The shaft is a crucial component in an electric motor . As the link between the motor and the equipment for electromechanical energy conversion, it supports rotating parts, transmits torque, and determines the relative position of rotating parts with respect to the stator. Therefore, the motor shaft must possess reliable strength and rigidity to ensure the realization of its pre-designed functions.
Types and applications of shafts
●Classification by whether the shaft has a step or not. Shafts can be divided into two types: plain shafts and stepped shafts. Plain shafts are often made of cold-drawn round steel, which reduces the machining time of the shaft's outer diameter; they are sometimes used in micro motors. Stepped shafts allow for convenient and reliable mounting of many different components; therefore, most motors use this type of shaft.
Stepped shafts can be further divided into unidirectional stepped shafts (where the step diameter decreases gradually from one end of the shaft to the other) and bidirectional stepped shafts (where the step diameter decreases gradually from the middle of the shaft to both ends) depending on the direction of the steps.
●Classified according to the manufacturing method of the shaft blank. It can be divided into round steel shafts (shafts machined from hot-rolled round steel), forged shafts (shafts made from forgings), and welded shafts (shafts with radial ribs welded on).
Shafts machined from hot-rolled round steel are the most common type of shaft in small and medium-sized motors. The material commonly used is 45 high-quality carbon structural steel. For low-power motors, Q235 ordinary carbon steel is sometimes used. The blank diameter needs to be selected based on the maximum diameter of the shaft plus a machining allowance. Therefore, the amount of material removed during machining is relatively large.
Shafts with a diameter of 100 mm or more are preferably forged. Forged steel has higher mechanical strength, and forging can roughly shape the stepped shaft, saving raw materials and machining time. For large shafts with high mechanical strength requirements, such as the shafts of steam turbine generators, alloy steel forging is commonly used.
Replacing the rotor support with radial ribs on the welded shaft can increase the ventilation area inside the rotor cavity. However, welding the ribs can easily cause shaft deformation, requiring annealing after welding. Furthermore, discontinuous cutting during machine tool processing is detrimental to the cutting tools. Due to the presence of weld seams, the shaft's fatigue strength is significantly reduced, making it unsuitable for high-speed motors.
●According to the way the shaft and core are joined, shafts can be divided into shafts with knurled center, heat-fitted shafts, and shafts with keyways in the center. Knurled shafts are used in small motors below 10 kW, eliminating the need to machine keys and keyways. However, when pressing the shaft into the core, it is prone to deformation. During motor operation, some knurled rotor cores exhibit axial movement. This shaft deformation is caused by an excessively tight fit between the core and shaft, while axial displacement is caused by insufficient interference fit.
The middle section of the heat-shrinkable shaft is neither knurled nor keyed. A certain amount of interference fit is left between the shaft and the inner bore of the core, allowing the shaft to be fitted into the core while it is still hot. As long as the interference fit is appropriately chosen, the connection between the rotor core and the shaft is highly reliable.
Shafts with a keyway in the middle can be further divided into those with a single keyway and those with two keyways. Shafts with a single keyway are used in small motors, while those with two keyways are used in medium to large motors. The rotor core (or support) is keyed to the shaft. The core is axially fixed, with a shoulder added to one end and an arc-shaped key securing the other end into the annular keyway on the shaft. This type of shaft can transmit larger torques and is often used in high-power motors or motors that require frequent forward and reverse rotation during operation or where the rotor core cannot be heat-fitted.
●Classification by shaft extension shape. Shafts can be divided into cylindrical shaft extensions, conical shaft extensions, and shaft extensions with half-couplings. Cylindrical shaft extensions are easy to machine and are the most commonly used in electric motors. Conical shaft extensions have fastening bolts, requiring more machining. However, the drive wheels they are fitted with are easy to install and remove, and they are mostly used in special motors. Shafts with half-couplings are mainly used in hydroelectric generators and large DC motors.
●Classification by shaft shape. Shafts can be divided into solid shafts, shafts with a deep hole at one end, and shafts with a central through hole. Solid shafts are the most commonly used in electric motors. Shafts with a deep hole at one end are mainly used in wound-rotor induction motors to connect the rotor leads to the slip rings outside the end cover through the hole. Shafts with a central through hole are mainly used in large motors: in double-water-cooled steam turbine generators, the central through hole also serves as part of the cooling water circuit.
●Classification by shaft magnetic conductivity. Shafts can be divided into magnetic shafts and non-magnetic shafts. Magnetic shafts are mainly used in steam turbine generators. Shafts in other types of motors typically do not require magnetic conductivity.
●Other classification methods. According to the number of shaft extensions, they can be divided into single-shaft extension shafts and double-shaft extension shafts; according to the number of bearings, they can be divided into single-bearing shafts, double-bearing shafts, and multi-bearing shafts.
