In product and mechanical design, the concept of the “safety factor” is crucial for preventing component breakage and failure. The safety factor is an indicator that specifies the margin of safety to be maintained relative to the actual load applied to materials or components, serving as a criterion for ensuring product reliability.
When performing motor selection, setting an appropriate safety factor for the anticipated load is essential for extending the lifespan of the product and reducing the risk of failure.
In this article, we provide an easy-to-understand explanation for beginners, covering everything from the basic definition of the safety factor to calculation methods, how to determine it in practice, and guideline values for mechanical design.
| Supervised by: C.I. TAKIRON Corporation Electronic Devices Sales Group This article has been supervised based on the advanced technical expertise and insights we have cultivated since our founding in 1919 as a leading company in plastic processing. Our department continuously analyzes market trends and the latest technologies in ultra-compact, high-precision micro motors, focusing on providing high-value-added information to designers and developers. As a team of experts with in-depth knowledge of product characteristics, we support our customers’ problem-solving and technological innovation by delivering accurate and practical content. |
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What Is a Safety Factor?
The safety factor is a fundamental concept in product design used to ensure the safety of materials and components. In actual operating environments, various uncertainties exist, such as loads exceeding those assumed during design, variations in material properties, and aging degradation. The role of the safety factor is to establish an appropriate margin of safety to prepare for these risks.
Contents of This Section
- Definition and Basic Meaning of the Safety Factor
- Why a Safety Factor Is Necessary in Product Design
A proper understanding of the safety factor enables the creation of products with high levels of reliability. Below, we will examine the specific definition of the safety factor and why it is necessary in design in detail.
Definition and Basic Meaning of the Safety Factor
The safety factor is a dimensionless value calculated as “material’s standard strength ÷ actual stress,” indicating the design margin. Standard strength includes properties such as tensile strength and yield point, serving as an indicator of the maximum stress a material can withstand. On the other hand, actual stress refers to the value calculated from the load applied to a component during use.
For example, a safety factor of 2.0 means that the material’s nominal strength is twice the actual stress. This ensures a safety margin against unexpected loads or changes in the operating environment. While a higher safety factor provides a larger margin of safety, an excessively high setting can lead to increased costs, so an appropriate balance is required.
Why a Safety Factor Is Necessary in Product Design
In actual operating environments, loads exceeding those anticipated during design may occur. Additionally, variations in material dimensions and strength can arise during manufacturing, and changes in material properties due to aging are inevitable.
Furthermore, there are many factors that cannot be fully predicted at the design stage, such as differences in user operation methods and installation environments. Setting a safety factor is crucial to address these uncertainties and ensure product safety and reliability. In particular, for medical equipment involving human life and Industrial equipment where failure can cause significant losses, it is required to set a conservative safety factor based on various standards and risk assessments.
How to Calculate Safety Factors
Calculating the safety factor is a critical step that forms the foundation of the design process. To perform accurate calculations, it is necessary to correctly understand the material’s ultimate strength and the stresses or loads that will actually occur. Here, we explain the formula for the safety factor and the specific calculation procedure.
Contents of This Section
- The Relationship Between the Safety Factor Formula and Allowable Stress
- Examples of Safety Factor Calculations
Understanding the calculation formula enables appropriate design decisions in practical applications. Below, we will explain everything step-by-step, from the basic method for calculating the safety factor to specific examples.
The Relationship Between the Safety Factor Formula and Allowable Stress
The safety factor (S ) is calculated as “design strength (σB ) ÷ actual stress (σ)” or “failure load ÷ service load.” The allowable stress is the design strength divided by the safety factor, and it represents the upper limit of stress that a component can withstand during design.
Yield strength includes tensile strength, yield point, and fatigue limit, and the appropriate value must be selected based on the operating conditions. Generally, the yield point is used for components subjected primarily to static loads, while the fatigue limit is used for components subjected to cyclic loads. Properly setting the allowable stress helps clarify the range within which the material can be safely operated without failure. Understanding this relationship is the first step toward a highly reliable design.
