共查询到20条相似文献,搜索用时 312 毫秒
1.
刘巽尔 《航空标准化与质量》1997,(5)
B/T16671-1996《形状和位置公差 最大实体要求、最小实体要求和可逆要求》是等效采用ISO2692《技术制图———几何公差———最大实体要求、可逆要求、最小实体要求(几何公差与尺寸公差的关系)》制定的。它给出了一系列新的概念和规定,并替代了GB1183-80中的“相关原则”部分。GB/T16671-1996根据GB/T4249-1996《公差原则》中对“相关要求”的原则规定,分别对最大实体要求和最小实体要求,以及可逆要求应用于最大、最小实体要求,作了详细的说明,并给出若干应用示例及图样表示方法。有关形位公差与尺寸公差的相互关系,在贯彻实… 相似文献
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李晓沛 《航空标准化与质量》1997,(4)
1 成立原因及背景 ISO/TC213“产品的尺寸和几何规范及检验”技术委员会成立于1996年6月14日。它是根据ISO/TC3“极限与配合”、TC10/SC5“尺寸和公差的表示法”、TC57“表面特征及其计量学”的工作任务,在ISO/TC3-10-57/JHG“联合协调工作组”的工作基础上成立的。ISO/TC3、TC10/SC5和TC57原承担着工件尺寸、几何特征的公差数值、标注原则、符号代号、检验及计量器具等方面的标准化工作,虽有各自的工作范围但又彼此密切相关,然而由于各TC多年来一直独立地开展工作,相互间缺少必要的工作沟通和协… 相似文献
3.
目前,工程尺寸驱动原理尚未涵盖三维模型中的隐式约束与几何约束,故公差尺素链分析法(GECM)的应用受到一定的限制。为此,本文对工程尺寸驱动原理进行了完善,并建立了面向虚约束模型(VCM)的公差分析方法。首先,分析工程尺寸不可驱动的原因,建立建模参数集可驱动工程尺寸集的充分必要条件;其次,针对工程尺寸不可驱动的模型,提出产品虚约束模型,给出虚约束模型的构建方法,并构造虚约束模型的公差分析算法;最后,通过实例测试,验证本文所提模型的正确性和算法的有效性。 相似文献
4.
李晓沛 《航空标准化与质量》1998,(6)
GB/T1800.2-1998《极限与配合基础第2部分:公差、偏差和配合的基本规定》(简称新国标)对标准公差等级代号、基本偏差代号、上偏差代号、下偏差代号,公差带的表示、注公差尺寸的表示、配合的表示,注公差尺寸的解释,配合基准制和配合分类,基准温度和... 相似文献
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夏晓理 《航空标准化与质量》2004,(4):33-37
介绍了智能化设计的特点,较详细说明了CAD智能化机械零件功能表面几何特征(尺寸、形位公差、粗糙度)设计思路,以及进行三者联合设计的步骤和关键技术。 相似文献
11.
A new method based on a Reverse Monte Carlo [RMC] technique and aimed at the inverse problem in the analysis of interstellar (intergalactic) absorption lines is presented. The line formation process in chaotic media with a finite correlation length (l > 0) of the stochastic velocity field (mesoturbulence) is considered. This generalizes the standard assumption of completely uncorrelated bulk motions (l 0) in the microturbulent approximation which is used for the data analysis up-to-now. It is shown that the RMC method allows to estimate from an observed spectrum the proper physical parameters of the absorbing gas and simultaneously an appropriate structure of the velocity field parallel to the line-of-sight.The application to the analysis of the H+D Ly profile is demonstrated using Burles and Tytler [B&T] data for QSO 1009+2956 where the DI Ly line is seen at za = 2.504.The results obtained favor a low D/H ratio in this absorption system, although our upper limit for the hydrogen isotopic ratio of about 4.5 × 10-5 is slightly higher than that of B&T (D/H = 30
-0.5
+0.6
× 10-5). We also show that the D/H and N(HI) values are, in general, correlated, i.e. the derived D-abundance may be badly dependent on the assumed hydrogen column density. The corresponding confidence regions for an arbitrary and a fixed stochastic velocity field distribution are calculated. 相似文献
12.
