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Final drive to the wheels was by sprocket chain. Some locomotives had a sprocket gear ratio of 19:19 and others 15:19. In addition, there was a two-speed gearbox but gears could only be changed while the locomotive was stationary. Tractive effort was:
Ratio 19:19, Low gear 12,600 lbf (56 kN), High gear 4,705 lbf (20.93 kN) (68154-68179 and 68184-68185)
Ratio 15:19, Low gear 15,960 lbf (71.0 kN), High gear 5,960 lbf (26.5 kN) (68180-68183)
References
Ian Allan ABC of British Railways Locomotives, 1948 edition, part 4, page 47