Retractable sheath for hypodermic needle

摘要

A retractable sheath for a hypodermic needle is shown for preventing accidental needle pricks. The sheath automatically extends to cover the needle and retracts to expose the needle without having to touch the sheath or activate any locking or release mechanisms. A barrel is mounted on the hypodermic needle or syringe with the needle projecting axially therefrom. A telescoping sheath is mounted in or over the barrel to extend and retract respectively to cover and uncover the needle. A cam and cam follower co-operatively formed in the barrel and sheath prevents the sheath from retracting by an accidental force on the sheath but allows the sheath to be cocked, so that a second deliberate force on the sheath allows the sheath to retract allowing the needle to be inserted into the skin. The cam has a first segment which allows the cam follower to slide longitudinally permitting the sheath to retract, a second segment which blocks the cam follower preventing retraction of the sheath, and an intermediate segment that moves the cam follower from the blocked position to the ready or cocked position, so that a subsequent force on the sheath allows it to retract automatically and then return to the blocked position.