The causes of wheel separation are well known. They include bearing failures; loose, broken, or missing wheel fasteners; damaged wheels, and to a lesser degree, failures of certain axle and suspension components.
For more information, please visit our website.
What’s not as well understood is why these events continue to occur.
The Canadian province of Ontario has done loads of research into wheel separations and is probably the most active jurisdiction in North America when it comes to wheel-separation investigation. More than 2,300 wheel separations were reported there between and , and 11 people lost their lives to separated truck wheels during that period, according to the province’s Ministry of Transportation.
Investigations conducted by the province determined that 26% of separations resulted from bearing or wheel-end failures, while about 65% were wheel-fastener related. In 83% of the reported incidents, repairs or maintenance had been performed on the suspect wheel(s) a short time prior to the separation. In one case, a vehicle traveled just 355 miles before a separation occurred following the installation of new brake linings at the suspect wheel position.
Despite the abundance of training material and step-by-step instructions provided by wheel component suppliers, industry associations and others, wheels continue coming off trucks. Keeping wheels where they belong should simply be a matter of process and procedure. Here are four reminders of what needs to be done when servicing truck tires and wheels to ensure they don’t wind up in a forest or a field – or the front seat of an oncoming car.
Bearing-related wheel-end failures are often caused by over- or under-tightened bearings or lack of lubrication. Under-tightening (excessive endplay) can cause the wheel to wobble on the spindle, damaging the seal, which can lead to a loss of lubricant. Over-tightening (excessive preload) can damage the bearing, causing overheating, seal failure, and lubricant loss.
While slight preload of the bearings is regarded as the optimum environment, determining it is almost impossible without special tools, such as Temper Axle’s Doctor Preload bearing adjustment tool, distributed by Meritor.
To combat inconsistency in adjustment, component suppliers publish installation instructions (not installation guidelines). There also are generic instructions such as Recommend Practice 618B from the American Trucking Associations’ Technology & Maintenance Council. This RP describes a nine-step installation and adjustment process, depending on the type of wheel-end assembly, finishing up with the use of a dial indicator to verify correct adjustment.
But industry sources say that because it’s finicky and time-consuming, the final step is often left out – despite being one of the most critical parts of the process.
“That’s a recommendation for an ideal situation,” admits John Heffernan, SKF key account manager. “Getting techs to use torque wenches was a huge step for the industry. Use of dial indicators is growing, and more fleets are adopting the practice, but we still have a long way to go.”
Jeremy Gough, director of national fleet maintenance at Bison Transport, one of the consistently safest fleets in North America, has a second technician check the work before the truck or trailer is released from the shop. “A second set of eyes goes a long way to ensuring the job is done right the first time,” he says. “It takes a bit longer, but it’s less time-consuming than dealing with a failure.”
One option is switching from conventional to pre-adjusted hubs, sometimes generically called “pre-set” hubs – although, much like people call any clear tape “Scotch tape” or any facial tissue “Kleenex,” that’s actually a brand name associated with ConMet (Consolidated Metco).
“This technology takes bearing adjustment out of the hands of the commercial vehicle manufacturer and guarantees that equipment leaves the factory with wheel bearings optimally adjusted for miles of service with a minimum amount of required maintenance,” says Roger Maye, ConMet’s national service manager.
With pre-adjusted hub assemblies, a precision-machined hub combined with tight tolerance bearings and spacer provide a fixed dimensional distance between the inner and outer bearing. Simply spinning on the axle fastener and torqueing exactly to the manufacturer’s specification sets the bearings properly every time, without the need to verify end play with a dial indicator, says Drew Coen, Stemco’s wheel-end product manager.
“Pre-adjusted hubs are easier to install and eliminate the need for fleets and repair facilities to have multiple parts to build a complete wheel-end,” Coen adds. “The pre-assembly of the components and ease of install saves the mechanics time and eliminates wheel end installation mistakes that could lead to costly and/or unsafe conditions as the vehicle is put back into service.”
