Seismic Shake Table

Tell us the story of your project.: 

As an engineer I have volunteered for National Engineer's Week for over 10 years.  Typically, when you visit a class of middle schoolers, you give a presentation explaining the breadth of an engineering career and how much enjoyment and lifestyle rewards it can bring to your life; and you then introduce a technical subject with some rudimentary engineering theory.   The students are then split into teams of 4-8 students and tasked with a project associated to this technical subject.  I have tasked classes with many fun and varied topics such as nuclear fusion where you throw a tennis ball into a crowd of students each holding two balls.  The one(s) who are hit throw their balls in the air prompting more "fissions".  By packing the students close and then spacing them apart you demonstrate a critical and then a sub-critical arrangement.  I have also talked to classes about mechanical impact absorption, descibing how crumple zones in cars, or soles of running shoes work, and then tasked teams of students to compete in an egg dropping competition, whereby they build the best method of packaging their egg using styrofoam cups, paper, straws etc,.  On one occassion we had to go outside the class into the school stairwell to tie break two particularly gifted team's efforts. 

I find that by taking some of the mystique out of the engineering discipline and giving the children a fun and achievable project to take part in you build a connection which hopefully will encourage some of them to dedicate themselves a bit more to their studies and possibly even help bridge future skill gaps in my profession.  Another project I have tasked the students with in the past is building the strongest tower from spaghetti and marshmallow based on meeting certain parameters - i.e. height and base area and load carrying capacity - i.e. number of pennies their tower can support.  Over the last couple of years I have upped the ante and designed and build a seismic shake table using predominantly 3D printed parts.  In my presentation I explain basic structural engineering including the virtue of triangles over rectangles and  seismic considerations such as the importance of the balance between stiffness and flexibility.   Students build their towers on little plastic trays which are crossed with ridges in both directions to better adhere to the base marshmallows and with notches in each corner to allow the tray to be locked onto the shake table.  They must adhere to a few specifications and will be disqualified if they do not.  These include ensuring a supplied plastic monkey is placed on the tower at least 7" above the base and does not fall off during the shaking test.   The shake table features a standard powered screwdriver type 1/4" hexagonal drive, which when driven oscillates two cams on the drive shaft which in turn oscillates two linkages back and forth.  These two linkages and two parallel non-powered linkages are connected in turn to the base plate and the top plate of the table, so that collectively they form a pair of synchonized four bar linkage which oscillates the top plate horizontally in proportion to the shaft rotation speed.  Four lock tabs on the top plate allow the tower support tray to be engaged for testing and disengaged afterwards.  I use twin speed 18V cordless drills to drive the shake tables.  After some trial runs each team's tower is tested for ten seconds at low speed, and then each surviving tower is tested at high speed for ten seconds.  Usually a few towers survive this and they are retested for up to thirty seconds and the surviving team(s) are appointed as winner(s). Through some trial and error, I have perfected the shake table including a number of enhancements which have greatly increased its reliability and its ability to withstand the brutal ministrations of my 18V drills.  These enhancements have included: the use of roller and ball bearings on the drive shaft (as plastic on plastic quickly galls); the use of brass pins turned to size on my lathe to connect the parallelogram linkages to the top and bottom plates; and the use of bridge nylon as the material for my drive shaft as it is far tougher than ABS and other available filament materials.

The STL files and full instructions for my shake table are freely available at https://www.thingiverse.com/thing:2820772/makes and are readily printable on any 3D printer with a platten size of 6" x 6" or greater.

How-to: 

Full instructions and stl files available to Thingiverse URL above

Difficulty: 
Collaborators: 
barty3d
Number of Forks: 
0
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3D printer
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1
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Lathe
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1
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CordlessDrill
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1
Materials: 
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8x22x7 608 bearing
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1
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Koyo BH-108 needle bearing
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2
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3D printer Filament
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as required
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Brass rod 1/4"
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as required
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Brass rod 3/8"
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as required
Team Members: 
Team member name: 
barty3d
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Teaser: 
I have taken part in National Engineer's Week for the last 10+ years. I devised a shake table to test the strength of teams of student's towers build from spaghetti and marshmallows to see who's tower can survive the shake tower of terror!
Aha! moment: 
Most of the problems involved the drive shaft. Making something that is 3D printable, strong enough, and can be assembled without loss of strength took a few iterations. I eventually realized that making the bearing housing split and using steel rolling element bearings and toughening the shaft by printing it in nylon greatly increased it's life.
Uh-oh! moment: 
Destroying the drive shaft on both my shake tables at the final of the three classes I visited last year. We had to resort to trying to shake the school desks instead - not very successful! I am now pondering carrying spare shafts and redesigning the split bearing housing with screws to allow on-the-fly shaft replacement.
Show & Tell video as default: 
Creation Date: 
Friday, February 22, 2019 - 19:03

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