A motor spindle is a directly driven spindle, where the motor, which is normally a three-phase asynchronous motor, is located directly on the main spindle between the spindle mounts. Here, the rotor is shrink-fitted directly onto the spindle shaft. The heat arising from the high power losses is partially dissipated via liquid cooling, or occasionally via air cooling over the stator.
Compared to conventional drives, motor spindles are characterised by a high degree of torsional stiffness and most commonly belt-driven or gear-driven designs. Further advantages of the motor spindle are the compact construction of the motor, the quiet and precise running of the spindle even at very low rotational speeds, as well as the detection of the motor speed and the spindle position via a rotary encoder. [WECK06]
Depending on the spindle diameter, motor spindles with roller bearings are currently able to reach maximum speeds of over 30,000 rpm, whilst motor spindles with air bearings attain speeds of up to 120,000 rpm [GMN]. In addition, maximum torques of up to 750 Nm can also be achieved [WECK06].
The selection of the mounting is therefore heavily influenced by the purpose of the spindle. Synchronous and asynchronous motors are used.
As a result of the often very compact construction and system integration, the complete spindle unit can be quickly replaced. However, the high level of integration conceals the risk that if a single component fails, such as the motor, the clamping system or the mounting system, this can result in a total failure of the spindle system.
The heating of the rotor must be closely observed when used in production. During operation, the power loss of the three-phase motor leads to high temperatures at the spindle, the bearings and the housing and this can be very difficult to dissipate between the rotor and the housing. This results in thermal deformations (spindle shaft extension), which reduce the working accuracy and the service life of the bearings. For this reason, some machines are equipped with a positional sensor in the factory so that thermal effects on the accuracy can be balanced out within limits through the machine control system.