Smith Corona leaf-spring-over-membrane

Smith Corona leaf-spring-over-membrane
Manufacturer	Smith Corona
Switch type	Progressive rate
Sense method	Spring-over-membrane
Keycap mount	Custom inverted cruciform
Switch mount	Moulded to mounting plate
Patents	US4818828 (1988)

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Description

Smith Corona leaf-spring-over-membrane is a provisional name for a low-cost membrane switch used in various Smith Corona electric typewriters. It is a leaf-spring-over-membrane design. The cross-shaped slider is integrated into the keycap, of which serves only to guide the keycap. An integrated plastic arm on the corner of the keycap presses down onto a leaf spring, which presses down onto a conductive rubber dome. This closes the circuit and registers a keypress. The leaf springs are moulded into the assembly with plastic rivets and cannot be removed.

The small arm on the keycap is very fragile and breaks quite easily, especially on older, used examples. Remove keycaps with caution. The safest way to remove keycaps is to separate the plastic arm on the keycap from the leaf spring, and push it through the slot that the arm navigates through.

These switches look similar (although not identical) to those used in the DEC LK201.

Key feel

The keyfeel is fairly unusual, with smoothness varying widely depending on condition. Clean switches in good condition are fairly smooth, but more heavily used ones are very rough, scratchy, and prone to binding. Actuation occurs before bottoming out; this is common in most spring-over-membrane designs. The force curve is progressive, which gives this switch a very odd keyfeel compared to a standard linear switch. In some instances, the leaf spring may produce a distinct ping noise during the upstroke.