Which hinge would be least appropriate for heavy-duty applications?

The pin hinge would be least appropriate for heavy-duty applications due to its design and construction. Pin hinges are typically smaller and simpler in design, which makes them suitable for lighter tasks or applications where minimal load-bearing capacity is required. They do not provide the same level of strength and stability as the other types of hinges listed.

In contrast, butt hinges are commonly used on doors and larger gates and can handle significant weights and stresses. Strap hinges are designed for heavier applications as they have a long leaf that distributes weight, making them ideal for gates or barn doors. Piano hinges, or continuous hinges, run the full length of the door or lid, providing strong support and distributing weight evenly, making them an excellent choice for heavy-duty applications.

Therefore, while pin hinges have their uses, they fall short in situations requiring robust support, durability, and the ability to handle heavy loads.