Is it true that Bolts only experience tensile stress.?

Bolts also experience significant shear stress due to external forces and torsional stress from tightening and vibration.

Is it true that Friction in bolts is constant.?

Friction depends on the normal force, the coefficient of friction, and can be affected by the thread geometry (e.g., the helix angle).

Mechanical Fasteners: Bolts and Screws

Understanding the forces, stresses, and friction involved in bolted connections is crucial for ensuring structural integrity and preventing self-loosening.

Core Principles

Friction is key to bolt function: It resists relative motion between threads and under the bolt head, preventing loosening.
Self-loosening occurs when the axial force (Q) tends to cause slipping along the thread incline, overcoming friction.
Bolt strength calculations involve shear stress, bearing stress, and equivalent stress, considering factors like thread angle and friction coefficient.

Action Steps

Calculate the axial force (Q) acting on the bolt.
Determine the friction force (F_f) using the normal force (F_N) and friction coefficient (μ).
Calculate the tangential force (F_t) required for tightening or loosening using the axial force and thread/friction angles.
Evaluate the stresses (tensile, shear, equivalent) in the bolt core to ensure it meets strength requirements.

Formulas

Friction Force: $F_f = an(\varphi) \cdot F_N$
Torque for Tightening/Loosening: $F_t = Q \cdot \tan(\alpha \pm \varphi)$
Equivalent Stress (Strength Hypothesis): $\\sigma_{eq} = \sqrt{(\\sigma_t)^2 + 4(\\tau_t)^2}$
Torsional Shear Stress: $\\tau_t = \frac{M_t}{Z_0}$
Torsional Section Modulus (for circular cross-section): $Z_0 = \frac{\\pi}{16} \cdot d_1^3$

Key Terms

Axial Force (Q): The force acting along the axis of the bolt, typically due to clamping or external loads.
Friction Angle (φ): The angle whose tangent is the coefficient of friction (tan(φ) = μ), representing the resistance to sliding.
Helix Angle (α): The angle of the screw thread, related to the pitch and the mean diameter of the thread.
Equivalent Stress (σeq): A calculated stress value that combines tensile and shear stresses into a single value for comparison against material strength limits.
Self-loosening: The tendency of a bolted joint to loosen under vibration or cyclic loading due to the axial force overcoming static friction.

Pro Tips

The friction coefficient (μ) can be adjusted by factors like thread angle (β), leading to a modified friction coefficient (μ').
Self-loosening is prevented if the thread angle (α) is less than or equal to the friction angle (φ).

Pitfalls to Avoid

Ignoring friction leads to underestimating the torque required for tightening and overestimating the tendency for self-loosening.
Using simplified stress assumptions (like the basic assumption) instead of more accurate strength hypotheses can lead to inaccurate safety assessments.

Myth vs Reality

Bolts only experience tensile stress.: Bolts also experience significant shear stress due to external forces and torsional stress from tightening and vibration.
Friction in bolts is constant.: Friction depends on the normal force, the coefficient of friction, and can be affected by the thread geometry (e.g., the helix angle).

Real World Examples

Tightening a bolt on a machine part.: The applied torque creates an axial force (Q) and friction forces (Ff) that hold the parts together. The torque must be sufficient to overcome thread friction and achieve the desired clamping force.
A bolted joint subjected to vibration.: Vibration can reduce the effective normal force, potentially causing the axial force to overcome static friction, leading to loosening if the helix angle is too large relative to the friction angle.

Statistics

Standard friction coefficient (μ) for steel on steel: Typically between 0.15 and 0.25
Torsional shear stress limit (τt) for bolt core: Must be less than or equal to Z0