星载激光通信终端轴承预紧力及加载结构研究

星载激光通信终端通过方位、俯仰轴系的运动实现目标跟踪。轴系回转精度是二维转台实现良好工作的前提。通过对角接触球轴承模拟分析,确定卫星发射以及在轨运行时转台轴承的最优预紧力大小,通过轴承预加载结构实现轴承从高预紧状态过渡到轴承低预紧状态,解决了传统固定式预紧力不能同时满足两种工况的难题,即发射时大预紧力满足高刚度与基频,在轨运行时低预紧力以降低平均摩擦,扭矩噪音以及轴承接触应力。     

Surface Characterization of Vulcanized Rubber Treated with Sulfuric Acid and its Adhesion to Polyurethane Adhesive

Modifications produced on a vulcanized styrene –butadiene rubber surface by treatment with sulfuric acid were studied and several experimental variables were considered.

The treatment of R1 rubber with sulfuric acid produced a noticeable decrease in contact angle which was mainly ascribed to an increase in surface energy due to the formation of sulfonic acid moieties and C?O bonds, and the removal of zinc stearate. The rubber surface swelled and became brittle as a result of the treatment, and when flexed microcracks were created. A rubber surface layer modification was produced with a consequent decrease in tensile strength and elongation-at-break values. The treatment enhanced the T-peel strength of R1 rubber/polyurethane adhesive joints and the locus of failure was cohesive in the rubber.

The optimum immersion time in H₂SO₄ solution was less than 1 min., and the reaction time in air was not found to be critical; the neutralization with ammonium hydroxide and the high concentration of the sulfuric acid (95 wt%) were essential to produce adequate effectiveness of the treatment.     

Contact between a smooth microsphere and an anisotropic rough surface

This article discusses the effects of asperities on elastic and adhesive contact between a smooth sphere and a rough surface. Two numerical methods are introduced: an asperity-superposition method and a direct-simulation method. In the first method, geometric parameters such as asperity heights, orientations, and radii of curvature are identified by a least-squares regression of neighboring surface heights measured using an atomic force microscope. The rough surface is reconstructed by the superposition of these asperities. The modeling of adhesive and elastic contacts begins with the modeling of a single parabolic-shaped asperity contact. A generalized JKR model for an arbitrary parabola is developed to suit this purpose. The contact between the rough surface (represented by the supposition of parabolic-shaped asperities) and the sphere consequently is modeled bythe mapping and integration of individual asperity contacts. In the second method, pure-elastic contact is modeled by half-space elastic theory. A contact-search algorithm is used to find solutions on the displacement and the contact-pressure distribution that satisfy both the load-displacement equation and the contactboundary conditions. Results from both methods are compared to reveal the effects of asperities on adhesion and elastic-contact pressure.     

WHEN LESS IS MORE: EXPERIMENTAL EVIDENCE FOR TENACITY ENHANCEMENT BY DIVISION OF CONTACT AREA

Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.     

Theoretical Method to Predict the First Microcracks in a Scarf Joint

In this work, we have calculated the theoretical threshold (F _d) of the first microcracks in the scarf joint. This threshold is particularly important because it marks the end of the elastic behavior of the bonded structure. At this point, the mechanical behavior of the adhesive is nonlinear and becomes dependent on the type of loading applied (dynamic tests, fatigue). Our method takes into account the geometrical variations of the joint as the scarf angle varies. We have used and adapted to our study a theoretical model based on the asymptotic-expansion method. The comparison between the theoretical values and the experimental thresholds determined by strain gauges and acoustic-emission techniques showed that the model accurately forecasts the microcracking of the joint provided that the scarf-angle value is more than 10°. When α is smaller than 10°, the theoretical model can no longer predict the very complex micromechanical behavior at the extremities of the joint, where the sharp ends of the adhesive layer and the metallic adherends coexist and react among themselves.     

