Buckling spring

A buckling spring is a type of keyswitch mechanism that was popularized by IBM through its keyboards for the PC, PC/AT, 5250/3270 terminals, PS/2, and other systems. It was used by IBM's Model F keyboards, and the more common Model M. It is described in and, both now expired. According to the original patent: "A non-teasible, snap action, tactile feedback, key mechanism of extreme mechanical simplicity and high reliability is achieved."

Operation

The coil spring tensed between the keycap and a pivoting hammer buckles at a certain point in its downward traverse, providing auditory and tactile feedback to the keyboard operator. Upon buckling, the hammer is pivoted forward by the spring and strikes an electrical contact which registers the key press. In a Model M, the electrical contact is a membrane sheet similar to that of a modern dome switch keyboard.