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IBM beam spring

  • Updated 10 July 2021
  • Est. Reading Time: 1.61 minutes

Contents

Background

The IBM switch button with a snap mechanism (commonly known as the IBM beam spring) was a clicky switch employed by IBM in the 1970s as their primary switch mechanism for most of their discrete keyboards. It was invented by Richard Hunter Harris and Robert John Wolfram2 in 1971 and was put into production and employed the same year for the IBM 3277 Display Station Typewriter Keyboard. The switch takes the form of a rather tall design that wouldn't have been unusual for the time, rated capable of actuating 100 million times and has a bottom-out force of about ~44gf.

Design

The beam spring is a capacitive switch that actuates when the switch's bottom plate coated with capacitive material is lifted from a sensing PCB called the pad card in IBM's terminology. The lifting occurs when the user pressing on the switch causes a leaf spring that is typically arched upwards to invert. Patent GB1363777A describes the total part count of the switch:

Patent figures<sup>1</sup>
Patent figures1
  1. Plunger
  2. Leaf spring
  3. Blade spring
  4. Leaf spring-blade spring connection
  5. Capacitive plate
  6. Casing
  7. Keytop
  8. Plunger return spring

Some of the components have alternative names as per various other IBM documentation and community vernacular. The capacitive plate and blade spring couple are typically referred to as the fly plate and the leaf spring is typically called the beam spring, giving the switch its common name of beam spring or the beam/fly spring interlock.

Gallery

Sources

  1. IBM - TOGGLE MECHANISM [accessed 2021-07-10].
  2. IBM - SCHALTTASTE MIT EINER SCHNAPPMECHANIK [accessed 2021-07-10].