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"Point contact"
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A novelty mandrel supported thin-wall tube bending cross-section quality analysis: a diameter-adjustable multi-point contact mandrel
The mandrel plays a vital role in metal tube bending limit improvement and high-quality achievement. For the bending forming process of metal tubes with different diameters, the corresponding mandrel needs to be customized, which has the disadvantages of a long design cycle, high manufacturing cost, and low reuse rate. Therefore, a novelty concept of diameter adjustable mandrel (DAM) based on the multi-point contact was proposed to adapt different diameter tube bending processes. Since the point-contact characteristics of the DAM can lead to irregular cross-section deformation, the cross-sectional full profile radial error (FP-RE) model was proposed. As a more comprehensive evaluation model, the FP-RE model is used to measure the deformation degree of the whole bending cross-section. A diameter-adjustable DAM was adopted in the AISI 304L tubes (the inner diameter = 40–56 mm) bending experiments. The maximum absolute error of FP-RE was 0.11% compared with the ones obtained from FEA in Abaqus/explicit. In addition, the effects of the DAM characteristic parameters, viz., the support blocks amount, the effective magnification, and the effective support diameter, on the tube forming quality were discussed. The results indicated that (a) an even number of support blocks could prevent the collapse of the outer convex tube wall and improve the quality of the inner concave tube wall. (b) When the DAM was used in the larger inner diameter tube bending, the larger non-contact area would cause an increase in the roundness section distortion. (c) A larger DAM diameter would lead to a more minor cross-section deformation degree for the tubes with the same diameter, while the wall thinning was just the opposite. Finally, the result also proved that the DAM (the minimum effective diameter is 40 mm with six support blocks) could be used for tubes with an inner diameter of about 40–56 mm, and the forming quality was acceptable.
Journal Article
A dynamic model of a three-point contact ball bearing-rotor system: numerical and experimental verification
Three-point contact ball bearings (TPCBBs) belong to the main bearings of aeroengines. Their contact state will change significantly with external forces, which will affect the nonlinear vibration responses of rotor systems. This paper presents a quasi-static model for a 5-DOF TPCBB, taking into account various external conditions. Using the structure of a certain aeroengine as a reference, both the TPCBB’s quasi-static model and a cylindrical roller bearing model are introduced into the dynamic model of the TPCBB-rotor system. An innovative solving method, which combines the Newton–Raphson method and the Newmark-HHT method, is proposed. The solving method is used to analyze the acceleration responses, displacement responses, and contact forces of the TPCBB-rotor system with varying axial and radial forces. Furthermore, the validity of the proposed model is verified by a comparison of simulation and experimental results. The results indicate that the three-point contact state and the two-point contact state of the TPCBB will switch with the change of axial and radial forces. Compared with the three-point contact state, the acceleration amplitude and displacement amplitude of the system in the two-point contact state are smaller. And the contact forces between the balls and one of the inner rings will become 0 N.
Journal Article
A Combined Experimental and Atomistic Investigation of PTFE Double Transfer Film Formation and Lubrication in Rolling Point Contacts
Solid lubricants such as polytetrafluoroethylene (PTFE) are used in rolling-element bearings (REBs) when conventional lubrication (i.e. by fluids or greases) cannot be applied owing to extreme operating conditions (e.g. high temperatures or vacuum). Often a double transfer film mechanism is used with a cage acting as a lubricant reservoir resupplying the REB with solid lubricant by cage wear. An increase in service life of such bearings requires a better understanding of the transfer processes in the sliding and rolling contacts. Here, we investigate the effect of PTFE resupply on friction and lubricant film formation in a steel/steel and steel/glass rolling contact by tribometry and classical molecular dynamics (MD). A ball-on-disk tribometer is enhanced by a pin-on-disk sliding contact that transfers PTFE to the disk. The experiment allows simultaneous in situ measurement of friction and film thickness by white light interferometry in the rolling point contact. Increasing the pin load results in an increased PTFE film thickness in the rolling contact accompanied by a significant decrease in friction. To elucidate the observed film transfer and friction mechanism, sliding MD simulations with a newly developed density-functional-based, non-reactive force field for PTFE-lubricated iron oxide surfaces are performed. A strong adhesion of PTFE chains to iron oxide drives transfer film formation, whilst shear-induced chain alignment within PTFE results in reduced friction. The simulations reveal an anti-correlation between PTFE film thickness and friction coefficient—in agreement with the experiments. These investigations are a first step towards methods to control PTFE transfer film formation in REBs.
