Knots

What kind of knots are you supposed to use to attach a rope (dyneema) to a drum for a winch? I read somewhere that a knot is where the rope will fail most often so I want to try and make sure that the knot we use doesn’t fail. We plan on riveting the knot to the drum.

Paging @RoboChair (who was demonstrating knots and splices at LAN)

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Our winch drum is an aluminum tube 1.25" outer diameter. We fitted it with hex-bore hubs on each end to keep the cord in place, and drilled a hole through the tube diameter that the cord passes through. Then we tied a stopper knot (figure-eight) in the cord to keep it from running out.

Knots always weaken a rope. A figure eight is probably the strongest.

" …the figure-eight follow-through, which, when pull-tested, breaks at 75 to 80 percent of the rope’s full strength" - Climbing Magazine

Resident Eagle Scout checking in.

Go with a climbing knot. That’s a double figure 8 or a bowline. Neither are particularly hard to tie (double figure 8 on a bite will be easiest.) You’ll definitely need a post or hole or something to tie to though. You’re not gonna get enough friction on a cylinder that you can just tie around it.

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@Richard_Wallace @BryceHanson. So to secure the knot, I would drill all the way through the tube, and make sure the knot is on one side of the hole. Correct?

Actually, a figure eight is not ideal in dyneema line. It’s too slippery for most knots to be really effective. I use this stuff all the time in camping equipment (hammocks, flys, etc.) so I’ve had a lot of experience with it. The best knots for it are either the EStar stopper (for a stop knot) or the buntline hitch (for a loop) since both have the least slip and most strength in dyneema. If you follow the links, you’ll find the Animated Knots pages that will show you how to tie them.

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Go with this person’s advice.

As for stopping: Drill through the tube in one place (doesn’t have to be all the way through), feed the rope in, and tie a stopper knot. That way your knot will be inside your tube and out of the way.

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We have 3 winches. One to raise the arm, one to extend it and one to lift the robot. The two smaller winches use the AndyMark Large Standoff Winch Drums so the Dyneema is tied around one of the standoffs with a square knot. The lifting winch drum is a 1/2" hex rod. Two big, beefy shaft collars are used to keep the bearings in place. A hole was drilled in one of the shaft collars and the Dyneema was tied through it with a square knot.

I will be checking out the links provided by @Strategic later. What do you recommend for preventing the cut ends of the Dyneema from fraying? So far, we have been tying a half hitch.

For the two winch drums, use the buntline hitch to put a small loop around a standoff. For the lifting winch, use the EStar to put a stop knot on the line after it’s pulled through the hole in the shaft collar.

The best thing to do with dyneema to stop fraying is to carefully melt the end of the line to fuse the fibers. A soldering iron is probably your best tool for this. Just tying a knot in the end won’t really stop the fraying very well.

Thanks for your suggestions.

What about soaking a short section near the end of the Dyneema with runny super glue (cyanoacrylate) to bind the fibers together?

Otherwise, I will get out my soldering iron. The students tried using a BBQ lighter but didn’t get very good results.

The superglue will work okay. Lighters (the traditional thing to use on synthetic fiber lines) don’t do so well on dyneema. It tends to burn and shrivel rather than melt and you don’t get good fusion of the fibers. That’s why I recommend a hot tool like a soldering iron, which will fuse but not burn them.

That is exactly what happened. Thanks again for your insights.

We used this eye splice in 2016, it’s pretty slick:

I don’t think it is very applicable to smaller diameter line, it’s a little to tiny to handle. If you make the non-locking version of the Brummel splice, then you have a rope with an eyelet at each end that is an adjustable length.

You should also check-out the Capstan Equation. We run 3-4 turns on the winch spool at full extension of our climber, at that point the knot sees a very small holding force.

For years we’ve been using spectra spearfishing line for winch climbing. Our preferred method is to wrap it directly around 1/2" hex shaft. To attach to the shaft, we take a 2-piece heavy duty shaft collar and split it open. Wrap one or two wraps of the line around the collar. Then clamp the collar back onto the shaft with screws slightly longer than the originals. No knots to slip, no drilled holes in either the shaft or collar to weaken them, and no sharp edges to chafe the line. Plus, an inexpensive 1/2" ratchet wrench can be used on one end of the shaft to keep your climber from backdriving.

What did the loop attach to?

That splice was used for a mechanism that wasn’t a winch.

In 2018/2019, we attached an eye loop (I don’t recall the knot) to our spool like this, we tapped the spool body and looped the eye around a bolt/spacer. Gaffer’s tape held the line in place near the eye loop, but the strength for the cable came from multiple wraps around the spool.

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