Zip Line Challenge

My spousal unit, who actually writes this blog, is an author and illustrator of books for kids. She ran across this website for aspiring engineers, and thought it would be interesting to some of my readers. DiscoverE (formerly the National Engineers Week Foundation) helps to unite, mobilize, and support engineering and technology volunteer communities. They hope to increase the collaborative footprint in K-12 education and celebrate with the public as it discovers the value of engineering education and careers.

zipline

I was especially intrigued by their Zip Line Challenge for kids. It’s actually a model that challenges kids to transport a ping-pong ball down a zip line from start to finish in 4 seconds or less. The activity discusses many of the considerations when designing and building a zip line. I’ve uploaded the PDF so you can download it from here, or you can get it from the website listed above. There are a lot of other creative engineering related activities there, all free to download and use in your classroom, home, summer camp, whatever.

Check it out: zip-line-challenge_091316

One of these years I hope to finish my zip line. All is ready, but now the brush has grown up so much I have to hire a tree climber to clear the 420 foot long pathway.

ZipLanding

View from the zip line tower to the landing 420 feet down the hill.

Thanks for stopping by! Be sure to check out our bridge book if you’re thinking about a DIY suspension bridge. Here is the link: Building a Small Cable Suspension Bridge with the Cable Locking System.

And if you’re curious about Robin Koontz’s books, look her up on Amazon as well: Books by Robin Koontz

Images, diagrams, and text copyright 2013-2017 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.

Evolution of the Cable Locking System and an Alternative Plan

When the idea of the cable locking system first came to me, it was to assemble and disassemble the means of holding the beams for the deck of a suspension bridge. How it was to be produced and what that might cost were of secondary consideration then. After working out just how I would have it made, I found a manufacturer that would do the job, and it was a reasonable cost.

It was a long process getting the patent for the cable locking system, but once that was accomplished we set about writing a book – Building a Small Cable Suspension Bridge with the Cable Locking System. The book was meant to promote the cable locking system and talk about all the steps we took to build the bridge.

I never thought there was much of a market for small suspension bridges, after all, how many people have a creek or ravine that they need to cross on their property? But after we published the book, there was a fair amount of interest in the cable locking system, mostly in the United States, but also from around the globe. And we had put in the book, and on our website, “contact us if you are interested in the CLS.”

About a year later, some people were asking about purchasing CLS components. We had three companies that could do the job, but we were only considering about 50 “units.” After some waiting they gave quotes on 100 or 300 units. The prices seemed expensive, and after informing the interested buyers, we heard no more from them.

The cost of production seemed to be a problem and we weren’t willing to buy and store parts in case someone ordered them. So I wondered: Can I simplify the CLS so that it could be produced more cheaply, and by just about anyone, with a minimum of tools? I came up with a possible answer.

The “inverse” CLS employs the cable locking system but places it somewhat in reverse of the
original application. It eliminates some of the materials and simplifies the manufacturing process while still using the patented cable locking system. Here’s a drawing:

CableLockingNew

The new system has some drawbacks, in that it has to be placed beneath the beam during assembly as opposed to slipping the beam into the original CLS. Also, the beam has to be pre-drilled and a PVC tubing should be inserted to protect the cable from corrosion. But overall, assembly is not all that different from the original cable locking system design.

Picture a swing: hung by two ropes (chains, cables, etc) one on each side. To assemble the “inverse” system, while cradling the beam in your lap, grasp the first of the two suspender cables, push it through the pre-drilled hole ( with tube protector) such that the cable can be inserted through the “inverse” CLS. Pull tight. Proceed to other side and repeat. A screw through the side of the inverse will lock it to the beam so that it can’t move about. I actually use a screw in my original design, if you noticed in the book.

Hopefully this design will be something more affordable and something that people can have made locally without a lot of hassle. I prefer the original set-up for ease of assembling “on the fly” and less intrusion to the beam, but this is an option for you to consider.

Thanks for stopping by! Be sure to check out the book. It includes some cool ideas that apply to other projects, like how to put a really tall post into a deep hole when you aren’t that tall. Amazon has the book on sale for about $12.00 right now. Here is the link:

Building a Small Cable Suspension Bridge with the Cable Locking System

Images, diagrams, and text copyright 2013 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.

Dealing with the Tedious

Let’s face it, a lot of construction work, any work, can be tedious and boring. There are things that you can accomplish while your brain is elsewhere i.e. digging a ditch, stuffing envelopes, cleaning horse stalls. But some mundane tasks need you to be thinking about what you’re doing, no matter how boring the tasks are. Those are the worst.

There were roughly 600 Douglas-fir siding boards to stain for our house project. I stained all the surfaces: front, back, edges, and ends. And as I worked, I sorted the boards according to quality and thickness so that when I’m ready to nail them to the house, things should go pretty fast. So, it wasn’t a task I could just do brainlessly. I had to pay attention.

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But my mind could wander a bit as I brushed on the stain. One of the things I pondered was a similar mundane task I had in my childhood: shucking and shelling corn. We had animals on the farm that thrived on dried corn. Ears of semi-dried corn got tossed into a 20x20x6 foot bin and my job was to shuck and shell the stuff. The bin was full: a bottomless pit of corn cobs all summer long. The job couldn’t be done in an hour, a day, a week, a month. It seemed endless. And summers were long in Texas.

