INDUSTRIAL TRACTORS These are machines designed for hauling trailer trucks over comparatively smooth surfaces such as railroad terminal and ware house floors, steamship piers, etc. They were developed from industrial trucks which began to replace hand trucks on steam ship docks and railroad platforms about 1904. Such tractors would haul a trailer load ranging from 5 to 20 tons, depending upon the sizes, and so reduced to a great extent the manual labour required in handling freight and baggage at the terminals and warehouses.
Industrial tractors for the above purposes generally were elec trically driven, since the low fire hazard and the absence of exhaust gases characteristic of this system of drive recommended it for indoor work. The wheel tread usually was around 3o in., and the wheelbase ranged from 4o to 48 in., while the wheels in most cases were 20 in. in diameter and equipped with solid rubber tires. Although the earliest industrial tractors were steered by the front wheels, and were driven by the rear wheels, later designs fre quently were provided with a four-wheel drive and a four-wheel system of steering; the former to give a sufficient traction even on slippery (oily or icy) surfaces, and the latter to manoeuvre the trains more advantageously in restricted spaces.
Four-wheel-drive tractors were usually equipped with four wheel brakes, which made it possible to stop them quickly in an emergency. Speeds ranged from 6 to 8 m.p.h. when light, and around 4 m.p.h. when loaded. On fairly smooth surfaces the trailer trucks required a drawbar pull of approximately 5o lb. per ton. With the usual construction the normal continuous draw bar pull obtainable was about io per cent of the total weight of the tractor with battery. The weight of the battery amounted to 3o to 35 per cent of the total tractor weight.
Generally the operator steered by means of a lever or tiller which folded out of the way when he left his seat. Three speeds in both directions were obtained by means of a controller. For safety purposes, the main switch, controller and brake were interlocked. Before the tractor could be started a pedal had to be depressed. When the operator withdrew his foot from the pedal the main switch opened and the brake was applied auto matically. This tended to prevent accidents, as the tractor would come to a stop if the driver were thrown from his seat. The battery voltage used to drive the tractors was either 32 or 48 volts.