Jean Audouze 《Space Science Reviews》1995,74(3-4):237-243
The most recent developments in the very light element abundance observations, especially the determination of a high (D/H) ratio 3 10–4 on a remote QSO line of sight, are analyzed and some of the consequences on the Big Bang nucleosynthesis (BBN) models, on the predicted baryonic density and on the galactic evolution schemes are reviewed. Should this (D/H) determination be confirmed by further observations, the simplest standard BBN model would be the most successful in these abundance predictions: the baryon density should be close to the luminous mass density
B
(5–8)10–3 if H=100 km s–1 Mpc–1; the challenge is now to devise galactic evolution models able to account for a large D destruction and avoiding to overproduce3He and metals. 相似文献
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Excellent HST/GHRS spectra of interstellar hydrogen and deuterium Lyman- absorption toward nearby stars allow us to identify systematic errors that have plagued earlier work and to measure accurate values of the D/H ratio in local interstellar gas. Analysis of 12 sightlines through the Local Interstellar Cloud leads to a mean value of D/H = (1.50 ± 0.10) × 10-5 with all data points lying within ± 1 of the mean. Whether or not the D/H ratio has different values elsewhere in the Galaxy and beyond is a very important open question that will be one of the major objectives of the Far Ultraviolet Spectroscopic Explorer (FUSE) mission. 相似文献
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The mass spectrometric determinations of the isotopic composition of helium in the solar wind obtained from (1) the Apollo Solar Wind Composition (SWC) experiment, (2) the Ion Composition Instrument (ICI) on the International Sun Earth Explorer 3 (ISEE-3), and (3) the Solar Wind Composition Spectrometer (SWICS) on Ulysses are reviewed and discussed, including new data given by Gloeckler and Geiss (1998). Averages of the 3He/4He ratio in the slow wind and in fast streams are given. Taking account of separation and fractionation processes in the corona and chromosphere, 3He/4He = (3.8 ± 0.5) × 10-4 is derived as the best estimate for the present-day Outer Convective Zone (OCZ) of the sun. After corrections of this ratio for secular changes caused by diffusion, mixing and 3He production by incomplete H-burning (Vauclair, 1998), we obtain (D + 3He)/H = (3.6±0.5) × 10-5 for the Protosolar Cloud (PSC). Adopting 3He/H = (1.5±0.2) × 10-5 for the PSC, as is indicated from the 3He/4He ratio in the planetary gas component of meteorites and in Jupiter (Mahaffy et al., 1998), we obtain (D/H)protosolar = (2.1 ± 0.5) × 10-5. Galactic evolution studies (Tosi, 1998) show that the measured D and 3He abundances in the Protosolar Cloud and the Local Interstellar Cloud (Linsky, 1998; Gloeckler and Geiss, 1998), lead to (D/H)primordial = (2 - 5) × 10-5. This range corresponds to a universal baryon/photon ratio of (6.0 ± 0.8) × 10-10, and to b = 0.075 ± 0.015. 相似文献
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ESO 3.6m Caspec spectra of the LMC luminous blue variable (LBV) taken at minimum have been analysed using NLTE model atmospheres and line formation calculations to derive atmospheric parameters and chemical composition. Using the silicon ionization balance and the hydrogen Balmer lines we deriveT
eff
=17250, log g=1.80 and a microturbulent velocity of 15–20 km/s. The analysis yields abundance ratios by number of approximately 0.43 for He/H, 0.03 for C/N and 0.14 for O/N, implying that enrichment of the atmosphere by processed material has taken place. We have re-evaluated the reddening of R71 using IUE low resolution data and published UBVRIJHKL photometry and derive a value for A
V
of 0.63. We also construct an extinction curve using archive IUE data for mid-B LMC supergiants and show that the extinction is anomalous; the 2175A bump being almost absent and the far UV rise very pronounced. A comparison of our model flux in theV-band with the observed (dereddened)V magnitude and the D.M. of the LMC (18.45), implies that the bolometric magnitude or R71 is –9.9. This is significantly higher than the value of –9.0 usually adopted for R71 and suggests that this object may not in fact be a subluminous LBV. 相似文献
17.
Oscar Buneman 《Space Science Reviews》1985,42(1-2):103-112
Increasing the number of dimensions calls for significant changes in simulation techniques. Demand on computer time and space
increases by orders of magnitude and hardware development affects the feasibility. Gridless and Fokker-Planck codes are possible
in ID, but one needs grids and PIC codes in 2D and 3D. This imposes limits on Debye lengths, particle size and spacing, and
on resolution. Non-spectral (local) E-M codes also suffer a Courant restriction on δt, in addition to the usual ωp δt restriction. Spectral methods therefore have an advantage; they also permit convenient filtering, particle shaping and
control of resolution. 2D and
codes are well advanced and documented (4,5). 3D codes are in their infancy. Data management, rather than physics or numerical
analysis, becomes the major problem (10). Machine-independent 3D codes are too limited in resolution and speed. Parallelism
helps greatly but makes the 3D codes machine-dependent. A present-day limit is attempted in a 2*128**3 grid code for CRAYs
which process ∼ 5 million particles in ∼ 2 minutes per time step. Layering is employed to break up the 3D problem into many
2D problems. Fields and particles are packed and buffered in and out of core. Diagnostics are limited by the large volume
of information accumulated in a run. Results of runs with 3D codes have tended to show that the third dimension, treated as
“ignorable” in 2D simulations, should not have been ignored. With the next generation of highly parallel multiprocessors (which,
however, call for the abandonment of spectral and implicit methods) one may hope to do very realistic 3D simulations. 相似文献
18.
A series of observations of the Virgo cluster of galaxies with the Medium Energy experiment on board EXOSAT are presented. These take the form of two orthogonal scans meeting at M87. The observed count rates have been compared with those obtained with the IPC on board the EINSTEIN observatory and are found to be consistent with the the IPC surface brightness distribution. This fact together with spectral data obtained with Exosat indicates that the region around M87 is more or less isothermal (T 2–3 keV) out to > 80 arc min from the centre. Although a slight hardening is seen near the optical centroid of the cluster the results do not support a model which involves a hot ( 8 keV) intra-cluster medium. A mass of 5 × 1013 solar masses is derived for the inner 90 arc minutes of M87. 相似文献
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The most recent chemical evolution models for D and 3He are reviewed and their results compared with the available data.Models in agreement with the major galactic observational constraints predict deuterium depletion from the Big Bang to the present epoch smaller than a factor of 3 and therefore do not allow for D/H primordial abundances larger than 5 × 10-5. Models predicting higher D consumption do not seem to be able to reproduce other observed features of our galaxy (e.g. SFR, abundances, abundance ratios and/or gradients of heavier elements, metallicity distribution of G-dwarfs).Observational and theoretical 3He abundances can be reconciled with each other if the majority of low mass stars experience in the red giant phase a deep mixing allowing the consumption of most of the 3He produced during core-hydrogen burning. 相似文献