Basic Question on Return and Return to Tank on Open Center Systems

[bigmarley98]

So I am new to Hydraulics but am in the process of adding rear remotes to my LX2610 that would come off the power beyond supply lines I use for my BH77 backhoe. I think I have it figured out, but came across something doing my research that I would like to understand - I don't think I need it for my configuration but it has been nagging at me.

My backhoe, the BH77, has two lines. One is the power coming in, and the other is the return which is under pressure. In doing some reasearch I came across someone that did something similar with a BH70 backhoe, but the difference is it has three lines - the Power, Return (under pressure), and a third line with is return to tank and from what I understand low pressure.

So - why the difference between these two backhoes? Why does one need a return to tank and not the other? The only thing I have come across is that some hdyraulic valves/cylindes/motors use oil not just for power but for lubrication, or there may be some leakage outside of the hydrualic circuit and they need to get rid of it. Would that be the case here - the BH70 has different compenents that need to get rid of excess oil, but the BH77 does not? I just want to make sure I am not missing anything here.

 

[ericm979]

Are you sure the single return from the BH can see full pressure? On my backhoe the valve stack has only one output and it's not a full pressure output. It's only good for about 750 psi. Since the backhoe loop is between the loader valve and the three point hitch, you're not supposed to operate the three point hitch with the backhoe mounted. Which is ok because you can't use it with the backhoe in place.

The difference between the two backhoes is that with one they spent more money on the valve stack and included a power beyond. That's the "right" way to do it but leaving it off is common because it doesn't matter in practice.

 

[bigmarley98]

That's a good point. The return is probably not full pressure on the BH77 as there is likely a drop after the backhoe, but it is still only two lines, where the BH70 has three lines, and I believe the pressure in the third return to tank line is less than 25 psi. As you mentioned it might have something to do with the different valve setup.

 

[K5lwq]

If you are adding a new valve for rear remotes, your new valve needs to have Power Beyond and you need 3 lines.

You will want to feed the backhoe with the Power Beyond of your new valve. You can tee the tank line of your new valve into the tank line from your loader.

 

[rScotty]

That's a good point. The return is probably not full pressure on the BH77 as there is likely a drop after the backhoe, but it is still only two lines, where the BH70 has three lines, and I believe the pressure in the third return to tank line is less than 25 psi. As you mentioned it might have something to do with the different valve setup.

In a two hose open center hydraulic system, there should not allow any restriction on the return to tank line. In fact, the return line hose is often a size larger diameter to make sure of that.
As you saw, that return line may have high flow.....and it may look like it could develop pressure...and it could if blocked....but it is important that it not be restricted. Just run it directly to the sump.

The reason to plumb the return line so it has no restriction is because of the way that the control valves work. When the control valve pressurizes the working side of the piston in a hydraulic cylinder, the other side of that piston is forcing displaced oil back to the control valve where an aux. circuit adds the unpressurized displaced oil into the return flow. For best operation, It is important that this return flow remain unrestricted so that no back pressure tries to push the cylinder back the other way.

Open center systems with 3 hoses use a different plumbing scheme that adds a Power Beyond capability. That PB is useful if you want hydraulic pressure available for some other task. Regardless of how many hydraulic implements are diasy-chained in series with an open center hydraulic circuit, the final hydraulic control valve - in your case this is the backhoe - will always be a two hose type, just pressure & return.
hope this helps,
rScotty

 

[bigmarley98]

Thanks, that helps - but while it answered my first question, it raised another that I thought I had understood . I will think on that one but may have to post a different thread. Just when I think I am certain on the hydraulics there is a new wrinkle....