The new FAG spindle bearings RS have large size balls, a nominal contact angle of 20 degrees and an internal construction for optimized friction.
The new thermally resilient FAG cylindrical roller bearing TR combines, for the first time, the reliable non-locating bearing function of cylindrical roller bearings with suitability for high speeds and varying temperature differences.
Where very high requirements for speed capacity and machining performance are present, the main spindles of a machine tool are generally designed as direct drive units or so-called motor spindles.
Comparison of conventional and new design of a motor spindle – working side
Comparison of conventional and new design of a motor spindle – rear side
Mounting of spindle bearings
Head of Communications
Schaeffler Group Industrial
Tel. +49 9721 91-3400Martin Adelhardt
2011-09-19 | 000-003-078 DE-EN
SCHAEFFLER GMBH, HANOVER/SCHWEINFURT
For motor spindles, Schaeffler Group Industrial has succeeded in combining very high speed capability with high load carrying capacity and the ability to withstand changing operating conditions by means of its new high performance spindle bearing series RS and the thermally resilient cylindrical roller bearing TR. It is thus possible to achieve significant simplification of bearing supports for direct drive motor spindles compared to conventional solutions.
The new FAG spindle bearings RS have large size balls, a nominal contact angle of 20 degrees and an internal construction for optimized friction. Due to these characteristics, RS bearings are robust in relation to changes in internal clearance and combine the speed capability of high speed series with small balls and the robust design and load carrying capacity of bearings with large balls. The new thermally resilient FAG cylindrical roller bearing TR combines, for the first time, the reliable non-locating bearing function of cylindrical roller bearings with suitability for high speeds and varying temperature differences.
Where very high requirements for speed capacity and machining performance are present, the main spindles of a machine tool are generally designed as direct drive units or so-called motor spindles. In this design, the motor is located directly on the precisely supported shaft of the spindle. In order to achieve optimum radial runout and high guidance accuracy, the bearings must be preloaded and are therefore typically located on both sides of the motor for optimization of the static and dynamic rigidity. Due to the high performance density, temperatures of up to 150 degrees Celsius occur through electric losses in the rotor during operation. These lead to substantial temperature gradients between the shaft and housing that must be compensated by the bearing support. In conventional bearing supports, the locating and non-locating bearings are provided in the form of a spindle bearing preloaded by springs. The preload is applied by means of springs between the bush and the housing. Sliding motion of the outer ring can compensate changes in shaft length and changes in the overhang of the bearings as a result of speed and temperature influences. Due to the use of sliding bushes, a drawn cup ball bearing and springs, the adjacent construction is costly to realise and mounting is complicated. The sliding function of the bush may cease to operate as a result of tilting and where there is too little clearance due, for example, to temperature differences.
Simpler construction and mounting
The new high performance series RS and the thermally robust cylindrical roller bearing TR open up new possibilities in the design of main spindle bearing supports. Where a rigidly preloaded arrangement is used, RS bearings are significantly less sensitive to influences arising from interference, speed and temperature than conventional spindle bearings. Where RS bearings are used in conjunction with an improved speed capability, it is therefore possible to use rigidly preloaded arrangements, which are of a much simpler design than spring preloaded systems. This gives a simplified spindle design as well as increased rigidity.
The thermally resilient cylindrical roller bearing can also be reliably operated even where very large temperature differences are present. It is fitted with a flexible outer ring that can compensate the distortions due to changes in the radial internal clearance. The outer ring, which is still of standard dimensions, has two undercuts. The outside diameter has a slight radial recess between the undercuts. As a result, the central portion of the outer ring can expand in a radial direction in response to varying temperature differentials. This means that the contact force and thus the contact pressure on the outer ring increase significantly more slowly than in the case of a standard cylindrical roller bearing. The thermally robust cylindrical roller bearings are typically fitted with a PEEK cage with optimized friction that is guided on the outer ring. In this design, they can achieve outstanding speeds with grease lubrication that are 60 percent higherthan cylindrical roller bearings with brass cages guided by the rolling elements.
Optimized lubricant distribution
In addition, the PEEK cage is only guided on one side on the outer ring. This facilitates optimum lubricant supply. Excess lubricant can be carried significantly faster out of the bearing. This decreases the time required for the grease distribution process. The risk of damage to the grease by overrolling is significantly reduced. After running-in, a lower temperature level with reduced variation is achieved.