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Fujitsu leaf spring

Contents

Overview

“Leaf spring” refers to a number of keyboard switch types from Fujitsu. Historically, Fujitsu leaf spring types have been divided into three generations. However, tentative evidence now suggests that these “generations” do not represent actual product history. Very few dated examples of Fujitsu mechanical keyboards exist, but date information at present gives a very rough idea.

Despite the “generation” classification, the first two “generations” are closely related in age. Fujitsu filed quite a number of patents and utility models within a period of around three years at the end of the 1970s, and these significantly blur the different types together. Additional patents and utility models were filed in the 1980s

This page is presently a rough overview of the product range pending the availability of more detailed information.

Historic classification

Leaf spring switches were classified as follows:

First generation
Discrete, plate mounted switches (FES-360 Series) — these are still in production for existing customers
Second generation
Dual-discrete switches (discrete switches combined with separate discrete slider assemblies)
Third generation
DIN-compliant dual-discrete switches with lower profile slider assemblies; these are also still in production in FKB2500 Series keyboards.
Fourth generation
Membrane-based keyboards with a single-piece top housing holding the leaf springs

Patent summary

Document Type Title Filed Published Switch Notes
JP S54-63279 A Unexamined patent 押釦スイッチ (pushbutton switch) 1977-10-28 Uncertain Covers the switch assembly and external spring type
JP S54-150428 U Utility model フラットキーボード (flat keyboard) 1978-04-12 1979-10-19 Sheet key
JP S55-49496 U Unexamined utility model (unreadable) 1978-09-29 Uncertain Covers the switch assembly and external spring type
JP S58-29871 Y Utility model 押釦スイッチ (pushbutton switch) 1978-09-29 1980-03-31 Uncertain Covers the switch assembly and external spring type
JP S55-178927 U Utility model (unreadable) 1979-06-11 1980-12-22 FES-360? Sheet key and dual-discrete are also depicted, but apparently only as illustrations of prior designs for comparison
JP S56-96418 A Patent Keyboard switch and method of manufacturing same 1979-12-10 1981-08-04 FES-360
US 4370533 Patent Keyboard switch and process for production thereof 1980-12-05 1983-01-25 FES-360
JP S58-90618 U Unexamined utility model 押釦スイッチ (pushbutton switch) 1981-12-15 1983-06-20 Dual-discrete? The drawings are not quite a match for any type
JP 59-99325 U Unexamined utility model キーボード (keyboard) 1982-12-24 1984-07-05 Dual-discrete?; some unknown type
JP S59-186214 A Patent Pushbutton switch 1983-04-08 1984-10-23 Membrane types
US 4529849 Patent Push-button switch and a keyboard comprising the same 1984-04-03 1985-07-16 Membrane types This has different drawings to the Japanese patent, and shows the full membrane keyboard designs

Types

Based on findings from Kiyoto (in particular the Japanese patents), the different switch series are now treated as follows. The names, where unofficial, are largely arbitrary.

The three major types—flat/sheet, dual-discrete and discrete—were all designed within a few years at the end of the 1970s, with flat/sheet being the oldest of the types known to have been produced, and discrete (“first generation”) appearing to be the last (there are other types depicted that have yet to be seen in a finished product). Dual-discrete appears to have been updated around 1982 or so to reduce the height in line with German standardisation (forming the so-called “third generation”), but this is only speculation.

The types below are listed in the approximate order that they appear to have been designed (which is not necessarily the order that they entered production, assuming they entered production at all). However, all this information is speculative due to there being insufficient information available to clearly understand what was made and when. Please treat all this information as preliminary.

External spring

The oldest patent that Kiyoto found covers a switch similar to “third generation”. However, it has low leverage, and derives its stiffness from an external helical spring. Unexamined patent S54-63279 A from 1977 covers this. It is also shown in a later utility model from 1978, in both its unexamined (S55-49496 U) and examined (S58-29871 Y) versions. This utility model appears to be a revision to the patent, and revises the contact assembly structure design to cover melted lugs for securing the leaf spring, in addition to clip-on attachment of the spring.

The outlines within the drawing above match the image in the patent, but the colouring is a guess, as the drawing is not clear enough to understand, and is not backed up by additional views to show how it is assembled.

