Design considerations
Design considerations The majority of super-precision bearings are used in machine tool spindles. Most of the information required when designing a bearing arrangement for maximum bearing perform ance can be found in the following sections.
Bearing arrangements A bearing system, which is typically used to support a rotating shaft, generally requires two bearing arrangements. Depending on the requirements, such as stiffness or load directions, a bearing arrangement consists of one or more (matched) bearings. Bearing arrangements for heavy loads Lathe spindles are typically used to cut metals at relatively slow speeds. Depth of cut and feed rates are usually pushed to the limit depending on the required surface finish. In a lathe, power is normally transmitted to the spindle by a pulley or gears, resulting in heavy radial loads at the non-tool end. On the tool end of the spindle, where there are heavy combined loads, a high degree of rigidity and high load carrying capacity are important operational requirements.
In a lathe spindle, it is common to have a double row cylindrical roller bearing in combin ation with a double direction angular contact thrust ball bearing at the tool end and a double row cylindrical roller bearing at the non-tool end († fig. 10). The outside diameter of the thrust bearing housing washer is manufactured to a special tolerance. This tolerance enables the bearing to be radially free when mounted in a housing of appropriate bore diameter tolerance for the adjacent double row cylindrical roller bearing. This prevents the thrust bearing from carrying any radial load. This bearing arrangement provides a long calculated life and a high degree of rigidity and stability, both essential to the manufacture of good quality workpieces. A good rule of thumb is to have the distance between the tool end and non-tool end bearing centres in the range 3 to 3,5 times the bore diameter of the bearing(s) at the tool end. This rule is particularly important when heavy loads are involved. For additional information, refer to System rigidity († page 66).
Fig. 10 Belt-driven CNC lathe spindle for large diameter bar stock
NN 3020 KTN9/SP + BTW 100 CTN9/SP
NN 3018 KTN9/SP
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