Abstract: A door lock system for the rear doors of the trailer of a tractor-trailer rig is disclosed. The lock system includes a locking bar fixedly secured to one door and attachably secured to a second door. A pin projects from the second door so that it may pass through an opening in the locking bar. A pin cover locks over the pin for securing the bar to the second door. In a preferred embodiment the pin is formed identical to a conventional trailer king pin or coupling pin so that the pin cover may be used either to lock the trailer doors or as a theft prevention device covering the king pin to prevent unauthorized coupling of unattended trailers.

Abstract: The coupling pin of the trailer of a tractor-trailer rig is provided with a theft prevention device for preventing the unauthorized coupling of unattended trailers. The device includes a truncated conical housing having a cylindrical sleeve which is secured to the inner base of the cone and is adapted to receive the coupling pin. The coupling pin is held within the sleeve by means of a pair of opposed, spring-biased locking jaws which normally protrude through a pair of opposed slots in the sleeve. In order to engage the reduced diameter portion of the coupling pin. The locking jaws are mounted on a pair of U-brackets. The brackets carry a pair of camming surfaces which are engaged by the wing cams of a key-operated lock. The lock is preferably recessed within the housing so that it cannot be sheared off during use. The invention has the additional utility of serving as a safety device for preventing the movement of a trailer during unloading operations.