| Competence gives security |
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Gerald Nonnast, Schaeffler KG
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Extremely high machining forces, workpiece mass and machined material volume, high radial, axial and tilting moment loads, highly dynamic accelerations, speeds that were inconceivable some years ago, integral measuring systems capable of measuring angles to an accuracy of a few seconds, very high machining accuracies and surface quality values, long tool life, very low system costs: this is often the content of a design brief when it comes to bearing arrangements for rotary tables, face plates, machining heads and high precision swivel axes. The following paper gives a focused overview of the design and product competence of the Schaeffler Group on bearing units for rotary axes.
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The right bearing for every application
The fact that not every bearing is equally suitable for such complex and multilayered requirements as described in the introduction is understandable. We will not therefore satisfy this challenging area of application with just one design. This would show insufficient thought and would seriously contradict our principle of seeking the best solution in both technical and economic terms for the bearing arrangement. We therefore have the most comprehensive range of rotary table bearings.
Based on this product range, proven on many occasions, our customers can select the appropriate bearing for their application. This gives security, a technical advantage and a high level of cost-effectiveness. In the machine tool, only the right bearing arrangement will ensure perfect machining results and the minimum of non-productive time. In bearing selection, we offer support through both our knowledge and our many years of experience in this field, see guide to bearing selection.
A guide to bearing selection
Rotary table bearings have, due to their geometry and design, quite specific characteristics. For the design of the bearing arrangement, this means: if it is to be operationally secure, low-maintenance or maintenance-free and economical, the correct bearing type of the correct size must be used. This will prevent over dimensioning and at the same time ensure that the bearings have a long operating life.
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Step-by-step process
In the design of the bearing arrangement, it is necessary to take account of and weigh against each other a number of performance parameters that, in some cases, differ from each other and, in none too rare cases, conflict with each other. Nevertheless, standardization can often be achieved within certain limits, since many rotary axis bearing arrangements can be traced back to similar essential parameters.
Based on our long experience in the design of such bearing arrangements, selection is generally found to be carried out in the following stages:
● Preparation of a design brief in which all influences on the bearing arrangement are recorded. The more precisely these values are known and the more comprehensively they are taken into consideration, the higher the operational security and cost-effectiveness with which the bearing position can be designed. The influences that come into question here are as follows: loads, speeds, operating temperatures, mounting space, environmental conditions, required rating life, friction, lubrication and maintenance, fitting and dismantling of the bearings, and annual quantity required.
● Preliminary bearing selection in accordance with the catalogue. Specific technical publications provide all the information that is required for preliminary selection of a bearing.
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● Design of the bearing arrangement using BEARINX . With the aid of this modern calculation program, it is possible to achieve secure dimensioning of the bearing. This takes account of factors including: - the machining forces and the forces arising from the drive system, as well as the workpiece mass - the rigidity of the entire system, see also Overall rigidity of the bearing unit- the influence of heat distribution in the subassembly on the bearing preload, see Thermal simulation - the influence of lubrication on the basic rating life - the holistic consideration of all bearing positions in the machine tool. If required, the bearing positions for the feed and main spindle bearings, rotary table bearings and linear guidance system can be calculated in a system model. The selection and arrangement can be additionally optimized by the calculation of bearing variants.
Overall rigidity of the bearing unit
The overall rigidity of a bearing position is a description of the magnitude of the displacement of the rotational axis from its ideal position under load. The static rigidity thus has a direct influence on the accuracy of the machining results. Normally, rolling bearing catalogues only state the rigidity of the rolling element set. Since the deformations are often very small and the bearing rings are very rigid compared to the rolling contact, this is sufficient for most applications. The situation is different for bearing arrangements in machine tools, for example in the case of rotary axes. Here, standard practice is to use bearing units mounted by means of screws, such as our YRT bearings. In such cases, the rolling element rigidity alone is no longer adequate to describe the rigidity of the bearing position relevant to actual practice. It is rather the overall rigidity of the bearing unit that is decisive here. Naturally, this is the value that we state in the product documentation. It is of greater assistance to our customers if we give them rigidity values that are relevant to their application, see Figure 2. The diagram shows clearly the substantial differences and the significance of using data that are not comparable with each other. Furthermore, transparent and comparable statements stand for secure operation of the bearing arrangement and the required quality of the machined products. It is thus possible to achieve the development goals, i.e. the precise and dimensionally stable machining of workpieces even under high machining forces.
