Recently, there was a big hullabaloo in my neighborhood regarding the swings in the park maintained by our HOA. Someone posted a picture on our neighborhood Facebook page showing two of the swings hanging by only one chain.
As you might imagine, the neighborhood keyboard warriors responded immediately. They vehemently claimed “Obviously they’ve been vandalized by someone,” “We need cameras, this is getting out of hand.” Some even threatened physical violence. They seemed to be suggesting that the teenagers in the neighborhood had gone out with bolt cutters and cut the swings down. As if that was the only possible explanation.
Now, cyber-bullying is a real thing, and most everyone needs to relax a little when they are typing. We have lived in this neighborhood for a while, and we love it. We know most of the teenagers. They are really good kids. They stop and talk with us at the park. They play pickle ball and tag with our younger kids. They write motivational quotes on the running path that goes through the neighborhood. We have paid some of them to babysit our kids and watch our dog over the years. These kids would NOT go out and cut down swings just for the fun of it.
It made me a little angry to see people blaming the neighborhood teenagers, without any evidence. I was also curious to see why the swings were breaking. Failure analysis and mechanical engineering expert witness work are a large part of what I do for my day job, so I thought I might be able to shed some light, and some actual evidence on the situation. The park with the swings is only about 100 yards from my house, so I walked over to look at the chains and see if I could find the broken link to look at the failed surfaces.
Even though it was dark out when I read the post and walked over to the park with a flashlight. Something the keyboard warriors couldn’t be bothered to do, but it didn’t matter because they were already convinced… or so they thought. After a quick look at the swings, the chain, and the broken (not cut) link, figuring out why the chains were breaking and what might be done to fix it, was very simple. So I posted my pictures and findings to our neighborhood site, which, as it turns out, people were rather grateful for. It turns out that people really didn’t want to blame the teenagers, but sometimes that is easier than actually trying to figure out what the problem really was. My post looked very similar wo what follows.
Hey neighbors, after seeing the post about the broken swings, I walked over to take a look and gather some evidence rather than jump straight into blaming people and accusing them of vandalism.
If you look at the design of these swings, there are large bearings at the top of the chains that allow the swing to rotate. The chain should remain relatively motionless relative to the bearings taking a load that is almost pure tension (the chain experiences negligible torsion or bending during the anticipated use).
I also noticed that the swings are also set up so that the swing is about 12 inches off the ground, quite low for anyone over 3.5 feet tall to use. So, when a taller user (accused older kids) wants to use the swing, they raise the level of the swing by throwing it over the top bar. This is how the still functioning swing was set up when I saw it tonight. According to a quick google search, swings should generally be set up 18-20 inches off the ground.
When the swing is used with the chain wrapped around the bar the chains rotate and twist on the links rather than the bearing. Not only does this cause wear on the links as well as the top bar, it also applies bending and torsion (twisting) loads to the chains. I actually did some testing on twisted chains a while back and found that it can reduce the strength by 70%! (https://youtu.be/clj42no4DxY). You can see the wear on the chains in the pictures.
So, it seems that the adults and older kids who use the swings with them wrapped around the top bar are responsible for the failures, but it is certainly a stretch to call it vandalism. Perhaps an innocent misuse would be a better description.
A good solution might be to find a height for the swings that is high enough for an adult user but not so high as to be dangerous for a toddler… a happy medium on height… and set the swings there, so they can be used properly and allow the motion to come from the bearings. Likely somewhere between 18 and 20 inches.
I am a licensed professional engineer and most of my work is forensic failure analysis so this is the type of thing that I do for work on a regular basis. I am often doing similar analyses on garbage trucks, mobile cranes, aerial lifts, construction equipment like skid steers and back hoes, and other manufacturing equipment. However, if you think my 5-minute analysis and the evidence provided is incorrect and continue insist that it is vandalism, I’d be happy to discuss it with you.
The post was met with gratitude and there were no further comments about vandalism.
About the Author: David Smith is a licensed professional mechanical engineer, a certified safety professional in comprehensive practice and the Vice President of Alpine Engineering and Design, Inc. He specializes in product development, machine design, design safety reviews, risk analysis and expert witness work. If you would like to speak with David about your equipment or a new case, please reach out through www.AlpineEng.com