Aportal axle (orportal gear lift) is anoff-road vehicle suspension and drive technology in which theaxle tube or the half-shaft is offset from – usually above – the center of thewheel hub and driving power is transferred to each wheel through a simple gearbox built onto each hub.[1] It has two advantages over a non-offset axle: first, ground clearance is increased, particularly beneath the low-slung differential housing of the main axles; second, any hub reduction gearing allows the axle half shafts to deliver the same power but at reduced torque (by using higher shaft speed), which reduces load on the axle crown wheel and differential.
The portal gear configuration is also sometimes called adrop gear ordrop gearset configuration[2][3] (which, despite its similarity to the termdropped axle, produces the opposite effect).
Compared to a normal layout, portal axles enable the vehicle to gain higherground clearance, as both the axle tube anddifferential casing are tucked up higher under the vehicle.
Due to thegear reduction at the wheel, which lessens thetorque on all the otherdrivetrain 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 thecenter of gravity for a given ground clearance. In a vehicle that also requires a reduced top speed in at least one gear, the gear ratio can also have this effect. The militaryKübelwagen of WWII used a ratio of 1.4:1 to provide a 2.5 mph walking speed in first gear,[4] as well as a useful lift of 50 mm.
As they require a heavier and more complex hub assembly, however, portal axle systems can result in increasedunsprung weight and require robust axle-control elements to give predictable handling. In addition, at higher speeds the hub assembly can overheat.[5]
They are also used inrailways andlow floor buses[6][7] 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, theinverted 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 (ordrop 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 orUnimog 404).
Vehicles fitted with portal axles include:
A related development is the use of anepicyclic hub gearbox. This is mounted in-line with the half shaft, so that there is no change in ride height. They are often used for large and heavy vehicles, where the wheel diameter already gives adequate ground clearance. The reduction gearbox allows the half shafts to turn faster than the wheels, thus requiring lesstorque for the same power. This permits a smaller and lighterhalf shaft and internaldrivetrain.
Hub gearboxes were an iconic feature of the Alvis FV600 chassis vehicles, such as theStalwart andSaracen. The FV600 used a version of theDAFH-drive, with a singledifferential 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 'wind-up' 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.
Today, portal gear lifts are manufactured for use inutility task vehicles (UTV).
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