Bill Hicks, SAF-Holland’s director of product planning, says fleets are slowly shifting to pre-adjusted hubs for the labor savings and the relative ease of installation.
“We’ve seen slow but steady growth in orders for pre-adjusted hubs on our axles,” he says. “They cost more because of the more precise machining and the tighter tolerances involved, but it becomes a value equation for the fleet. They are easier and less time-consuming to install, and they usually come with longer warranties. As well, when the technicians follow the installation instructions, the bearing is pre-tightened properly every time. It takes away some of the technician’s ‘discretion’ in how the hub and bearings are installed.”
Hicks cautioned that technicians may still need some training to help them identify a pre-adjusted hub. There’s risk of damage to the bearings and the bearing spacer if excessive torque is applied to the retaining nut during installation.
Bearing condition can be checked during regular preventive maintenance inspections by jacking up and safely supporting the wheel and rotating it while listening for noise. Maye suggests placing a hand on the steering knuckle or brake chamber to check for vibration when the wheel is rotating. Next, check for movement or “chucking” by placing a pry bar under the tire and, with one hand on top of the tire, lift the wheel-end and feel for movement between the hub and the spindle.
“Make sure to grease the kingpin on steer axles before this test, as kingpin wear can feel like wheel-end chucking,” he says. “If excessive movement is found, wheel-end service is required.”
Trailer hubs lubricated with semi-fluid grease should be inspected annually by removing the hubcap to check the presence and condition of the lubricant. Add additional lubricant if the ends of the bearing rollers are not covered. If the lubricant appears dried out, the wheel-end should be removed, cleaned, reinstalled, and new lubricant added to the proper level. Maye suggests either using the fill-plug magnet on PreSet hubs or placing a handheld magnet into the lubricant well to check for metallic particles. “If metal bits are found, the hub should be disassembled to find and correct the source of the issue.”
Before you even think about putting the wheel back on the truck, be certain the contact surfaces between the hub and the inner and outer wheels are absolutely free of dirt, rust, grease and other contaminants.
“A buildup of foreign material on the wheel-end mounting surfaces causes extra thickness in the joint,” explains Brandon Uzarek, Accuride’s field engineer for wheels. “The foreign material may settle or work its way out of the joint, causing the tension in the bolt to decrease, resulting in a loss of clamping force.”
The contact areas must be rigorously cleaned with a wire brush to remove all foreign material before mounting the wheel. Wheel studs should be cleaned with a wire brush as well to rid the thread grooves of rust and foreign material that can affect the torque on the nut.
Extra care is needed with painted steel wheels. Rust can develop under the paint if the finish is broken, causing flaking. This material can drop out over time, reducing the clamping force. The coating thickness on a painted wheel is important, too, as coatings thicker than 3–3.5 mils can affect the mounting surfaces between the bolt holes.
While there’s little published research on the impact of hardware aging and degradation caused by use and environmental factors, research by Ottawa, Ontario-based Woodrooffe and Associates, published in the paper “Heavy Vehicle Wheel Separations: Exploring the Causes,” suggests that re-using degraded flange nuts could cause problems.
“Anecdotal tests have shown that as [two-piece] flange nuts age, or when they are re-used, their torque/clamping force characteristics can be as little as 50% of the design value,” the report notes. “Given that hub-piloted wheels depend exclusively on clamping force to prevent the wheel from separating from the hub, such a reduction in clamp force characteristics represents a significant risk to wheel separation.”
Tighter is not better, especially when re-using studs and nuts. Many technicians attempt to achieve maximum clamping force by applying more than 500 lb-ft of torque without realizing the possible consequences.
“If a wheel stud is subjected to excessive torque, it is possible to stretch the stud past its yield point,” cautions Accuride’s Uzarek. “If this occurs, the joint will act as if it is under-torqued – and there will be lower, not higher, clamping force.”
