Hydrodynamic bearings work by having a film of fluid separating the sliding surface (shaft/journal or runner) from the bearing surface. The hydrodynamic lubrication theory explains that pressure develops in this film when the oil film is squeezed by the sliding surface over the stationary surface in the direction of rotation. The fluid film provides stiffness and damping to the rotor-bearing system.

• Journal applications are those where the rotor is to be laterally supported by a bearing
• Thrust applications are those where the rotor is to be axially supported by a bearing

When a load is applied to a stationary body, its displacement (movement) is in the same direction as the applied load. The stiffness governing this motion is its direct stiffness.

When a rotor is rotating in a journal bearing, the bearing exhibits direct stiffness due to the fluid film. Additionally, however, it also exhibits a cross-coupled stiffness i.e. displacement perpendicular to the direction of the applied load. This is due to the gyroscopic effects of the rotating rotor. The presence of cross-coupling makes the rotor system system susceptible to instability.

Fixed geometry bearings such as a plain sleeve or lobed bearing inherently have high cross coupling thereby limiting their use to less critical applications. We design and manufacture both plain and taperland thrust bearings for less critical applications.

Tilting pad journal bearings (TPJBs), as the name implies, have variable geometry wherein the bearing surface is able to tilt perpendicular to the plane in which the sliding surface moves. This eliminates the cross-coupling almost completely. Therefore, tilting pad journal bearings are heavily used in rotordynamically critical applications. Most often these are machines that operate at speeds above the critical speeds of the rotor.

D&S Engineered Products makes two types of tilting pad journal bearings - Rocker Back TPJB and Pliant Support TPJB.

Rocker Back TPJB

Rocker Back TPJBs were first introduced almost a century ago. It is produced by assembling a set of journal pads onto the inside diameter of a bearing shell. The outside diameter of the pads is slightly smaller than the inside diameter of the shell so the pads can "rock" on the shell ID. Nozzles assembled between the pads provide passage for the lubricating oil to be injected close to the journal surface.

Pliant Support TPJB

Pliant Support TPJBs are a relatively new design. The bearing is made from a monolithic piece of material. The journal pads are made by electric discharge machining (EDM) process. The pads are inherently attached to the "shell" via an I-beam like elastic pivot. The pivot is elastically pliant and it allows the pad to tilt under load. Nozzles are drilled in the material left between the pads during the EDM process to provide passage for the lubricating oil to be injected close to the journal surface. This design provides significant advantages over the rocker back TPJ design in many applications.

A comparison of characteristics for both types of TPJ design is presented in the table below.

A comparison of suitability (1- not suitable to 5 - highly suitable) to different sizes is presented in the table below.

Both types of TPJBs can be integrated with various types of thrust faces to make a combination journal/thrust bearing. The thrust faces could be plain or bumper, taperland, rocker back tilt pad or pliant support tilt pad types.

D&S' engineering staff can advise you on the most suitable and commercially viable journal bearing design for your application. Please contact us to discuss your application.

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