Hard Rolling for Increasing the Strength of Functional Surfaces subjected to Rolling Loads
As part of a funded project, HEGENSCHEIDT-MFD GmbH has developed a hard rolling process in close cooperation with research institutes, rolling bearing manufacturers and metallography companies. Through the use of the associated machining concepts and tool variants, residual compressive stresses are induced in the running surfaces of the bearing rings, which results not only in hardening and smoothing, but also in a demonstrable increase in the service life of rolling bearing raceways in particular. HEGENSCHEIDT-MFD GmbH, a medium-sized mechanical engineering company within the NILES-SIMMONS-HEGENSCHEIDT Group (NSH) with an international focus, has been engaged for many decades in the smooth and deep rolling and roll straightening of rotationally symmetrical components.
HEGENSCHEIDT-MFD GmbH demonstrated in the three-year research project “Hard rolling of bearing rings” (funding code EFRE-0800344) in close cooperation with other project partners that the bearing rings of anti-friction bearings achieve a significantly longer service life as a result of the process sequence of hard turning and hard rolling with a hard rolling process that had been specially adapted for this purpose.
Compared with the conventional process chain for hard machining, hard rolling therefore offers a resource-saving and cost-effective alternative. The longer service life of the bearings contributes to a further increase in efficiency and therefore reduces the costs not only for the manufacturer, but also for the user. This may be the avoidance of early breakdowns of production machines or the extension of maintenance intervals, for example in the energy supply field. From tool and process development to the finished anti-friction bearing, all of the necessary skills were available within the project consortium. Determination of the process requirements was followed by the iterative development of the rolling tools and process parameters, with this being followed in turn by trials on analogue and real components.
Hard rolling is a forming production process in which hardened areas of components are plasticised and shaped by means of a rolling element in the contact area. The aim is both work hardening and the introduction of residual compressive stresses into the border zone, as well as the smoothing of the surface, which results in an increase in the service life of functional surfaces that are subjected to rolling loads. The advantages of the hard rolling process lie in its cost-effectiveness and resource efficiency, as it can be easily integrated into existing production lines with a short machining time and low energy requirements. The high variance in the specially adapted rolling tools and process parameters also enable the machining of a large number of different anti-friction bearing variants.
There are currently only isolated approaches to using the process for anti-friction bearings, although the anti-friction bearing industry offers a high application potential for the use of hard rolling. Anti-friction bearings are among the most frequently used machine elements and often have a significant influence on the operating behaviour and service life of production machines and systems.
The process is shown schematically in the image below. The rolling element generates a contact pressure due to the normal force, which leads to plastic forming of the workpiece surface and levels the roughness profile, as well as inducing additional residual compressive stresses in the border zone.
It has been demonstrated in several different studies that such residual compressive stresses increase the service life of anti-friction bearings. The residual compressive stresses counteract the progress of cracks in the rolling contact and therefore contribute to an increase in the resistance to surface disintegration and an improvement in rolling-contact stability. The smoothing of the surface counteracts any insufficient lubrication.
The induction of residual compressive stresses in bearing rings requires the development of a new tool concept, as none of the existing ones offers accessibility of the tool.
3. Rolling Process and Tool Development
For the hard rolling process, HEGENSCHEIDT-MFD has developed a suitable variable tool that is used in the feed rolling process. Relatively small rolling elements are used for this. The contour of the bearing ring is traced with the rolling element using a defined feed rate. In this way, the usual raceway contours can be machined very flexibly. The force-based support in several spatial axes, as well as the variable tool design, enable the machining of a wide range of designs of inner and outer rings. The robust and durable mechanical design of the rolling element bearing does not require any additional media. Furthermore, it is possible to monitor the process by means of interfaces and thereby carry out a resource-saving adaptation of the machining task.
4. Experimental Investigation
The first investigations were carried out on axial washers as simple test specimens, which enabled a high level of reproducibility and therefore comparability of the machining. The characterisation process comprised features such as the geometry, surface quality and shape and position tolerances on the one hand, and boundary zone properties such as residual stresses and a hardness profile on the other. On the basis of the analogue components it was shown that induced residual compressive stresses lead to a significant increase in the service life.