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Accuracy
Bearings for rotary tables have high running accuracy. In order to transfer this as effectively as possible to the machine subassembly, clearance-free fits on the rotating component are necessary. This is generally only possible by matching the adjacent components to the actual bearing diameter. However, this maximum running accuracy is not required in all applications. It is therefore important that, to begin with, the bearing tolerances have a level of precision appropriate to practice. If the diameter tolerance is too large, the good running accuracy cannot be transferred to the subassembly.
As a result, this will reduce the accuracy of the workpieces to be machined. In selection of the bearing, both accuracies (running accuracy and diameter tolerances) should therefore always be checked, in order to eliminate the need for any additional work, such as grinding of the shaft.
Location
Our rotary table bearings are screw mounted directly to the adjacent construction by means of fixing holes. This gives a considerable reduction in the mounting work required. The number of screws is defined such that the INA/FAG bearing units are successful with the smallest possible number but, at the same time ensure maximum frictional locking against slippage of the bearing and the highest rigidity of the screw connection. If fewer fixing screws are used for the same thread diameter, the rigidity will be considerably reduced and there is a risk that the bearing will reep in a radial direction. In a high accuracy application, the results would be catastrophic.
Thermal simulation
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Rotary table systems, combined with direct drives and new bearing technologies, give a clear advantage in dynamic terms. This in turn gives a significant expansion of the application spectrum of rotary axes. The performance capacity of such systems and thus the operational security of the bearing arrangements is often restricted, however, by thermal factors.
In order to minimise the risks in this area, two thermal simulation models of differing complexity were developed as part of a development project for high speed rotary tables. These were a simple model based on analogies from the field of electrotechnical engineering and a complex FEM model incorporating BEARINX , see Figure 3.
The latter also takes account of values such as temperature development, thermal conduction, convection and thermal radiation. For example, the distribution in the subassembly can be checked and, if necessary, modifications can be made to the motor cooling system or a thermal separation between the bearing and motor can be provided.
As part of a test-based validation, the different calculation methods can then be compared, see Figure 4.
These tools provide an economical means of supporting the design of highly dynamic rotary axes. It is thus possible to achieve rapid, reliable selection of the bearing, drive, motor and measuring system on the basis of a draft design or to check an already selected combination for its performance capacity and operational security.
Comprehensive product documentation
Comprehensive documentation is available on INA/FAG rotary table bearings. Catalogue HR 1 and Technical Product Information TPI 120 contain comprehensive descriptions of the bearings. TPI 103 provides all the information required for fitting and maintenance of the high precision bearings. Initial operation of the measuring system is described in MON 18, see Figure 5. In conjunction with the comprehensive dimensional and load data in the tables, the designer thus has all the information required for the correct design and reliable operation of the bearing arrangement.
Many errors are possible at the fitting and maintenance stages. If the necessary data are not available, fitting and maintenance defects may easily occur that have a harmful effect in operation. This can be prevented by use of our documentation, which is of course provided both in printed form and as an online version. There is also a series of special papers and publications covering the subjects of current research relating to bearing arrangements for rotary tables. These publications show fundamental development work and can be obtained upon request.
All-round service based on systems approach
Our bearings are modelled (simulated) using FEM, thoroughly checked in all aspects and optimized in tribological terms by an experienced lubricant engineering team. Their function is assured by means of long term tests on in-house test rigs that are designed exclusively for the testing of rotary table bearings. We analyze the friction in the bearing and apply targeted measures to reduce the friction values in the entire system, see Figure 6. We of course incorporate additional functions in the product, such as precise measurement of the angular position. Through its IDAM subsidiary, the Schaeffler Group has direct drive technology available as in-house know-how across organizational boundaries. We carry out measurements directly on customer machinery, run customer specific application tests and offer support on-site in the mounting of bearings.
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