The Impact of Torque, Radial, and Axial Loads on Motor Selection
Valentin Raschke | Jul 07, 2023
Selecting a motor for a specific application can be a challenging task. Many factors must be considered, including the application’s required voltage, maximum current and diameter, speed, efficiency, and power, among others. Balancing these considerations with other unique application requirements will help ensure the selection of the ideal motor. Collaboration between the motion solution and design engineering teams is vital from the outset.
When first approaching a supplier for assistance in miniature motor selection, a vital question must be addressed: What is the application’s specific working point, or the required torque and speed? Understanding the load on the motor will help pinpoint the necessary motor power and the required accessories. It is also important to discuss how the motor will be integrated into the application. Different types of loads will have a significant impact on the required motor design, and the motor’s lifetime and reliability.
Torque, radial, and axial load are different types of loads present in common miniature motor applications. They are crucial to the motor selection process.
Torque is the amount of rotational force generated by a motor during its operation. (Please see Figure 1 above.) The typical purpose of a motor is to convert electrical energy (Pelec = voltage x current) into mechanical energy (Pmech = torque x rotational velocity), so torque load is present in almost any application for rotary motors.
Simply selecting a motor based on the required torque value “x” is usually insufficient. The required torque (and velocity) in an application must be understood over the entire motion cycle, as the motor needs to provide the necessary mechanical power without overheating. The motion supplier will often ask you to provide the exact motion cycle the motor is expected to undergo; this allows for the analysis of the maximum temperature the motor can reach before overheating. An example of this is provided below in Figure 2.
Figure 2: Typical motion cycle of a motor used in an industrial power tool.
Keep in mind: Selecting the right motor requires not only understanding the required torque value, but also the torque/speed profile over an entire motion cycle and its duty cycle.
Figure 3: Example of a belt drive.
In certain applications, the motor or gearbox must not only provide a certain torque to drive the load, but it must also support a radial load. This is a force that acts radially on the motor shaft. One example is a belt drive (Figure 3, at left), which is used to drive an axis parallel to the motor. The tension force must be considered as a radial load acting on the motor shaft, especially if the belt is pre-stressed.
A second example is a diaphragm pump. A piston moves up and down to either a positive or negative pressure in the valve to facilitate the flow of a liquid or other material. Mounted on the motor shaft, the piston creates the movement and applies a radial load on the motor.
Radial load is relevant to motor selection given its impact on bearing options. In the example of a brush DC or stepper motor, there are two standard bearing options—sleeve bearings or ball bearings. Sleeve bearings typically support a lower radial load and provide less lifetime, but these negatives are offset by their lower cost. Depending on the motor’s total cost, using two sleeve bearings instead of ball bearings can significantly reduce expenses. However, for applications where a radial load is present like the belt drive and diaphragm pump, utilizing at least one ball bearing for the front bearing of the motor helps ensure a reasonable lifetime and is thus the better choice.
Figure 4: Radial load on a BLDC motor using two ball bearings.
In contrast, brushless DC motors typically use two ball bearings and can be driven at much higher speeds than DC or stepper motors. A motor manufacturer will recommend a maximum radial dynamic force at which a minimum lifetime of the motor at a specific speed can be achieved. The maximum radial dynamic force will depend on the size of bearings used, the distance between the two ball bearings in the motor (Figure 4: Distance “B”), and the position where the radial load applies (Figure 4: Distance “A”). A long motor with oversized ball bearings typically supports a larger radial load than a shorter motor (Figure 4).
Keep in mind: When selecting a motor, the radial load depends on its position on the motor shaft, the motor bearings, the required lifetime, and the speed of the application.
There are two types of axial loads—dynamic and static.
If an application requires a 90° rotation of the rotating movement at a lower speed, a worm gear—a worm shaft featuring a spiral thread that is driven by the motor—can be an ideal solution. The worm shaft drives the worm wheel with a reduction ratio as small as roughly 2:1 or larger. Following the spiral thread on the worm shaft, a radial and axial load must be supported by the motor.
Figure 5: A worm gear consisting of a worm shaft (driven by the motor) and a bronze worm wheel.
Sleeve bearings are not meant for supporting significant axial loads. Equipping the motor with ball bearings is often mandatory. Similar to the radial load, the maximum recommended dynamic axial load of a motor will depend on the ball bearings and preload used, the distance between the two ball bearings in the motor, and the lifetime requirement.
In the example of a typical axial brushless DC motor design, the dynamic axial load is supported by the front ball bearing, as the inner diameter of the bearing is bonded to the motor shaft. If an axial push load acts on the motor, the preload on the front ball bearing is reduced. This can lead to additional radial play, negatively impacting the motor’s lifetime, vibration, and noise. In the case of an axial pull load, the load acts in the same direction as the internal preload, increasing its stress. Motor manufacturers will usually restrict the recommended dynamic axial load to a certain limit that can be supported by a bearing without negatively impacting its lifetime.
Figure 6: Typical spring-based preload design for a BLDC motor (shaft bonded to the front ball).
Keep in mind: Depending on its direction, a dynamic axial load impacts the motor’s bearing assembly differently. The bearing assembly or the bearings themselves must be improved if axial loads are greater than the recommended specification.
In addition to a dynamic axial force acting on the motor during its operation, there is also a possibility that at least once during its lifetime, a static axial load will be applied to the shaft. This usually occurs when an additional component (for example, a pinion) is press-fitted onto the motor shaft of the assembled motor (Figure 7). Shock loads are another example, such as when a motor is used in a handheld device and dropped on the floor.
In the example of a motor equipped with ball bearings, a press-fit operation’s recommended limit is typically much higher than for a dynamic axial load. In fact, the limiting factor is only the ball bearings’ elastic limit! As long as the static load applied to the bearing is below its elastic limit, there will be no permanent deformation of the bearing balls or raceway. Similarly, exceeding the maximum recommended load can lead to permanent deformation of the bearing balls and raceway, resulting in reduced life and increased noise and vibration of the bearing.
Figure 7: Path of the force during a press-fit assembly (shaft bonded to the front ball bearing ID).
An additional difference to consider is whether the shaft of the motor can be supported during press-fitting, as shown in Figure 7. Certain motors are closed or are equipped with an encoder on the back side, which prevents access to the motor shaft. Without support, the force applied during the press-fit is directly transmitted to the front ball bearing, whose inner raceway is usually bonded to the motor shaft to absorb the axial loads. Supporting the rear shaft allows for a higher force during press-fitting, as the flow of force is through the motor shaft, not the bearings.
Keep in mind: Excessive static axial loads can permanently damage a motor’s ball bearings, negatively impacting its lifetime, noise, and vibration.
We have reviewed a selection of applications and examples where other forces, in addition to the torque load normally present, act upon a motor. The most common are radial and axial loads, which must be considered in relation to their impact on a motor and the motor selection process. A dedicated motion solution provider like Portescap supports customers in calculating the loads experienced by a motor in a given application and is fully equipped to develop the most suitable motion solution that fulfills—and even exceeds—application and device requirements.
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