swallow

The swallow is the opening or channel in a pulley block through which a rope or line passes around the sheave. It sits between the inner curved surface of the sheave itself and the outer shell or frame of the block. In a properly designed block, the swallow provides just enough clearance for the rope to run freely without binding, yet stays narrow enough to support the sheave and contain the load effectively.

Swallows are shaped according to the rope they will carry. A steel wire rope needs a deeper, V-shaped swallow with a tight radius to grip and protect the wire. Natural fiber rope and synthetic cordage require a broader, more rounded swallow to distribute pressure across the rope strands and prevent crushing. The swallow depth typically runs 10 to 15 millimeters deeper than the rope diameter to allow for rope stretch and minor debris.

Variants and wear patterns

Single blocks have one swallow, double and triple blocks have multiple swallows stacked side by side or on different axles. The swallow opening on a metal block can wear or score from repeated rope movement, particularly if the rope is undersized, loaded off-axis, or contaminated with grit. A scored swallow will grip the rope inconsistently and accelerate wear on both rope and sheave.

The term comes from the shape: the opening resembles a bird's mouth or throat at the moment of swallowing. It appears consistently in British and European maritime vocabulary; in North American usage, the same feature is sometimes called the throat of the block, though "swallow" remains standard in naval and commercial shipping contexts.

When selecting or inspecting a block, check that the swallow matches your working line in material and diameter. A mismatched swallow is a common cause of rope slip, uneven wear, and unsafe load handling. Swallow damage is grounds for removing a block from service.