“GEARBOX
The Maybach-Olvar gearbox provides eight forward speeds and four reverse speeds with pre-selective hydraulic engagement. The gears are arranged on a main shaft, lay shaft and reverse lay shaft, although, in fact, both main shaft and lay shafts consist of four short lengths of shaft each carrying a gear and each able to be engaged or disengaged by a dog clutch. The rearmost dog clutches on both main shaft and lay shafts are controlled by selection forks mounted on a common shaft in such a manner that when one dog is engaged, the other is free. The movement of these selector forks is controlled by a double acting hydraulic cylinder mounted on the top of the gearbox. Likewise the central pair are operated by another cylinder. The remaining dog clutch on the lay shaft is operated by a third cylinder, but has engagement with a different gear at each end of its travel (i.e. no free position). The remaining dog clutch on the main shaft and the dog clutch on the reverse are operated together by a hand lever, so that in the forward position of the hand lever the dog on the main shaft is engaged and that of the reverse shaft free, with the exact opposite for the rear position of the hand lever, whilst in the intermediate position of the hand lever both dogs are free. Thus for forward drive, once the hand lever has been placed in the forward position, all changes of gear are effected through the operation of the hydraulic cylinder. The movement of the gear change lever through its quadrant causes the rotation of a valve which determines the distribution of oil pressure to the appropriate hydraulic cylinder for the ratio required. Side pressure on the gear change lever then admits pressure to that cylinder and effects the gear change. In reverse the procedure is similar but only the four lowest ratios are available. With the exception of the final output gear and the main shaft and reverse shaft meshing with it, all gears are single helical. An extension of the final output shaft carries a spiral bevel for the main drive, whilst an extension of the main shaft carries a small straight bevel for controlling the speed of the steering differentials. At the input end of the main shaft is a multi-disc centrifugal clutch which can, however, be disengaged by the admission of hydraulic pressure to a cylinder, the piston of which operates a withdrawal fork.

Cutaway of the Maybach-Olvar gearbox.

The same withdrawal fork is operated by the clutch pedal. A small cone clutch on the input side of the main shaft and the rear end of the lay shaft are geared together and operated by hydraulic pressure giving synchronization of the lay shaft with the engine speed. The input shaft also drives, through two gears, a fore and aft shaft on which are mounted two oil pumps, which provide the pressure necessary for hydraulic operation. Another unit driven from this shaft is the engine rev counter. Hydraulically operated brake shoes are provided for the slowing of the main clutch and the second gear from the input end on the main shaft, and a hydraulic device adjusts the throttle opening as required. An easily accessible oil filter is provided which serves both gearbox and steering unit which have a common oil level; the level is checked by a dip-stick on the gearbox and replenished through a filler cap. The oil capacity is 32 litres (7 gallons). A metal strip on the gearbox cover bears the following figures:

which can be explained as follows: The figures within the diamonds correspond to the hydraulic cylinder of each selector. The figures before the diamonds in each case represent the number of the gear engaged with the selectors in question in the forward position. The figures after the diamonds represent the gear engaged with the selectors in the rear position. The table thus gives the position of each set of selectors for each gear.

The overall ratios from gearbox input flange to steering unit output flanges, including therefore spiral bevel reduction on the gearbox output shaft, are:

The four ratios of reverse correspond to, but are lower than, the four lowest forward gears.

STEERING UNIT
Steering is effected by imposing different speeds on to the sun wheel of an epicyclic, whose annulus is positively driven by a bevel meshing with the main gearbox output bevel, and whose planet carrier carries the output flange to the final drive. There is one of these epicyclics at each end of the bevel shaft meshing with the gearbox output bevel.

The direction and speed rotation of the sun wheel and the choice of left or right hand sides are governed by the engagement of two or four hydraulically operated multi-disc clutches. These clutches connect the sun wheels through a lay shaft meshing with the small bevel on the extension of the gearbox input shaft. In this way a choice of two speeds both proportional to engine speed, is imposed at will upon the sun wheel of the epicyclic whose annulus is already rotating at a multiple of engine speed according to the gear engaged. In addition, in neutral with no output from the main gearbox bevel, a drive is still obtained from the bevel on the extension of the gearbox input shaft, and this produces with the engagement of the appropriate steering clutch opposing rotations of left and right sun wheels. As the annuli are unable to rotate in opposite directions owing to their being secured to the same shaft this opposite rotation of the sun wheel, of necessity, produces opposite rotation of the output flanges, resulting in a neutral turn. For one full revolution of both output flanges the gearbox input flange performs 117 revs.

Oil pressure from a pump driven by an extension of the fore and aft gearbox shaft is admitted to the required steering clutches through ports in the steering assembly casing, the opening of which is controlled by a piston valve. Movement of piston valve is governed by the driver's steering wheel. In this way two distinct radii of turn are available for each gear engaged, and identical for left and right turns. A mushroom shaped knob on the gearbox casing operates hydraulically a brake on one section of the main shaft, thereby eliminating clutch drag and making the neutral turn more positive.

The tank can also be steered when required by steering levers on each side of the driver which operate the right and left brake assemblies.

Cutaway of drive sprocket and internal gearing.

FINAL DRIVE
The final drive assemblies are bolted directly to the hull from the outside and contain two stages of reduction.
(a) A straight spur reduction, and
(b) An epicyclic
The steering unit output shafts drive the small pinion of the spur reduction which meshes with the large pinion. The large pinion is mounted on the same shaft as a smaller pinion, which is the sun wheel of the epicyclic reduction on which the planet carrier mounts the driving sprocket, and the annulus is rigidly secured to the final drive casing. The overall ration is 10.55 to 1. Each final drive casing, which is a steel casting, has its own drain plug and filler plug, and a third plug which gives the correct oil level. Oil capacity 8 litres (14 pints).”