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A portal axle (or portal gear lift) is an off-road vehicle suspension and drive technology where the axle tube or the half-shaft is offset from – usually above – the center of the wheel hub and where driving power is transferred to each wheel via a simple gearbox, built onto each hub. It gives two advantages: ground clearance is increased, particularly beneath the low-slung differential housing of the main axles — and secondly, any hub reduction gearing allows the axle half shafts to drive the same power but at reduced torque (by using higher shaft speed). This reduces load on the axle crown wheel and differential.
The portal gear configuration is also sometimes called a drop gear or drop gearset configuration (which, despite its similarity to the term dropped axle, produces the opposite effect).
Due to the gear reduction at the wheel which lessens the torque on all the other drivetrain components, the size of the differential casing can be reduced to gain even more ground clearance. Additionally, all drivetrain elements, in particular the transfer gearbox and driveshafts, can be built lighter. This results in lowering the center of gravity for a given ground clearance. Where a vehicle also requires a reduced top speed, in at least one gear, the gear ratio can also have this effect. The military Kübelwagen of WWII used a ratio of 1.4:1 to provide a 2.5 mph walking speed in first gear, as well as a useful lift of 50 mm.
As they require a heavier and more complex hub assembly, however, these systems can result in an increased unsprung weight and require robust axle-control elements to give predictable handling. In addition, at higher speeds the hub assembly can overheat. Exaxt 4WD
They are also used in and low-floor bus Railway Technology - Portal axle with SAB V60 rubber-sprung wheels and sound absorbers ArvinMeritor Reveals New Second Generation Inverted Portal Axle although, in the case of the latter, the device is engineered in the opposite way to those fitted to off-road vehicles - the axle is below the center of the wheel. Thus, the inverted portal axle allows the floor of the bus to be lowered, easing access to the bus and increasing the available cabin height.
Bolt-on portals (or drop boxes) are a housing with a set of gears which bolts onto the final flange of the axle tube. This approach allows existing vehicles to be converted to use portal gear lifts without modifying the axles (Volvo C303 or Unimog 404).
Hub gearboxes were an iconic feature of the Alvis FV600 chassis vehicles, such as the Alvis Stalwart and Alvis Saracen. The FV600 used a version of the DAF Trucks H-drive, with a single differential between sides and all wheels on each side linked by an internal driveshaft and bevel gearboxes to the half shafts. This has no differential axle between wheel stations on each side and so 'driveline windup' was a regular problem for these vehicles when driven on roads. If individual wheels were out of phase with their neighbours, possibly caused by cornering or slightly varying tyre diameter, this could place a considerable force on the gearbox, leading to breakages. For this reason, it was regular practice when driving on tarmac to bump the vehicle over a kerb or other object at times, to allow this wind-up to be released. On slippery surfaces like sand or mud there is enough slippage between the tires and ground to dissipate the tension.
They are now found on many large quarrying dump trucks and heavy plant vehicles.
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