Selecting the best fastener for an application has a major effect on its functionality and operational lifetime. This includes things like intended use, material, operational environment, and more.
In the case of bolts and screws, another critical consideration is thread engagement. This is the extent to which two threaded components, such as a bolt and nut, interconnect or overlap when the fastener is installed, usually expressed as the length or percentage of engagement.
Keep reading to learn how calculating and choosing fasteners with optimal thread engagement adds significantly to joint and application integrity.
How to Calculate Minimum Thread Engagement
Click to Enlarge
When designing a joint that uses threaded fasteners like bolts and nuts, it’s important to determine the minimum amount of engagement, or overlap, that is needed to ensure a strong joint without stripping threads or damaging components.
Thread engagement is expressed in two ways:
- Length of Thread Engagement. This refers to the actual length of the threaded section of a bolt that will be inside the nut when fully screwed in. For example, a bolt with 1.00 in. of threads and a nut with 0.5 in. of threads will have 0.5 in. of engagement when fully screwed together.
- Percentage of Thread Engagement. This is the size of the nut or mating hole vs. that of the fastener and the proportion of the height of the bolt’s threads that are engaged.
In calculating the minimum thread engagement required for a specific joint, the goal is to ensure the fastener will be the failure point rather than the nut or component. To do this, it’s necessary to calculate the tensile stress area and shear area on the bolt in the cross-section where the bolt and nut overlap. The tensile stress area is the physical area that provides strength under tension applied to the bolt. The shear area is the physical area of the bolt that resists shearing forces applied to it. These two calculations include factors of major diameter, minor diameter, and pitch diameter.
The ideal minimal length of engagement will use as much of the bolt’s tensile strength as possible before thread shearing occurs. This generally means that the bolt will fail or break instead of the nut or other internally threaded component being stripped. If the fastener and nut are made of materials with different properties and tensile strengths, this affects how the length of engagement is calculated.
Some rules of thumb are helpful when making these calculations for certain materials, such as:
- For aluminum threads, the minimum thread engagement should be 2.0 to 2.5 x the bolt’s or screw’s base nominal diameter
- For brass or cast iron threads, the minimum thread engagement should be 1.5 to 2.0 x the bolt’s or screw’s base nominal diameter
- For steel threads, the minimum thread engagement should be 1.0 to 1.5 x the bolt’s or screw’s base nominal diameter
Why Is Thread Engagement Important?
Thread engagement is a way to add to the strength and durability of a joint, which in turn improves the functionality and useful life of the application. The key is to strike a balance between the ideal bolt length and depth of the bolt in the nut member, as this will prevent breakage and stripping.
If thread engagement is inadequate, threads in the nut or tapped component may be stripped due to high stress and shear forces. The joint also may not be strong enough to withstand other stresses from opening and closing, vibrations, or other use conditions.
Greater thread engagement length generally increases the tensile strength of the joint; however, specifying a bolt that is too long or has a length of engagement that is excessive may result in additional cost or wasted material. A very high length of engagement can also cause high levels of torque for driving in the bolt or screw, which can compromise strength or cause the fastener to break during assembly or installation.
Source Standard and Customer Fasteners from Associated Fastening
Thread engagement is critical in designing a strong joint that will resist shearing and failure over an application’s lifetime. Associated Fastening Products, Inc. provides a comprehensive selection of fasteners, cable ties, hardware kits, O-rings, washers, and fittings. We also offer CNC and Swiss screw machining, water jet and laser cutting, and metal stamping services.
Contact us today to learn more about our products and services or request a quote to start your next project.