Not a day passes by without someone bringing over a gear
pump to see if it is ok. The usual story is - the machine has lost
pressure and, of course, the gear pump is the primary suspect. The
first thing most machine owners do is dismount it from the machine and
take it to local hydraulics dealers to get their professional opinion.
Well, after a year of working at a hydraulic workshop, I
knew for sure how to evaluate a gear pump just by looking at its parts.
Five years later, however, my visual evaluations became much more
reserved, as I had seen gear pumps with severely scored parts
presenting reasonable efficiency, and I had seen pumps, that looked
brand new, everybody betting their reputation and years of experience
that the pumps should work, and still the pumps would fail to supply
flow at pressures higher than 100 bars.
It's obvious that when a gear pump has excessive wear,
there is not much you can do but replace it. But before dismounting
any component from a machine, you must make sure you're dismounting the
"root" of the problem. Experience tells me that indeed, in most "not
enough pressure" cases the pump is the guilty one, BUT, experience also tells me that most cases aren't all
cases. If every time, when you encounter a "high pressure sick"
machine, you go for the pump without confirming its malfunction, there
will be unjustified downtimes involved...
There is, however, a very simple way to see if a gear pump
is lacking volumetric efficiency. In any worn-out fixed displacement pump,
when system pressure rises to a certain level, ALL of the produced flow
goes through the excessive clearances to the casing. Whenever
there is a pressure drop without producing work, heat is generated,
and in the particular situation of "reaching the maximum attainable
pressure" when there is zero flow to the outlet, all of
the input energy is transformed into heat. In a piston pump with case
drain connection, great part of the generated heat is carried
away by the drain oil flow. But with the gear pump things get somewhat
funnier, as gear pumps have no case drain. So when a damaged gear
pump hits its maximum head, all of the generated heat is
concentrated inside the relatively small body, causing instant local
overheating, which is easily detectable and is the instant "busted pump"
indicator.
Imagine, for example, a 16 cu.cm. pump, driven by an
electric motor at 1450 rpms, producing 23 litres per minute flow.
Say, the system pressure doesn't go higher than 100 bars. If the
efficiency were the problem, we'd see 23*100/600=3.8 KWA of heat
generated inside the small body. This is the power of a decent
industrial heater!
The scientific approach would be attaching a contact
temperature sensor or using an infrared thermometer to detect the overheating.
The "real life" approach is much more fun - you let the machine
cool down to "warm", then you start it, put your hand on the body of
the pump, and raise the pressure (by extending one of them jacks till
it hits the end of stroke, for example). If the pump is the "guilty one",
you will feel the pump temperature increase
instantly - in a matter of seconds you'll have to take your hand off of
it. However if the pump is not the problem, you'll feel no significant
temperature rise.
I can imagine there are people who will find
measuring temperature of components "by hand" unsafe, and I understand
this, and even agree, but only to some extent. I am not talking about
putting your hand on a high pressure hose here, I am talking about
putting a hand on a COLD surface that is ABOUT TO get hot. If you're
not retarded, you'll know when to take the hand off.
So, whenever you detect insufficient pressure in a gear
pump hydraulic circuit, check the pump body for "instant
overheating". If you don't detect it, there's a good chance the
problem is elsewhere.
For the record, the infrared thermometer is one of my most favourite tools I never leave home without...
