Introduction: Industrial Strength POGO STICK
First Prize in the
I Could Make That Contest
If you want to bounce 200 pounds three feet in the air, you're going to need some serious power. And, if you're like me, you're going to want that power for as cheap as possible. The traditional way to get that power is through a metal spring, and if you go to Walmart for a pogo stick, that's what you will see. You will also see a label that says "Maximum Weight Capacity: 80 lbs".
It turns out that metal springs aren't all that effective for doing what a pogo stick needs to do (more detail in the theory section). People have tried other methods to get the big bounce, from compressible air (as in the $369 "Vurtego") to bow-and-arrow-ish bending sheets (as in the "Bowgo" developed at Carnegie Mellon). But the simplest and cheapest way (and the way that the very popular $349 Flybar uses) is to use rubber bands.
But why pay $349 for a Flybar, when you can do one better for less than $90? Big, powerful rubber bands are actually pretty cheap. You can get the two you need for this project for less than $20 at McMaster Carr, who sells them for keeping pallets together while shipping. With a few other parts from McMaster Carr, a trip to the scrap metal yard, and a little bit of time welding and machining, you can have a pogo stick that will send you flying.
Step 1: Theory (For Interest Only: Completely Skippable)
The purpose of a pogo stick is to jump as high as possible... without breaking your legs in the process. This means that your pogo stick needs to do two things:
- Store as much energy as possible
- Do so without imparting too much force to your legs
Energy You Need = (Height You Want to Jump) x (Your Weight)
So if you weigh 200 lbs and you want to jump three feet (36 inches), you need 36 x 200 = 7,200 inch-pounds of energy.
How much energy does a spring give you? That depends on the stiffness of the spring (in pounds per inch) and the maximum deflection of the spring (how much the spring can be squished or stretched, in inches). The equation is also pretty easy:
Energy a Spring Gives You = (0.5) x (Stiffness) x (Max Deflection)2
So let's say you buy a big spring (like this spring-tempered steel jumbo compression spring, which is 12" long and almost 5" around and costs $31.21). The spring is rated for 875 pounds, so it looks promising. But let's do the calculation. The spring rate (i.e. stiffness) is 127 pounds per inch. The maximum deflection is 6.9 inches. That means that the spring can store 0.5 x 127 x 6.92 inch pounds, or 3,023 inch pounds of energy. That's only enough to bounce you 15 inches high, and it's going to be putting 875 pounds of hurt on your feet when it does so.
What's the problem here? How can we get more energy without putting even more force on our feet? The key is in the equation for energy. Notice that the maximum deflection is squared. That means that the maximum deflection, NOT the strength of the spring, is the most important factor to look at.
Say you have two springs. Spring A is twice as stiff as Spring B, but Spring B has twice the maximum deflection of Spring A. Which will store more energy? The answer is that Spring B will store TWICE as much energy. Go ahead, do the math: Energy A = 0.5 x 2 x 12 = 1 ; Energy B = 0.5 x 1 x 22 = 2. This isn't a math trick, it's the way energy storage works, and it's why a rubber band (with low stiffness but massive deflection) will bounce you much higher than a metal spring, and it will do it with much less force.
QUALIFYING NOTE 1: You can actually jump much higher than the spring on your pogo stick bounces you. This is because you're jumping: you're adding the energy in your muscles to the energy in the spring. The height calculated in the equation is the height that your pogo stick could bounce a rock. You, being a person with muscles, will be able to bounce even higher.
QUALIFYING NOTE 2: Rubber bands don't follow the spring equation perfectly, because as they stretch, their stiffness actually changes. But I did quite a few experiments (involving hanging weights off my balcony), and the stiffness doesn't change too much. You can still use the energy storage equation to get pretty close.
Step 2: Gather Your Materials
You'll need a few types of steel. You can get it online, but I recommend going to a local shop to avoid some hefty shipping charges.
- 11.5 ft of 1" OD steel tubing with 1/16" wall. If you can get chromoly steel (which is used in bike frames), it will be a little stronger, but I ended up using SAE 1018 (sortof a generic carbon steel), which according to my stress calculations and some pretty rough use of the pogo stick is plenty strong enough.
