ServoFluid Control Bearing

Until recently, few had challenged the assumptions regarding fully lubricated, externally pressurized bearings on high-speed machinery, and fewer still had actually examined the behavior of such bearings over sufficiently large pressure ranges. (When incompressible fluids are used, operating pressures of several hundred psi are typically adequate for proper bearing performance. Regardless of the application, Bently Nevada limits the use of incompressible fluid pressures to 2000 psi or less to ensure safety.) Not Bently Nevada. For the last decade we've been learning what happens when you fully lubricate a bearing and then pressurize it enough to force the lubricant to flow primarily along the shaft, rather than its natural tendency to be pulled into rotational motion around the shaft as with low-pressure designs. The result is our ServoFluidTM Control Bearing - superior bearing technology for today's, and tomorrow's, machines.

The Bearings of the Past
Rotating machinery has made tremendous strides over the last fifty years. At the heart of this machinery, however, is bearing technology that has remained relatively unchanged during that same fifty years. Bearings that are intentionally operated with partial lubrication at very low pressures - just enough to move the lubricant through the bearing's supply, cooling, and filtration system with adequate flow. Why? Because it was erroneously believed that full 360-degree lubrication and external pressurization resulted in an unstable bearing when used in high-speed turbomachinery. The fundamental principles governing fully lubricated, externally pressurized bearings were simply not well-understood. As a result, industry relegated these bearings to static or quasi-static applications, such as rotating/tracking mechanisms for large telescopes, unaware of the tremendous potential that existed for improvement when applying this technology to high-speed applications. They remained convinced there was no alternative to the limitations of conventional bearing technology - limitations such as:

• Machines that continue to suffer from fluid-induced instabilities. Innovations such as tilting pad bearings helped, but did not really address the underlying system's stability.
  
• Constraints on shaft diameters and lengths imposed by conventional L-to-D ratios to achieve adequate stiffness and damping characteristics.
  
• Very few options for working fluid choices, besides conventional hydrocarbon-based lubricants.

The Bearings of Today - ServoFluid Technology
Announcing Bently Nevada's new ServoFluidTM Control Bearing. Its innovative design uses pressure to overcome the limitations of conventional bearings and is ideal for addressing applications such as:

• Machines with instability problems, or those with other rotor dynamic problems.
  
• Machines where an alternative to hydrocarbon-based lubricants is desired.
  
• Bearing retrofit applications where tilting pad bearings are being considered. ServoFluid technology can be installed cost-effectively instead, with superior results.
  
• Slow-speed, heavily loaded machines. These machines are often fitted with rolling element bearings because the rotational speeds do not allow hydrodynamic action to generate an adequate film. ServoFluid bearings require less space, work even at zero rotational speed, and have better load-bearing capacity than comparable rolling element or conventional fluid-film bearings.
  
• Machines with poor damping at operating speeds. ServoFluid technology provides essentially constant damping over the entire range of a machine's rotational speeds, and typically provides higher damping at operating speeds.

Exceptional Stability
The bearing's design keeps the journal near the center of the bearing clearance with an attitude angle near zero degrees. The result is an extremely stable bearing.

Independently Adjustable Stiffness and Damping
Bearing stiffness and damping can be pre-selected and independently adjusted to best support an application and to address rotor-related malfunctions that may be of concern. High stiffness can be provided at low rotor eccentricity and is precisely maintained over a long life.

Improved Stiffness
A pressurized bearing is a stiffer bearing ­ one with extremely high load density properties, allowing an entirely new generation of machines to be developed with shorter rotors, smaller bearings, and the reduction or even elimination of seals.

Excellent Heat Removal
The bearing utilizes fluid flow to remove unwanted heat. The flow rate can be designed to support the application.

Efficiency
Improved bearing stiffness and stability maintains a precise rotor position near the center of the bearing. This allows for tighter clearances (rotor-to-seal, blade tip, etc.) and improved efficiency. These bearings also reduce fluid-related mechanical losses (drag).

Flexible Working Fluid Choices
While ServoFluid technology can operate using conventional lubricants, it is also applicable to many other fluids - including incompressible fluids such as water, and compressible fluids such as air, nitrogen, and carbon dioxide. This flexibility allows bearing fluids to be selected based upon process compatibility or to minimize environmental risk. In some cases, even the process media itself may be used.

Increased Rotor/Bearing/Seal Life
Because the bearing centers the journal in the bearing clearance prior to startup and through coastdown, the rotor does not contact the bearing during any phase of operation. This eliminates wear typically associated with hydrodynamic bearings. It also significantly reduces seal wear and startup torque requirements.

Machine Size Reduction
The ServoFluid Control Bearing has a very high load density. This may enable the use of smaller bearings or shorter rotors, and the reduction or even elimination of seals.

Wide Applicability
ServoFluid technology can be applied in radial, thrust, or combination radial/thrust bearings.

Supports Both Retrofits and New Machine Designs
Bently Nevada can design, install, and support this advanced bearing technology as a retrofit project for existing machinery, where an installation will typically include two radial bearings and a thrust bearing for each machine case. Original Equipment Manufacturers can benefit as well. Bently Nevada will help you incorporate our bearing technology into new machine designs resulting in improved stability while allowing smaller, more efficient designs than previously possible.

Safety Backup
If high-pressure supply fluid is lost, the bearing simply reverts to operation as a conventional hydrodynamic bearing, allowing the machine to be shut down safely.

Pressurized fluid is delivered to the bearing through recessed inlet ports. Each port directs fluid to a bearing pocket (the number of pockets may vary with rotor diameter). Fluid flow through the bearing is restricted by each inlet port and also by the journal/bearing clearance. As the rotor attempts to move in the bearing, pocket pressure increases in the direction of motion and decreases in the direction opposite motion. This effect creates a pressure differential in opposing bearing pockets, and a corresponding restoring force to the rotor.

When speed is increased (for example, from S1 to S2 in the figure at left), a bearing will eventually become unstable. However, by increasing the fluid delivery pressure adequately (from P1 to P2 in the figure), the bearing again becomes stable. For each increase in speed, an increase in pressure can keep the bearing in its stable region. By properly designing and pressurizing a bearing, it can be made stable for all operating speeds of interest. This simple principle is what gives the ServoFluid Control Bearing its outstanding stability.

For additional information you can e-mail us at servofluid@bently.com, phone us at (775) 782-3611, or simply contact your nearest Bently Nevada sales professional.

Bently Nevada Corporation, 1631 Bently Parkway Avenue, Minden, NV 89423. Tel: 775-782-3611; Fax: 775-782-9337.