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Mechanical Enterprises LFW

Contents

Overview

LFW is a series of mechanical keyboard switches from Mechanical Enterprises, which falls under their “Gold V-bar” family. LFW is the full-travel gold V-bar type; it was complemented by LM series low-profile switches for keypads. US patent 4004121 filed on the 4th of April, 1974 covers both switch types.

Few details about these switches have been recovered to date. Both Type LFW and Type LM “Gold V-bar” switches were advertised by Tekdata in Electronics Today International, November 1974, page 7, sadly without any clear illustration. The advertisement indicates that these switches were provided “either in push-button version or with moulded keytops”. The patent depicts LFW has having the legend inscribed directly onto the plunger, which will be the “push-button version”. The version depicted in TV Typewriter Cookbook has a keystem of the same design as that of T-5 series, which accepts normal keycaps.

From the patent, it would seem that this is a linear switch. The patent does mention that a “further object is to provide a key or push button operated switch with improved touch or feel in depressing and releasing the switch key or push button”, but unlike with the original Mercutronic keyswitch design, there is no indication that any aspect or component provides tactile feedback.

Part LFW-CT was given in the April 1974 issue of Popular Electronics, which covers construction of an SWTPC (Southwest Technical Products Corp) keyboard kit. The switches are given in the parts list as “Mechanical Enterprises LFW-CT”. The photographs of the assembled keyboard do not depict the switches clearly, but the appearance is similar to that of T-5 series. The corner locating posts from T-5 and M-5 are present, but the terminals are located close to the other two corners, owing to the arrangement of the stationary contacts inside.

The same author, Don Lancaster, also wrote a book TV Typewriter Cookbook (1976, fourth printing 1977), of which page 136 depicts a Mechanical Enterprises switch (captioned solely “Courtesy Mechanical Enterprises”). The text refers to Mechanical Enterprises “T5” switches, but the switch depicted is not T-5. The illustration instead depicts a design with the terminals in the same locations as seen on the SWTPC keyboard. This type is likely to be LFW (and possibly also LFW-CT). The design is simpler than that shown within the patent, with no clear indication of how it functions.

Operation

LFW is a bridge contact design: a narrow gap between the two stationary contacts is bridged by the movable contact. The switch contains a thin, stiff wire spring that runs diagonally across the inside, which functions as the movable contact. Positioned perpendicular to this wire rod are two downward-facing J-shaped pieces of thick wire, which form both the terminals and the pair of stationary contacts. A small gap is left between the two wire pieces. The spring wire is pushed down into this gap during actuation, bridging the gap. The plan view of the arrangement can be seen in the following illustration.

In the patent, a “spring rider” connects the plunger to the contact wire; the design is somewhat complicated. This spring rider is absent from the illustration in Don Lancaster’s book, with a flat disc assembly taking its place. Such details are omitted from the diagrams below; refer to the patent for more details.

The diagrams below are taken from the patent, and preserve all the dimensions therein, hence the improbably short solder terminals. Each diagram is a diagonal cross-section through the switch; each cross section angle is indicated in the diagram above. The first diagram shows how the wire contact rod is moved in between the stationary contacts:

The second diagram shows the motion of the contact rod. The patent does not appear to indicate that anything secures the two ends of the rod. The return spring, and the spring rider, are deliberately omitted.

The stiffness of the contact rod is greater than that of the return spring. When the plunger is depressed, the return spring initially absorbs the force without the contact rod being affected. Overtravel is presumably achieved by further compression of the return spring once the contact rod has reached its limit position. In the design as given by the patent, a long vertical rod inside the plunger presses down onto a pair of inward-facing cantilever-spring prongs across the top of the “spring rider” to cause (from the point of contact onwards) extra pressure to be felt by the spring rider. This extra pressure on the spring rider ensures that the contact rod receives sufficient force at the correct moment, which means that the contacts close at the correct pretravel distance even when there is variation in the characteristics of the return spring.