Vibration reduction of pneumatic percussive rivet tools: mechanical and ergonomic re-design approaches

This paper presents a systematic design approach, which is the result of years of research effort, to ergonomic re-design of rivet tools, i.e. rivet hammers and bucking bars. The investigation was carried out using both ergonomic approach and mechanical analysis of the rivet tools dynamic behavior. The optimal mechanical design parameters of the re-designed rivet tools were determined by Taguchi method. Two ergonomically re-designed rivet tools with vibration damping/isolation mechanisms were tested against two conventional rivet tools in both laboratory and field tests. Vibration characteristics of both types of tools were measured by laboratory tests using a custom-made test fixture. The subjective field evaluations of the tools were performed by six experienced riveters at an aircraft repair shop. Results indicate that the isolation spring and polymer damper are very effective in reducing the overall level of vibration under both unweighted and weighted acceleration conditions. The mass of the dolly head and the housing played a significant role in the vibration absorption of the bucking bars. Another important result was that the duct iron has better vibration reducing capability compared to steel and aluminum for bucking bars. Mathematical simulation results were also consistent with the experimental results. Overall conclusion obtained from the study was that by applying the design principles of ergonomics and by adding vibration damping/isolation mechanisms to the rivet tools, the vibration level can significantly be reduced and the tools become safer and user friendly. The details of the experience learned, design modifications, test methods, mathematical models and the results are included in the paper.     

Effect of cation contamination and hydrated pressure loading on the mechanical properties of proton exchange membranes

Perfluorosulfonic acid (PFSA) polymer membranes are widely used as electrolyte thin films to transport protons in proton exchange membrane (PEM) fuel cells. The mechanical degradation of the membrane represents a common failure mode that limits the operational life of the fuel cells. In the present work, effect of contamination related to cation exchange on the mechanical reliability of PEMs was investigated. We applied the bulge test technique to assess the mechanical properties of Nafion^® PFSA membranes simulating pressure loading on hydrated PEMs in fuel cells. The corresponding elastic moduli of Nafion^® before and after cation exchange were analyzed and compared with the results measured by uniaxial tension experiments at selected humidity conditions, showing increasing stiffness with the increase of cation radius. We also used the out-of-plane tearing test method to characterize the fracture behaviors of PEMs. The effects of cation exchange and water absorption on mechanical and fracture properties of PEMs at different temperatures are discussed in terms of cation and water interactions with the molecular structure of PFSA polymers.     

Preparation,characterization, and swelling and drug release properties of a crosslinked chitosan‐polycaprolactone gel

For applications in biotechnology to prepare biopolymers containing functional groups is essential. In addition, these materials have to be strong to provide physical support for practical applications. Recently, chitosan, polycaprolactone (PCL), and their various combinations were used for this purpose. In this work, we described the preparation and characterization of a new biodegradable polymeric gel containing chitosan and PCL. The gel preparation reactions were performed in suitable acetic acid solutions to obtain the products in high yields. A crosslinking agent was added to produce crosslinked gels. Swelling behavior of chitosan/PCL gels in different compositions was studied, and the results were compared. The chitosan/PCL gels show a rather large equilibrium swelling in water and in the phosphate buffered saline solution. Acrylic acid (AA) was added to these gels during preparation process to obtain a stable material for various applications. These polymeric gels were characterized by Fourier transform infrared. Their physical and morphological properties were investigated by using differential scanning calorimeter and scanning electron microscope techniques, respectively. Cell growth experiments indicate that chitosan, a positively charged polysaccharide, is not suitable for cell proliferation studies. On the other hand, the drug release studies were successful and, 59% of lidocaine, was released from a chitosan/PCL/AA hydrogel in buffer solution at pH = 7.4 at 37°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Zinc and tin xanthates in the coordination polymerization of propylene oxide

The polymerization of propylene oxide with zinc and tin xanthates was studied. Polymerization with both systems was found to be a zeroth‐order, nonterminating process, where the molecular weight was controlled by transfer reactions. It is discussed that these observations were consistent with a mechanism in which the rate‐determining step was the addition of monomer on the growing chain rather than the coordination of the monomer to active sites. These catalysts turned out to be quite stable, even under semiclosed conditions. With zinc isopropyl xanthate, high conversions in short polymerization times could be obtained. The product had a very broad molecular weight distribution and could be split into crystallizable and amorphous fractions. The crystallizable fractions consisted of stereoregular segments separated from each other by stereoirregularities or regioirregularities. The degrees of polymerization of stereoregular segments (S₁'s) were estimated from melting point measurements. It was found that the melting points and, hence, average S₁ values changed in different fractions. It was concluded that there was only one type of active center; however, the rates of wrong additions (e.g., head‐to‐head addition) of incoming monomer controlled the average S₁ value of the chain. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Modeling the Influence of Immersed Objects on Pool Fire Burning

Fire Technology - The study presents a heat transfer model for prediction of the effect of immersed objects with high thermal conductivity (refluxer) on the mass burning characteristics of a pool...     

Interference mitigation in heterogeneous networks with simple dirty paper coding

In heterogeneous networks where macro cells and metro cells use the same frequency band to communicate with their respective users, the major problem limiting performance is the interference caused by macro cells to metro cells. The information theoretic result known as dirty paper coding provides a way to address this problem and significantly improve the performance of heterogeneous networks with co-channel deployment. In this paper, we show how a simple dirty paper coding scheme employing Tomlinson-Harashima pre-coding with partial interference pre-subtraction can be employed by metro cells to mitigate the interference caused by macro cells. A performance study included in this paper shows that the proposed dirty paper coding scheme can lead to significant improvement in user rate statistics.

     

Determination and Improvement of Deposition Parameters of TiO2 Thin Films via ALD

In recent years,with the development of technology,interest in microelectronics and thin film devices has increased considerably.Future improvements in microele...