Stretched studs will have deformed threads. Check the condition by hand-threading a new nut and observing any unusual resistance to hand-turning. That could indicate stretching. When fastening the nuts to their final torque, use only a calibrated torqueing device.
With all that said, there remains one step in this process that fleets rarely, if ever, comply with: the wheel-fastener retorque. The manufacturers of heavy truck wheels recommend that wheels be checked for correct torque between 50 and 100 miles after wheel installation. Doing that is extremely difficult in a logistics sense for most fleets, but there are alternatives.
Some suggest running the truck around the yard with a loaded trailer, doing a series of tight right and left turns and hard brakes, to give the wheels an opportunity to settle onto the studs and against the mounting face of the hub, and then retorquing them. Others recommend flagging the unit for a retorque the next time it arrives in the yard.
Neither situation is ideal, but both are better than completely ignoring the requirement to retorque the wheels after installation.
A wheel hub assembly on a heavy duty truck has many moving parts, all critically connecting the wheels on the road to the rest of the truck. When something goes wrong with this system, it can be disastrous.
The good news is these situations are often preventable with proper inspection and appropriate maintenance practices.
It is important to first understand the basics of the wheel-end system, and how to identify the different types of wheel-end systems. Proper inspection – whether that be pre- and post-trip inspections or more in-depth service – is also critical to the continued performance of wheel-end systems.
Some 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.
“There is not much room for error in bearing adjustments,” says Russ Kolany, heavy duty key account manager of bearing and seal manufacturer SKF. “Around the thickness of two human hairs is the margin.”
After installing the outer bearing and nut systems of the hub assembly, fleets should adjust wheel bearings in accordance with TMC RP 618, to ensure proper endplay measurement is within 0.001” and 0.005”.
“These clearances are critical to wheel-end health and are a key indicator of how the wheel-end is performing during inspection,” says Vincent Purvis, product manager - engineered products, Stemco. Stemco offers 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 five one-thousandths of an inch (0.001” and 0.005”). Technicians must use a dial indicator to accurately measure endplay.
“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,” Purvis explains.
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 wheel-ends, on the other hand, require proper bearing adjustment any time the wheel-end is reinstalled on the vehicle.
There are three types of wheel-end systems used on heavy duty trucks: manually-adjusted, pre-adjusted and unitized.
Sometimes referred to as traditional or conventional, manually-adjusted wheel-end systems are the most common in use today. These wheel-end systems have an adjustable fastening system and follow traditional methods for greasing and servicing. TMC Recommended Practice 618 provides detailed instructions for manual-adjusted wheel-end system installation procedures.
If you are looking for more details, kindly visit BWHY.
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 specific dimension for setting preload,” SKF’s Kolany says. The TMC RP 640 advises these wheel-end systems are typically field serviceable, but advises fleets should still refer to manufacturer’s service recommendations.
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 recommendations.
“Pre-adjusted and unitized wheel-end assemblies control their bearing adjustment through computer verification 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 wheel-end,” 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 to TMC RP 640 for instructions on how to properly identify these systems.
It is important for technicians be able to identify the wheel-end system, to ensure proper maintenance procedures.
There are some key items to look for when identifying the wheel-end system, Maye says. First, look for the chassis decal or wheel-end identification on the hub cap, to confirm wheel-end type and lubrication specs. If this information is unavailable, the technician can disassemble the wheel-end to inspect the unit. Maye says many manually adjusted and some pre-adjusted wheel-ends will use a variety of different types of spindle nuts, and those nuts should include steps for proper installation.
The key indicator of a pre-adjusted wheel-end is the presence of a bearing spacer.
“If the wheel-end includes a bearing spacer, disregard the spindle nut installation instructions specific to manual bearing adjustment and follow the instructions from the manufacturer of the pre-adjusted wheel-end assembly,” Maye adds.
The wheel-end system type dictates the components replaced in the system during service. Rick Domin, Lead Technical Trainer for wheel hub and bearing supplier Timken, advises manually-adjusted wheel-ends require the replacement of cups and cones. For pre-adjusted wheel-ends, in addition to the cups and cones, the technician will need to replace the bearing spacer.
