» Once reinstalling the wheel end unit, it
is critical to follow proper torque specs
for stud nuts, proper measurement for
endplay and proper lubrication levels.
Photo courtesy of SKF
» Differences in lubrication types and
performance standards for axles mean
different suggestions for maintenance.
Photo courtesy of SKF
June 2018 | VehicleServicePros.com 25
End-play and preload
Basic knowledge of wheel-end systems is required
in order to understand proper servicing. In particular,
bearing adjustment is critical to the safe and
continued operation of the wheel-end system.
“Th ere is not much room for error in bearing
adjustments,” says Russ Kolany, heavy duty key
account manager for bearing and seal manufacturer
SKF. “Around the thickness of two human
hairs is the margin.”
Aft er installing the outer bearing and nut
systems of the hub assembly, fl eets should adjust
wheel bearings in accordance with TMC RP 618,
to ensure proper endplay measurement is within
0.001” and 0.005”.
“Th ese clearances are critical to wheel-end
health and are a key indicator of how the wheelend
is performing during inspection,” says Vincent
Purvis, product manager - engineered products,
Stemco. Stemco off ers a range of wheel end components,
braking and suspension products to the
commercial vehicle industry.
Known as endplay, this measurement allows
for slight degrees of clearance with the bearings
– within one one-thousandth and fi ve
one-thousandths of an inch (0.001” and 0.005”).
Technicians must use a dial indicator to accurately
“While this is the industry accepted range for
manually adjusted wheel bearings, component
manufacturers agree that the lower end of this
range into slight ‘preload’ is the optimum range
for wheel-end performance,” says Roger Maye,
national service manager, Consolidated Metco
Inc. (ConMet). “Preload is a condition that removes
any excess ‘endplay’ in the bearing through a load
or force being applied to the bearing, creating an
axial interference.” ConMet supplies the commercial
vehicle industry with wheel ends, aluminum
castings and other components.
It is important to not have too much preload
though, Maye adds.
“Too much preload can result in large amounts
of heat being generated in the bearing, leading to
damage and failure,” he says.
“Technicians with normal tools are not able to
measure the amount of preload on the system,”
All shops should have a calibrated torque
wrench and dial indicator in order to complete
this work on manually-adjusted wheel-ends.
In addition, there are specialty tools technicians
can use that provide assistance with proper bearing
and seal installation.
It is also important to follow bearing and
adjustment procedures during wheel-end system
installation, and also to incorporate regular
inspection during PMs. It is recommended to
record the endplay measurement as a requirement
for every PM.
Pre-adjusted and unitized wheel-ends come
from the factory calibrated with the proper
amount of endplay. Manually-adjusted wheelends,
on the other hand, require proper bearing
adjustment any time the wheel-end is reinstalled
on the vehicle.
Types of wheel-end systems
Th ere are three types of wheel-end systems used
on heavy duty trucks: manually-adjusted, pre-adjusted
Sometimes referred to as traditional or conventional,
manually-adjusted wheel-end systems are
the most common in use today. Th ese wheel-end
systems have an adjustable fastening system and
follow traditional methods for greasing and servicing.
TMC RP 618 provides detailed instructions for
manual-adjusted wheel-end system installation
Pre-adjusted wheel-end systems provide a
preset amount of preload during component
assembly by the manufacturer. Pre-adjusted
systems have “an aluminum hub with a spacer
between the inner and outer bearings used to set
a specifi c dimension for setting preload,” SKF’s
Kolany says. TMC RP 640 advises these wheelend
systems are typically fi eld serviceable, but
advises fl eets should still refer to manufacturer’s
Unitized wheel-end systems go a step beyond
pre-adjusted, in that the system includes bearings
and seals installed, as well as lubricant,
during component assembly at the manufacturing
facility. It is a sealed unit, and care should
be taken to follow the manufacturer’s service
“Pre-adjusted and unitized wheel-end assemblies
control their bearing adjustment through
computer verifi cation at the factory before the
wheel-end is shipped to the end user, resulting
in a wheel-end assembly that the operator only
needs to torque the spindle nut to the proper
installation torque to correctly install the wheelend,”
ConMet’s Maye explains.
Manually-adjustable wheel-ends require the
most attention, due to the additional variables
technicians are responsible for reviewing during
the installation and maintenance process. Refer
» Technicians must use a dial indicator
to accurately measure the slight degree
bearing clearance, known as endplay.
Photo courtesy of SKF