December 2019 Newsletter Loose screws resulted in train doors being open whilst the train was moving
On 22 August 2019, a train service from London in the UK travelled 16 miles at speeds of up to around 80 mph while a coach door was open. At the time, the coach was occupied by passengers. The doors were remotely operated by the driver.
A bracket connected the drive mechanism to the door and screws with wedge-lock washers were used to secure the door brackets. The two screws fixing its bracket to the drive mechanism had worked loose and fallen from the bracket. Without the door attached, the rod moved to the door closed position when the driver operated the 'doors close' control so the interlock incorrectly indicated that all the train doors were fully closed.
The screws that fell out should have been 'witness marked' (a yellow mark across the screw head, washer and bracket) to indicate that they had been tightened. The service company also required that a second person should witness the tightening, check 50% of the screws by applying 80% of the specified torque and then apply a blue dot to the screws which they had checked.
There were no witness marking on the two screws which fell from the connection between the bracket and the drive belt, and no yellow mark adjacent to the corresponding holes in the bracket. Both these screws carried a blue verification dot. Yellow marks were present on the screws which secured the bracket to the door and one of these also carried a blue dot.
Following this incident, checks on the fleet of trains were completed. Of the 480 doorways on the refurbished units, it was identified that at least one screw was deemed to be loose on at least 60 doorways. The train operator has introduced an enhanced door bracket screw monitoring programme and will undertake further remedial action if necessary, until it is satisfied that the issue has been resolved.
Some thoughts: A wedge-lock type washer relies upon the bolt preload to prevent self-loosening. Short grip length joints (L/D ratio <2) are prone to preload loss from embedding. In such cases, the correct tightening torque can be critical in preventing initiation of self-loosening. Once loose, screw detachment can occur.
Information regarding this incident is taken from the RAIB Safety Digest 10/2019: Hockley