Mecartex SA - Why Flexures

WHY FLEXURES ?

Like plain or rolling bearings, flexures are joints connecting solid members and permitting relative motion in some directions while constraining motion in others. Flexible bearings are mechanical linkages just like conventional bearings. In the latter, linkage is assumed by friction or a rolling motion between two solid bodies.

comparison between standrad and flexure joints

But whereas the two former types of bearings rely upon the friction or rolling of solid bodies on each other, flexures use the elastic properties of matter. This brings numerous advantages for high precision mechanisms :

· Absence of solid friction : plain or rolling contact between solids inevitably generates friction which alters the joint's functioning. Friction dissipates energy, provoking mechanical hysteresis. At low speed it causes halting motion due to the "stick & slip" phenomenon which limits the resolution of the movements. Finally friction is at the origin of wear. Flexures are free from any solid friction. Solely remains the internal friction of matter which is practically negligible.

· Absence of wear : wear reduces the precision of plain and rolling bearings because it alters their geometry and increases their mechanical play. Moreover, it is the principal factor limiting their life-time. Flexures do not suffer from these drawbacks and have their life-time limited only by the eventual fatigue of the material. Good design maintaining stress below the fatigue limit allow to guarantee almost infinite life-time.

· Absence of mechanical play : to reach high precision, plain and rolling bearings often require complicated play compensation. By definition, flexures have no play.

· High rigidities : the more rigid the mechanical structures of machines, the more precise they are statically (when external loads are applied) and dynamically (when vibrations occur). The rigidity of rolling bearings depends on the pressure of rolling elements on top of a rolling surface. With small bearings, the small radiuses of the rolling elements limit rigidity. Well designed flexures can be much more rigid than rolling bearings.

· Compact and monolithic structures : plain and rolling bearings are made of many mechanical parts which need to be assembled. This assembly increases their bulkiness and reduces their construction precision. Wire-electrodischarge machining allows to manufacture very complex flexible structures monolithically, thus providing high compactness and precision.

· Immunity to contamination : the wear and required lubrication of plain and rolling bearings frees particles of matter which can pollute the air of clean rooms. On the contrary, when used in dirty environments, dust can easily hinder or even block these bearings. Flexures are perfectly clean and are not affected by dirt.

As it has been clearly noticed, the main limitation of flexures is their short range of motion. It is due to the stresses in the flexures which must be kept below the yield stress of the material. The need for long strokes calls for flexures of ever thinner cross-sections. Wire electro-discharge machining (wire EDM) has shown to be one of the most suited manufacturing processes for this purpose, for it allows the manufacturing of necked down sections of various shapes with thicknesses thinner than 50mm, geometrical tolerances of the order of ±1microns and low surface roughnesses. Moreover, the machined pieces are not subject to any mechanical stresses which could alter their geometry, and materials with very high elastic limits can easily be machined.