The document summarizes literature on expandable and collapsible winding mandrels. It begins with an introduction to winding mandrels and their importance in the coil winding process. It then reviews 14 US patents related to expandable and collapsible winding mandrel designs. Common features of mandrels are identified including dividing plates, winding baffles, and supporting bars. The review analyzes and classifies the patents based on whether the mandrel is expandable or collapsible. Expandable mandrels are expanded before winding and collapsed after to remove the coil. Collapsible mandrels are wound on and then collapsed to remove the coil. The conclusion proposes changing from a circular to elliptical mandrel design to reduce transformer size by 15% while maintaining