Know Your Axles!

Potential buyers of extra-heavy commercial vehicles (XHCVs) have a choice between a wide variety of different manufacturers, models, and specifications. Included in the  process is the choice between single-axle reduction and hub-reduction rear-drive axles.

In order to ensure that the vehicle is suitable for the job that it will be undertaking, it is important to understand both of these rear axle drive types.

To fully understand the pros and cons of both types of rear-drive axles, we first need to examine the basic functions of a drive axle:

• The distribution of torque to the driving wheels
• Reduction
• Compensation of different rates of wheel rotation
• Deflection of the power flow by 90° from the drive shaft to the axle.

Single-reduction axles – also known as hypoid-type axles – are fitted to many modern trucks. One of the major advantages they provide is the mechanical efficiency of the single-reduction gears, which in turn results in good fuel consumption.

Due to the design of single-reduction axles, heavy-duty vehicles fitted with this type of component usually have a limited gross combination mass (GCM) rating of between 56,000 and 70,000kg. This means that  they are not capable of hauling abnormal loads that exceed these ratings.

Hub-reduction axles – also known as planetary axles – split the torque between the differential and the planetary gears in the wheel hubs. As the torque is spread over several gear cogs, this design is extremely strong and well-suited for extra-heavy-duty work. Due to the strength of the hub-reduction design, vehicle manufacturers that fit these axles have the choice of increasing the GCM rating of the vehicle, making it suitable for abnormal loads.

Some vehicle operators are of the opinion that vehicles equipped with hub-reduction axles use more fuel than those fitted with single-reduction axles. However, according to manufacturers that supply vehicles using the hub-reduction type, modern trucks fitted with these components are just as fuel efficient as those with single-reduction axles – provided the drivetrain and gearbox ratios are correctly matched.

While both single-reduction and hub-reduction axles are generally well-suited for most normal vehicle operating conditions in South Africa, for applications where vehicles are required to operate beyond the legal GCM rating of 56,000kg, a vehicle fitted with hub-reduction axles will clearly be the better choice.

In addition, vehicles fitted with hub-reduction drive axles offer better ground clearance, making them well-suited to any applications that involve rough or uneven terrain.

The decision-making process for which type of rear axle reduction is best for an operation is often clouded by truck sales personnel, who obviously promote and endeavour to sell the type of axle drive that is fitted to a truck that is readily available. Operators would be well advised to do their homework before heading to the dealership – to ensure they know exactly what they are looking for and how well it will match the job at hand.