Which type of metal coupling is designed to have a one-piece construction and does not require lubrication?

The correct answer is bellows coupling. This type of coupling is specifically designed with a one-piece construction that allows it to maintain its shape and integrity, even under varying loads and misalignment conditions. The bellows are typically made from a flexible material capable of accommodating some degree of axial, angular, and parallel misalignment while providing a high level of torque transmission.

Additionally, the design of a bellows coupling eliminates the need for lubrication. This is particularly advantageous in applications where sealing is crucial or where maintenance accessibility is limited. The absence of lubricants means there is reduced risk of contamination and the associated maintenance issues that can arise, making bellows couplings a reliable choice in many industrial applications.

Grid couplings generally consist of two hubs and a grid element that transfers torque and allows for misalignment but do require lubrication. Sleeve couplings are simpler and can involve multiple components that sometimes need additional maintenance, while chain couplings, which also consist of multiple parts, typically require lubrication to function effectively. Thus, the bellows coupling stands out for its robust design and lubrication-free operation.