Everyone may know the story ——The Tortoise and the Hare at a very young age. Everyone may remembered below plot:

...Hare ran down the road for a while and then and paused to rest. He looked back at Slow and Steady and cried out, "How do you expect to win this race when you are walking along at your slow, slow pace?"
Hare stretched himself out alongside the road and fell asleep, thinking, "There is plenty of time to relax." Slow and Steady walked and walked. He never, ever stopped until he came to the finish line. The animals who were watching cheered so loudly for Tortoise, they woke up Hare. Hare stretched and yawned and began to run again, but it was too late. Tortoise was over the line...

In this instructables, I will show you how to design a mechanical toy make the Hare never catch up with the Tortoise. It is just pretty basic mechanical device that turn rotary motion into more of a linear motion. Turn the crank will make the hare jump up and down, and the tortoise move. I modeled them all in 123D Design Desktop, printed with Makerbot Replicator 2 and Dremel. Then, assembled them together.

Before start to modeling, you need to prepare some mechanical knowledge for two questions: why they can jump? Why the tortoise is slower than hare?

Step 1: Why the Hare Can Jump Up and Down?

"The Scotch yoke is a reciprocating motion mechanism, converting the linear motion of a slider into rotational motion, or vice versa. The piston or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a pin on the rotating part. The location of the piston versus time is a sine wave of constant amplitude, and constant frequency given a constant rotational speed."

In this toy, we need two for the hare and tortoise. But there is a little difference, the hare jump higher, the tortoise don't need jump so high. So, we need to design different diameter of the circle made by the crank pin, large diameter for hare and small diameter for tortoise.

Step 2: Why Did the Tortoise Is Slower Than the Hare?

"When two gears mesh, and one gear is bigger than the other, a mechanical advantage is produced, with therotational speeds and the torques of the two gears differing in an inverse relationship."

So, I used two gears to make the hare and tortoise can move together, but different diameters of gears that can make the hare and the tortoise can move in different speed. The small one for hare, the big one for tortoise.

Step 3: Drawing 2D Draft to Calculate Basic Dimensions

Drew the draft sketch to calculate basic dimensions, that is very important step before you start modeling. The draft sketch and the basic dimensions will be very helpful if you want to create the models accurately.

Please ignore my poor painting skills. From the sketch, you can get:

1. the yoke length a > 2a1.

2. the single piece rob length l >= a1 + h.

3. the gears radius is the top first dimension you should have, since the teeth must match.

So, let's start from the gears.

Step 4: Get the Gears

You don't need to design the gears by yourself. Go to 123dapp.com, search 'gear' under 123D Design, you will get many gears appear, choose two different sizes to use them, download .123dx file format which you can import into 123D Design Desktop and modify it.

Then, let's modeling in 123D Design Desktop.

Step 5: Gears and Washers

1. Design a I-shaped stand frame to place all the components, and put the gears in correct position

2. Create s short crank on gear, and a long crank for another

3. Create a hole in center of the gear with radius is 2.5mm

4. Sketch -> Project, project the hole profile on the front face

5. Create center-line cross the circle sketch as reference line

6. Create a concentric circle sketch with diameter is 6mm

7. Extrude and subtract the I-shaped frame

8. Using the similar ways to create washers

Step 6: Slot

1. Create rectangle sketch on the existing reference center line

2. Mirror the rectangle sketch

3. Extrude for 12mm

4. Chamfer the bottom corner with distance 12mm

5. Project the top face profile

6. Offset with distance 2.5mm

7. Drag the sketch line to edit the profile

8. Extrude and subtract to make a rectangle hole as the slot

Step 7: Measure the Length for Yoke

1. Project the gear profile on plane

2. Measure the distance between two center points

3. Measure the distance between the gear's center point and the top edge of the slot

After get these two distance, you can calculate dimension of the yoke.

Step 8: Sliding Yoke

1. Create rectangle sketches

2. Offset the bottom rectangle with 2.5mm

3. Trim unnecessary sketch lines

4. Extrude the profile with 5mm

5. Fillet the corners make it looks smoothly

Step 9: Create Another Half Using Same Ways

Using the similar steps to create remaining part. Separated the components and put on canvas make them in correct direction easy to print. Then, you can printing them out using different color materials.

Note: The two yokes have different dimension.

Share the models here, you can download to print out.

http://www.123dapp.com/123D_Design/HareAndTortoise/4340794