一类二簧系统的力分析

利用 Dixon结式给出了解决一类二簧系统力的逆分析的新方法 ,利用该方法 ,可方便地得到一个关于弹簧位置角度的一元六次方程 ,即封闭形式的解。两个弹簧的长度和另一个位置角等变量可相应求出。并用一个数值例子验证了该方法。另外通过分析系统势能随外力的变化规律 ,得到了机构灾变点的判别式 ,该判别式说明当机构靠近灾变点时 ,判别式值趋于零。推导过程使用了微机数学软件MATHEMATICA。     

Temperature Dependences in the Tomlinson/Prandtl Model for Atomic Sliding Friction

The temperature dependence of the Tomlinson/Prandtl model for nanoscale sliding friction is analyzed by considering the properties of the initial and final states between which the tip can move, as well as the energy barrier between them, for various sliding regimes defined by the value of the corrugation factor γ. When γ < 1, the friction force tends to zero, defining a so-called superlubricious regime. The most commonly observed behavior is found for γ > 4.603, where the friction force increases monotonically with increasing sliding velocity up to a critical value equal to the value of F ^* (lateral force at T = 0) and monotonically decreases with temperature from F ^* at T = 0. However, completely different behavior is found when 1 < γ < 4.603. The temperature dependence of the lateral force in this regime is investigated using Monte Carlo simulations. The friction force still tends to F ^* as T approaches 0 K, but in contrast to the behavior found when γ > 4.603, the friction force increases with increasing temperature from F ^* , reaches a maximum value, and then decreases monotonically as the temperature rises further. Such behavior has been observed in atomic force microscopy friction measurements.     

Remaining Useful Life Model and Assessment of Mechanical Products: A Brief Review and a Note on the State Space Model Method

The remaining useful life(RUL) prediction of mechanical products has been widely studied for online system performance reliability, device remanufacturing, and product safety(safety awareness and safety improvement). These studies incorporated many di erent models, algorithms, and techniques for modeling and assessment. In this paper, methods of RUL assessment are summarized and expounded upon using two major methods: physics model based and data driven based methods. The advantages and disadvantages of each of these methods are deliberated and compared as well. Due to the intricacy of failure mechanism in system, and di culty in physics degradation observation, RUL assessment based on observations of performance variables turns into a science in evaluating the degradation. A modeling method from control systems, the state space model(SSM), as a first order hidden Markov, is presented. In the context of non-linear and non-Gaussian systems, the SSM methodology is capable of performing remaining life assessment by using Bayesian estimation(sequential Monte Carlo). Being e ective for non-linear and non-Gaussian dynamics, the methodology can perform the assessment recursively online for applications in CBM(condition based maintenance), PHM(prognostics and health management), remanufacturing, and system performance reliability. Finally, the discussion raises concerns regarding online sensing data for SSM modeling and assessment of RUL.     

A Note on the Stability of Flow in Loaded Journal Bearings

If two nonconcentric cylinders are rotating, a circumferential pressure gradient is created. The effect of this pressure gradient flow on the stability of the local velocity profiles is discussed. It is concluded that in the most critical region the flow due to the pressure gradient has a stabilizing effect.     

A Note on Interpreting Tool Temperature Measurements from Thermography

Thermography (thermal imaging) is a well-established experimental method for studying cutting tool temperature distributions. In one form, cutting edge temperatures within the chip / tool contact area are deduced from thermal images of tool faces normal to the cutting edge but offset from the contact region. In general practice, the offset is made as small as possible (<< 1 mm) and it is assumed that the observed temperature is the same as that within the contact. In this short communication an approximate analytical model is developed for the influence of the offset on the observed temperature. The predictions from the model are compared with previously unpublished existing results on the machining of Ti alloys (Ti6Al4V and Ti5Al4V) and on steel (AISI 4140). It is shown that ignoring the offset may introduce underestimates of cutting edge temperature of ≈ 30% or more. This is large compared to the usually considered uncertainties of ± 5% from camera and tool emissivity calibration. There is a need for a dedicated study of this effect.     

