Vehicle Axles: Structural Features

Axle 1
Axle
An axle is a central shaft for a rotating wheel or gear.
In some cases the axle may be fixed in position with a
bearing or bushing sitting inside the hole in the wheel
or gear to allow the wheel or gear to rotate around the
axle. In other cases the wheel or gear may be fixed to
the axle, with bearings or bushings provided at the
mounting points where the axle is supported.
Sometimes, especially on bicycles, the latter type is
referred to as a spindle.
Vehicle axles
Axles are an integral structural component of a wheeled Train wheels are affixed to a straight axle, such that both wheels
rotate in unison. This is called a wheelset.
vehicle. The axles maintain the position of the wheels
relative to each other and to the vehicle body. Since for
most vehicles the wheels are the only part touching the ground, the axles must bear the weight of the vehicle plus
any cargo, as well as acceleration and braking forces. In addition to the structural purpose, axles may serve one or
more of the following purposes depending on the design of the vehicle.
• Drive: One or more axles may be an integral part of the drivetrain. A mechanical system (typically a motor)
exerts a rotational force on the axle, which is transferred to the wheel(s) to accelerate the vehicle.
• Braking: Conversely a vehicle may be slowed by applying force to brake the rotation of the axle. Most vehicles'
brakes are part of the wheel assembly and then exert torque to the wheels directly, but engine braking may still be
effected via the axle.
• Steering: The front axle of most automobiles is a steering axle. The vehicle is maneuvered by controlling the
direction of the front wheels' rotational axis relative to the body and rear wheels.
Structural features
A straight axle is a single rigid shaft connecting a
wheel on the left side of the vehicle to a wheel on the
right side. The axis of rotation fixed by the axle is
common to both wheels. Such a design can keep the
wheel positions steady under heavy stress, and can
therefore support heavy loads. Straight axles are used
on trains, for the rear axles of commercial trucks, and
on heavy duty off-road vehicles. The axle can be
protected and further reinforced by enclosing the length
of the axle in a housing.
In split-axle designs, the wheel on each side is attached

to a separate shaft. Modern passenger cars have split 0 Series Shinkansen Wheel
drive axles. In some designs, this allows independent
suspension of the left and right wheels, and therefore a smoother ride. Even when the suspension is not independent,
split axles permit the use of a differential, allowing the left and right drive wheels to be driven at different speeds as
the automobile turns, improving traction and extending tire life.
Axle 2
A tandem axle is a group of two or more axles situated close together. Trucks designs will use such a configuration
to provide a greater weight capacity than a single axle. Semi trailers usually have a tandem axle at the rear.
Drive axles
An axle that is driven by the engine is called a drive axle.
Modern front wheel drive cars typically combine the transmission and
front axle into a single unit called a transaxle. The drive axle is a split
axle with a differential and universal joints between the two half axles.
Each half axle connects to the wheel by use of a constant velocity (CV)
joint which allows the wheel assembly to move freely vertically as well
as to pivot when making turns.
Splines on a front drive axle. In rear wheel drive cars and trucks, the engine turns a driveshaft which
transmits rotational force to a drive axle at the rear of the vehicle. The
drive axle may be a live axle, but modern automobiles generally use a split axle with a differential.
Some simple vehicle designs, such as go-karts, may have a single driven wheel where the drive axle is a split axle
with only one of the two shafts driven by the engine, or else have both wheels connected to one shaft without a
differential.
Dead axles/lazy axles

A dead axle, also called lazy axle, is not part of the
drivetrain but is instead free-rotating. The rear axle of a
front-wheel drive car may be considered a dead axle.
Many trucks and trailers use dead axles for strictly
load-bearing purposes. A dead axle located
immediately in front of a drive axle is called a pusher
axle. A tag axle is a dead axle situated behind a drive
axle. On some vehicles (such as motorcoaches), the tag
axle may be steerable. This dump truck has an airlift pusher axle, shown in the raised
position.
Some dump trucks and trailers are configured with lift
axles (also known as airlift axles or drop axles), which may be mechanically raised or lowered. The axle is lowered
to increase the weight capacity, or to distribute the weight of the cargo over more wheels, for example to cross a
weight restricted bridge. When not needed, the axle is lifted off the ground to save wear on the tires and axle and to
increase traction in the remaining wheels. Lifting an axle also makes the vehicle perform better on tighter turns.
Several manufacturers offer computer-controlled airlift, so that the dead axles are automatically lowered when the
main axle reaches its weight limit. The dead axles can still be lifted by the press of a button if needed.
Full-floating vs. semi-floating

The full-floating design is used in heavy duty applications as opposed to many of the lighter-duty differentials that
are semi-floating. A full-floating differential can be identified by the large hubs that stick out the end of the axle and
have the axle shafts bolted to them. These axles are able to carry more weight then a comparable semi-floating
differential because the hubs on the ends of the axle have their own bearings that carries the load of the vehicle. The
semi-floating design carries the weight of the vehicle on the axle shaft, there is simply one bearing on the end of the
axle tube that carries the load from the axle and that the axle rotates through. In the full-floating design the hubs
carry the weight of the vehicle while the axles are only used to transmit torque and rotation from the carrier in the
Axle 3
differential to the hubs on the ends of the axle. Full-floating axles are held in place by bolting a flange on the end of
the axle to the hub. With a semi-floating axle, the axles are held in place by c-clips that are in the differential carrier.
If an axle or c-clip ever breaks it is possible that the wheel of the vehicle could leave the vehicle. The full-floating
designs, however, do not have c-clips, but can still break axles. If the axle shaft breaks, the wheel will not go
anywhere since it is still bolted to the hub.
See also
• Klien-Lindner axle
• Portal axle
• Wheel and axle
• All pages beginning with "Axle"
• All pages with titles containing "Axle"

Vehicle Axles: Structural Features

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Axle 1

In split-axle designs, the wheel on each side is attached

Dead axles/lazy axles

Full-floating vs. semi-floating

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