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Motor Sizing Basics Part 4 - How to Calculate Radial Load and Axial Load
Beyond load torque, acceleration torque, speed, and load inertia, failing to consider specific sizing parameters during the motor sizing process can significantly impact the performance and lifespan of your machinery.
A motor’s operational capabilities are defined by its torque and speed specifications. These parameters tell us whether a motor can accomplish a given task. On the other hand, the motor's structural strength is determined by its ability to handle radial and axial loads. These load specifications indicate how long a motor can maintain performance under a particular workload.
The robustness of a motor depends on the combined strength of its casing, flange brackets, output shaft assembly, and, in the case of gear motors, the gears and additional bearings. The bearings closest to the load bear the brunt of this stress. As such, the radial and axial load specifications of a motor or gearhead are closely tied to its bearing design.
Below, we outline the internal structure of an AC motor and its accompanying gearhead. The principles of radial and axial loading apply universally across different motor types.
| Internal Structure of AC Motor | Gearhead Structure |
| --- | --- |
| The rotor and shaft assembly, supported by two ball bearings, is the only moving part. Surrounding the rotor is the stator, while the flange bracket and motor case encase the entire structure. | In a gearhead attached to the motor's pinion shaft, each gear and output shaft is supported by individual bearings. The input shaft remains supported by the motor bearing. The bearing supporting the gear shaft (and load) is notably larger. |
Radial Load (Overhung Load)
Radial load refers to the maximum force that can be applied to the shaft in a direction perpendicular to the motor shaft axis. It’s also called the “overhung load†because it resembles a load hanging off the shaft. Radial load capacity decreases with distance from the bearing.
Axial Load (Thrust Load)
Axial load pertains to the maximum force applied along the motor shaft axis. It’s also referred to as “thrust load,†as it acts along the same axis as the thrust force. Typical axial loads are around half the motor's weight, though this has risen over time.
These forces can occur in various directions. For instance, a motor with a 100 N axial load specification can support a 100 N load on its shaft when facing upwards, equivalent to approximately 10 kg.
Why Are These Specifications Important?
Radial and axial load specifications reflect the mechanical rigidity of the bearings, shaft, and case assembly. Exceeding these limits can lead to severe consequences, such as bearing flaking, rolling element damage, or even shaft breakage.
| Motor Damage Due to Excessive Radial/Axial Load | Gearhead Damage Due to Excessive Radial/Axial Load |
| --- | --- |
| Images illustrating motor damage caused by exceeding radial or axial load limits. | Images showing gearhead damage due to similar overloads. |
For example, exceeding the permissible radial load can cause the shaft to bend and eventually break. If the permissible axial load is surpassed, the motor or gearhead bearing might degrade and fail. Either scenario leads to motor malfunction or reduced lifespan, with the nearest support component often failing first.
Tip: A Simple Ball Bearing Test
To assess internal motor or gearhead damage, disconnect power, disassemble the motor from the gearhead, and manually rotate the shaft clockwise and counterclockwise. If the motor or gearhead is damaged, you’ll notice varying resistance, abnormal noise, or inability to rotate the shaft.
The earlier the lifespan ends, the greater the extent to which these specifications are exceeded and the longer the overload persists. For instance, our ball bearings are rated for 10,000-hour life, so exceeding either radial or axial load by 10% could reduce its life by about 1,000 hours.
If you're keen on estimating service life based on bearing life, our technical support engineers are ready to assist.
How Are These Specifications Shown?
Manufacturers present these specifications differently. Generally, a table lists permissible radial and axial loads according to gearhead size and gear ratio. Axial load remains constant, while radial load varies with the "distance from the end of the gearhead output shaft."
| Permissible Radial and Axial Load Table | Radial & Axial Load Illustration |
| --- | --- |
| Example table showing permissible radial and axial loads for common geared AC motors. | Visual representation of radial and axial load forces. |
The “Seesaw/Fulcrum†Effect (for Radial Load)
In the table above, you’ll notice that the permissible radial load changes with the "distance from the end of the gearhead output shaft." This distance is from the load shaft end to the point of force application. As this distance increases, the load gets closer to the support bearing inside the gearhead flange, allowing more load to be supported. This is akin to a seesaw or fulcrum.
| Fulcrum Diagram |
| --- |
| Visual explanation using a seesaw analogy. |
Static and Dynamic Radial and Axial Loads
Like dynamic and static moment loads, radial and axial loads also have both static and dynamic components. The table above helps determine both.
For example, static radial load includes the weight of the pulley and belt tension at rest. Dynamic radial load, requiring calculation, includes forces from the same pulley weight and belt tension during motion. Static axial load is the weight of the pulley if the motor shaft is vertical. Dynamic axial load is typically lower than static axial load, so only the static axial load is usually considered.
