KBSM is the part number prefix for a series of mechanical keyboard switches from Raytheon. The earliest discovered mention of a Raytheon mechanical type is in a December 1967 advertisement; see Documentation below. This advertisement appears to depict KBSR, but does not name any of the series or models. Another advertisement a month later names three models: KBSR-1, KBSR-2 and KBSM-1. With the reed switches, “S” denotes switches with a standard 10° sloped base, while those with a flat base use “F”. As such, a KBFR series may also have existed.
The switch uses a bridge contact arrangement, with wiping contacts. Bounce time is said to be less than 10 milliseconds, and the rated lifetime appears to be the same 10 million cycles as the reed type. A single confirmed example of this switch exists, depicted in eBay listing “1 vintage new in box NOS Raytheon KBSM - 1 …”, already sold in August 2020.
The 1967 advertisement indicates that double-level wiping-action switches were also available, so there should be a KBSM-2. This model was not listed in the 1968 advertisement where the model numbers are given.
Meryl Miller collected a sample of a very similar switch, which he reported as having a glass shell:
(At the top left is MEI T-15 sealed; below this is what appears to be Cherry M62-0100 from before 1980, an unknown type, and finally what appears to be Raytheon.)
The single, low-resolution photograph is hard to make out, and it is not clear whether or not the design is quite the same as the one shown on eBay.
The following diagram was drawn based on analysis of Meryl’s example, and it matches up in principle with the example later discovered on eBay:
Specifically, the plunger itself acts as a bridge that connects the movable contacts to each other. A tapered section keeps it clear of folded projections in the movable contacts. As the plunger is depressed, the width of the plunger at the height level with the projections becomes sufficient to make contact with them, and current can flow from one movable contact to the other via the plunger. To allow for ovetravel, the movable contacts flex to accommodate the plunger, providing a wiping action that keeps the contact surfaces clean. The contact shapes were a best guess, and indeed they may be drawn incorrectly.
Both Meryl’s example and the example on eBay have sloped bases, which is a less common way of achieving a stepped arrangement. Instead of an angled plunger, the base of the switch itself is stepped. This can also be seen in Navcor, George Risk KB and Fujitsu FES-5 reed switches.
No patent is known for this design either. In August 1971, Raytheon filed a patent for a bridge contact switch. In 1973 they were granted US patent 3725625 “Mechanical pushbutton switch with pointed end coil spring contact”. One of the illustrations from the patent is reproduced below:
The shell is described as being from a material such as “glass, plastic or ceramic”. A shorting bar is brought down over a pair of coil springs; these have sharpened ends so that they do not slide across the shorting bar, itself suggested to be roughened for the same reason. The use of coil springs provides overtravel: they can be compressed once the shorting bar makes contact with them. This suggests that these switches may have a progressive rate with knee force curve.
All documentation below was scanned by Bitsavers unless otherwise noted.