Swing Project - Engineering Advice Needed

[sand_man]

So the kids want a tree swing. None of the trees on our property have an outward branch, fit to hang a swing from. The style of swing is the classic, single-seat, wood bench with rope on either side. My plan is to stretch a line between two trees and hang the swing from that. We'd like to keep it natural looking, so trying to avoid cable or straps. I was thinking about using a 1" diameter hemp rope (or similar style) wrapped/looped around each trunk, and secured with knots (bowline, taught line hitch, timber hitch, etc.). I was even thiking about doubling the line between the trees (two passes of rope) and using U-bolts to keep them joined together. The U-bolts could also double as stops to keep the swing ropes from sliding along the horizontal line, between the two trees. I'll alo be sure the tree trunks are protected from chaffing.

Here are some pics of the trees in question. Additionally, here is a napkin sketch of the plan. Both trees have a branch that I could utilize as a vertical stop for the rope. I will also be sure to check the forward and rearward travel distace of the swing, to ensure there's clearence...don't want anyone to hit the fence!

An additional requirement: this needs to support the weight of teenagers and adults.

So what say you? Will this work? I'm trying to keep it simple. I thought about running a beam between the two trees, but that might be overkill. I want it to be safe, but not over-engineered or overly complex. My one concern is even with a taught line, how much sag will there be on the center of the line, with weight on the swing?

[sand_man]

Also, to help reduce friction on the horizontal line, maybe can employ some carabiners on the ends of the swing ropes...

[vash]

without doing the math.

no it wont work. two things. it will be a B to get that horizontal line tight enough. and when you pull down from the center, you add force to that horizontal line exponentially.

extreme (magic) example..if a truck was stuck in the mud..you could feasibly get a long enough strap..lonnnggg...like miles long. assume zero stretch in this magic strap. tie the end to something rigid..hike the many miles to the exact center length..grab that strap and pull it off to the side and you could pull out that stuck truck. i remember this problem in school when we were solving vectors.

[sand_man]

Math? Aw man, nobody told me there'd be math...

[sand_man]

You know, in looking at my first picture, that one limb might be strong enough...

[vash]

just for fun.

how far apart are the trees and how much does your heaviest kid weigh ??

i think i have my old math book around here somewhere.

[KFC911]

Originally Posted by sand_man

Math? Aw man, nobody told me there'd be math...

As a redneck engineer who didn't go to NCSU, I'd rig a good thick chain between the trunks above the branches....use an O-ring or nut through the links to take out slack....one run across. Take pics

[sand_man]

Originally Posted by NotaBRG

I agree, there will be too much sag. Even if the rope had no stretch you would pull those two trees together enough for the swing to get too close to the ground.


I think you need a beam.

Yeah, that's why in that crappy napkin drawing I had the smaller tree staked to the ground to help oppose any bending forces. But "X'd" it out...

[sand_man]

Originally Posted by KC911

As a redneck engineer who didn't go to NCSU, I'd rig a good thick chain between the trunks above the branches....use an O-ring or nut through the links to take out slack....one run across. Take pics

HHHMMM, chain! Yeah I know it goes against the "natural" requirement (women ), but might have to use something like that

[KFC911]

Originally Posted by sand_man

HHHMMM, chain! Yeah I know it goes against the "natural" requirement (women ), but might have to use something like that

Camo paint

[sand_man]

So, now re-thinking how to employ a beam...

[KFC911]

Seriously, why not try a taught chain....easy to rig up. If one end comes loose, I didn't study law at Duke either....so this ain't legal advice

[vash]

this was way more interesting than what i am doing at work.

i just did a free-body diagram.

assuming the trees are 10 feet apart. and a kid weighs 50lbs. pulling down in the middle, when that rope displaces 3" (assuming no stretch - like a chain). you are pulling one tree to the side 1000lbs.

(unless i forgot how to do vortex problems...but i think it is just a similar triangles thing)

[sand_man]

Originally Posted by vash

this was way more interesting than what i am doing at work.

i just did a free-body diagram.

assuming the trees are 10 feet apart. and a kid weighs 50lbs. pulling down in the middle, when that rope displaces 3" (assuming no stretch - like a chain). you are pulling one tree to the side 1000lbs.

(unless i forgot how to do vortex problems...but i think it is just a similar triangles thing)

Sorry, I meant to respond. Yes, trees are 10' apart and assume an adult of 190lbs (I might want to have a go).

[sand_man]

Originally Posted by KC911

Seriously, why not try a taught chain....easy to rig up. If one end comes loose, I didn't study law at Duke either....so this ain't legal advice

LOLz! And I haven't stayed at a Holiday Inn...

[flipper35]

Why not just put the rope from one branch to one side of the swing and then the other side to the other tree? That way the force vector is not horizontal between the trees but angled to the swing/ground and you won't snap the rope/pull the beam out when the wind blows the trees around.

[Aerkuld]

Why not this (in yellow)?

[sand_man]

Originally Posted by Aerkuld

Why not this (in yellow)?

Interesting. Is that what flipper35 was getting at?

[vash]

that would make holding on to the chains odd feeling no? not bad of an idea tho..

how tall are those anchor points. ?

[flipper35]

You could put a chain/rope across 4' above the seat to pull them together to hold onto easier.

Yes, that pic is what I was getting at.