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Journal Article
Cage Dynamic Analysis of Four-point Contact Ball Bearing for High-speed Railway Traction Motor
The cage, serving as a pivotal element in rolling bearings, possesses dynamic characteristics that have a direct bearing on the overall performance of the bearing. This paper caters to the requirements of dynamic and strength analysis of the four-point contact ball-bearing cage in high-speed railway traction motors. A rigid-flexible coupling dynamics model of the bearing is established on the ADAMS platform and its dynamic analysis was carried out under variable axial forces and with different guiding clearances. The results show that the variable axial force has little effect on the bearing cage stress, but has a great effect on the bearing cage motion. The guiding clearance has little effect on the stress of the cage, but it affects the displacement and speed of the cage obviously.
Journal Article
The Applicability of the Hertzian Formulas to Point Contacts of Spheres and Spherical Caps
Hertzian formulas are commonly used for the evaluation of deformation and pressure distribution of non-conformal and slightly conformal mechanical pairs to estimate component stiffness and durability. For the sake of simplicity, their use is extended even to those cases in which Hertz’s hypotheses do not hold. This paper summarizes Hertz’s theory and compares the results obtained with theoretical and finite element analysis of the point contact of non-conformal and conformal pairs made of spheres, caps, and spherical seats. This study was motivated by the non-Hertzian behavior of a tilting pad bearing ball-and-socket pivot conforming contact observed by the authors in previous experiments. In particular, the displacement and force relation were investigated by varying the geometrical parameters, the materials, the boundary conditions, and the friction coefficient. In the case of non-conformal contact, the parameter variations had negligible effect in agreement with Hertz’s theory while for conformal contact, the cap and seat height and width and the relative clearance were the most influential parameters on the non-Hertzian behavior. These novel results indicate that in conformal pairs, such as for tilting pad bearing ball-and-socket pivots, whenever Hertz’s hypotheses are not satisfied and the assessment of contact stiffness is crucial, Hertzian formulas should not be applied as done in common practice, instead more accurate numerical or experimental evaluation should be made.
Journal Article
Three‐Phase Electrolysis by Gold Nanoparticle on Hydrophobic Interface for Enhanced Electrochemical Nitrogen Reduction Reaction
Electrochemical nitrogen reduction reaction (NRR) provides a facile and sustainable strategy to produce ammonia (NH3) at ambient conditions. However, the low NH3 yield and Faradaic efficiency (FE) are still the main challenges due to the competitive hydrogen evolution reaction (HER). Herein, a three‐phase electrocatalyst through in situ fabrication of Au nanoparticles (NPs) located on hydrophobic carbon fiber paper (Au/o‐CFP) is designed. The hydrophobic CFP surface facilitates efficient three‐phase contact points (TPCPs) for N2 (gas), electrolyte (liquid), and Au NPs (solid). Thus, concentrated N2 molecules can contact the electrocatalyst surface directly, inhibiting the HER since the lowered proton concentration and overall enhancing NRR. The three‐phase Au/o‐CFP electrocatalyst presents an excellent NRR performance with high NH3 yield rate of 40.6 µg h−1 mg−1 at −0.30 V and great FE of 31.3% at −0.10 V versus RHE (0.1 m Na2SO4). The N2‐bubble contact angle result and cyclic voltammetry analysis confirm that the hydrophobic interface has a relatively strong interaction with N2 bubble for enhanced NRR and weak electrocatalytic activity for HER. Significantly, the three‐phase Au/o‐CFP exhibits excellent stability with a negligible fluctuation of NH3 yield and FE in seven‐cycle test. This work provides a new strategy for improving NRR and simultaneously inhibiting HER. A three‐phase electrocatalyst with abundant three‐phase contact points (TPCPs) is achieved by direct in situ fabrication of Au NPs on hydrophobic carbon fiber electrode, demonstrating high ammonia yield rate (40.6 µg h−1 mg−1) and high Faradaic efficiency (31.3%).
