Hydraulic Question

[minidozer]

My understanding of a simple hydraulic system is that the hydraulic pump sucks fluid from a sump and pumps it to a pressure relief valve that adjusts the pressure of the system up to the capacity of the pump. Fluid from the relief valve goes back to the sump. To operate equipment, a branch off the pressure side of the pump controlled by a valve creates another pressure loop ending in a return to the sump.

My question is: As long as the relief valve is working, how can the hydraulic pump deadhead when a coupling comes apart?

 

[EFC]

The pressure relief is at the end of the system furthest away from the pump not at the pump. When the disconnects come apart the pressure relief is not in the system.

buck

 

[minidozer]

That sounds like the coupling is in series not parallel. Why would you design a system that way especially with couplings? For safety reasons, shouldn't a person with a coupling in series add another relief valve?

 

[RonMar]

It depends on the type of system. A system such as you described would provide hydraulic pressure all the time but when you were not actually using the hydraulics, the engine would still be under load pressurizing the system to the pressure set by that relief valve.

Most systems are open center. In those systems, the hydraulic fluid flows with little restriction through the pump and the open center of any control valves and back to the sump. When the hydraulics are needed, the valve diverts the supply side of this flow to one side of a hydraulic cylinder or motor, the pressure builds, the cylinder or motor moves and work is accomplished. At the same time the control valve connects the other side of the cylinder/motor to the return line to receive the fluid displaced by the moving cylinder/motor. Each of these valves has a safety valve in it that will connect the input from the pump directly to the output and bypass the load if the cylinder or motor is overloaded or at the end of it's travel. This type system only places a load on the power source when the hydraulics are actually being used.


As an example, my tractor has 3 safeties on it. The first is on the loader valve. The next thing in line is the diverter or flow divider valve that sends fluid to the steering system or 3PH as needed. That diverter has a safety on the steering circuit that returns directly to the resovoir if I should overload the steering cylinder. From the flow diverter the main fluid flow is back to the 3PH valve which in the lower or neutral position flows directly back into the resovoir tank. There is a safety in that valve assembly that flows back to the resovoir if the 3PH lift limit is exceeded when the valve is in the lift or neutral(hold) position. In my system, the first thing after the pump are the quick connects that connect the input and output of the loader spool valve. As stated, if you disconnect these, this blocks the flow completly. On a completely blocked flow without a safety bypass to the resovoir, that pressure will find a "non-standard" route to vent itself or will stall the powerplant.

 

[RonMar]

</font><font color="blue" class="small">( That sounds like the coupling is in series not parallel. Why would you design a system that way especially with couplings? For safety reasons, shouldn't a person with a coupling in series add another relief valve? )</font>

You are correct it is in series and yes, a relief at the pump output would be a good thing. I recall a thread on this forum not too long ago about an individual that did just that. On the other hand, valves and plumbing cost money. If you know not to disconnect these fittings under load or leave them disconnected when attempting to run the machine and you take precautions to keep the quick connects from being opened accidentally(tiewraps on the QC collars, enclosure around the fittings) then these things have and will continue to operate simply and efficiently.

 

[Soundguy]

</font><font color="blue" class="small">( Why would you design a system that way especially with couplings? For safety reasons, shouldn't a person with a coupling in series add another relief valve? )</font>

Makes ya wonder if that money ya saved was really a savings /forums/images/graemlins/grin.gif

Soundguy

 

[minidozer]

Ron, Thanks for your answer. I understand why you would not want to pressurize the system when not needed. If I wanted to use a log splitter or add a cylinder to provide downward force on a post hole digger or box scrapper on my dozer, how would I plumb it considering this "open centered system"?

 

[RonMar]

Well the specifics depends on how your system is configured but basically you would need to put a single spool open center control valve in series with the main flow line of your system. The input port on this valve would of course be on the side coming from the pump and the output port on the valve would be on the downstream side headed for the resovoir. The two working ports would go to the ports on a dual acting cylinder. You can see some examples of these on the northern hydraulics catalog/website. The Prince Hydraulics website has some good basic system drawings also. http://www.princehyd.com/LinkClick.aspx?link=products/valves/RD5000InstS.pdf

The valve would of course need a safety valve in it that bypasses the fluid from the input to the output side if overloaded. Does your hydraulic system have any accesory ports on it or a installed quick connect in series that is not being used? Many tractors come with a quick connect right off the hydraulic pump that is just connected right on to the rest of the system. This allows for a quick installation of a front end loader spool valve in series with the rest of the system. Some tractors have a supply and return port on the rear of the tractor either with a jumper installed between them to allow flow when not being used to power an accesory or they are driven off the 3PH valve and require some valve setup to apply pressure to the supply port. And some systems such as the one pictured in the above link can be configured with a "Power Beyond" port to allow an additional circuit.

 

[minidozer]

My hydraulic system is "hard" plumbed and consists of a gear type G5-12 pump (6.6gpm) and a 4 spool control valve with relief valve set at 2230psi. There are no add on ports or couplings that I can see which is the way I would want it since I assume it is an open centered system.

I see that Northern sells an adjustable relief valve for $26.99.

It looks as if I need to go back to school. I have FM 5-499 Hydraulics which should keep me busy for a while.

 

[EFC]

Soundguy, you are right but it is almost impossible to make the tractor "customer proof". I had no end of blown pumps even though we sat down with each customer and went through what would happen if they opened the disconnects. I even made up a handbook that talked about how the system worked and what not to do. Didn’t matter one bit. So I ate the cost and started installing pressure relief’s at the pump. I had to set the pressure on the relief higher than the working load of the system. It worked fine except customers would pull the disconnects apart while taking the loader off or the like and let it run for 20 min or so. To overcome the pressure relief the pump has to go to almost full load and it gets hot after a short period of time. I was actually replacing the same amount of pumps with the same amount of mad customers. I guess I could put in a flashing light that pops up out of the dash, hmmmm.

Buck