Insane Hydraulics
Bold and audacious blog about fluid power
"Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."
Albert Einstein
I have another tip for you today. Two tips, actually, and the first one is - if you want to be a professional - stick to doing what you are good at, and leave the rest for your off-work time. Here's a neat picture to hook you even further:
How about that for an attention-grabber?!!
And now let me make everything I just said make sense and leave you with a small (but real) mechanical tip in the process.
I am a guy who works primarily with hydraulic stuff - and yet people often ask me for help with mechanical problems - which is OK, of course, but only to a certain degree - because when I am responsible for charging a client for the time I spent solving his problem - I want to charge a high fee and I want to feel good about it. And it never feels good billing several hours for a job that an experienced professional would finish in fifteen minutes. But life is a complicated thing, and sometimes circumstances align in a way that makes it impossible for me to refuse a "cry for help" with things that are outside of my direct field of expertise, and naturally, in such cases there's always a chance for committing the most ridiculous goof-up... or a series of goof-ups... like, for example, that time when I was asked to fix a malfunctioning milling machine... (yes - a freaking milling machine - see The Tale of the Golden Safety Rule for amusing details).
So, today, I have another one of these for you, and please, don't feel bad if, while reading this short story, you feel the urge to say something along the lines of "What an idiot!!!" out loud - I totally get it.
Anyhow, I was asked to "have a look" at a Dana Spicer two-speed planetary gearbox that "got stuck" after it had been assembled with a new drive shaft. I'd done enough final drives to know my way about a planetary gearbox - but first - it was something I didn't do every day, and second - I'd never worked on the model that the client asked me to fix, but hey - since I couldn't turn this one down anyway, I just said to myself: "How hard can it be? I guess I'll see soon enough" - and proceeded with taking it down.
I quickly discovered what was wrong with the gearbox, though - the non-original output shaft was slightly longer than needed, which was enough to jam it, but here's the "interesting" part - after digging for information online (which took a lot of time, because there wasn't much information on that model - T2A5C43S - the only "decent" thing I found was this Eider Fordia manual with a highly pixelated cutaway view of the gearbox) I determined that there were supposed to be two washers inside the planet carrier - and this gearbox contained only one! This here is a carrier from a similar gearbox. You can see the two washers inside - over the planet gears, and they are supposed to be the sliding interface between the sun gear and the carrier.
But the technical details on what these are for are not important - what's important is that there were supposed to be two washers inside the planet carrier, and the last assembler left only one. So, I spoke to the client, and we decided that we would cut the shaft down to the original length and install a second washer to make the gearbox as original as it could be. The only problem was the fact that adding the second washer would require removing one of the planet gears, and the shafts of the gears were secured with tight spring pins installed in what seemed like blind holes:
You can see that the hole does not go through, which means that these pins are easy to hammer in but are virtually impossible to pull out, especially considering how small and tight they are!
If you are used to repairing such gearboxes and you know the right way to work through the pin and take the gear out (which takes about two seconds), I am sure that you are cringing right now! I am with you on that, brother, I am cringing myself as I am typing this. What can I say? I can only promise that I will show the right way after I tell you about the unsuccessful (and pretty idiotic) attempts to pull the damned thing out. In my defense - when I showed the carrier to the mechanic who'd brought the gearboxes over and mentioned that removing the gears would be tricky because of the whole "sprig pin in a blind hole business," he said - "Just drill them out, mate! You'll be all right!" which, kind of, set my mind to it right from the start.
Before anything - I decided that I would leave the drilling as a last resort because the pins were hardened, which meant I would need carbide bits for that, and with the diameter that small, I would most likely end up with a piece of a carbide bit stuck in a blind hole, which would be even worse.
So, I Googled "removing a spring pin from a blind hole" and tried the most common suggestions:
Tap and Pull with a slide hammer (alternatively - use a self-tapping screw to grab and pull the pin). The pin was too hard for that - the tap would not bite into it no matter how hard (pun intended) I tried.
Hydraulic Action - filling the cavity with oil or grease and then hammering the fluid with a tight dowel the size of the hole in the pin so that it would push the pin out. Maybe something like that could work for a larger pin, but not for this one - too small for that. There were suggestions to use an even thicker substance for the "work fluid" - like a paper towel mushed into a slurry or even corn flour - but after the attempts with grease failed, I decided not to experiment in this direction any further.
Erosion methods were, quite obviously, out of my reach, like the often suggested plunge EDM (electrical discharge machine). Maybe someday.
Chemical methods - in other words - dissolving or weakening the pin with acid. I didn't even want to think about that!
Using super glue (cyanoacrylate) to bond a suitable pullable piece of metal to the pin to pull it out. I was actually going to give it a shot, but I decided to try another method first, namely:
Welding something to the pin to grab it and pull it out. I can see how this could totally work with a larger pin, but with this one (only 4 mm in diameter), there was no way I would be able to stick, MIG or TIG weld something to the pin and not the steel around it. However, being a "creative thinker," I thought of using my other "secret weapon" for that - a transformer from an old microwave oven which I occasionally use for rudimentary spot welding (among other things):
In case you don't know - if you cut off the secondary of a common microwave transformer and replace it with a couple of turns of thick (25 mm²+) copper wire - you'll get yourself a low-voltage/insane-current capable source, which you can use for all sorts of "resistive heating missions." In this case, the idea was to either spot weld a nail to the end of the pin or, if it would not work, to maybe heat the end of the pin enough for it to soften and become tap-able:
So, feeling like an adventurous pioneer I commenced pioneering. And of course, nothing good came out of this. The marvelous picture that I showed you in the beginning of this post was my attempt to take a shot of the nail being welded to the pin, but since I was holding the camera with one hand and pushing the transformer button with the other (which is why the nail end was actually just resting on top of the pin, without any downward pressure) - my coordination got scrambled and I "overdid the button ", so to speak... Luckily the camera took the shot right at the moment the nail evaporated. Still - you have to agree that the picture looks awesome!
To resume - the improvised spot welding didn't work either. The nail would not weld despite all of my attempts, and the pin remained hard, too. The part would heat up, the nail end would glow red and all, but it would not weld. Then I decided to try something else - I decided to tap on the nail head with a hammer as I would turn the current on, hoping that the compressing pressure of the tap would aid the welding process (like a forge welding of sorts) - and, believe it or not, this idea saved the day!
But not the way you think! No, the nail didn't weld to the pin. However, when I tapped on the nail hard a couple of times - I saw the pin sink into the hole for a good couple of millimeters - and then it dawned on me - the "stupid engineer" who designed this joint was not stupid at all! He (or she) was a genius! Because he/she/they used a short spring pin in a long blind hole, and all I needed to do to remove the shaft was to push the pin in, instead of pulling it out!
So, in certain assemblies, a spring pin needs to be pushed in instead of being pulled out for the part to be removed. Next time, I'll take a piece of small-diameter rigid steel wire, bend its end into a small hook, and feel the end of the pin to confirm if it's shorter than the hole or not.
In any case - even if you already knew all this, I hope that you are closing this page with a smile on your face because you just read how someone attempted to disassemble a gearbox with a piece of microwave oven!