What is Tribology and How it Relates to Ball and Roller Bearings

Tribology is a term that came about in 1966 by Peter Jost dealing with wear, friction and lubrication however the concept had been understood for thousands of years prior. The earliest mention of tribology is from the Bible when a reference is made about Noah building the Ark and sealing it internally and externally with asphalt/bitumen.

Bearings are all about wear, friction and lubrication so the terminology describing Tribology are one and the same. It is important to understand all the factors concerning Tribology in bearings.

Wear may be influenced by external forces and or internal design features and lubrication issues. Bearing race material, cage type, Internal clearance, fits, shields, seals, internal geometry for raceways, preloads, and lubrication all combine to create the dynamic of a rolling bearing. So the term Tribology in bearings involves more analysis than just lubrication as it involves careful design and testing to yield the best result.

Demonstrates Egyptians used animal fats (tallow) to lubricate chariot axles. This is a good way to understand the science of Tribology where friction is mitigated with the use of lubrication. 

Good rolling contact is a requirement when it comes to rolling bearings however contact between two surfaces should be minimal because contact leads to friction. The goal of Tribology is to find optimal methods to eliminate friction and thus reduce wear so that everything operates smoothly.

INA and FAG Bearing Companies have a great deal of expertise in rolling bearing lubrication. Pacific International Bearing Company gives its customers expert advice in the selection of the right lubricant for any application. It is possible If no suitable lubricant can be found, we can test new lubricants for you. Bearing Companies like INA and FAG actually have Tribology departments which work to create new grease or oil blends to find suitable results for almost any environment or application.

For customers who have very specific operation requirements for smoothness, we can determine which oil or grease has the right application characteristics. When your requirements include a predetermined service life we can assist in estimating it based on certain lubrication properties. Tribology like many disciplines must be aware of the need for environmental concerns and so the use of biodegradable lubricants is available in many cases.

More About Tribology in Bearings

The pervasive use of ball and roller bearings is because it is proven that rolling friction is less than sliding friction. Friction is a combination of issues involving material where limits of elasticity exist. Loads acting on the bearing can cause deformation and compression causing increased friction. It is known that the relation between load and deformation is not the same when the load is increasing compared to it decreasing even when the limit of elasticity of the material is not exceeded. When the load increases, a given deformation corresponds to higher stress than when the load declines. Here elastic hysteresis exists analogous to the magnetic hysteresis in the science of electricity.

Friction within bearings is a key factor that affects operational efficiency and energy consumption. Design strategies aim to optimize the frictional characteristics to balance force transmission and minimize energy losses.

Pacific International offers Name brand ball bearings from stock like SKF, FAG, INA, TPI and NMB. Visit our website to select sizes, download PDFs and order samples.

PIB offers the 6000, and 6200 series bearings, such as TPI bearing part number 6203-LLU or 6203-LLB for reducing friction through seal type selection. TPI ball bearings are specifically designed to manage friction effectively. These bearings meet the standard for a super surface finish and use specialized lubricants to achieve a lower friction coefficient, which directly contributes to their enhanced performance and life. Learn more about this quality high value bearing brand.

Wear mechanisms and service life management in bearings

No bearing will have an unlimited service life. However, life can be extended through good practices for design and maintenance where possible. If a bearing is exposed to moisture or dirt in its operating environment it may rust or wear prematurely. If the bearing can be properly protected with a good seal design and proper lubrication and handling then its life can be extended.

Fatigue

The remaining issues for the bearing include fatigue from the material due to repeated stresses under rotation. Bearing wear is a critical area affected by tribological principles. Fatigue wear can significantly not only reduce life but also affect the bearing’s performance.

Handling of bearings

Another often overlooked area of Tribological consideration is the influence of mounting and dismounting procedures that will affect performance and life.

For given operating conditions there exist multiple options for design, mounting, and assembly. The assembler must be able to perform installation without difficulty and not damage the bearings. A bearing improperly handled and installed will increase its failure potential.

The importance of fits

Continuing with the thought on what causes bearing to fail would be Fits. To achieve proper bearing function the fit between the inner ring and shaft and the outer ring and housing must be calculated for each application. Operating conditions determine the choice of fit. For any given application the direction of the load in relation to the bearing ring is of the greatest importance.

Tribological Calculations in Bearing Design

These calculations involve assessing the interactions between surfaces in motion, specifically focusing on friction, wear, and lubrication dynamics. 

Film thickness and lubrication regime

One of the key aspects of tribological calculations in bearings is determining the lubrication regime, which is dependent on the lubricant film thickness relative to the surface roughness of the contacting parts. The lubrication regime can be categorized into three main types:

• Hydrodynamic lubrication

This occurs when the lubricant film completely separates the contact surfaces under load, preventing direct metal-to-metal contact.

• Mixed lubrication

Characterized by partial contact between the asperities of the surface and the lubricant film.

• Boundary lubrication

This occurs when the film thickness is insufficient to separate the surfaces, and the friction and wear are controlled by the properties of the surface layers.