5. Service Life Investigations
The results of trials with real anti-friction bearings are shown in the following image:
If the results of standard hard-turned samples are taken as a reference to 1, the hard rolling process results in an increase in the service life by a factor of 3.2 according to the current status.
6. Rolling Results
Example of a clearly identifiable improvement in the surface roughness levels:
The measurement report shows that the bearing raceways are uniformly well smoothed, which also includes the uniform generation of the residual compressive stresses.
Roughness values of a hard-rolled anti-friction bearing ring:
The generated contour accuracy corresponds to the common requirements of the anti-friction bearing industry.
7. Potential Workpieces
In principle, hard rolling with the newly developed tool base and the tool variants derived from it can be used for all rolling bearing raceways. Also for linear bearings and guides, as well as ball caster units and the like. Furthermore, it can provide significant performance improvements for more complex structural units with integrated rolling element raceways and for the components which are treated accordingly.
Although the benefits of the surface rolling of bearing raceways have been demonstrated several times in various studies, they have not yet been implemented as standard because suitable process and tool concepts for the machining of typical bearing geometries have been lacking up to now. The hard rolling process developed by HEGENSCHEIDT-MFD for rolling bearing raceways closes this gap. The variable design options enable its application in a large number of different types of anti-friction bearing units. The integrated interfaces for process monitoring, as well as the robust and durable design of the rolling element bearing, ensure the precise positioning of the required rolling forces. The trials carried out so far have shown both high machining qualities and very good values for the increase in the service life. Several projects are currently underway with different partners to prove the suitability of the process for series production.
Deep and finish rolling of the wheel running surface of rail vehicles
M.Sc. Alexander Rudi (former employee), Dr.-Ing. Jandrey Maldaner,
Dipl.-Ing. Heiner Muhr, HEGENSCHEIDT-MFD, Erkelenz
M.Sc. Patrick Schneider, Dipl.-Ing. Michael Kölker (former employee),
Univ.-Prof. Dr.-Ing. Christian Schindler, RWTH Aachen University, Institute for Rail Vehicles and Transport Systems, Aachen
The wheel-rail system is subject to various wear mechanisms through permanent sliding and rolling movements. The resulting change in the wheel running surface profile poses not only a risk of material failure for the wheelset and adjacent components such as the wheel bearing and running gear, but also an acoustic source of disturbance for passengers and the environment. The profile contour of each wheelset profile is to be reprofiled at regular intervals by a machining production process. However, this repair procedure reduces the service life of the rail vehicle wheels due to material removal. Deep and finish rolling of the wheel running surfaces, on the other hand, is one way of reducing wheel profile wear and thus extending the repair intervals and thus the service life of a rail vehicle wheel. Together with the Institute for Rail Vehicles and Transport Systems (IFS), the deep and finish rolling of the wheel running surfaces of rail vehicles was systematically investigated by means of computer simulations and test bench trials. Final field tests on the routes of a German light rail operator are to be carried out to assess the wear behaviour of rolled wheels and thus demonstrate the potential of rolling processing.
2. Rolling process and machining
The deep and finish rolling of the surface is a purely mechanical forming of the component edge layer. Rolling bodies are guided over the surface of the component under defined contact pressure. The direct component contact area is plastically deformed. Depending on the contact conditions, only the surface is smoothed, small notches are flattened and the material is specifically hardened in the plastically deformed volume. The forming process removes harmful residual stresses from the pre-machining and introduces strength-promoting residual compressive stress into the boundary area. Along with the increase of the boundary layer hardening, this can achieve a reduction of wheel running surface wear and thus an increase in the mileage of the rail vehicle wheels. The formation and progression of cracks is impeded significantly.