Sheet key

“Sheet key” (シートキー) refers to a flat panel arrangement with keys printed onto a membrane cover. Below this are individual plungers over leaf spring contact assemblies: the plungers are pushed through the membrane. This design is a hybrid of pure membrane types (such as Datanetics batch fabricated array and early Commodore PET and Sinclair keyboards) and full-travel mechanical. Switch travel is a mere 1.3 mm (compared to 3.8 mm of full-travel leaf spring keyboards) and switch lifetime is only 10 million instead of the 50 million of the full-travel switches. These keyboards remain in limited production, and are snap action (clicky).

These keyboards are covered by Japanese utility model S54-150428U, filed in April 1978 and published in October 1979. According to Kiyoto, 雑誌FUJITSU 36(4)(207) reports that sheet keyboards came before “block” keyboards; this document is only available in print in selected libraries.

The datasheet for FKB3000 Series keypads refers to these as “FES-301S type”.

A later utility model (S55-178927U, below) gives a cross-section illustration of the sheet form:

Dual-discrete (original)

Dual-discrete keyboards (so-called “second generation leaf spring”) use switches comprising two separate discrete parts: a high-leverage leaf spring switch assembly soldered to a PCB, and a plate-mounted plunger assembly. The high leverage of the leaf spring reduces the plunger travel of around 3.8 mm down to the short travel of the metal dome actuation, and this results in the switch assemblies being offset horizontally. (The discrete switches stack opposing leaf springs to bring the switch assembly under the plunger.) Japanese utility model S55-178927U, filed in June 1979 and granted in December 1980, depicts these switches (and the aforementioned sheet type), but apparently as prior art:

Original dual-discrete seems to be rare, and DIN-compliance may have led to a rapid change of design to reduce keyboard height.

FES-360 Series

The aforementioned utility model also depicts a fully-discrete switch (so-called “first generation leaf spring”):

This type went into production as FES-360 Series, which at the time of writing (April 2019) is still in production, although only for existing customers. The above illustration appears to be an approximate guide; FES-360 is covered far more precisely in Japanese patent S56-96418A (filed in December 1979) and US patent US4370533 (filed in December 1980). These documents show for example how the leaf spring passes all the way through the plunger and into the keystem (which may in fact offer it some reinforcement against being snapped).

FES-360 has the advantage of being able to used fully discretely, unlike all of the other variants. The contact assembly is a clip-in block that attaches to the bottom of the switch, which implies that it was not the original design. The suggestion is that Fujitsu wanted to re-use the existing contact assembly design in a discrete switch, although they still had to significantly reduce the leverage of the actuator leaf to bring the switch assembly within the space of a single-unit 19 mm grid switch. The US patent indicates that this switch was designed for ease of assembly.

Integrated membrane leaf spring

US patent 4529849 (filed in April 1984 and preceded by Japanese patent H0445924 in April 1983) covers two products. Chiefly it documents integrated membrane leaf spring. This type has never been observed: it replaces the metal dome contact assembly with one that uses a single key three-layer membrane that connects to a pair of solder terminals.

It is possible that Fujitsu decided to use the new plunger housing and single spring in conjunction with FES-360 to form third-generation leaf spring. (The plunger housing depicted varies greatly between drawings due to gross errors, but it is clearly the third-generation DIN-compliant design in figures 2A and 2B.)

Non-discrete membrane leaf spring

This is the second type introduced with the aforementioned patents. This is a standard three-layer membrane keyboard, using the revised leaf springs to press down on the membranes. This did enter production, and was used in some of Fujitsu’s desktop and laptop keyboards.

Detailed photos exist of at least two keyboards using these switches:

DIN-compliant dual-discrete

So far, no patent has been found for the “third-generation” switches. These are effectively the same as the earlier dual-discrete switches, but reduced in profile. The only mention discovered to date of DIN compliance is in the FKB2500 Series datasheet. These are still in production at the time of writing (April 2019).

Unlike with FES-360, these are documented to both linear and snap action. The plunger assemblies are identical to that depicted for integrated membrane leaf spring.

There is tentative evidence to suggest that third-generation is FES-301 Series.

Semi-discrete DIN leaf spring

This is a hybrid between non-discrete membrane and dual-discrete DIN. A PCB contains FES-300/301 switch units, but the top case is a uniform moulding. This can be seen in the Fujitsu FM77AV keyboard, dated to 1985.