Rolling bearing lubrication: Basics, selection and condition monitoring for maximum operational safety

Introduction

The operational reliability and service life of rolling bearings are crucial for the performance of industrial systems. Correct lubrication is the most important factor and influences up to 80% of premature bearing failures. Missing or incorrect lubrication leads to increased friction, wear and overheating, which significantly increases downtime and maintenance costs. Choosing the right lubricant, adhering to precise relubrication intervals and using modern lubrication systems are therefore essential for reliable production in the DACH manufacturing sector. UNITEC-D, as a proven supplier of industrial components, offers a wide range of solutions to optimize bearing lubrication.

Basic principles of bearing lubrication

The primary task of lubrication is to reduce friction and wear between the meeting surfaces in the rolling bearing. This is achieved by forming a stable lubricating film. The thickness and stability of this film are critically important and are influenced by the kinematic viscosity of the lubricant, the peripheral speed of the bearing components and the load carried. There are three main lubrication states, as shown in the Stribeck curve:

• Limiting friction: At low speeds or high loads, the lubricating film is incomplete. Direct metal-to-metal contact leads to high wear and friction. This is an undesirable operating condition.
• Mixed friction: A partial lubricating film separates the surfaces, but local metal-metal contact still occurs. Special additives in the lubricant (EP additives) are required here to minimize wear.
• Hydrodynamic friction (full lubrication): A fully supporting lubricant film completely separates the surfaces. This is the optimal operating condition with minimal friction and wear. The viscosity of the lubricant plays a dominant role here. Rolling bearings are referred to as Elastohydrodynamic (EHD) lubrication, as the elastic deformation of the contact surfaces under load is taken into account.

The kinematic viscosity (unit: mm²/s or cSt) is the most important property of a lubricant and is specified at 40 °C and 100 °C. The Viscosity Index (VI) describes the dependence of viscosity on temperature. A higher VI means less change in viscosity with temperature fluctuations, which is beneficial for constant operating conditions.

For lubricating greases, another important characteristic is the consistency, classified according to the NLGI (National Lubricating Grease Institute) class from 000 (very liquid) to 6 (very solid). Most industrial greases are in NLGI 1, 2 or 3 grades.

Technical specifications & standards

The selection and application of lubricants are subject to strict national and international standards to ensure performance and safety. These standards define classifications, test procedures and quality requirements.

• DIN 51825: This German industrial standard specifies the requirements and classification of lubricating greases for rolling bearings. It categorizes greases according to their service temperature, water resistance and corrosion protection properties (e.g. KP2K-30: K = rolling bearing grease, P = EP additives, 2 = NLGI class, K = temperature range -20 to +120 °C, 30 = low temperature test at -30 °C).
• ISO 3448: This international standard classifies industrial fluid lubricants by viscosity. It defines viscosity classes (ISO VG) at 40 °C (e.g. ISO VG 46 for an oil with a kinematic viscosity of 46 mm²/s at 40 °C).
• ISO 281: This standard deals with the dynamic load capacity and service life of rolling bearings. Precise lubrication is an essential input parameter for the service life calculation and influences the service life factor a_iso.
• DIN EN 13306: This European standard defines maintenance terms and is relevant for planning lubrication intervals as part of preventive and predictive maintenance.
• VDI 2206 / VDI 2207: These VDI guidelines cover the diagnosis of machine damage through vibration analysis and the monitoring of rolling bearings.

Certifications such as CE Mark confirm conformity with European health, safety and environmental standards. TÜV certificates prove the testing and certification of products and systems by an independent body. For potentially explosive areas, products with ATEX certification (Directive 2014/34/EU) are required, which regulate the use of equipment in such environments.

Selection & sizing of lubrication

The correct selection of the lubricant and the determination of the relubrication intervals are crucial for the bearing life. The decision between grease and oil depends on operating conditions, bearing design and maintenance strategy.

