齿间摩擦对机床渐开线齿轮支座振动之影响

由于不计齿间摩擦,机床机械传动领域内一直近似认为,只要主动轮输入扭矩保持恒定,渐开线齿轮支座就不会产生振动。研究表明,由于齿间存在摩擦,齿轮支座反力属性生财藏一变化,为此,机床渐开线层座会产生一定程度的强迫振动。文章指出,降低齿间摩擦系数并适当减小啮哈 角是减缓甚至避免机床齿轮支座振动的有效措施。     

Support Structures for High‐Resolution Optical Systems

High‐resolution optical systems require support structures that isolate the optical elements from mechanical and thermal loadings. Because the requirements for precise fabrication of the support structure are generally much less rigorous than they are for the optical element surface, the optical quality of the system can be degraded if the optical element is rigidly attached to the support structure. For small optics that can be treated as rigid elements, kinematic (statically determinate, point support) mountings may be used. For larger optics, however, finite contact surfaces (pads) are required to reduce contact stresses so that a semikinematic support results. If the contact surfaces are not coplanar, the optical element can be distorted. To circumvent this latter problem, support pads with self‐aligning designs may be used. These designs may either be mechanisms or flexures. Flexures are used more generally because they are free of both friction and hysteresis. For large optics, the optical elements require numerous supports so that the support system is highly statically indeterminant. These systems require analysis using the finite element method to meet optical deflection design criteria.     

Buckling of Circular Mindlin Plates with an Internal Ring Support and Elastically Restrained Edge

This paper is concerned with the elastic buckling problem of circular Mindlin plates with a concentric internal ring support and elastically restrained edge. In solving this problem analytically, the circular plate is first divided into an annular segment and a core circular segment at the location of the internal ring support. Based on the Mindlin plate theory, the governing differential equations for the annular and circular segments are then solved exactly and the solutions brought together by using the interfacial conditions. New exact critical buckling loads of circular Mindlin plate with an internal ring support and elastically restrained edge are presented for the first time. The optimal radius of the internal ring support for maximum buckling load is also found. An approximate relationship between the buckling loads of such circular plates based on the classical thin plate theory and the Mindlin plate theory is also explored.     

Strength and stiffness requirements for intermediate ring stiffeners on discretely supported cylindrical shells

Silos in the form of a cylindrical metal shell are often supported on a ring beam which rests on discrete column supports. This support condition produces a circumferential non-uniformity in the axial membrane stresses in the silo shell. One way of reducing the non-uniformity of these stresses is to use a very stiff ring beam which partially or fully redistributes the stresses from the local support into uniform stresses in the shell. A better alternative is to use a combination of a flexible ring beam and an intermediate ring stiffener. Recent research by the authors has identified the ideal location of the intermediate ring stiffener to provide circumferentially uniform axial membrane stresses above the stiffener. To be fully effective, this intermediate ring should locally prevent both radial and circumferential displacements in the shell. This paper explores the strength and stiffness requirements for this intermediate ring stiffener. Pursuant to this goal, the cylindrical shell below the intermediate ring stiffener is analysed using the membrane theory of shells and the reactions produced by the stiffener on the shell are identified. These reactions are then applied to the intermediate ring stiffener. Vlasov's curved beam theory is used to derive closed form expressions for the variation of the stress resultants around the circumference to obtain a strength design criterion for the stiffener. A stiffness criterion is then developed by considering the ratio of the circumferential stiffness of the cylindrical shell to that of the intermediate ring stiffener. The circumferential displacements of the ring and the shell are found for the loading condition previously obtained to determine the required strength. A simple algebraic expression is developed for this intermediate ring stiffness criterion. These analytical studies are then compared with complementary finite element analyses that are used to identify a suitable value for the intermediate ring stiffness ratio for practical design.     

