Mechanical Enterprises Series LM
Series LM is a series of mechanical pushbutton switches from Mechanical Enterprises, which falls under their “Gold V-bar” family. Few details are known for certain. They are lower in profile, and can be packed in more tightly, than the larger full-travel LFW series gold V-bar switches. Both types were advertised by Tekdata in Electronics Today International, November 1974, page 7, sadly without any clear illustration.
LM switches appear to be targeted towards applications such as calculator keypads, where tight spacing and low profile matter more than switch travel, or where short travel is preferable.
Two switch types were covered in US patent 4004121 filed on the 4th of April, 1974, which are almost certainly LFW and LM series. Various advertisements for Series LM, such as Instrument and Apparatus News volume 21 issue 4 (April 1973) describe the design as follows:
Switching contact is made by moving a gold-wire beam spring into the vee formed by the conical points of two gold-plated contact rods. Contact arrangement combines best features and reliability of wiping and crosspoint contact modes. Spring-on-spring design also provides high hysteresis desired in keyboard switching.
The patent claims:
Each of the two switch contacts, ending in solder terminals 24, is formed by an inverted J member 34. The long leg of each J member protrudes through the base of the housing to provide a terminal 24. … Their upper ends curve away from each other and join intermediate legs 38 of the J members. The curved portions define a generally V-shaped cavity into which the wire beam spring 32 is flexed to engage the short legs 36 of J members 34 and thereby effect circuit closing. The intermediate leg 38 of each J member extends away from this generally V-shaped cavity, joining with the long leg which forms terminal 24.
Comparing the patent and the advertisement appears to demonstrate that Series LM is the shorter type in the patent. Other advertisements show the outline form of Series LM to be virtually identical to the patent illustrations, and for the switch body to have the same quarter-inch height as stated in patent. From this, it appears that one can infer that the taller type in the patent would be LFW, for which far fewer details exist.
The patent does not give any indication of the switch having hysteresis, or any clear suggestion as to how this may occur. “Spring-on-spring” switches generally achieve this by having an over-centre snap-action spring placed below a linear spring, but there is no indication that the “beam spring” in these switches exhibits this behaviour. Nonetheless, there are indeed two springs: a coil spring above the “spring rider”, which in turn presses on the “beam spring”, a design suggested only to condense the switch travel down to a small amount of contact spring movement, not to provide hysteresis. Some unidentified switches believed to be LFW were described by the person who discovered them as “linear, very mushy”, which is more in line with what the patent depicts.
Travel is given in the Instrument and Apparatus News advertisement as 0.070″, equivalent to 1.8 mm.