共查询到20条相似文献，搜索用时 78 毫秒
1.
在铸造过程中,由于铸件种类繁多,具体生产工艺都有各自的特点,浇注系统设计结果的准确性难以保证.本文以轴承磨床床身铸件为例,分析消失模铸造机床麻身铸件的工艺,设计了生产轴承磨床床身铸件的工艺参数,并就可能出现的缩孔缩松问题提出了对策. 相似文献
2.
对立轴磨床多轴加工进刀轨迹优化控制模型的设计,提高模型的加工精度,为评价数控立轴磨床多轴联动的误差补偿提供依据。传统的进刀轨迹优化控制模型采用稳态预测误差控制的控制算法,导致磨床进刀的交叉耦合轨迹预补偿误差较大。提出一种基于直线轨迹的轮廓误差补偿的立轴磨床多轴加工进刀轨迹优化控制模型。以CAD为预处理软件工具,对加工模具进行空间曲线生成实体模型,构建三维造型,进行立轴磨床的多轴加工进刀轨迹控制模型设计,正确地选择加工刀具,合理地设置切削参数,构建刀具进刀的时间控制轴。构建复杂的零件加工控制系统解耦得到两个独立的控制子系统,实现进刀轨迹三轴联动控制,构建立轴磨床多轴加工进刀轨迹控制系统的轮廓误差向量最小化目标函数,实现控制模型改进。仿真结果表明,该模型能有效提高加工精度,减小误差。 相似文献
3.
普通磨床的数控化改造是利用现有资源提高加工效率、精度和安全性的最有效方法。本文在机械结构、传动方式和控制方式几个方面给出了普通磨床进给系统的数控化设计,实际证明本设计具有结构简单、控制精度高等优点,为磨床进一步数控化起到推动作用 相似文献
4.
在垂直循环基本运动原理的基础上结合机械设计原理与自动控制理论,通过对垂直循环立体车库结构的创新与改进,文中介绍了一种新型混合式立体车库,利用链轮链条啮合转动提供动力保证了存取车辆的稳定性与安全性。垂直升降运动通过含有转向圆弧的导轨及辅助导轨的设计来完成,传动机构与车库框架的整合有效提高了立体车库结构的紧凑性,使得存取车辆的效率与土地利用率得到极大提高。 相似文献
5.
本文提出了一种应用在大、中型数控机床回转工作台上的开式静压导轨及定心轴结构,其特征是应用液体定量式静压原理,在常规回转定心轴结构基础上增加推力轴承结构,一方面控制开式静压转台的浮起量,同时也控制了液体静压导轨副的初始预负荷,增加油膜刚性,提高开式静压转台的运动平稳性、运动精度及抵抗偏载能力,实践证明使用该结构的重载转台运动及定位性能稳定可靠。 相似文献
6.
7.
单双柱立式车床的横梁,以及2m以上的滑枕等,均属刚性较弱的长导轨零件。这些零件在加工中,在夹紧力和自重力的作用下,变形较大。为了解决这些问题,我们通过对细长导轨加工变形等问题的认真分析,得出了保证加工精度的有效方法——自然夹紧发。为保证细长导轨的加工质量找到了有效途径,本文将重点分析刚性较弱的细长导轨的加工。 相似文献
8.
随着计算机技术和数值分析方法的发展有限元方法在机械结构分析中得到广泛应用,取得了令人瞩目的成果,已成为现代机械产品设计的一个重要工具。运用有限元软件Nastran对某公司提供的一轮胎集装葙式龙门起重机的结构进行了强度分析。首先利用Patran建立有限元模型。通过Nastran的后处理得到静栽、跨中纵移、跨中横移、端部纵移、端部横移五种工况下的应力云图和位移分布图,根据最危险工况下应力和位移分布情况,得出该龙门起重机满足结构强度和刚度要求的结论。本论文旨在运用有限元思想对龙门起重机的结构进行强度分析。 相似文献
9.
10.
11.