Journal Article
Comparative Study of Wheel Profile Influence on Multi-Point Wheel–Turnout Contact Using Kalker’s Theory
Turnouts represent a critical element of railway infrastructure, and are subjected to some of the highest mechanical stresses due to the discontinuity of the track geometry. Failures in this area generate high maintenance costs and may compromise traffic safety. This study investigates the effect of wheel profile geometry on the wheel–turnout interaction in the presence of multi-point contact. Two standard wheel profiles, S78 and S1002, are compared using numerical simulations based on Kalker’s three-dimensional rolling contact theory, implemented in the CONTACT program. The methodology includes parametric analysis of the contact stresses, adhesion/slip distribution, and frictional power density for typical operational conditions. It was observed that the choice of wheel profile significantly influences the shape and load distribution of contact patches, with direct implications for wear mechanisms and guidance safety. These findings provide valuable insight for optimizing wheel–rail interface design and for reducing turnout maintenance costs.
Journal Article
Multi-Point Contact Dynamics of a Novel Self-Centring Mechanism for In-Space Robotic Assembly
Autonomous in-space assembly using a free-flying robot can lead to residual vibrations and positioning errors of the target modules during the grasping process. This places stringent demands on end-effectors, which must tolerate large misalignments while maintaining high positioning accuracy. In this regard, this paper presents a novel self-centring mechanism, which consists of two self-centring fingers mounted on the end-effector and a double V-groove mechanism attached to the target module. The proposed compact structural design passively corrects substantial parallel offsets and angular misalignments between the end-effector and the module. A multi-point contact model consistent with this mechanism is then developed using the virtual sphere layer method to describe the self-centring process. This model incorporates a normal contact force model and a three-dimensional bristle frictional force model to characterise the multi-point bouncing contact behaviours during the self-centring process. Numerical simulations and experimental tests involving the grasping of a module with a single robotic arm confirm that the self-centring mechanism effectively eliminates initial misalignments, achieving sub-millimetre positioning accuracy. The measured parallel offsets and contact forces align closely with numerical predictions, with minor discrepancies attributed to environmental noise and vibrations from the elastic bungees in the gravity compensation system. Finally, the self-centring mechanism is applied to grasp two modules with a dual-arm robot in the Space Proximity Operations Test facility. The centroid displacements of the robot closely match the simulation results, further validating the accuracy of the proposed multi-point contact model.
Journal Article
Study on Comprehensive Performance of Four-Point Contact Ball Slewing Bearings Based on a Bearing Support Bolt-Integrated Model
To investigate four-point contact ball slewing bearings, a bearing support bolt-integrated model was created with HyperMesh and ANSYS software, and its accuracy was theoretically confirmed. This study examines how the rolling element number Z, contact angle α, bolt number N, bolt pre-tightening force coefficient P, and radial load-overturning moment angle θ affect the comprehensive performance of four-point contact ball slewing bearings and connecting bolts. The study found that increasing Z, α, N, P, and θ reduces overall bearing, ring, rolling element, and contact load deformations. The maximum deformation and stress of bolts rise with P but decrease with Z, α, N, and θ. The degree of influence of each parameter on the deformation of the inner and outer rings, the deformation of the rolling element, and the contact load of the rolling body from large to small is ranked as follows: α, N, Z, θ, and P; the degree of influence on bolt deformation and bolt stress distribution uniformity from large to small is ranked as follows: N, α, Z, θ, and P; the degree of influence on the overall deformation of the bearing from large to small is ranked as follows: N, θ, α, Z and P; the degree of impact on the maximum stress of the bolt from large to small is ranked as follows: P, N, Z, α, θ. To improve the overall performance of a four-point contact ball slewing bearing, increase α, N, Z, and θ.
Journal Article