HOW IT WORKS
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THE MAXWELL four-speed unit is simple in concept and designed for ease of maintenance. It comprises three main elements: gear trains, clutch packs and bevel drive take-off. These are grouped into a twocompartment construction with the front section holding an assembly of constant-mesh spur gears grouped around a central input shaft.
Only radial loads are present in the transmission so it is only necessary to support the gear trains on heavy-duty roller and ball bearings.
Housed in the rear section are oilfed multiplate clutch packs at the end of each layshaft. These are designed for easy replacement. It is not necessary to remove the gearbox.
Selection of gears is via the clutches which arc controlled by a programmed electronic control unit and electro-pneumatic valves.
Reverse drive is selected by using first gear and an idler cog.
Because the clutch drive relies on the shear of oil rather than on
friction, a copious amount of oil is necessary. This is supplied by a Dowty pump, now mounted at the rear of the input shaft, which allows the use of a much smaller sump than on earlier boxes.
Output to the drive axle is through a bevel gearbox fed by a spur gearwheel. Depending on the engine installation, the box can have either a 90 degree drive or a side output.
A major feature of the Maxwell box is that these drive clutches also double as constant torque hydraulic retarders. The electro-pneumatic control unit allows partial engagement of first and second gears during a third or fourth gear operation, bringing the vehicle smoothly to rest.
When used as retarders, the clutches usually absorb around 37kW (50hp), but during stall tests figures of 97kW (I3Ohp) have been reached over a limited period without apparent damage.
High temperatures are generated in the gear oil but this is dissipated by an oil/water exchange unit.