The Mark V star was relatively slow to manœuvre, owing chiefly to the amount of track-bearing surface on the ground.
The Medium Mark A or “Whippet” Tank ([Plate III]—see [p. 176])
The Medium A tank, known also as the “Chaser” and “Whippet,” was the British standard light-type machine, and it differed altogether from its heavier relatives. Its weight, equipped, was about 14 tons, whilst it was 20 ft. long, 9 ft. high, and 8 ft. 7 in. wide, carrying a crew of three. It could attain a maximum speed of about 8·3 m.p.h., and could span a trench approximately 7 ft. in width.
On this machine the tracks were not carried “overhead” as in the case of the heavy tanks, but the two trackways existed as such only, and formed the road members of what may be described as the chassis upon which the engine-room and fighting cab were mounted. There were no sponsons, and the tank was driven and fought from the cab at the rear of the machine, provision being allowed for an armament of three machine-guns.
Each machine was fitted with two 45 h.p. 4-cylinder Tyler engines with an autovac petrol feed, and cooled by means of a tubular radiator provided with two fans driven by chains from each crankshaft.
The power of each engine was transmitted through separate cone clutches, leather flexible couplings, and four-speed and reverse gear-boxes, to a casing, at the rear of the machine, containing two worm gears.
The two worm-wheel shafts of these gears were in line, with their inner ends nearly touching, and each carrying the keyed-on half of a jawed coupling, one of which could be slid along at will, to engage with the other, thus locking the two shafts together.
One of the shafts carried a friction-clutch arrangement, designed to limit the power transmitted from one shaft to the other to about 12 h.p.
It will be seen, therefore, that either worm-wheel shaft could be driven independently by its own engine, or the two could be locked together so as to rotate at the same speed, driving the tank straight ahead, provided that there was not more than 12 h.p. difference between the developed powers of the engines. Extensions of each worm shaft carried a band brake, as well as a fan for forcing air into the cab of the machine.
Returning to the details of the transmission system, each cross-shaft from the worm case terminated in a “driving chain pinion shaft,” the outer ends of which were supported by ball bearings mounted upon the sides of the track frames. The chain pinion carried by this shaft transmitted the drive, through a roller chain, to the final track-driving wheels, which, engaging with the slots in the track links, drove the tank along the track. Each track consisted normally of sixty-seven plates or shoes, and rollers served to support the weight of the tank upon the track, as well as to carry the track over the top of the trackway. Adjustment of the track was effected by movement of the front “idler” wheel as in the case of the heavier machines.