Schaeffler Group Industrial has recently introduced solutions for the bearing of main spindles under the FAG product brand with the high-performance RS series and the thermally robust TR cylindrical roller bearings, which in many cases also enable the bearings to be adjusted rigidly for motor spindles. As a result, parts of the surroundings are partially unnecessary and the assembly of the bearings is greatly simplified.

Motor spindles not only place the highest demands on the speed of spindle bearings. Due to the high power dissipation in the rotor, large temperature gradients can occur between the shaft and the housing. Spindle bearings in directly driven main spindles are therefore typically elastically preloaded in order to enable the compensation of different changes in length due to changing temperatures.

Typical construction of motor spindle bearings

If the main spindles of a machine tool have to meet the highest demands in terms of turning capacity and cutting performance, these are now usually designed as directly driven units. Such spindles are commonly referred to as motor spindles. In this design, the motor sits directly on the precisely mounted shaft of the spindle. It has become generally accepted due to its compact design, high power density and dynamic advantages due to low masses and mass moments of inertia.

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The bearings, which must be preloaded due to the high demands on concentricity and guidance accuracy, typically sit on both sides of the motor to optimize the static and dynamic stiffness.

Due to the high power density, electrical losses in the rotor cause temperatures of up to 150 ° C during operation. These lead to large temperature gradients between shaft and housing, which must be compensated by the storage.

Working side storage

On the working side of the bearing of a main spindle, the shaft must be guided by the bearings axially and radially as accurately as possible. In addition, the forces must be absorbed from the processing.

Figure 1 shows a typical mounting structure of a motor spindle in the form of a spring tandem O bearing. The two front bearings are axially fixed both on the shaft and in the housing. The outer ring of the anvil is mounted in a socket which is slidable to the housing.

The bias voltage is generated by springs between the socket and housing. Due to the sliding movement of the outer ring, changes in the length of the shaft as well as changes in the overhang of the bearings due to speed and temperature influences can be compensated. The preload in the bearing changes only insignificantly, since the stiffness of the springs is much lower than that of the bearings.

Back storage

The rear bearing mainly serves to guide and support the shaft. In the case of long-protruding tools, radial forces must also be absorbed from the machining.

Figure 2 shows a typical structure of the rear mounting of a motor spindle in the form of a spring-biased spindle bearing. Again, the outer ring is fixed in a socket which is displaceable to the housing.

The rear bearing is the actual floating bearing. The shifts that occur are greater than on the working side, as the entire change in length of the shaft due to temperature changes must be compensated for. For this reason, the sliding bushing is often located in a spherical bushing, which enables safe sliding movement even over long distances and under preload.

Advantages and disadvantages of spring-loaded bearings

The advantage of spring-mounted bearings thus lies in the ability to compensate for projections and length changes without excessive forces on the bearings occur. On the other hand, this solution also has a number of disadvantages:

Sliding bushes, ball bushings and springs make the surrounding construction complex and the assembly complicated. The sliding function of the socket can fail due to tilting and too little clearance, for example due to temperature differences.

Stress in the effective direction of the spring can be absorbed only up to the amount of biasing force of the spring. The shaft is suspended very softly in the effective direction of the spring and thus susceptible to vibrations in the axial direction.

The new high performance series RS and the thermally robust cylindrical roller bearing TR combine the highest speed suitability with high load capacity and insensitivity to changing operating conditions. In sum of their properties, they open up new possibilities for constructive design of motor spindles.

Robust and durable RS spindle bearings with high speed suitability

RS spindle bearings are large-spherical bearings with a nominal pressure angle of 20 degrees and friction-optimized internal construction (Fig. 3). They combine the speed suitability of small-sized high-speed series with the robustness and load capacity of large-spherical bearings.

Figure 4 shows a comparison of the speed limit and the dynamic load rating of various spindle bearing designs of size 7014.

Universal spindle bearings are manufactured with a defined radial clearance. This determines the pressure angle. A defined projection determines the preload class. A reduction in the radial clearance in the bearing due to influences from assembly, speed and temperature leads to an increase in the supernatant. As a result, preload increases in rigidly set bearings (Fig. 5).

The optimized internal construction, the large ball cross section and the pressure angle of 20 degrees make RS bearings robust against changes in the radial clearance.

Figure 6 compares the increase in preload due to the reduction in radial clearance in conventional large-spherical bearings, small-ball high-speed bearings and the high-performance RS series.

In comparison, the relative change in the preload force is the strongest in the small-spherical design. For the high-performance RS series, it is only about half the size.

Image 7 shows measured operating temperatures of a fast rotating motor spindle, where bearings were made with rigidly fixed ceramic ball bearings. Despite the high speed characteristic of almost 1,5 million mm / min for this bearing arrangement and for the grease lubrication, the measured temperatures at the outer ring remain stable below 60 ° C. In this case, this corresponds to an over-temperature of 35 degrees with respect to the environment.

Thermally robust cylindrical roller bearings N..TR -

As an ideal floating bearing, the cylindrical roller bearing has undergone decisive further development in terms of its speed suitability. However, due to its sensitivity to temperature differences, it is rarely used in motor spindles. The new thermally robust FAG cylindrical roller bearing N ..- TR- combines for the first time the safe floating bearing function of cylindrical roller bearings with the suitability for high speeds and changing temperature differences. It will therefore represent the ideal floating bearing for a large number of motor spindle applications and will have a strong influence on the designs there in the future (Fig. 8).

Safe floating function

Single-row precision cylindrical roller bearings are ideal as pure radial bearings, which are designed without collar on the outer ring, ideal as a floating bearing. Inner ring and roller ring can be adjusted freely relative to the outer ring in their position. Since the relative displacement takes place within the bearing between the rotating rolling elements and the outer ring, a failure of the floating bearing function is not possible.

