8 Use of “Managed Stiffness” in Design

In many cases mechanical systems benefit from increasing stiffness of their critical components. However, stiffness increases are often achieved by “beefing up” the structural components, increasing contact areas and preloading forces (to increase contact stiffness), thus requiring larger preloading devices. Such changes usually lead to greater costs and to undesirable increases of size and weight of the unit being designed. While in many cases the required stiffness can be reduced if the damping is enhanced, see Appendix 2, the desirable results, usually achieved by stiffness enhancement, can be also achieved by much less costly techniques if the role of the stiffness parameter were clearly understood. Often, stiffness is enhanced to just reduce deformations in the system. Obviously, the deformations can be reduced either by stiffness enhancement, or by reduction of forces causing the deformations.

There are many important cases where stiffness reduction is beneficial or where there is an optimal range of stiffness values. Some examples of such cases are as follows: generation of specified, constant in time, forces, e.g., for preloading bearings, cam followers, etc.; vibration isolators, force measuring devices (load cells) in which a compromise must be found since the reduction of stiffness improves sensitivity of the device but may distort the system and/or the process being measured; compensating resilient elements for precision over-constrained systems; reduction of stress concentration; improvement of geometry and surface finish in metal cutting operations by intentional reduction of stiffness of the machining system; use of elastic elements for limited travel bearings and guideways; use of anisotropic components having significantly different stiffness in different directions; trading off stiffness for addition of damping into the system; etc. This chapter addresses some of these issues many of which deal with machining systems, but also with general mechanical systems and design components.