Simple hydraulic question (I think)

[TSL]

Greetings all. I am new to the forum so forgive me if this has been asked before.
I am trying to design an attachment for my JD 955 compact utility tractor which will have 4 double acting cylinders on it and I need to be able to activate multiple cylinders at the same time. The 955 has an open center hydraulic system which, if I understand correctly, does not lend itself well to activating more than one valve at a time because they have to be plumbed in series. So, my thought was to add a second hydraulic system which would be of the closed center type utilizing a fixed displacement pump connected to the tractor's PTO. My question is: Since this type of pump is pumping at capacity all of the time, how do you relieve the pressure when none of the valves are activated? (Please be gentle when replying as my understanding of hydraulics is rudimentary at best!)

BTW, this is a great forum and I have already learned a lot just by reading some of the posts.

Thanks,

Larry

 

[J_J]

You can't use constant pressure pump unless it is used with an unloader.

Closed center circuits exist in two basic configurations, normally related to the regulator for the variable pump that supplies the oil

Constant pressure systems (CP-system), unloaded. Same basic configuration as 'standard' CP-system but the pump is unloaded to a low stand-by pressure when all valves are in neutral position. Not so fast response as standard CP but pump lifetime is prolonged.

Load-sensing systems (LS-system) generates less power losses as the pump can reduce both flow and pressure to match the load requirements, but requires more tuning than the CP-system with respect to system stability. The LS-system also requires additional logical valves and compensator valves in the directional valves, thus it is technically more complex and more expensive than the CP-system. The LS-system system generates a constant power loss related to the regulating pressure drop for the pump regulator.

JD955 hyd.

John Deere 955 Power:
Engine: 33 hp [24.6 kW]
Drawbar (claimed): 23 hp [17.2 kW]
PTO (claimed): 27 hp [20.1 kW]

Hydraulics:
Type: open center
Capacity: 4.5 gal [17.0 L]
Pressure: 2500 psi [172.4 bar]
Valves: 0 to 2
Total flow: 7.2 gpm [27.3 lpm]


Why would you want to mix systems, as that involves new pumps tank, valves, regulators. etc.

Your JD 955 has only so much HP to power a hyd pump for attachments.

If you want more hyd, you need a larger engine, larger pump.

You can get additional hyd by using a PTO pump, but you are still limited by the HP/PTO HP.

 

[TSL]

J.J.:
Thanks much for your reply. I may, indeed, be going down the wrong rabbit hole. I really don't care if I use an open or closed center system as long as I can activate more than one valve at a time. I was under the impression that this is not possible with an open center system. Is there a way to use the 955's hydraulics and still be able to use two or more valves simultaneously? Also, I will need to use electrically activated valves tied into laser receivers. Don't know if this makes any difference but thought I should mention it.
Thanks again,
Larry

 

[J_J]

In an open center hyd system, the fluid flows through all valves in series.

The first valve will have priority and if it is using all the flow, there will be none left for the other valves.

If you are only using some of the flow by small lever movements, you will pass the remaining flow for downstream valves.

The last valve in the system will work if other valves are not using the fluid.

You could however use a valve called a priority flow divider to ensure one circuit will get a certain GPM and the remaining flow will go to the other valves.

If you are using solenoid valves, they open fully using all available fluid.

 

[TSL]

J.J.:
You have confirmed what I thought I understood. So, given that the 955 has an open hydraulic system and that I need to simultaneously activate more than one valve, and that I need to use electric valves, what do you suggest? BTW, you clearly have a much greater knowledge of these things than I do. I very much appreciate you taking the time to help.
Larry

 

[TSL]

J.J.
I should have mentioned that at least two of the valves must have equal flow capability at all times. In other words, I don't think that the priority flow divider you mentioned would work.

 

[J_J]

This valve will provide a 50/50 split from 0 to 8 GPM's.

Surplus Center - 3/8" 0-8 GPM DIVIDER VALVE

So if the output from this valve goes to two solenoid valves, they will see the same flow.

Each valve will get half of the flow.

 

[Zebrafive]

What are you making that will use four cylinders? Why is must cylinders be able to operate at the same time? Might be more than one way to do what you want it to do.

 

[TSL]

J.J.:
Wow! This just might work. So let me see if I have this straight: I should divide the load into 2 open center circuits, each of which contains one of the cylinders which must be able to move at any time, placing their valves furthest upstream (closest to the pump) and then also include one of other lesser priority cylinder/valves. Then place the Divider Valve between the pump and these two circuits and use it to supply each circuit. If I knew how, I'd draw a picture and attach it---might save a lot of words. Anyway, do I have this about right?
Larry

 

[TSL]

Zebrafive:

I am building a grader blade to go between the two axles. Two lift cylinders (one on each side), one cylinder to rotate the blade and one to side-shift the blade. The two lift cylinders are the ones that must be able to move up or down all the time, i.e. at the same time. It would be nice if the rotating and side-shift cylinders were could also move all the time but isn't absolutely necessary.
Larry