Table 1: Comparison of lubricating grease and lubricating oil

Calculation of relubrication intervals for grease-lubricated bearings

The lifespan of a grease is limited by aging and wear. The relubrication interval (t_f) can be approximated according to ISO 281 or VDI 2207-3 (Section 3.3):

t_f = K * (n_ref / n)^(x) * (d_m / d_ref)^(y) * f_T * f_P * f_V

• t_f: Relubrication interval in hours (h)
• K: Basic factor (typ. 1000 - 5000 h)
• n_ref: Reference speed (often 1000 min⁻¹)
• n: operating speed (min⁻¹)
• d_m: Average bearing diameter (d+D)/2 (mm)
• d_ref: reference diameter (often 50 mm)
• x, y: Exponents, depending on the bearing type (typ. x=0.5, y=0.3)
• f_T: Temperature factor (0.5 at 80 °C, 0.25 at 90 °C, 0.1 at 100 °C)
• f_P: Load factor (1 at C/P > 10, 0.8 at C/P = 7)
• f_V: Vibration factor (0.5 for strong vibrations)

Example: A deep groove ball bearing 6205 (d=25mm, D=52mm, B=15mm, d_m=38.5mm) runs at n=3000 rpm and 85 °C. C/P = 15. K=2500, x=0.5, y=0.3. Reference speed n_ref = 1000, reference diameter d_ref = 50.

t_f = 2500 * (1000/3000)^0.5 * (38.5/50)^0.3 * 0.4 * 1 * 1

t_f = 2500 * (0.333)^0.5 * (0.77)^0.3 * 0.4

t_f = 2500 * 0.577 * 0.92 * 0.4 ≈ 530 hours

This corresponds to a relubrication interval of approx. 22 days. Such calculations serve as a guideline and must be clarified based on experience and condition monitoring.

Selection of oil viscosity

The optimal operating viscosity (ν) of the oil at operating temperature depends on the bearing diameter (d_m) and the speed (n). It is often determined via the ratio κ (kappa), which represents the ratio of the actual viscosity to the required minimum viscosity. For good lubrication conditions, κ should be between 1 and 4.

Required minimum viscosity ν₁ at 40 °C for deep groove ball bearings:

ν_1 ≈ (5000 / n^0.5) * (d_m)^0.5

Example: deep groove ball bearing with d_m = 50 mm, n = 1500 min⁻¹, operating temperature 60 °C. An oil with a VI of 95 has a viscosity of 30 cSt at 60 °C. The viscosity at 40 °C for an ISO VG 46 oil is 46 cSt.

ν_1 ≈ (5000 / 1500^0.5) * (50)^0.5 ≈ (5000 / 38.7) * 7.07 ≈ 129 * 7.07 ≈ 912 cSt (at operating temperature)

This value is the minimum viscosity at operating temperature. To achieve this viscosity at 60°C you typically need an ISO VG 220 or ISO VG 320 oil, depending on the VI of the oil. The exact viscosity at operating temperature must be determined using viscosity-temperature diagrams or software tools. An ISO VG 46 oil would be significantly too thin for this example.

Installation & commissioning best practices

Proper installation and commissioning is essential to maximizing bearing life with lubrication in mind.

• Cleaning: Storage areas must be thoroughly cleaned before assembly. Particles as small as 5 µm can have a damaging effect. The cleanliness according to ISO 4406 is crucial here (e.g. 16/14/11).
• Initial greasing: For grease-lubricated bearings, the correct initial greasing quantity is crucial. Typically the warehouse is filled to 30-50% of the free internal volume. Overfilling can lead to a rise in temperature due to flexing, while underfilling can lead to insufficient lubrication.
• Alignment: Incorrect shaft or housing alignment leads to increased stress and premature wear. Tolerances of a few hundredths of a millimeter are critical (e.g. < 0.02 mm). Laser alignment systems are an industry standard.
• Seals: Intact seals protect against the ingress of contaminants and the escape of lubricant. Selecting the correct seal type (contact or non-contact seals) is application specific.
• Storage: New bearings should be stored in their original packaging, cool, dry and with little vibration to avoid corrosion and contamination. The maximum storage time is up to five years, depending on the storage type and preservation.

Failure modes & root cause analysis

Bearing failures are costly. It is estimated that around 70-80% of all premature bearing failures are due to poor lubrication. Identifying the specific error modes enables targeted cause analysis and prevention.

