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Causes / Effects

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.

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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 loads.

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 experienced

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.

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Wheel Detachments
Legal Liabilities
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.

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