Experimental Contribution to the Mechanics of Herringbone Grooved Journal Air Bearings

Gas Lubricated Herringbone Grooved Journal Bearings (HGJB) have long been used in equipment such as navigation systems, optical scanners for laser printers and high speed cameras and disk spindle for flyhead testers. All such applications are characterized by a relatively small size (journal diameter <10 mm, rotor mass <100 g), benign duty cycle (mainly start-and-stop) and reliance on the inherent high stiffness of HGJB to maintain a stable axis of rotation. The oil lubricated version of Helical Grooved Journal-Thrust Bearing cartridge has been adopted by the disk drive industry for disk stack support; known as the hydrodynamic spindle, it makes possible a substantial increase of track density by eliminating self-noise and there is much interest to up-grade to an Air Lubricated version to reduce heat generation. Load carrying capacity per se is not of concern in these applications; albeit high-g shock testing is a procurement requirement as safeguard for service reliability. Despite the variety of applications, curiously missing from the roster is its use as the main support bearing for energy conversion machinery even though HGJB owes its heritage to the Whipple thrust bearing, which was invented for rotating machines for operation in hermetically sealed loop of radio-active environment. These machines are typically supported by pivoted pad journal bearings. Nevertheless Helical Grooved non-contact shaft seals are used in energy conversion machines; high radial and angular stiffness ensure uniform radial clearance and flow leakage is kept at a very low level. There is an immediate interest for the deployment of HGJB in the machine tool industry; specifically a high speed machining spindle appears to be very desirable. In order to enhance early realization of this application, research is needed to address issues related to increased size and rotor mass, and an assessment of how HGJB can satisfy the more demanding duty cycle. Although many analytical/numerical methods – including Narrow Groove Theory (NGT), Finite Difference Models, and Finite Element Models (FEM) – have been developed to calculate stiffness and load carrying capacity of HGJB, very scant experimental results are available to validate model predictions. This dissertation presents an experimental study performed on air lubricated HGJB, which attempts to fill this gap. For these experiments a novel four degree of freedom piezo-actuated loading system has been developed. The novelty of the loading system resides in the fact that both – the shaft and the bearing – are rigidly supported, so that bearing whirl instability is suppressed in the experiments, which makes their interpretation significantly simpler. Loading of the HGJB is performed by imposing relative displacements between shaft and journal by means of piezo actuators driven by a specially designed multivariable control algorithm. We evaluated several conventional and non-conventional manufacturing technologies to introduce shallow spiral grooves with controlled profile on a shaft, and selected the most appropriate one – electrochemical etching. Measurements performed with the novel test facility during the experiments on HGJBs allow determining the load carrying capacity and stiffness of a journal bearing as a function of compressibility number. This correlation is established at different speeds and different eccentricity ratios. The use of a shaft diameter of 60 mm combined with a zero-speed radial clearance of 14.5 µm leads to compressibility numbers of up to 35, for shaft speeds of 40'000 rpm. This compressibility number range enables accurate characterization of the performance of a HGJB over most useful operating regimes. Different grooving configurations have been tested with a constant bearing length to diameter ratio, equal to one. By measuring bearing reaction forces in two perpendicular planes we determined the bearing attitude angle at different levels of eccentricity ratio. We also monitored the temperature inside the lubricating film. The experimental results are compared to predictions obtained from available analytical/numerical models. Conclusions are drawn regarding the domain and conditions of validity of each of the models (namely regarding the number of grooves and eccentricity ratio). We observed that the NGT over predicts performance of HGJBs. The influence of the number of grooves on performance characteristics is observed to be lower than what is predicted by the FEM analysis. Furthermore, the increase in the radial force coefficient with the increase in the eccentricity ratio is also shown to be lower than what FEM analysis predicts. We concluded that the power dissipated by a gas lubricated HGJB is well predicted by the modified Petrov's equation. All this information contributes to putting design and analysis of HGJB on a more solid and reliable basis, which is the ultimate goal of this dissertation.


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