Motor shaft basic requirements
(1) Have sufficient strength. That is, under normal load and specified special conditions (such as sudden short circuit, etc.), no part of the shaft can produce residual deformation or damage.
(2) Have sufficient rigidity. That is, the deflection of the shaft must be within the allowable range.
(3) There should be enough difference between the critical speed and the working speed to avoid resonance.
Before performing mechanical calculations, the geometry of the shaft should be initially determined. A convenient method that is commonly used is to directly refer to the shaft size of the motor that has been produced. The relevant dimensions in the electromagnetic design can also be used, or the diameter and the journal size of the middle of the shaft (ie, the iron core) can be determined based on empirical data and mechanical formulas, and then the length of the shaft is determined according to the structural sketch of the designed motor.
According to the statistics of the produced motor, the diameter of the central part of the shaft is about 1/3 to 1/4 of the outer diameter of the stator or the outer diameter of the armature for motors below 1000 kW. Asynchronous motors have a small air gap and are generally close to 1/3 in order to increase the stiffness of the shaft.
In the preliminary design, the bending moment value is not known. At this time, the concept of the torsional strength can be approximated first, and the minimum diameter dmin of the rotating shaft in the transmission torque portion is determined according to the torque T, and the calculation is as shown in equation (1):
TN - rated torque of the motor, TN = 9565PN / nN (cow / meter), PN is the rated power (kW), nN is the rated speed (rev / min).
In small and medium-sized vertical motors (such as various vertical motors for pumps, fans, and centrifuges), the diameter d0 of the shaft of the shaft is usually 10 to 15% larger than the minimum diameter of the shaft of the transmission; In a motor, d0 is generally close to this minimum diameter. The diameter of the journal is usually between the diameter of the shaft and the diameter of the shaft. The specific dimensions should be determined after the bearing (rolling) or bearing lubrication calculation (sliding).
