An old invention with new possibilities

The first inventions related to tilting-pad bearings were made more than a century ago by the pioneers Anthony Michell and Albert Kingsbury. They developed at the beginning of the 20th century, and independently of each other, a water-lubricated axial thrust bearing of the tilting-pad type. These early tilting-pad bearings had better load capacity and durability characteristics than existing bearing solutions. Hence, it did not take long before water-lubricated tilting-pad bearings started to appear in a range of applications, such as steam turbines, centrifugal pumps, and hydroelectric generators. The next technology step was to replace the water film with a thin oil lubrication film. This did work just as fine, as Michell showed around 1916 with the world first oil-lubricated tilting-pad journal bearing^1,2.

A couple of years later, studies into air-lubricated tilting-pad bearings started to appear. For example, W. Stone demonstrated the load-carrying capacity of an air-film by spinning a glass disk against several pads made from quartz³.This experimental setup, apparently suggested by pioneer Michell, proved the self-alignment effect of tilting-pads. Around 1922, Albert Kingsbury further confirmed the feasibility of compressible gas as a lubricant with his own demonstration model of an aerodynamic tilting-pad thrust bearing⁴.

In 1946, Arthur Hagg became one of the first to recognize the advantages of tilting-pad bearings for suppressing self-excited rotor vibrations in journal bearings⁵. He even suggested that tilting-pad bearings were probably the only type of fluid bearings that could completely suppress whirl instabilities. However, it would take until the 1960s for the first industrial implementation of gas-lubricated tilting-pad bearings started to appear. The main driving force at that time was the demand to operate fast-rotating equipment in very hot, cryogenic, or radiation environments. It is therefore no surprise that one of the first real implementation of a tilting-pad gas journal bearing was in a helium circulator to cool a nuclear reactor⁶. These new gas bearings were developed by the French company Société Rateau and apparently performed in a robust manner with outstanding start-stop behavior.

Based on this early success, research on tilting-pad gas bearings started to accelerate in the mid 60s, which resulted in numerous theoretical models for predicting their behavior. During this era, Jørgen Lund provided an important contribution to this field by publishing a method to analytical compute the stiffness and damping coefficients of tilting-pad bearings⁷. His method is now known as the Lund's assembly method and has been implemented in many models since then.

Unfortunately, interest in gas bearings for fast-rotating machinery started to diminish in the late 70s. At that time, attention was shifting more toward gas-film lubrication problems encountered in tape and hard-disk storage systems. It was not until the late 90s that research programs on high-speed gas bearings reemerged. This new interest was mainly driven by the need for oil-free and small-scale turbomachinery such as turbo-compressors, air-cycle machines, and micro gas-turbines. Most of these reemerged research programs focused on the recently developed foil bearing. However, several other research programs continued with the old tilting-pad concept. The leap in computing power of this era led to numerous publications on tilting-pad modeling. It was however Yang Lihua who published in 2007, as one of the first, a mathematical model to study the dynamic characteristics of tilting-pad gas bearings⁸. Unfortunately, Lihua's results were still limited to very basic bearing geometries and, as a consequence, unsuitable for practical use. However, in the last decade a rapidly improvement of mathematical models took place, leading to more advance bearing designs with real practical implementations⁹.

Nowadays, the existing modeling tools and design methodologies could be fully explode to design tilting-pad gas bearings which have ideal characteristics for small-scale turbomachinery and other high-speed machinery^a. As such, it seems only a matter of time till we will see novel tilting-pad bearings designs in an increasing number of applications, like micro gas-turbines, turbo-compressors and high-speed motors.

1) T. Dimond, "A review of tilting pad bearing theory", 2011
2) J. Simmons, "Michell and the development of tilting pad bearing", 1987
3) W. Stone, "A proposed method for solving some problems in lubrication", 1921
4) D. Dowson, "History of Tribology" p458, 1998
5) A. Hagg, "The influence of oil-film journal bearings on the stability of rotating machines", 1946
6) J. Powell. "A review of progress in gas lubrication", 1970
7) J. Lund, "Spring and damping coefficients for the tilting-pad journal bearing" 1964
8) Y. Lihua, "Dynamic stiffness and damping coefficients of tilting-pad journal bearings", 2007
9) A. Rimpel, "A Rotordynamic, Thermal, and Thrust Load Performance Gas Bearing...", 2017
a) M. Nabuurs, "Tilting-pad gas bearings for high-speed applications...", 2020