In re Donaldson Co.

The Donaldson Company (plaintiff) owned a patent application directed to air-filtering devices, commonly called dust collectors. The dust collector disclosed in Donaldson’s patent application sought to overcome a problem with conventional dust collectors, namely, hardening of dust accumulating in the hopper of the collector and interfering with downward movement of the collected dust. At least one wall of the hopper of Donaldson’s dust collector was constructed of a flexible material that could expand outward with temporary air pressure increases occurring in a dirty-air chamber of the dust collector. This movement would cause any accumulated dust in the hopper to break up, and it would cause expansion of the volume of air inside the dirty-air chamber, allowing air pulses to impinge the filters more vigorously. Claim 1 in Donaldson’s application recited an air-filter assembly comprising various elements, the last of which was written in means-plus-function (MPF) format: “said [lowermost] portion having means, responsive to pressure increases in said chamber caused by said cleaning means, for moving particulate matter in a downward direction . . . .” In 1991, the Board of Patent Appeals and Inferences (the Board) of the United States Patent and Trademark Office (USPTO) (defendant) issued decisions sustaining a USPTO examiner’s rejection of claim 1 under 35 U.S.C. § 103 as being obvious over a prior patent (the Swift patent). The Board did not limit the scope of the MPF limitation to the flexible diaphragm-like hopper wall disclosed in Donaldson’s specification, reasoning that it is axiomatic that details appearing in a specification are not to be read into the claims. The Board therefore held that Donaldson’s flexible hopper wall could not be cited as a feature to distinguish claim 1 over the Swift patent, even though the Swift patent failed to disclose such a structure. Donaldson appealed.