“If the technician removes the wheel-end and, after removing the retaining nut, notices the entire hub assembly must be removed and there is an inner and outer seal, they are most likely dealing with a unitized bearing and must replace them as a whole assembly,” Domin adds.
It is also important to evaluate the benefits of repairing versus replacing the wheel-end unit. As is the case with aged or high-mileage vehicles, the hub casting, bearings or bolts may be severely worn or damaged.
“Many times a technician is not properly trained to determine if these components are suitable for re-use,” Maye advises . “When the cost of components to rebuild the wheel-end, the labor involved, the additional time out of service for the vehicle and the liability for the repair is considered, wheel-end replacement is often the best option. The new wheel-end can be quickly installed and often times comes with an extended warranty. The new assembly will put the wheel-end back to as new condition.”
“The three critical components for wheel-end performance are bearings, lubricant and the seal,” explains Stemco’s Purvis. “TMC’s recommended lubricant practices show that lubricant condition is a leading indicator of wheel-end health.”
Lubricant leakage is symptomatic of a larger issue – primarily due to seal failure because of improper wheel bearing adjustment. This goes back to the importance of measuring proper endplay.
When it comes to lubricants, there are two types of lubrication processes for traditional wheel-end systems – oil bath wheel-ends and semi-fluid grease wheel-ends.
It is important to review information from the manufacturer on the process for lubricating pre-adjusted and unitized hub assemblies.
While it is the fleet’s decision on what type of lubricant to use, TMC and manufacturers have suggestions on how to select the right lubricant for the intended application.
When considering lubricants for manually adjusted wheel-ends, either synthetic- or petroleum-based oils or greases are acceptable to use. First, note if the wheel-end is part of a driven or non-driven hub assembly. Different non-driven wheel-ends include steer, dolly, trailer, pusher and tag axles. According to TMC’s Recommended Practice 631, fleets can lubricate non-driven wheel-ends with their preference of oil or grease. For drive axles, however, only oil is recommended.
Differences in lubrication types and performance standards for axles mean different suggestions for maintenance (see sidebar "Pre- and post-trip checklist for wheel-ends").
Service interval ranges from 100,000 miles to five years in over-the-road service, depending on the axle type, manufacturer recommendations and lubricant performance capabilities.
“Consult with your vehicle, wheel-end and lubricant manufacturers to determine the lubricant that is best for your particular application,” ConMet’s Maye advises.
When inspecting lubrication for wheel-ends, Maye also says inspection should note the level and condition of the lubricant.
It is important to address improper wheel-end installation, unit contamination, overloading or lubrication issues.
Taking into consideration the integral aspects of identifying and servicing wheel-end systems mentioned earlier, fleets can be sure to properly implement maintenance programs to service this area of the vehicle.
In addition to verifying proper endplay, fleets should note proper wheel nut (including axle nut, spindle nut, lug nut, etc.) torque measurements. Whether under- or over-torqued, improper torqueing accounts for many wheel-end failures.
Inadequate torque can cause the wheel bearings to fail – the most common cause of wheel-end failures, says SKF’s Kolany. This is due to the loss of clamping force on the lug nuts, or amount of pressure applied to keep the wheel and hub intact.
Conversely, excessive torque can cause additional stress on components by causing stretching and fracturing of the lug nut stud.
“If a lug stud is broken or missing, the damaged lug stud should be replaced along with the two studs on either side the broken stud,” Kolany says. “If more than one stud is broken or missing, all lug studs should be replaced on the hub. Lug nuts should be cleaned and attached.”
In addition, Kolany notes the washer should spin freely on the nut. If it doesn’t, he advises to replace the lug nut stud.
