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THE SNAIL / DROP CAM

V. Ryan 2006 - 2009

 

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Eccentric, heart shaped and pair shaped cams generally allow for a slow rise and fall of the follower. However, a snail drop cam is used where the drop or fall of the follower must be sudden.
The example snail/drop cam shown opposite rotates in an anticlockwise direction. Rotating in a clockwise direction would probably lead to the entire mechanism jamming. This highlights one possible disadvantage of using this type of cam profile. Also, to ensure the rotation is smooth, the vertical centre line of the snail/drop cam is positioned slightly to the left of the slide (see diagram).

The diagrams below shows the rotation of the snail/drop cam. When rotating for one complete revolution the follower stays level for approximately the first 120 degrees (diagrams 1 to 4). The follower then rises slowly (diagrams 5 to 6) and then suddenly drops when it reaches and passes the peak (diagram 7).

   

 

   

The mechanical toy seen below has a snail/drop cam as its main part. The follower is connected to the characters arm by a wire link. As the cam rotates, the follower rises and the wire link lifts the characters arm. This gives the appearance of the character lifting a fork full of food towards his mouth.

As the cam continues to rotate the follower suddenly falls and also the characters arm and fork.

QUESTIONS:
1. Design a mechanical device/toy/model that relies on the use of a snail/drop cam. Draw two designs and add notes that explain how the device works.
2. A snail/drop cam usually works efficiently in one direction only. With the aid of diagrams and notes explain why this is the case.

   
Another example of a drop cam is seen below. As the cam profile rotates the foot of the model rises slowly and then suddenly drops.
 
   
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