Tilting pad journal bearings can operate under a wide range of conditions of speed, load, load angle, oil temperature, and oil viscosity. Each one of these variables has an effect on the oil film, and an influence on whether the film is laminar, turbulent, or in transition. Other parameters affect the film as well including geometry such as the bearing diameter, length, clearance, preload, the number of pads and their orientation.
Where: ρ is density; μ is velocity ;h is film thickness: γ is viscosity The general form for film thickness is equation 2:
Where: l is length; p is unit loading, C are the constants. For a given bearing geometry at constant speed and load, the film thickness (h) varies with the square root of viscosity (√γ), while the Reynolds number varies indirectly with viscosity (1/γ). Combining the equations, we find that Reynolds number varies by 1/(√γ) for a given bearing geometry at constant speed and load. For the above example of increasing pad temperature between 16 and 24 gpm [60 and 90 l/min] at 300 ft/sec [90 m/sec], increasing the oil flow rate cools the oil films thereby increasing the viscosity. This gives a smaller Reynolds number or, in other words, a higher transitional speed before turbulent transition. The higher flow in this case resulted in a change in the film from turbulent to laminar flow with higher associated temperatures. Based on the equations and data, it can be deduced that the increased flow rates keep the bearing in laminar operation longer, increasing the threshold speed before turbulent transition. Operation in the transient area may lead to unexpected results, so it is important know where the transition occurs. If you are experiencing behavior of this nature, we encourage you to consult our sales engineers for advice on developing a retrofit bearing to address the specific needs of your application. Please call us at +12158244000 or send us an email via sales@kingsbury.com.