Influence of the calcination temperature on the structure and reducibility of nanoceria obtained from crystalline Ce(OH)CO3 precursor

A high yield green method was developed for the preparation of reactive nanotextured ceria (CeO₂). The preparation method is based on the oxidation of a crystalline Ce(OH)CO₃ precursor that decompose at relative low temperature (ca. 250 °C) yielding CeO₂ nanocrystals initially rich in Ce³⁺. After increasing calcination temperatures (in the range 350-650 °C), PXRD analysis show a slight crystal growth after calcination temperatures up to 550 °C, however cell contraction in such case denotes the definitive oxidation of remnant Ce³⁺ centers. XPS results confirm Ce³⁺ fraction diminution as calcination temperature increases. TPR profiles of ceria samples show two reduction events being the low temperature one (at ca. 500 °C) related to a surface process in which approximately only one cerium monolayer is involved. Catalytic activity tests for COPROX reaction were performed under differential reactor conditions to evaluate their activity in the temperature range 100-300 °C. The optimum activity recorded for the sample calcined at 450 °C accounts for the compromise between oxide’s activation and surface preservation.     

改进的最大值约束的多最小支持度挖掘算法

数据挖掘是从大量不完全的、有噪声的、模糊的、随机的数据中,提取隐含在其中的,事先不为人知的,但又潜在的有用的信息和知识的过程.使用一种挖掘算法,用簇集表格来存储数据库中的项目,利用最大值约束下的多最小支持度挖掘算法综合以得出关联规则.     

管道支吊架设计应注意的若干问题

总结了管道支吊架、特别是大型支吊架的设计经验,介绍了一些基本的设计规定,并对在设计中应注意的问题做了说明。     

Aqueous synthesis of high surface area metal oxides

By applying high throughput synthesis and characterization technologies, we have been optimizing common dry or aqueous synthetic routes for the preparation of high surface area metals and oxides, such as precipitation and modified Pechini methods. For wet combustion synthesis, we have been screening a variety of organic acids as dispersants and developed proprietary recipes for individual metals. By resorting to easily decomposable organic acids (as opposed to citric acid in the original Pechini combustion method), such as glyoxylic acid, oxalacetic acid and ketoglutaric acid, it is possible to obtain high surface area materials for many metals after careful optimization of acid/metal ratio and calcination conditions. Examples are Sn, In, Co, Ru, Ni, Fe, Mn, Y, Ce and Rare Earth oxides and their mixtures. After calcination in the temperature range of about 300–400 °C, surface areas >150 m²/g could be obtained for Er, Tm, Co, Ru, and Nb; >200 m²/g for Sn, Fe, Mn, and Y; >300 m²/g for Ce; and >400 m²/g for Ni oxide. Noteworthy are also >140 m²/g for La₂O₃, >80 m²/g for CuO, and 75 m²/g for ZnO. For V, around 40 m²/g was possible for the nearly carbon-free V₂O₅, whereas up to 90 m²/g was obtained for a 90% V–10% carbon composite (by incomplete combustion of the organic acid). Residual carbon helps in stabilizing the porous oxide against sintering. Thus, conventional aqueous routes (precipitation, Pechini) can be competitive to more elaborate and costly methods such as those using organic solvents, sol–gel, supercritical drying or template/hydrothermal synthesis. Combustion synthesis is well suited for the preparation of mixed oxides from mixed metal solutions in aqueous organic acids. Bulk porous Co and CoRu mixed oxides have been screened for liquid phase alcohol oxidations and CoRuCe oxides for CO oxidation and VOC destruction, and doped NiO has been reduced to the metal and tested for various hydrogenations.     

一种基于多重支持度的缺省规则挖掘算法

Rough集理论提供了一种新的处理不精确、不完全与不相容知识的数学方法。从不一致决策表中快速而有效地挖掘出缺省规则是决策规则挖掘研究的一个热点。MDRBR算法采用单一的规则支持度阈值进行缺省规则的挖掘,这不利于有效地挖掘出用户感兴趣的缺省规则。为此,该文对MDRBR算法进行了改进,并提出了一种基于多重支持度的的缺省规则挖掘算法-MSMDRBR算法,MSMDRBR算法可依据多重支持度阈值合理地取舍决策规则,因而具有一定的实用意义。     

石油加氢脱硫催化剂载体的研究

介绍了目前加氢脱硫催化剂载体的种类和研究现状,包括氧化物载体、介孔分子筛载体、活性炭载体、酸碱载体等。分析了不同载体所具有的的优缺点,并展望了未来载体的研究方向。