Rolling with less friction

Rolling bearings are key components for mechanical engineering, where every detail has to be right. For this reason, almost all of their components are ground. In order for this to be as efficient as possible, partners are needed that use procedures and tools to design optimal processes.
Rolling with less friction
Rolling bearings are key components for mechanical engineering, where every detail has to be right. For this reason, almost all of their components are ground. In order for this to be as efficient as possible, partners are needed that use procedures and tools to design optimal processes.
 
Rolling bearings have the task in technical systems to absorb radial and/or axial forces while allowing the rotation of an axle. In contrast to lubricated slide bearings, they use rolling bodies, reducing frictional resistance and thus achieving high efficiencies. In order to meet the requirements of different industrial applications in full, there are a variety of types of bearings. Depending on the specific application and existing framework conditions, the various bearings can react to varying load directions or load capacities.
 
The basic design forms of rolling bearings are ball, cylinder, needle, tapered roller and spherical roller bearings. Most widely used are the variant-rich ball bearings; designed as angular contact ball bearings, they can absorb both radial and axial forces.
 
Grinding the critical process in rolling bearings manufacturing
When the requirements for the maximum load capacity increase, cylindrical or tapered roller bearings are used. If they are equipped with conical roll barrels, the radial and axial load capacity can be increased further. Such bearings are often used as wheel bearings in vehicles. Needle bearings are another extremely important type of bearing for the automotive industry. Due to its compactness, it is primarily suitable for gearboxes and motors.
 
Because bearing characteristics – such as the magnitudes and directions of load, accuracy, durability, friction, and low-noise operation – are directly dependent on their surface and edge zone properties, which in turn are significantly influenced by the finish, grinding is one of the most important process steps in bearing manufacturing.
 
Bond development is a key competence for abrasive makers
The focus of grinding is on the functional surfaces of the outer and inner rings as well as the roll barrels themselves. Different grinding processes and bonded abrasives are used in each case, and as a result, a high level of grinding technology expertise is required to produce the necessary surface quality with damage-free edge zones.
 
One of the core competencies 
of a grinding tool manufacturer is bond development. For example, Hermes Schleifmittel specialises in ceramic bond systems for conventional as well as high-hardness grinding wheels. Conventional wheels are tools with aluminium oxide, sintered aluminium oxide or silicon carbide as the cutting material, whereas the term high hardness refers to the use of cubic boron nitride (CBN) or diamond.
The bond has the task to fix the cutting materials and connect together. At the same time, the amount of bond in combination with the cutting material concentration determines the resulting porosity of the tool.
 
For the grinding process, micro-splitting of the grains is sought, so that continuous self-sharpening of the tool takes place. This property also depends largely on the type of bond used and the bond volume. An optimally designed grinding tool results in impressive profile retention and performance along with a corresponding excellent component quality. The porosity must be large enough to transport chips and coolant, but at the same time not too large, because otherwise sufficient bond volume is no longer available.
 
A solution to this conflict of objectives can be achieved, at least in part, with the help of newly developed bonds. Such high-performance bond systems – for example Hermes Vitra – have a higher specific strength than usual bonds and an optimised microstructure. High porosity and low tool wear are no longer a contradiction. In addition, it is possible to work at grinding rates up to 125 m/s. As far as high-hardness grinding wheels are concerned, such grinding rates are not uncommon and essentially represent the state of the art. But even with CBN abrasives, bonds with higher specific strength act as a key to greater performance, because here too increased porosity is possible with consistent profile retention.
 
Grinding process for roll barrels
Although the basic shapes of rolling bearing – and thus also of the roll barrels – differ, they are fundamentally rotationally symmetrical. Outer surfaces of roll barrels, for example, are therefore often highly productively ground centrelessly. Depending on the shape of the roll barrels, either centreless plunge or centreless through-feed grinding is used. The tools used for this are grinding tool sets, which consist of several individual wheels. As a rule, these are conventional grinding wheels whose progressive bonding enables them to achieve high performance.
As with many other grinding processes, an increase in performance is possible with the help of ceramic-bonded CBN tools. Despite the higher price, these high-performance abrasives are impressive due to their short grinding times and, above all, a significantly longer tool life. As a result, costs per component are typically lower. If a CBN grinding tool set is chosen with a significantly lighter CFRP core body, you also benefit from a reduced spindle load and simpler handling and installation.
 
Finalising outer rings in just three grinding steps
For grinding outer rings, three different grinding methods and corresponding bonded abrasives can be used. Centreless grinding is mostly used for grinding the circumferential surface of the outer ring. The rings are then often brought to their final dimension using through-feed grinding with conventional abrasives. The inner diameter of the outer ring, which forms the race of the roll barrels and thus must have optimum friction properties, is ground using special race grinding wheels. Due to the large specific contact surface during internal cylindrical grinding and the resulting high tool loads, ceramic-bonded CBN tools are used for this process.The required end-face parallelism of the outer and the inner ring is realised by dual-face grinding of the side surfaces with resin-bonded aluminium oxide abrasives.
 
For the internal cylindrical grinding of the bore of the inner ring, similar requirements apply as for the race grinding of the outer ring. Again, high-hardness grinding wheels are primarily used in this case. Grinding of the lateral boundary of the race on the inner ring, the rib, is performed with profiled conventional abrasives.
 
Subsequent grinding of the race is external cylindrical grinding using a plunge process and is also performed with conventional grinding wheels. A special feature: High-performance bonds are used for grinding rates up to 125 m/s. In this way, conventional tools can also be operated in part in the process variable range of CBN grinding wheels. The result is highly productive, extremely economical grinding.
 
High-hardness abrasives reduce inner ring grinding times
As can be seen, grinding in the manufacture of rolling bearing components is a complex process that is critical to their final quality and requires a high level of expertise. The sequence of different grinding processes and the different types of tools must be coordinated so that the required surface finish quality is always achieved with high process reliability and defined edge 
zone properties.
 
Because, apart from quality, focus is increasingly placed on the productivity of the individual production processes for rolling bearing manufacturers, continuous development of abrasives, process expertise and customer-specific services are essential for grinding tool manufacturers. Hermes Schleifmittel takes account of this fact with specially developed high-performance bonds as well as with its consulting expertise bundled in the HAI – the Hermes Abrasives Institute. In this facility, experienced application engineers provide customers with comprehensive support in analysing and optimising their processes.
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About the Author:
Dr Tim Göttsching is the Head of Industry Management Bonded at Hermes Schleifmittel. He can be reached on email:t.goettsching@hermes-schleifmittel.com
Frank Wolf is the Head of Product Engineering at Hermes Schleifmittel. 
Michael Engers is the Application Engineer at Hermes Schleifmittel.
To find all the information about products and production processes that companies need to best solve their market-specific tasks, visit www.hermes4bearings.com
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