Insane Hydraulics
Bold and audacious blog about fluid power
Today I want to talk about pressure-testing of LS+PC pump controls separately from pumps - a technique that has become near and dear to me, but, from what I see and hear, is not very popular. Also - since I made such a nice base plate for Rexroth A11VO DRS controls last week (with extremely well-finished o-ring grooves, might I add), its story would be incomplete if I didn't put it to good use, so I will, naturally, be showcasing the test, too.
But before I do that, let me tell you how I came to love testing common open-loop pump controls (pressure compensator + load sensing) "on their own."
The most common "pump control protocol" I know of is "inspect and relay" - i.e. - the controls are disassembled and inspected, and whenever an especially scored spool is found, the responsibility is transferred to the client with something along the lines of "...visible wear marks on spools indicate there's a possibility of an increased internal leakage in the control module, which can lead to loss of efficiency, overheating, and even incorrect operation of the unit..." and then the client gets to choose if he wants to invest in a new (and expensive) control or, basically, roll the dice.
To be honest, I've never been too fond of this approach. Mainly because I don't like describing internal leakage with generic terms like "excessive" or "abnormal" - I prefer tangible figures, i.e., liters per minute at given pressure - something I can transform into energy units and then make an educated call about the replacement. But the truth is - I only saw the potential behind the separate control testing when I came across an A10VO DFR control that dumped almost 20 liters per minute (5 gpm!) into the pump casing through its own internal leakage.
The tests that I've been practicing are simple - I pressurize (individually and controllably) the "P" and the "LS" ports of a control, and then monitor the pressure in the servo-pressure port and the flow in the "T" port. Unfortunately, I don't repair as many pumps and motors as I used to when I was a 100% shop worker, but even with the few tests that I have done so far, I can say that:
Quantifying and recording the internal leakage of a pump control gives me the numbers I can present to the client. For example, this DRS control has pretty worn spools, and in my test, I saw that it leaks 8 l/min at 260 bar and also bypasses some pressure into the servo port.
Note how informative thermal imaging can be in such cases. You can see the hot spot "light up" in the pressure compensator area, you can see the T line is hotter than the pressure supply lines (this control has actually two of those, and if you want to know why, you should look it up in the catalog, it's all there. As always, whenever you come across a port or a hole you have no explanation for - you must investigate!), and you can also see that the LS line is cold, which means that there's no bleed orifice in it. All that info from a single FLIR shot, how cool is that!
Now, eight liters per minute may not seem like much in comparison to the max flow of 200+ lpm, but it is, especially for this control. Why? Because a brand new DRS control leaks only about 0.4 l/min, and the servo-pressure remains at a stable 0 bar until the PC spool shifts!
Getting the approximate settings of the LS and the pressure cut-off in such a test is, among other things, a good way to see if the settings have been tampered with, which may sometimes be important.
And control's "controlling loss" is the control's internal leakage increase when the spools shift. You won't find this information in catalogs, but it is quite interesting to see how different controls behave. The brand new DRS from the test above, for example, increases the leakage to about 2 l/min when the pressure cut-off spool shifts (which you can easily see by monitoring the pressure in the servo-port). A brand new DFR control from a sub-100cc A10VO has virtually undetectable leakage before the compensator spool shifts, but when it does, the leakage flow goes up to almost 3 lpm at 290 bar. This Parker P1-100 L0 control from a unit I opened this week turned out to be even "leakier:"
The spools looked and felt new, and the control presented virtually undetectable leakage under high pressure, but when the PC spool shifted, the T flow went up to full 5-6 l/min (at 220 bar)! I believe that this behavior is due to the underlapping spool (the spool land is narrower than the hole leading to the servo-piston), and, to be honest, I don't know yet what to make of it because I don't have a brand new L0 control valve to test and compare. But you know what I will do? I will record the numbers and make sure I compare them to my next test of a similar control valve from a different (or a new) P1.
In any case, my point is that testing of pump controls separately from pumps is a valid technique and a good way to learn something new, so I intend to continue using it and collecting the leakage data for future reference.