Bearings

12-2000


Design Innovation

Pumped pressure makes smooth rotations

Tom Shelley reports on hydrodynamic and aerodynamic bearings with outstanding performance and the potential to improve the efficiency of gas fired power stations

A new class of bearings supports shafts on a 360 degree cushion of high pressure pumped liquid or gas.

Fluid is pumped in from points around the periphery and rotors self centre even under heavy load.

Target applications are heavy duty pumps, compressors and electric generators but they are suitable for almost any application requiring high speeds, heavy loads and quiet running.

ServoFluid Control Bearings are the brainchild of Donald Bently, owner and Chief Executive Officer of Bently Nevada, based in the state of that name. The company is best known for its $200 million turnover machinery protection and management systems business. Bently is also the inventor of the company’s original shaft observing proximity probe, 45 years ago.

In this new concept, the rotor acts as a valve servomechanism. Fluid is delivered through ports to pockets round the periphery of the rotor. The number of pockets depends on the diameter of the bearing. The minimum number is three. Fluid flow through the bearing is restricted by an orifice at each inlet port, and also by the journal/bearing clearance. As the rotor moves closer to, or farther from, the bearing fluid inlet ports, a pressure differential is created. This arises because the pressure is highest when the rotor is close to the fluid inlet ports and pockets, and less when it is further away. The pressure difference tends to push the rotor back towards its equilibrium central position.

Steve Sabin, the company’s marketing manager says, "In essence, it is a purely mechanical servo mechanism, in the same way that you can float a ping pong ball on a jet of air without any electronic feedback control or gears or levers. As the ball moves away from the centre of the jet, the flow along the periphery of the airstream nudges the ball back toward the centre.

Have pressurised bearings been around previous to this? "Of course they have," Sabin admits. "There have been low pressure lubricated bearings around for years, and most engineers are familiar with the ‘Offset Half’ and ‘Lemon Bore’ types." The real story here is not that this is the first time bearings have been pressurised but rather this is the first time anyone has rigorously studied the effects of pressurised bearings in high speed turbo machinery applications. "Most people will tell you that pressurisation of high speed bearings results in instability. We found just the opposite: insufficient pressurisation is generally worse for stability than unpressurised designs but when adequate pressure is supplied to a properly designed, fully lubricated bearing, it behaves much better than unpressurised bearings in a number of respects."

Barry Nurdin, the company’s senior applications engineer in the UK says: "The nearest oil lubricated bearing to the ServoFluid Bearing is a multi-pad journal bearing which has the capability to increase bearing stiffness." He explains that these designs rely on the historical remedy of introducing discontinuities to break up the fluid flow, thus reducing fluid whirl. "Not only do they introduce inefficiencies, they tend to be rather expensive," he comments. "The ServoFluid Bearing offers better stiffness and control as well as adjustability, flexibility in fluid choice, fewer frictional losses, no moving parts and a better load carrying capability." Nurdin adds that its increased stiffness reduces seal, disc and blade running clearances in gas turbine generating sets, improving thermodynamic efficiency.

The company compares and contrasts its bearings with active magnetic bearings. These require electronic controls, windings and ferromagnetic laminations to convert current to magnetic flux. This adds mass to the rotor dynamic system and reduces its resonant frequency. The ServoFluid bearing, furthermore, has a more appropriate response to eccentric perturbations. If the rotor gets close to the periphery, the restoring force tending to centre the shaft dramatically increases in magnitude. A magnetic bearing does not work in this way, and its restoring force may disappear altogether if magnetic saturation occurs.

Development at Bently Rotor Dynamics Research Corporation took some three years. The study subject was a Clark 1M6 turbo compressor driven by a 97kW electric motor. The compressor was retrofitted with two radial ServoFluid Control Bearings and one thrust SFC bearing. Operating oil pressures were 1000 psi (69 bar) for the supply and 700 psi (48 bar) for the bearing ports.

F-Sfluid 2a.jpg (84267 bytes)

To give the bearings a real test and to investigate the likely effects of problems, the test rig configuration included the fitting of a ‘perturbation wheel’ on the rotor drive end. This wheel could rotate on the shaft independent of the main rotor speed and direction, and could accommodate weights to provide out of balance forces.

Tests were conducted with the compressor stopped and running at 7,000 rpm. At these compressor speeds, the wheel was run with and without a 14.2g weight at 63.5mm radius at rotational speeds from zero to 10,000 rpm. 14.2 g may not sound much, but at 10,000 rpm, the centrifugal out of balance force would have been 9891N or one UK ton force.

Despite the severity of the tests, the displacement of the rotor was found to be tiny, and the bearing behaved in a very stiff manner. As well as tests with oil or water, which give the best results, the bearings are also found to behave as required when run with compressible gases such as air, nitrogen or steam as working fluids. Should the high pressure fluid supply be lost, the bearing functions as a conventional hydrodynamic bearing, allowing for a controlled machine shutdown.

The original experimental bearings were 80mm in diameter. There are now power station generator sets with bearings 300 to 500mm in diameter.

Sabin says that he feels the breakthrough advantages are: the simplicity of the bearing construction; its efficiency (lower power losses); its exceptional stability; improved stiffness; its ability to use compressible and non compressible fluids; and size reduction in both machine length and clearances. Marco Alcalde, the firm’s engineering manager adds that, "Unlike partially lubricated fluid film bearings, ours are also applicable to vertical machines, not just horizontal machines, and we completely eliminate whirl instability problems that commonly plague vertically oriented machines." Bently says: "The bearing’s working fluid can be chosen based on its compatibility with what the machine is handling and the hazardous environment the machine may be in - in some case, we can even use the machine’s process media itself."

When asked about the range of possible applications, the company replies: "Anything that goes round!" More information can be found at www.servofluid.com .

Design pointers

The new bearings are much stiffer and more stable than conventional hydrodynamic and aerodynamic bearings. Possible working fluids include: oil, water, air, nitrogen and steam

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

Bearings can be mounted horizontally or vertically

Other advantages include: simplicity of bearing construction, efficiency (lower power losses) and size reduction in both machine length and clearances

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