How is a puller mandrel utilized in cold expansion?

A puller mandrel is utilized in cold expansion by being pulled through a split sleeve, which effectively enlarges a pre-drilled hole and cold works it to improve its mechanical properties. This process involves inserting the mandrel into the sleeve and pulling it through, causing the sleeve to expand outward and increase the diameter of the hole. The cold working that occurs during this operation enhances the material's strength by introducing residual compressive stresses in the surrounding area of the hole.

This technique is particularly beneficial for fastener holes in aerospace structures, where increased load-carrying capacity and resistance to fatigue are critical. The alignment of the sleeve with the hole ensures that the expansion occurs uniformly, which is vital for the performance and reliability of the assembly. By increasing the hole diameter without heat, the integrity of the surrounding material is preserved, maintaining the desired properties without compromising the component's structural integrity.