When a trailer is coupled to a tractor unit a pin extending from the underside of the chassis on the trailer is locked into a coupling device on the tractor unit known as a fifth wheel. The fifth wheel is basically a'C'shaped metal plate mounted on the chassis of the tractor unit towards the rear, which has retractable jaws to lock the trailer pin in place. When a trailer is uncoupled from a tractor unit support legs are extended from under the trailer. The trailer is unlocked from the fifth wheel coupling device on the tractor unit, by releasing the jaws at the rear of the fifth wheel. The tractor unit is then driven away from the trailer. The trailer is then left supported by the support legs towards the front of the trailer and the rear wheels. Theft of the trailer and its load is therefore easy, as a thief only has to couple up to the trailer with a tractor unit and drive away. Several items have been marketed to prevent trailer theft including devices, which lock around the coupling pin. Unfortunately the coupling pin has to be heavily greased, which results in the anti-theft device also becoming very greasy.

This invention aims to prevent the theft of articulated lorry trailers by blocking the access to the trailer coupling pin, using a retractable locking pin, which is permanently fitted to the trailer, and can only be operated by use of a security key.

A specific embodiment of the invention will be described with reference to the following drawings in which: Figure 1 shows a side view of a simplified lorry trailer, with load carrying body and ancillary components removed, to show typical positions of coupling pin and fitting position of anti-theft device.

Figure 2 shows view from below of simplified lorry trailer.

Figure 3 shows a view from above of a sectioned device fitted to trailer in locked position.

Figure 4 shows a view from above of a sectioned device fitted to trailer in unlocked position, with lock barrel removed and locking bolt withdrawn.

Figure 5 shows a sectioned device in locked position, shown from side.

Figure 6 shows a sectioned device in unlocked position, shown from side.

Referring to the drawings, the device is permanently attached to the trailer chassis 2, by means of welding, brazing or other method. All components are within the internal structure of the chassis 2, with only the locking pin 5, end of outer pipe 16 and end of actuating handle 14 protruding outside. The device consists of a hollow metal tube 16 which is machined internally. The outer end of the tube 16 and is machined internally to allow fitment of a standard lock barrel assembly 12. An annular recess is machined internally 11 to allow fitment of a rubber or plastic'0'ring 9. Another annular recess is machined internally 13 to allow locking pin 8 on lock barrel assembly 12 to lock in place. The tube 16 is then machined out the rest of its length to allow locking bolt 4 a close sliding fit. Locking bolt 4 is machined a short distance at its outer end to allow it to protrude into the lock barrel 12 recess, when lock barrel 12 is removed. A spring 7 is fitted around the smaller diameter of locking bolt 4, the tension of which keeps the bolt 4 to the inner end of tube 16. A pipe 6 is then welded, brazed or otherwise attached to the inner end of tube 16 as shown in drawings. A hole is machined in the side of the pipe 6 to allow locking bolt 4 to pass through it. Locking pin 5 is located in this pipe 6 in a close sliding fit. The locking pin 5 has a machined lip to prevent it being removed from the pipe 6, when pipe 6 is welded to tube 16. A groove is machined around locking pin 5 to allow the end of locking bolt 4 to seat. A slot 15 is machined in the tube 16. The actuating handle 14 passes through the slot 15 and is attached to the locking bolt 4. The outer end of the actuating handle 14 passes through a hole 17 in the chassis 2.

The device is operated as follows. Starting from position as shown in figure 6. The key 10 is inserted in lock barrel 12 which is seated in the end of tube 16 protruding through side of trailer chassis 2. The key is turned which withdraws lock barrel pin 8 from annular recess 13. The'0'ring 9 in recess 11 grips the barrel to prevent the barrel 12 from just turning. With the pin 8 withdrawn the lock barrel assembly can now be withdrawn. The actuating handle 14 is pulled which withdraws the bolt 4 from the recess machined in locking pin 5. Pin 5 will now drop, due to gravity, within pipe 6 until its machined lip seats on internal seat in pipe 6 as in figure 5. The bolt 4 can only be withdrawn because with lock barrel assembly 12 removed, the smaller machined end of bolt 4 can pass into the lock assemble 12 recess as shown in figure 4. The actuating handle 14 can then be released. The spring 7 then forces the bolt 4 inwards. The inner end of bolt 4 passes through the hole in pipe 6 and prevents locking pin 5 from being pushed back into pipe 6. The lock barrel assembly is then inserted into its recess in tube 16. The key is turned to the locked position and its lock pin 8 enters annular recess 13. The key can now be withdrawn. In this position the trailer coupling pin 1 cannot be engaged by the fifth wheel, as locking pin 5 is in the way. Unlocking the device is the reverse process, except when actuating handle 14 is pulled back, the locking pin 5 is pushed by hand upwards into pipe 6. The handle 14 is released, and spring 7 forces the end of bolt 4 to seat in the machined recess in pin 5.