Wetting Behavior of Ibuprofen Racemate Surfaces

The wetting behavior and detailed surface energetics of a racemate crystalline system were measured via contact angle measurements and inverse gas chromatography at finite concentrations. The advancing contact angles for water, diiodomethane, formamide, and ethylene glycol were measured on specific facets for racemic ibuprofen and S-(+)-ibuprofen single macroscopic crystals, and were found to be facet dependent for both systems. This observation demonstrates that variation in molecular orientation within the crystal lattice results in variations in exposed surface chemistry for differing facets, which results in anisotropic wetting behavior as previously reported. Surface free energy profiles of the ibuprofen racemates determined using a novel inverse gas chromatography method showed that powder samples (75–150 µm particle diameter) exhibited relatively homogeneous surface energies, with similar values of γ_SV ^d to those obtained by the contact angle analysis. These results lead us to conclude that ibuprofen exhibited a low level of surface heterogeneity, with the dominant facet of these powders exhibiting a low γ_SV ^d, with high energetic sites estimated to be < 3% of exposed available surface.     

Use of Ion Scattering and Secondary Ion Mass Spectrometry to Characterize Apparent “Adhesive” Failure in an Adhesive Bond

Frequently it is not easy using visual or even microscopic examination of an adhesive joint to determine after physical testing whether an apparent adhesive failure occurred at the original interface due to improper wetting or at some new interface leaving behind a thin layer of adhesive. Elemental analysis techniques such as ion scattering spectrometry (ISS) and secondary ion mass spectrometry (SIMS) are easily capable of determining the locus of failure in an adhesive joint. The use of these two techniques in combination is shown for investigating adhesive bonding phenomena. The operating parameters as well as advantages and disadvantages of each are summarized. ISS-SIMS data are shown for two adherend surfaces which broke in a lap shear test by apparent cohesive failure in both the adhesive and adherend.     

Influence of Drying Method on Enzymatic Stain and Contact Angle of Maple

Lumber enzymatically stained has a significantly reduced value. The process involved in producing this degradation has not been well characterized. The influence of temperature and fluid flow transport of sap solutes were probed using contact angle analysis. Temperature effects on surface chemistry, as detected by polar components, was identified but was found not to attribute to staining. Dispersal components, used to indicate fluid flow transport of solutes, pointed to a positive influence on the production of stain. A more detailed layer-by-layer analysis was discussed to provide a more definitive conclusion of attributing the degree of staining to fluid flow transport of sap solute.     

Evaporation of Ethanol-Water Mixture Drop on Horizontal Substrate

This study investigates the evaporation of sessile drop comprising ethanol and water mixture on horizontal poly methyl methacrylate surface. The contact angle (θ) and contact radius (R) of ethanol-water mixture drop are recorded with time, considering the impacts of presence of ethanol. With excess ethanol, the drop evaporation is principally controlled by a phase in which both the contact angle and contact radius are falling. A diffusional model assuming linear relation between contact radius and time is proposed as θ = eR ^?3 + cR ^?1, where e and c denote fitting coefficients. This model correlates with the experimental data.     

Performance Analysis of Contact Baking of Flat Bread in an Indirect-Heating Oven

A significant portion of Iranian flat breads are produced in semi-industrial, indirect-heating ovens. Therefore, an efficient oven design and a proper selection of operating conditions are crucial to improve the product quality and reduce energy consumption. In the present study, a mathematical model is developed to simulate a semi-industrial, indirect-heating, continuous oven performance during contact baking of an Iranian flat bread, referred to as Taftoon. Individual modes of heat transfer are considered among various components of the baking system to estimate the system performance and the bread quality in terms of design and operating conditions. The predictions of this model are in good agreement with the experimental data. Numerical results indicate that conduction is the primary heat transfer mechanism. Furthermore, the effects of dough thickness, conveyer speed, and input air velocity on the quality of the bread are studied and appropriate ranges for the parameters are determined.