This paper discusses the combined use of quantitative analysis equations and knowledge-based techniques in the design of machine elements. We introduce the method of design compatibility analysis to 1) guide the application of correct analysis equations, 2) interpret the results of analysis, and 3) suggest modifications to improve the design. As an example vehicle, this paper focuses on design issues related to contact stresses and lubrication between two surfaces. The compatibility approach covers not only quantitative performance measures, but also the qualitative design issues such as suitability of surface materials, machining methods, surface roughness, lubricants, etc. To verify the utility of this concept, we developed DECS (Design Expert for Contact Stress), a PC-based design-aid for contact stresses and lubrication. DECS uses Prolog for top-level management and inference, while C provides and efficient computation of the quantitative equations. DECS also utilizes some recent developments in the computations of contact stress and elastohydrodynamic lubrication. Our method would be useful in many other design applications.