When it comes to journal bearings in the Turbomachinery industry, the tilt pad bearing is used for the high speed rotor assemblies of centrifugal compressors because of their design flexibility. Tilting pad bearings usually come configured with 3 to 6 pads. The most commonly used configuration used in the turbomachinery industry is a 5 pad Tilt Pad Journal Bearing (TPJB). There are two main pad orientations that are used to describe how these pads are loaded by the shaft. These are load-on-pad (LOP) or load-between-pads (LBP).
You can easily verify which configuration is present for your rotor. Look at your TPJB and if one pad is centered with the very bottom centerline of your rotor, then you have a LOP orientation. If two pads straddle the bottom centerline of your rotor, then you have a LBP orientation.
When designing a tilt pad you have a few common design parameters such as: pad clearance (Cpad), bearing (assembly) clearance (Cbrg), journal (shaft) diameter, and preload.
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| Typical Tilt Pad Bearing Bores |
Bearing (assembly) clearance (Cbrg)
This is equal to the diametral clearance from the assembly bore diameter minus the journal diameter. To get an approximate real life assembly clearance, a lift check can be performed using a dial indicator on the bearing and multiplying this lift by an appropriate factor based on number of pads and orientation.
Pad clearance (Cpad)
This is equal to the diametral clearance from the machined pad bore diameter minus the journal diameter.
Preload (m)
One of the most common design parameters is called preload. As the oil wedge clearance changes during operation, a bearing's damping and stiffness changes. Preload is introduced to describe these variations. Preload is the fraction of distance between the bearing clearance (when concentric) to the pad clearance. Preload can typically vary from 0 to 0.75.
*If you want to calculate your own preload, you can use my own Preload Calculator.
Preload (m) = 1 - Cbrg/Cpad
Preload is considered positive if the pad clearance is greater than the bearing clearance. When these clearances are equal then your Preload is zero. The downside of using zero preload is that when the shaft is not centered with the bearing, only some pads will create a hydrodynamic pressure which loads the bearing against the housing. The unloaded pads or lightly loaded pads may become unstable and lead to pad "flutter".
To find preload you must start from somewhere. You typically know the shaft journal diameter where the bearing pads will ride. From there, you will have to establish a reasonable bearing clearance (radial gap) when the shaft journal is concentric with the bearing, between the shaft journal and the pad at the pivot position. Pad clearance is chosen based on achieving either a positive or negative preload, where positive preload is typically desired. The pad clearance is the difference between radius of curvature of the pad profile and the shaft journal radius. Next, you choose a pad thickness with a desirable amount of babbitt thickness for the size of your bearing. Thereby, setting your cage bore where your bearing pad will have its pivot contact area.
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