The following story is not related to oil hydraulics per se, but the golden safety rule that it so well underlines is of extreme importance to everyone who operates and/or services heavy hydraulic equipment. I will tell you the story of how I "let it slip" due to mental inertia, and despite all of my "industrial experience" came within a hair's breadth of seriously injuring myself. I was lucky to come away unharmed and with a lesson learned, but it definitely could have been the other way around..
So - I get asked to have a "quick look" at a vertical milling machine that stopped working...
Yes - you read it right - a milling machine. A bike shop was asked to fix a car. A house painting contractor was asked to restore an old painting. A freaking ophthalmologist was asked to perform open-heart surgery!
What do I know about milling machines? Zilch! We have an old lathe in the shop, and I may occasionally make a new rod, or maybe a cylinder gland, but this doesn't happen too often - only when I do repairs for friends or "VIPs". Top it off with a drill press, a band saw, and some welding equipment - and you'll get a good view of how far my "metalworking abilities" extend.
And this one was a Knuth Servomill UWF15, which is, technically, a conventional vertical milling machine, but its axes are servo-motor driven. A fly-by-wire system of sorts, in which there's no mechanical connection between the controls and actuators.
Normally I refuse such requests because I never work on stuff I am not familiar with, but that time it was different. I don't want to go into the details of why I did the assistance on a piece of expensive equipment I knew nothing about - all I can say in my defense is that it was one of those jobs that "one can't refuse".
At least the information that I had at my disposal was reassuring, as it seemed to point at a simple mechanical problem - i.e. something I felt I could solve.
Let me tell you about the malfunction now. The Z-axis of the mill stopped working. The client, obviously, asked for assistance - and the company responsible for the after-sales service support sent an electrician to check the electrical stuff (electric motor driven axes - remember?) So, when the electrician "verified" the machine, he reported back saying that the motor "was turning", but the worktable was not moving.
Very strange, yet not an uncommon piece of news for an industrial mechanic, isn't it? We deal with hydraulic systems and running into a broken electric motor coupling or a sheared shaft key is a routine thing. So, after a quick "over the phone conference," it was decided that the mechanical link between the actuator motor and the table lifting screw was broken, and therefore a mechanic was needed to inspect and rectify the problem. I happened to be the mechanic that was around and available.
So, I thought to me-self - I can do this. I've worked with mechanical stuff for years, I've replaced a ton of shaft couplings, and even if the mill is servo motor driven - so what? I'm a certified "electronic tinkerer" who's done his share of hobby projects to feel relatively comfortable around stepper motors and whatnot - so, I'm not scared at all! I'll replace the freaking coupling in a heartbeat!
So, filled with much confidence, I gathered my "good" tools, jumped in the service vehicle - and took an hour and a half drive to see the broken mill.
Upon arrival, I took some time to get familiar with the machine and the shop manager was very kind to give me a crash course in its controls. To be honest - I found it quite simple, which filled me with even more confidence than before! Of course, the servo motor controlling part was complicated enough, but mechanically it was the common three-axis milling system, and since the axis-driving motor was working, I was sure I'd nail the mechanical problem in an instant.
Now... about the motor that was turning... As soon as I was shown the "perfectly working motor" an alarm went off in my brain - "This doesn't seem right... This doesn't seem to be right at all!."
Are you telling me that this is the motor the electrician said was "rotating just fine?" - Yes - that's the one! I may not be a milling machines guy, but even I could tell that a tiny three-phase squirrel cage motor could never power a precision linear drive! A quick inspection showed that it was connected to a small lubrication pump, and naturally, as soon as the auto-feed was turned on, it would start to rotate - that was what the electrician saw and ticked off as "perfectly functional".
