I visualize a way to modify - but principally leave intact - (only) one component of an existing patented, mult-component, commercial, marketed product whose conceived, intended, advertized, and actual use is completely unrelated to the use to which my modified version of this one component (of this existing patented item) would be put.

E.g., taking the chassis of a name-brand automobile off its manufacturer's assembly line, significantly modifying it (in appearance & function) to instead use it as the structural foundation/base frame for a large commercial air conditioning unit (of the size typically used on the roof of a large building).

Given the "chassis" is already existing then it is prior art in terms of your patent. The tricky question is whether only 5/6ths of your concept is patentable, or the whole lot of it. If the "chassis" is only an extant example of a component required to make your concept functional, then generally the whole concept would be patentable, the relevant claim needs to refer to that part by its function, and note that parts of that type are currently available. The only time it would get difficult is where there are (a) specific features of the "chassis" that are claimed in the original, and (b) these features aren't obvious to "someone skilled in the art" , and (c) you must have these features. The original patent needs to be referred to in "prior art", and in the description section you can identify the specific part. A sloppy claim for the "chassis" in your patent may make it easier for others to circumvent your patent.

For example you might be using the "chassis" for your prototyping and experimentation because it's convenient, but in mass production you would make your own parts with a similar interface. You wouldn't think to make your own screws and nuts and washers, and ignition coils or pumps or plumbing connectors, at some point you have to draw the line between buying sub-assemblys and making them in-house.

A secondary issue arises as to whether you could actually make and sell your product. The actual spare part may not be available on the market to buy. The original maker might raise the price. The original maker might sue you for "defacing" his product. If you make an identical copy of the part then you will be breaching copyright. If you make something that is functionally similar to the "chassis", but retains say the dimensions of the mounting holes, and otherwise designed by yourself from the ground up and looks vaguely the same from a distance, then copyright is not infringed. The functionality of the "chassis" is irrelevant from a copyright perspective.

You need to draw a line between inspiration and imitation, so you can't patent tractor tubes as pool toys, but inflatable donut shaped pool toys could be patented (assuming no one has been seen in public using tractor tyres in the pool prior), even if they are the same dimensions as a standard tractor tube.

A great number of patents come about from the application of an existing technology in new and unexpected ways. I am not a lawyer, but I'm guessing so long as the application is completely different, you are free to use the devices design. For instance the inventors of the laser probably didn't envision its use in flow cytometry. You may not be able to just buy the patented device and apply it in some other way as there might be limitations imposed by the original manufacturer. There may be good reasons for those limitations as the original manufacturer may want to reduce exposure to law suits that could arise from the new application.

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