A general framework for modeling equipment aging

For many types of devices and systems, age is not well described by duration of ownership. The most obvious example of the distinction between age and clock time occurs when a device may be turned off or is not used continuously. More interesting cases are those in which age is accumulated at a variable rate as a consequence of the operational profile the device experiences. In this paper, we develop a general model that permits the analyst to portray three specific features of equipment use in the calculation of age accumulation. The three features are (1) variation in intensity of use, (2) variation in ambient operating conditions and (3) changes in device technology associated with different vintages of the equipment. The general model allows for any or all of these aspects of aging to be represented. Several analytical examples are constructed and analyzed. The analysis shows that the model provides greater sensitivity to operating profiles than previously used aging models and also that those previous models can be obtained as special cases of the model developed here. Thus, previous models are approximations for the model defined here. It is shown that an important implication of the more precise computation of equipment age is the ability to construct more effective preventive maintenance plans.