Lubrication of rolling bearings: grease against oil, replenishment intervals and automatic systems

1. Introduction

Reliability studies of industrial equipment on Ukrainian enterprises in the machine-building and mining and extraction sectors show that up to 36% of premature failures of rolling bearings are caused by lubrication problems. Insufficient amount of lubricant, its degradation, contamination by foreign particles, or use of inappropriate viscosity class lead to dry friction, micro-welding of rolling surfaces and thermal seizure. Operating conditions on metallurgical plants or cement plants require engineers to have deep knowledge of tribology to achieve the target MTBF (Mean Time Between Failures) over 50 000 hours.

Specialists at UNITEC-D GmbH daily face challenges of component selection and development of maintenance schedules for complex production conditions. In this technical article, we will consider the physical fundamentals of choosing between grease and liquid oil, provide calculation formulas for determining replenishment intervals according to current standards and analyze the efficiency of automatic lubrication systems (Centalized Lubrication Systems).

2. Fundamental Principles of Bearing Tribology

The main purpose of lubrication in rolling bearings is to create a separating oil film between the rolling elements (balls or rollers), the raceways of the rings and the separator. The thickness of this film is determined by the parameter of specific film thickness $\\lambda$:

\\lambda = \frac{h_{min}}{\\sqrt{R_{q1}^2 + R_{q2}^2}}

Where $h_{min}$ — minimum oil film thickness (μm), and $R_{q1}, R_{q2}$ — root mean square surface roughness of the contact surfaces (μm). The value $\\lambda$ determines the friction mode in the node:

• $\\lambda > 3$: Full hydrodynamic or elastohydrodynamic (EHD) film. There is no metal-to-metal contact, wear is practically zero.
• $1 < \\lambda \\le 3$: Mixed friction. Partial micro-contact of roughness is observed. Requires EP (Extreme Pressure) and AW (Anti-Wear) additives.
• $\\lambda \\le 1$: Boundary friction. High risk of scuffing, plastic deformation and fatigue damage of the surface (micropitting).

Base viscosity of the lubricant material ($\ u_1$) at operating temperature is a critical parameter to achieve $\\lambda > 2$. According to ISO 15312, it is calculated based on the average bearing diameter $d_m = (d+D)/2$ and rotational speed $n$. If the bearing operating temperature is +70 °C, and the calculated kinematic viscosity is 30 mm²/s, the application of grease of NLGI 2 class based on mineral oil ISO VG 100 will ensure stable operation up to $n \\cdot d_m = 300 000$ mm·rev/min.

3. Technical Specifications and Standards

Design and maintenance of friction units are regulated by a complex of international and national standards. Compliance with these norms guarantees the safety of operation and compatibility of spare parts supplied by UNITEC-D GmbH:

• ISO 281: Rolling bearings. Calculation of dynamic load rating and nominal life (taking into account the modified fatigue strength factor $a_{ISO}$ for lubrication conditions).
• DIN 51825: Lubricating greases type K. Classification of greases by application (for example, KP2K-30 for high loads).
• ISO 6743-9: Lubricants, industrial oils and related products (class L). Classification of systems and lubricants (group X for semi-solid lubricants).
• DSTU GOST 21150:2009: General-purpose grease (technical requirements, harmonized with European standards).
• IEC 60034-9 / DIN 51818: Classification of grease consistency according to NLGI (National Lubricating Grease Institute) penetration degree from 000 to 6.

4. Selection and calculation: grease or oil

Selection between grease and oil depends on the rotational speed ($n$), load ($P$), operating temperature and constructive features of the housing. Below is a comparative matrix for engineering decisions.

Calculation of the grease replenishment interval ($t_{f}$):
For standard radial ball bearings, the SKF empirical formula for determining the replenishment interval (in hours) is as follows:

\\log t_f = 6.54 - 2.6 \\cdot \frac{n}{N_{F}} - 0.03 \\cdot d

Where:
- $n$ — working rotational speed (rpm)
- $N_{F}$ — grease limiting speed (rpm)
- $d$ — inner diameter of the bearing (mm)

Example: For the bearing 6312 ($d = 60$ mm, $D = 130$ mm) at a operating speed of 2000 rpm and a lubricant limit speed of 6300 rpm, the calculated replenishment interval will be approximately 3800 hours under normal conditions (temperature up to +70 °C, absence of vibration).

