To fit in with the strict geometrical integrity and ensure dimensionally consistent fabrication of the welded aerospace structures. the low stress no distortion(LSND)welding, a technique for thin materials, was poinee...To fit in with the strict geometrical integrity and ensure dimensionally consistent fabrication of the welded aerospace structures. the low stress no distortion(LSND)welding, a technique for thin materials, was poineered and developed to provide an ininprocess active control of welding distortion. Satisfactory distortion free results were achieved in both welding of jet engine cases of heat-resistance alloys and rocket fuel tanks of aluminuim alloys, and there need no. reworking operations for post-weld distortion correction. Based on the 'static' method a newly developed method for dvnamic in-process control is also discussed in this paper. Both methods provide quanutiative in-process control of incompatible strains in weld zone and low stress no distortion welding results.展开更多
To prevent buckling distortions of thin-walled elements, Low Stress No Distortion welding techniques have been pioneered and developed for product engineering and component manufacturing of aerospace structures with m...To prevent buckling distortions of thin-walled elements, Low Stress No Distortion welding techniques have been pioneered and developed for product engineering and component manufacturing of aerospace structures with material thickness less than 4 mm. In this paper, the nature of Low Stress No Distortion (LSND) welding techniques using thermal tensioning effects is described and special emphases are given to the mechanism of localized thermal tensioning effect. The fundamental principle of Low Stress No Distortion welding is to create active in-process control of incompatible (inherent) plastic strains and stresses formation during welding to achieve distortion-free results implying that no post weld costly reworking operations for distortion correction is required. Finite element analysis is applied to predict and optimize the localized thermal tensioning technique with a trailing spot heat sink coupled to the welding heat source. Comparisons of the thermal elastic-plastic stress-strain cycles are given between conventional gas tungsten arc welding and GTAW with a trailing spot heat sink.展开更多
The recirculating planetary roller screw mechanism(RPRSM)is a transmission mechanism that engages the screw and nut threaded by multiple grooved rollers.In this paper,frstly,the design method of RPRSM nut threadless a...The recirculating planetary roller screw mechanism(RPRSM)is a transmission mechanism that engages the screw and nut threaded by multiple grooved rollers.In this paper,frstly,the design method of RPRSM nut threadless area is proposed,and the equations related to the structural parameters of nut threadless area are derived.On this basis,the cross-section design method of roller,screw and nut is constructed according to the actual situation of engagements between the screw/nut and the roller.By adjusting the gap between the two beveled edges and that between the arc and the beveled edge,the accuracy of the thread engagements between the screw/nut and the roller can be improved.Secondly,to ensure the engagements of the screw/nut and the roller,the distance equation from the center surface of the diferent rollers to