Correct motor or not - my mind's inertia was already running at full speed. I came here to find an electric motor that was driving an axis and remove it from the mill to verify the condition of its coupling. So - that's what I did next. The heavy servo motor that was driving the Z-axis (vertical movement of the worktable) was placed on the left side of the machine, under the white shroud:
I had to use the photo from the KNUTH's own site here. I did shoot a couple of videos, and I managed to pull a couple of decent frames from them, but none show the motor placement, unfortunately. You will see why I wasn't in my "picture-taking mood" in a minute.
The mechanical arrangement of the Z-axis drive was simple. I even managed to remove the control knob cover and peek inside the frame - which allowed me to see the coupling, which looked perfectly intact by the way. But my mind was already dead set in the "coupling exchange mode", and so I blindly continued with the "game plan".
I stuck my hand inside and tried rotating the coupling, and it was completely stuck. I couldn't rotate it at all, not even with a tool, although the opening was very small, so I didn't have enough space to apply significant torque. Then I decided to pull the motor out, just as I had planned, and see if I could move the coupling then. A thought crossed my mind - "even if the coupling is OK, maybe something got jammed in between the gears or something..."
All of this seemed like a pretty familiar and very straightforward mechanical system to me. At no point did it feel that I was doing something dangerous. And the fact that I had done so many successful interventions on industrial equipment was filling me with just enough confidence to be doing all of the nonsense with the sweet feeling of "I know exactly what I'm doing".
My next "strategic" move was removing the heavy servo motor. Take another look at the way the mill is built, and the place where the servo motor is seated, and tell me how you think I approached the motor removing task. Of course - like any field mechanic would! There was just about enough space to squeeze myself under the said motor, rest it firmly on my left shoulder, remove the four screws securing it in place, and then gently "disengage" it from the machine by prying it off with a screwdriver. Although small, the motor was a solid chunk of metal. Just by wiggling it about as I loosened the screws, I could tell it was pretty heavy.
So, here I am, kneeling down under the milling table with a heavy electric motor firmly rested on my shoulder, rocking the flange as I pull it back, secretly proud that I am still fit enough to be doing this single-handedly, and as I finally pull it out... nothing happens. Even though the motor is much heavier than I expected, I keep a poker face, and carefully place it onto the previously prepared wooden block as if it weighed a couple of pounds:
Then, while still in the same position - i.e inelegantly crouched under the worktable - I place my hand on the coupling inside the frame and try rotating it. Still nothing. So, I grab a wrench and try to very awkwardly rotate it - still nothing. Seems like I need a bigger wrench, don't I? I finally remove myself from under the table and the instant I do that I hear z-z-z-z-z-z as the screw gears over-speed and the heavy worktable flies crushing down till it hits a wooden block lying under it!
I got covered with sweat in a second! It's summer in Portugal now, and the temperatures are indeed very high, but it was not the high temperature! It was the realization of what could have happened if the heavy table decided to "release itself" when I was still holding on to the heavy motor on my shoulder crouched under it, with my "back-knee arrangement" forming a straight and firm line resting on the floor in my attempts to support the heavy servo-motor with a straight face!
Do you know the weight lifting capacity of this machine? I looked it up. Knuth states that you can put works of up to one ton one table. One freaking ton! I am not sure how much the table frame weights, but I am sure it is enough to do serious damage to my spine!
So - what do we have here and what's the golden safety rule I mentioned earlier?
We have a mechanic, who's working on a system that is very simple, yet completely new to him, which makes him forget the most basic safety rule of them all - "What goes up must come down!"
I have this rule I never ever skip - before doing any work on a hydraulic system, any work at all, no matter how harmless a system may seem - forget about everything, look at the machine from all angles, see what is "in the air" - and then bring it down! Any work at all must begin with this simple step, as hydraulic systems move heavy things, and if they are "up", as soon as you remove or relieve their actuators - they will come "down"!
Starting from today - I am changing my golden safety rule from "before doing any work on a hydraulic system..." to "before doing any work at all..."!
And what about the mill? It turned out to be an easy fix in the end. All I had to do was re-upload the controller software (kindly provided by the OEM) and fix a loose wire in the control board!
I did learn my lesson!