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Question

Difficulty: MediumFasteners and Joints

When fastening a rigid steel bracket to a softer aluminum base, a wide, flat metal washer is typically placed beneath the bolt head and nut. Which of the following best describes the primary mechanical function of this flat washer?

  1. It distributes the compressive clamping force over a larger surface area to prevent crushing the softer material.Answer
  2. B
    It acts as a sacrificial shear plane to absorb lateral forces before they can damage the bolt shaft.
  3. C
    It provides continuous spring-like tension against the bolt head to prevent loosening from vibration.
  4. D
    It increases the contact area between the fastened plates to generate higher friction and prevent slipping.

Answer

It distributes the compressive clamping force over a larger surface area to prevent crushing the softer material.
The correct answer accurately identifies that a flat washer increases the bearing area under the fastener. Because mechanical pressure is equal to force divided by area (P=FAP = \frac{F}{A}), increasing the contact area reduces the localized compressive stress on the softer material, preventing the bolt head from sinking into or crushing it.

Step-by-Step Solution

1
Analyze the forces acting on a joint when a bolt is tightened.
Tightening the bolt creates tension within the bolt shaft and downward compressive force against the clamped materials.
Understanding the primary stresses in a bolted joint is necessary to evaluate component functions.
2
Evaluate the material properties given in the scenario.
Aluminum is softer than steel and has a lower compressive yield strength, making it vulnerable to high localized pressure.
Identifying the weak point in the assembly explains why additional hardware is needed.
3
Apply the mechanical relationship between force, area, and pressure (P=FAP = \frac{F}{A}).
The flat washer has a much larger diameter than the bolt head. By spreading the same clamping force over this larger area, the overall pressure is significantly reduced.
This calculation proves that a larger surface area prevents the compressive stress from exceeding the aluminum's yield strength.

Key Concept

Load Distribution and Surface Pressure in Bolted Joints
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Topics

Fasteners and Joints

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