Anti-friction bearings
Hydro-dynamic bearings
Magnetic bearings.
Anti-friction bearings rely on rolling elements to carry the load of the equipment and reduce the power losses. In general anti-friction bearings are used for equipment of low horse-power (say, below 500 kW) or for special rotating machines such as aero-derivative gas turbines where light-weight design is absolutely necessary.
Hydro-dynamic bearings rely on liquid film, usually lubricating oil, to carry the load of the equipment and minimize friction.
Magnetic bearings use modern digital control techniques to offer contact-less, oil-free, compact, light, reliable and robust bearings. The shaft location is identified with advanced sensors. Digital controls are used to analyze deviations from the anticipated shaft center and calculated magnetic forces are applied to correctly position the high-speed rotating shaft. Magnetic bearings do not require lube oil. Position and corrective force signals can be used for condition monitoring of bearing. In other words, magnetic bearing technology offers the most reliable bearing type, the lowest power losses as well as built-in condition monitoring. To date, their high cost along with a lack of references and the conservative nature of the large-block power generation industry, limit the widespread use of magnetic bearings in power generation trains.
For the near-term at least, anti-friction and hydro-dynamic bearings will hold a large portion of the power generation-train market while magnetic bearing applications grow in special purpose rotating machine market.
Radial bearings are responsible for supporting the main static and dynamic loads (including rotating assembly weight, fluid forces and so on). Dynamic forces for gas turbines and steam turbines are usually in order of 10 to 30 percent of static loads. For gear units, radial load component is made up principally of the meshing forces of the gear teeth and the load will vary from zero to maximum torque. Gear units require careful bearing sizing and design. For all applications whether rotating machine itself or gear unit, there are specific oil film pressure limits which dictate the bearing dimensions (length and diameter).
Bearing life depends directly on the forces acting on the bearing to the third power. As an example, if the forces are twice the design values, the life of the bearing would be reduced by around eight times. Sources of bearing forces include the following:
Misalignment or unbalance
Increased pipe loading on machine (poor piping layout, unequal flow distribution, improper stress study and so on)
Machine fouling
Foundation forces (consider soft-foot, different settlement and so on)
Thermal expansion (change in cooling loop, operating temperature exceeding limits and so on)
Improper assembly or installation clearances.
The bearing should be installed in accordance with manufacturer instructions. Differences in bearing design, model, type and manufacturing should be respected. Major bearing reliability factors include:
Minimum external pipe loads
Minimum external foundation forces.
Proper alignment.
Exceeding the specified limit leads to reduced equipment reliability and shortened machine life. Extra safety margins on the limits outlined above mean increased reliability and expected life.
Bearings continuously support forces by providing sufficient bearing area and require oil flow to remove the generated frictional heat. Bearing forces, bearing area, oil flow and frictional heat should be carefully checked. Regarding condition monitoring, bearing housing vibration, bearing housing temperature and bearing lube oil conditions (mainly water content, particle content and so on) are continuously monitored.
Thrust bearings are usually vulnerable components considering their critical roles, their relatively fragile structures and their relatively low capacity (considering large axial forces in high-pressure machines). There are many reports about thrust bearing failures. The main reasons are more load than anticipated or insufficient bearing area, incorrect installation and unclean, insufficient or hot oil supply (sometimes even incorrect oil type or viscosity).
More load than anticipated is reported as a main reason for thrust bearing failure. This type of failure can be the result of higher operating pressures than design, insufficient clearances or operation errors.
