Portal Drop Axle | Words of Engineer

Portal  Drop Axle

Generally, vehicles can be classified into two groups based on its application area-

1. On-Highway Vehicles

2. Off-Highway Vehicles

Simple explanation of difference between On Highway and Off Highway vehicles is, On highway vehicles are used on normal terrain and Off Highway vehicles are used in rough terrains like rocky, mud, agriculture land,mining and construction sites,etc.

The concept of power transfer from engine to wheel is same in both vehicle types and done via Engine, Gearbox, Driveshaft, Axle and finally to wheel.Definitely, specifications of those drivetrain components may vary depending on the requirements of On Highway and Off Highway vehicles.Gear Reduction,ground clearance,speed are the few examples of requirements.

From drivetrain components,axle is the only component which is directly in contact with wheels and used to transfer power to wheels. There are different types of arrangement between Axle and wheels are available for On Highway and Off highway vehicles.

In On highway vehicles, Axle tube centerline and Wheel centerline is mostly same,i.e.No offset between them.

Normal Axle Layout

Whereas,in Off Highway vehicles, sometimes,there is offset (usually above ) in axle tube centerline and Wheel centerline.This distance between the two centerlines is called as "Drop Distance" and measured in mm.

Portal Drop Axle 

Along with the drop distance,these Drop axles has simple gearbox installed at both wheel ends which is used as gear reduction. So, these axles are also called as "Two Stage Reduction axles".
First reduction occurs in differential by Ring & Pinion drive gear and second reduction is done at wheel end by simple gearboxes.

Portal Drop Axle Graphics

( Image Source - Pinterest

Image Credit - Respective owners )

There are various advantages of Portal Drop Axles, which made Off Highway vehicles to adopt this design,and are mentioned below -

Advantages of Portal Drop Axles -

1. Two Stage Reduction

Due the addition of gear reduction at wheel end, torque reduction strain on other drivetrain components can be lowered,which provides room for design of  smaller size Ring and pinion gears, differential case for same gear reduction.

2. Ground Clearance

As compared to normal layout, Portal axles provides higher ground clearance to the vehicles as size of differential is reduced,axle tube and differential case are mounted above the wheel centerline.

Disadvantages of Portal Drop Axles -

1. Slightly Increased maintenance cost.

Due to increase in number of gears in axle, oil needed for the lubrication is increased, which also needs periodic check of oil at every 8000-10000 km.

2. Not easy for installation

Portal Drop Axles are not easy to install, expertise is needed for correct installation to avoid potential safety and cost issues.

That's about " Portal Drop Axle" from Words of Enginner.