the end surface of cam ring is given.Thirdly,combined with the working principle and structural composition of RPRSM,the component model is established according to its relevant structural parameters,and the virtual assembly is completed.Finally,the 3D model is imported into the ADAMS simulation software for multi-rigid body dynamics.The dynamic characteristic is analyzed,and the simulated values are compared with the theoretical values.The results show that the contact forces between the screw/nut and the roller are sinusoidal,mainly due to the existence of a small gap between the roller and the carrier.The maximum collision forces between the roller and cam ring are independent from load magnitude.Normally,the collision force between the roller and the carrier increases as the load increases.When RPRSM is in the transmission process,the roller angular speed in nut threadless area begins to appear abruptly,and the position of the maximum change is at the contact between the roller and the convex platform of cam ring.The design of the nut threadless area and the proposed virtual assembly method can provide a theoretical guidance for RPRSM research,as well as a reference for overall performance optimization.展开更多
目的探讨新生儿重症监护病房(neonatal intensive care unit,NICU)高危新生儿床旁视频脑电图(video electroencephalography,VEEG)监测的护理效果.方法前瞻性纳入2017年12月至2018年6月在北京大学第一医院儿科NICU住院并进行VEEG监测的...目的探讨新生儿重症监护病房(neonatal intensive care unit,NICU)高危新生儿床旁视频脑电图(video electroencephalography,VEEG)监测的护理效果.方法前瞻性纳入2017年12月至2018年6月在北京大学第一医院儿科NICU住院并进行VEEG监测的新生儿为研究对象,由经培训的NICU护士进行电极粘贴并记录新生儿基本信息及VEEG监测过程中不良事件,分析VEEG监测对临床操作的影响.结果共完成245例次多导联多参数VEEG监测,患儿出生胎龄(32.1±3.6)周,出生体重(1879±757)g;监测时校正胎龄(33.8±3.3)周,最小25^+5周,体重(2008±716)g,最低520g.监测时长(4.9±2.4)h,范围3~20 h.监测期间无创呼吸机支持80例次(32.7%,80/245),气管插管机械通气43例次(17.6%,43/245).电极粘贴期间出现血氧饱和度下降8例次(3.3%,8/245),监测过程中电极粘贴部位皮肤发红4例次(1.6%,4/245)、电极脱落2例次(0.8%,2/245).监测期间影响临床操作18例次(7.3%,18/245).电极粘贴和监测过程无严重不良事件发生.结论经过培训由NICU护士对高危新生儿行VEEG电极粘贴工作可行,无严重不良事件发生,护理效果满意.展开更多
目的探讨极早产儿生后7d内视频脑电图监测的背景活动特点.方法选择2017年6月至2018年4月在北京大学第一医院新生儿重症监护病房住院、胎龄≤31周且在生后7d内完成床旁视频脑电图检查的极早产儿进行回顾性研究,排除头颅影像学严重异常、...目的探讨极早产儿生后7d内视频脑电图监测的背景活动特点.方法选择2017年6月至2018年4月在北京大学第一医院新生儿重症监护病房住院、胎龄≤31周且在生后7d内完成床旁视频脑电图检查的极早产儿进行回顾性研究,排除头颅影像学严重异常、放弃治疗、死亡或脑电图提示有发作的病例.收集早产儿的基本信息、临床诊断、围生期病史.从脑电图背景活动连续性、睡眠-觉醒周期等方面进行分析.结果共纳入56例,胎龄(29.4±1.6)周,出生体重(1306±316)g;诊断窒息31例,5 min Apgar评分均>5分,脐动脉血气pH 7.29±0.07.所有病例首次头颅超声及校正足月时头颅超声均提示在正常范围,随访至校正6月龄时Gesell评分各项均在正常范围.随着校正胎龄增加,极早产儿脑电图逐渐呈现正常连续/不连续图形.脑电图最长暴发抑制间歇95%置信区间可达95 s.脑电图睡眠-觉醒周期在校正胎龄26周即可出现,但为间断不成熟的睡眠-觉醒周期,随胎龄增加睡眠-觉醒周期逐渐趋于成熟.结论本组相对正常的极早产儿生后1周内视频脑电图提示暴发抑制间歇可能较长,在校正胎龄26周即可出现睡眠-觉醒周期.展开更多
文摘To fit in with the strict geometrical integrity and ensure dimensionally consistent fabrication of the welded aerospace structures. the low stress no distortion(LSND)welding, a technique for thin materials, was poineered and developed to provide an ininprocess active control of welding distortion. Satisfactory distortion free results were achieved in both welding of jet engine cases of heat-resistance alloys and rocket fuel tanks of aluminuim alloys, and there need no. reworking operations for post-weld distortion correction. Based on the 'static' method a newly developed method for dvnamic in-process control is also discussed in this paper. Both methods provide quanutiative in-process control of incompatible strains in weld zone and low stress no distortion welding results.
