In this article I want to talk about shop repairs of hydraulic components (i.e. repairs done in the comfort of your shop, with no time pressure and limitations of field conditions), and reveal a simple tip that has the potential to instantly make you a better hydraulic repairman (unless, of course, you are already applying it), talk a little about the "shop learning" curve, and, of course, provide a couple of real-life examples to illustrate my point. The tip is very simple, and it goes like this:
When you overhaul a component, take apart ALL and inspect ALL.
And in case you were wondering - it makes you a better tech because most shop techs do NOT take apart ALL and inspect ALL. So if you take apart ALL and inspect ALL, you immediately "rise above the crowd".
But why would a mechanic do that? Because mechanics are people, and making assumptions and "cutting corners" is natural for all human beings.
After repairing hundreds of components (be it pumps, motors, or valves) a technician realizes that in nearly all repair/overhaul scenarios the complete disassembly of a component is not necessary. So, after seeing a certain amount of recurring failure patterns, it is natural for a human mechanic to assume that it will always be like that and then elaborate a series "disassembly patterns" that include "cutting corners", and by cutting corners I mean leaving parts of the component not disassembled and/or not inspected.
Take, for example, variable displacement bent axis hydraulic motors. Taking apart the servo-cylinder assembly can be tricky for some models, and involve "more advanced" disassembly techniques, like undoing the thread glue by heating parts, using special tools, and whatnot. So you spend an extra 15 minutes going through this process - and you find zero damage or wear. Then you do it 10, 20, 30 times in a row - and the result is always the same - you find perfectly re-usable parts.
So - next time you strip down such a motor - you skip this process and boom - you just saved yourself at least half an hour! Great! And what do you know? It works! Zero problems! "Hm..." - says your subconsciousness - "Not doing is easier than doing! Duly noted!" So you repeat this again, and again, and again... And it always works! That is - till that one time it doesn't, and you spend all of the precious repair-time that you saved by "cutting corners" on lengthy troubleshooting or, which is even worse, an expensive assistance call under warranty.
Another example - blanking plugs that you find in pumps, motors, manifolds, and anything hydraulic that has a machined oil passage in it. It can be very tempting to just "leave them be". They look so nice, especially when painted over! Inserting an Allen key would ruin that shiny finish, plus - you don't really need to take any of them out to replace the rotary group of the unit you are overhauling anyway! "Ahh!!!" - says your subconscious self - "Unnecessary work! We know better, don't we?" I can relate to that, I get it. Only there will be that one time when there's an obstructed orifice or o bunch of stuff that should not be there under the plug, and you'll miss it trying to save a couple of minutes...
Check out this combination. This is what I call a "proper DCV". When you take out such a spool - it may seem nice and clean, and removing the threaded end caps that are locked with Loctite seems like an unnecessary task and too much work - so it's easy to "let it be". This particular spool, however, houses more valves inside of it, and this particular time they got completely blocked with contamination. Take another look at the size of this manifold, and now imagine it inserted in a 60-ton dumper truck. If you take into account all of the work and the logistics it would take to mount it on the machine, run it, start production, then stop production because it is malfunctioning again, dismount it, bring it over back to your shop, and then redo the lengthy overhaul once again - you realize that spending an extra quarter of an hour to remove the internal spools from the main spool isn't a "waste of time" no matter what your subconsciousness tells you!
And how about these swash plates from Rexroth 10VO085s I worked on last week. Nice and shiny - they seem to present no problem whatsoever. Lap'em and slap'em back, lads! However if you used a blowgun to check if the internal lubrication passages are clear - you'd see that in one of the plates the suction side lubrication channel is obstructed. And, of course, nobody likes removing the break-off plugs that are not supposed to be removable in the first place. (Please, note that I am not advocating the removal of all break-off pugs you find! I am encouraging further investigation when you detect something unusual- and in that case saying something like "... I would check it, but this plug is not supposed to be removed..." is not an excuse!)
When I did remove the plug, I found the internal orifice plugged. I must say that I hate this design. No orifice should be "hidden". And, by the way, I came across recurring failures caused by this design solution before.
In any case - If you failed to detect the blocked orifice - the pump could be reassembled like this and, which is even worse, could pass the bench test, and then fail on the machine when the swash-plate would get stuck due to insufficient lubrication. I saw many situations where a simple blowgun could instantly detect a blocked orifice of a channel during disassembly and cleaning, and yet the component would go through the complete overhaul process without detecting the failure. Sometimes even worse - the failure would be detected, but then nobody would act on it simply because this had never happened before and there's no "investigate when something doesn't seem right " culture in the shop.
I've done enough repairs to know how tempting it is to skip some steps, or cut corners to make the whole repair process faster and, maybe, less tedious. And the truth is - 99 times of a 100 it will work. However the real truth with hydraulic components is - anything can break and eventually everything will break. So if you are used to seeing a component or a part of it without failures - it doesn't guarantee that you will see this forever. Sooner or later it will break!
A mechanic's learning curve goes through three stages:
1) I know nothing (greenhorn mechanic)
2) I know everything (mediocre mechanic)
3) I know nothing (good mechanic)
where the second step is when you become pretty good at "corner cutting" and the third step is when you finally realize that it's probably not worth it. Not all reach stage three, you know.
My advice then is - jump straight to stage three by adopting the "zero corner cutting" policy and applying the "take apart ALL and inspect ALL principle". Do not assume that a part is "good", do check if it is. Assumption, my friends, is the mother of all you-know-what-ups!