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With most spindle bearing applications, the use of non-locating bearings is a vital necessity in order to compensate for thermal expansion. The use of cylindrical roller bearings and Floating Displacement bearings (FD bearings) makes it possible to carry out this necessary length compensation within the bearing. Cylindrical roller bearings boast high load carrying capacity with medium speed capacity while FD bearings offer maximum speed capacity with lower load capacity that is sufficient for a wide range of applications.
Thanks to consistent friction minimizing measures, Schaeffler was able to develop a high-speed cylindrical roller bearing with steel rollers that can achieve speed levels 80 percent higher than those of conventional cylindrical roller bearings and even surpasses the speeds achieved with hybrid bearings and brass cage. Nevertheless, its load carrying capacity is only slightly lower. The bearing boasts optimized contact geometry and an outer lip riding PEEK cage of considerably less friction than rolling element riding machined brass cages. Due to lower operating temperatures resulting from lower friction, the stress on the lubricant in the rolling contact zone is lower too. This results in longer grease life.
The Floating Displacement bearing is the ideal non-locating bearing for maximum speeds if the bearing is not subjected to heavy external loads. Compared to highspeed hybrid cylindrical roller bearings, it offers a much more cost-efficient solution. Another advantage is that it is less sensitive to changes in radial clearance during operation. In the newly developed universal Floating Displacement bearing, the manufacturing radial clearance of the bearing is now adjusted depending on the bore diameter. As long as a specified tolerance is adhered to for the shaft diameter, the radial clearance will be within the desired, safe zone after assembly. In this way, mounting and stock keeping become much easier. Furthermore, it is also possible to mount several Floating Displacement bearings next to each other in order to increase the load carrying capacity.
In main spindles with high radial load carrying capacity, there are often several bearings combined to form tandem or triple sets on the locating bearing end. As all bearings in one set show the same protrusion, they also have the same preload and carry the same loads. During operation, however, differences in temperature due to heat input from the drive rotor will arise, which cause changes in preload. In particular, the inner ring is usually warmer than the outer ring, which leads to higher preload in the bearing closer to the motor because the outer bearing is unloaded due to the axial expansion of the shaft. In these solutions, which are especially optimized for one particular application, the bearing closer to the motor has a defined radial clearance on the outer ring. Due to this characteristic, the outer ring of this higher loaded bearing can yield radially. In the newly developed, thermally robust tandem set, the higher loaded bearing, which is closer to the motor, also deflects more readily than the outer, less loaded bearing. This, however, is achieved through a well-aimed adjustment of the bearing rigidity. While the universal tandem set carries equal proportions of load at the outer and inner rings at identical temperatures, differences in preload arise under these conditions with the thermally stable tandem set. In the scope of temperature differences between the inner and outer rings typical of motor spindles, however, the preload differences in a thermally stable tandem set are lower than in the universal tandem set. The more even load carrying of both bearings reduces the stress level in the rolling contact zone of the inner bearing. At the same time, the dynamic and static rigidity of the spindle is improved. This results in higher operational reliability, improved spindle performance and, where applicable, better workpiece quality.
When starting up a grease-lubricated spindle, a grease distribution run ensures the optimal distribution of grease in the bearing. The duration of the grease distribution run can be reduced through skilful pre-distribution of grease. The new greasing methods enable the positioning of grease selectively, close to the points where it is supposed to be after grease distribution. This reduces the grease distribution run period and thus costs too. In addition, the grease distribution run becomes more reliable especially when only low centrifugal forces occur with comparably low maximum speeds, so that it takes longer for the grease to abut the outer ring.
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