I repair a lot of closed-loop transmissions, and whenever I deliver an overhaul to a client, I attach a checklist containing things they should and should not do when re-starting their recently (and very expensively) repaired units. To top it off, I always tell them that despite the very long list and all of my verbal "calls for caution", we still regularly see such transmission return to our shop because they don't survive the recommissioning.
I think the main reason for that is the lack of training, which prevents mechanics from singling out closed-loop transmissions that may require "additional attention" during the recommission. I say this because I often see experienced mechanics look at me as if I was crazy when I insist on by-passing actuators, installing cumbersome external pressure filters for loop cleaning during start-ups, or projectile clean what they call "already perfectly clean" hoses/lines. And very often I hear things like "We installed pumps and motors on that other closed-loop machine, did none of this crap and it went great!"
I know well how much closed-loop overhauls cost, and I know all too well how little it takes to scrap such transmission to bits and pieces, literally. Let me tell you a "scary story" that I had to whiteness this week. Just an episode for me, nothing I haven't seen before, and I am sure I'll see more of those in the future.
I was asked to diagnose a farm sprayer transmission, that had done about 50 hours after a major overhaul of all components, and started making a strange noise and lacking torque. Mechanically, it was a simple closed-loop circuit, consisting of a single Danfoss series 90 pump, four axial-piston wheel motors, and a couple of on/off flow dividing manifolds for the "hydraulic differential lock" function. A perfect solution for a small-series farm vehicle.
Now, the owner of the sprayer had spent about 15K on the transmission overhaul, and being a long-term hydraulic equipment owner he went all the way to contract a "hydraulic specialists" for the re-commission, to make sure things went as smooth as possible. He even had all the transmission hoses replaced, which, truth be told, were in pretty bad shape. The idea was - to repair it all and get a brand new transmission that would last him for another 5 to 10 years headache-free. Preventive maintenance and due capital investment at their best!
I get to see a lot o crazy and inexplicable malfunctions in my practice, but I also see tons of repeating stuff, and this one was a classic "Deja Vu" for me:
I come to the machine and hear it struggle... I already know the charge pressure is down.
I install the pressure gauges and confirm the charge pressure plunge - a sure sign that something is busted.
But I already know this because when I heard about the new transmission hoses and asked if they were cleaned I was told that they were blown through with a shop compressed air blowgun.
I already know this when I look underneath the frame and see a "rat's nest" of hoses covered with dirt and sand.
I already know this when I see R2AT hoses where 4SH should have been used...
Still, I am not the "repairing party" here, so I have to present the assertive proof that the pump is undoubtedly damaged, so I spend another couple of hours checking and recording the motor drain flows, the braking circuit, the charge pump suction line, and its filter, and in the end, record the test with the pump disconnected from the system. And I do all that knowing that the pump is done for...
It will be removed next week for inspection. I'm almost certain I'll be hearing about particle damage to the rotary group components, and I guess its severity will determine the extent of further "corrective actions". I'll make sure to post it if I get this information.
There's no fault on the owner's part here, but I can absolutely see how even experienced mechanics can overlook the importance of "paranoid cleanliness" when you deal with closed loops that use extensive hose connections and work in dirty environments, which can't be "removed from the equation" during the recommissioning due to logistics.
This particular case is a very good example of such a system. One pump feeding four motors and two anti-skid manifolds take much more lines than a couple of short hoses you normally find in a single pump/motor configuration.
When all you have to install is a couple of two-foot hoses to close the loop off from the external world, a lot of corners can be cut. But when you have ten plus hoses, that have to be routed along a dirty frame in a silica-filled environment (lots of dirt and sand, which is a must for an agricultural vehicle, especially for its under-frame) - anything less than "immaculate-clean" will lead to a very expensive failure.
Environment isolation (think Dexter Morgan), pressure-filter flushing, mechanical cleaning, zero uncapped lines policy - anything you can think of as excessive - is, actually, not!
So, I guess the lesson of today is - if you recommission a closed-loop transmission where a pump sits next to the respective actuator- go fast. But when the lines are "numerous, ample and winding", and the environment around you makes you "feel sand on your teeth", make sure to turn the "paranoid-clean" mode on, if you don't want to do the re-commissioning twice.