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Difficulty: MediumFasteners and Joints

Review the following mechanical fastening scenarios. Match each scenario to the primary type of stress or characteristic failure mode it is most likely to experience.

  • Two overlapping metal plates secured by a single bolt and pulled forcefully in opposite, parallel directions.Single shear stress across the cross-section of the fastener.
  • A bolt and nut assembly tightened significantly beyond its recommended torque specification.Excessive tension resulting in fastener stretching or stripped threads.
  • A pulley attached to a rotating drive shaft using a square machine key.Torsional shear stress as rotational force is transferred.
  • A threaded rod subjected to continuous, alternating bending cycles during machine operation.Metal fatigue leading to progressive cracking and eventual fracture.

Answer

Overlapping pulled plates experience single shear stress; over-tightened bolts suffer excessive tension; keyed pulleys transfer torsional shear stress; and alternating bending cycles lead to metal fatigue.
Each scenario describes a distinct physical loading condition. Overlapping plates pulled in opposite directions slice across the bolt (single shear). Torquing a nut stretches the bolt shaft along its length (tension). A rotating shaft transferring power through a key creates a twisting sideways force (torsional shear). Continuous bending back and forth causes microscopic damage over time (fatigue).

Step-by-Step Solution

1
Analyze the overlapping plates scenario.
Identified that parallel pulling forces on overlapping joined plates create a slicing or cutting action (shear) across the fastener.
This is the classic definition of a lap joint in single shear.
2
Analyze the over-tightening scenario.
Identified that tightening a nut draws it up the bolt threads, pulling the bolt shaft lengthwise.
Applying torque translates into clamping force, which places the bolt shaft under tensile stress (tension).
3
Analyze the keyed pulley scenario.
Identified that the key prevents the pulley from slipping on the spinning shaft, transferring rotational force.
The transfer of rotational motion creates a twisting shear force known as torsional stress on the key.
4
Analyze the alternating bending scenario.
Identified that continuous, repeating stress cycles cause microscopic wear within the metal structure.
Cyclic loading is the primary cause of metal fatigue, especially where stress risers like threads exist.

Key Concept

Identifying the specific types of mechanical stress (shear, tension, torsion, fatigue) applied to fasteners in different operating environments.
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Topics

Fasteners and Joints

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