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Cam Driven Ratchet

   A purely mechanical movement has been designed and constructed as a semester project for manufacturing and failure analysis. This small machine, adapted from a combination of common mechanical actions, will convert a constant-speed cranking motion into an intermittent rotary output. Despite its delicate appearance, its mechanical elements have been engineered and machined in order to withstand the hoisting of a modest 5lb load. To set its gear train into action, observers are encouraged to wind its crank.  

An Overview of Selected Design:

   Early inspiration was taken from the 18th edition of 507 Mechanical Movements (Henry T. Brown, Watchmaker Publishing, 2010) while searching for a machine which produced a unique motion. Of immediate interest were devices which used ratchets and pawls to create one-directional or intermittent motion.




    Seeking similar designs which featured ratchets, a larger collection of devices was consulted. The 4th Edition of Mechanisms and Mechanical Devices Sourcebook (Neil Sclater, McGraw-Hill Publishing, 1991) included a very small picture of a ratchet being pushed by the action of a cam.

 

   The final design, constructed from a mix of mild steel, stainless steel, and aluminum, was designed to lift a load of 5 lb. with an all-around excessive safety factor of 10. The device consists of 24 individual pieces, not including fasteners.. The overall action is this: the operator rotates a handle which has been fixed to an eccentric circular cam. For each full revolution, the cam pushes the ratchet with the upper pawl just enough to advance the ratchet by one full tooth. As the upper pawl returns with the cam, the ratchet is held in place by the lower pawl. As this cycle continues, motion of the ratchet is transferred to a two-gear speed reduction on the backside which slowly lifts a 5 lb. weight which hangs about the lifting drum by a string.
 



Special Considerations


   Strength of materials must be considered. Most of the material used in steel.



   A spring was bought from the hardware store and was tested with some known weights for its spring constant. This helps relate physical extension to its resistance force.



   The upper pawl is placed in compression as it is driven into the ratched by the cam arm. I have modelled it as a column subject to a buckling condition. Of course, forces are small and buckling is unlikely for even this small element in compression. Nonetheless:



  The lower pawl is a little more complicated. This pawl experiences forces excreted by both a spring and ratchet gear, as shown below (shear and moment diagrams depicted):



   The spring arm, is shown below. Its experiences forces from several points and all are labeled. The arm is slender but is able to withstand the loads applied.





   Most of the elements were cut on a waterjet machine from the 2D geometry. The layout files used at the waterjet are shown below:



Construction






Under Construction




 

  







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