Dear experts in fasteners simulation : Bolt with friction

Hello Forum, I am a teacher from France, and I've been utilizing CREO 5.0.3.0 to develop a course on fastener design and calculations. My objective is to illustrate the correlation between calculations and simulation to my students. To introduce various parameters, I've opted to use a straightforward model – two components connected by a bolt.

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During a simulation involving an axial force, I achieved results consistent with my calculations (I have attached a PDF in French).

Now, I aim to conduct a simulation with a tangential force. This scenario is more complex. I have anchored the pink part and applied a tangential force (500 N) to the yellow segment. This represents the bolt definition with a preload of N.

I have defined an interface with infinite friction and set a friction indicator at 0.5.

Upon analyzing the results: for simplicity, the preload is N, and the friction indicator is 0.5. Theoretically, the maximum tangential force should be equal to N before slippage occurs. Surprisingly, the shear force on the fastener is low, approximately 10 N, which seems unusual, and slippage is evident.

According to CREO documentation, it states:

Create Slippage Indicators. Creates three slippage indicator measures. It is available for selection only if you select the

Infinite

These measures assess whether slippage occurred in the contact area during a static analysis with contacts. If any of the slippage indicator values are positive during such an analysis, a warning appears in the summary report (.rpt file), indicating that the surfaces in contact may have slipped relative to each other, thus questioning the infinite friction assumption.

I have observed that the identifier for any slippage in interface1 reports a positive value.

The mathematical expression states that slippage does not occur when the value of Si is less than or equal to zero. Creo Simulate computes Si across the surface and employs Si for calculating slippage indicator measures:

A positive value indicates that slippage has occurred at least at one point in a contact region.

Your insights into this matter—and any guidance you can provide—would be immensely appreciated, as I seek to resolve these anomalies. Thank you for taking the time to read my post.

Best regards,

VV

Dear Autopians. It's time. Time to delve deep into the world of engineering. Have you ever been curious about the physics that governs the functioning of bolts? They essentially operate like springs. How do manufacturers determine the appropriate bolt size? How is fastener installation torque specified? Do you ever wonder how much of the force applied through a ratchet actually contributes to creating clamp load versus merely overcoming thread friction? Let us embark on this enlightening journey to explore the seemingly mundane yet intricately fascinating world of fasteners.

[Welcome to Huibert Mees' column, where the former Ford GT/Tesla Model S suspension engineer shares his expertise on The Autopian. -DT]

So, what defines a bolt? At its core, a bolt is simply a clamp – a miniature version of a C-clamp. If you aim to temporarily hold two objects together, you resort to a C-clamp. For a more permanent bond, a bolt serves as the best option. Formally stated:

A bolt provides a clamping force between two or more surfaces ensuring they cannot move relative to each other under typical service loads.

Bolts are tasked with clamping together two main types of joints: Shear and Tensile. In shear joints, the bolt utilizes friction to prevent movement between surfaces. In tensile joints, the bolt generates sufficient preload, ensuring that surfaces do not separate under normal loads.

Understanding Shear Joints

A bolt relies on friction to maintain the connection between objects, preventing movement when normal forces are applied. It's critical for a bolt to prevent any displacement; even minimal movement signifies failure, indicating the joint's weakening or breaking. Thus, the bolt must be appropriately designed to provide the necessary friction to withstand the expected forces.

Bolt as a Spring Analogy

How does a bolt create the friction needed? A bolt functions similarly to a spring. Whenever a spring is pushed or pulled, it develops a force governed by:

F = K x X

Where X denotes the displacement of the spring. For example: if a spring exhibits a stiffness of 15 lb/in, it demands 15 lb of force to compress it by one inch. To compress it by two inches, you would need:

F = 2 x 15 lb/in = 30 lb

The same principle applies for bolts. Tightening a bolt results in the threads attempting to draw the nut toward the bolt head. The clamped object intervenes, necessitating the bolt to stretch like a spring. This process becomes increasingly challenging, akin to a spring where added tension escalates resistance to tensioning.

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Upon tightening the bolt, the forces exerted by the bolt and objects are equal, generating friction that maintains their relative positions.

Calculating Bolt Capabilities

Having understood how a bolt generates friction, the next step involves determining its adequacy. Begin with establishing the resisting force of the joint.

As an example: Assume two steel plates require bolting together, capable of withstanding a pulling force of 1,000 lb. The friction between two surfaces is formulated as:

F_friction = mu x N

Where N signifies the force applied to maintain the surfaces together (in this scenario, the clamping force from the bolt) and mu stands for the ‘coefficient of friction’ of the materials involved. The characteristics of friction rely heavily on the materials in contact – steel on concrete versus steel on Teflon show significant variance in sliding resistance.

The coefficient of friction generally ranges from 0 to 1, influenced by empirical testing across different materials, with typical values for steel against steel around 0.2.

Returning to our earlier example, knowing mu (0.2) and required friction (1,000 lb), we seek N. Rearranging gives:

N = F_friction / mu

Expanding our calculations leads to N = / 0.2, yielding N = 5,000 lb. Subsequently, we select a robust bolt capable of withstanding the calculated force.

Torque Specifications for Tightening

Once the appropriate bolt is selected, we need to determine its tightening measure. Multiple resources offer guidelines on bolt tightening; however, precise tightening requires physical testing. We must prepare prototypes that mirror final products. Through rigorous testing, we can ascertain torque requirements ensuring longevity and reliability of bolt joints.

For further inquiries or detailed discussions on Friction Bolt, feel free to contact us today!