Nonlinear Dynamics Analysis of a Rolling Bearing

A numerical analysis of the nonlinear bearing model about two degrees of freedom has been presented in this paper. The contact between a ball and ring by Hertz theory is described. Influence of the number of balls, shaft rotation and clearance on the acceleration were investigated in detail. Three numbers of balls from 11 to 16 were analyzed. The clearance level in the range of 0-71μm has been studied. It has been shown that the acceleration responses are different, depending on the vibration direction and are usually higher when the radial internal clearance and the shaft speed are increased. The higher ball number caused that the accelerations decreased in both directions. Moreover, two dynamic indicators that can be used for comparison bearing dynamics have been proposed. These obtained results are useful for understanding the vibration response mechanism from a practical point of view.

Mixed thermal elastohydrodynamic lubrication analysis with dynamic performance of aero ball bearing during start-up and shut-down

In this study, a coupling model is developed to include the aero ball bearing dynamic performance in the mixed thermal elastohydrodynamic lubrication analysis, and the low-speed and heavy-load conditions during start-up and shut-down are involved. Based on the bearing quasi-dynamics, the inside motion state of the main loading surface is obtained, and the mixed thermal elastohydrodynamic lubrication is conducted to get bearing lubrication state and properties. The numerical lubrication model under low-speed and heavy-load conditions is validated against published tested data, which reveal well consistency in central film thickness. The lubrication properties between a single ball and inner race during start-up have been studied, which indicate the lubrication film transforms from boundary lubrication to unsafe mixed thermal elastohydrodynamic lubrication, and then goes into safe lubrication. The lubrication properties of the balls at different azimuths have been investigated during shut-down and compared with those in start-up, which have a similar opposite changing trend, but not a simple invertible process. The time in boundary lubrication region during shut-down is shorter, and the ball number in boundary lubrication region gets less, which means the lubrication properties are relatively better. At last, the parametric study on mixed thermal elastohydrodynamic lubrication properties during shut-down has been carried out. It is found that the small bearing curve coefficient and increasing ball number can reduce the boundary lubrication time and improve the bearing lubrication.

Recalculation of the basic static load of ball screws

Based on the correction factor of accuracy applied in ISO standard 3408-4:2006, this article proposed a modified method to calculate the effective ball number and basic static load rating of ball screws. Meanwhile, the basic static load rating of ball screws was obtained through a new measurement method presented in this article. The experimental results, agreeing well with the theoretical values calculated by the modified method, showed that there is a large discrepancy between the basic static load rating calculated by the traditional method and the measured result, which proves that the new modified method is valid. This study provides a more accurate method to obtain the basic static load of ball screws, which is significant for predicting the performance and service life of ball screws.

Influence Factors Comparison and Analysis of Axial Contact Stiffness between Single-Nut and Double-Nut Ball Screws

Axial contact stiffness increase of ball screws can reduce the vibration and enhance machining precision of CNC machine tool during machining process. On the basis of Hertz contact theory, calculation formula of axial contact stiffness was deduced with consideration of helix angle for 5010 single-nut and double-nut ball screws, influence factors comparison and analysis was carried out, and contrast curve was obtained. The result shows that the axial contact stiffness of preloaded double-nut ball screws is higher than that of single-nut ball screws. Ball number, contact angle and curvature ratio has obvious influence on the axial contact stiffness of double-nut ball screws: ball number increase can enhance the axial contact stiffness; increase of contact angle and curvature ratio can lower the axial contact stiffness. The axial contact stiffness of single-nut and double-nut ball screws does almost not vary with axial force and helix angle. The conclusion provides certain theoretical basis for design and selection of ball screws.

Forming Process and Mechanism of Microcapillary Grooves of Miniature Cylindrical Heat Pipe

Miniature heat pipe is one of promising means of removing high heat flux for electronics cooling in limited available space. How to produce narrow and deep microcapillary grooves is the critical issue in promoting performance of axially grooved miniature heat pipe. In this work, the microcapillary grooves for miniature cylindrical heat pipe was fabricated by using oil-filled high speed spinning method. The forming process of microcapillary grooves was investigated. The influence of technical parameters on characteristics of microcapillary grooves was analyzed and the optimal technical parameters were found out. The experimental results show that drawing speed ranging from 48cm/min to 64cm/min, spinning ball number of 4 and big distance between the end of multi-tooth mandrel and the center of spinning balls within 3.7 mm should be chosen to obtain narrow and deep microcapillary grooves. In addition, the forming mechanism was investigated by observing microstructure of cross-section of microcapillary grooved tubes.

Damping in Vibration Transfer Through Deep-Groove Ball Bearings

Damping characteristics in vibration transfer through deep-groove ball bearings were measured precisely using a single ball bearing (equivalent to the 6200 type) and the compliance transfer function resulting from the impulses applied to the bearing. To eliminate uncertainties caused by bearing assembly and preloading, a monolithic-type ball bearing was specially fabricated by unifying its inner ring with a shaft, and its outer ring with a rotor. This structure enables preloading without the need for any solid contact using an externally pressurized air bearing. Experimental damping ratios were found to range from two to four percent and damping coefficients 0.15-0.35 Ns/mm. Damping was found to decrease with an increasing preload, and to be larger for a larger clearance and for a larger ball number. The decreases in damping resulting from superimposing the tilting mode vibration upon the lateral mode and from an increasing clearance indicated more ball rolling motion rather than slipping motion to be the cause of decreased damping. Also, the bearing rotation effect is found to be small. Addition of grease hardly provided any additional damping during rotation.