The rolling machining follows on from the reprofiling of the wheel running surfaces. A rolling unit developed by HEGENSCHEIDT-MFD (see Fig. 2) is suitable for both underfloor wheelset lathes (shown in Fig. 1) and overfloor wheel lathes and can roll the entire wheel profile or parts of it deep and smooth.
By means of several rolling tests on an underfloor wheelset lathe from HEGENSCHEIDT-MFD the influence of the rolling process on hardening, smoothing and hardness dispersion over the circumference was determined. Depending on the material properties, an increase in surface hardness of up to 36 % is achieved. The process is accompanied by a smoothing of the surfaces (waviness and roughness) and a homogenisation of the hardness dispersion over the wheel circumference.
With the help of the rolling tests, a correlation matrix was created with which a statement can be made about the desired final hardness after rolling, taking into account the wheel material used (see Fig. 3 and 4).
3. Roller test bench trials and modelling
The wheel-rail contact can be simulated with the help of the IFS roller test bench tests. With increasing rollover, the hardness in the contact area increases up to a saturation value. This applies to both the unrolled and the rolled wheel. It can be seen that the hardness increase during the overrolling process is significantly higher for the previously rolled wheel than for the reference wheel. Here, an increase of approx. 60 HV (extrapolated) compared to 35 HV can be expected (see Fig. 5).
Furthermore, the inspection could establish that the residual stresses in the boundary layer up to a depth of 300 µm are positively influenced by deep rolling. Figure 7 shows the difference between the unrolled and rolled wheel in the running track area from the test bench test.
The rolled wheel shows no tensile stresses, but residual compressive stresses at the edge. As a result, stress cracks cannot migrate from the surface into the interior of the component, which suggests that the mileage is extended.
In order to estimate the resulting hardness development of the wheel tyres on the vehicle and the resulting material removal, these were calculated for the first 170 km of the mileage of the vehicles used by the cooperating light rail operator with the aid of a multi-body simulation model in the SIMPACK software. For this purpose, the correlations between hardness development and the number of overrolls determined in the test bench tests were implemented in the model. It was found that a lower scattering of the initial hardness along the wheel surface led to more uniform wear. In addition, wear is reduced in accordance with current laws by an increased surface hardness.
4. Field tests
Field tests will be carried out to determine whether deep and finish rolling has a positive effect on the wheel’s properties. In these tests, a rail vehicle is observed over a certain period of time on a representative route of the light rail operator. Figure 8 shows the averaged hardness development on the wheel running surface over the mileage.
Besides the hardness development, the calculated wear masses are shown in figure 9.
In addition to hardening, rolling also causes a homogenisation of the hardness dispersion. The wear model also shows more uniform wear behaviour with homogeneous hardness distribution. A correlation between hardness and wear could be shown both in the wear model and in the field test. The hardness curve in the field test does not correspond to that from the test bench test. So far, no significant differences in the hardness development between the rolled wheels and the reference wheels could be detected in the field test. Similarly high wear mass of unrolled and rolled wheels. The execution of the field tests in passenger operation has not yet been completed.
Hegenscheidt-MFD: 15 new APPRENTICES starting their working lives
15 new apprentices are now striding through the factory gates of the Erkelenz machine tool company, Hegenscheidt-MFD, each morning. The company is therefore continuing a tradition that has now lasted for decades: The training of specialists for their own, constantly growing need for specialists.
For decades now, the company has been training an above-average amount of young people in various occupations that require training. Alone in the last 10 years, over 100 youths were able to successfully conclude their training programs. The group of Hegenscheidt apprentices currently accounts for a good 11% of the overall workforce. In total, 48 young people are currently completing a training course at Hegenscheidt-MFD in the following professions: industrial mechanic, cutting machine operator, mechatronist, electronics technician, technical product designer and industrial clerk. The occupations that require training, i.e. industrial mechanic, mechatronist, electronics technician and technical product designer are also offered in the form of combined studies.
