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[August 2007]
Outstanding Benefits Achieved With Ceramic Linear Bearings
After more than 20 years of research and testing PM Bearings says it has proved that ceramic linear bearings maintain longer and more reliable performance. Now available from Micromech, the exclusive UK agents, these bearings are non-magnetic, have reduced friction, retain their stiffness and operate at higher temperatures. On top of that, they have an exceptional lifespan but require little or no lubrication.For many years, ceramic bearings have been well accepted in demanding ball bearing applications being superior to steel for density and hardness. Because of the non-magnetic properties this is an ideal material for use in non-magnetic or ultra-high vacuum environments, as is often required nowadays in the new generations of machines. But now PM Bearings, who are said to be the first linear bearing company in the world to realise practical use of ceramic linear bearings, can supply its high precision positioning stages to operate under extreme environments.
The bearings are very lightweight as the ceramics density is about 40%-60% of traditional bearing steel DIN 1.3505. Because force is directly proportional to mass, the low density silicon nitride used significantly reduces the starting force which is required to move the slide/stage. This enhances operations in high dynamic applications where the reduction of the mass-moment of inertia is an ongoing issue.
High rigidity is achieved because ceramic ways and balls (silicon nitride and alumina) have a Youngs Modulus which is about 50% higher than steel, making them an ideal linear bearing material. These are the qualities needed for use in high-speed XY-stages as for wirebonding machines in the semi-conductor industry where rigidity and precision at high speed is required.
The lifespan of linear bearings is directly related to hardness, the most critical mechanical property of a bearing material. With an impressive Rockwell C 78 hardness, this is twice as hard as many bearing steels. They also have a high compressive strength, about 5 to 7 times that of steel; therefore silicon nitride improves wear resistance, minimising the damaging effects of repeated surface contacts.
The mechanical properties of ceramic bearings do not change in extremely low (cryogenic) or extremely high temperatures, which makes it very suitable for use as a heat resistant material. A major problem caused by such high temperatures is lubrication as oil and grease can normally be used in temperatures of up to 300°C. When temperatures are higher, only solid lubricants can be used, but even then they can only be used for temperatures up to 500°C. However, ceramic bearings can operate at temperatures up to 800°C, exceeding the best high temperature bearing steels by a factor of three.
Other characteristics such as vacuum compatibility show they exhibit less particle generation about one-third in both air and vacuum compared with stainless steel. Chemically inert, corrosion resistant and electrical non-conductivity can also enhance linear bearing performance in special applications.
As ceramics are extremely hard and have outstanding wear resistance they outperform most other materials in bearing life, and without lubrication. In the rigorous comparison tests it was shown that the rollers in the steel linear bearings exhibited flaking and a loss pre-load was observed after only 40% of the calculated life while the ceramic linear bearings were still operating after twice the lifespan of steel. Inspection of the rails and rolling elements revealed no wear was found and the accuracy of travel was still the same. The test confirmed that the life without lubrication is dramatically longer for ceramic linear bearings than for steel.
Ceramic linear bearings and slides are manufactured on a made-to-order basis. In this way, the ceramic bearing specifications can be matched with the customer's system to meet their exacting requirements of speed, operating temperature and accuracy.
www.micromech.co.uk
ENDS