The TN12 transmission was originally designed by Self-Changing Gears Limited but development, production and marketing are now carried out by David Brown Defence Equipment Limited for the Chieftain MBT. Development has extended the range to include designs capable of input powers in excess of 1,000 hp in both remote or close-coupled configurations.

TN12 series transmissions have been installed in all production Chieftain, Vickers Mk 1 and Mk 3 MBTs. All incorporate epicyclic geartrains in the change speed section to give six forward and two reverse speeds. Vehicle steering is achieved by a fully regenerative Merritt-Brown triple-differential system which maintains power through turns and provides an axis steer capability for high manoeuvrability.

By eliminating unnecessary clutches, all TN12 units feature high mechanical efficiency, giving higher sprocket power for a given gross engine power. In addition, a corresponding reduction of burden on and therefore size of, cooling group can often be achieved. Designs of intermediate versions between TN12 and TN12-1000 exist for both new vehicles and matching to replacement engines in existing TN12 installations such as Vijayanta and Chieftain MBTs.

The latest generation of this range, designated T-1200, utilises the TN 12-1000 core to which is applied the latest electronics and materials technology to control, steer and start modules - giving an optimum match to today's high torque rise turbocharged engines.

The normal TN12 series is semi-automatic, but a fully automatic version is now available.

Production. By mid-1999 production of the TN12 transmission amounted to over 5,000 units with production continuing. It is installed in the Vickers Mk 1 and Mk 3 MBTs, all Chieftain MBTs and variants.