This circuit is used to charge the brake accumulator then the dump/hoist and steering pilot circuit etc. on a 7.5 yard scoop. I'm a little green and trying to make sense of the print but don't understand the priority flow control valve symbol. Anyone care to take a crack at explaining it to me?
What is the TP1 and the INLET port connected to?
TP1 is a test port, a gear pump feeds the INLET port.
The pump flow comes in at port #8. Flows through orifice 4 to the check valve at the charge valve. Tees out at the charge valve spool and goes down to the park brake valve, hand pump check check valve, ( to release the park brake in case of pump failure), to the brake accumulator. The brake accumulator will charge automatically as long as there is pump flow. Once the accumulator if fully charged the pressure builds and will spool the charge valve against the spring and open the pump flow to the rest of the machine. The purpose is to maintain brake and steering pressure before the rest of the machine. May not be exact but that's the general idea.
What the priority valve symbol means in detail I dont know. I've not seen a priority valve symbol like its before but in general, its job is to take pump oil in at the left side then send oil firstly to the right side to make sure the accumulator charging valve (item 5) has the brake accumulator fully charged. Once that's happened, the accumulator charging valve will close. (The pressures acting on both sides of the spool will equalise, but the spool will be pushed to the left by the bias given by the adjustable spring you can see on the left side) Now the priority valve is going to send flow out of the bottom to the fan motor. This then returns to the valve to go through a filter (8) and become pilot pressure limited by relief valve (7). Excess oil from this relief valve goes to the cooler whose internal pressure is limited by relief valve (6).
Cmark is pretty much spot on with the operation of the priority valve and the rest of the circuit as well. And to agree with him as well, as I view the schematic, I see nothing that indicates it operates anything other that the brake circuit (along with accumulator on that circuit), the charge valve (which is nothing more that a circuit pressure regulator), and the fan motor, which as Cmark noted, return oil from fan motor goes on to become pilot system pressure supply. I see nothing on this diagram that indicates it powers steering or loader functions other than possible pilot controls of these functions. Back to the priority valve symbol, it is a priority style flow regulator first of all, non adjustable orifice. The line coming out of the side of the orifice going to the fan motor is a by-pass (or secondary) circuit. Priority flow first goes through the orifice to the brake circuit, once that circuit is satisfied with pressure, the valve allows oil to the by-pass circuit (fan motor). The arrows going perpendicular through the priority and by-pass circuits indicate they are pressure compensated, meaning, their ports open or close to account for changes in input pressure/flow (such as throttling of engine) or changes in demand downstream to maintain a constant flow/pressure on the circuits. The thing I haven't seen before is the fourth line (additional by-pass circuit) going to the charge valve. That circuit is not pressure compensated, rather it goes through the .020 orifice before going to the charge valve. That line is basically a sensory circuit that is part of what makes the charge valve do its job of regulating circuit pressure. Here's a typical diagram from Parker of a priority pressure compensated flow regulator with a secondary circuit.
Thanks Willie. Looking again at the "sensing" line from the priority valve - orifice - charging valve I'm thinking it's maybe to "switch over" the priority valve, that is... When the accumulator pressure is low [zero], the charging valve spool moves to the right, this allows the sensing line to drain to tank which (presumably) somehow causes the priority valve to stop flow to the bottom (fan) circuit. As the accumulator pressure rises, the charging valve spool moves to the left (as previously explained) which blocks the sensing line, which (also presumably) tells the priority valve to send flow to the fan circuit. As the accumulator pressure slowly drops through natural leakage or brake application, the pressure is maintained through the orifice in the priority valve and the check valve in the charging valve and the charging spool doesn't move. However if there is a large drop in accumulator pressure, the spool should move to the right, draining the sensing line and switching the priority valve fan circuit off. The problem is, I don't see how the charging valve can ever work as drawn. If the charging spool is to the right, the sensing line is drained to tank and can never build up pressure on the right to equalise the pressure at the left end of the spool. And if the charging spool is to the left, the sensing line has nowhere to drain to to allow the spool to move right. Does this make any sense??
Yes, it makes sense Cmark as obviously we can only discuss possible theories of operation since we're not privy to exactly how that priority valve is operating. Genie uses a similar priority valve on some of their aerial boom lifts, but they're more like the Parker illustration, an input, a priority output, and a secondary output that receives flow/pressure after primary priority circuit is satisfied. The fourth line in this one I've not seen before and can only guess what it's doing, especially since the priority output and secondary are pressure compensated and the fourth port circuit is not. Really weird valve. I get what your saying about pressure changes, especially rapid drop in pressure on brake circuit and how it would affect charge valve, but don't forget about the adjustable spring on the charge valve forcing the spool to the left thereby putting it in the blocked outlet to tank position. Also, the .020 orifice is going to always have the sensor line lag behind the pressure changes in the brake circuit. Additionally, we don't know what pressure that charge valve is set at to overcome the spring pressure and dump pressure in the sense line to tank. What we can tell from the drawing is the oil goes to the fan motor, then it returns to supply the pilot circuit and to a designated relief valve set at 350 psi. By reason of that relief valve we can assume the pressure returning from the fan motor is below 350 psi, otherwise the relief would always be cracked open creating heat. This, of course, assumes there is not some other form of pressure regulating valve after the oil leaves the pilot lines of this manifold drawing. Assuming this is correct, that's what prompted me to think the charge valve is operating as a pressure regulator, keeping pressure below the 350 psi of the fan motor return circuit relief valve. Very peculiar circuit for sure.
