FRICTION generated in mechanical systems, even when originating from seemingly small contact areas, has a surprisingly large global impact. Past assessments indicate that approximately 23% of worldwide energy consumption is tribology-related, with about 20% devoted to overcoming friction in mechanical contact and an additional 3% associated with wear, spare parts, and maintenance.

As a result, bearing friction torque directly contributes to thermal behaviour, fatigue life, and overall energy consumption. While local tribological mechanisms are well understood, the system-level behaviour of complete bearing units operating in real-world service conditions has been examined far less extensively. Rotating component resistance accounts for a stable share of total train resistance, around 7%, and becomes particularly important at low speeds and during frequent acceleration and braking in start-stop operations. Reducing real energy consumption therefore requires accurate assessment of bearing friction torque in order to reduce it at a system level.

Beyond contact-related mechanisms, further losses originate from the lubricant itself.

In a grease-lubricated railway wheelset bearing, friction torque does not originate from a single mechanism, but from several concurrent energy dissipation processes acting at different interfaces within the bearing. Rolling-related losses arise primarily from the contacts between rolling elements and raceways.

In addition, significant sliding-related losses are present at specific internal interfaces. These include sliding at the roller end-flange contacts in tapered roller bearings, interactions between rolling elements and the cage, cage guiding surfaces, and seal contacts. Sliding at these interfaces contributes directly to frictional heating and is strongly influenced by load, speed, and lubrication conditions.

Beyond contact-related mechanisms, further losses originate from the lubricant itself. In grease-lubricated bearings, grease churning and drag losses arise from the displacement and macroscopic flow of grease within the bearing and can make a substantial contribution to friction torque, particularly during start-up and transient operations.

Friction, temperature, and grease viscosity are therefore strongly coupled within a complex tribological system. Frictional losses generate heat, which alters the temperature distribution within the bearing and surrounding components, subsequently modifying the rheological properties and base oil viscosity of grease. This, in turn, influences the frictional behaviour of the bearing.

A critical aspect of this interaction is the thermal inertia of the bearing system. Due to the thermal mass of the bearing, shaft, housing, and lubricant, changes in frictional losses and lubrication conditions are not immediately reflected in temperature measurements. As a result, friction torque and temperature may evolve at different rates during start-up or variable-speed operations, before a stable thermal equilibrium is reached.

Frictional behaviour

The frictional behaviour of rolling bearings cannot be described by a single deterministic value, but inherently exhibits a certain degree of dispersion. This variability arises from manufacturing tolerances that are intrinsic to all bearing components, leading to differences in internal geometry, contact conditions, and load distribution.

Wheelset bearings are typically characterised by wider tolerance fields when compared with high-precision rolling bearings and, in many cases, also compared with bearings used in drive systems. Therefore, the resulting friction torque is subject to a corresponding spread, even under nominally identical operating conditions.

This dispersion is further amplified by lubrication conditions. Wheelset bearings are almost exclusively grease-lubricated, and grease behaviour during operation is known to be highly variable, particularly when compared with systems featuring a continuously-supplied oil film. Grease redistribution, churning, and local starvation phenomena introduce additional variability in frictional losses.

In addition, batch-to-batch variations in key grease properties, such as consistency or oil bleeding behaviour, can further contribute to the observed spread in frictional behaviour, even for nominally identical grease specifications.

Consequently, the frictional response of wheelset bearings should not be represented by a single fixed value, but rather by an interval of possible values reflecting the combined effects of manufacturing variability and the complex, time-dependent behaviour of grease lubrication.

Existing methods for estimating friction torque range from simplified analytical formulations to highly-detailed quasi-static and dynamic simulations, and more advanced numerical methods. These approaches are well established and provide valuable support for analysis and design activities.

Wheelset bearings are typically characterised by wider tolerance fields when compared with high-precision rolling bearings.

However, the practical standardisation of friction torque assessment for wheelset bearings requires a complementary perspective, focused on measurement rather than prediction. From an industrial point of view, a standardised method must be simple, robust, and easily implementable by all bearing suppliers, while at the same time delivering consistent, accurate, and reliable results. This requirement is particularly relevant in view of the very low friction torque levels involved, typically in the range of a few newton-metres (Nm), as well as the inherent dispersion arising from manufacturing tolerances and grease lubrication behaviour.

A meaningful standardised approach must therefore be based on a well-defined and transparent test procedure, employ equipment that is widely available within the railway bearing industry, and ensure a high level of repeatability. To achieve this, friction torque measurements should be performed at predefined load and speed operating points, once thermal equilibrium has been established, in order to reflect representative operating conditions. Artificial cooling of the bearing during testing should be avoided as it would disturb the natural thermal balance of the system and lead to temperature, lubricant viscosity and friction conditions that are not representative of in-service operation.

Furthermore, the measurement method must be easy to calibrate and verify by direct measurement. This implies that a known friction torque must be experimentally reproduced and confirmed through the measurement system itself, rather than being inferred or validated solely through analytical calculations or model-based assumptions.

These requirements are fulfilled by the two load cell measurement principle, implemented on a standard bearing test rig such as that specified in EN 12082 and combined with application-specific load and speed measurement points. The standardised method proposed by SKF in the framework of the DIN technical specification for friction torque assessment follows this approach and is based on standardised test cycles for specific railway applications, as defined in EN 50591. The objective is to provide a simple framework, applicable industry-wide, for the practical determination of friction torque in wheelset bearings.

Design rules

To achieve maximum reduction, specific design principles must be applied. These are grounded in a deep understanding of the physical phenomena within rolling bearings that contribute to friction. The key principles are:

  • selecting a seal type that generates minimal friction torque, while providing sufficient sealing efficiency for the application
  • optimising the overall geometry of the roller set (macrogeometry), notably the geometrical proportions of the roller and flange angle, is essential to achieve an optimal load distribution and at the same time minimising friction torque
  • improving the cage design to enhance roller guidance and reduce roller skewing by minimising cage deformation, and
  • applying surface finishes and optimised raceway profiles (microgeometry) to contact surfaces to enhance lubrication conditions and minimise friction torque.

In the context of the worldwide objective of reducing energy consumption and minimising CO₂ emissions, wheelset bearing friction torque is becoming an increasingly important parameter.

Although the wheelset bearings are not the largest contributor to the overall energy consumption of the entire railway vehicle, their impact is nonetheless significant. The evaluation of wheelset bearing friction torque is therefore essential for both designers and operators of rolling stock.

*Martina Giaveno is product development manager and Jan Babka is competence center manager at SKF.