When designing axle assemblies for any type of vehicle, one of the first questions to ask is, "How will this thing move?" Below, we'll look at some examples for simple vehicles such as rail carts and pull wagons—not just commercial vehicles. Most axle designs fall into one of two categories: Drive axles and dead axles—also known as "lazy axles."
In a nutshell, their names describe their most basic characteristics. Drive axles are your powerful go-getter types. They're self-propelled and hungry to get moving. Dead axles need a bit more external motivation. Without an added propulsion mechanism, they'll remain stationary while the world passes them by.
Drive axles are basically any type of axle that receives torque from an internal drivetrain—typically from a drive shaft or transaxle connected to an engine. In these assemblies, the drive axle is what transfers drive power to the wheels. In turn, with enough friction and grip, the wheels will propel a vehicle forward under its own power.
In its most basic form, a drive axle is a solid bar with wheels affixed at either end. Though, because turning radii affect the speed at which each individual wheel turns, this configuration is typically only used for fixed axles in rail applications.
For self-guiding vehicles (i.e., those with a steering wheel) a driven axle is typically split by a differential, which allocates power according to which wheel is on a tighter radius. In some applications, many go-karts for example, a single drive wheel is common. In this configuration, one wheel is fixed to the drive axle while the other is allowed to rotate freely—preventing slippage when rounding corners.
There are a couple of considerations to note here.
- To transfer torque from an axle to its wheels, the wheels must be fixed to the axle. This can be done with a full hub assembly or with a keyed shaft in simpler applications.
- To support the weight of a vehicle while delivering torque, bearings are needed. In a drive configuration, the inner bearing surfaces rotate with the axle, while the outer bearing surfaces remain stationary—fixed to the vehicle body.
Since driving force is limited to where each drive wheel touches the ground, terrain can be a major consideration in drive axle design. Rough or slippery terrains create challenging environments for drive wheels. Uneven terrain can separate drive wheels from the ground, leaving them spinning in the air with no point of contact. Slippery surfaces can create the same effect. 4×4 or all-wheel-drive configurations use multiple drive axles to apply driving power to more contact points.
Dead axles are mechanically simpler than drive axles. Dead axles are basically wheel assemblies that aren't connected directly to any internal powertrain. Instead, they roll freely with a vehicle's motion.
Even though dead axles are much simpler than drive axles, there are a few design elements to consider. Namely, what turns and where.
Most often, the simplest dead axle design is to use a fixed axle with wheels that turn independently. With this configuration, each wheel runs on its own bearings. Inside bearing surfaces remain fixed with the axle, while the outer bearing surfaces rotate with each wheel. Allowing wheels to rotate independently means a differential isn't needed to avoid wheels slipping while going around corners.
Dead axles that use this configuration can also be easier to service and maintain. Movement typically causes wear, which in turn requires maintenance. Wheel bearings are most often more accessible and easy to replace than axle bearings.
The big question with dead axles is how to get them moving. Often dead axles are installed as load-bearing axles to complement driven axles. But vehicles can also be propelled by external power sources as well. The simplest example is that of a wagon or pull-cart.
In industrial or commercial applications, a winch or cable-pull mechanism can be effective—especially where a vehicle is propped on a set of rails and doesn't have any independent steering capacity.
A cable pull can also be beneficial where rail conditions are less than ideal. For rails that are bent or uneven—or that may be covered in a slippery solution—wheels on a dead axle don't rely on friction to propel a vehicle. For applications where rails may be bent at opposing angles, wheels can also be designed to float across portions of an axle—allowing them to track with the curvature of each rail.
Axles and their wheels
Axle design is an important component to any vehicle assembly. Not only are they responsible for mobility, they must also bear all of the weight and associated punishment from dutiful loads and uncompromising ground surfaces. At their most basic function, they are valued for smooth, frictionless movement, helping you get from one point to another.
For information on installing Reliance Foundry wheels, or to learn more about keyed or bearing customizations, contact our Sales Department.