Electric Motor Manufacturer : University Question On The Motor Shaft


The shaft is one of the important parts in the motor. S […]

The shaft is one of the important parts in the motor. Since most of the damage has fatigue properties, the requirements for general motor shaft materials are: high enough strength, low sensitivity to stress concentration and good processing performance. Except for the unimportant and less stressed shaft, Q235A, Q255A or Q275 ordinary carbon steel is generally made of 45 quality carbon steel, and then re-tempered if necessary. Today,     Electric Motor Manufacturer    talk to you about the most basic requirements of the shaft. The shaft that seems to be relatively simple has a lot of knowledge.


When the diameter is less than about 10 cm, round steel is often used as the blank; for 10 cm or more, the stepped shaft blank is forged. Large motors and motor shafts subjected to impact loads are subject to higher material requirements due to their increased importance. Most of them are alloy steel forgings.


In recent years, the electroslag welding process has become more and more widely used in the motor industry. The shafts with extra large diameters (even small diameters) tend to use welded hollow shafts composed of multiple sections to reduce weight and save large forgings and reduce costs. .


n operation, the form of stress on the shaft varies with the type of motor and the transmission. Generally, in a vertical motor (such as a hydro-generator) that uses direct drive, the bearing is subjected to tensile stress caused by the weight of the rotor (sometimes with water thrust), shear stress generated by torque, and bending stress caused by one-sided magnetic pull force; In a horizontal motor that transmits torque, the rotating bearing is subjected to bending stress caused by the weight of the rotor, the unilateral magnetic pulling force, and an external force acting on the shaft extension (for example, belt tension) and the shear stress generated by the torque.


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 transmitting torque portion is determined according to the torque T.


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).