Newbie with hydraulics question

[realtaildragger]

Really new to tractors. I understand my 'bota' needs to use open center hydraulic valves. Just guessing, but i surmise that the hyd pump is always in operation and must be bypassed from pressure to return when the pressure is not being used. Is this correct thinking? I have a FEL now, but if i add another valve must i observe the logic that at least 1 valve must bypass when no pressure is being used and that the valve NOT being used (ex FEL valve) must NOT bypass when the alternate valve (ex plow angle) is being used. Hope y'all are still following here /forums/images/graemlins/smile.gif

Any recommended reading or other sites for gaining hydraulic knowledge available?

Thanks.

 

[MadReferee]

Open center systems are typically used in applications where the pump is continuously running, like your tractor. Fluid will flow thru the valve when the spool control levers are in neutral position.

When a spool is activated some amount of fluid will flow thru the work port to the cylinder. The remaining fluid will continue to flow thru the valve and be available to other spools or valves downstream.

If you have more than one valve then it needs to have power beyond (PB) capability. Your loader valve has PB. All valves must be connected in series with the PB from one valve connecting to the inlet of the next valve. All valves will also have a return to tank outlet for fluid that is expelled from the work ports.

It all sounds complicated but in reality it is really simple. I am sure there are several internet sites that can be used for a reference on open center systems.

Many people have added aux valves on their tractors. Do a search here on TBN, sit down with your favorite beverage, and read away.

 

[techman]

You are correct in your thinking. The "open center" design hydraulics is based on the use of a fixed displacement pump, like a gear pump on the Bota. It works reasonably well, except if you want to be able to run several circuits simultaneously, you need to start adding what are called "priority" or "flow divider" valves. Since the valves are basically in series, the first valve in the line can steal all of the pressure from the pump unless you have a valve to prevent that from happening. This is how the steering circuit gets a gauranteed amout of oil through the use of a priority valve, giving a minimum fixed amount of oit to the steering. It would be bad if you raised the bucket while driving and lost power steering.

Open center designs start to become a problem on bigger machines. The pressure drop through all of the series connected valves, at higher flows, becomes a heat issue. Most bigger machinery uses a closed center, or pressure compensated design. In this configuration the pump is a variable displacement pump (much more expensive) the will provide a constant outlet pressure. Each circuit can then tap off of that high pressure header to run its cylinder, without effecting the others. Better control and lower losses, but a much costlier system.

paul

 

[MadReferee]

</font><font color="blue" class="small">( Works reasonably well, except if you want to be able to run several circuits simultaneously, you need to start adding what are called "priority" or "flow divider" valves. Since the valves are basically in series, the first valve in the line can steal all of the pressure from the pump unless you have a valve to prevent that from happening. This is how the steering circuit gets a gauranteed amout of oil through the use of a priority valve, giving a minimum fixed amount of oit to the steering. It would be bad if you raised the bucket while driving and lost power steering.)</font>

Kubota solved this problem by having seperate pumps for their power steering and attachment hydraulic circuits on B and L series and bigger tractors.

The flow divider valves you mentioned are only to seperate the PS from the attachment circuit. Once you are in the attachment circuit they have no significance when talking about adding aux hydraulic valves for rear remotes on the attrachment circuit for the smaller tractors.

There will always be flow thru the loader valve and all other downstream valves even if one spool is left open since a work port does not typically flow the same gpm as does the main valve gallery. This is what allows the loader to both curl and lift/lower at the same time (both spools activated).

I am guessing that the original poster was trying to educate himself on what to buy in order to add rear aux hydraulics for remotes, not so much as to learn the various design issues with different hydraulic circuits. The whole rear remote problem is not overly complex and is rather easy to solve, at least on Kubotas.

 

[btolle]

</font><font color="blue" class="small">( It all sounds complicated but in reality it is really simple. I am sure there are several internet sites that can be used for a reference on open center systems.)</font>

That's easy for you to say but I have had a heck of a time getting it straight in my mind!

Look at Bareco Hydraulic Schematic page

That page helped me a lot. Look at the third schematic down "OPEN CENTRE CIRCUIT WITH POWER BEYOND" to see how it works with 2 valves.

Bill Tolle

 

[techman]

Your comment isn't quite correct. On the BX tractors there is only 1 pump. With the steering circuit, if it were first in the circuit, steering could cause loss of pressure for other functions. If it were downstream, impements could steal flow/pressure from steering. The BK's use a priority (flow) valve (item 8 on Kubota schamatic T12960HY00101). It connects directly to the output of the pump and has an output for the implements, and another output for the steering.

This valve gaurantees that the steering circuit will always have "priority" in getting flow. If the implement circuit tries to take too much oil, the flow control chokes the flow to the implement so that a minimum is fed to the steering. It is needed to provide operational safety. Here is an example of a similar valve:

Click here for link.

It is quite easy to swallow all of the available oil in one cylinder. The "galleys" are not the flow restriction, but rather the valve spool openings. Normally the drops are approximately equal in all flow paths and keeping drops as low as possible is a goal to limit heat. Add a little load induced pressure and the full flow can be consumed by one cylinder circuit. You can easily see this on the BH circuits. No matter how many valves are in the circuit, an upstream circuit can generally grab all of the flow available.

paul


EDIT: Shortened long link.

 

[MadReferee]

Techman,
I specifically said B and L series and larger tractors. I am well aware that the BX series has only one pump.

 

[realtaildragger]

Wow... you folks are a great resource. Thanks soo much for all your input. I am getting the gist of the open center with power beyond, but still need to study the diagrams a bit more to be confident. MadReferee hit the nail in the head when he guessed that i was trying to add on remote outlets. I checked out the PBY outlet of the FEL valve and was surprised to find a hose on it even though it is the only valve... then i realized it must be feeding back to the 3 point hitch valve. Also, this tractor seems not to have power steering unless it is hidden internally.

Thanks again to all.

 

[btolle]

</font><font color="blue" class="small">( Also, this tractor seems not to have power steering unless it is hidden internally. )</font>

Tractor power steering runs off the internal pump. It is not like a car where you have a dedicated power steering pump mounted externally on the engine.

I am not familiar with the L245DT that you have so I don't know if it came with power steering or not.

Bill Tolle

 

[MadReferee]

You will have no problem connecting up rear remotes for your tractor. I have posted several times the correct method for connecting everything up. And yes the loader is first in the circuit and the 3pt is last on your tractor, that's why the loader vavle has PB.

If you are looking for a ready to go, fairly inexpensive kit with 2 remotes, then check out the TNT kit that CCM sells. You can get it for around $600 and includes just about everything you will need.

CCM TNT kits