Bolt (Fasteners) and MEMBER STIFFNESS in Just Over 10 Minutes!

Summary

This video explains the principles behind fasteners, focusing on bolts, screws, and their ability to create non-permanent joints under various loads. It introduces key concepts like pretension (preload), clamping force, and the stiffness of both fasteners and the materials they secure. Using examples and equations, the video demonstrates how to calculate bolt and member stiffness through the application of springs in series. The stiffness of the bolt and the members are crucial for understanding load distribution. The video also previews future discussions on proof strength and calculating safety factors.

Takeaways

• 🔩 Fasteners are used to create non-permanent joints that can resist tensile, moment, and shear loads between objects.
• ⚙️ Clamping force, also known as preload, is generated as a nut is twisted and stretches the bolt, compressing the members being fastened.
• 🔧 Fastener stiffness, including bolt and screw stiffness, is determined by both threaded and unthreaded zones within the grip.
• 🧮 The stiffness of a bolt or screw in the grip can be calculated using the concept of springs in series for both threaded and unthreaded sections.
• 📏 Member stiffness, like bolt stiffness, is also calculated as springs in series and involves various sections such as washers and plates.
• 🛠 The cross-sectional area and thread lengths for bolts can be easily found in online resources, like vendor websites, or through ANSI charts.
• 📐 Member stiffness is calculated using conical stress distribution, often simplified to a cone with a 30-degree half-apex angle for practical calculations.
• 🔍 To calculate member stiffness, the cross-sectional area at any point is determined using the dimensions of the fastener and related geometry.
• 💡 Member stiffness calculations depend on the thickness and material of each component within the grip, and different values for stiffness must be calculated for each section.
• 🔑 By understanding both fastener and member stiffness, it's possible to determine how preload or external loads are distributed between the members and fastener.

Q & A

What is the purpose of using fasteners in mechanical assemblies?

Fasteners are used to create non-permanent joints that resist tensile, moment, and shear loads between two or more objects.

What is preload or pretension in the context of fasteners?

Preload or pretension is the initial clamping force generated when a nut is twisted and stretches the bolt, producing a reaction force that clamps the members together.

How is clamping force generated in screws and bolts?

In bolts, the clamping force is generated as the nut stretches the bolt after initial contact with the clamped member. For screws, such as cap screws, the screw's body stretches when tightened, generating a clamping force.

What role do washers play in fastened joints?

Washers contribute to the total grip thickness in fastened joints and are part of the member stiffness calculation. Their thickness is included in the overall member and bolt stiffness considerations.

What is bolt stiffness, and how is it calculated for different fasteners?

Bolt stiffness is the measure of resistance to deformation in the bolt and is calculated using separate equations for the threaded and unthreaded portions of the bolt. It can be modeled as two springs in series, where the total stiffness depends on the combined stiffness of the two zones.

How does member stiffness influence fastener performance?

Member stiffness refers to the resistance of the clamped objects to deformation. Like bolt stiffness, member stiffness is also modeled as springs in series. The stiffer the members, the more they resist deformation, which affects the distribution of loads between the fastener and the members.

What is the significance of calculating the area as a function of distance in the member stiffness equation?

In member stiffness calculations, the area changes along the conical profile of the clamped members. The area at any point affects the displacement and stiffness, making it important to calculate the area as a function of distance from the bolt hole.

Why is a conical stress profile used in member stiffness calculations?

A conical stress profile is used because the stress distribution within clamped members forms a cone shape as the cross-sectional area increases away from the bolt hole. This allows for a more accurate representation of the load transfer through the members.

What factors determine the member stiffness when multiple materials are clamped?

When multiple materials (such as washers and plates) are clamped, each material contributes a different stiffness value based on its thickness, outer diameter, and the bolt's nominal diameter. The total member stiffness is calculated by considering each material’s stiffness separately.

How can you find bolt dimensions like threaded length or diameter when designing a fastened joint?

Bolt dimensions, such as threaded length and diameter, can be found online from vendors or through standards like ANSI charts. These sources provide necessary values for calculating bolt stiffness and ensuring the correct dimensions for specific applications.