Roller burnishing is an extremely efficient process for increasing the fatigue strength of mechanically loaded components. The process itself is very easy to carry out and can be used on any turning or milling machines. It is one of the mechanical surface treatment methods and ensures that the surface roughness is smoothed and the near surface area is strengthened in a targeted manner. 

The effects of roller burnishing, which is also called deep rolling, are a plastic deformation of roughness peaks and the associated reduction in roughness, an increase in dislocation density and the associated work hardening as well as the introduction of compressive residual stresses. 

The latter in particular leads to an increase in fatigue strength during dynamic loading because the formation of cracks is stopped or greatly slowed down by the existing compressive residual stresses. Deep rolling naturally has the greatest effect at the points where the greatest mechanical loads are applied. From a mechanical point of view, this is the case at notch points and in radius transitions on shafts or shaft-like components. For this reason, the process is used here particularly often. 

How notch stresses affect the service life of your shafts

Shafts are loaded in different ways. These can be tensile or compressive loads, torsional loads or even bending loads. In all cases, the greatest stress occurs at the transition between two shaft diameters or at the transition into the flange area. 

The picture shows a simplified wheel carrier that is subjected to a bending load. It can be clearly seen that the stresses are highest in the transition area and that failure of this component will start from this point. 

Significant increases in service life can be achieved with targeted roller burnishing. Stefanie Günther presented extensive studies on this at the VDI conference "Shafts and shaft-hub connections" 2024 and was able to show load increases of up to +93% [1]. 

Tool selection and process optimization for maximum fatigue strength

During deep rolling, a rolled body is pressed onto the surface of a component. It then rolls off and causes plastic deformation of the surface and the underlying near surface area. The decisive parameters for the process are geometry of the roller, feed rate and, above all, the rolling force. This determines what level the introduced compressive residual stresses can reach. 

There are various tools on the market for deep rolling. These often differ in the geometry of the individual rolling elements. The tool - and in particular the roller - must be matched to the processing task. A roller like the one shown in Figure 2, for example, is not suitable for processing radii and other free-form surfaces. Instead, it is ideal for smoothing cylindrical inner or outer surfaces, as it enables very high feed rates. 

Single-roller mechanical tools in which the roller is self-supported are particularly suitable for deep rolling radii. Figure 3 shows, for example, a single-roller mechanical tool of type ECOROLL EG45 tool with a 40M roller. The roller is directly supported and can absorb axial and radial loads. By positioning the roller at 45°, machining over an angle of 90° is possible. This makes this roller variant very suitable for machining radii on shafts.

The roller can be guided along the contour via CNC control and thus apply an optimum deep rolling force at any point. However, a special feature of the tool type must be considered when programming. The force is applied in the tool via spring elements that can deflect in one direction; in this case at 45°, i.e. at right angles to the roller axis. 

The spring direction must be taken into account when evaluating the rolling force. If a cylindrical surface is machined, the real rolling force becomes the spring force at an angle of 45°. The latter is displayed on the dial gauge. If a constant rolling force is applied over an entire radius, the displayed spring force changes. It would therefore be wrong to program the process in such a way that the dial gauge displays the same value over the entire radius. 

This correlation exists for all tools in which only one spring element is installed, although the machining direction changes over the component contour. It is important to know and understand this correlation. However, the question arises as to how important this correlation actually is. This is to be reassessed from application to application. In some cases, it is essential to determine and control the rolling force very precisely, which is why this must also be considered in the programming. Often, however, the programming effort for the user is too great and it is absolutely sufficient to work with a constant infeed over the entire radius. What exactly is important for the process must always be weighed up separately and individually by the user. 

[1]      S. Günther, B. Muhammedi, T. Werner, B. Schlecht, A. Hasse, A, Brosius: Evaluation of the accuracy of the strength verifications of notched components. VDI Reports No. 2443, VDI Conference on Shafts and Shaft-Hub Connections, Munich, 2024