• Fatigue (pitting): Material fatigue of the raceways and rolling elements, visible as pittings. Often caused by excessive load, inadequate lubricating film (viscosity too low), contamination or insufficient oil film thickness (κ < 1).
• Wear: Wear of the contact surfaces. Can be caused by abrasive particles in the lubricant (abrasive wear), insufficient lubricating film (adhesive wear) or fretting corrosion due to micro-movements under load. Typical indicators are rough surfaces, material removal and increased bearing clearances. A high concentration of iron particles (>100 ppm) in the oil indicates wear.
• Corrosion: Rust formation on the bearing surfaces. Caused by water in the lubricant (even 100 ppm of free water can be critical), aggressive media or a lack of corrosion protection additives. Visible as reddish or brownish discoloration.
• Grease aging / oil oxidation: High temperatures and oxygen cause grease and oil to age. The fat becomes hard and brittle or oily, the oil thickens and its additives are used up. This leads to reduced lubrication performance and can be recognized by discoloration (dark coloring) and changes in the odor of the lubricant. An increase in the acidity (AN value) in the oil above 0.5 mg KOH/g is an indicator of oxidation.
• Electrical erosion: Current passage through the bearing can lead to tiny fretting on the raceways, often visible as a washboard pattern. This can be caused by lack of grounding or VFD controlled motors.

Predictive maintenance & condition monitoring

Modern maintenance strategies rely on condition monitoring in order to detect impending bearing damage at an early stage and avoid unplanned downtime. DIN EN 13306 describes the maintenance terms that apply here.

• Vibration analysis: One of the most effective methods for detecting bearing damage. Changes in the vibration spectrum (e.g. increased amplitudes at bearing natural frequencies such as BPFI, BPFO, FTF, BSF) indicate the onset of fatigue or imbalance. An increase in the vibration amplitude by 6-10 dB above the reference value is a warning signal.
• Temperature monitoring: Elevated bearing temperatures (> 80 °C) are an indicator of inadequate lubrication, overload or friction. Thermographic cameras or permanently installed temperature sensors (PT100) enable continuous monitoring. A temperature increase of more than 10-15 °C above normal is critical.
• Oil analysis: For oil-lubricated systems, regular analysis of the oil provides valuable information about the wear condition of the bearing and the condition of the lubricant.
• Spectroscopy: Detection of wear particles (Fe, Cr, Ni, Cu) and impurities (Si for dust) in ppm.
• Ferrography: Analysis of the size and shape of wear particles to identify the wear mechanism.
• Viscosity: Viscosity control at 40 °C and 100 °C. Significant deviations (>10% from new) indicate oil oxidation or contamination.
• Water content: Monitoring of the water content (e.g. with Karl Fischer titration). Target is <100 ppm for critical applications.
• Acid number (AN): Indicator for oil oxidation, target value usually < 2 mg KOH/g.
• Acoustic emission analysis (AE): Detects high-frequency noises that occur when micro-damage and friction begin. Particularly sensitive to early stages of fatigue.

Comparison matrix: lubrication systems

Choosing the right lubrication system affects not only the efficiency of lubrication, but also maintenance effort, personnel safety and operating costs. Modern, automated systems are becoming increasingly important.

Table 2: Comparison of lubrication systems

UNITEC-D offers a selection of components for automated lubrication systems, from single point lubricators to high-performance metering units for central lubrication systems. The use of these systems improves occupational safety, avoids unplanned downtime and optimizes lubricant consumption, resulting in a reduction in total cost of ownership (TCO). Precise dosing extends bearing life and contributes to compliance with relevant environmental standards.

Conclusion

Technically precise and standard-compliant lubrication is the cornerstone of the reliability and efficiency of industrial rolling bearings. From the basic principles of lubricant film formation to the selection of the appropriate lubricant according to DIN 51825 and ISO 3448 to the implementation of predictive maintenance strategies and modern lubrication systems - every aspect contributes to maximizing system availability. The consistent application of proven practices and the use of high-quality components, such as those offered by UNITEC-D as a trustworthy partner for the industry, minimize downtime and optimize operating costs. For detailed information about our products and solutions, visit our e-catalog at https://www.unitecd.com/e-catalog/.

References

• DIN 51825: Lubricants – Lubricating greases K – Minimum requirements and tests
• ISO 3448: Industrial fluid lubricants – ISO viscosity classification
• ISO 281: Rolling bearings – dynamic load ratings and service lives
• DIN EN 13306: Maintenance – Terminology
• VDI 2207 Sheet 3: Operational stability of machine elements - rolling bearings - service life calculation and application