It's interesting stuff isn't it Willie. IMO we shouldn't have to guess how the valve works. Schematics are to help people troubleshoot and should be accurate. It wouldn't be the first time I've come across errors (even in Cat drawings ) I still say that, as drawn, the priority spool can never move right as there's nowhere for the pressure on the right hand end to drain to, and whoever drew it is a big panty-waist with carburettor breath.
LoL, agreed. There's just too much missing info, not seeing the rest of the "big picture", and no info on adjustment of the charge valve. Interesting though that it's actually called a "charge valve", which in typical applications is a valve that regulates or maintains some form of circuit charge pressure.
It sounds like you guys know what you're talking about and thanks for a better understanding! The service manual says when the accumulator pressure drops below 1600 psi the charge vale kicks in and sends oil to the accumulator. Once the pressure is charged to 2000 psi, the charge valve kicks out and oil bypasses the valve. Cmark, you mentioned something about the priority valve switching the fan circuit off? The fan motor does slow while the brake accumulator charges. I will try to find another print that shows more of the circuit.
This is as detailed as it gets..
Thanks for the expanded drawing roddy, something more to study. Cmark, upon closer inspection, that 4th line that leaves the priority valve, goes through the .020 orifice to the charge valve, that's a dashed line, doesn't that indicate that it's a drain line from the charge valve?
I though a dashed line was just a pilot signal, though I could be wrong. Either way, it should show a tank symbol somewhere. I've noticed that the spring chamber seems to be connected to tank. Maybe there should be a line connecting the spool end to the spring chamber? Although even then it would still need an orifice smaller than .020 after the spool end to make it work.
You're probably right that the dash line from priority valve is considered pilot control circuit. I'm fairly certain the dash line coming from the spring chamber is for venting internal leakage at spring chamber to tank. And I certainly don't claim to know exactly how the priority valve is working. I do declare this is one of the most complex brake control systems on a simple machine that I've ran across. :yup
A few links... if you've got about a month to read... lol http://hydraulicspneumatics.com/oth...sue-11-its-really-just-bunch-lines-and-shapes http://hydraulicspneumatics.com/other-technologies/chapter-4-iso-symbols http://hydraulicspneumatics.com/ebooks/fluid-power-ebook-fluid-power-circuits-explained http://hydraulicspneumatics.com/other-technologies/chapter-10-directional-control-valves-part-1 Home Page... http://hydraulicspneumatics.com/ OCR
Well, the entire drawing that you posted roddy, it' too small to be legible if one tries to enlarge it. The only way we could work with that drawing would be if you posted it full size at a picture hosting website.
I used photobucket to upload the others. The original is kind of small to begin with. I will do my best to get a better image posted, unfortunately I wont be able to work on that or get that up for a day or so..
What exactly is the problem roddy, or are you just curious as to how it works? (Either option is fine.)
Just trying to educate myself and get a better understanding of how the system works..
Which is exactly the way you do it roddy, kudos for your curiosity. :drinkup
Probably one of the worst schematics I've seen in a long time. The direction of the arrows and the extra arrows provide no facts as to the logic of each component. Who is the manufacturer of this machine? When I glanced at the drawing my first thought was inlet oil is hitting an orifice and there is a straight T going to the fan motor. That would mean the fan motor has priority over the brakes. Not supposed to happen! Second item you guys discussed was somehow controlling the fan motor on the return side of the motor which again is a huge no no. Restricting down stream flow will blow the seals in the motor. Many engineers seem to treat schematics as some kind of art project and forget that it is supposed to be the interpretation of a function. I'm wondering if someone can design a software program to animate how one of these work? Even better would be to be able to set up a mapping program where you click on a line and it colors it all through the schematic.
pilot lines I tried to add a schematic showing the difference of a pilot pressure line and a pilot drain line BUT got the run-around with the computer, SO basically a pilot pressure circuit will drawn/should be drawn with a longer dashed line. Pilot drains will be a shorter dashed line, In heavy industry they will also be noted a X/pilot pressure and Y/pilot drain. Have to say i'm glad I don't have priority valves to deal with in my world. Looks like the engineer here used pilot pressure symbol/lines in places where its definitely a pilot drain circuit. good luck Maytag
OMG, how I would love to have something like that, especially on large diagrams, including electrical! :notworthy
I can view the original and zoom in at photobucket but not here.. I am uploading this copy to the site hopefully it will be better.