Employees as the strongest success factor
The Hegenscheidt-MFD managing partner, John Oliver Naumann, was also very satisfied about the commitment shown towards high-quality vocational training. “In the potential of well-trained specialists, we are not only seeing a significant locational advantage in a high-wage country like Germany, but also a necessity: It is only with well-trained specialists that a company like us, which is producing technologically sophisticated products, will survive on the world markets.” A large amount of those currently employed, he added, were recruited via the vocational training courses offered by the company. Many families are also represented by several specialist generations at Hegenscheidt-MFD. “Once vocational training has been completed successfully, many paths are open to the graduates within our company, right up to management level. This is why today, former trainees are holding positions such as department manager, foreman, group and team leader and shift supervisor through appropriate advanced training.
At the moment, six employees of the company – of those, two are full-time employees – are occupied with the training courses for the various departments. These instructors are also committing themselves to activities outside the company, i.e. through their commitment on the selection boards at the Chamber of Industry and Commerce, for high-quality vocational training. This is one of the reasons why Hegenscheidt APPRENTICES complete their training with very successful results and because of the extraordinarily intensive support they are given. One or even several apprentices regularly receive awards as the best of his/her age-group at the Chamber of Industry and Commerce in Aachen.
The company supports young people even beyond the classic combined vocational training. Each year, a large number of trainees are accepted by the various education organisations, advanced technical colleges and universities as well as the general education schools for career choice orientation.
Technology with tradition: As a machine construction company, Hegenscheidt-MFD is one of the world market’s leaders in the railway and automotive business areas. The company is part of the Niles-Simmons Hegenscheidt Group (NSH Group), which is among the 50 largest machine tool manufacturers in the world and which combines more than 175 years of experience of machine tool construction in Germany and the US. Hegenscheidt-MFD currently employs 437 people at the Erkelenz site; of those, 48 are trainees.
Photo (from left to right)
Mr Michael Königs – Personnel manager
Mr Andreas Hetterle – Instructor
Mr Frank Risters – Mechanical design manager (instructor)
Mr Fabian Schulz – Industrial mechanic apprentice
Mr Jens Beckers – Industrial mechanic apprentice (combined studies)
Mr Tobias Kox – Cutting machine operator apprentice
Mr Mario Engels – Industrial mechanic apprentice
Mr Alexander Schmitz – Industrial mechanic apprentice
Mr Kai Lausberg – Cutting machine operator apprentice
Mr Hendrik Abzug – Cutting machine operator apprentice
Mr Lars Müller – Technical product designer apprentice (combined studies)
Mr Thomas Hilgers – Industrial clerk apprentice
Mr Peter Krekels – Mechatronist apprentice
Mr Christian Mülders – Technical product designer apprentice (combined studies)
Mr Lars Paassens – Mechatronist apprentice
Mr Thomas Zaunbrecher – Electronics technician apprentice (combined studies)
Ms Hannah Gier – Technical product designer apprentice (combined studies)
Mr Nils Märtens – Mechatronist apprentice (combined studies)
Dr Winfried Büdenbender – CEO
Mr Achim Heldens – Manager for industrial vocational training
Mr Christian Frentzen – Instructor
Jacqueline Drescher, Press and Public Relations, Hegenscheidt-MFD
Contact: firstname.lastname@example.org – mobil: 0177.33 93 630
European Football Championships 2012 – Lambertusmarkt Erkelenz on 09 June, 2012 – The Germany vs Portugal game Impressions of Public Viewing – Sponsored by Hegenscheidt-MFD
The President of the Chamber of Industry and Commerce, Bert Wirtz, awards examinees with top marks Hegenscheidt-MFD examinees achieve the grade “very good”
With outstanding results, the apprentices Michael Bronckhorst, industrial mechanic, and Heinrich Pfaffenrot, cutting machine operator on lathes, have achieved something that all apprentices dream about: They have completed their training with the near-perfect grade of “very good”. For this special performance, they received an award in a ceremony together with 95 other “very good” examination candidates from the President of the Chamber of Industry and Commerce, Bert Wirtz, at the Chamber of Industry and Commerce in Aachen.