The Ground Pushback crew did not give a B767 Captain the 'Set Brakes' signal and so after pushback the aircraft rolled into the tow bar which required Maintenance action.

Narrative

After pushback the towbar was disconnected without the brakes being parked. When the tug driver came on the intercom; and during the pushback; we had normal clear communications with the driver. He pushed us back and then stopped and we did not hear any communications for several seconds. There were several other aircraft moving around us; and initially I thought he was pausing the push for traffic. The First Officer was also listening to the intercom and neither one of us heard the driver request the brakes to be parked. After approximately 20 seconds or so I began to hear the normal thumping sounds associated with the towbar disconnection and I called the drive and said 'Do you want the brakes parked?' I did not get a reply and said it again. Still no reply. I flashed the nose taxi light twice; still no reply. I was about to call him again when we felt the airplane start to move forward. I said the First Officer 'I guess he is going to pull us forward;' and for a second I thought the noises I had heard were just the tug starting to pull rather than push. But something didn't feel right about the movement and I said 'I don't think he's pulling us; I think we are rolling.' I put on the brakes and called the driver again. Still no response. I flashed the nose taxi light again and after awhile one of the ground crew appeared in front of the airplane and gave me the hand signal for parking the brakes. I confirmed that they were parked. After several minutes someone else came on the interphone; loud and clear; and said that after they had disconnected the towbar the aircraft had rolled forward jamming the towbar between the nose tires. The mechanics came out and got the towbar unstuck and towed us back to the gate where they replaced both nose wheels and tires as a precaution and inspected the nose gear. The nose gear was not damaged. When I talked to the driver later he said that he called for brakes parked. Neither of us heard it. He said that he thought he later heard me say 'brakes parked.' This may have been when I asked if he wanted the brakes parked. But that was after I already felt the towbar being disconnected. The driver also said he heard other talking over the intercom and he didn't know if we were talking to him or not. On some airplanes; the driver can also hear ATC communications or intercom communications from the cabin. During the pushback; the #1 Flight Attendant called on the intercom. I answered the call since it could have been an emergency (smoke in the cabin; etc). She wanted me to know that the video player wasn't working; which means they have to do a manual safety demo. This takes longer and means I may have to delay the takeoff. I told her to let me know when they were finished. That conversation took a few seconds. The driver may have heard that; or the First Officer talking to Ground Control. But we didn't say anything on the radio or intercom that sounded like 'brakes parked; you are clear to disconnect the intercom.' The aircraft was later taken out of service; but that was due to an electrical problem that developed; which was completely unrelated to the nose gear issue.