Put the photobucket hyperlink here roddy.
The original is 574kb, over 10x the size.. not sure how to make it bigger but I will email it if you private message your email addy.
http://i28.photobucket.com/albums/c209/hadesrider/BrakeCircuit_zps436a236a.jpg
That worked roddy, I was able to download the original diagram to my computer.
pilot lines Hopefully you can see this print. If you can look at the dashed lines that are circled, longer dash is the pilot pressure, short dash is the pilot circuit drain.
Someone tell me if my eyeglasses are just dirty, or am I really seeing what I'm seeing. Check out this brake control valve on that full drawing. It has not only foot pedal manual control, but hydraulic pilot control as well. That in itself (having two forms of control) isn't the weird part, although not common, but look at the check valve in the pilot control line. Someone explain to me like I'm a 6 year old just how that would work? Additionally, the way it's drawn, the valve is already in the shifted position, the lines should have been drawn connected to the spring end of the valve. :crazy
This is a SAHR braking system. It uses a reverse modulation brake valve. Please keep in mind I am learning to read prints (and not the best ones at that. To me it looks like the check valve would help keep the brakes in released position until the brake pedal is pressed releasing oil back to tank?
Taking a closer look I think I am way off on that check valve!
No worries roddy, we commend you in "teaching yourself" in reading schematics, that's the way it's done and we've all been there. And I think I can speak for others that we'll do all we can to assist you. But at the same time...you picked (through no fault of your own) a helluva diagram to learn from.
And I know what your saying about the "reverse modulation". It's sort of like a P&H T-300 crane I worked on once. Throttle from upper cab to lower engine was controlled by air. But it took air "pressure" from the upper control valve to make engine idle. Mash the foot valve or hand valve in the upper to "throttle engine up" meant it actually took air away from the engine actuator. Talk about a brain twister to diagnose problem with upper control valve and foot pedal. :dizzy
Guys, I found another oddity. This is the redundant parking brake solenoid valve on the full drawing. At first glance it looks like a closed center 4 way valve, but it's shown shifted to the end section, and it shows a spring on one end and a solenoid coil on the other end. In all my years of looking at diagrams I have never seen something drawn like this. >
Which 7.5 yd scoop is this from roddy? Care to name and shame the manufacturer?
symbol That is a two position valve with an "all ports blocked condition" as the spool transitions from one position to another. On some applications it won't matter, on others it will. Been on the receiving end of some bad engineering with an 'all ports blocked" condition where the rod gland would be blown. fortunately Rexroth offers the same two position valve with an "all ports open" condition. Have a good day, Maytag
You may be right Maytag as I've never ran across an "all ports blocked" situation, but I have never seen a valve like that on mobile applications and I've looked at a bunch of drawings over the years. :crazy
Willie, Here is a sheet from rexroth from their catalog for DCVs. Pay particular attention to any envelope(square) that has the broken lines. Any time a broken line is used it denotes a "condition" rather than a "position", a condition the the valve and all downstream components see momentarily as the spool transitions from one position to another. Hope this helps explain, Maytag
Closed center hydraulics have all the center ports blocked and are most of the time three position. There are two position valves for things that are simply on or off. The park brake in a loader is usually set up that way. The example shown is a closed center valve. All ports open is referred to as a motor spool. Usually there are straight lines that designate the sections of the spool. What is hard to grasp sometimes from a line drawing is that there are cut outs in the valve spools that smooth the transition from no flow or center flow to one direction of the other. The page above probably shows the concept but It comes out too small for me to see it. On yellow iron I've never seen it in a schematic. Service manuals usually explain it but don't provide some kind of graphic to make the function easier to understand. It would be nice for each line drawing to have a legend the one that maytag shows above.
I didn't clear much up with my download. using a new computer system that is supposed to be a combination of the best features from the Mac and Windows but there is a steep learning curve for me. Anyway Rexroth is good to provide the details like this. Never seen one of these symbols used on a schematic but info is useful in design and especially failure analysis. I believe that was what the engineer was trying to convey on the print the original poster provided.
Not sure which question you still need an answer to. For the Reverse modulating SAHR brake pedal, the check valve is for reverse free flow. ie; when you dump the supply, you dump everything downstream, and the brakes apply. Basically, the valve is a pressure reducing valve, which was how we originally drew them when designing the systems. There is no real ISO or ANSI symbol, so all the folks at Mico, Rexroth, Wagner, and all, sort of took some liberties, and made up their own symbols.... more or less. They threw in the lines for 'sliding spool' among other things, but they do make sense if you gaze at 'em a bit. Compare it to a run of the mill PRV with reverse free flow. It'll make more sense. (attached is ISO of PRV with reverse free flow) Bill