I'm not an engineer (ok, I am, but not that sort. And even if I was that sort, I'm not your engineer. Give me money and we'll talk.). None of this should be taken as advice. Not even "I did it and it worked for me.". I haven't done it. I'm planning to do it, but this might be more "cautionary tale" than "good advice". I am "some wacko on the internet". Don't rely on me.
Also, I did google around. The best thing I found was (approximately 100 links to) this blog post which isn't really what I'm going for.
Some months back I was able to buy a power rack. It's a piece of junk. It had virtually no attachments, what it did have is junk, it's some non-standard size, close to 2" but not close enough to use most 2" attachments,... But it was in stock, and was actually fairly cheap. I'm planning to replace it once production catches back up with demand, but that might be a few years.
Anyway, the safeties are 1" (I'm pretty sure actually 25 mm) round steel bar They're a pain to use, they're kinda heavy for the kids to adjust, they scrape up the bar... I was thinking that strap safeties would be nice, but, given the fact that my rack is not a standard size, I can't find any that would fit my rack. And even if I could, such things seem to go for $150-ish.
Which made me think about making them. The standard design requires welding and/or bending very thick metal, neither of which I do, and anything named "safety" strikes me as a bad first project. But there's an older design that doesn't look that hard to fabricate.
Random aside, I'm pretty sure the strap in that picture is upside down.
Pounds are the correct unit for once
From here on out, to be clear, I'm talking lbs, and that's not just because I'm American. In this case it's actually right. IME Europeans (TBH, I don't know many non-European non-Americans) typically talk about "kg of force", and that's wrong. It's Newtons. Now tell me without checking how many Newtons your squat PR exerts on the ground at your altitude. I'll wait.
Okay, enough defensiveness, the plan
For the strap I was thinking of getting something like this which has a breaking strength of 10,000 lbs and a SWL of 3,330 lbs. Now that's static not impact, but umm, yeah, 3,330 lbs is in the "safe working load"...
The strap pins
For the pins to attach them to the shackles I was thinking something like this which is J684 rated, which seems to mean that it can tow up to 10,000 lbs (ok this is a weird one where lbs are wrong, but it's not kg either. It's both mass & force on the ground x friction,... It's weird.). TBF, this is inside a coupling that is different from what I'm planning. But I've only ever squatted 410 lbs, and a towing standard has to assume some level of dynamic load. I think I'm safe.
Then there's making the shackles themselves. This is the only part of it that looks like doing something. My plan is to make 4 2" wide flattened V shapes out of 16g steel.
According to this chart cold rolled steel has a yield strength of 51 ksi. Per this chart, 16 gauge steel is .06" thick. That seems to mean that 1" wide 16g cold rolled steel should have a yield strength of 3,000 lbs, and a tensile strength about 20% past that. So 2" x 16g steel 6,000 lbs. I'm pretty sure that the shackles are by far the weakest part of my planned safety, and they should still be about 15 x stronger than I'll ever need.
That said, if I can find slightly thicker plate, I'll use it. Though I think the only real reasons are to limit deformation near the clevis pin holes, and aesthetics. But 16g seems to be what local hardware stores sell. Also, TBH, much thicker than that, and you start to need the right tools, which I don't have. I'd definitely use 15 or 14g if I can find it, but I'm not convinced I could use 11g if I found it.
If I had a welder I'd weld on a fender bolt on either side of the clevis pin holes and I'm pretty sure I'd be fine.
The shackle upright pin
I'm planning to just cut a rod of 1" round to length. I'm pretty sure I could use schedule 80 pipe if I can't find the rod. The issue here is how it fits in the hole in the upright, not its strength.
So my idea is to
- cut out 4 2" wide flattened V shapes from the 16 g steel
- drill a 5/8" hole through near the ends
- drill a < 1" hole through the middle
- bend them around my rack uprights (with some plastic (my current thought is a cut-up milk jug) in between)
- using some sort of anvil with a slightly > 1" hole, hammer a rounded end 2" piece of 1" round into the just shy of 1" hole from the inside out. Leave an inch or so sticking out to make it easier to grab
- steel stick epoxy that in place? Honestly I think that the friction will hold that fine
- cut the D rings off of 2 of the axle straps (be careful, don't cut the strap with your saw, or with the sharp no-longer-D-ring end as you're removing it)
- put the shackles around the upright, with the 1" bar in an upright hole
- put clevis pins through
- a washer
- shackle end
- axle strap eye
- shackle end
- then R-pin it in place
I'm omitting some "file sharp corners off", "deburr steel", "test fit with cardboard", "paint the steel", ... steps that I hope are understood by anyone planning to try this.
This looks to me as though it will cost about $50 in materials, and doesn't look like the tooling will be anything that you don't have already. And you'll have a spare pair of axle straps that, umm, will be useful later? Trade them with a friend who's doing the same thing?
Random other thoughts
- I'm thinking about also getting a set of 36" straps so I have more flexibility. If so, I'm planning to get them in a different color so it's exceedingly clear which is which. Because I've never made dumb mistakes about how long something is, or how much weight is on each side, no no, not me. But I'm told it can happen, you know, to other people.
- I'm pretty sure that if I build this as planned, the rack itself will be the weakest link. Did I mention that it's a piece of junk? But A) I'm already depending on it, and B) I'm pretty sure it will fail plastically and be destroyed, rather than kill someone.
Request For Comments
So, does anyone see any problems with my idea? Does anyone think it's great? Has anyone tried this? Did it work for you? Does anyone have ideas to make this work better? Is my math bad? Did I rely on the wrong numbers?