Holes, strategically placed, can be added to most parts with little effect on durability. In fact, it's done all the time to reduce weight (yes, that's my favorite subject). I
did it in my build with Fluxor's tire carrier, which was too heavy for my taste. The basic idea is to not have sharp changes (which we engineering-types call "stress concentrations") in how forces are distributed internally in an object under load. SendCutSend has a good basic explanation
here. Take the most simple example of a hole in a part under tension:
View attachment 703235
The bigger you make the hole, or the closer you get to the edges, the closer those red lines get to each other, and stress is directly proportional to the closeness.
However, in the case of an axle, the force distribution is not only complex, but it's dynamic (changing all the time). It's not so obvious to visualize those red lines. In this case, you have to just make an educated guess, or you have to do an engineering analysis to better visualize the red lines. In this case, I'm advocating for the educated guess method, since I don't have time to do any analysis.
You should use these factors to determine where to put the hole, and how it should be shaped:
- Keep the hole(s) as close to the geographic center of whatever face it's on.
- Keep the hole(s) as small as possible.
- It's okay, and sometimes preferable, to make the hole something other than a circle. Sure, it's harder to make the hole that way because you can't simply drill or use a hole saw, but offsetting from the outside edges is a good way to do it - usually. Like this (parden the shakiness - using a mouse):View attachment 703238
- Don't put any sharp corners in the holes. If you use holes like above, use the largest radius you can at each corner.
- Stay away from other stress concentrations. For example, in your last picture in Post #15,014, you have two stress concentrations close to where you're pointing: The bend radius of that piece, and the witness mark from the press brake knife just under your finger. Both of those have a double-whammy on stress in that area: first, they cause forces to be concentrated, as just mentioned, but second, the forming has work-hardened the material, which makes it stronger, but more brittle. In a part subject to dynamic loading, work hardening and brittleness is not good because cyclic stresses cause fatigue failures. Regarding the press brake knife witness mark, I've seen those cause a failure. They seem like just a cosmetic defect, and 99% of the time that's all they are. Stay away from them, and that 99% goes to 100%.
- Don't create a bunch of small stress concentrations by leaving a very rough cut line. Use a file to smooth things up if you need to. A bunch of small, jagged bumps can add up to a significant stress concentration. I spent a LONG time cleaning up my cuts in that tire carrier for this reason (and I still wasn't satisified with it, but it's stainless and very hard to file.) It's also why I love having parts laser cut! Side note: even with laser-cut parts, I have a bit of edge clean-up. There's always a "tag" left somewhere on each perimeter. It's there because they need the cut parts to stay put and not move around while the toolhead is still traversing the workpiece. If not, the toolhead hits a loose part, and now the laser alignment is screwed up, or worse - the machine breaks. It's ever so small - just large enough to hold the entire workpiece together, but small enough to twist and break the cut parts off the workpiece. I still file them off unless I KNOW they won't be a crack instigation point!
All that said, I always look to Mr. Blaine's work for cues because he not only understands these things (many times better than I do), but he also understands the loading parts see, and how things break. I understand the engineering concepts, but I am out of my element with TJ axles and suspension. My LJ is the first vehicle I've worked on with solid axles, so his expertise and genius-level solutions are always my first stop when approaching design of a part for the LJ. Look at his mid-arm truss (picture credit
here):
View attachment 703240
See how inconsequential those vertical faces are? That tells me that those faces don't see much stress, so that's where I'd put the hole(s) for access. You should be able to do something like this to one or both sides of those vertical faces and get in there with a combination wrench (maybe with a flex-head?) and your cigar-stained digits to remove/replace the fitting:
View attachment 703243
Let me know if I've glossed over anything, and I'll clarify.
