Static rotational friction: does diameter matter?

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SUMMARY

The discussion centers on the impact of washer diameter on static rotational friction in bolted joints. It establishes that while the friction force remains constant regardless of diameter, the moment is proportional to the diameter of the washer. The torque required to tighten the bolt is influenced by whether the bolt head slips on the washer or the washer slips on the joint. For accurate calculations, one must use the average diameter of the slipping surfaces and ensure that compressive stress is evenly distributed.

PREREQUISITES
  • Understanding of static friction principles
  • Knowledge of torque calculations in mechanical systems
  • Familiarity with bolted joint design
  • Basic concepts of moment and lever arm effects
NEXT STEPS
  • Study "Torque Calculation for Bolted Joints" in machine design textbooks
  • Research "Moment of Inertia and Its Impact on Rotational Systems"
  • Explore "Friction Models in Mechanical Engineering" for deeper insights
  • Learn about "Compressive Stress Distribution in Bolted Connections"
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Mechanical engineers, design engineers, and students studying bolted joint mechanics and static friction principles.

ShearStress
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Imagine a bolted joint with a washer between the bolt and the surface.

Assuming the washer is always covered by the bolt head so it's getting a consistent load, does the washer's diameter impact the static friction being imparted on the surface?

I see two conflicting ways of viewing this:
1.) Friction doesn't care about surface area, so no the washer diameter doesn't matter (perhaps this is more relevant for linear friction, not rotational?)
2.) The larger the washer is, the higher the moment is since the friction is acting further from the axis of rotation

What's the answer and how can this solution be expressed mathematically?
 
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I don't see a conflict, the friction force stays the same but the moment is proportional to the diameter (assuming the normal force is equally distributed along the washer's diameter, which is not certain).
 
Your question is unclear:

1) If you are asking about the force to make the joint slip linearly, then your statement #1 is correct.

2) If you are asking about the torque to tighten the bolt, then your statement #2 is correct. The procedure is:
A) Determine if the bolt head is slipping on the washer or the washer is slipping on the joint.
B) Calculate the torque using the average diameter of the slipping surfaces, assuming the compressive stress is evenly distributed.

A quick search for bolt torque calculations did not find this, but it is in any book on machine design. It's in the chapter on bolted joints.
 

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