In this article, I will be talking about twins - i.e. two similar things that are a pair. Hydraulics is filled with twins, both in the prime mover and actuator departments. Whenever I get two equal pumps that work on the same engine and supply a single function, two motors that are connected in parallel and drive the same gearbox, or two cylinders that move the same load - I call them "twins". Theoretically, "closely related" pairs of pumps and actuators also belong here (like tandem pumps, or track motors, which are not connected in parallel, but serve the same function and have mirror duty cycles). I wonder if I should call them "cousins" rather than "twins"?.. In any case - I am sure you get the picture.
There's nothing wrong with twins (or triplets, or quadruplets, or as-many-as-there-are-uplets) aside from the fact that they can fool a mechanic if he takes a "lazy" approach to the overhauling. I consider myself a moderately good tech, and even I sometimes fall for the "twins' charms", despite my experience. Let me give you a concrete example to explain what I mean.
This week I received a pair of Rexroth A6VM250 hydraulic motors for a re-seal:
They come from a raise-boring rig, where they are connected in parallel and drive a single gearbox. As always, the overhaul was "moderately urgent". So, what do I do? Naturally, as an experienced tech that I am, I tell the client that not only will I replace the seals, but also thoroughly inspect the motors and report the detected wear so that he can make a decision about future overhauls or even a possible motor replacement.
So, I open the first motor, inspect the parts, and see nothing but normal and surprisingly minor wear, despite the 15000 hours of drilling, which doesn't surprise me, because I know how well the rig is maintained, and, naturally, since I know that the motors are the same model, and the fact that they are connected in parallel to the same gearbox means that they worked under equal conditions and literally dished out an equal number of revolutions, I already believe that I will encounter equal wear when I open the second motor. And there you have it - I have just fallen the victim to the "twins' charms".
To make things worse, the client called that very moment and inquired about the motors, and of course, I told him that they were "apparently OK", and pretty pleased with myself continued with the second motor, and as I cracked it open... Well, see for yourself:
Yep, I screwed this one up, didn't I? While the first motor looked absolutely fine (I am sorry but I have no pictures of the "good" motor, I hope you can imagine what undamaged parts look like), the second one had the rotary group completely busted. While there was little wear on the sealing surfaces of the cylinder block and the valve plate, the piston sides and the barrels were practically destroyed.
But.. how can this be? The motors have the same age, they are connected in parallel both mechanically and hydraulically - so the wear must be equal, must it not? I agree that it is natural to think that way when you face such "paired" components, but the truth is:
a) you never have all the information, so you can't be 100% sure that during the life of the system there was a "compromising event" of some sort (like a bad start-up, a ruptured hose, or a "transportation predicament") that affected only one of the twins, and
b) sometimes even exactly the same things have different lifespans. That's the mechanics for you. Have you ever heard of the B10 life of a roller bearing? B10 life is the point at which 10% of units of a population of bearings will fail due to fatigue. Here's another fun fact - some of the bearings from this very batch will last 10 x times more before failing. And this very principle can be applied to the rest of the hydraulic pump/motor components.
So, the lesson here is - despite the very natural urge to evaluate the condition of a population of equal hydraulic components (especially rotary) that operate in the same system, basing your opinion on the condition of a single unit, you should only make your mind after you have opened and thoroughly inspected every single one of them.
I guess this is just another way of saying that if you want to be a good overhauler, you should avoid cutting corners, no matter how small they seem.