5. Practical Recommendations for Installation and Commissioning

Incorrect initial lubrication of the bearing often causes failure already in the first weeks of operation. Follow these engineering standards:

1. Lubricant Fill Volume: The rolling bearing must be filled with grease to 30-50% of the free space in the housing. Overfilling (more than 70%) causes intensive grease mixing, sharp increase in hydraulic losses, heating and grease leakage through the seal.
2. Сумісність загусників: Категорично заборонено змішувати мастила на різних типах загусників. Наприклад, змішування літієвого комплексу з сульфонат-кальцієвим або бентонітовим може призвести до руйнування структури мастила (розрідження та витікання) через несумісність мильних баз. Звертайтеся до технічних паспортів виробників перед дозаправкою.
3. Cleanliness during installation: Keep lubricating materials in sealed cartridges or sealed packaging. Use syringes and dispensers intended exclusively for a specific type of lubricant to avoid contamination by abrasive particles.

6. Typical types of failures and root cause analysis (RCA)

Failure analysis performed in UNITEC-D laboratories allows to identify three main degradation scenarios of bearings due to lubrication problems:

• Thermal aging (carbonization): The grease loses its base oil, turning into a dense waxy or black solid mass. The cause is exceeding the maximum operating temperature (for example, above +120 °C for standard lithium greases) or prolonged operation under high load conditions without cooling.
• Brinelling and False Brinelling: Formation of pits on raceways in the form of marks from balls or rollers due to vibration of non-operating equipment (for example, standby pumps). The lubricant film is squeezed out by vibration, causing micro-pitting. Solution — use of EP anti-wear additives and periodic shaft rotation.
• Abrasive Wear (Contamination Wear): Entry of dust, swarf or process sludge through worn seals. Lubricant acts as a grinding paste, wearing the rolling elements and increasing the bearing radial clearance.

7. Predictive Maintenance and Condition Monitoring

To prevent emergency stops at critical nodes (main fans, crushers, extruders), modern industry applies a complex set of monitoring methods:

• Vibration Analysis (High-Frequency Acceleration / SPM - Shock Pulse Method): The shock pulse method allows the detection of damage to the oil film and the beginning of surface microcracks before an increased overall vibration level appears. An increase in the dBm indicator indicates dry friction.
• Acoustic Emission and Thermography: Non-contact temperature measurement of the bearing housing using infrared pyrometers and thermal cameras detects local temperature peaks indicating lack or excess of lubricant.
• Tribological Oil Analysis (Oil Condition Monitoring): For liquid lubrication systems, wear spectroscopic analysis (determination of Fe, Cu, Cr concentrations in ppm) and determination of viscosity drop over 10% from the initial value according to ISO 3104.

8. Comparison Matrix of Modern Lubrication Systems

For optimizing maintenance costs of large enterprises, it is advisable to transition from manual injection to automatic systems. Comparison of the main options is shown below:

9. Conclusion

Correct selection of lubricant materials and adherence to replenishment schedules are the guarantee of uninterrupted operation of industrial equipment. Engineers from UNITEC-D GmbH are ready to provide expert support in the selection of bearings, high-temperature lubricants and components of automatic lubrication systems that comply with strict European quality standards.

Familiarize yourself with our complete range of industrial components and bearings in electronic catalog UNITEC-D.

10. References

• ISO 281: Rolling bearings — Dynamic load ratings and rating life. International Organization for Standardization.
• DIN 51825: Lubricants — Greases K — Specification of greases for rolling and plain bearings.
• SKF General Catalogue: Bearing Lubrication and Maintenance Guidelines, Publication 10000 EN.
• ISO 15312: Rolling bearings — Thermal speed rating — Calculation and coefficients.
• IEV (International Electrotechnical Vocabulary) - IEC 60050: Industrial Tribology and Maintenance Terminology.