Fluid Pivot: 10 Reasons to Consider its Application
1. Vibration Control: Squeeze Film Damper Built In, Not Added-On
A self-generating hydrostatic pad support creates a squeeze-film damper arrangement behind each pad. This provides Pioneer’s Fluid Pivot bearing with substantially more damping than mechanically pivoted tilting pad bearings, especially at light load and high speed. A mechanical pivot introduces an undamped connection between the pad and ground which limits the effective bearing stiffness and damping. The Fluid Pivot principle introduces a well damped connection.
2. Superior Ability to Accommodate Dynamic Misalignment
The Fluid Pivot bearing with cylindrically seated pads (our Style JC) has good capability for accommodating misalignment. But our advanced Style JS Fluid Pivot bearing, with spherically-seated pads, extends this capability much further to more than 1 inch in 12 inches, an astonishing 5.0 degrees. This provides our customers unprecedented dynamic alignment, not alignment merely for the limited purpose of installation. Because either style pad is continuously floated by its hydrostatic oil film, the pads have more tolerance for operational misalignment, thermal distortion, load stresses, etc. than conventional, essentially dry, spherical bearing seats.
3. No Pivot Damage, Wear, or Fretting
In service, mechanical pivots are subject to distortion, wear, damage, and fretting. (We see them all the time–just ask our repair departments.) Whether mild or extreme, these mechanisms introduce additional, and generally unknown, clearance into the bearing. Increasing clearance can raise temperatures, de-tune stiffness, and reduce damping. Experience shows no such damage associated with the long term operation of a Fluid Pivot bearing.
4. Cooler Pad Operation: Dual-film Cooling
The individual pads operate with convected oil films on both the hydrodynamic and hydrostatic surfaces. Moreover, the through-the-pad oil port guides the oil through the center of the pad, typically near the hottest portion of the pad. These features result in more uniform oil exit temperatures and avoid the temperature extremes generated by other designs.
5. Increased Overload Capacity
Both the cylindrically-seated and the spherically-seated pads have superior overload capacity compared with mechanically pivoted configurations. Observations show that under overload, the pads can seat against the bearing shell and provide the increased load capacity of a fixed geometry bearing. When the load is reduced, the pad returns to a floating equilibrium. Equally important, bearing load capacity is not limited by contact stress considerations of a mechanical pivot.
6. Retrofit Options: Compact Design
For a given journal size and load capacity, the Fluid Pivot bearing requires a shorter length (no end seals), and an appreciably smaller shell O. D. than conventional, mechanically pivoted tilting pad journal bearings. In new machine designs, this smaller envelope may offer cost and performance benefits. Where existing journal bearings have been unsatisfactory from the standpoint of vibration, misalignment or power loss, the Fluid Pivot bearing can usually be retrofitted without altering the housing bore.
7. Accepts Load in Any Direction
The Fluid Pivot journal bearing will operate effectively when oriented at any angle relative to the anticipated load direction. Traditional Load-Between-Pad (LBP) or Load-On-Pad (LOP) orientations are employed depending on rotordynamics requirements. Testing of the bearing in numerous positions verifies its performance at low loads to be only slightly sensitive to orientation.
8. Reduced Life-Cycle Costs: Initial, Operating and Maintenance
Initial Costs:The basic simplicity of the directly-lubricated, Fluid Pivot journal bearing – no odd pivot shapes for pads or shell, no seals, fewer miscellaneous parts – enables this sophisticated design to be cost competitive with conventional, mechanically pivoted bearings. Conventional ball seated bearings call for spherically bored housings. With Pioneer’s Style JS design using spherically-seated pads, the machinery manufacturer can bore the bearing housings straight, not spherical, to receive the Fluid Pivot bearing’s cylindrical outside diameter with resulting savings in machining costs and totally effective dynamic alignment capability.
Operating Costs: The savings possible by using bearings with lower power loss can be substantial often enough to pay back the total cost of the bearings in one year. Lower power loss offers environmental benefits.
Maintenance Costs: Simplicity of design enables Fluid Pivot bearing pads (manufactured in interchangeable sets) to be easily inspected and replaced. Pads are interchangeable by design. In an emergency, Style JC babbitted pads can be relined, but Pioneer does not recommend this because of the precision sizes and preload geometry involved. The Fluid Pivot bearing avoids mechanical pivot fretting. Better alignment capabilities minimize edge loading and contribute to lower effective unit loading. There are no seals to wear or replace. All these factors reduce the operating and maintenance costs over the life of the bearing and machine.
Easy to measure: Because the pad outside diameters are precision surfaces, any bearing wear can be readily measured with conventional wall thickness readings. The assembled bearing’s inside diameter can be directly measured using anvil expanding micrometers, eliminating the complexities, inaccuracies and added expense associated with special fixtures for gauging mechanically pivoted 5-pad designs.
9. Efficiency: Reduction of Power Losses
Our Fluid Pivot bearing has employed directed pressure lubrication since its introduction in the 1970’s. With oil provided to the leading edge of each pad, the need for end seals to retain oil in the operating cavity is removed and churning losses associated with flood lubricated bearings are virtually eliminated. Several years of test and service experience shows the Fluid Pivot bearing exhibits only 40-60% of the power loss of comparable flood-lubricated tilting pad journal bearings. This power savings can be used to lower the bearing operating temperature or run a smaller bearing at the same temperature. Further, the Fluid Pivot bearing has always employed a trailing-edge scraper arrangement to limit hot oil carryover.
10. Effective Sound Attenuation
The isolating and damping effects of the self-generated hydrostatic oil film behind each floating pad make the Fluid Pivot journal bearing particularly quiet-running. This feature can be of significance in both commercial and military machinery.