The λ ratio (Lambda ratio) is a critical parameter used to determine the lubrication regime. It is defined as the ratio of the lubricant film thickness (ℎ) to the combined surface roughness of the bearing surfaces (σ). The formula to calculate the λ ratio is:

Where:

• ℎ = lubricant film thickness
• σ = combined surface roughness of the contact surfaces

Example Calculation:

Suppose a ball bearing operates with a lubricant film thickness of 0.5 micrometers and the combined surface roughness of the rolling elements and raceways is 0.1 micrometers. The λ ratio would be:

A λ ratio greater than 3 typically indicates hydrodynamic lubrication, ensuring minimal wear and friction.

Contact pressure and load capacity

The contact pressure between the rolling elements and raceways significantly affects bearing life and performance. The Hertzian contact theory is often applied to calculate the contact pressure in ball and roller bearings. The maximum contact pressure (p max ) for point contact in ball bearings can be estimated using the formula:

Where:

• E = modulus of elasticity of the bearing material
• W = applied load
• d = diameter of the ball
• ν = Poisson’s ratio of the bearing material

Wear rate calculation

The wear rate of a bearing is also an important consideration, particularly in determining the service life and maintenance intervals. The wear rate (WR ) can be approximated using Archard’s wear equation:

Where:

• k = wear coefficient
• W = normal load
• H = hardness of the softer contact surface
• v = sliding velocity

These tribological calculations are integral to the design and selection of bearings, allowing engineers to predict the operational characteristics and optimize the bearing for specific applications. 

Conclusion

Tribology, the science of wear, friction, and lubrication, is fundamental to understanding the performance and life of mechanical systems. 

Bearing performance can be significantly impacted by tribological challenges, which arise from the complex interactions of mechanical loads, material properties, and environmental factors. Understanding and addressing these challenges is critical to optimizing bearing life and functionality. 

Wear mechanisms in bearings

Bearings are subjected to various wear mechanisms that can deteriorate their functionality and reduce their service life. The most common wear types impacting bearings include:

• Adhesive wear

This occurs when asperities on contact surfaces adhere during motion, leading to material transfer or galling. It is prevalent in high-load applications where boundary lubrication conditions may exist.

• Abrasive wear

Caused by hard particles or asperities that cut through the bearing surface, this type of wear is common in environments with high contamination.

• Fatigue wear

This happens under cyclic stress conditions where surface or subsurface cracks form and propagate, leading to material flaking or spalling. It is particularly critical in applications involving high dynamic loads or vibrations.

Each of these wear mechanisms can drastically affect bearing performance, necessitating specific design and maintenance strategies to mitigate their effects.

Lubrication failures

Lubrication minimizes direct contact between bearing surfaces, thereby reducing wear and friction. However, lubrication failures can occur due to several reasons:

• Inadequate lubricant film

Insufficient lubricant film thickness can lead to metal-to-metal contact and increased friction and wear.

• Lubricant degradation

Over time or under extreme conditions, lubricants can degrade due to thermal breakdown, oxidation, or contamination, losing their effectiveness.

• Improper lubricant selection

Using a lubricant that does not match the specific requirements of the bearing application (e.g., viscosity too low or too high) can lead to inadequate lubrication and increased wear.

Managing lubrication effectively involves selecting the right lubricant type, ensuring proper lubricant application, and maintaining the lubrication system to prevent degradation and ensure consistent performance.

Surface degradation

Surface degradation can occur due to several factors:

• Corrosion

Exposure to corrosive environments or corrosive media can lead to oxidation and chemical degradation of bearing surfaces.

• Surface fatigue

Repeated loading and unloading of bearing surfaces can cause fatigue, leading to pitting and eventual spalling.

• Thermal degradation

High operating temperatures can alter the microstructure of the bearing materials, reducing their hardness and resistance to wear.

To combat surface degradation, bearings are often made from corrosion-resistant materials or coated with protective layers. Additionally, thermal management strategies such as heat sinks or cooling systems are used to maintain optimal operating temperatures and prevent thermal degradation.

Design and Engineering approaches yield the use of materials and employing advanced surface treatments utilized to combat these issues.

For example, tapered roller bearings, specifically part number 32216, incorporate high-strength steel and tailored heat treatments to enhance load distribution and reduce surface fatigue. This design approach effectively extends the bearing’s operational life by mitigating wear at high-contact stress points.

FAG’s deep groove ball bearings, model 6208, are commonly equipped with polyurea-thickened grease, which maintains its performance characteristics across a wide temperature range, providing reliable lubrication under variable load and speed conditions.

Technical specification:

• Bearing Model: FAG 6208
• Type of Lubricant: Polyurea-thickened grease
• Application Temperature Range: -20°C to +160°C

The field of tribology significantly impacts the efficiency and longevity of bearings through advanced understanding and innovations in friction management, wear reduction, and effective lubrication. By incorporating tribological principles into bearing design and maintenance, manufacturers can substantially enhance the performance and reliability of these critical components. For those interested in further exploring or ordering high-quality bearings, visit the online catalog of PIB Sales or direct inquiries to [email protected].

Visit Pacific International Bearing for assistance in design, selection and assistance.