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Question

Difficulty: EasyFasteners and Joints

A solid metal drop-pin is inserted vertically through the hitch of a tractor and the tongue of a trailer. When the tractor drives forward to pull the heavy trailer, which of the following best describes how the resulting load primarily acts on the pin?

  1. It creates a cutting or slicing force across the pin's middle section.Answer
  2. B
    It pulls and stretches the pin outward from its top and bottom ends.
  3. C
    It crushes and squeezes the pin inward from its top and bottom ends.
  4. D
    It twists the upper half of the pin in the opposite direction of the lower half.

Answer

The load creates a cutting or slicing force across the pin's middle section.
The tractor pulls forward while the trailer resists backward. Because the pin is positioned vertically, these opposing horizontal forces push across the pin's midsection in opposite directions. This attempts to slide the top half of the pin past the bottom half, creating a slicing action known in mechanics as shear stress.

Step-by-Step Solution

1
Identify the direction of the forces acting on the joint.
The tractor pulls forward horizontally on the top part of the joint, while the trailer's weight pulls backward horizontally on the bottom part.
Understanding the orientation of the applied forces is required to determine the type of stress.
2
Determine how these forces interact with the vertically positioned pin.
The opposing horizontal forces act across the cross-section of the vertical pin, pushing its top half forward and pulling its bottom half backward.
The relationship between the direction of the force and the orientation of the fastener determines the specific stress type.
3
Classify the resulting physical action.
Forces that attempt to slide one part of a material past another create a cutting or slicing action, known as shear stress.
Shear stress is the primary failure mode for pins used in horizontal towing applications.

Key Concept

Identifying Joint Loads and Shear Stress
Estimated Time:45s
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Topics

Fasteners and Joints

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