Bolted joints are used everywhere, but how much do you really understand about how they work? How are these joints able to transfer large forces and resist cyclic loads? It’s not quite as straightforward as it might seem.

The strength and effectiveness of a bolted joint all comes down to how it is assembled. When the nut is tightened the threads on the bolt and the nut engage, and rotation of the nut pulls the bolt threads down, which causes the bolt to stretch. This stretching creates a tensile force in the bolt, pulling the two joined members into contact with one another and compressing them.

This tensile force in the fastener is called preload. It is intentionally applied to the bolt or screw before any external loads that act on the joint. Preload is surprisingly powerful. It makes the joint stronger, more resistant to fatigue damage, and much less likely to fail.

The video below covers this in detail, and explains why preload is a key consideration when working with bolted joints.

Geometric dimensioning and tolerancing (GD&T) is a tolerancing technique that is used in the mechanical design process to control the fit and function of parts and assemblies. It complements traditional “plus and minus” dimensional tolerancing by letting you control 14 different geometric characteristics.

The geometric characteristics can be split into 5 different categories:

• Form Tolerances – used to control flatness, straightness, circularity and cylindricity.
• Orientation Tolerances – used to control parallelism, perpendicularity and angularity.
• Location Tolerances – used to control position, concentricity and symmetry.
• Profile Tolerances – used to control profile of a line or profile of a surface.
• Runout Tolerances – used to control cirular runout or total runout.

Unlike traditional dimensional tolerancing, that applies tolerances to dimensions, GD&T applies tolerances to features instead. Geometric tolerances are applied to features by adding feature control frames to the relevant feature on the engineering drawing.

The video below uses 3D animations to explain the different geometric characteristics in detail. It also covers other key concepts in GD&T, including:

• Datums, with an explanation of their importance, and how they are defined as used
• Material modifiers – how the Maximum Material Condition (MMC), Least Material Condition (LMC) and Regardless of Feature Size condition (RFS) are used to gain bonus tolerance depending on the actual size of a feature.
• The Envelope Principle, also called GD&T Rule #1, that is used by default in the ASME drawing standard, and the Independency Principle that is used by defaul in the ISO drawing standards. These principles establish the rules used to define whether the shape of a feature is controlled by its size.

Engineering drawings are key tools that engineers use to communicate, but deciphering them isn’t always straightforward. The video below covers the fundamentals, including the different types of views, first and third angle projection methods, dimensioning, tolerancing, best practices when creating drawings. It also provides a brief introduction to GD&T (geometric dimensioning and tolerancing).