- 2 ft of 1"x2" rectangular tubing with 1/16" wall.
- 2 inches of 7/8" OD steel bar (or thick-walled tubing)
- A scrap of 1/16" thick sheet steel (about 5 x 5 inches will be plenty)
Step 3: Cut Rectangular Tubing to Length: One 14" Piece and One 4-1/4" Piece
Use a bandsaw. If your perfectionist bone is strong, face off the ends with a mill. Just make sure you end up with one piece that's 14" long and another that's 4-1/4" long. These will become the step and the brace.
While you're at it, cut four 1-inch by 2-inch rectangles out of your 1/16-inch sheet metal. These will be caps for the rectangular pieces.
Step 4: Cut Holes in the Rectangular Pieces
Each rectangular piece will have two 1" holes (for the frame tubes) and one 1.25" hole (for the flanged sleeve bearings). The dimensions are shown in the drawings below.
If you don't have such a big end-mill or drill-bit, you can use a boring bar to make the holes big enough.
Alternatively, you could drill a small hole, run the blade of a coping saw or thin band-saw through the hole, and then cut out the holes by hand.
Step 5: Check Your Fits
The 1" tubing should fit into the 1" holes. If not, make the hole a little bigger.
The sleeve bearing should fit into the 1-1/4" hole. If not, make the hole a little bigger.
Step 6: Turn a Groove Into Each Flanged Bearing and Install Retaining Rings
In order to get the bearings to stay in place, you'll be using an external retaining ring. This ring sits in a groove and keeps the bearing from sliding out of place.
Chuck the bearing in a lathe and machine the groove as shown in the drawing: 1" from the flange, 0.056" wide, and 0.037" deep (to a 1.176" diameter).
When you're done, check your fit by installing the bearings in their holes and clipping an external retaining ring into the groove. There's a special tool made for installing these rings (called, creatively, retaining ring pliers), but you can use a sharp pair of needle-nosed pliers too.
Step 7: Cut Three 36-inch Pieces of Round Tubing and Machine "fish-mouth"s Into Them
Use a bandsaw to cut three lengths of tubing to 36".
Then use a 1" end mill (or a band saw, or a cutting torch) to make a round "fish-mouth" on one end of each piece of tubing. The nicer you make this cutout, the easier your welding will be later on.
Step 8: Drill a Hole in the Fish-mouth of One Piece (the Shaft)
The central shaft will have a hole in the fish-mouth to secure another piece with. The hole is 3/16" diameter and is placed 3/16" from the fish-mouth tip.
Step 9: Cut and Slot Two 5-inch Pieces of Round Tubing
Use a bandsaw to cut two 5-inch lengths of the round tubing you have left. Machine or cut out a slot (2 inches wide, 1/2 inch deep) in the center of each piece, and check to make sure that the rectangular tubing fits snugly into the slot.
These pieces will hold the rubber bands on the step.
Step 10: Cut Two 1-1/2 Inch Pieces and Two Ovals for the Top of the Shaft
Use a band-saw to cut a 1-1/2-inch piece of the 1" tubing.
Use a band-saw to cut a 1-1/2-inch piece of the 7/8" bar, and use a 1-inch end mill to fish-mouth the bar.
Use a band-saw to cut two ovals (1 x 1-1/2 inch) from the sheet steel. While you're at it, cut four more for the bottom pieces (so six ovals total).
Make sure these four pieces fit together well. They will hold the rubber bands to the top of the central shaft.
Step 11: Cut a 14-inch Piece of the Round Tubing for the Handle
Use a band-saw to cut a 14-inch piece of the round tubing for the handle. Cut two 1-inch circles from your sheet metal to cap the ends if you like.
The cutting is now finished! Get ready for some welding.
Step 12: Assemble the Frame
Put it all together. Make sure everything fits.
You don't have to put on all the end-caps yet, but make sure you have them all at hand.
Step 13: Prepare the Frame for Welding
Assemble the frame and get everything lined up.
GET EVERYTHING LINED UP. Install the bearings and make sure the shaft slides well without binding. Use a square, or a level. Make sure everything is nice and steady so that when you start tack welding, you don't throw off the alignment.
