Metamorphic Flexure Bearings for Extended Range of Motion

Abstract

Flexure bearings provide precise, low-maintenance operation but have a limited range of motion compared to conventional bearings. Here, we introduce a new class of bearing-the metamorphic flexure bearing-that not only retains the advantages of precision, low wear, and low hysteresis over its limited flexure-bearing range but also provides an extended range of motion as needed. This extended range of motion is achieved via a position-activated transition to a conventional sliding or rolling bearing. To demonstrate the operating principles of this new class of bearing, we describe, design, assemble, and test a linear-motion metamorphic flexure bearing using three categorically different transition mechanisms: a compression spring, a constant-force spring, and a pair of magnetic catches. This design paradigm has the potential to provide various benefits (e.g., reduced wear, reduced downtime, cost savings, and increased safety) in areas ranging from precision manufacturing to healthcare robotics to biomedical implants.

Keywords: compliant mechanism synthesis; compliant mechanisms; conventional bearings; extended-range; flexure bearings; mechanically-programmed mechanisms; position-activated transition.

Fig. 1

Metamorphic flexure bearings. From left to right, these diagrams describe linear-motion metamorphic flexure bearings that use linear-force retention, constant-force retention, and retention handoff, respectively. The first row shows the bearings in the flexure-bearing mode, the second row shows the bearings after transition into their intermittent conventional-bearing mode, and the third row displays the theoretical force profiles corresponding to each retention mechanism, showing the force required to hold the bearing at any given position. The fixed grounded body has a sliding surface contact to an intermediate body, which is connected to the stage through a linear-motion parallelogram flexure bearing. In the flexure-bearing mode, the preload is maintained by a retention mechanism. Note that on each force profile plot, from left to right, the range of motion increases while the required force on the stage for a given conventional-bearing mode position decreases.

Fig. 2

Design of a metamorphic flexure bearing with selectable and tunable retention mechanisms. We designed a cylindrical-form-factor linear-motion metamorphic flexure bearing. The top left image shows the physical bearing, and the center image shows a cross-sectional view of the design. In the cross section, the mechanical ground consists of a square linear shaft with two end caps held on by a bolt running through the shaft. The intermediate body rides on this shaft via two square linear plain bearings (comprising the conventional-bearing portion of the mechanism). The intermediate body is connected to the stage through a linear-motion flexure bearing (attachment rings omitted in detailed view for ease of viewing). The retention mechanisms hold the intermediate body to the grounded body's home side (left side). Though we used only one retention mechanism at a time during testing, this figure shows all three simultaneously for brevity. To tune the compression spring retention force, we swapped the compression springs for springs of different lengths and fine-tuned initial compression using in-line washers. To tune the constant-force spring retention force, we attached a subset of the constant-force springs to hooks on the intermediate body. To tune the magnetic-catch retention force, we altered the number of parallel, identically aligned magnets between the steel plates, and we tuned the stage-side magnetic catch in the same manner. Grip fixtures on each end facilitate attachment of the bearing to a universal testing machine for force testing.

Fig. 3

Layout of parts used in the assembly. This figure shows all the parts we used in this work in the implementation of the metamorphic flexure bearing. The retention mechanism components are shown at left, the conventional-bearing components are shown at top right, and the flexure-bearing component is shown at bottom right.

Fig. 4

Universal machine testing setup for force profile collection. We mounted the bearing vertically between two wedge action grips to measure the quasi-static force throughout its full range. For a time-lapse of a test of the bearing with each of the retention mechanisms, see Supplementary Movie S2.

Fig. 5

Force profile of the metamorphic flexure bearing with linear-force retention. The force profile of the metamorphic flexure bearing is shown over its full range when using a compression spring for retention. A legend at right shows the compression springs used with vertical bars marking their resting lengths and arrows in corresponding colors indicating the initial compression of the spring during the flexure-bearing mode, as inserted into the mechanism. Stiffness is also shown, with a shared extension axis (stiffness data smoothed to remove noise). Note that there is a force step at approximately +9.1 mm in all curves except the curve corresponding to the lowest preload level. Images from the Instron test (see Supplemental Movie S2 for full test) correspond with the extension axis and include a dashed line (shaded to match its corresponding force profile curve, the 4.9 N preload case) indicating the equilibrium point. All shortening data are plotted here with an alpha value of 0.5 to distinguish them from the lengthening data. See Table 1 for transition data corresponding with this plot.

Fig. 6

Force profile of the metamorphic flexure bearing with constant-force retention. The force profile of the metamorphic flexure bearing is shown over its full range when using constant-force springs for retention. The legend in the bottom right corner of the figure shows the number of constant-force springs used in each case. Stiffness is also shown, with a shared extension axis (data smoothed to remove noise). Note the force step at approximately +9.1 mm in all curves except the curve corresponding to the lowest preload level. Due to the constant-force nature of this retention mechanism, the lowest constant-force spring never fully enters the conventional-bearing mode, but stays in a combined mode after transition (the stage never contacts the intermediate body). Images from the Instron test (see Supplemental Movie S2 for full test) correspond with the extension axis and include a dashed line (shaded to match its corresponding force profile curve, the 6.5-N preload case) indicating the equilibrium point. All shortening data are plotted here with an alpha value of 0.5 to distinguish them from the lengthening data. See Table 2 for transition data corresponding with this plot.

Fig. 7

Force profile of the metamorphic flexure bearing using retention handoff. The force profile of the metamorphic flexure bearing is shown over its full range when using magnetic catches for retention. The number of magnets used in the ground catch and stage catch for each case is shown at right, with the colors of the swept-force catch corresponding to the colors of each curve and with an asterisk indicating data that are shown in both plots. The plot above the test images (from Supplemental Movie S2) shows the extension and return curves for a sweep of ground-catch contact forces at a constant stage-catch contact force, and the plot below shows the extension and return curves for a sweep of stage-catch contact forces at a constant ground-catch contact force. Insets show detail of the swept preload values at the ground and stage detachments. For alternative zoomed-in insets showing changes in attachment forces, see Supplemental Fig. S7. As noted earlier, gravitational retention assisted the system in the particular mounted orientation used (for more on gravitational retention, see Supplemental Section S3.2). Images from the four-magnet-ground-catch six-magnet-stage-catch case Instron test correspond with the extension axis and include a dashed line indicating the equilibrium point. The constant-detachment-force data curves are plotted with an alpha value of 0.5 to distinguish them from the detachment-force swept curves. The direction of travel is indicated in the zoom insets with gray arrows. See Table 3 for transition data corresponding with the plots.

Fig. 8

Bidirectional metamorphic flexure bearing (linear-force retention). A second conventional bearing enables range extension at both ends of the flexure-bearing range of motion. The previously mechanically grounded stage becomes a second intermediate body nested within an outer mechanically grounded stage. A second preloaded spring serves as a retention mechanism to hold the second intermediate body against the opposite side of this new grounded body.