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#1
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Re: PC1800 Front Shovel (1/12)
The hydraulic stuff can get confusing very quickly. Yes, a fixed displacement pump will deliver same volume for same pump speed, but if divided between circuits, each circuit gets only a part of that volume, so each function will be slower than each individually. Also, for a given valve opening, oil volume is directly related to pressure on that circuit. If using two circuits at once and both valves are opened the same amount, the circuit with the least resistance (and lower required pressure to move it) will move first. Only when that circuit bottoms out or it's valve is closed, will the other (higher required pressure) function move. Throttling or feathering the two valves during operation is the only way to compensate so both functions can move simultaneously while operating at different circuit pressures. Pressure-compensated (and usually with variable displacement pumps) take care of this for operators, but are much more complicated systems. Yes, the open center system does reduce heat and horsepower needed when in neutral. But having some "standby" pressure, as Mario stated, does give quicker response when activating. Also, open center may allow a function to drop a bit when first pressurizing a circuit - I don't think our model circuits have load-check valves to prevent this.
Ken
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#2
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Re: PC1800 Front Shovel (1/12)
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Here is what I understand....if a pump at a given RPM moves 2 gal/min is attached to a cylinder that has a total volume of 2 gallons it should take one minute to fill the cylinder....if you have two of these 2 gallons cylinders it will take longer to fill them both, in this case twice as long. The larger the total volume of the cylinders being used the longer it takes to fill them all and the cylinder movement speed will be reduced accordingly. What I don't understands is why this isn't happening like it should....this pump supplies much more volume than what is really needed....there should be a surplus of volume left over when using multiple cylinders....but where is this surplus going??? I think that in this valve body there is always some flow allowed to bypass the spools and flow to the return port even when the spools are totally open to the cylinders!!?? One would think that all the flow would be channeled through the spools that are being used and only allowed back to the return through the return hoses on the cylinders. Yes/No ??? |
#3
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Re: PC1800 Front Shovel (1/12)
Interesting?!?! Have you confirmed the volume of oil being supplied by the pump? Disconnect the return line at the tank and put a container that is calibrated or that you can measure the actual quantity easily. Depending on how large your reservoir is, run the pump without actuating any circuits and measure the amount returned. May have to run for only 15 or 30 seconds, but confirm pump delivery. What pressure is being displayed during this test? Confirm system maximum pressure will make 275 PSI. Bottom a cylinder (either direction) by fully stroking control valve and see if relief valve actually is 275 PSI. Also, at this condition, return oil volume should match max output of pump. (You may see a slight decrease if the motor reduces RPM while under relief pressure, plus no pump is 100% efficient. Most big gear pumps can be up to 98% efficient when new. I'm guessing yours should be near that.) When you saw the system operate at 210 PSI, was that while cylinders were actually moving? Should go to 275 PSI when cylinder hits end of stroke. If there is a question about individual circuit flow, disconnect a cylinder line and put it into a container for measuring. Stroke valve fully and measure volume delivered to circuit. At same time, see if any oil is being returned via the valve return to tank. Should be practically nothing. If is significant amount, the valve may have something going goofy. Also, observe system pressure at this condition.
Ken
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Big iron is awesome! |
#4
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Re: PC1800 Front Shovel (1/12)
The best thing for me to do is share the results of all the different tests so you all know what is happening.....
The pump "should" be moving 3000cc/min or 50cc/sec. The total combined volume of the two boom cylinders is 241cc, so at 50cc/ sec it should take 4.82 seconds to fully extend both cylinders. Keep in mind this does not account for any slip in the pump and would be with no load or restrictions. 20lbs was used for the load in all the tests First test 1/4" ID suction line tank to pump, the blue1/4" OD tubing from pump to valve body, the ID of the tubing is .180", the same 1/4" OD tubing was used on the return from pump to tank. PRV set at about 275 psi on a 0-600 psi gauge. System pressure with no load and the controls in neutral was about 60 psi. It took 7.5 seconds to fully extend both cylinders and pressure was about 220 psi.....funny not what I was hoping for...lol Second test Changed the suction from the tank to pump up to 3/8" ID and changed the return line from valve to tank up to 3/8" ID as well. Pressure in the neutral position was now zero, and operating pressure dropped to about 200 psi. The time to extend the cylinders increased to just over 9 seconds. So I was getting some restriction in the return using the smaller tubing...interesting. Third test Increased the feed line from the pump to valve body to 3/8" ID tubing....no changes Forth test Check the flow rate of the 5/32" OD tubing (.106" ID)....removed the 3/8" return line and installed one 5/32" OD tubing. With the controls in neutral I turned the pump on for 30 seconds (220psi) and got almost 1500cc which is 3000cc/min, again not what I was hoping for...lol....really thought the small tubing was restricting the flow to the cylinders but that is not what is happening. Test 5 Increased pump speed 600 rpm to 2000....again very little change....scratching a hole in my head with this one...lol Last test Dropped the pump RPM back down to 1400, increased suction to 1/2" ID used the same 3/8" line pump to valve but reinstalled the blue 1/4" OD tubing on the return....the result where nearly the same as the first test. Which makes me think that some of the flow in this open center valve is still bypassing the spools even with them fully open to the cylinders. The pump is supplying the proper amount of flow but if all of that flow isn't getting to the cylinders where is it going?? (and no there isn't a leak some where...lol) |
#5
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Re: PC1800 Front Shovel (1/12)
Oops....need to correct something in my last post.....in the first test....it should read return from valve to tank, not "return from pump to tank"
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#6
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Re: PC1800 Front Shovel (1/12)
I noticed in a couple pictures the suction from tank to pump you had to use a reducer. The larger the suction line the better otherwise the pump is trying to suck through a straw and will drastically effect output.
One other thing that quickly comes to mind is the prv you are using. It is the original valve relief? What is the maximum working pressure of the valve? The reason I ask is, if you have a circuit that uses a 5,000. Psi relief but you install an attachment like a clam or grapple which only needs let's say 2,200 psi, you can't just turn the 5,000 psi relief way down to the lower pressure. It must be replaced with a lower value relief. Long story short, maybe the relief is rated too high and working at 275 psi is causing it to not work properly. |
#7
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Re: PC1800 Front Shovel (1/12)
Been pondering this... I was looking at Test #4 and you said it was delivering 3000 cc/min at 220 PSI, in neutral. I don't understand that with the valve in neutral the system is loaded so much. I'd expect a much lower back pressure (maybe 10-30 PSI) - appears there is a restriction going back to tank that is too high. Do you have a filter in the return line? If so, have you tried removing it for testing? Vented tank? Could vent be restricted? Did you get a schematic of the valve? Might look at it to see if anything unusual or unexpected in the guts of it.
Ken
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#8
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Re: PC1800 Front Shovel (1/12)
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