Rolling bearing lubrication: grease versus oil, relubrication intervals and automatic lubrication systems

1. Introduction: The challenge of rolling bearing lubrication for system reliability

The lubrication of rolling bearings is a critical engineering task that has a direct impact on the reliability, service life and operating costs of industrial systems. Statistics show that up to 80% of all premature bearing failures are due to inadequate or faulty lubrication. This results in unexpected downtime, increased maintenance costs and lost production. Precise and standard-compliant lubrication is therefore essential to ensure the functionality of machines and to optimize productivity in the manufacturing industry. The selection of the appropriate lubricant - grease or oil - as well as the correct determination and compliance with the relubrication intervals are fundamental aspects of effective maintenance management according to VDI 2200.

2. Basic principles of rolling bearing lubrication

2.1 Physical mechanisms of action

The main function of a lubricant in a rolling bearing is to reduce friction and wear between the moving components (raceways, rolling elements, cage). This is achieved by forming a stable lubricating film. There are primarily three states of friction:

• Boundary friction: Direct contact between surfaces with a small lubricating film. High wear and tear.
• Mixed friction: Partial contact of the surfaces; Lubricant film is partially stable.
• Fluid friction (hydrodynamic/elastohydrodynamic): Complete separation of the surfaces by a lubricating film. Lowest wear and optimal operating conditions. In rolling bearings, the elastohydrodynamic state (EHD) is dominant, in which the lubricating film deforms elastically under pressure.

The viscosity of the lubricant is a key factor here. The kinematic viscosity ν (measured in mm²/s or cSt at 40°C and 100°C according to ISO 3448) must be sufficiently high to ensure a stable lubricating film at operating temperature, but not so high that it causes excessive frictional heat and energy losses. The required viscosity depends on the operating temperature, speed and bearing size. The lubricant film thickness is largely described by the κ value (kappa value), which indicates the ratio of the actual viscosity to the required viscosity. A κ value between 1 and 4 indicates optimal EHD conditions.

2.2 Properties of lubricating greases

Lubricating greases are dispersions of a base oil, a thickener (e.g. lithium soap) and additives. The consistency of greases is classified by the NLGI (National Lubricating Grease Institute) grade according to DIN 51818, with values ​​ranging from 000 (very soft, flowable) to 6 (very hard). NLGI Class 2 is the standard for most industrial applications. The advantages of grease are easy to handle, good sealing properties and a lower risk of leakage. The disadvantages are the limited heat dissipation and the tendency for the lubricant to age.

2.3 Properties of lubricating oils

Lubricating oils primarily consist of base oil and additives. They offer better heat dissipation and can be continuously cleaned using filter systems, which leads to longer service lives. Oils also enable higher speeds. The disadvantages are the higher design effort for seals and supply lines as well as the potential risk of leakage. The selection of the base oil (mineral oil, synthetic oil, PAO, esters) and the additives (EP additives, corrosion protection, antioxidants) is based on specific application requirements.

3. Technical Specifications & Standards

3.1 Standards for rolling bearings and lubricants

• ISO 281: Regulates the dynamic load capacity and service life calculation of rolling bearings. Correct lubrication is a fundamental prerequisite for the validity of the service life formulas.
• DIN 51825: Classification of lubricating greases for rolling bearings based on parameters such as base oil viscosity, service temperature range and water resistance. For example, KP2K-20 characterizes a lithium soap grease with EP additives, NLGI 2, for temperatures from -20°C to +120°C.
• DIN 51502: Classification of industrial lubricating oils (e.g. CL, CLP).
• ISO 15306: Describes the testing of lubricants with regard to their performance in rolling bearings.

3.2 Classification and evaluation criteria

Lubricants are classified according to their performance, physical properties and area of application. Important evaluation criteria are:

• Viscosity: Determines the lubricant film thickness (DIN 51562).
• Dropping point (for fat): Temperature at which the fat becomes liquid (DIN ISO 2176). A dropping point of >180°C is desirable for many applications.
• Walking stability: Resistance of the fat to changes in consistency under mechanical stress (DIN ISO 2137).
• Corrosion protection: Ability to protect components from rust (DIN 51802).
• Wear protection (EP properties): Necessary for high loads (DIN 51350, FZG test).
• Oxidation resistance: Resistance to aging processes under the influence of temperature (DIN 51806).

4. Lubrication selection and sizing guide

The decision between grease and oil lubrication as well as the choice of the specific lubricant depends on various factors. A systematic approach is crucial.

4.1 Fat or oil?

The following table summarizes the primary selection criteria:

4.2 Calculate relubrication intervals

Determining the optimal relubrication interval is crucial for bearing life. An interval that is too short will result in excessive lubricant consumption and potential overfilling, while an interval that is too long will result in inadequate lubrication and premature failure. The calculation is typically carried out according to the manufacturer's specifications, often taking into account ISO 281 and VDI 2200.

A simplified SKF grease formula can serve as a starting point:

t_f = K * (1 / (n * d_m)^(1/2)) * f_T * f_P

• t_f: Relubrication interval in hours
• K: Bearing constant (from manufacturer tables, e.g. 200-500 for deep groove ball bearings)
• n: Speed in min⁻¹
• d_m: Average bearing diameter in mm (d_m = 0.5 * (d + D))
• f_T: Temperature factor (e.g. 1 at 70°C, 0.5 at 90°C, 0.25 at 100°C)
• f_P: Load factor (e.g. 1 at P/C < 0.1, 0.5 at P/C = 0.2)

For a precise calculation, the bearing manufacturers' specific tables should be consulted. The UNITEC-D specialists support you in selecting and calculating suitable lubricants and components.

