WESLEY D. CHESTNUT
Emergency vehicles are some of the heaviest vehicles on the road today. With water, aerial devices, and various types of equipment, some emergency vehicles can weigh 80,000 pounds.
This massive amount of weight rests on the tire and wheel assemblies of these vehicles, which are held to the axles by several wheel-attaching nuts, more commonly known as lug nuts. Having something so small, in comparison to the overall size of emergency vehicles, to keep critical components attached to the vehicles is quite impressive. It takes all of the lug nuts working together at the proper torque to keep the tire and wheel assemblies attached to the vehicle.
Importance of Proper Tightening
The required torque equates to clamp load, which keeps the wheel attached to the emergency vehicle. It takes appropriate torque on all lug nuts to keep the wheel attached. A single lug nut coming loose may have a domino effect on the remaining lug nuts or lead to damage on the wheel or wheel stud.
Inspecting the lug nuts and verifying torque are critical to the integrity of the wheel mounting. If a loose lug nut goes without notice and damage to the wheel stud or wheel occurs, a wheel could separate from the axle. The National Highway Traffic Safety Administration (NHTSA) has investigated previous incidents in which this may have occurred, and manufacturers have issued safety recalls in certain cases.
New Vehicles
During assembly, a new emergency vehicle's wheels and tires are installed on the axles, then tightened to the appropriate torque. In many cases, the axles' supplier may specify the appropriate torque for the lug nuts. These torque values equate to the clamping force intended to keep the tire and wheel assembly attached to the axles. Manufacturers may use a calibrated torque tool to achieve the appropriate torque in the appropriate sequence. After the emergency vehicle has gone through the assembly process and subsequent road testing, manufacturers ensure the lug nuts are tightened, again using a calibrated torque tool.
In certain cases, an emergency vehicle may be built in two or more stages, with the chassis cab being built in one facility and final assembly occurring at a separate facility that may be miles away. The chassis cab manufacturer may recommend that the lug nuts be inspected and torque verified on all lug nuts after receipt of the chassis cab and prior to delivery to the end customer. This is a precautionary measure to ensure the lug nuts have remained properly tightened to their recommended torque value.
Once "In Service"
After the vehicle is placed in service, it is even more important to inspect, or verify torque on, the lug nuts. The wheel studs and lug nuts often experience dynamic conditions such as impacts from traversing various types of terrain and heat generated by vehicle braking. Over time, these conditions may adversely affect the wheel, wheel studs, or lug nuts.
National Fire Protection Association (NFPA) 1911, Standard for the Inspection, Maintenance, Testing, and Retirement of In-Service Automotive Fire Apparatus, indicates that lug nuts are to be torqued to the manufacturer's recommendation. The frequency of this inspection may depend on an emergency vehicle's use. However, there is guidance within the standard that suggests verifying torque between 50 and 100 miles after wheel removal.
Federal regulations promulgated by the Federal Motor Carrier Safety Administration (FMCSA) include a minimum periodic inspection standard that departments can use as baseline criteria for emergency vehicles. This standard includes inspecting wheels and fasteners for loosening, cracks, stripping, or elongated holes in the wheels.
Lug Nut Torque Indicators
There are various methods of inspecting for loose lug nuts that include visual inspection or applying a torque wrench. As technology has advanced, there are various types of lug nut indicators available on the market for installation. These indicators allow for an easy, quick-glance inspection to determine if the lug nuts are loose. Using these indicators provides a more efficient means of inspection on a more frequent basis.
Keeping the Wheels On
Emergency vehicles by nature can be very heavy and subjected to extreme conditions that can impact the very foundation on which they sit-their tire and wheel assemblies. As a result, lug nuts may be subject to loosening over time. There are standards and guidelines that identify the need to inspect lug nuts to identify and correct issues with them early to prevent more serious consequences.
Technologies related to motor vehicle safety continue to advance, and these advancements do not exclude lug nuts. There is a wide variety of lug nut torque indicators for emergency vehicles that provide a quick and easy means of checking lug nut torque. Using this method, as well as the more traditional methods, and following the manufacturers' recommended torque procedures and values can help mitigate future damage. Fire Apparatus Manufacturers' Association (FAMA) member companies can help with identifying information related to lug nuts and identify torque indicators that best suit a department's needs.
FAMA is committed to the manufacture and sale of safe, efficient emergency response vehicles and equipment. FAMA urges fire departments to evaluate the full range of safety features offered by its member companies.
WESLEY D. CHESTNUT has been involved with the fire service for more than 20 years. He is co-chair for the chassis subcommittee under the Fire Apparatus Manufacturers' Association (FAMA) Technical Committee and is employed as compliance lead for Spartan Motors, Inc. He interacts with federal agencies like the National Highway Traffic Safety Administration and the Environmental Protection Agency on behalf of FAMA when federal rules are being developed.