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    Years of troubleshooting hydraulic machinery have taught me that anything can break or malfunction, no matter how simple and seemingly unbreakable it is. That is why I made it a habit of mine to check everything, whenever I am disassembling a hydraulic system component, and I mean EVERYTHING. Sometimes  people would say about me - This guy is nuts! Why the hell is he checking this ___ (insert an unbreakable component)? It's just a waste of time, surely this ___ (insert again) can't break!..

    Well, my friends, the truth is IT CAN, no matter what. That is why spending an extra minute to check all the components of a hydraulic assembly is not a waste of time. This is especially true when you deal with variable displacement pump controls. Normally, when a client brings you a malfunctioning pump to repair, he will not be able to tell you if the problem is control related. Sometimes, skipping a few simple checks of apparently unbreakable parts of a control module can cost you and the machine owner loads of downtime and unnecessary work (and make YOU look incompetent).

    One of the components in pump displacement controls that gets overlooked quite often is the boost pressure shuttle valve (or two check valves, performing the same function). Some (not all) mechanics will skip checking it because they either aren't familiar with its function and don't know it is there, or think that such a simple thing as a ball/seat check valve is an unbreakable component, hence checking it is a waste of time.

    I personally have witnessed more than once pumps get complete overhaul and continue to malfunction due to broken boost pressure shuttles that had "dodged" analysis. I mean way more than once, so I am hoping the following article will help some pump repair dudes give those valves a little bit more attention and, hopefully, save them from possible trouble.

     First of all their function. The main purpose of these valves is to supply the displacement control with servo pressure from an external pressure source (normally a relatively small gear pump, supplying pilot pressure for the rest of the machine's hydraulic controls) when there is no pressure available from the pressure line, and, at the same time make sure that the high pressure from the pump's outlet doesn't escape to the low pressure pilot line. Take a look at the schematics, these are the two most common designs of such a system.

     More often then not, such system will be used in pumps with proportional displacement controls and stroke limiter controls. With these controls, when the pump is at low or zero displacement and/or  there is no resistance to the outlet flow (no load), there is no oil pressure at the outlet to guarantee the function of the servo system (the cylinder or cylinders that change the swashplate angle) or provide an adequate response time. The existence of this simple system makes sure the servo system has a pressurized oil supply at all times, thus guaranteeing the correct function of the control.

    You can also find this system in large pumps with closed center load sensing controls. Although, by definition, a closed center load sensing control is supposed to have at least the delta P setting pressure at the outlet, sudden system flow demands can create situations when the pressure at the outlet drops abruptly, and, due to the fact that the swasplate and the servo cylinders are massive and the volume of oil in the servo chambers is relatively big, the response time is too long even with the spring loaded swashplate. The boost pressure system improves dynamic response of the control in such situations.

    The most common design is the ball type checks or shuttle, but other designs (e.g. spool type) are possible. As you can imagine, these valves go "smashing around" all day long from one seat to the other, which normally results in wear of the die cast body that secures the ball (spool) in its path. After many thousands of hours the ball literally "eats away" part of the control body, making a cavity sometimes several times as big as the initial one. Then several outcomes are possible. Either the ball falls into the new cavity blocking or obstructing the servo pressure supply, or, which is more common for spool type shuttles, the spool places itself sideways in the widened cavity, and the shuttle becomes a tee, connecting the high pressure to the pilot pressure supply, thus creating a huge leak of high pressure oil through the pilot pressure limiter.

     Recently I repaired three AA11VLO250 pumps from a Caterpillar excavator, that all three had this problem. You can see the shuttle here and here. You wouldn't believe how deep the small spool dug into the cast body, finally placing itself sideways. If such a problem is not detected during disassembly, there is a chance it will not be detected during bench tests. If a technician opts for a test with the boost pressure port plugged (enough to check the rotary group efficiency) the pump will pass the test with distinction. If a technician opts to connect the boost pressure port to a pilot pressure supply, he might confuse the decrease of flow at the outlet, caused by oil escaping through the malfunctioning shuttle and then through the pilot pressure limiter, with the torque limiting function of the displacement control. There will be no excessive case drain flow, so there will be no doubts as for the efficiency of the rotary group. The pump will pass the test, go to the machine, and there you will have it - its majesty unexplained overheating! Then the consequent troubleshooting will be based on the fact that there is no problem in the recently overhauled pump, and then... Well, let's say the future isn't bright...

    On these pumps, by the way, the problem is easily solved by machining the cavity and applying a small cartridge type shuttle valve, of which there are plenty on the market. Other "on the knee" solutions are possible, though, like applying an external non-return valve, which will prevent the high pressure oil from entering the pilot side, at least till you find a "more technical" solution. Although the pilot oil will still partially escape through the broken shuttle to the pressure side when the outlet pressure drops.

   Here and here you have the same system in a Liebherr LPVD pump control, which uses two check valves.  As you can see, the high pressure side check ball has machined a nice round cavity, which he then  plugged completely, resulting in control malfunction (no high pressure limiter function). A possible solution is here - machining the seat and applying a smaller ball with a guide.

   Always check schematics (when available) before evaluating a control's condition, if you see that the control has boost pressure system, make sure to look for and check the shuttle. You should do the same whenever you deal with an unknown control, and feel it is a proportional displacement control or a stroke limiter.

    Boost pressure system operation depends on the correct function of a very simple component - shuttle valve, which, while simple and robust, has wear and can break. Checking the shuttle valve condition is a quick step which mustn't be skipped, as the consequences of not detecting a malfunctioning boost pressure shuttle can be dramatic and expensive.
Boost pressure shuttle valve, A11VLO250
A11VLO250, shuttle spool
Liebherr LPVD, high pressure check valve
LPVD check valve
Possible solution