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Fasteners are often the most inexpensive and overlooked component in engineering and mechanical product design. We use them around the home, in construction and in mechanical engineering projects, such as building the complex engines that power our vehicles.
When looking into both fastener and screw facts it is important to look at the history of these useful tools. The earliest concepts of screws as fasteners date back as far as 200 B.C., but the modern, threaded-metal screw we all know today didn’t originate until between the 14th and 17th centuries.
Around the time of the Ancient Greeks, screws were originally designed for use in olive and grape presses. By employing the threaded mechanism of the screw, they could exert more pressure than a human hand would permit, giving the user the ability to use minimal effort when pressing olives and grapes. By the middle ages, these same mechanisms were used for both the paper and printing presses of the day.
As mentioned above fasteners cover a large variety of styles and designs, but all are essentially required for the adjournment of two components, or affixing something to an object through mechanical interaction. Below we will look at how fasteners work, and what features they hold in terms of function. Mechanical properties such as strength, durability and materials are all essential when selecting the right type of fastener for a project.
For high-grade performance capabilities, manufactures often employ carbon alloys. Corrosion is also another factor to consider, and stainless steel fasteners offer unique advantages, as they are more resistant to specific wear and tear. There are over 1000 alloys of stainless steel alloys available, and the performance and longevity will greatly depend on the specific ratio of metals used.
In general, most fasteners consist of a low to medium form of carbon steel, but other metals, such as stainless steel, brass, nickel and aluminum are also added to create resistant alloys. The quality of the alloy or metal used is extremely important to prevent unwanted fatigue or cracking. In addition, component metal can also be plated or coated with additional materials such as zinc, cadmium, nickel or chromium for addition resistance against the elements.
One of the most important properties when selecting a fastener is the type of material used in its manufacturing. While most fasteners are made of metal, the type of metal, its alloys and metallurgical properties are all vital in how it will perform under stress. Different types of materials offer a variety of options for specific applications. Temperature, strength, movement, vibration and fatigue are all important factors when deciding which type of fastener you will be using.
The different properties fasteners will determine the best purpose for your specific application. Below we discuss tensile strength, fatigue, and torsion among other factors to consider when selecting a fastener as well as quality control specification. When it comes to sheer strength in the component, tensile strength is the most common mechanical property reviewed.
Tensile strength is the maximum amount of applied force or load that the metal can withstand before it fractures. Tensile strength is vital to consider when selecting fasteners that will have to endure extreme wear and tear, and can be dependent on both the material used and how the fastener is used.
Fatigue in a single fastener can be a critical factor in some applications, and like the ancient proverb suggests, may result in a catastrophic failure if not employed correctly. When looking at fastener performance, other occurrences can lead to fatigue and cause a weakening strength due to operating conditions. If you exceed the maximum amount of stress, even if tensile strength is designed to withstand the general load requirements, cyclical and repetitive operating conditions can lead to fracture as well.
Quality control is important to maintaining consistency and industry standards across a wide range of applications. For high strength, fasteners such as screws and bolts with more than 150,000 psi provide the tensile strength requirements to meet the specific grade of the industry.
Failure of a single fastener could result in injuries or extreme expense in some instances. Therefore, high strength fasteners with a Grade 8 are commonly employed. Grade 8 standards include pure tensile strength, metallurgical composition of carbon alloy steels and testing and adherence to a variety of industry standards. The primary standards are tensile strength, hardness and design of the bolt head.
These standards are set by organizations like the American Society for Testing and Materials (ASTM), as well as the American Society of Mechanical Engineers (ASME). Other organizations include the American National Standards Institute.
Threaded fasteners require suitable tightening and clamping for proper function. This proper amount of tightening is required to prevent breakage. If a fastener is too loose it will not perform properly — if it is too tight it may be prone to fatigue and fracture. For fasteners subject to repetitive loads, fatigue can result if they’re not tightened properly.
Quality Control and Product Standards
When any torque is applied, fasteners become subject to overcoming friction and elongation. To reduce friction and torsion on a fastener during the tightening process, lubrication is often applied. Recommended torque is useful when trying to properly tighten a fastener, which can help with fatigue loading and reduce a loose connection.
The more energy or torque required, the more friction the bolt will be subjected too and need to overcome. All fasteners, regardless of what material they are, have a slight elasticity to them.
All bolts need to stretch slightly when subjected to a load. Each thread will in turn have a different load applied. At its point of contact, the highest percentage of the load rests on the thread and decreases from each thread onward.
By enduring extensive testing regarding these mechanical properties like toque and elongation, the use of high strength fasteners can receive a grade indicating they are suitable for high performance purposes, such as automotive engines.
The benefits of coarse and fine threaded fasteners depend on the application. Finer threading provides a larger stress area than coarse threading of the same diameter. If you want a greater strength over the same area, a finer thread is a better choice. Finer threading also allows for better adjustment accuracy.
However, coarse threading is also advantages, as it is less likely to become cross-threaded. This allows for quicker assembly. For projects that will need to endure harsh operations, conditions or chemicals, coarse threaded fasteners can be advantageous because they provide better resistance.
Among the most common type of fastener is the screw. Screws are a type of threaded fastener that includes bolts and studs, but like other fasteners come in a range of different types. Some are for wood and adhere to the wooden surfaces, while others are more suited for machines. Threading for screws can be tapered, straight or parallel. As mentioned above, screws also are subject to many of the same mechanical and physical properties associated with other fasteners.
Understanding some of the facts about screws can help you better understand the mechanisms of each type. Screws come in many different sizes and varieties, but all serve to join to objects together. Since the evolution of the earliest mass production of screws in the late 19th century, manufactures still employ many of the same thread rolling processes.
The design of a screw comes in three different parts — the driver, the pitch and sizing. The length of a screw, however, is not included in sizing, but rather the thickness and wideness of the screw. The driver is the type of indentation found on the head and can come in a range of types, some of which are more secure than others. The two most common driver types are flathead and cross head. When referring to the pitch of a screw, pitch is determined by a number indicating the inclined plane degree of the screw’s threading.
There are two processes commonly used to manufacture screws. The thread rolling method must first be proceeded by a process called cold heading. Cold heading is a process in which metal wire feeds into a machine used for straightening.
The straightened metal is then cut at whatever the desired length is needed before a die cuts a blank, which becomes the head of a screw. This process can produce up to 550 screw blanks per minute. From there the thread rolling process can begin as the blanks are put into a thread-cutting dies.
As with other fasteners, quality control standards are conducted through testing. Today, the International Standards Organization, or ISO, determines the quality control standards for production. They factor the number of threads, pitch and diameter sizes of the screws when evaluating for compliance.
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