Safer transport for new motor vehicles
When the failures occurred, some of the cars plunged down on each other, damaging the vehicles, thus incurring insurance claims. Fortunately the situation did not cause any personal injuries, other than the necessity to recover from an incident that need not have occurred.
The major cost by far was the recall of all the transporter trucks throughout the country. Furthermore, a huge amount of labour was required to take the structures apart and then mechanically lift the cars off the transporter.
Following this, replacement of all the fasteners had to be overseen and then, finally, the carrying units within the trucks had to be reassembled.
The tilting mechanism was carried out manually through an actuated screw-drive system enclosed within the steel structural members. Holding the members together were the failing fasteners. They were 5 / 16 ” -18 x 3 /4” alloy steel socket countersunk head cap screws. All failures were at the fillet radius. The products had a manufacturer’s identification mark on the head.
A conversation with the employees revealed that the first indication of trouble was when some of the personnel were in the vicinity of the assembled structural units, which were being stored outdoors, and kept on hearing an occasional pinging sound at high pitch, like metal hitting metal.
Walking through the area, they found dozens of broken heads on the ground. A laboratory evaluation was conducted and it was found that the heat treatment was improper and that the fasteners were carburised, thus having a very high surface hardness. Using the electron microscope small amounts of red corrosion product and an inter-granular type of fractured surface at the failure point was observed.
Submitted by Pat Monaghan - Technical Support Manager SPS Unbrako.