For precise guidance of the shaft and to prevent slippage damage, when mounting cylindrical roller bearings in high-speed main spindles, the radial clearance must be set so that the bearing runs without clearance or with slight preload.

The radial clearance changes cause a radial tension in the bearing of cylindrical roller bearings. Due to the increased surface pressure, the friction in the rolling contact increases. The rolling element heats up. This increases the preload in the bearing and increases the friction further. If the resulting additional heat can no longer be dissipated, the radial tension leads to bearing failure.

Radially compliant system

The thermally robust cylindrical roller bearing is equipped with a flexible outer ring, which can compensate for tension by changing the radial clearance. The outer ring, which still has standard dimensions, has two grooves (Fig. 9). Between the punctures, the outer diameter is slightly withdrawn radially. As a result, the central region of the outer ring can radially expand with variable temperature differences.

Due to the resilience of the outer ring, surface pressures increase less when the radial clearance changes during operation.

Figure 10 shows contact force and surface pressure as a function of the temperature difference in a clearance-mounted N1011-K-PVPA1-SP bearing in standard design and a corresponding bearing with yielding outer ring N1011-K-TR-PVPA1-SP.

Due to the resilience of the outer ring, the contact force and thus the surface pressure on the outer ring in the thermally robust cylindrical roller bearing rise much slower than in the standard cylindrical roller bearing. Even with a temperature difference of 40 K, the surface pressure is still well below the fatigue limit of 1500 MPa.

Lower lubricant load

The lower contact forces and surface pressures result in lower friction and less stress on the lubricant. The compliance of the outer ring prevents uncontrolled increase in bias by heating the rolling element. Non-backlash mounted thermally robust cylindrical roller bearings can thus be operated safely even with temperature differences of 40 K.

Suitability for high speeds

The maximum speed of bearings is limited primarily by the friction generated in the bearing. If the heat generated during operation can no longer be released to the environment, the storage temperature will continue to rise. The failure of the lubricant ultimately leads to bearing failure.

At the EMO 2007, the Schaeffler Group Industrial introduced FAG High Speed ​​Cylindrical Roller Bearings with a speed capability that was 60% higher than the previous standard. These bearings achieve speed characteristics with steel rolling elements, which until then were only possible with ceramic rolling elements. The speed increase was achieved by consistently minimizing friction at the two major heat sources in the bearing, the rolling contact and the cage friction.

As standard, precision cylindrical roller bearings are equipped with a brass cage guided on the rolling elements. In contrast, high-speed cylindrical roller bearings feature a friction-optimized PEEK cage guided on the outer ring. Furthermore, the friction in the rolling contact was reduced by an optimized contact geometry on the roller and inner ring.

In the version with PEEK cage, thermally robust cylindrical roller bearings have the same internal design optimization as high-speed cylindrical roller bearings and also achieve their outstanding speeds.

Figure 11 shows the measured operating temperatures of a thermally robust cylindrical roller bearing N1011-K-TR-PVPA1-SP. The bearing, which, according to the catalog, has a limit speed of 26.000 rpm with oil-air lubrication^-1 was able to operate reliably with 34.000 min^-1 operate.

Optimized lubricant distribution

Cylindrical roller bearings are sensitive to over-lubrication. Especially with grease lubrication, it is important that the grease is distributed quickly and excess grease between the rolling elements is removed, so that no damage due to high flexing work during the rolling occurs.

A PEEK cage guided on both sides of the outer ring ensures low friction during operation. Regardless of the position of the inner ring, at least one cage web is always guided on the outer ring. The possible displacement of the inner ring is characterized in both directions the same size and is - as in the cylindrical roller bearing guided on the rolling element cage - limited by the width of the raceway on the outer ring.

Disadvantage of the cage guided on both sides, however, is that excess lubricant between the rolling elements can only be transported through the small guide gaps to the outside. This prolongs the grease distribution run in grease-lubricated bearings and there is a risk of lubricant damage due to overrolling.

On the non-locating side of motor spindles, the displacement during operation always takes place in only one direction. Due to the heat losses from the engine and the external cooling, the shaft is warmer than the housing. As a result, the inner ring moves away from the engine. This allows the use of a single-sided guided cage, since the cylindrical roller bearing is always mounted so that the larger diameter of the inner ring bore on the motor side is (Figure 12).

With a cage guided on one side, excess lubricant can be conveyed much faster from the bearing. This reduces the time required for the grease distribution run. The risk of damage to the fat by rolling over is significantly reduced.

Customer experience shows a faster fat distribution run with lower maximum temperatures. After the inlet results in a lower temperature level with less scattering.

New design options for motor spindle bearings

The new high-performance series RS and the thermally robust cylindrical roller bearing TR open up new possibilities in the design of main spindle bearings.

For rigid applications, RS bearings are much less sensitive to influences of overlap, speed and temperature than conventional spindle bearings. If the electrical losses on the rotor can be limited so that the occurring temperature differences between the shaft and the housing remain moderate, the use of RS bearings in conjunction with the improved speed suitability instead of spring adjustment can result in structurally simpler rigid settings (Fig. 13).

This simplifies the spindle design and at the same time increases the rigidity. The user of spindle bearings thus receives greater design freedom. The thermally robust cylindrical roller bearing can be safely operated even at very high temperature differences.

The safe non-locating function with simple construction, the insensitivity to temperature changes and the safe and fast grease distribution run make the thermally robust cylindrical roller bearing in combination with the suitability for highest speeds in the sum of its properties the ideal floating bearing now also for motor spindles (picture 14).

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