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In the recent past the basic design has been optimized to improve performance. A partial list of these features includes:
- Pad shape variations: The basic size and shape of a bearing pad has an influence on the oil flow patterns affecting the hydrodynamic forces. Computer modeling of many variations has proven effective in changing operating characteristics.
- Material variations: Under certain conditions, bearing pads are subject to forces large enough to temporarily distort the original shape. Pad distortion can have an adverse effect on design characteristics. Heat transfer also is a major consideration in choosing pad backing material.
- Directed lubrication: The horsepower loss associated with rotating a shaft in a flooded bearing housing is significant. The concept of directing a controlled amount of oil to a specific location has proven to be advantageous.
- Various pivot designs: Concentrated loads are hard to avoid when evaluating pivot designs, primarily due to the size limitations. Many innovative variations have proven to be durable
- Hydrostatic lift oil: Prolonged rotation on turning gear or at speeds significantly less than synchronous can cause premature bearing wear, which will ultimately affect performance. Lift oil is effective in forming a minimal oil film at reduced RPM.
Generally speaking, field inspection of a bearing designed to minimize oil whirl is limited to the following areas, depending upon the specifics of the design.
- Crush or pinch of mounting fit: Clearance in a bearing mounting is high on the list of variables that must be quantified when troubleshooting vibration issues.
- Visual inspection of Babbitt: The effects of vibration induced by oil whirl are sometimes obvious at a glance.
- NDE of Babbitt: Verification of Babbitt bond integrity by ultrasonic and dye penetrant inspections is essential to avoid a future problem.
- Visual inspection of pad pivots: Load concentration will compromise the structural integrity pad pivots and adversely affect a bearings characteristics over time.
- Visual inspection of lift oil ports: Faulty lift oil systems can lead to premature wear while on turning gear and during start-up/shutdown.
- Physical measurements: Diametric clearance and pad wear validation is possible by using an inspection mandrel ONLY if the design of a tilt pad bearing is confirmed to have central pivots and zero preload.
Dimensional validation of a fixed bore bearing is possible if the design is confirmed to be a cylindrical, elliptical or pressure dam.
Most bearings designed to minimize oil whirl are constructed with unique features and machined to specific geometries. Inspection of these bearings is best done in a machine shop with precision measuring instruments and the proper machine tools. A critical element of the inspection is the experience factor of the personnel overseeing the process and evaluating the data.
A rule of thumb regarding shop inspection of tilting pad bearings – specialty bearings require unique fixtures and techniques. It is a prudent practice as the owner of a tilt pad bearing to research and confirm the design specifications prior to self-performing or sub-contracting an inspection and potential repair.
Steve DeCrow is the founder and president of Renewal Parts Maintenance, a division of Mechanical Dynamics & Analysis, Ltd. He has been active in the maintenance and repair of turbine generators for the last 35 years while employed by an OEM, an electric utility and currently as an aftermarket service provider. You may contact him by e-mailing email@example.com.
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