The shaft is one of the most important components in an electric motor . Since most damage to shafts is fatigue-related, the requirements for general motor shaft materials are: sufficiently high strength, low sensitivity to stress concentration, and good machinability. Except for less critical shafts and those subjected to low stress, which use ordinary carbon steel (Q235A, Q255A, or Q275), shafts are generally made of high-quality carbon steel (45#), and may undergo further tempering treatment when necessary. Today, Ms. Can will discuss the most basic requirements for shafts; what seems like a simple shaft actually involves a considerable amount of knowledge.
For diameters less than approximately 10 cm, round steel is commonly used as the blank; for diameters greater than 10 cm, forged stepped shaft blanks are used. Large motor shafts and motor shafts subjected to impact loads, due to their increased importance, have higher material requirements and are mostly made of alloy steel forgings.
In recent years, electroslag welding technology has been increasingly widely used in the motor industry. Large diameter (or even not very large diameter) shafts tend to use welded hollow shafts composed of multiple sections to reduce weight, save on large forgings, and lower costs.
During operation, the stress on the shaft varies depending on the type of motor and the transmission mechanism. Typically, in vertical motors using direct drive (such as hydro generators), the shaft bearing experiences tensile stress caused by the rotor weight (and sometimes water thrust), shear stress generated by torque, and bending stress caused by unilateral magnetic pull. In horizontal motors that transmit torque, the shaft bearing experiences bending stress caused by the rotor weight, unilateral magnetic pull, and external forces acting on the shaft extension (such as belt tension), as well as shear stress generated by torque.
Basic requirements for motor shafts
(1) It must have sufficient strength. That is, under normal load and under specified special conditions (such as sudden short circuit), no part of the shaft should suffer residual deformation or damage.
(2) It must have sufficient rigidity. That is, the deflection of the shaft must be within the allowable range.
(3) There should be a sufficient difference between the critical speed and the operating speed to avoid resonance.
Before performing mechanical calculations, the geometric dimensions of the shaft should be preliminarily determined. A common and simple method is to directly refer to the shaft dimensions of already manufactured motors. Alternatively, relevant dimensions from electromagnetic design can be used, or the diameter and journal dimensions of the middle section of the shaft (i.e., the core section) can be determined based on empirical data and mechanical formulas, and then the shaft length can be determined according to the structural sketch of the designed motor.
According to statistics on manufactured motors, the diameter of the shaft center is approximately 1/3 to 1/4 of the stator inner diameter or armature outer diameter for motors below 1000 kW. For asynchronous motors, due to the smaller air gap, this value is generally closer to 1/3 to increase shaft rigidity.
In the preliminary design, the bending moment value is not yet known. At this time, it can be approximated by the concept of torsional strength. The minimum diameter dmin of the shaft in the torque transmission part is determined according to the torque T, and calculated according to formula (1):
TN—Rated torque of the motor, TN=9565PN/nN (Newtons/meter), where PN is the rated power (kilowatts) and nN is the rated speed (revolutions/minute).
In small- to medium-capacity vertical motors (such as various vertical motors used in water pumps, fans, and centrifuges), the diameter d0 of the shaft core is typically 10-15% larger than the minimum diameter of the drive shaft; in large vertical motors, d0 is generally close to this minimum diameter. The diameter of the journal is usually between the diameter at the midpoint of the shaft and the diameter of the shaft extension, and its specific dimensions should be determined after selecting the bearing (rolling type) or performing bearing lubrication calculations (sliding type).
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