Taking it to the limit
Leading experts tell Tom Shelley about the factors that limit the performance of rolling element bearings and how the frontiers are being pushed back
Ultimate bearing performance is limited by physics and materials science, but this can be pushed back if the application demands it.
Limiting factors depending on usage. In aerospace, bearings may be required to run so fast that there is a tendency for the inner race to expand and lift off the shaft. There is then a need to use inner rings made of tool steels of sufficient stiffness to ensure that the inner race still grips.
Another issue at very high speeds is getting lubricant in and out of the bearing. Issues which need to be considered include windage. There is often more of a problem getting the lubricant out than getting the lubricant in. If lubricant builds up inside the bearing, friction rapidly increases with consequent evolution of heat. One way of improving extraction is to employ special shapes in the construction of the bearing which throw the lubricant out.
Gerald Rolfe, engineering manager at SKF UK, says that the fastest running rolling element bearing he has seen so far had a speed-times-diameter product of more than 3 million DmN (rpm x mm) which was used for an undisclosed military application. Nonetheless, bearings for machine tool spindles are now approaching this level of performance.
In very high accuracy bearings, there is a danger of bearings going into ball slippage instead of ball rolling. A problem at high speeds is gyroscopic motion tending to make balls spin as well as roll. Slippage leads to increased friction and wear. The solution is to apply enough pre-load to prevent spinning.
High speeds require special materials, special designs and small balls which are lighter, or ceramic balls which offer the same advantage as well as reducing wear. Such bearings are used in jet engines, turbo machinery pumps, machine tools and rocket motor fuel pumps where they are only required to last for a few very arduous minutes. The highest speeds can be endured by air and magnetic bearings, but load-carrying capacity is limited.
Heavy loads bring other problems. The heaviest loads are usually borne by plain bearings: steel on steel, steel on polymer composite and polymer composite on polymer composite. The problem with plain bearings is friction. Rolling element bearings, on the other hand, have friction coefficients of less than 0.002, which means that they normally require no external cooling. Load carrying capacity is limited by maximum contact stress, usually around 4,000 MPa, above which the steel goes from elastic to plastic deformation and the bearing behaves as a miniature rolling mill. Maximum loads are borne by bearings made of very hard high alloy steels.
Very high temperatures lead to lubrication problems. Solid graphite cages which supply their own lubrication are used for working temperatures of up to 500 deg C. Extremely low temperatures also cause problems, since liquid lubricants solidify. Again, the solution is to turn to solid lubricants.
It's a gas
Dr Steve Lacey, product engineering and development manager at NSK-RHP, says that the most strenuous performance demands are placed by main shaft bearings on gas turbines. Speeds are currently up to 3.5 million DmN diameter. He expects manufacturers to demand performances of up to 4 DmN in the next five years. Smaller sizes in gas turbines are likely to lead to demands for smaller bearings capable of carrying increased loads.
In order to meet such demands, he thought it would be necessary to develop better materials, capable of longer life and greater materials. He said that the main requirements were not for exotic new alloys, but cleaner steels with more uniform grain structures.
NSK-RHPs European Technology Centre at Ruddington has conducted a great deal of research into hybrid bearings with ceramic balls. The balls are silicon nitride made by hot, isostatic pressing. Such bearings are being looked at urgently for use in gas turbines in the next five years.
Such bearings are already in use in machine tools, especially in Japan. Machine tool speeds are up to 2.2 million DmN using air/oil lubrication. Hybrid bearings allow higher speed operation, usually between 15 and 25 per cent depending on spindle design, resulting in better cutting performance. They are also available in sizes which are dimensionally interchangeable with conventional steel balled bearings. This means that when retrofitting, re-engineering is not usually required to accommodate them.
The density of ceramic balls used in NSK-RHP bearings is about 40 per cent that of chrome carbon steel. Under high speed running, stresses in the outer ring of the hybrid bearing are reduced and internal dynamic performance is improved.
A further advantage of using ceramic balls is that modulus of rigidity is 50 per cent higher than that of steel. This raises the stiffness of the bearing by about 12 per cent when stationary, and at speed, there is less fall off in stiffness, a condition which results from changes in ball/raceway contact conditions. Depending on speed, bearing stiffness using ceramic balls may be raised by anything from 12 to 100 per cent in comparison with the stiffness provided by steel balls.
Because of the stiffness of the ceramic material and its lower density, benefits are also gained in terms of running temperatures, wear and vibration. These result from the reduction of internal forces at high speeds, forces which in a conventionally balled bearing would tend to alter the position of the ball/raceway contact points so that they are no longer diametrically opposite one another. When this happens, pure rolling cannot take place, and the resulting high speed sliding causes heating, rapid wear and may result in catastrophic failure of the component.
Ceramic balled bearings have a lower spin-to-roll ratio at high speeds, a factor which provides much more advantageous rolling contact conditions, and benefits in terms of lower friction and heat generation. Lower operating temperatures prolong the life of lubricating grease or, alternatively, permit higher speeds while retaining grease lubrication. The current limit for grease is about 1.2 million DmN. Air/oil lubrication is required at higher speeds.
In space, under vacuum and at high and low temperatures, NSK has used silver, gold and graphite solid lubricants. For aerospace use, bearings have silver plated cages for reduced friction and increased life.
Lower friction additionally realises benefits in terms of wear. Under high speed operating conditions, where loading is mainly internally generated, bearing life may be increased by two to five times.
Researchers led by Dr Frank Muller at the University of Applied Sciences in Mittweida in Germany are studying rolling element bearings that will run at more than 20,000 rpm. They see cage materials as being one of the limiting factors. They are therefore experimenting with cage materials made of PEI/PTFE reinforced by carbon fibre and also PEEK/PTFE reinforced with carbon fibre. More than a few research laboratories are looking at the consequences of coating bearing raceways with synthetic diamond and diamond like carbon. In the far future, rolling element bearings may well use even more exotic materials.
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