Tanks in the Great War, 1914-1918

by J. F. C. Fuller

The tail-shaft from the gear-box carried a worm which drove the crown wheel of a large reduction gear, this gear also serving as a differential to enable the track driving wheels to rotate at different speeds, as when steering the tank on its track brakes. A device was provided, under the driver’s control, for locking the differential when it was desired to steer a dead-ahead course, or when negotiating a trench or other obstacle. With the differential locked, the gear became, so to speak, “solid” and obviated the risk of one of the tracks slipping in bad ground, a condition very apt to cause a tank to slip sideways into a trench and become ditched.

Some trouble was caused through breakages of this locking muff in the earlier days, but latterly the arrangement was considerably improved and strengthened.

The gear-box tail-shaft terminated in a brake drum, the band of which was operated by means of a pedal at the driver’s foot. It may be of interest to point out here that the whole of the items so far referred to, i.e. engine, gear-box, and differential, formed the standard power unit of the pre-war Foster-Daimler tractor, and thus provided a known quantity around which the rest of the detail was designed. This greatly facilitated production.

On either side of the differential case projected cross-shafts, the outer ends of which were supported in bearings mounted upon the outside wall of the tank, and, between the inner and outer walls of the hull, two sliding pinions were carried on a splined portion of the cross-shaft, one pair of pinions on each of the right and left hand sides of the tank.

In describing the remainder of the transmission, it will suffice to deal only with one side of the machine, the detail on either side being identical.

The sliding pinions, already alluded to, were operated by means of short levers by two gearsmen, whose sole duty it was to assist the driver, who signalled to them his requirements from his seat in the front of the tank, the two gearsmen being accommodated towards the rear of the machine on seats placed over the primary gear-box. The sliding pinions were of two sizes, known as the high-speed and low-speed pinions, and immediately in their rear was mounted another pinion assembly, also carrying two gear-wheels of different dimensions, with which the sliding wheels could be engaged at will—in other words, on each side of the tank there existed what were known as secondary gear-boxes, each offering a choice of two speeds.

Thus it will be seen that the whole arrangement provided a range of four speeds. Assuming the secondary gears to be at “low,” the driver had the option of using either first or second speed by manipulating the control to the primary gear-box, whilst in order to obtain third or fourth (top) speed it was necessary for him to signal the gearsmen to alter their gears to “high,” and to assist them in the process by a great deal of intelligent clutch work. It need hardly be pointed out that this arrangement was exceedingly clumsy, and often involved much loss of time and temper. It might also be mentioned here that the reverse gear, already alluded to, was considerably higher than the lowest forward speed, so that there was little possibility of driving backwards, clear of any obstruction which might have ditched the tank.

Hand-operated brakes, under the control of the tank commander in the front of the tank, alongside the driver, were incorporated with the secondary gear-box. These brakes, by checking one or other track, enabled the tank to be steered in some measure with the differential unlocked, whilst, by locking the differential and placing, say, the right-hand secondary gear in “neutral,” the machine could be swung to the right, practically upon its own axis, by applying the right-hand brake. To swing to the left, the right-hand secondary gear was engaged, the left-hand being placed in “neutral,” the differential locked and the left-hand brake applied.

From the secondary gear-box a Coventry chain transmitted the power to an assembly, at the rear of the hull, which carried, on either side of the chain sprocket, two heavy pinion wheels, in constant mesh with the final sprocket wheels, which in turn, engaging with the links of the track plates, drove the hull along the track.

Each track was composed, normally, of ninety plates or road shoes, the separate plates being coupled together by means of links (two per plate) and link pins, the links themselves being recessed so as to engage with the driving wheels as shown above.