Technology Hybrid ball bearings
Properties of ceramic
The ceramic material silicon nitride Si3N4 is excellent for use in precision ball bearings. A comparison between silicon nitride and conventional bearing steel 100 Cr6 is shown in the diagram:
Further advantages of ceramic are:
- Low chemical affinity to 100 Cr6
- Low friction coefficient
- Low heat conductivity
- Corrosion resistant
- Non-magnetic
- Electrically isolating
Advantages for the user
Longer service life
Experience shows that twice the service life in comparison to conventional bearings can be reached by using hybrid bearings. Depending on the operating conditions even longer service life can be achieved.
The reasons for this are:
- Low surface adhesive wear
The lower affinity to steel reduces the adhesive wear, which is caused by the cold welding effect on irregularities in the raceway and ball surface. - Low abrasive wear out
With steel balls, contaminants and particles from the process of running in are embedded into the surface. With every revolution of the ball, these foreign particles damage the raceway. These particles make little impact on the extremely hard ceramic ball. - Insensitivity to poor lubrication
Low adhesion and friction allow the hybrid bearing to perform well even under poor lubrication. - Longer grease service life
Lower operating temperature and favourable tribological features extend the service life of the grease.
Higher speeds
The attainable speeds depend above all on the thermal conditions in the bearing. Because of lower friction, the hybrid bearing generates less power loss. Therefore the speed limit is clearly increased. Depending on the application, speed rises of up to 30% are possible compared to bearings with steel balls.
- Low rolling friction
The rolling friction is reduced, as the centrifugal force of the lighter ceramic ball is less. The contact ellipse is less because of the higher Young's modulus. - Low sliding friction between ball and raceway
At high speeds, sliding friction is responsible for most of the total friction. One of the criteria for the sliding friction is a low spin/roll ratio. The service life is effected by values above 0.25. The diagram shows the advantages of ceramic balls. - Avoid ball skidding
The balls skid on the raceway if the preload between the rings is too small. This process usually occurs in case of an insufficient preload of the bearing or an excessive acceleration. With hybrid bearings the minimum preload can be reduced as they have a smaller inertia and generate a smaller spinning moment.
Low cost lubrication
Grease lubrication can be used in higher speed ranges.
The limiting speed for minimum oil lubrication increases significantly. In many cases, it can replace the expensive oil jet lubrication.
Higher rigidity
- The radial rigidity of ceramic hybrid bearings is approximately 15% higher at low speeds because of the higher Young's modulus.
- With higher speeds, the centrifugal force affects the internal load distribution and the dynamic rigidity is reduced. The diagram shows reduced loss of rigidity for hybrid bearings.
- A high rigidity improves the accuracy and shifts the critical fundamental frequency (inherent frequency) of the bearing arrangement.
Improved machining accuracy
The following factors lead to an improvement of the surface quality and accuracy of machined parts.
Load ratings
DIN/ISO standards do not specify any calculation methods for the determination of load ratings of hybrid bearings. If the classical fatigue theory is used, the load ratings and the service life will be lower than those for steel balls. However, experience shows that the actual service life is significantly longer. Due to this, GMN uses the same load ratings as for conventional bearings.
Examples of application
- Spindles for machine tools
State of the art machining processes like high speed milling require a new concept of bearing arrangement for spindles. The application of hybrid bearings has resulted in a remarkable improvement in performance. For some years we at GMN have successfully used many spindles with hybrid bearings for our own production processes. - Special bearing arrangements
With vacuum pumps, for instance, reliability of the bearings is of utmost importance, as breakdown can result in high costs. Today for pumps predominantly hybrid bearings are applied.
More applications are:
- Medical equipment like X-ray tube bearings
- Touchdown bearings for magnetic bearings
- Bearings for aeronautic and aerospace
Summary
When conventional bearings fail, the solution is often to use hybrid bearings. It is important always to take the whole system into consideration and to carry out a "weak point" analysis. GMN's application engineers would be pleased to assist you.