Marjorie Armstrong-Stassen Margaret Landstrom Ramona Lumpkin 《The Information Society》1998,14(2):153-164
This study examined how students who had no prior experience with videoconferencing would react to the use of videoconferencing as an instructional medium. Students enrolled in seven different courses completed a questionnaire at the beginning of the semester and again at the end of the semester. Students at the origination and remote sites did not differ in their reactions toward videoconferencing but there was a significant difference for gender. Women reacted less favorably to videoconferencing. Compared to the beginning of the semester, students reported significantly less positive attitudes toward taking a course through videoconferencing at the end of the semester. There were no significant differences in students' attitudes toward videoconferencing across courses at the beginning of the semester but there were significant differences across the courses at the end of the semester. The results suggest the need for better preparation for both students and instructors. 相似文献
12.
A joint study by Prof. ZHANG Zhibin from the CAS Institute of Zoology his co-workers from Norway US Swiss have indicated that historical outbreaks of migratory locusts in China were associated with cold spells suggesting that China's projected climate warming could decrease the pest's numbers. The study was published in Proceedings of the National Academy of Sciences on September .…… 《中国科学院院刊(英文版)》2008,22(1):6-7
Ajoint study by Prof. ZHANG Zhibin from the CAS Institute of Zoology and his co-workers from Norway, US and Swiss have indicated that historical outbreaks of migratory locusts in China were associated with cold spells, suggesting that China＇s projected climate warming could decrease the pest＇s numbers. The study was published in Proceedings of theNational Academy of Sciences on 17 September, 2007. 相似文献
13.
Caroline Haythornthwaite 《The Information Society》2001,17(3):211-226
A computer-mediated group is a complex entity whose members exchange many types of information via multiple means of communication in pursuit of goals specific to their environment. Over time, they coordinate technical features of media with locally enacted use to achieve a viable working arrangement. To explore this complex interaction, a case study is presented of the social networks of interactions and media use among members of a class of computer-supported distance learners. Results show how group structures associated with project teams dominated who communicated with whom, about what, and via which media over the term, and how media came to occupy their own communication niches: Webboard for diffuse class-wide communication; Internet Relay Chat more to named others but still for general communication across the class; and e-mail primarily for intrateam communication. Face-to-face interaction, occurring only during a short on-campus session, appears to have had a catalytic effect on social and emotional exchanges. Results suggest the need to structure exchanges to balance class-wide sharing of ideas with subgroup interactions that facilitate project completion, and to provide media that support these two modes of interaction. 相似文献
14.
A research group led by Prof. ZHAI Qiwei from the Institute for Nutritional Sciences under the CAS Shanghai Institutes for Biological Sciences has discovered that even relatively low doses of resveratrol-a chemical found in the skins of red grapes in red wine-can improve the sensitivity of mice to the hormone insulin according to a report in the October issue of Cell Metabolism.…… 《中国科学院院刊(英文版)》2008,22(1):14-15
Aresearch group led by Prof. ZHAI Qiwei from the Institute for Nutritional Sciences under the CAS Shanghai Institutes for Biological Sciences has discovered that even relatively low doses of resveratrol--a chemical found in the skins of red grapes and in red wine--can improve the sensitivity of mice to the hormone insulin, according to a report in the October,2007 issue of Cell Metabolism. As insulin resistance is often characterized as the most critical factor contributing to the development of Type 2 diabetes, the findings“provide a potential new therapeutic approach for preventing or treating” both conditions, the researchers said. 相似文献
15.
Electronic data interchange (EDI) provides means for interorganizational communication, creates network externalities, requires an advanced information technology (IT) infrastructure, and relies on standards. In the diffusion of such innovations, institutional involvement is imperative. Such institutions contain governmental agencies, national and global standardization organizations, local government, and nonprofit private organizations like industry associations. The last type of organizations we call intermediating institutions. They intermediate or coordinate ("inscribe") the activities of a group of would-be adopters. Unfortunately, little is known of how these organizations shape the EDI diffusion trajectory. In this article we examine one specific type of intermediating organizations?industry associations?and how they advanced the EDI diffusion process in the grocery sectors of Hong Kong, Denmark and Finland. We identify six institutional measures, placed into a matrix formed by the mode of involvement (influence vs. regulation) and the type of diffusion force (supply push vs. demand pull), that can be mobilized to further the EDI diffusion. Industry associations were found to be active users of all these measures to varying degrees. Their role was critical especially in knowledge building, knowledge deployment, and standard setting. Furthermore, institutional involvement varied due to policy and cultural contingencies and power dependencies. 相似文献
16.
