An electric motor is an electromechanical device that converts electrical energy into mechanical energy. Electrical energy input from the power grid is converted into electromechanical energy through the magnetic field in the air gap inside the motor, and the mechanical power output from the motor shaft extension drives various devices. Simply put, the air gap inside the motor is the invisible medium for converting electrical energy into mechanical energy, while the shaft extension is the physical connection between the motor and the mechanical transmission system. Today, we'll discuss the connection types between motors and mechanical transmission systems, the characteristics of each type of connection, and why a particular connection method must be chosen.
Theoretically, the connection methods between motors and mechanical transmission systems can be divided into two main categories: detachable and non-detachable. However, in almost all application systems, the motor exists as a universal independent module and must be interchangeable; therefore, only detachable connection structures are possible. Precisely because of this fundamental characteristic of detachability and interchangeability, motor shaft extensions only come in three types: cylindrical shaft extensions, conical shaft extensions, and splined shaft extensions. The actual output connection forms are divided into cylindrical shaft extension flat key connections, conical shaft extension flat key connections, cylindrical shaft extension tangential key connections, and spline connections. Among these, spline connections are designed as rectangular spline shaft extensions and involute spline shaft extensions depending on the load and precision requirements.
Common shaft extension types for motors
Cylindrical shaft extension key connection
The most common type of cylindrical shaft extension is the flat key connection. Unless otherwise specified or by default, cylindrical shaft extensions are connected by flat keys. This type of shaft extension does not require highly advanced machining equipment or operating skills. Surface roughness, dimensional accuracy, and alignment accuracy are easily guaranteed, which can meet the structural strength and fitting accuracy requirements of most applications.
Conical shaft flat key connection
Tapered shaft extensions with flat keys are often used in applications requiring frequent forward and reverse rotation, vibration resistance, impact resistance, and heavy-load starting. They typically have a 1:10 taper and a locking device at the shaft end. Ordinary machining equipment and basic operating skills are sufficient for machining tapered shaft extensions. Their application is second only to cylindrical shaft extensions, and together they serve as the standard shaft extension connection type.
Cylindrical shaft extension tangential key connection
Tangential key connections for cylindrical shafts are mainly used in heavy machinery with shaft diameters greater than 100mm, where centering requirements are not high and loads are large. A tangential key consists of two ordinary wedge keys with a 1:100 angle. The two parallel narrow faces are the working surfaces, one of which lies in the plane passing through the shaft centerline. During operation, the compressive force on the working surface acts along the tangent of the shaft, thus transmitting torque through the compressive force of the working surface. A single tangential key can only transmit unidirectional torque. To transmit bidirectional torque, two tangential keys must be used, staggered by 120-135 degrees and installed in opposite directions, as shown in Figure 1.
Spline shaft extension
When the strength of the flat key cannot meet the design requirements, a spline shaft extension should be considered. A spline connection consists of multiple key teeth and keyways evenly distributed circumferentially in the shaft and hub bore, and is divided into rectangular splines and involute splines.
Splines have numerous teeth, a large working area, and high load-bearing capacity; the keys are evenly distributed, resulting in more uniform stress distribution on each tooth; the tooth and shaft are integrated with shallow tooth grooves, minimizing stress concentration at the tooth roots, leading to high strength and reduced weakening of the shaft; and the shaft components have good centering and guiding properties. All these advantages, unmatched by flat keys, make splines the most suitable for use as motor shaft extensions. However, spline machining requires specialized equipment and cutting tools, making inspection and measurement difficult and costly. Therefore, splines are only used in high-precision, special applications, or when the strength of ordinary shafts cannot meet design requirements.
Figure 2 shows the rectangular spline structure, and Figure 3 shows the keyway cross-sectional shape. The rectangular spline uses a minor-diameter centering method, meaning the minor diameters of the external and internal splines serve as the mating surfaces. Features: 1) High centering accuracy and good centering stability; 2) Deformation caused by heat treatment can be eliminated by grinding. The markings for rectangular splines include the number of keys N, minor diameter d, major diameter D, and key width B. For example, a spline specification of 6×23×26×6 indicates 6 keys, a minor diameter of φ23, a major diameter of φ26, and a key width of 6. The spline pair is: 6×23H7/f7×26H10/a11×6H11/d10.
Figure 4 shows the cross-sectional dimensions of the involute spline shaft. Its characteristics are: 1) It has a large load and requires high centering accuracy, and the tooth profile is involute; 2) When under load, there is a radial component force on the teeth, which can play a self-centering role; 3) The load on each tooth is uniform, with high strength and long service life; 4) The pressure angle is 30°, 37.5°, and 45°. The larger the pressure angle, the smaller the load capacity and the stronger the self-centering ability; 5) The module is 2.5~10, and the minimum number of teeth is 10.
The involute spline pair with 24 teeth, module 2.5, 30° round tooth root, tolerance grade 5, and fit category H/h is represented as: INT/EXT24z×2.5m×30R×5H/5h.
In short, the motor shaft extension is directly connected to the equipment, requiring very high dimensional precision and involving many issues related to motor quality.
Specific problems caused by improper shaft extension
Shaft extension diameter
The equipment is connected via either an axial coupling or a belt drive. Regardless of the connection method, the fit between the shaft extension and the coupling is crucial. If the diameter is too large or the fit with the coupling, pulley, or other equipment interfaces is too tight, it can lead to installation failure or severe damage to the mating surfaces. Ultimately, poor fit results in low overall equipment efficiency. Conversely, if the fit is too loose, the motor and equipment will wobble, and radial relative movement will occur between the shaft extension and the coupling. This can damage the connecting key and cause severe noise. Furthermore, the keyway on the motor shaft extension and the coupling key will be damaged, potentially rendering the equipment inoperable.
Shaft extension length
The shaft extension length is a critical dimension determining the axial connection space between the motor and the equipment. If the shaft extension is too long and an axial connection method is used, the relative space between the equipment and the motor will inevitably be too tight, resulting in severe axial stress on the motor and equipment, and ultimately, bearing damage. This problem is relatively less severe for belt-driven systems, but as a motor manufacturer, we must strictly control this dimension to meet the requirements of various customer installation methods.
Shaft root treatment
This is a common problem encountered by many motor manufacturers. Firstly, can the root treatment ensure proper coupling installation? Secondly, improper machining of the root transition fillet can easily cause stress concentration. Some motor manufacturers handle this fillet very carelessly during shaft machining, some even using a cutting tool, ultimately leading to excessive stress concentration and, as a result, shaft breakage at the root when the motor is under load. It's worth noting that not only does stress concentration at the shaft root cause shaft breakage, but the bearing seat root also frequently experiences shaft breakage due to the same problem.
Shaft extension keyway width, depth and symmetry
If the keyway width or depth is out of tolerance, the shaft and coupling cannot be installed. Conversely, if the dimensions are out of tolerance, the keyway may be damaged due to key movement when the motor is running under load, and may even cause the motor and equipment to rotate out of sync. Regarding symmetry, if the requirements are not met, it will be impossible to connect with the coupling.
The shaft extension is the interface between the motor and the equipment. The interface connection method directly determines the load capacity. When selecting it, the principles of suitability, practicality, safety and reliability should be followed, and the customer's needs should be met as much as possible through the most economical and reliable means.
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