where are the nerds at?!?!

Discussion in 'Technology / Science' started by redivider, Dec 8, 2017.


    redivider Well-Known Member

    I consider myself pretty handy with engineering type deals but with this I need help.

    I am putting up a DIY zip line on a family property.

    Need to calculate the loads on the ends of the line.

    1/4" Steel Cable-

    44 meter span.

    1.1 meters of sag.

    2.5% incline.

    Max weight = 135 KGS

    So I need the tension for 135KGS suspended from the lowest point, static.

    I need to know the amount of tension this thing will be exerting on the ends so I can figure out the type of anchors. I have two trees identified for the anchors, one is 2 feet in diameter the other is 1.5 foot eucalyptus. the trees are more than sturdy to hold the cable.

    I just need to figure out if I need to do a sling wrap on the tree, cable wrap on the tree or a through bolt with a nut and washer. I do not want to wrap the tree if possible but I need the numbers....

    I am thinking a 1" threaded rod (used as a throughbolt) with large washers and a locknut should be enough, and a threaded eye on the other end with a locknut securing it in place. that should be plenty strong but since my nieces, daughter, wife, possibly aunts and a whole host of other family members are going to be using it I want to be sure.

    anybody here knows physics or has experience with zip lines?????

    ginjawarrior Well-Known Member

    i dont know answer to your question

    eucalyptus are notorious for having poor root systems i would be very wary of using one as an anchor point especially if it is high up on the tree. It might be fine but the dynamic stress caused by 135kg traveling down the ziplines will get quite high and worse case sencario you could pull tree over ontop of a beloved family member
    redivider likes this.

    redivider Well-Known Member

    well fack. ok so I figured out at max speed. I'm looking at a heart pounding 12km/hr.

    that's with the heaviest rider and thus minimum 'loss' of acceleration due to friction between the trolley and the cable.

    the low speed is due to the incline. So they recommend a 3% incline for 'non-braked lines' and a max 6% incline for braked zip lines.

    I'm using 1.9% incline on mine-

    a kid as small as my nieces and own daughter will get to a heart-stopping 4-5km/hr max speed

    I found a zip line calculator from some university for these numbers. I found that with a 3% incline the top speed for a small rider tops out at 12km/hr which is slightly faster than an average jog... heavier riders top out at 16 km/hr which again - which is slow - but I don't want or need this any faster at the moment.

    so I have yet to find good info online on the zip line installation. the manuals don't state how big the through bolts are or anything.

    what I am thinking is a 1" threaded steel rod (custom made I haven't found one long enough for the size of the tree that I will use as a lower anchor) - with 2 of the largest washers i find, and two nuts on the 'back side' of the anchor.

    a 1" piece of hardened steel will withstand close to 40 thousand pounds of force before yielding. so I think the tree will break before the rod does.

    a 'Y' type yoke on the other with a 1" steel pin (with double cotter pins on the outisde as safety). the Yoke would be threaded into the bolt and securred with 3 nuts (one towards the tree and two towards the cable) using two nuts tightened upon each other and bathed in loctite thread locker is a safe way to go I think. finding large cable yokes is not hard - just go to any 'metal fabriated building' or 'industrial steel building' supplier and they use these types of yokes.

    then the zinc sleeve and cable passed through this yoke. the cable gets tied with 4 brackets.

    I will have a 'backup' safety cable tied around the tree, through the eye of the cable that passes thorugh the yoke, through a set (not sure if this is the word you guys use, here that refers to a screw that's put directly into the wood without any predrill or anything- and it does not pass through) eye bolt in the trunk on the far side of the tension (1 meter above the anchor), then tied with 4 brackets. this creates a redundant system in case the rod/yoke or something fails. by putting the set bolt on the far side of the zip line the line tension will not be ripping the bolt from the tree, but it will push it into the tree, so the bolt only has to hold the weight of the rider until they reach the end. calculating a ride length of 12 seconds. the largest eyebolt I find is around 1/4 inch, if I remember correctly 1/4 inch of steel should hold close to 14 thousand pounds before yielding......

    not sure if anybody is keeping up with this thread, justa project I'm going to be finishing next week I thought I'd share...

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