Puay Tang 《The Information Society》1998,14(1):19-31
The increasing prospects of digital piracy has prompted the perceived need by electronic publishers to adopt technical systems of protection, and governments to reform their copyright laws. This article is a preliminary study of the management of intellectual property by electronic publishers, defined as those involved in the production of online databases, and CD-ROMs. It focuses on three main issues: (1) how electronic publishers view the increasing threat of piracy; (2) the methods of protection employed to protect intellectual property in digital format; and (3) the importance of technological protection of intellectual property in electronic publications. The analysis is based on a sample of 23 UK electronic publishers. The interviews revealed an interesting assortment of protection methods and did not show that technological protection was a preferred way. Instead, the means of protection, in addition to copyright law, comprised niche markets, pricing, trust, bad publicity, and nontechnical and technical means. 相似文献
17.
Milton Mueller 《The Information Society》2001,17(3):151-163
Long-standing conflict between domain name registrants and trademark holders prompted the Internet Corporation for Assigned Names and Numbers (ICANN) to create a global, mandatory arbitration procedure known as the Uniform Dispute Resolution Policy (UDRP). The UDRP has been used in 2166 cases involving 3938 domain names as of 1 November 2000. The policy gives the initiator of a complaint, generally a trademark holder, the right to choose which ICANN-accredited dispute resolution service provider (RSP) will handle the case. During the preparation of the UDRP,some feared that complainant selection would lead to "forum shopping" that might bias the results. This article performs a statistical assessment of the forum-shopping thesis and finds support for it. There are statistically significant differences in the various RSPs' propensity to take away names from defendants; there are also major differences in the number of cases brought to each RSP. RSPs who take away names have the larger share of cases. The study examines other variables that might explain differences in market share, such as price, the plaintiff's nationality, or the time taken to decide a case. It finds that nationality and time also are correlated with market share. The study concludes that shopping for a favorable outcome is an important factor in the UDRP. 相似文献
18.
Prof. Raymond C.K. Chen a neuropsychologist with the CAS Institute of Psychology has made novel progress in his studies of schizophrenia. His work has been reported by a recent issue of Behavioural Neurology.…… 《中国科学院院刊(英文版)》2008,22(1):15-15
Prof. Raymond C.K. Chen, a neuropsychologist with the CAS Institute of Psychology, has made novel progress in his studies of schizophrenia. His work has been reported by a recent issue of Behavioural Neurology. 相似文献
19.
Active biological molecules functional structures can be fabricated into a bio-mimetic system by using molecular assembly method. Such materials can be used for the drug delivery disease diagnosis therapy new nanodevice construction.…… 《中国科学院院刊(英文版)》2008,22(1):22-22
Active biological molecules and functional structures can be fabricated into a bio-mimetic system by using molecular assembly method. Such materials can be used for the drug delivery, disease diagnosis and therapy, and new nanodevice construction. 相似文献
20.
《中国科学院院刊(英文版)》2014,(3)
<正>In Xishuangbanna,one of China’s most biodiverse regions,landscape has changed dramatically during the past three decades due to the conversion of tropical rainforest to rubber plantations.In steep areas,terraces are often constructed before planting rubber trees,which causes two important changes in the soil:the destabilization of soil in the bench terraces and the increased vulnerability of unvegetated riser faces to erosion.Few studies have documented the nature and intensity of erosion on bench terraces.Prof.LIU Wenjie and his colleagues from the Xishuangbanna Tropical Botanical Garden(XTBG)conducted a study in Menglun County(21°5′39″N,101°15′55″E),Xishuangbanna to evaluate the influence 相似文献