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.