And make sure the fish-mouths on the top are lined up. They should be parallel so that the handle fits into them nicely. You'll be kicking yourself if you weld the frame only to realize these nicely-machined beauties are cock-eyed. (But if they are, you can always throw the frame back on the mill and correct them... as I did.)
Step 14: Begin Tack Welding
It's ok to leave the bearings in to make sure the shaft is well lined-up for the first couple tack welds, but you'll want to take them out soon. If you're using oil-impregnated bearings, as I did, the heat will cause them to leak oil everywhere.
Excuse the quality of my welds; I'm a newb.
Step 15: Remove the Bearings and Continue Welding
Step 16: Weld on the Bottom Rubber Band Holders
Use as little heat as possible on this step. Too much heat will tend to bend the tubes and the step. If this happens to you (as it did to me), you may end up having to file the hole a little bit to make the bearing fit back in again.
Step 17: Weld on the Handle
Here's your chance to shine. If the fish-mouths are machined and aligned properly, this step will be a breeze. If not... it will not.
Step 18: Weld on the Rectangular Caps
These are the candy of welding: external corners, which even I found that I could weld tolerably.
Step 19: Weld on the Circular Caps and Flanges on the Bottom Rubber Band Holders
Step 20: Assemble and Weld the Topper for the Central Shaft
You could arguably weld this onto the central shaft instead of making it removable, as I did. I chose to make it removable so that if the central shaft bent, I could easily replace it.
Unless you're a much better welder than I am (which is more than likely), you'll have to do quite a bit of grinding to make the topper fit into the shaft. But that's ok. Once it does, drill a hole in the topper so that the screw fits through.
Step 21: Grind
Use a belt-sander and/or a bench grinder (or a file if you want to be really manly) and clean up all your bad welds. Just don't go too deep, or you'll weaken the joints.
Step 22: Sand, or Sand-Blast
Ok, you can try it out now. Make sure everything fits and works, and that you can jump as high as you'd ever dreamed. But then get back to the shop and sand this thing. You've gotten this far; you might as well finish it.
Step 23: Paint or Powder-coat
There are some good resources out there for painting bike frames. The same principles would apply here. Unfortunately, I was nearing the end of the semester, and people in class were getting a group together to send parts out for powder coating, so I gave in, forked over about $60, and had a local shop do the powder-coating for me. It turned out really nice, but I bet you'd get a lot more satisfaction from painting it (or powder-coating it) yourself.
Step 24: Install the Bearings
Install the linear bearings and snap on the external retaining rings. You're close, now. Smell the cumulonimbus.
Step 25: Install Two Rubber Bands, Two Rubber Handles, and a Rubber End-cap
Each rubber band will start at the bottom step, wrap over the top, down to the bottom step again, over the top again, and back down to where it started.
That means each rubber band will be quadrupled.
Divide the length of the rubber band into fourths and make a mark at each fourth. When you install the rubber band, try to make each mark line up with one of the bends, so that the tension is even.
You can also cut one of the extra rubber feet into a bumper to absorb some of the shock of the top piece (see assembly drawing in step 2).
If you're still confused, watch the video at the end of this instructable.
Step 26: Jump
If you've never jumped on a pogo-stick before, start slow. This one is quite powerful. Put one foot on the step, then stand up and balance for a second. Once you've got the feel of that, start taking little hops. You should probably wear a helmet if you're prone to landing on your head.
Since this pogo stick is so powerful, it may be difficult to bounce if you're really light. If that's the case, remove a rubber band, or wrap them fewer times.
If you're heavier, or want to bounce even higher, you can always add more rubber bands; just be careful, because with too much force that shaft will bend. But that just means another trip to the shop.
Step 27: Gallery
Here is a brief video highlighting the design and building process. You'll notice quite a few prototypes, from chopstick pogo-sticks to one made out of 2x4s (which worked!) to the final product. This is part of Stanford's design mantra; prototyping early and often leads to better designs.
I'm also including several pictures of the prototypes and a few iterations of the final pogo-stick that I ended up abandoning.
Thanks for reading! If you liked what you saw, and you can bear to move your mouse six more inches, I would appreciate your vote in the "I Could Make That" contest.
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