Grace's Guide is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 142,980 pages of information and 229,144 images on early companies, their products and the people who designed and built them.
The complex infrastructure of Barry Docks included a swing bridge and two sliding (retractable) bridges survives. Large retractable bridges are rare, and one of the Barry Docks survives as a listed structure, along with its control cabin. . The cabin is in very delapidated condition, while the bridge is suffering from corrosion (2019).
See photo above and here.
The bridge was traversed by wire ropes actuated by two hydraulic 'jiggers'. The arranagement of ropes and chains served to multiply the travel of hydraulic ram. In this case each ram had four pairs of pulleys. 2007 photo here.
Photo here shows a device which presumably acted to shut off the hydraulic supply to the rams at the outward end of the bridge's travel. Judging by the fractured casting, it wasn't always successful.