions but thought I'd ask on this thread anyway.

Tom

snipped-for-privacy@hotmail.com (surftom) wrote in news: snipped-for-privacy@posting.google.com:

I suggest the open center valve, since constant flowing fluid requires less energy than running everything through the pressure relief back to tank. There will still be some go through the relief, depending on your pump and the rest of the plumbing. The open center, to my understanding, will, as you move the spool off- center, divert flow from the T line to either A or B ports. when the valve returns to center, all flow goes to T. Remember, unlike pneumatic systems, you have a constant flow of fluid. It has to flow somewhere. If it doesn't flow back to the tank through the spool, it will have to go through the pressure relief, meaning your engine will have to produce enough power to maintain the max system pressure at all times, not the best way...you only need pressure when there is work to do.

The Army hydraulics manual is at:

and there are some other interesting links at under the links to learning section.

The Army manual and supplier catalogs, and deja are the things which I have found the most useful in figuring out stuff. I don't think it's all that hard, you just have to understand the lingo (I think ;) )

don

Okay, a brief description:

It's going to be a self contained unit for a compact/garden tractor (PowerKing). So I will be purchasing a dedicated pump and all the associated plumbing for the project, since the current hydro pump is of much lower flow (like 4-6GPM I think). The tractor has a 14HP motor, so I should be able to run at least 10GPM at 1500PSI, possibly slightly more. I believe that most loaders have 10-12GPM pumps. Since it's a relatively small tractor the lifting limit is going to be limited by the size of the tractor (i.e. it'll fall over forward) rather then by the hydraulic system, but I would like it to be fairly quick to go up/down. (Unless convienced otherwise) I was planning on using an open-center system since that seems to be more popular and common. Seems like it might be a bit simplier also.

I guess that's it...any advice is welcome.

Thanks,

don

Your best bet is the simplest type of open-center system. This has a pump that pumps a constant volume and in the "idle" condition, with all the spools at center position, there is free flow through the system and the only power consumed (and heat generated) is from friction loss in the pump and plumbing. Almost any other type of system is far more expensive and complicated.

Open-center valves are so common that the term has come to mean what your old text calls "tandem". In the center position there is free flow from P to T, and A and B are shut off and blocked, so that whatever cylinder is connected to A and B is locked in position.

There is typically only a single pressure relief in the system, built in to the valve body, set at whatever the safe limit is for the pump, hoses, valve body, cylinders etc. This could be anywhere from 1500 to 3000 psi for a minimum-cost system. Won't find it in minimum-cost equipment.

If your cylinder bores and mechanical linkages are sized properly, there is no need for separate pressure relief settings for each cylinder. Pressure and flow limiting comes into play only in equipment where the operator can plausibly break something, and for some reason it's easier to protect the equipment by pressure limiting than by sizing the cylinder.

The limitation of open-center is that to get predictable response at the cylinders, you can operate only one spool at a time. For example raise the boom, tilt the bucket, raise the boom, tilt the bucket.

In an open-center system, with more than one spool open there is nothing to distribute or balance the pressure between the active spools and their cylinders, so all the flow will go to whichever cylinder is presenting the least resistance to work. In an extreme case, the fluid might even flow backwards through a spool. For example raising a boom (lots of effort) while lowering a bucket (low effort) may result in the boom at first falling instead of rising, until the bucket has tilted to its extreme.

But the cost advantage is so strong that you just deal with it by operating only one at a time. with all your spools

Closed-center can mean a number of different types of systems. It can be the same constant-volume pump that in the "idle" condition is pumping its full output against a pressure relief valve set at a full system pressure of say 2500 psi. The advantage of this design is to the extent that the pump can supply the required volume, multiple spools can be opened with predictable flow in response to each's position independent of the load on the cylinders. Burst-flow at full pressure can be had by adding an accumulator, that is a bladder pressure tank. This is probably the second-cheapest system design and is used for equipment that requires precise operation of multiple spools at the same time while still keeping the cost reasonable. Its behavior has more in common with compressed air systems. The main disadvantage of this type is that the system is running continuously at full power and the entire output of the engine and pump are going into heat that must be blown off in a radiator.

More sophisticated closed-center systems use variable-volume pumps that maintain constant pressure at all the spools while wasting far less power. There are also more sophisticated multiple-spool valves that behave like open-center valves when all the spools are at center.

A "load check" is a one-way flow valve that prevents backwards flow through a spool when multiple spools are open in an open-center system. It has nothing to do with pressure relief or with whether the valve blocks flow or "floats" when at center.

The only disadvantage of using a spool that is rated for a much higher flow than your application is that it may provide less of a "fine touch" control than a smaller valve. It should not be a problem for what you describe.

"Normal" multiple-spool valves are parallel flow which has the issue described above, with all flow going the path of least resistance when multiple spools are active.

