I like recalling the following episode as another good example of how
bench tests sometimes fail to simulate real life conditions.
At that time I was employed at a company that sold (among other brands) losi (www.loesi.de)
steering units and hydraulic motors. A client was claiming that a brand
new steering unit was causing the oil of his tractor to overheat,
although the power steering function was ok. He brought the unit
over to our shop, and it was immediately hooked up to a bench test. The
steering was of a very basic type - simple open circuit, non-reaction,
with a relief valve and no anti-shocks - the type any hydraulic shop
sells a dozen a week... The test itself was also the classic no-brainer
- the steering was connected to a fixed displacement pump (P
port), tank (T port), and a hydraulic jack (ports A and B). The unit
functioned flawlessly - everything worked one hundred percent ok
without anything even remotely suggesting overheating. The client, as
it would be expected, was advised to look for a problem somewhere else
in the hydraulics of his machine.
A couple of days later the client sent the unit over again
and said that it was still overheating the oil. The power steering
function was fine, meaning that the wheels turned and the effort on the
steering wheel seemed to be normal, yet the oil would overheat. When
the new unit was replaced with the old one - the overheating problem
disappeared. Understandably the man was pretty upset, because in an
honest effort to "fine-tune" his tractor he'd paid good money for the
new steering, which was now causing him downtime without any fault from
his side. Anyway - the unit was reconnected to the test bench, a
steering wheel was attached to it, and a mechanic spent a good hour
"playing Schumacher" in attempt to see if any signs of malfunction or
overheating would appear - and yet again without any result - the
damned steering was working fine... Working pressure was normal,
pressure limiter was functioning fine, the delta P between the P port
and the T port was normal - nothing, absolutely nothing suggested that
the unit was problematic in any way. Still the owner was ardent that it
was the steering that was causing overheating. The fact that when he
replaced the unit with the old one the overheating problem went away,
and when he re-installed the unit back the problem reappeared, was
pointing to the unit as the source of all evil.
Luckily fate intervened - the frustrated Schumacher gave
the steering wheel a violet kick, and then suddenly the unit went
"s-s-s-s" and the stand-by P to T free flow differential of 0.5 bar
suddenly went as high as 40 bar!!! Obviously something was
strangulating the oil passage. When the pump was stopped and re-started
- the "s-s-s-s" disappeared, and when the wheel was turned violently
again - reappeared! Hurray! If you see the problem you can solve it!
It turned out that the P line check valve, which was
of an "inverted umbrella" design, was not in the correct position
(too much in - see schematics)
which allowed it to function as a some king of a hose rupture
protection valve (speed fuse, if you will). When the oil was flowing
through it at normal speed, the valve was opening normally, but when
the flow would increase abruptly, the poppet would "glue" itself
against the bottom oil passage, thus strangulating the oil flow and
causing the overheating, without jeopardizing the assisted steering
function, as the oil was still passing through the valve. Normally,
when the operator started the tractor, he would accelerate it for a
short time, thus causing the increase of flow big enough to put the
check valve in the "heating mode", which then would stick in that
position till the engine was turned off. The problem was quickly solved
by fixing the check valve in correct place.
Moral of the story (yet again):
- even the simplest component (like a check valve) can malfunction
- sometimes it is next to impossible to simulate machine working conditions on a test bench
- even a brand new component can be faulty
- want to be a good hydraulic technician? - Start opening your mind!
