The purpose of this paper is to present a functional schema which can be us
ed to develop computer tools to assist designers during the early stages of
the mechanical product design process. The schema is based on an analysis
of the meaning of function. Five main types of mechanical function are desc
ribed: performance, assembly, manufacturing, marketing and maintenance. The
paper focuses on the specification and representation of performance funct
ions. These are defined using a natural language, which is an extension of
the verb-noun pair technique. Three levels of performance functions are ide
ntified: overall, embodiment and geometric. The relationship between these
levels of functions and the physical structures required to implement them
is discussed in detail. An architecture to implement the schema using objec
t-oriented technology is described. A research prototype based on this arch
itecture has been developed in the programming language Incr Tcl/Tk.