Experimental study of the influence of using polyurethane cushion to reduce vibration received by a wheelchair user.

PURPOSE: The aim of this experimental study was to compare the ability of polyurethane cushions of three arbitrary selected thicknesses to minimize vibrations transmitted from the wheelchair to its user. METHODS: Measurements were made during passive motion on five different surfaces often found in public spaces. Two tests were carried out during the measurements. In the first test, the sensor was located directly on the surface of the wheelchair seat. In the second test, a polyurethane cushion was placed on the seat, on which the measuring sensor was then placed. RESULTS: The study showed that regardless of the surface on which the wheelchair user moves, the threshold defined in the ISO standard for frequencies in the range of 4-40 Hz was exceeded. However, thanks to the use of polyurethane cushions, vibration damping was visible for frequencies ranging from 10 to 40 Hz. The impact of the user's weight on the magnitude of the perceived vibrations was also observed. CONCLUSIONS: Studies show that wheelchair users are exposed to whole body vibration that can negatively affect their health. Cushions made of polyurethane seem to be a promising solution to reduce whole body vibration in the frequency range that is burdensome and harmful to human health.

Using the Fourier Methods for Cycle Counting of Bimodal Stress Histories with Variable in Time Amplitudes of Components.

The application of the Fourier methods to perform cycle identification and description for different cases of bimodal stress histories are presented and discussed in the paper. The direct spectral method and the modified direct spectral method, previously co-proposed by the author, together with the aspects of the use of Fourier methods discussed in this article, offer a unique alternative to the methods known in the literature for identifying and counting various types of bimodal stress variations in time (with constant or variable amplitudes; completely reversed, reversal, or pulsed type). The application of the Fast Fourier Transform (FFT) and the Short-Time Fourier Transform (STFT) is discussed. The method is useful, especially in cases when it is known that due to the form of work of the mechanical system the two existing components of vibrations with various frequencies can be identified, of which the one with higher frequency has a vibration amplitude lower than a component with a lower frequency and, above all, a variable in time.

Using the Effect of Compression Stress in Fatigue Analysis of the Roller Bearing for Bimodal Stress Histories.

A new approach based on the direct spectral method for fatigue analysis of elements subjected to bimodal stress histories, including high compression effects, is proposed. A correction factor, taking into account the influence of the mean compressive stresses, is used in the proposed method. Equivalent amplitude is estimated, based on criteria proposed by Smith, Watson, and Tooper, and by Bergmann and Seeger. The method is presented with example of a thrust roller bearing. Two cases in which the rollers were subjected to constant force 206 N (where constant amplitude stresses occurred in the rollers) and cyclic force (where bimodal stresses with variable amplitudes occurred in the rollers) are studied. It is observed that multiaxial fatigue criteria (Crossland, Papadopoulos) do not include the influence of bimodal stresses and should not be used for such loading conditions. The proposed method includes both kinds of stress waveforms in the fatigue analysis and can be applied for the accurate identification of stress components and the determination of fatigue life. The damage rate calculated by the proposed approach for rollers subjected to a cyclic force (equivalent load equal to 151 N) was 0.86, which is in good agreement with the recommendations provided in the literature. The obtained accuracy of the proposed method is above 95%.

[Computer simulation of the isolated lesion of tibiofibular an syndesmosis using the finite element method]. / Komputerowa symulacja izolowanego rozejscia wiezozrostu strzalkowo-piszczelowego metoda elementów skonczonych.

The aim of this study was to construct a computed simulation of an isolated lesion of tibiofibular syndesmosis on typical clinical range of value. The analysis was made using the method of finite elements with a simplified plain model of a bone and assuming material of bone and ankle joint as isotropic and homogeneous. The distraction processes were modelled by external generalized forces. The computed programme ANSYS was used. For evaluation obtained was the computed image of changes of anatomy in relation to forces.