drawhead

A drawhead is the enlarged, flanged terminus of a drawbar, the steel rod that couples railway vehicles together. It sits at the outer face of a locomotive or wagon, projecting slightly beyond the buffer, and serves as the mechanical anchor point where the coupling link or hook engages. The flanged design, typically a cast or forged steel ring 200 to 300 mm in diameter, resists the side and vertical loads transmitted during coupling and train movement. The drawhead bears the full tensile force of the train; a freight locomotive may transmit 300 to 400 tonnes of pull through its drawhead during starting or climbing grade.

The drawhead works in tandem with the coupling link, a separate U-shaped or oval steel member that loops around the drawhead of one vehicle and through the drawhead of the next, creating a flexible articulated joint. This arrangement allows slight angular movement between vehicles without binding, critical on curves and over undulations. The flanges on the drawhead prevent the coupling link from slipping sideways off the rod, a failure that could derail the entire train.

Wear and failure patterns

Drawheads are prone to three main failure modes. Fatigue cracks develop in the cast material where the flange meets the drawbar shank, initiated by millions of impact loads from coupler slack and jerking as brakes release and locomotives pull. Wear of the drawhead's outer face occurs where the coupling link rides against it during oscillation, gradually thinning the flange until it can no longer contain the link. Shearing occurs under exceptional impact, particularly when coupling at high speed or when a freight train is jerked suddenly by an air brake application.

The term 'drawhead' distinguishes the flanged end from the plain drawbar itself, which extends back into the locomotive or wagon frame where it is bolted or welded to the draft gear. On modern freight stock in North America, the drawhead is integrated into a single-piece draft system with a shelf coupler or E-coupler, reducing the separate links and making the whole assembly more robust. On older vehicles and many European railways, the separate link remains standard.

Inspection of drawheads is routine on every locomotive and freight car at depot and between shifts. Cracks are detected by magnetic particle inspection after the drawhead has been ground and degreased. Worn or cracked drawheads are cut out and replaced by re-boring the drawbar shank and pressing in a new forged drawhead, a process requiring accurate alignment to prevent eccentric loading during service.