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Heavy Duty Division
Loose Wheel Nut
Loose wheel nuts can occur for a variety of reasons, including undertorquing,
overtorquing, differential thermal contraction, and improper mating surfaces.
Both undertorquing and overtorquing are common when air impact wrenches are used
because the actual torque that gets applied depends on so many variables.
A common approach is to purposefully overtorque wheel nuts, in part to combat
this variability and in part based on the reasoning that "more is
better". However, overtorquing actually reduces (not increases)
clamping force in many cases, by stretching the studs or threads beyond their
ability to respond - especially when this is done repeatedly. Overtorquing
can also cause other problems such as cracked, seized, or cross-threaded nuts
(which cannot apply the appropriate clamping force), and increases the frequency
of stud failure and cracked wheels.
Differential thermal contraction
can occur when wheels are mounted at shop temperatures in cold climates.
As the wheel components cool to ambient temperatures, clamping force is
lost. Wheel nuts which experience any rotation (i.e. back off) during this
time will not regain their original torque / clamping force values.
Improper mating surfaces include both damaged and contaminated mating surfaces.
Proper clamping force cannot be achieved with non-flat mating surfaces such as
damaged or bent hubs and wheels, or worn or elongated bolt holes (raised
metal). Contaminants such as excess dirt, sand, rust, metal burrs, and
paint on mating surfaces can wear away with use, causing a settling
effect. When present on the threads or between a nut and the wheel
surface, these contaminants can also change the clamping force / torque
relationship, resulting in "false torques" where much of the torque
applied is used to overcome friction and is not converted into clamping force.
A loose wheel nut can originate from any of these sources individually, or more
probably, from a combination of these sources - which makes the task of eliminating all loose wheel
nuts very difficult.
Stretches the Studs
Wrench Use & Maintenance
Loose / Missing Wheel Nuts
Wheels are subjected to a variety of forces,
including vertical forces from the vehicle and its cargo, road vibration and
shock forces, cornering forces when the vehicle turns, and rotational forces
from the turning of the wheel, especially during acceleration and braking.
When a wheel nut loosens these forces are
redistributed among the remaining nuts and studs, but preferentially to the nuts
and studs adjacent
to the loose nut, causing these nuts to back off at reduced wheel force levels.
This loosening process accelerates with each successive nut that loosens, as the
total clamping force drops and the stress concentration at the remaining nuts
and studs increases. At this point these studs can fracture due to fatigue
or overstress, accelerating the process further - especially with heavy cargo
When the wheel forces
exceed the clamping force of the remaining nuts and studs, the wheel will move
relative to the hub which results in side loading and loosening of the remaining
nuts, bending fatigue failure of studs, elongated bolt holes and wheel
pilots on the hub, fretting fatigue cracks between the bolt holes, and wheel
separation if not detected quickly..
The rate of this process, and thus the potential for detection prior to wheel
separation, depends on the type and magnitude of the wheel forces being
Fines / Impound
Wheel / Component Damage
Downtime / Road Calls / Late Deliveries
Loose or missing wheel nuts, and the wheel
and component damage caused by them, can result in both fines and impound.
Penalties vary from "points" and a few hundred dollars to several
thousand dollars and 15 days impound for first offenses, with much stiffer
penalties for subsequent infractions.
Wheel, hub, and mounting hardware
damage caused by past loose wheel nuts or overtorquing greatly increase
the risk of future loose wheel nuts and wheel detachments, and should be
corrected. Repair or replacement can easily reach well into the hundreds of
dollars when considering both part cost and the labor involved.
In addition to the direct costs
for inspection infractions and wheel/component damage, downtime is required to
correct these problems. If not corrected in the shop, the downtime for
both vehicle and driver can extend to several hours and result in road call
charges while the driver or inspector waits for the problem to be
corrected. These charges also can reach well into the hundreds of dollars at
typical hourly downtime and road call rates.
Collectively, the total cost of a single event
caused by loose or missing wheel nuts can be substantial. This is
especially so if the event results in a missed delivery time, the penalties for which can
sometimes reach into the thousands of dollars per minute range.
Keep Your Wheels Rolling Strong
Liberal Seeks to Speed
Up Truck Law
Defects Indicate Need for Greater Safety Efforts
2000 Truck Safety Inspections
Wheel detachments occur at an estimated rate of 40 to 60 per week in the
U.S., resulting in about 20 reported accidents per week. Studies in both
the U.S. and Canada have indicated that about 45% to 50% of these wheel
separations are caused by loose wheel nuts.
Those separations which do not
result in accidents can still be expensive. The total costs for an
extended road side service call, the driver/vehicle downtime, the replacement of
a wheel and/or hub and mounting hardware, and a tire if the wheel is lost, can be
substantial - not to mention the potential for late delivery penalties.
And fines for wheel separations can be
stiff in some regions - for example, up to $50,000 per separation in Ontario.
The wheel separations which do
result in accidents can create devastating consequences, sometimes maiming or
killing those involved. The majority of these accidents result from the
detached wheel bouncing along the highway until it impacts oncoming
traffic. A small percentage result from the detached wheel hitting
pedestrians, or causing the vehicle itself to overturn.
Those that result only in
vehicular damage can be covered by liability or collision insurance, albeit
with increased premiums in the future. Those that result in personal
injury or the unfortunate loss of life can generate legal suits reaching into
the millions of dollars, affecting the fleets, their maintenance staff and
management, and their customers and vendors.
Bald Facts (Tire & Wheel Negligence)
Trucker in Fatal Wheel-Loss Crash Likely to
Man Hit by Flying Truck Wheel
Fatality Renews Truck Safety Concerns
State Finds Problems with Trucking
What Are Loose Lug Nuts Worth? Try $6.25