Jamie Untersinger, Business Development Manager, Vehicle Inspection Systems, recommends the “R.I.S.T.” checklist for the removal and replacement of wheels, to help reduce the chance of a wheel-off situation:
Vehicle Inspection Systems promotes brake and undercarriage inspection as well as wheel and rim safety products for the commercial vehicle industry.
Whether it's pre-and post-trip inspections by drivers, or assessing the wheel-end system as part of regular preventative maintenance, visual checks can provide warning signs to issues. Fleets should look for any broken or damaged components, and for any leakage around seals and hub cap gaskets. Depending on the type of lubrication method, Stemco’s Purvis recommends inspecting the hub for the following:
For pre-and post-trip inspections, “the lubricant levels are easy to check,” Timken’s Domin confirms. “Additionally, look for wetness on the back of the wheel which could indicate a seal leak.”
“When performing a wheel-end inspection, the technician should check the wheel bearing seal for signs of excessive leakage, the hub assembly for excessive endplay and the hubcap or axle flange for signs of lubricant indicating a leaking gasket or possible damage to the hubcap,” Domin adds.
SKF’s Kolany says it is critical to have drivers properly trained to inspect wheel-ends. In addition to educating drivers on proper techniques on confirming possible lubricant leakage, they can visually inspect for slipping or missing lug nuts and other aspects of the unit.
“If the lug nuts are loose or backing off you will notice rusty discoloration or lines as if they’re slung from the wheel bolt area,” Vehicle Inspection Systems’ Untersinger adds. “This is a good indication torque should be rechecked immediately.”
Drivers should also physically touch the wheel-end assembly during a post-trip inspection to confirm heat generated from the unit.
“Warm to touch is normal but when too hot to touch, there is a problem” Kolany says.
Regarding operating temperature, it is recommended the wheel-end temperature record at no higher than ambient temperature-plus-150 degrees F. In other words, if it is 50 degrees outside, the temperature should not exceed 200 degrees F.
“Heavy braking or a dragging brake can also effect wheel-end operating temperature,” ConMet’s Maye says.
High temperatures can be indicative of a more widespread problem with other areas of the vehicle like the braking system.
“Overheated brakes can cause excessive heat to the wheel hub that can damage the seal and cause the seal to leak lubricant onto the brake shoes,” Domin says. “In such instances, the brakes will be less effective and cause increased stopping distances, which will in turn apply more heat to the wheel-end and cause more seal damage.”
Other indirect causes of bearing adjust issues or failures, as it relates to wheel-ends, include premature leaking seals, ABS fault codes and abnormal brake wear.
When techs conduct inspections in the shop, they have access to additional tools for conducting PMs.
“TMC RP 631, 651, and 655 provide an excellent starting point for determining the PM schedule that fits your fleet,” Maye advises. “Your actual inspection program should be based on your own fleet performance history and will be different based on equipment specifications and application.”
Maye provides the following suggestions for PM schedules, dependent on the needs of the fleet:
When a complete teardown is required, Kolany suggests the following: “Wheel hubs, wheels, lug nuts, lug studs and center pilots should be cleaned, inspected and wire brushed during removal. Inspect hubs and wheels for any cracking, missing or loose lug studs along with checking for excessive end play in the wheel bearings.”
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. Once the unit is back on the vehicle, “final inspection after installation should be a road test of vehicle and then a recheck of torque,” he adds.
Training is integral to proper installation and maintenance of wheel-end systems. Many manufacturers can provide wheel-end installation training to technicians. Additional training and certification is also available through the Tire Industry Association (TIA).
“The best prevention is proper training for technicians so that they can properly identify the type of wheel-end being worked on, and know how to service each system correctly, including knowing the proper procedure for manual adjustment and knowing what components to use for proper installation of pre adjusted systems,” Timken’s Domin says.
With regular inspection and proper maintenance, many wheel-off situations are preventable. Inspecting and detecting wheel-end conditions is less time-consuming and more efficient in the long run to preventing damage.
For more Truck Bearingsinformation, please contact us. We will provide professional answers.