US Patent 7201533 Sewer grate locking mechanism and method of installing same

A conventional sewer grate has a locking mechanism for preventing removal of the grate from its mounting frame by unauthorized persons. The locking mechanism includes a grate hold-down device, a threaded nut attached to the mounting frame below the hold-down device, and a bolt extending downwardly through the hold-down device into threaded engagement with the nut. The head of the bolt is specially constructed so that a specially-formed wrench is required to unscrew the bolt from the nut.