It's amazing how far wheel refinishing has come since it appeared in the market in the mid-1980s.
Today wheel refinishing is a major revenue producer for most retreaders that have installed wheel-refinishing lines in their plants. Usually, standard coated steel wheels require rust removal and refinishing after two to three years of service and may end up being refinished three or four times, and perhaps more, over their lifetime.
In the early days of wheel refinishing, it was common for wheels that had been refinished to show signs of rust again in only a few weeks while still stacked on the pallets they were returned on from a retreader. Since then refinishing systems, coatings and processes have gotten much better, but mistakes are still made that can ruin a paint job or make a wheel unserviceable. Improper wheel reconditioning also can be the reason wheels have to be scrapped.
The Technology & Maintenance Council (TMC) of the American Trucking Associations (ATA) has written several recommended practices (RPs) that deal with wheel inspection, refinishing and out-of-service criteria for wheel corrosion and pitting. This was in response to the increased frequency with which wheels are being reconditioned and concerns regarding their integrity and safety due to their refurbishing.
The information in these RPs is vital to truck operators running steel and aluminum wheels as well as for tire dealerships that refinish steel wheels. So let's take a look at the reconditioning process from stem to stern.
Inspection is a critical component to wheel maintenance as well as an integral step in the refinishing process. All wheels should be thoroughly inspected for any damage or out-of-service conditions before mounting tires on them or reconditioning them.
Fleet maintenance personnel as well as tire dealership technicians must always be vigilant when it comes to determining whether wheelsaluminum or steelhave been damaged due to excessive heat, which can change their metallurgy and cause them to lose strength and change dimension.
Factors that you, the tire dealer/retreader, may not be aware ofsuch as brake malfunctions or abuse, wheel bearing failure, running a tire flat for an extended period of time, tire fires and improper mounting and inflation procedures that use ether, propane or other flammable materialscan shrink wheels so that they no longer have the ability, proper contour and dimension to hold the tire bead on the wheel while under pressure. As a result, tires can blow off wheels with explosive force when inflated and maim and kill people.
That's why it is always imperative that all wheels are inspected for proper rim flange contour and dimension as well as other signs of heat damage prior to refinishing (as well as mounting). Damaged wheels typically appear charred, burned or discolored. Check the valve hole and labels for evidence of charring, melting, blistering, cracking or burning and the rim flange area for a scalloped appearance. If found, scrap the wheel immediately.
Excessive heat usually affects the bead seat and rim flange on the open side of the wheel since this side slips over the brake drum. To determine whether a wheel has undersized bead seat(s), roll the wheel on a smooth flat floor for a minimum of 10 feet. Any deviation from rolling in a straight line is an indication that the dimensions of the wheel have changed, and it should be scrapped.
Sometimes both bead seats may have shrunk and the wheel may not turn when rolled. However, in these cases usually a visual inspection will indicate damage to the rim flanges.
If it is not possible to do a roll test, use a framing square to check the wheel. Place the long leg of the square on the disc face, aligned through the centers of the two opposing bolt holes. Drop the short leg of the square down to contact the rim flanges squarely across the rim. Check for a gap between the square and either rim flange. If the gap is 0.03-inch or more (the thickness of a credit card), then scrap the wheel. Check multiple locations on the wheel. A gap of 0.03-inch or larger anywhere on the wheel is cause for scrapping it.
Another out-of-service criterion for wheels is the inability to find or read the stamping on the wheel. The stamping must be legible before and after the refinishing process since, like the federal Department of Transportation (DOT) code on a tire, it is required by the DOT. If the wheel stamping is not legible, then a significant amount of metal is missing and the wheel should be scrapped.
Some surface pitting on the wheel mounting face is acceptable as long as the mounting surfaces remain flat.
Flatness can be checked with a straight edge and a 0.020-inch feeler gauge. Other than burrs around bolt holes, never grind metal off the mounting surfaces.
Uneven surfaces may result in eyebrow cracks that start at the edge of the wheel nut.
If the wheel is stud-piloted, check the bolt holes with a bolt-hole chamfer wear gauge. If the wheel does not pass the gauge testspace is evident between the nut and gaugeor the bolt holes appear to be distorted or elongated, scrap the wheel. If material is removed from the mounting surface, if there is severe pitting or an illegible wheel stamp, that wheel should also be scrapped.
In the rim area between the bead seats, light surface pitting with minimal metal degradation is acceptable. Wheels with more extensive corrosionespecially in the critical drop center areashould be scrapped.
