Very often I get to see the following situation- a hydraulic machine
has a problem, normally the "not enough force" problem. A series of
troubleshooting attempts are made, leading to the final "it probably is
the pump/it must be the pump/there's a chance it is the pump"
conclusion (it can also be the "motor" variant). Then, as no "advanced"
testing gear is available, the aforementioned pump is dismounted from
the rig and is brought over to a local workshop for a check-up. At that
point, as the troubleshooter is not a hundred percent sure that the
pump is the root cause of the evil, he decides it is best to test the
pump in the workshop test bench first, which sounds like a logical
choice. Indeed, it is a quick way to make sure the diagnostics was
correct, and if the pump turns out to be OK, having it on the test
bench is a good way to make all the adjustments, if necessary.
Most of the hydraulics workshops will be happy
to test a pump for a client, charging a not so modest fee, of course.
For them it's a win-win, if the pump is OK, they'll charge the test, if
it is not OK, they'll charge the test and the following repair, and
another test in the end. The bottom line here is - in any case they get
to charge something.
A smart mechanic, however, in such
situation, before blindly agreeing to shove a pump into the test rig,
will perform three simple checks, which, if failed, will guarantee that
consequent tests are a waste of time.
Number one - rotate the shaft test. You take a
necessary tool, most of the times a pipe wrench, and rotate the shaft a
couple of turns. An experienced mechanic will easily detect jerky
torque resistance or excessive play/vibration - sure symptoms of
serious failure in the rotary group or the bearings.
Number two - air in the pressure line (open
circuit), or pressure lines (closed circuit) test. You take an air gun,
improvise some kind of a sealing accessory (a piece of cloth) and blow
compressed air into the pump's inlet. If there is a serious damage
to the rotary group, causing excessive internal leakage, it will be
easily detected by the generous quantity of air passing through to the
case drain port or suction port (or the opposite line of the closed
loop pump). Some previous experience might be needed to determine how
much exactly is "generous" for a certain pump model. Of course, in case
of a vane pump this test is useless.
Number three - flashlight test. You take a small LED
flashlight, and peek inside the casing through the case drain ports.
Any signs of metal wear, like chips, raspings, shavings and other wear,
like shamelessly broken parts staring at you, are all tell signs of a
busted pump.
These three simple tests take under a minute to
perform, and are a fast way of detecting damage without performing any
hydraulic tests. For example, let's imagine that the pump you are about
to test has a valve plate like this.
In this case you won't detect any uneven or excessive torque resistance
on the shaft, nor will find noticeable wear debris inside the casing,
but you will see a large amount of air passing through the pressure
line to the casing, something a pump in a good condition wouldn't allow!
Unfortunately, these three very simple one-minute checks
are often overlooked, which at some occasions leads to downtimes, that
could have been avoided.
This is just another reminder that no matter what a
system or a component you are troubleshooting, you should always
perform simple checks first. A few extra minutes you'll spend doing so can potentially save you many hours of work!
P.S.
Of the above three, the air test is my favorite. In
fact, knowing how to correctly use compressed air to test hydraulic
components can potentially save you a lot of time.
Most importantly you MUST know
exactly where to blow and what to
expect. "Mastering the air gun techniques" allows a technician to
detect some malfunctions during disassembly of a component (this is
especially true in case of displacement controls, like checking the
correct function of check-valves, shuttle valves, existence of oil
passages, etc..) which in turn speeds up the whole repair
process, eliminating the need of faulty component disassembly during
"wet" tests.
