Editing IBM buckling spring (section)

==Membrane==
{{Main|IBM Model M}}
[[File:ModelMBucklingSpring.jpg|100px|thumb|right|Patent diagram of the membrane buckling spring]]
This was a revision upon the earlier capacitive-style buckling spring, designed by Edwin T. Coleman and patented by IBM in 1983 as the '''"Rocking switch actuator for a low force membrane contact sheet"'''<ref name="_PAT_4528431" />. It is the most common form of buckling spring mechanism, and for all intents and purposes, when someone talks about buckling springs, they are referring to this mechanism in particular. It was used in the Model M style keyboards manufactured by IBM, Lexmark, Maxi Switch and Unicomp, as well as some electronic typewriters such as the Wheelwriter series. A series of buckling spring keyboards made by AT&T may have used this design, or a slight variation upon it.

From a technical viewpoint, there is not much to be said for them in terms of how they work – they are exactly the same as the capacitive type buckling spring, except instead of the hammer hitting capacitive plates to actuate the key, the hammer strikes down on contacts on a plastic [[membrane]] assembly, which completes a circuit. In this regard, they operate similarly to a rubber dome keyboard.

Multiple sources outline the motivation for changing from capacitive switching to electrical switching using a membrane: cost. The patent for the mechanism claims that the cost of producing a buckling spring keyboard with membrane contacts to be half of producing a capacitive one. The large reduction in cost was done at the expense of durability and reliability – the membrane buckling spring mechanism was rated by IBM for 25 million key presses. The membrane is far more susceptible to contamination than the capacitive PCB. Failing membrane traces are not an uncommon problem with old Model Ms.

There are subtle tweaks to adapt the mechanism to use with a membrane.  The hammer is much smaller, and the springs are of the same length as those used in the capacitive mechanism, but have less coils. As consequence of the latter, the typing feel is slightly different. The patent talks about the force required to mash the two contacts on the membrane together, and it is quite likely that the springs had to be designed to ensure that the hammer struck the membrane with just the correct amount of force.

<gallery widths=250 heights=187>
IBM Model M (1996) -- buckling springs.jpg | Buckling springs and plastic barrel plate from a Greenock 1996 blue label Model M
Buckling spring comparison.jpg             | A comparison between a Model F hammer & spring (left) and their Model M counterparts (right)
</gallery>