5. Installation and commissioning best practices

Correct installation and commissioning maximizes bearing life.

• Purity: The ingress of contaminants is the most common cause of bearing damage. Work areas must be clean; Tools and hands must be cleaned.
• Correct amount of lubricant: Overfilling with grease can lead to overheating due to flexing. Underfilling leads to insufficient lubrication. Manufacturer instructions regarding the degree of filling (typically 30-50% of the free space) must be strictly adhered to.
• Miscibility: Different lubricating greases and oils can be incompatible and lead to saponification, change in consistency or destruction of the additives. Before changing, the miscibility must be carefully checked or a complete cleaning carried out.
• Initial Filling: For new bearings, a careful initial filling with the correct lubricant and quantity should be carried out, often with gentle rotations to ensure even distribution.
• Sealing: Defective seals must be replaced immediately to prevent dirt from entering and lubricant from escaping.

6. Failure modes & root cause analysis

Bearing failure analysis provides important information to optimize lubrication and maintenance. Common failure modes are:

• Fatigue (pitting): Surface damage due to material fatigue under repeated loading. Can be accelerated by insufficient lubricating film. Visually recognizable by small pits and breakouts.
• Wear: Material removal due to direct contact between surfaces (adhesion, abrasion). The cause is often lack of lubrication, the wrong lubricant or contamination. Recognizable by shiny, rubbed surfaces or discoloration.
• Seizure damage: Local or large-area welding and subsequent detachment of material due to extremely high frictional contact and overheating. Occurs when the lubricant film breaks completely. Visuell durch massive Materialübertragungen und Blaustich erkennbar.
• Corrosion: Rust formation due to water ingress or aggressive media. Protective additives in the lubricant can fail. Recognizable by red-brown discoloration.
• Overheating: Thermal damage due to excessive friction (overfilling, incorrect viscosity, excessive speed) or external heat supply. Causes discoloration (tarnishing), loss of hardness of the material and destruction of the lubricant.

A detailed root cause analysis requires examining the bearing, lubricant and operating conditions. Documentation is essential here.

7. Condition monitoring & predictive maintenance

Modern maintenance strategies rely on condition monitoring to evaluate the condition of the lubrication and bearings and to plan maintenance measures in advance. This minimizes unplanned downtime.

• Vibration analysis: Changes in the vibration spectrum indicate the beginning of bearing damage, often long before it is visually noticeable. The analysis according to ISO 10816 is standard.
• Temperature measurement: An increase in bearing temperature is a direct indicator of friction and lack of lubrication or overfilling. Infrared cameras or permanently installed temperature sensors are used.
• Oil analysis (ferrography, spectral analysis): With oil lubrication, the condition of the oil (viscosity, additive degradation, water content) as well as the wear particle content (iron, copper, etc.) can be analyzed in order to assess the bearing condition and determine the optimal oil change interval.
• Acoustic Emission: High-frequency noise may indicate early damage or inadequate lubrication.

7.1 Automated lubrication systems

Automatic lubrication systems represent a reliable and efficient solution for applying precise amounts of lubricant at defined intervals. They make a significant contribution to increasing system availability and reducing maintenance costs. Systems range from single-point lubricators to centralized multi-point systems.

• Single-point lubricators: Autonomously operating units that continuously release a predefined amount of grease over a period of time (e.g. 1-12 months). Ideal for bearings that are difficult to access or require individual lubrication.
• Multi-point lubrication systems: A central unit supplies several lubrication points with lubricant via lines. Enables the simultaneous supply of an entire machine or system.
• Central grease lubrication systems: For large systems with many lubrication points. Control via PLC, precise dosage.
• Central oil lubrication systems (circulating lubrication): Deliver oil to the lubrication points, collect it, filter it and supply it again. Provide excellent cooling and cleaning.

8. Lubrication system comparison matrix

The following table compares common lubrication systems in terms of their features and areas of application.

9. Conclusion

Selecting and applying the right lubricant as well as maintaining precise relubrication intervals are not trivial, but are crucial for the operational safety and cost-effectiveness of rolling bearing applications. By considering bearing type, operating conditions, lubricant properties and implementing appropriate lubrication systems, component life can be significantly increased, failures minimized and overall equipment effectiveness (OEE) optimized.

UNITEC-D GmbH is available to you at any time as a certified specialist supplier for competent advice and the procurement of high-quality rolling bearings, lubricants and lubrication systems that meet the highest industry standards (DIN, ISO, CE certification). Our e-catalog offers a comprehensive selection of solutions tailored to your specific needs.

Visit our e-catalog for more information and products: https://www.unitecd.com/e-catalog/

10. References

1. ISO 281:2007. Dynamic load ratings and rating life for rolling bearings.
2. DIN 51825:2004. Lubricants – Lubricating greases K – Classification and requirements.
3. VDI 2200:2008. Lubrication technology – selection of lubricants for machines.
4. SKF lubricants and lubrication systems catalog.
5. FAG/Schaeffler rolling bearing manual, chapter lubrication.