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Difficulty: HardFasteners and Joints

In a mechanical flight control linkage, a solid steel pin is used to connect a single control rod (tang) inside a U-shaped bracket (clevis). A heavy tensile force is applied to the control rod, pulling it directly away from the fixed bracket. Which of the following best describes the primary type of stress experienced by the pin and how its load-bearing capacity compares to a simple two-plate lap joint secured by a single identical pin?

  1. The pin experiences double shear stress, distributing the load across two cross-sectional areas and making it significantly stronger than a lap joint.Answer
  2. B
    The pin experiences single shear stress, concentrating the load at one cross-sectional area and providing the same strength as a lap joint.
  3. C
    The pin experiences pure tensile stress because the control rod is being pulled, making it more susceptible to stretching than a lap joint.
  4. D
    The pin experiences compressive stress as the sides of the bracket squeeze inward, providing greater rigidity than a lap joint.

Answer

The option stating the pin experiences double shear stress, distributing the load across two cross-sectional areas and making it significantly stronger than a lap joint.
The pulling force on the control rod acts perpendicular to the longitudinal axis of the connecting pin, which creates shear stress (a slicing force). Because the U-shaped clevis bracket supports the pin at two distinct points—above and below the control rod—the pin must be sheared in two separate places for the joint to fail. This configuration is known as 'double shear.' Because the load is distributed across two cross-sectional areas of the pin instead of just one, a double shear joint is significantly stronger than a simple lap joint, which only puts the fastener in single shear.

Step-by-Step Solution

1
Analyze the direction of the applied force relative to the pin.
The control rod is pulled horizontally, which applies a force perpendicular to the longitudinal axis of the vertical connecting pin.
Forces acting perpendicular to the axis of a fastener create shear stress, not tensile stress.
2
Examine the physical structure of the joint.
The joint consists of three overlapping sections: the upper arm of the clevis bracket, the central control rod (tang), and the lower arm of the clevis bracket.
Identifying the number of overlapping components determines how many places the pin could be severed.
3
Determine the number of shear planes.
Because the control rod pulls in the middle and the bracket resists on both the top and bottom, the pin would have to be sliced in two separate places to fail.
Two shear planes indicate 'double shear', which distributes the force over twice the cross-sectional area of the pin compared to a standard two-plate lap joint (single shear).

Key Concept

Identifying load distribution and shear stress (single vs. double shear) in mechanical joints.
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Topics

Fasteners and Joints

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