This post is for discussing homemade weblock-style webbing anchors
Posted May 9, 2012, 12:56 AM UTC
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Wow that looks pretty nice - I'm assuming you're pretty confident in the strength of each component? What do you estimate the load limit is?
Also, is that KMIII-max?
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I'm not a mechanical engineer, but as a physicist I took a shot at estimating the strength of the components. Multiplying the yield strength of the alloy in PSI by the smallest cross sectional area that takes the full load gives me a yield strength of 12500 lbs. It probably wouldn't even break at that point, just deform. The bolts have a double shear strength that is even higher.
From this number, I'm going to treat it as if it has a working load limit of 2500 lbs. That is higher than I would want to go with my type-18 webbing in any case.
Yes, that is KMIII-max. I got it because of its single strand weave, which I thought would be good for reducing friction. It does noticeably reduce friction, especially through the grigri1 that I am using as a brake. However, it is a little trickier to release tension because of that same property. It wants to just fly through the brake if I'm not really careful.
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There was a little comment thread in another post about these things, which I thought deserved its own post.
http://i.imgur.com/u23xCh.jpg
I made this one out of 1/8 inch 4130 alloy steel. I shaped the side plates with a mill in a machine shop that I have access to, and drilled the bolt holes with a drill press. The bolts are 1/2 inch alloy steel shoulder screws from McMaster. There is an aluminum spacer on the middle bolt that you can't see in the picture, 1.25 inch diameter so that the webbing goes around a bigger radius at the point of highest tension.
As you can see, I didn't bend the plates so a shackle could be attached. I find that it works pretty well to attach it directly to a pulley. There are some washers in there to keep the pulley centered on the bolt.
This was a super fun project, and it works great! Anyone else built anything like this?
edit: spelling
Thanks for posting about this! I've been making my own web locks for over a year now, and I've finally started using SolidWorks to design them. I just picked up my 7th generation sideplates from the machine shop today. They're plasma-cut by the shop (cost me one 6-pack of beer), with 1/2" holes and soft-pad sanded edges.
I included a picture of the SW drawing so that the geometry was easier to interpret. I formed the design around having the anchor- or pulley-side hole directly centered on the direction-of-pull of the line. The barrel bushing (around which the webbing wraps) and the locking pin are aligned so that they don't interfere with the direction of pull, meaning that the webbing stays flat and unimpeded on the surface. The barrel bushing shown in the photo is 1", which is just a temporary stand-in for a 1-1/4" aluminum bushing to come from the machine shop. The bolts are 1/2" alloy button-head socket screws, with nylock nuts on the back side.
The first picture in the album shows my 6th generation side plates, and you can see that the newer model is reduced more than 1" in length.
http://imgur.com/a/8CQEN
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Haven't done real load testing yet, but SolidWorks has some stress analysis functions that I'm exploring right now. It's possible this device is well over 60 kN MBS, but I'll be testing that soon.
would you be able to make another one? I'd like to buy one from you! :D
Nice! Thanks for posting that solid works drawing. Is the image "actual size"?
Aside from the hole geometry, how did you decide on the shape of the plates?
The device in my photos is my second one, and I'm still at the point of just estimating the shape. I haven't made any detailed plans. More or less by luck, the geometry has turned out as you've described, so that the webbing makes a straight line from the top of the bushing without being deflected by the lock pin.
Also, how did you get that pin? I tried to order directly from Jergens, but they didn't have the size I needed in stock, and said it would take a while to backorder it.
I'm not sure if it is actual size relating to the pixels or screen resolution... the plates are right around 4.5" in length and 3" in height. I went with 1/2" x 1.5" pins from Jergens... sorry, but I bought the last two in stock at that length. You might be able to get www.balancecommunity.com to sell the pins to you instead.
I based the geometry of the device on a lot of factors. The circles and lines in the image are each a functioning point of reference. On the anchor-side hole, you can see two concentric circles around the anchor hole. The inner circle is a guide for the 1/2" plate around the anchor bolt, and the outer is 5/8" guide for the SMC pulley block head, so that I can know how much space I have to shove my line through and around the barrel. The small circle that is tangential to the 5/8" anchor circle and one of the circles around the barrel bushing is so that I can easily input a new radius on that small circle and SW will immediately rebuild the geometry of the plate for the new gap.
The multitude of circles around the center barrel is to help reference not just the barrel bushing, but the number of wraps of webbing around the barrel. I used 3/32" as a general webbing thickness under tension.
The diagonal line from the barrel to the front pin is another quick calculation point where I can input the angle of the line (in this case 65°) and SW will rebuild the geometry around it.
The stub nose above the front pin was so that the webbing is completely perpendicular to the exit point of the device. I had one generation of line lock that allowed the webbing to creep up on the side plate due to minor cross-loading, and this seemed to have the potential for increased abrasion.
There's a few more points of geometry that I used for reference, but I'm getting hungry for some breakfast. My main goal was to minimize the size of this generation, and I think it came out alright. My next generation may be 1/8" longer, but there you go.
Thanks again for starting this thread! I'm glad to see others are doing the same thing.
If you're interested, I'd be happy to email the SW file to you. Just send me a private message.
Edit: 65°, not 70°