The structure has a driven element (10) which is connected to a coupling housing (22) that is provided with a chain tunnel (28) and several bars (30), and are hydraulically moved into the chain tunnel. The hydraulic accumulator is connected to bars through a conduit system in communication with a closed system. A check valve is provided in parallel to a pressure control valve. A stop (38) is provided for limiting the retraction of each bar into the chain tunnel. Independent claims are included for the following: (1) method for operating chain drive structure; and (2) apparatus for operating chain drive structure.