The use of porcine or bovine pericardium biological cardiac valves has as its main disadvantage a relatively short lifespan, with failures due to calcification and fatigue. Increasing these valves' durability constitutes a great challenge. An understudied phenomenon is the effect of flutter, an oscillation of the leaflets that can cause regurgitation and accelerate calcification and fatigue. As a starting point to study how to reduce or prevent these oscillations, a method was developed to quantify the flutter frequencies occurring at the point of the valve's full opening. On a test bench that simulates the heart flow, the cusp behaviors of eight biological valves were filmed with a high speed camera at 2000 frames per second at different flow rates and motion capture software obtained the frequencies and amplitudes of the vibrations of each leaflet. Oscillations in the range of 200 Hz with average amplitudes of 0.4 mm were found; larger nominal diameter valves obtained lower values, and bovine pericardial valves had superior performance compared to porcine valves. A dimensionless analysis was performed to find a relationship between the geometric and mechanical properties of the valves with the critical speed of the onset of fluttering. This relationship inspired a method to predict whether flutter will occur in the bioprosthesis. This method is a new tool for the consideration of maximizing the life of prosthetic valves.
Biological prosthetic valves are known for having good hemodynamics and resistance to clot formation, but they also have the disadvantage of possessing a short lifespan. An understudied effect is the fluttering of cusps, which is associated with calcification, hemolysis, and fatigue. The present study used a mathematical model of eigenvalues calculation to predict the critical speed of flutter onset in porcine and bovine pericardium valves, comparing the results with experiments on a test bench. Most results were below the speeds found experimentally, an outcome usually found in other flutter analytical theories. The analytical method demonstrated that pericardial valves have greater resistance to the onset of flutter than porcine valves, which agrees with experimental results. The sensitivity analysis showed that the internal diameter has a high impact on the critical speed, while thickness has greater importance when considering critical flow. The same analysis demonstrated that the higher thickness and elastic modulus values of pericardial valves explain why it has an increased resistance to cusps oscillation in comparison with porcine. The mathematical model in this paper is the first flutter analytical theory focused on heart valves. It can assist in new bioprosthesis projects that can be more resistant to oscillations and early fatigue failure.
For an adequate design of radial cams with translating roller followers, it is necessary to maintain the constraint that maximum pressure angle cannot exceed the empirically accepted value of 30°. The direct calculation of this parameter is difficult and it is usually sought to reduce it by trial and error. Nomograms that exist so far only calculated this angle for cams with no eccentricity between follower and cam. This research presents two nomograms with this additional parameter, which allows for greater design freedom using all available variables. The charts are for cycloidal and harmonic motion curves, and can be used for their full and half curves. The study discusses the advantages of using nomograms and the methods to obtain low values of maximum pressure angle by satisfactorily combining available parameters. Although nomograms are no longer a widely used tool in the industry, they still have didactic functions in textbooks and could be useful for preliminary analysis in engineering projects.
High pressure angles in cams with roller followers still pose as a problem in cam projects. The main objective of this work is to deduce the maximum pressure angle equations for full and half cycloidal and harmonic motion curves for translating roller followers in radial cams and create nomograms that can calculate this parameter by simply connecting two points by a straight line in a chart. The nomograms were made for the mentioned curves via an open source software and it is discussed the limitations of the given charts and the advantages of using the current method. Additionally, the paper reinforces the need for authors to make it clear that another well known and frequently used nomogram cannot be used as is for half curves. Although use of nomograms has fallen into disuse, it still has applications in textbooks and quick preliminary analysis in real life projects.
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