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Question: Can a spring be designed so that it functions as both a compression spring and an extension spring?
Answer: Actually it can be. The key elements of the design are that there has to be space between the coils so that the spring can be compressed and the ends of the spring need to be designed so that they stay firmly attached when the spring is extended. A common example of a spring designed to function as both a compression spring and an extension spring is the spring within the lancet used by the American Red Cross. The lancet is used to prick your finger when you donate blood and the critical component within this medical device is the compression/extension spring. (You are a blood donor - right?) When the device is loaded, the spring gets compressed between the outer shell and the shuttle that holds the needle or blade. In this position the spring is ready to thrust the shuttle forward. When the latch release lever is pushed the shuttle is released and the compressed spring pushes the shuttle with the needle forward. The momentum of the shuttle carries the needle forward just enough to prick your finger. Then the spring, which gets extended as the shuttle flies forward, pulls the shuttle and needle safely back into the shell. Therefore, no sharp points or edges are left exposed and the lancet can be disposed of safely. So the next time you donate blood, you’ll understand exactly how the compression/extension spring actually works within the lancet.