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post #1 of 17 Old 01-31-2013, 12:43 PM Thread Starter
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Hydraulics 101

While considering a comment I wanted to make in another forum, I had to stop and consider my basic understanding of hydraulics. Some basic information that I have regarding how hydraulics work, that once seemed clear enough to me, all of a sudden became more fuzzy. Basically, there is what I think I know, and then there is reality. And being aware that reality is rather more complicated then my simple understanding of how hydraulics work, I'd like to try and fill in, maybe just a bit, the gaps in that knowledge base.

I assume in the case of tractors in general, that system PSI is held constant by means of a pressure compensator, regardless of Flow rate or Engine RPM. If system pressure can be maintained at a set working value, I then assume that a maximum quantity of (Work = force x distance), can also be accomplished regardless of Flow rate or Engine RPM.

Example. If a FEL is rated to pick 1000 LBS maximum to full height, then so long as operating PSI is maintained, that FEL should be able to lift that Maximum load of 1000 LBS to full height regardless of Flow rate or Engine RPM. The difference being the time required to move that force through the maximum distance. (Lift Height).

Thus, the ability to do work remains constant where the (Power = Work/Time) is variable.

Power is therefore dependent on flow rate, and flow rate is dependent on the pumps capacity to produce flow, which is in turn is dependent upon the Engines RPM's.

It then follows that two different hydraulic configurations could produce the same amount of work, but not necessarily the same amount of hydraulic power. The importance of which comes into play where a given piece of equipment requires a set amount of Power in order to operate effectively.



This is my general understanding of how hydraulics work. Can someone confirm, deny or help fill in the gaps here. I realize not every hydraulic system is built exactly the same. So I tried to keep this a general as I could. The million foot view if you will.
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post #2 of 17 Old 01-31-2013, 02:41 PM
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Re: Hydraulics 101

For a closed center hydraulic system, you are essentially correct. GTs and the smaller end of the bigger tractor spectrum use open center circuitry which is less expensive and simpler.

For an open circuit system, you have the right idea for flow being controlled by engine rpm, but pressure is controlled by how much work is being done and limited by the relief valve which is there to:

a) Protect the system components from over pressure situations.

b) Limit the amount of work that can be accomplished to protect the equipment (tractor).

For instance; Commercially produced FELs for dedicated GT use generally have a low setting (400-600 psi) for relief pressure, or have small bore cylinders with a higher relief setting, and are designed to protect the tractor from overload (read dangerous) conditions. Homebuilt FELs do not have the same protection since most relief valves are built into the valve set and have a low setting of 1500 psi instead of 400 - 600 psi. That difference can easily overload the front tires, spindles axle, axle pivot, and the front frame on some tractors. Mostly, what will happen is that the rear wheels will come off the ground until enough counterweight is applied to keep them on the ground with a given lift, or until so much counterweight is applied that the rear axle is damaged from overload.

At the top of the thread list for the Hydraulics forum are a couple of stickies. One of them links to what used to be the Vickers Hydraulic Manual. It will help you achieve a better understanding of hydraulics principals.

Bob

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post #3 of 17 Old 01-31-2013, 03:00 PM Thread Starter
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Re: Hydraulics 101

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Originally Posted by TUDOR View Post
For a closed center hydraulic system, you are essentially correct. GTs and the smaller end of the bigger tractor spectrum use open center circuitry which is less expensive and simpler.

For an open circuit system, you have the right idea for flow being controlled by engine rpm, but pressure is controlled by how much work is being done and limited by the relief valve which is there to:

a) Protect the system components from over pressure situations.

b) Limit the amount of work that can be accomplished to protect the equipment (tractor).

For instance; Commercially produced FELs for dedicated GT use generally have a low setting (400-600 psi) for relief pressure, or have small bore cylinders with a higher relief setting, and are designed to protect the tractor from overload (read dangerous) conditions. Homebuilt FELs do not have the same protection since most relief valves are built into the valve set and have a low setting of 1500 psi instead of 400 - 600 psi. That difference can easily overload the front tires, spindles axle, axle pivot, and the front frame on some tractors. Mostly, what will happen is that the rear wheels will come off the ground until enough counterweight is applied to keep them on the ground with a given lift, or until so much counterweight is applied that the rear axle is damaged from overload.

At the top of the thread list for the Hydraulics forum are a couple of stickies. One of them links to what used to be the Vickers Hydraulic Manual. It will help you achieve a better understanding of hydraulics principals.
Would it be foolish to assume, as I have, that SCUT's such as the BX25 and the GC1700 series have a "Closed Center Hydraulic System", where by a set operating PSI is maintained by a Pressure Compensator? I actually have an email into AGCO to inquire about this as it applies to the GC1700 series. I have looked through all readily available literature on the GC Series, and called my local Dealer who was fuzzy at Best on this, and I have not be able to find anything more then the the fact that the GC has an operating PSI of 1920. But no details specifying if this is a constant PSI, or as you suggest in an "Open Center Hydraulic System", the pressure is determined by the work being requested, and limited by a Relief Valve.

This all matters with regards to my understanding of the GC, or for that matter any other tractors "True Capacity". One has to have a solid understanding of how the Hydraulics operate with regards to how you run the Engine in order to see the larger picture. Specs on a sheet are fine, but specs alone can "Color" the reality of what a tractor is actually capable of.

Do you have any sense with regards to the BX25 and or GC Series of what kind of Hydraulic system we are talking about? As I said, I Assumed a Constant Working PSI.
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post #4 of 17 Old 01-31-2013, 03:38 PM
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Re: Hydraulics 101

No, they don't have a constant working pressure. I have a GC2310, and it is open circuit.