Bob

On 28 Jan 2004 10:46:21 -0800, snipped-for-privacy@hotmail.com (surftom) brought forth from the murky depths:

Try your local library. I found a fun little 1,000 page book titled Audel's "Pumps, Hydraulics, Air Compressors; A practical guide covering theory, constructon, and operation of modern pumps, hydraulic machinery, air compressors and blowers" (How's that for a title?) Author: Graham, Frank Duncan

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On Wed, 28 Jan 2004 22:32:42 GMT, Anthony vaguely proposed a theory ......and in reply I say!:

No kidding there. If you run a hnydraulic FEL, and let it keep pumping when the ram has reached full stroke, you feel even a quite powerful machine (120HP) hesitate. I have been caught by the crowd a couple of times (the lift is a little more obviuos ). A little tractor will die, at anything other than something approaching full revs, trying to run the max pressure of a hydro system of any grunt. I learned this when I took a backhoe off a Massey 40 tractor with FEL/backhoe and forgot to join the out and in hoses. They have an auto stop on them. I wanted to move the tractor a few yards, and it just would not run.

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Spike....Spike? Hello?

Don, What you described is doable, there is a good description of a basic hydraulic system below. You would need a pump, tank, double spool valve, cylinders press relief,filter and plumbing. Sounds a lot like a script for an upcoming Monster Garage......:-) While I can't say I am familar what your specific unit is.... but check this out

This has a very well designed hydraulic system, closed center I do believe, compact and integrated, the pump is an axial piston servo type, a spitting image to the big ones used in the heavy ag stuff. I had one apart years ago IIRC it couples internally to a motor for the drive. and has two fittings for the remote lines. You can see the valve handles along the steering wheel. If this would fit into your unit it might be a lot more cost efficient not to mention simpler than buying just the pump. There seems to be a fair supply of these around the series1 Kohler engines evidently had a serious problems staying together. I know loader att. were used on these units. email for more info if needed Good luck Dan

We used both Industrial Hydraulic Technology by Parker and Industrial Hydraulics Manual by Vickers in a hydraulics class I recently took. Both books cover basic hydraulics and include color drawings indicating pressures in a variety of circuits. The Vickers book has an additional chapter on basic electricity and electronics.

Nate

--

Can an automotive power steering pump be used for hobby uses such as a press?

Gunner

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If I was doing this I would not use oversize valves or pump . With small bore rams it will go up and down too damned fast . You will never be able to feather the loader like you will need at times . That plus smaller components will be a lot cheaper to buy . Ask any industrial supplier and they will be able to help you size the components to where they will work with what is already proven acceptable . Ken Cutt

On Thu, 29 Jan 2004 01:20:19 GMT, "Toolbert" vaguely proposed a theory ......and in reply I say!:

Lovely. Seriously. I have learnt all this after much searching. It is beautifully put.

Only thing....

A large flow spool with little feel can be a problem if the unit is to be used as a "crane" (and if you say it wont' you LIE ), and you don't have the feel.

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On Thu, 29 Jan 2004 01:20:19 GMT, "Toolbert" vaguely proposed a theory ......and in reply I say!:

OK. You give a good impression of being able to put the truth across.

I have a backhoe. I find that some of the controls for the digger (slew in particular) are way too fast. I wanted to place a flow limiter in the circuit, preferably adjustable within limits, so that I do not have to be _so_ careful with the slew controls when working, especially in confined sirrounds.

I was told this would not work, as it would force fluid back through the pressure release/return all the time and heat the fluid too much.

The system is quite large for the machine (industrail, not farm setup on a little Massye 40) and has about 30-40 litres of tank capacity. I spend a lot of time "feathering" the controls anyway, which is going to have the same effect, no? Is there really a problem?

Thanks for you attention and any help.

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Spike....Spike? Hello?

I haven't had the pleasure of operating anything with high-quality fine touch control. It's second nature to just throttle down. I spent a bit of last summer operating a little excavator within inches of a building and tried hard to not punch any extra holes. Spent a lot of time near idle.

I think with a farm tractor you just deal with its limits. It'll have enough other limits. If the hydraulic system is not properly sized or designed it'll still get the job done, just with more risk of breaking something. (An engineeered system will have all the cylinder bores sized to the mechanical design so full pressure won't bend things.)

Adding a flow limiter is beyond my depth. I like the suggestion about going with smaller hoses, though there is more risk of blowing the hoses with pressure surges from mechanical shock.

Bob

On Thu, 29 Jan 2004 09:24:15 -0600, Roy J vaguely proposed a theory ......and in reply I say!:

I wondered about that or longer runs, but was concerned about the same "problem" that was presented by a reducing valve.

I would not be surprised if the hoses are the wrong size anyway . Old machine

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