Tip: Remember to Include Belt Tension as Radial Force
Don’t overlook belt tension! In my experience as a technical support engineer, excessive belt tension frequently caused motor issues.
To ensure proper handling of all radial and axial loads, make sure:
- Static radial load is under the chart value.
- Dynamic radial load is under the chart value.
- Static axial load is under the chart value.
The Equation for Dynamic Radial Loads
For radial loads, there’s an additional dynamic component, which is the radial load during motion. Ensure the calculated value stays under the chart value.
When pulleys, belts, gears, sprockets, chains, etc., are used as transmission mechanisms, dynamic radial load is calculated with the following equation:
W = T / y
With a belt conveyor, motor torque generates the driving force, represented by T (torque in N·m). If y (effective radius in meters) is the pulley radius, we can calculate radial load or W (workload).
The actual equation is more complex, factoring in load coefficient and service factor.
| Radial Load Formula for Motors | Load Coefficient Based on Drive Method (K) |
| --- | --- |
| Formula for calculating radial load. | Chart showing load coefficient based on drive method. |
Service factor, related to operating conditions, considers factors like frequent starts/stops and changing rotation direction affecting radial load.
| Service Factor (f) for Radial Load Calculation |
| --- |
| Chart illustrating service factor impact on radial load. |
Time for Practice
Example: Calculating Radial Load of a Conveyor
Consider the belt conveyor application below. How would you calculate the required radial load value from the motor?
I’m working on a chain and toothed (sprocket) conveyor using a 2IK6 motor with a 360:1 gearhead. I need 10 N·m of torque on a 0.1-meter (effective) diameter sprocket. I estimate chain tension to be about 10 N. I plan to rotate in one direction only. My sprocket is mounted 10 mm from the end of the shaft.
Can my gear motor handle the radial and axial load from my application?
First, I ask myself: What equation do I use (we know this), and do I have all the variables?
Assuming a perfect world, all necessary variables are provided in the correct units. In reality, it usually takes more effort. For example, load coefficient and service factor weren’t given, but drive method and load type were. Values might also require unit conversion.
We need to:
Ensure these conditions are met:
- Static radial load is under the chart value.
- Dynamic radial load is under the chart value.
- Static axial load is under the chart value.
| Permissible Radial and Axial Load Table |
| --- |
| Example table showing permissible radial and axial loads for 2IK6 motors. |
Static Radial Load = OK
Static radial load is belt tension. My estimate of 10 N is the best information available. At 10 mm from the end of the shaft, the maximum radial load for the 2IK6 motor is 200 N, so we’re fine here.
Dynamic Radial Load = OK
Plug in values for every variable in the dynamic radial load equation.
| Belt Conveyor Example | Radial Load Equation |
| --- | --- |
| Image showing variables for calculation. | Formula for dynamic radial load. |
W = What we’re solving for (radial load in Newtons).
K = 1; chain and toothed belt conveyor.
T = 10 N·m.
f = 1; uniform load/unidirectional continuous operation.
y = Effective radius of pulley = 0.1 meter.
W = K × T × f / y
W = 1 × 10 × 1 / 0.1
W = 100 N
100 N is below the 200 N permissible radial load value, so we’re good here.
Static Axial Load = OK
Static axial load is unknown, but looking at the application image, we shouldn’t have much static axial load, and it should definitely be below the 40 N value in the chart, so we’re fine here too.
Conclusion
The 2IK6 motor with a 360:1 gear ratio gearhead will be able to handle both radial and axial loads.
Most motor sizing software doesn’t consider axial or radial loads, so don’t forget to confirm your radial load and axial load after sizing your motor.
Let Us Help!
In most cases, thorough motor sizing requires more effort than anticipated. If your time is as valuable as we think it is, let our experts help!
To begin your motor sizing consultation with us, use our Motor Sizing Calculator, select a common application, then fill in the blanks. A sizing report, including calculations, can be generated. Our technical support engineers are happy to analyze your report to ensure you buy the right motor the first time.
Tips for Motor Selection
In the next post, I’ll provide tips for motor selection. Here’s also a post explaining how to search for a motor using your sizing results.
Previous Posts:
- Motor Sizing Basics Part 1 - Load Torque
- Motor Sizing Basics Part 2 - Load Inertia
- Motor Sizing Basics Part 3 - Acceleration Torque (and RMS Torque)
Tip: Is There an Easier Way to Size Motors?
Use a motor sizing tool. After completing a motor sizing, our technical support engineers can guide you through the product selection process if necessary.
Example: Index Table Application
Try Our Motor Sizing Calculators