You will find that engine rpm is what you will be more concerned with than the pressure. The pressure increases automatically with the amount of work being done. The speed at which the work is done can be scary and very uncomfortable if the throttle is set too high.

As long as the work being attempted does not require pressure in excess of the relief setting, it will get done. After time you will gain enough experience to recognise what can or can't be done by the tractor by looking at the load and estimating the weight involved. For a GC, that's about 900 lb max to get it off the ground and transport it. It will let you know if it's too heavy. It won't lift the load and it will scream like a banshee if you keep trying.

Open circuit means the oil has a free flow path from the pump, through the valve and back to tank. When the valve is moved, the oil is redirected to the cylinders to do work, and that is where the pressure is developed. Oil going over a relief valve creates heat for no functional purpose.

Bob

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post #5 of 17 Old 01-31-2013, 03:52 PM Thread Starter
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Re: Hydraulics 101

Thank you Tudor. That go's a long way to helping me understand what is going on. There are many things that I have "Assumed" based on what my dealer has been able to tell me, or not tell me, and the available literature. I have not read anywhere or told directly by the dealer that this was an Open Circuit Hydraulic System. This changes my understanding of what the potential differences between the 1720 and 1710 are. That of course leads to more questions. But I think I will post those questions in another thread already running. Back to the drawing board here for me.

And thanks again for clearing that up. Maybe you should start selling these instead =)
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post #6 of 17 Old 01-31-2013, 07:08 PM
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Re: Hydraulics 101

Hydraulically, there isn't any difference between the 1710 and 1720, other than the higher rpm creating slightly more flow on the 1720.

I leave the selling to others and spend my time answering those questions that I can answer, thank you. It's a lot easier on the nerves.

Bob

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post #7 of 17 Old 03-18-2013, 02:55 PM
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Re: Hydraulics 101

Hello all,,,
Great form.. I have worked with Hydraulic systems, but just replaced parts.. Could someone tell me what is the difference between an "open" an "closed systems"?????

Thanks

Bord
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post #8 of 17 Old 03-18-2013, 03:42 PM
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Re: Hydraulics 101

Tudor, you ARE "Da Man"! I always appreciate your posts!
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post #9 of 17 Old 03-18-2013, 04:45 PM
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Re: Hydraulics 101

No, just The Bord... With a bit of luck you might enjoy some of my posts,,,, but I need to get a bit better at finding my way around.
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post #10 of 17 Old 03-18-2013, 05:44 PM
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Re: Hydraulics 101

??? does a convertible top pump have a pop off on them
if they do i never found it
put one under my 7300 pound motor home to lift the back end to level it
could lift 3 sides with one ram 6"
thats why i ask

john
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post #11 of 17 Old 03-19-2013, 12:51 AM
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Re: Hydraulics 101

Quote:
Originally Posted by The_Bord View Post
Hello all,,,
Great form.. I have worked with Hydraulic systems, but just replaced parts.. Could someone tell me what is the difference between an "open" an "closed systems"?????

Thanks

Bord
The basic difference is in that open circuit systems, the fluid has a free flowing path from the pump, through all the valves in their center position, and back to the reservoir and the closed circuit system does not.

In a closed circuit system, other means (components) are used to reduce oil flow when it is not required, or otherwise redirect the flow back to the reservoir while at the same time, maintaining system pressure and reducing the return flow pressure without creating excessive heat.

Open circuit systems are simpler, less expensive to build and maintain, and are found in most small mobile applications. Closed circuit systems use more components and more expensive components, and are usually found in larger agricultural and commercial equipment.

Bob

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post #12 of 17 Old 03-19-2013, 01:12 AM
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Re: Hydraulics 101

Quote:
Originally Posted by no2498 View Post
??? does a convertible top pump have a pop off on them
if they do i never found it
put one under my 7300 pound motor home to lift the back end to level it
could lift 3 sides with one ram 6"
thats why i ask

john
I'm assuming that you are referring to the pump used to raise and lower the top of a car? The load for the designed task is not that great and there is a fuse or circuit breaker protecting the motor from overload. A relief valve is redundant.

I don't know what pressure they are rated for, but I would hazard a guess from comments that I've seen that they will handle maybe 300 psi and maybe even a bit more. The cylinders are thinwall aluminum tubing and the hoses have crimped ferrels holding them on the nipples, neither of which gives me a lot of confidence for a pressure rating much higher, but then, I may be too conservative. The car companies don't want hydrulic fluid spraying around in high end cars due to poor quality hoses and cylinders.

With that said, a cylinder with a 6" bore will lift 7300 lb at 258 psi.

Bob

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Last edited by TUDOR; 03-19-2013 at 01:19 AM.
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post #13 of 17 Old 03-20-2013, 01:14 PM
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Re: Hydraulics 101

no it was 6" long like 1 1/2 round
sorry about that
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post #14 of 17 Old 03-20-2013, 02:41 PM
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Re: Hydraulics 101

With a 1.5" bore, it takes 2065 psi to lift 3650 lb. That's pushing the envelope even with steel GT implement cylinders.

Bob

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post #15 of 17 Old 03-20-2013, 11:46 PM
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Re: Hydraulics 101

Hello Tudor did some changes to my fel hydo system was having a whineing sound from the pump and when powering the fel it would get louder Well after talking it over where I pur my hydro stuff decided it could be the lines had 1/2 from tank to filter to pump to spool valves then 3/8 hose and pipes . Replaced the 1/2 to 3/4 from tank to pump and no more wineing and fel moves lot faster. Much better now.
Now to finish up the snow blower 196 930 to my 990 it was from a vert drive system ,making it work off the rear pto added a 3rd pullely to it .

Want to extend my exhaust to the rear what size pipe would you sugest?

Bernard Camarro
Fairfield Conn
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