Design using flexures


Like bearings, flexure joints constrain relative motion in certain directions and allow motion in desired other ones, the degrees of freedom (DOFs). The DOFs are enabled by slender parts which deform elastically, shown in Figure 1. Due to their excellent repeatable motion (no friction, no backlash, no hysteresis) they are extremely suitable for precision applications. In addition, flexure joints are potentially low cost to manufacture by injection moulding and additive manufacturing, assembly and maintenance can be minimized, and the integration of functionality allows for an important reduction in mass of complex systems. Moreover, the flexure joints are contamination and lubricant free, which is important for vacuum chambers, and they can operate in hostile environments (moisture, dirt, cryogenic, chemicals, radiation, space). However, despite all the above-mentioned advantages over traditional bearings, the applicability of flexure joints has been restricted to applications with limited range of motion, in the order of 10-20 degrees, due to the strong decrease of stiffness at large deflections.

Figure 1. Example of flexure joint, which allows rotation in one DOF, and constrains all other five DOFs; a) State-of-the-art flexure joint Additive Manufactured by Selective Laser Melting of Ti6Al4V; b) maximum deflection of +/- 20° with sustained support stiffness.                                                              

This chapter starts with the introduction of the basic flexure-based constraint elements, such as the leafspring and the wire-flexure. Linear 2D models are derived and non-linear models 2D and 3D are shown. Next, a collection basic joints and linear guides are shown with design considerations. The basic joints include the 1 DOF cross flexure, cart wheel and notch pivots, the 2 DOF universal joint and the 3 DOF spherical joint. Special attention is devoted to the parallel leafspring mechanism with many improvement options. Special topics in this chapter are large range of motion, pre-curved flexures, tube flexures and flexures for low aberration. A separate section is devoted to manufacturing and assembly of flexures. Additionally, some cases are listed to give some insight in the design considerations using flexures.  


Design principles