文摘To prevent buckling distortions of thin-walled elements, Low Stress No Distortion welding techniques have been pioneered and developed for product engineering and component manufacturing of aerospace structures with material thickness less than 4 mm. In this paper, the nature of Low Stress No Distortion (LSND) welding techniques using thermal tensioning effects is described and special emphases are given to the mechanism of localized thermal tensioning effect. The fundamental principle of Low Stress No Distortion welding is to create active in-process control of incompatible (inherent) plastic strains and stresses formation during welding to achieve distortion-free results implying that no post weld costly reworking operations for distortion correction is required. Finite element analysis is applied to predict and optimize the localized thermal tensioning technique with a trailing spot heat sink coupled to the welding heat source. Comparisons of the thermal elastic-plastic stress-strain cycles are given between conventional gas tungsten arc welding and GTAW with a trailing spot heat sink.
基金Supported by National Natural Science Foundation of China(Grant Nos.52065053,51875458)Natural Science Foundation of Inner Mongolia(Grant No.2020BS05003)Inner Mongolia Science and Technology Project(Grant No.2020GG0288).
文摘The recirculating planetary roller screw mechanism(RPRSM)is a transmission mechanism that engages the screw and nut threaded by multiple grooved rollers.In this paper,frstly,the design method of RPRSM nut threadless area is proposed,and the equations related to the structural parameters of nut threadless area are derived.On this basis,the cross-section design method of roller,screw and nut is constructed according to the actual situation of engagements between the screw/nut and the roller.By adjusting the gap between the two beveled edges and that between the arc and the beveled edge,the accuracy of the thread engagements between the screw/nut and the roller can be improved.Secondly,to ensure the engagements of the screw/nut and the roller,the distance equation from the center surface of the diferent rollers to the end surface of cam ring is given.Thirdly,combined with the working principle and structural composition of RPRSM,the component model is established according to its relevant structural parameters,and the virtual assembly is completed.Finally,the 3D model is imported into the ADAMS simulation software for multi-rigid body dynamics.The dynamic characteristic is analyzed,and the simulated values are compared with the theoretical values.The results show that the contact forces between the screw/nut and the roller are sinusoidal,mainly due to the existence of a small gap between the roller and the carrier.The maximum collision forces between the roller and cam ring are independent from load magnitude.Normally,the collision force between the roller and the carrier increases as the load increases.When RPRSM is in the transmission process,the roller angular speed in nut threadless area begins to appear abruptly,and the position of the maximum change is at the contact between the roller and the convex platform of cam ring.The design of the nut threadless area and the proposed virtual assembly method can provide a theoretical guidance for RPRSM research,as well as a reference for overall performance optimization.
文摘目的探讨极早产儿生后7d内视频脑电图监测的背景活动特点.方法选择2017年6月至2018年4月在北京大学第一医院新生儿重症监护病房住院、胎龄≤31周且在生后7d内完成床旁视频脑电图检查的极早产儿进行回顾性研究,排除头颅影像学严重异常、放弃治疗、死亡或脑电图提示有发作的病例.收集早产儿的基本信息、临床诊断、围生期病史.从脑电图背景活动连续性、睡眠-觉醒周期等方面进行分析.结果共纳入56例,胎龄(29.4±1.6)周,出生体重(1306±316)g;诊断窒息31例,5 min Apgar评分均>5分,脐动脉血气pH 7.29±0.07.所有病例首次头颅超声及校正足月时头颅超声均提示在正常范围,随访至校正6月龄时Gesell评分各项均在正常范围.随着校正胎龄增加,极早产儿脑电图逐渐呈现正常连续/不连续图形.脑电图最长暴发抑制间歇95%置信区间可达95 s.脑电图睡眠-觉醒周期在校正胎龄26周即可出现,但为间断不成熟的睡眠-觉醒周期,随胎龄增加睡眠-觉醒周期逐渐趋于成熟.结论本组相对正常的极早产儿生后1周内视频脑电图提示暴发抑制间歇可能较长,在校正胎龄26周即可出现睡眠-觉醒周期.