Note on contributors

The influence of an external d.c. magnetic field on the diffusive wear of two rubbing bodies represented by idealized asperities was investigated. The magnetic field affects the fracture characteristics at the asperity junction giving different wear rates owing to the presence of the magnetic field. A simple model predicts how the hardness gradient at the surface of the asperity is of significance in affecting the wear characteristics of a sliding pair. The mass of a wear particle is directly affected by diffusivity, and the application of an external magnetic field to a deforming junction affects it considerably. Numerical calculations and experimental observations for mild steel rubbing against brass support the predictions that the application of an external magnetic field enhances diffusive wear at an asperity junction.     

Note on contributors

It is generally accepted that one particular wear mechanism involved in rolling contact between two cylinders is basically that of fatigue. This mode of wear is influenced by the maximum Hertzian stresses occurring in the contact zone of the cylinders. The equations usually employed to calculate the magnitude of the Hertzian stresses and the corresponding allowable working loads are based on an assumption that the contours of the contacting surfaces are perfect. In general practice however the profiles of the cylinders are always imperfect in form.In this paper cases of imperfect contact between bodies of nominally cylindrical form are analysed.     

Macroscopic versus microscopic description of friction: from Tomlinson model to shearons

In this Letter we investigate the response of an embedded system that is subject to an external drive, which is chosen to be either a constant velocity or a harmonic shear. Two approaches are introduced that cover macroscopic and microscopic aspects of the problem and mimic recent measurements on friction using the surface forces apparatus. The first approach is based on the Tomlinson model and its generalization, and allows for the investigation of the coupled lateral–normal response to a lateral drive. We propose a method for analyzing both linear and nonlinear response of confined systems driven harmonically. The method provides a way to deduce the microscopic parameters responsible for dissipation. The second approach focuses on the microscopic level. The shear is shown to excite “shearons”, which are collective modes of the embedded system with well-defined spatial and temporal patterns that dominate the frictional properties of the driven system. We demonstrate that the slip relaxation in stick–slip motion and memory effects are well described in terms of the creation and/or annihilation of shearons.     

Note on the form of wear laws

The effect of cutting speed and tool wear land length on the plastic strain distribution in the surface region of annealed 18% Ni maraging steel machined under lubricated and unlubricated conditions is determined by using the grid technique.The results show that the magnitude of the plastic strains in the surface region and the depth of the plastically deformed layer increase with an increase in the cutting speed or the tool wear land length or both. The presence of the lubricant in the cutting region results in considerable reduction in the subsurface deformation.     

Note s on contributors

The applicability of time series modelling techniques to the simulation of machined surface profiles is discussed. The methods are then illustrated for a second-order autoregressive model with a normal amplitude distribution. In order to allow generation of non-normal surface profiles the relations between the surface height third moments and residual third moments together with similar relations for the fourth moments, and hence skew and kurtosis, are also given for first- and second-order autoregressive models. Examples are then shown for each of these cases.     

Note: A portable pulsed neutron source based on the smallest sealed-type plasma focus device

Development and operation of a portable and compact pulsed neutron source based on sealed-type plasma focus (PF) device are reported. The unit is the smallest sealed-type neutron producing PF device. The effective volume of the PF unit is 33 cm(3) only. A compact size single capacitor (4 μF) is used as the energy driver. A battery based power supply unit is used for charging the capacitor and triggering the spark gap. The PF unit is operated at 10 kV (200 J) and at a deuterium gas filling pressure of 8 mb. The device is operated over a time span of 200 days and the neutron emissions have been observed for 200 shots without changing the gas in between the shots. The maximum yield of this device is 7.8 × 10(4) neutrons/pulse. Beyond 200 shots the yield is below the threshold (1050 neutrons/pulse) of our (3)He detector. The neutron energy is evaluated using time of flight technique and the value is (2.49 ± 0.27) MeV. The measured neutron pulse width is (24 ± 5) ns. Multishot and long duration operations envisage the potentiality of such portable device for repetitive mode of operation.