Example of Safety Factor Calculation
As a concrete example, let’s calculate the safety factor for a component made of steel with a tensile strength of 400 MPa and a yield point of 230 MPa, where a stress of 100 MPa is applied. When using the yield point as the reference, the safety factor is 230 ÷ 100 = 2.3.
This value indicates a margin of 2.3 times the actual stress and is often considered a reasonable benchmark in general mechanical design.On the other hand, if the tensile strength is used as the reference, the safety factor becomes 400 ÷ 100 = 4.0, allowing for an even greater margin of safety. In actual design, it is necessary to select the appropriate reference based on the usage conditions; for example, the yield point is chosen when static loads are predominant, while the tensile strength is selected for applications involving brittle materials or where fracture is not permitted.
How to Determine the Safety Factor and Reference Values
Setting the safety factor requires a comprehensive assessment of usage conditions and product characteristics, rather than simply applying a formula. Determining an appropriate safety factor allows for the optimization of the balance between product reliability and cost.
Contents of This Section
- Approach to Determining Safety Factors
- Guidelines for Safety Factors in Mechanical Design and Their Application to Motor Selection
In practice, safety factors are set by referring to industry standards and past performance while taking individual operating conditions into account. The following explains the specific approach to determining safety factors and provides guideline values.
Approach to Determining Safety Factors
When setting a safety factor, a comprehensive evaluation is conducted of the nature of the load (static, dynamic, or impact), material reliability and variability, operating environment (temperature, humidity, and corrosiveness), frequency of maintenance and inspection, and the impact of a failure.
If the load is static and predictable, a lower safety factor is generally acceptable; however, a higher safety factor is required when the load is impact-based or cyclic. Additionally, a higher safety factor should be set when there is significant variation in material properties or when the operating environment is harsh. More cautious judgment is required for components where maintenance and inspection are difficult, or for applications where failure could endanger human life. Since an excessively high safety factor leads to increased product weight and higher costs, striking the right balance is crucial.
Safety Factors in Mechanical Design and Motor Selection
In general mechanical design, safety factors must be set to appropriate values based on the materials used and the nature of the loads (static, dynamic, or impact). Since actual settings vary depending on standards and applications, it is essential to set values that comply with industrial standards and the operating conditions of the equipment in question.
When performing motor selection, it is important to consider the impact load during startup and the moment of inertia. In particular, for applications involving frequent start-stop cycles or requiring high-precision positioning, the motor’s performance characteristics significantly impact the overall reliability of the product.
C.I. Takiron Corporation’s Coreless motors generate virtually no cogging torque and offer smooth rotational characteristics, enabling high-precision control. With their low-vibration, long service life design, they are an ideal choice for applications requiring high reliability, such as medical equipment and industrial equipment.
Summary
The safety factor is an indicator used in product design to ensure safety; it is a dimensionless value calculated by dividing the standard strength of a material by the stress actually applied. In practice, it is determined by comprehensively evaluating factors such as the nature of the load, the operating environment, and the impact of a failure.
When performing motor selection, it is also important to consider various factors such as the load during startup and the moment of inertia. C.I. Takiron Corporation’s coreless motors combine low vibration, high precision, and high reliability, and are adopted for applications requiring high reliability, such as medical equipment and industrial equipment.
Product Information & Inquiries
For more details on C.I. Takiron’s micro motor products, please visit the website below.
- Product Site: https://cik-ele.com/en/
- Coreless Motors: https://cik-ele.com/en/products/list/coreless_motor/
- Brushless Motors: https://cik-ele.com/en/products/list/brushless_motor/
- Geared Motors: https://cik-ele.com/en/products/list/gearhead/
- Encoders: https://cik-ele.com/en/products/list/encoder/
If you are having trouble selecting a small motor for your product development, please feel free to contact us via the inquiry form. Our technical staff will discuss your application and requirements with you and propose the optimal solution.
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