Add to that the satisfaction of a finished product: there was none. When I shucked the corn and put each ear into the sheller and turned the crank, the product went directly into a bucket which went to the feed bins. I never got to look over a nice pile of shucked, shelled corn and think about how productive my day was, shucking and shelling all that corn. The results of my efforts were gone. Well, there was milk and meat on the dinner table, but that wasn’t the same thing to a kid who had sore fingers from shucking countless ears of corn.

So this mundane task, that took me about a month to finish, at least has something cool to show at the end. Neat piles of stained fir are lined up in the house, ready to install. Here’s a photo of the piles, a few of them anyway.

siding

While a tedious task, I’m glad to have something to show for my efforts.

Thanks for stopping by! Be sure to check out my book about building a bridge. It includes some cool ideas that apply to other projects, like how to put a really tall post into a deep hole when you aren’t that tall. Amazon has the book on sale for about $12.00 right now. Here is the link:

Building a Small Cable Suspension Bridge with the Cable Locking System

Images, diagrams, and text copyright 2013 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.

Can You Dig It?

There were a total of eight holes to be dug for my small cable suspension bridge. They would house four posts and four dead-men (more on dead-men later). The posts were going to be three feet into the ground, but the plan also called for concrete punch pads, so that meant another ten or so inches of hole to dig.

Why a punch pad? A two-foot wide pad of concrete would help to distribute the weight. The posts were bearing much of the weight of the loads. If I just set the post to the ground, the point load might exceed the bearing capacity of the native soil. If there were a big load on the deck, such as a tree that fell on it, the posts could sink into the ground and cause the bridge deck to be lower over the creek. The point load would be far less than if I distributed the weight in a concrete pad.

I figured 1,500 pounds of bearing load per square foot, so three square feet of concrete pad totaling twelve square feet would allow for a pretty hefty weight capacity. I also planned to make the pads about ten inches thick to give them more tensile strength in order to prevent cracking.

Meanwhile, the plan also called for backfill with 3/4 minus gravel, which would be tamped at regular intervals. The gravel would help to protect the posts from insects and water damage, plus it would help hold the posts more firmly in the ground.

OvalHole

To allow for the backfill, the holes needed to be about two feet in diameter. I set to work and soon discovered that digging a hole that was only two feet wide was awkward to work in. After digging the first 18 inches or so, I connected the two post holes, creating an oval around both of them. It was easier to work in, plus, there would be that much more gravel backfill to add more security to the posts.

I always use ready-mix concrete for small projects like these punch pads. The bags have the correct ratio of sand, gravel, and cement in them, so all one has to add is water and an energetic mixer. Some people don’t even bother to mix, but if you want a strong concrete base, you will want to mix it well.

I needed to mix about three cubic feet of concrete for each punch pad, which amounted to six bags of 60 lb. ready mix per hole. Rebar and drift pins were already set into place. The pins stuck up about two inches from the center once the pads were poured. When the punch pads were cured, it was time to set the posts.

CoveredHole-Funny
I kept the holes covered until the posts and gravel were done. We didn’t need a critter or human falling in there.

Be sure to check out my new book, Building a Small Cable Suspension Bridge. There is a link to purchase it here: http://www.wildcatman.com. There is also a link there to contact me.

Images, diagrams, and text copyright 2013 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.

The Catenary Curve on a Homemade Suspension Bridge

arch

The St. Louis Gateway Arch is built in the shape of an upside-down catenary curve.
Photo by HHsu/Shutterstock.com

An integral part of my suspension bridge was the catenary curve of the two main cables. The catenary curve is the curve assumed by a heavy uniform flexible cord hanging freely from two points. The curve of the main cables would evenly distribute the weight of the deck among all of the suspenders, which are the cables that run from the main cables to the deck assembly. The curve also determines the relative lengths of all the suspenders.

M-StyleIllustration

The catenary is the curve formed by a perfectly flexible inextensible chain of uniform density that is hanging from two supports

I had to figure out how the cables were going to curve in the most efficient and eye-pleasing manner to support the structure while still looking cool. The factors to consider for the curve can get complicated. There truly is a mathematical formula to get the “ideal” weight load distribution in a curve. Engineers call that ideal an “equal resistance catenary.” This is when the cable’s resistance to breaking is equal along its entire length. To accomplish this bit of perfection, we consulted my wife’s brother the engineer. He was a great help in getting a better understanding of the physics behind catenary curves.

But, being a hands-on guy, I created a scale model for determining the catenary curve for my bridge design. This way, I could draw up the posts and deck at the pre-determined height and length, respectively, then set the catenary curve to my liking by using pins and heavy nylon fishing line. I could double-check using the math, but it was nice to have something physical that could be adjusted.

curve2

This approach at determining the catenary, though being less precise than the mathematical one, can be used to solve undetermined configurations.

You can read more about the catenary curve and another aspect I had to consider, harmonic resonance, in my book, Building a Small Cable Suspension Bridge. There is a link to purchase it on my website: http://www.wildcatman.com.

Images, diagrams, and text copyright 2013 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.