Safety Precautions in Engineering Design

In this tutorial, you will learn about the various safety precautions undertaken before designing any component to ensure that it does not fail in application. You will understand the various factors involved in ensuring safety and learn the different considerations which influence defining the safety factors for the design.

Contents:

  1. What are Safe Designs?
  2. What are the Modes of Failure?
  3. The Factor of Safety
  4. The Margin of Safety
  5. The Reserve Factor
  6. Yield and Ultimate Strength
  7. Choosing the Design Factors
  8. Selecting Factor based on Application

What are Safe Designs?

Design Engineers always strive to create a ‘safe design’ for applications. A safe design includes integrating hazard detecting and risk assessment methodologies at the beginning of the design process to eliminate risks of injury through the use of the product.

  • A safe design approach takes the following decision at the beginning of the conceptualization phase.

    • The design and its purpose
    • The materials to be used
    • Methods of construction, maintenance, and operation
    • Engineering Standards to be followed during design and manufacture

  • There are five principles of safe design. They are illustrated in the figure below.

    Principles of Safe Design
    • Persons with control ensure that those involved in decision-making promote the health and safety of the design at the source.
    • Product Lifecycle involves eliminating the hazards and reducing the risks at the earliest stage possible.
    • Systematic Risk Management involves identifying the risks and minimizing them.
    • Safe design knowledge and capability must be demonstrated by those who actively contribute to the design.
    • Effective communication ensures information transfer regarding design documentation and risk control amongst everyone involved.
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  • It is predicted that intrinsically safe plants and equipment would save 5–10% of their cost by reducing hazardous material stockpiles, reducing the requirement for protective equipment, and lowering the expenses of testing and maintaining the equipment.
  • Retrofitting, workers’ compensation and insurance levies, environmental clean-up, and negligence lawsuits are all examples of direct expenses connected with dangerous design.
  • Because these expenses have a bigger impact on those who buy and use the product later in their lifetime, the motivation for these parties to influence and profit from safe design is likewise greater.

What are the Modes of Failure?

A mechanical component may fail, that is, maybe unable to perform its function satisfactorily, as a result of any one of the following three modes of failure. The following figure shows the common mode of failures

Modes of Failure
  • Failure by Elastic Deflection – The elastic deflection results in unstable conditions, such as buckling of columns or vibrations. The design of the mechanical component, in all these cases, is based on the permissible lateral or torsional deflection. In applications like transmission shaft supporting gears, the maximum force acting on the shaft, without affecting its performance, is limited by the permissible elastic deflection.
  • Failure by General Yielding – A mechanical component made of ductile material loses its engineering usefulness due to a large amount of plastic deformation after the yield point stress is reached. A considerable portion of the component is subjected to plastic deformation, called general yielding. The yield strength of a material is an important property when a component is designed against failure due to general yielding.
  • Failure by Fracture – The separation of atomic or molecule connections causes mechanical fracture of a material. According to the magnitude of the strain at the moment of breaking, it is divided into brittle and ductile fractures. The existence of a fracture in a part amplifies the stress in the area of the crack, which can lead to failure sooner than predicted by standard strength-of-materials techniques.

The Factor of Safety

While designing a component, it is necessary to provide sufficient reserve strength in case of an accident. This is achieved by taking a suitable factor of safety (fs).
fs = \(\frac{failure \,stress}{allowable\, stress}\)

  • The allowable stress is the stress value, which is used in design to determine the dimensions of the component. It is considered as a stress, which the designer expects will not be exceeded under normal operating conditions.
  • The number of assumptions made in design analysis, to simplify the calculations, may not be exactly valid in working conditions. The factor of safety ensures against these uncertainties and unknown conditions.
  • The use of a factor of safety does not imply that an item, structure, or design is “safe”.

