The smart yo-yo is a yo-yo that makes it very easy to return compared to traditional yo-yos.
How It Works
The concept behind the smart yo-yo is a brake that is disengaged as long as the yo-yo is spinning fast enough. A lever is attached to the hub of the yo-yo and is allowed to pivot from one end. A small metal weight is attached to the other end. In the middle of the lever is a spring which pushes the lever into contact with the bearing. This bearing rides on the center axle and is the mounting point for the string.
At low speeds the spring force is stronger than the centripetal force acting on the weight, and the brake is engaged. This stops the rotation of the bearing with respect to the outer shell (hub) causing the yo-yo to wind up as soon as it reaches the bottom of the string. Once the yo-yo is spinning fast enough the centripetal force of the weight overcomes the spring force and the lever moves away from the bearing, allowing the hub to spin freely so the yo-yo "sleeps." Additionally the mechanical break creates an automatic return condition where very little force is required to return the yo-yo.
In traditional yo-yos, the whole body of the yo-yo is one part and has a string wrapped around the middle. It "sleeps" by spinning on the string. Since this depends on many varying characteristics of the string, including how much friction is involved or how tight the string is tied, it is very unpredictable. However with the bearing this model allows for longer and more reliable spin and sleep times.
Computer models were created of each of the following parts using the current model as a guide and accepted values for the material properties. The system was analyzed using the Adams and ANSYS software packages and dynamics (hand calculations). In addition to these values the spring constant was found by measuring the deflection after the application of a force in compression on the spring.
Two Engineering specifications were analyzed: conditions for sleep and durability of the lever arm.
The ability of the yo-yo to sleep is primarily dependant on the centripetal force felt by the weight at the end of the leaver. This is in turn dependent upon the angular velocity of the yo-yo. This angular velocity was determined using a force analysis of the weight and lever arm for the point at which the brake will disengage.
Concerning durability, the stresses resulting from the loading of the lever were analyzed using ANSYS. The stresses cause bending and axial tension. While these stresses do not even begin to approach the limits of the material used in the part (ABS), under the cyclic loading expected failure from fatigue becomes a concern.
In addition an Adams model of the yo-yo was created to simulate the motion of the lever and the breaking condition. To simplify the calculations and the processing time the geometry in the model was simplified. This geometry allows for the same motion and essentially the same characteristics. This model generated the views and the animation found below.
The table belows lists the Bill of Materials for the Smart Yo-yo:
|Part #||Part Name||Category #||Function||Material||Picture|
|1||Hub||Support element||Houses internal components||Acrylic (Polymer)|
|2||Lever||Transmission||Applies friction to main bearing||ABS (Polymer)|
|3||Main Bearing||Support Element||Allows hub and axle to rotate, attaches hub to string||ABS (Polymer)|
|4||Axle||Support Element||Connects the two halves of the hub and rotates in the main bearing||Steel|
|5||Spring||Input||Keeps brake engaged||Steel|
Yo-Yo Views from Adams
Red = Hub/shell of yo-yo
White = Lever arm
Yellow = Metal weight
Black = Spring
Green = Bearing
This view of the yo-yo best displays the functional components of the breaking mechanism and the simplified geometry used in the analysis.
This view gives the best overall impression of the assembly of the yo-yo.
This image affords a better view of the bearing and the planes that the components of the yo-yo fall in.
View Yo-Yo in Motion
Angular Velocity and Constraint Calculations
The above calculations were used to determine the angular velocity necessary to disengage the breaking mechanism. In addition there are a few calculations that were needed to generate the ansys model attached to this document.
Values of Related Properties When Brake is Disengaged
Spring Force = .2756 lbs
Centripetal force = .5664 lbs
Angular Velocity = 17.8 rad/s
Maximum Stress = .48216 psi
Graphical Representation of the Stress
This is a screen shot of the ANSYS model of the lever arm. In this model there is a displacement constraint on the right hand hole, where a peg mounts the lever to the outer shell (hub) of the yo-yo. The forces due to the spring and the weight were then applied to their respective elements.
It is clear from stresses seen in this plot that the locations of maximum stress, and thus the areas of greatest concern, fall along the inner curve where the lever meets the bearing and along the ring holding the weight. (The brighter the color the higher the stress)