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A bolt is not simply a pin holding two parts together. When you tighten it, you create preload, and that preload is a major part of how a bolted joint behaves under service loads.
What Is Bolt Preload?
Preload is the tension created in the bolt during tightening. The clamped parts experience a corresponding compressive force. A properly designed joint uses this relationship to keep the assembly together.
Why Torque Is Not the Whole Story
Torque is commonly used because it is easy to apply on the shop floor. But much of the applied torque can be consumed by friction in the threads and under the bolt head. That means the same torque does not always produce exactly the same preload.
Think About the Joint
Joint stiffness, bolt size, lubrication, surface condition and tightening method all matter. For critical assemblies, engineers may use torque-angle methods, tension measurement or other controlled tightening methods.
A Common Design Mistake
Specifying a very high torque without checking the bolt grade and joint design can overload the fastener. Always check the applicable fastener specification and design requirements.
CAD Drawing Tip
Your CAD model may show the bolt perfectly, but the drawing and assembly documentation should communicate the actual fastener specification and tightening requirement when it matters.
Keep learning: Explore more practical engineering articles in the GetCADFile Design Guide.