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Post 58A – Parallelogram Linkage

Back in the day…(that is what us old guys are supposed to say 🙂 ) back in the day when things were much more mechanical oriented there were a lot of very interesting mechanisms.  One of the simplest and often used is called a Parallelogram Linkage.  A simple drawing of this shown in the first picture.
It is a type of four link mechanism and opposite sides of this are the same length  and remain parallel to each other.  If one side remains fixed, the top side in the case of this drawing, the opposite side will remain parallel, but can move.   One place where this was used was on drafting tables in place of the T Square.   A variation of it is used on desk lights with arms and it is also used on floor jacks to to make the lift point stay parallel to the ground.  A good picture of the floor jack can be found here.

In the drawing I am showing the arc of the left side (red arc) and the arc of the right side (yellow arc).  As you can see the line at the bottom of the parallelogram remains horizontal.    Sometimes this geometric fact can cause problems and we dealt with one of those way back there in Episode 19 – Fundamentals of Bracing,

An interesting thing happened this last Wednesday.  I took my granddaughter to a playground and one of the swings there is roughly pictured in the sketch to the right.  The top of the swing frame had two horizontal posts to attach chains and some kind of hard material formed the  “animals” with saddles for the swing part.  Two chains were used on the front, and one chain supported the back from a bent piece of pipe attached to the bottom of the animal.   The interesting thing that happened was if I pushed her and got the swing going very high on the back part of the cycle the back chain got some slack and then as the swing moved forward it will pull the chain tight again.This would make the swing bump… much to the delight of my granddaughter.

But being a nerd, I am not just happy knowing that it does it, I want to know why.  The real parallelogram link mechanism uses rods not chains, so it would maintain the swing part flat no-matter-what.  Obviously, somehow the back-end of this was moving upward in comparison to the front-end.  My guess is that my granddaughter was moving forward on the seat when the swing went back and it caused the center of gravity of the swing to get in front of the attachment points. This would cause the back side of the swing to rotate upward.   The chain could not supply any compressive force to resist this movement.

Next time I take her there I hope to bring a camera along, both for pictures of her smiling face and to answer my nerd curiosity.

Remember the first law of Engineering:  You can’t push a rope!

A couple of announcements:

I did check out the e-mail subscription software. It works correctly, just not exactly as I remembered.  there are a few minor things I want to change.  One is that after you hit enter on the form it takes you to the About page.  I shall nerd out and figure that one out (someday).

The 2nd one is I am still asking for e-mails to garyfox@create-and-make.com  I would like to hear from you all about what you would like to see out of this blog.  I am also curious how many of you are comfortable with mathematics, especially with manipulating equations so the unknown is on one side of the equal side and the knowns are on the other side.  I may create some videos if that will be helpful.

The 3rd one is that I have disabled comments to most of the pages.  Comments are allowed on posts.  It seems that somehow those links are getting passed around and people are commenting just to “get their name out there”.  I had exactly the same comment. (word for word exact) from about 6 people and it was obvious that they had not read anything on the page.   I hate to not accept comments, I want to encourage them, but I also hate games.  The easiest and best thing to do was disable comments on the permanent pages.

As always,  Thank you for your time and support



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