When it comes to selecting a tough fastener, many factors require consideration. With virtually endless configurations available, YOU can choose the right part for your connection at a cost that works for you. The “toughness” of your fastener depends on several key factors—let’s break it down!
GRADE: In the fastener industry, there are established oversight bodies including ASTM, ISO, SAE, DIN, and ANSI that define universal standards, ensuring consistency across manufacturers. This means that no matter where a fastener comes from, you can trust its specifications. One critical element that remains consistent is the fastener grade. The grade indicates the strength and material properties of the fastener and gives you a sense of how much load it can handle before failure. The higher the grade, the stronger the fastener. Let’s review hex cap screws as an example:
Best for high reliability under extreme conditions such as in heavy machinery or construction.
MATERIAL: The materials selected for manufacturing fasteners play a vital role in determining their performance across a range of conditions. The choice of material significantly influences the fastener's strength, durability, corrosion resistance, and overall cost-effectiveness. Additionally, it is essential to assess the environmental factors specific to the application, as these will directly impact the long-term efficacy and reliability of the fastener.
Steel: One of the most common materials, steel offers excellent tensile strength and is typically used in general-purpose fasteners. It’s available in different grades with additional elements (such as chromium or nickel) to further increase strength and durability. Given that its widely available, steel fasteners are a cost-effective option for most applications. However, plain steel is not able to withstand moisture or similar environmental conditions resulting in a decreased longevity than those applications indoors for instance. (Stay tuned for discussions about how plating can impact steel!)
Brass: Corrosion-resistant and non-magnetic, brass is often used in applications where aesthetics are a higher priority (such as architectural or decorative designs). It is often called for in electrical components because of its conductive properties as well. Brass is suitable for low temperature applications as it will become brittle with high heat or chemical exposure making it less reliable than steel and stainless steel.
Stainless Steel: Known for its excellent resistance to rust and corrosion, stainless steel is ideal for those applications with moisture, chemical, or other harsh environmental considerations exist. It also can withstand moderate to high temperature applications making it an excellent choice for exposure to heat. These factors allow stainless steel greater longevity in harsh environments, which comes with a higher cost than steel. Depending on your application, the initial higher cost is worth the increased maintenance cost of lesser materials.
SIZE: When selecting the best fastener for your application, size plays a critical role in the strength and reliability of your connection. But what exactly does “bolt size” mean, and how do you measure it? The size of a bolt consists of several components, let’s review.
E: The diameter is the measurement around the outside of the shaft of a fastener, just under the head. It’s also called the outside diameter, and it stays consistent whether the shaft is threaded or not.
H: The head height is straight forward but a critical component. A taller head allows for better engagement, which means more torque can be applied without failure. It also improves resistance to shearing.
L: The length is measured from just below the head to the end of the bolt. Bolts can be fully or partially threaded with a threaded length (LT) as well.
F& G: The head width across flats and width across corners doesn't directly impact the strength, but rather, impacts how you apply torque for installation. The size of the bolt's head will affect how the force is distributed, the larger size of head will allows more even force distribution.
The difference between the threaded and unthreaded shaft on a bolt has significant impacts to the strength as well, particularly in tension and shear. The threaded section of a bolt has a smaller root area which leads to a lower tensile strength. The threads are at risk to crack if too high of load is applied. The unthreaded shaft offers the full diameter, allowing for a smooth surface and greater shear strength.
The industry standard for identifying bolts is generally follows this format: Diameter-Thread/pitch x Length, followed by the type, material, and finish
EX) 3/8”-16 x 2” hex bolt, Grade 5, zinc plated
THREAD or PITCH: We know that the thread of a bolt is the spiral ridge going around the shaft to be inserted into base material. Let’s break down the options and determine what is best for your application to ensure a successful connection that will last.
Coarse Thread/Pitch: The ridge is deeper and turns wider spaced making it ideal for soft materials. The larger ridges are more robust with less turns required for a quicker installation.
Fine Thread/Pitch: The ridge is shallow and turns closely spaced making it ideal for hard material that requires a precise fit. There is more surface area to engage with the base material allowing for an even load distribution.
A bolt’s thread—or pitch—is typically specified immediately after the diameter (EX 3/8"-16). However, thread pitch is not uniform across all diameters, which is why it’s crucial to understand the context and how thread changes with bolt diameter. For instance, 16 is considered coarse for a 3/8"-16 bolt, but the same 16 is classified as fine for a 3/4"-16 bolt. A larger diameter bolt takes more rotations to achieve the same number of threads per inch compared to a smaller diameter, assuming the same TPI as in our example of 16. This highlights how the classification of coarse or fine threads is relative to the bolt's diameter. In general, a higher value means the threads are more closely spaced, which defines a fine thread. Note the distinction in the table below:
Additionally, an important factor that influences bolt strength is the length of the threaded portion. A bolt’s diameter is technically the major diameter as this measurement is taken on the outermost part of the shaft regardless of being threaded or not it will measure the same. On the other hand, the minor diameter which is the smallest measurement at the base of the threads.
Why is this distinction important? A bolt’s load is really carried by minor diameter. For bolts that are partially threaded, the minor and major diameters are the same for the unthreaded section resulting in a higher strength. Using partially threaded bolts will improve load bearing capacity for this reason.
