The formal CGS specification for an Australian produced cruiser tank was finally issued on the 11th of November 1940. The CGS specification was a lengthy document still cluttered by the technological indecision and tactical confusion that had prevailed previously.
The major points were as such. An immediate requirement for 340 tanks to equip one armored division and a further 119 tanks for the 1st Australian Corps and the AMF (84 and 35 respectively), an additional 400 tanks for 12 months reserve, with a total production of 859 tanks. No weight restriction was issued beyond the lowest possible allowing for all other requirements to be met, with armor required at 50 mm (1.97 in) minimum, although this was later amended to 65 mm (2.56) frontal with 45 mm (1.77 in) minimum sides and rear.
Dimensions were restricted to conform to the loading gauge of Australian railways, a maximum width of 9 feet 4 inches and maximum height of 8 feet 6 inches with no restriction on maximum length. In regards to mobility, it was requested that the tank be able to operate on sand or black soil, with no requirement for cold weather operation.
Minimum required top speed was to be 35 mph (55 km/h) on level ground, with a slope traverse of 45 degrees, and trench and vertical obstacle crossing capabilities of 6 foot 6 inches and 3 foot 6 inches respectively. Operational range was specified as a minimum cruising range of 150 miles (240 kilometres).
Armament was to be one
Ordnance QF 2 Pounder accompanied by one .303 machine gun mounted in the turret with another machine gun mounted in the hull considered desirable. Minimum ammunition load was set at 120 2-pounder shells and 5000 rounds of small arms ammunition for the machine guns (7000 if two guns were mounted).
With the tank project arriving at a relatively late juncture in the Australian munitions program there were no available facilities equipped to roll armored plate in the thicknesses required. The solution devised was to cast the hull in six sections which would be bolted or welded together to form a rigid whole. Additionally, Australian stocks of nickel, typically used in cast armour, were earmarked as critical war resources and thus unavailable.
To deal with the nickel problem metallurgists at Broken Hill Proprietary (BHP) developed a new armored steel formula, Australian Bulletproof Plate 4 (ABP4), utilising zirconium in place of nickel. Zirconium was chosen due to Australia possessing some of the world’s largest readily available stocks of naturally occurring zircon sand.
The engine presented yet another problem. In order to meet the army’s requirement of a 35 mph (55 km/h) top speed it was estimated that an engine of at least 300 hp would be needed. However, both the intended original engines, the radial Pratt & Whitney Wasp and the Guiberson diesel, were not available.
The Pratt & Whitney Wasp had been produced in Australia under licence by the Commonwealth Aircraft Corporation (CAC) since the mid-1930s, but immediate war needs meant that all Australian Wasp production was tied up in aircraft orders for Australia and the UK. To circumvent the engine issue, Watson proposed the use of three regular Cadillac V8 346 in³ 5.7L engines, arranged in a clover-leaf formation through a transfer case leading into a common driveshaft, giving a total output of 330 horsepower.
In April of 1941 the clover leaf Cadillac setup was assessed by Professor Burstal of the University of Melbourne and the Chief engineer of the Vacuum oil Company, Mr Alfred Reginald Code, with both men concurring that while the setup was less than ideal it would be workable as an expediency.
The last issue to be overcome was that of the gearbox. The US M3 Medium tank used a state of the art synchromesh gearbox with helical toothed hardened steel gears running on multiple bearing races, and was difficult to produce even in the USA. In Australia, the machinery required to cut gears of that type was not available and a shortage of bearings meant that the synchromesh gearbox could not be manufactured in Australia. The solution was to simplify the gearbox to a crash type design that used the same gear blanks and maintained the dimensions of the synchromesh type. This meant that the gearbox could be replaced with the more modern type should supplies become available from the USA. The gearboxes were produced by the firms Coote & Jorgensen, and Sonnedales.
While remarkable progress had been made in the first six months of 1941, the AC I was still on the drawing board and no closer to the arrival of a pilot model. Noting the inefficiencies of the established organisation Prime Minister Menzies again intervened in June of 1941 to create the Directorate of Armoured Fighting Vehicles Production (originally titled AFV division), with Alfred Reginald Code appointed as Director. Code was known both as a respected engineer and a skilled administrator and Prime Minister Menzies deliberately gave the DAFVP an unorthodox structure beyond both the Ministry of Munitions and the Army in an attempt to fast track tank production.
Code immediately set to building a skilled staff of designers and production engineers in order to simplify the tank design into the most viable design for Australian industry. Australian foundries had been emboldened by the idea of a cast hull for the tank and insisted that not only was it possible, but that it could be cast as a single large piece with only the axle housing and other external fittings being bolted on separately.
Proof of this concept was borne out in August-September of 1941, when the first test hulls were cast in foundries at the New South Wales city of Newcastle. The one piece cast hull reduced the amount of time required in machining and assembling the tank. Additionally, the new team at the DAFVP made several revisions to the design to streamline assembly. This included modifying the final drive design so that they could be installed from the side after the axle housing had been fitted and implementing a new ‘scissor’ type Horizontal Volute Spring Suspension (HVSS) bogie, resembling the French
Hotchkiss H35, which offered equal performance to the American type while being easier to manufacture.
The suspension changed configuration during the development stage, with prototype vehicles having a trailing return roller, and production vehicles having the return roller on top. The AC I was designed to be able to use either locally produced steel tracks of a similar configuration to those used on British Cruiser tanks or alternatively US produced rubber block tracks. Installation of each type of track required the fitting of a different drive sprocket. Australian manufactured road wheels were functionally identical to US produced M3 road wheels, however they can be identified by either 4, 6, or 8 holes drilled radially around the inner diameter of the wheel.
The turret was fully cast with a 54 inch (1.37 m) turret ring, very similar to the British cruiser design, and the hull kept a low profile as intended. Armament was also similar to the British tanks, with the Ordnance QF 2-Pounder (40 mm/1.57 in) as the main armament. Due to the BESA machine gun not being in production in Australia, secondary armament consisted of one coaxial Vickers .303 (7.7 mm) machine gun and one hull mounted Vickers .303 machine gun protected by a massive cast armor mantlet, encasing the machine-gun watertank.
Ammunition for the main gun was 46 rounds stowed horizontally in the rear of the turret with 74 rounds stowed vertically in two racks bolted to the hull floor under the turret basket. The turret was rotated either manually or via an electric power traverse. The traverse mechanism was a 40 volt system with current drawn from a dynamo in the transfer box linking the three engines to the main drive shaft. To ensure consistent power supply at variable RPM three shunt motors were provided to stabilize the current flow.
An industrial mould packing machine was purchased from the USA but shipping delays meant that it did not arrive until late 1942 at which point the Bradford & Kendall foundry men were confident that they could hand pack the moulds at the same rate as the machine. It therefore went unused. Assembly of AC I tanks was contracted to New South Wales Government Railways with work carried out at Chullora Tank Assembly Workshops at Chullora, Sydney.
The Chullora Tank Assembly Workshops were repurposed for construction and maintenance of rail stock post war, and still exist to this day. The first production vehicle (no. 8001) arrived in July of 1942. The first 12 vehicles delivered were found to suffer from temper brittleness, which caused certain pieces of casting to inconsistently harden during the tempering process. As a result, the armor of these tanks was deemed potentially unreliable, but they were still put on the railways to Queensland to fight the Japanese.
( Slightly modified from
http://www.tanks-encyclopedia.com/ww2/Australia/AC1_Sentinel.php which rocks)