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Making a Better Corkscrew

Before making a new corkscrew let’s examine two common corkscrews and review their relative advantages.  The corkscrews pictured have two different types of mechanisms.  The black one uses a rack-and-pinion mechanism and the red one uses a four-bar linkage (the bottle is the fourth bar). 


The mechanical advantage for the red corkscrew does exactly what you don’t want it to do.  It starts with a low mechanical advantage and then goes high.  The forces are high to start and low to finish.  This results in two problems.  First, getting it started can be difficult, since the mechanical advantage is low, and the force is high.  Second, often there is not enough throw to get the cork out of the bottle since there is a high mechanical advantage, not enough work is getting done.  Both problems are a direct result of the mechanical advantage gone wrong. 

The black corkscrew is considered better.  While it consistently removes the cork, it can be difficult at the beginning.  It also does not work well unless lubricated. The mechanism provides a constant mechanical advantage.  Constant is better than wrong, but the right way would be for:

The mechanical advantage should follow the force. 

To make a better corkscrew, we would need a force profile for the cork removal process.  It has not been measured but let’s assume that the forces are three times greater at the beginning compared to the end.  This means we need three times higher mechanical advantage at the bottom than at the top.   

Screw Motion.PNG

The figure below shows the movement and rotations for the components in new corkscrew.  There are many ways to approach the design.  In this case, the lever arm moves vertically at half the speed of the worm.  It could have just rotated, or it could have moved with the worm.  This design splits the difference. 

The important outcome is that the worm is moving three times faster at the top of the stroke than at the bottom.  This gives a three to one change in mechanical advantages that matches the change in forces.  Look carefully at the video and you can see this change in velocity. 



The parts in this system could be made from standard shapes, plates, and dowels, with molded plastic parts creating the rolling surfaces and guides.  The net would be to lower the cost relative to machining the guides and rolling surfaces.  An additional advantage would be that the plastic would give the mechanism a silky feel.  Lubrication would not be needed.   

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