This battle shows that there is always a solution to whatever
unsolvable hydraulic problem you may encounter, and also falls under
the category of "insane inventions".
A colleague of mine asked my opinion on a
malfunctioning open circuit A11VO190 pump from a John Deere 1270D
forest harvester. The pump had gotten a complete overhaul (new cylinder
block and everything...), but was acting "funny" on the test rig. When
I got to the shop and tested the pump, everything indicated an internal
leakage problem. Further digging revealed the problem inside the
control module - the load sensing part was severely damaged, creating
huge leak to tank and, at the same time, causing the control to
malfunction.
A few words about forest related machinery and
business. Foresters are just like farmers, they are always in a hurry!
When, for example, you are called to see what is wrong with a forest
crane that stopped functioning, on your arrival you already have a
truck waiting to be loaded and the business owner arranging for
another truck to come in a couple of hours, without
even knowing if you will be able to fix the problem. All of the
assistance calls I handled for "forest people" were super-urgent and
over-time over-loaded. Forest contracts have tight deadlines and harsh
penalties, so no wonder these guys take unplanned stops for back stabs.
Normally, when such a malfunction occurs, there
isn't much you can do to amend it, so you just replace the complete
control module. The first problem about it is the control's price,
which is almost the price of a new pump. This is not a standard
control, so it would've had to be bought at a John Deere representative
at the most ridiculous price. The second problem is terms of delivery.
In this particular case the owner of the harvester was ready to pay
whatever it took to put the machine in service, he even would have
bought a new pump IF there were such a pump available. The downtime he
was suffering was way too expensive and he was at risk of loosing the
contract.
The market was consulted and the fastest solution was a
new control module ready to be delivered in FOUR weeks! This would have
stopped many, but not the insane hydraulics back engineerist (yours truly),
genetically inclined to look for solutions to unsolvable problems and
invent insane inventions!
This control is a simple load sensing control with an electric proportional stroke limiter (schematics,
the orifices are omitted), probably used for power or speed management
by the onboard computer. Although the load sensing part was pretty much
busted, the stroke limiter was working fine, so, in theory, all I
needed to do was to isolatethe existing load sensing gallery, make external connections and use an external pilot valve to provide the load sensing function.
I called the client and asked his permission for the
control violation. The client gave it a "go" and I plunged into
work. This picture
shows you one of the construction steps. The isolation was made by
drilling holes, making threads, installing plugs and swearing in
Portuguese. The external load sensing part was "stolen" from a
Hydromatic A10VO140 pump I (lucky me) found in our stock. This control
is just perfect for "invention purposes" as it's separated from the
pressure compensator and is a standard NG6 size block, which can be
screwed onto any NG6 intermediate plate, making connections a
piece of cake.
After quite a few hours I ended up with this,
and, strangely, it worked, at least on the test rig. Immediately I
called the client and told him the "operation" had been a success. The
next morning the pump was already working in the field and everybody
was happy...
If you think it's the end of the story - it is not.
After two hours of work the operator called me and told me the elbow
fitting in the servo cylinder line
had failed, springing away and leaving the nut and the cut ring behind.
Suspecting deficient fitting I replaced it with a new one. A couple of
hours later the new fitting failed, splitting clean in the middle.
Obviously I was dealing with pressure spikes originated by the "new"
control.
My theory was that, during the violent operation of
the harvester head, the swash plate was subject to rapid movements,
and, probably, the new spool when dislocating was causing momentary
short time servo line blockages, thus creating high pressure spikes due
to high momentum of the massive swash plate. Anyway, I told the client
to remove the steel pipe and replace it with a hose, hoping that the
spikes would be absorbed by the hose "accumulator" effect.
This did solve the problem and the machine worked
fine for another whole day. By the end of the day the operator called
me, saying that the pump pressure hose started shaking violently,
especially at low rpms, and the machine began to overheat and lost
speed and force. I told him to stop the rig and wait for me, jumped in
the truck and, cursing everything around me, went off to see what the
hell was wrong, suspecting the worse, as jumpy pressure line is never a
good sign.
As I was driving towards the machine's location, I
was telling myself to stop inventing stuff, for my own good. When I
arrived at the location, and the operator turned the harvester on, the
pressure line, indeed, started to dance violently, going bu-bu-bu-bu at
the same time... When I say violently I mean VIOLENTLY. Obviously
something was seriously wrong with the rotary group, so we dismounted
the pump and it went back, yet again, to my shop.
By that time the commercial department was giving me
the crook eye, as, apparently, the nuthead invention had failed
(ha-ha). Pump inspection revealed that the brass layer of the new
cylinder block separated form the steel (pictures are due to appear in
the Kaboom
section). I was blaming Chinese quality (new white market supplier), as
this type of failure is not uncommon to after-market parts. The
commercial department was, of course, blaming my "insane invention". As
there were no more cylinder blocks available in stock, I had to go for
lapping the old cylinder block, which, luckily, hadn't gone to scrap.
The pump was tested and re-commissioned. The machine worked without
failures for a day, then for another day... I was finally starting to
calm down...
If you think that this is the end of the story,
well, not quite. A week later the operator called saying that the
machine had STOPPED. No movements at all. My blood pressure gave a 100
bars spike. The problem turned out to be a broken wire of the
proportional stroke limiter. No current - no displacement. Talk about coincidence, ha?
Well, it has been over a year, and just a few
days ago I spoke to the man. The machine seems to have been working
fine and non-stop, so I am guessing the "invented"control, despite all,
paid off. It was, by the way, a temporary solution, but I am guessing
it's bound to last...
Lessons learned:
- Inventing stuff (even when you fail) and new approaches is good, as you learn lots of things anyway
- Unbelievable coincidences do happen
- Some after-market cylinder blocks are crap, the only way to know is by breaking a few...
- No matter what others think, stick to your truth if you are sure about it
- No matter how thoroughly you plan things, something unpredictable will happen
- Pressure spikes suck
- Blood pressure should be monitored
- Oh yeah, when commercial department suddenly goes "technical" on you, it's OK to tell them go f@ck themselves.