The Margin of Safety

The usage of a margin of safety (MoS or M.S.) to express the ratio of the structure’s strength to the standards is required by several government organizations and businesses.
Margin of Safety = Factor of Safety – 1

  • Because there are two different definitions for the margin of safety, it’s important to know which one is being utilized for a specific application. M.S. can be used as a capacity metric, like the Factor of Safety. The other usage of M.S. is as a measure of satisfying design requirements.
  • It specifies how much more load a component can tolerate beyond its design load before failing. If the margin is zero, the component will not take any more load before failing; if it is negative, the part will fail before achieving its design load in service. It can sustain one additional load of equivalent force to the maximum load it was intended to support if the margin is 1.

The Reserve Factor

The reserve factor is a popular strength indicator in Europe (RF). The reserve factor is specified in one of two ways, depending on the industry, with the strength and applied loads represented in the same units:
RF = \(\frac{proof \,strength}{proof\, load} = \frac{ultimate \,strength}{ultimate \,load}\)

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Yield and Ultimate Strength

When working with ductile materials (like most metals), it’s common to evaluate the factor of safety against both yield and ultimate strengths. The safety factor will be determined by the yield calculation until the component begins to deform plastically. The safety factor will be determined by the last computation till failure. Because these values are frequently indistinguishable in brittle materials, it is typically acceptable to compute simply the final safety factor.

Choosing the Design Factors

The selection of magnitude of the factor of safety is one of the difficult tasks faced by the designer. The guidelines for the selection of quantitative values of the factor of safety are as follows

  • Sometimes, the failure of a machine element involves only a little inconvenience or loss of time, or there may be substantial financial loss or danger to human life. The factor of safety is high in applications where the failure of a machine part may result in serious accidents.
  • The factor of safety is low when the external force acting on the machine element is static. On the other hand, a higher factor of safety is selected when the machine element is subjected to impact load.
  • A higher factor of safety is necessary when the machine component is subjected to a force whose magnitude or direction is uncertain and unpredictable.
  • When the component is made of a homogeneous ductile material like steel, yield strength is the criterion of failure. The factor of safety is usually small in such cases. On the other hand, a cast iron component has a non-homogeneous structure and a higher factor of safety based on ultimate tensile strength is chosen.
  • The factor of safety increases with increasing reliability.
  • The factor of safety is low for cheap machine parts.
  • A higher factor of safety is necessary when it is not possible to test the machine part or where there is a deviation between test conditions and actual service conditions.
  • When the machine element is likely to operate in a corrosive atmosphere or high-temperature environment, a higher factor of safety is necessary.
  • When the quality of manufacture is high, variations in dimensions of the machine component are less and a low factor of safety can be selected. Conversely, a higher factor of safety is required to compensate for poor manufacturing quality.

Selecting Factor based on Application

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The magnitude of the factor of safety depends upon several factors. Generally, a high factor of safety is required in the following conditions.

  • The magnitude and nature of external forces acting on the machine component cannot be precisely estimated.
  • The material of the machine component likely has a non-homogeneous structure.
  • The machine component is subjected to impact force in service.
  • The failure of the machine part may hazard the lives of people (hoist, lifting machinery, and boilers) and substantial property loss.
  • The exact mode of failure of the component is unpredictable.
  • There is stress concentration in a machine component.
  • The machine part is subjected to high temperatures during operation.
  • There is a possibility of residual stresses in the machine component.
  • The recommended factor of safety is 3 to 6 based on the critical buckling load of components like pistons, rods, power screws, and studs.

Key Points to Remember

Here is the list of key points we need to remember about “Safety Precautions in Engineering Design”.

  • In a safe design, control mechanisms are integrated to remove or, if this is not reasonably possible, minimize threats to health and safety over the life of the building.
  • There are three major modes of failure, failure by Elastic Deformation, failure by General Yielding, and failure by Fracture.
  • A factor of safety, often known as a safety factor, is a measure of how much stronger a system is than it needs to be for the weight it is designed to carry.
  • The margin of safety describes how much additional load a component can withstand before failing beyond its design load.
  • The Factor of safety is not a guarantee of the structural strength of a design.
  • Several considerations are required when deciding the factor of safety of any particular component, including material, manufacturing methods, and application environment.

If you find any mistake above, kindly email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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