Hybrid Ball Bearings
For most ball bearing applications, steel balls are sufficient. As applications become more complex and specialized, alternative ball options become a necessity. The most widely available options for ball bearings are TiC and ceramic balls. TiC balls are 440C stainless steel balls with a titanium carbide coating while ceramic balls consist of silicon nitride, Si3N4, a "polycrystalline material with an amorphous (glassy) intergranular phase." Bearings equipped with these options are referred as "pseudo-hybrid" and hybrid bearing (see Table I).
| Ring Material | Rolling Element Material | |
|---|---|---|
| Conventional Bearing | AISI 440C/AISI 52100 | AISI 440C or AISI 52100 |
| "Pseudo-Hybrid" Bearing | AISI 440C/AISI 52100 | Mainly AISI 440C w/ TiC coating |
| Hybrid Bearing | AISI 440C/AISI 52100 | Ceramic (Si3N4) |
Material Properties
The properties associated with TiC and ceramic options have significant impacts on bearings (some of these properties are listed in Table II). These options can add benefits such as longer bearing life, higher stiffness, lower torque, and lower noise. To achieve these benefits, thorough understanding of the material properties and their relationships to ball bearings must be developed.
| Property | Bearing Steel | TiC Coated | Ceramic |
|---|---|---|---|
| Density (g/cc) | 7.8 | 7.8 | 3.3 |
| Elastic Modulus (MPa) | 200, 210* | 200 | 320 |
| Surface Hardness (GPa) | 7 | 30 | 20 |
| Surface Finish, Ra (nm) | 10◊ | 3 - 5 | 5 - 7 |
| Thermal Expansion (x 10-6 °C) | 10.1, 10.9* | 10.1 | 2.9 |
|
* - lower value denotes AISI 440C, higher value denotes AISI 52100 bearing material ◊ - steel surface finish is governed by the ball precision grade |
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It is not the intent of this article to delve deeply into the material properties. The brief explanations below were made to illustrate some of the effects these properties have on bearing performance. For more information please consult the respected material manufacturers and service providers.
Density
Density relates to weight. Materials with lower density are lighter than those with higher density. The low weight has a great impact on the reduction of rolling element centrifugal forces and gyroscopic moments at high speed operations. Overall weight of the bearing is also reduced by low density materials.
Elastic Modulus
Young's modulus of elasticity relates to rigidity. Higher elastic modulus materials yield higher system stiffness and lower bearing deflection. Under load, ball deformation is smaller thus the area of contact is reduced (for same size bearings with same internal geometry). Reducing the contact areas resulted in lower friction but higher contact stresses and the lower static load capacity of the bearings.
Surface Hardness and Finish
Surface hardness relates directly to bearing life and performance. High hardness materials have better wear (fatigue) resistance. This is crucial for the repeated surface contacts the bearing must endure in operation.
With high hardness material, excellent (ultra) surface finish is achievable (via polishing). Surface finish (roughness) relates to torque and noise. Smooth surface finish does not require high torque to rotate and eliminates excessive roughness that could lead to noise (and vibration).
Thermal Expansion Coefficient
Thermal expansion is an important aspect for any material. The changing of temperature, from ambient, must be considered to analyze the effects of material growth. In preloaded bearings, the differences in ring and ball growth could cause a change in preload.
TiC and Ceramic Balls
The advantages of using alternative ball options are more than material properties. Dissimilar materials, ability to operate with marginal lubrication, fatigue resistance, and corrosion resistance are all associated attributes that contribute greatly to the improved performance of the bearing. Below is a brief discussion of these attributes (shared by both options).
Dissimilar Properties
Bearings under high stresses will lead to adhesive wear (or micro-welded areas). This is due to the breakage of the boundary lubricant films that prevent metal-to-metal contact. The TiC coating and the ceramic composition do not have the metal-to-metal contact thus reduces this type of wear.
Marginal Lubrication
The ability to operate under marginal lubrication is a highly valued attribute. In many cases, studies have shown that "pseudo-hybrid" and hybrid bearings can operate "dry" (without additional lubricant). This is true due to the "natural" properties of the TiC coating as well as the ceramic composition. However, it is always advisable to have lubricant in all bearings.
Fatigue Resistance
Repeated surface contacts place high stresses on the bearing. The stresses lead to deformation of the contact region. The high hardness of the TiC coating and ceramic material, even at elevated temperatures , resists deformation. Fatigue resistance is thus enhanced.
Corrosion Resistance
Corrosion is the result of the reaction between the surface with the environment. This occurrence is due to the absence of protective film. TiC coated and ceramic balls have excellent protection from corrosion.
TiC versus Ceramic Balls
While the benefits of these ball options are tremendous, the differences and limitations could decide which option to take. Impacts such as different weight, increased stiffness, loss of preload, and electrical conductivity have decisive implications. Knowing these limitations should always be an integral part of the decision making process.
Weight (Density)
The material density yields different weight. "Pseudo-hybrid" bearings have the same weight as conventional bearing while hybrid bearings are lighter. As discussed earlier, the weight has a direct influence on the centrifugal forces and gyroscopic moments. Thus, ceramic balls are better suited for applications where reduction of centrifugal forces and gyroscopic moments are the requirements.
Stiffness (Rigidity)
Rigidity of the system is different for "pseudo-hybrid" and hybrid bearings (for the same size bearing with the same internal geometry). TiC coated balls retain the same elastic modulus as the parent material (440C stainless steel) thus it will have the same rigidity as the conventional bearings. Ceramic balls have higher modulus thus greater stiffness.
In converse, higher stiffness based on same internal geometry yields higher stresses. The high stresses lead to the lower load handling capability of the hybrid bearings, approximately 20% reduction. "Pseudo-hybrid" bearings with TiC balls do not have the reduced load ratings.
Thermal Expansion
Material growth due to heat will cause changes in the preload. Preload has a direct relation to system stiffness. TiC coated balls also retain the same expansion rate as the parent material (440C) thus the growth will be similar. Ceramic balls have lower expansion rate thus the preload will change. Maintaining the same preload at high operating temperature, especially for duplexed bearings, is a big advantage for TiC over ceramic.
Electrical Conductivity and Magnetism
For applications that are sensitive to electrical conductivity and magnetic field, ceramic is the only choice. Ceramic material is classified as an insulator thus it can reduce the amount of conductivity. Ceramic is also a non-magnetic material thus it is appropriate for magnetic field sensitive applications (especially instruments).
Costs
The biggest factor in the ball options is the cost. Balls in these pseudo-hybrid and hybrid bearings command a high percentage of manufacturing costs. As the demands for these options increase, mass produced TiC and ceramic balls could improve the costs.
Summary
Utilizing alternative ball options can have tremendous impacts on bearing performance as well as bearing price. Both TiC coated and ceramic balls offer excellent benefits such as dissimilar material properties, ability to operate with marginal lubrications, improved fatigue and corrosion resistance, and lower torque. The differences between the two choices will be a determining factor for the selection. Ceramic balls are excellent for high speed - low load applications while TiC balls are excellent for applications that require all of the benefits yet perform similarly to steel balls. The costs of these options will require justification between the benefits versus costs of bearing repair/replacement. For more information, please contact your NPB technical sales specialist.
Article excerpt reprinted with permission from New Hampshire Ball Bearing, Inc.