It's important to check the valve stem hole for burrs and pitting in this stem area that can cause further corrosion and leaks. Use a fine emery cloth to clean up the valve hole seating area if necessary.
Check flanges with a flange wear gauge to ensure that the flanges maintain their integrity. If they don't pass the gauge test, scrap the wheel.
The weld that attaches the rim to the wheel should also be inspected. Wheels should be removed from service if there is more than light surface pitting with minimal metal degradation at the weld. Wheels that have been improperly cleaned in this area may develop premature corrosion after refinishing. If noticeable material has been removed, the wheel should be scrapped.
Once wheels have passed an initial inspection, they should be given a pre-wash to remove existing dirt, grease or debris prior to cleaning and removing the old paint. Skipping this step may negatively impact the durability of the coating that is applied as well as contaminate the blasting media used to remove the existing wheel coating.
The old coating is then removed without damaging the wheel or rim surfaces. If the stamped date code or part number stamping are not easily legible or the surface is severely pitted after the coating is removed, the blasting method or media may be too aggressive or the wheel could be worn out and must be scrapped.
Most retreaders use a blasting cabinet to remove old wheel paint. A variety of media ranging from metal shot to glass beads and other materials can be used.
The TMC advises that using grit alone tends to reduce wheel durability and recommends that smaller, less aggressive steel shot, typically less than S330 be used.
If you have a machine designed to remove only solvent or water-based coatings, it may not effectively remove powder coat finishes without removing an excessive amount of metal, too, thus damaging the wheel. Using a blend of grit and steel beads is best in this case although the grit content should not exceed 25 percent of the media used.
The bottom line, regardless of the media used, is that it is critical that at the end of this step the mounting surfaces and bead seats are clean, bare metal with little removal of the steel.
Burn-off ovens are not recommended to remove old wheel coatings since the high temperatures used in this operation may weaken the wheels.
Once the wheel coatings are removed, technicians should handle bare wheels only if they are wearing clean, lint-free, dry gloves. Bare wheels should never come in contact with the floor or anything that may contaminate their cleaned surfaces and should be coated promptly to prevent flash rusting.
After wheel coatings are removed, a really thorough inspection of the wheels must be made again for out-of-service conditions including legibility of the wheel stampings. With the paint removed, cracks are much easier to see as are the wheel stampings. The TMC's User's Guide to Wheels and Rims is an excellent reference to have at this station. If cracks, excessive metal removal or other out-of-service conditions are found, the wheels should be scrapped.
Not all wheel refinishers perform these next steps, but after the coating removal process, some steel-wheel refinishing systems use solvent/alkaline cleaning solutions or other methods to prepare the bare metal surfaces for coating.
This step adds an additional cost, but it has been found to significantly extend coating life. Coating wheels with a primer or base coat also will improve the finish performance and durability. While they, too, add more cost, these steps have a positive impact on wheel life and are always done by new wheel manufacturers.
The final coat of finish is then applied, but technicians must be extremely careful that the coating thickness is a total of no more than 3.5 mills. As part of the procedure, measure the coating thickness midway between two bolt holes on the mounting surfaces in five locations on both sides of the wheel. It is vital that there are no runs or excessive coating thickness on the bolt circle, wheel/rim mounting or bead seating surfaces because that can compress when the wheel is mounted on a vehicle and result in loose fasteners and wheel loss.
The next-to-last step is curing. All powder-coat wheel-refinishing systems use an oven or other heat source to cure the coating. Wheels that have air-dry coatings applied typically require as much as three days to cure completely while baking will speed up this curing time. Your coating manufacturer will provide the curing specifications for the product you use.
Whatever type of coating you use in your process, it is imperative that the wheels you refinish are cured properly before returning them to your customers. Under-cured coatings will have the same effect as excessive coating thickness since the soft coating will be squeezed from under the wheel nuts when they are tightened, resulting in loose fasteners and wheel loss.
After the wheels are cured, they must be inspected again for any runs or excessive coating thickness and to ensure that the wheel stamps are legible. If the coating has been improperly applied, the wheels must be redone and if the stamps are not legible, the wheels must be scrapped.
As you can see, there is a lot of emphasis on inspecting wheels for out-of-service conditions including loss of material, which is commonly exhibited in pitting and illegible wheel stampings. With steel wheels being refinished multiple times over the course of their lives, it's vital that only minimal material is removed along with the previous coating and that wheel strength and integrity are maintained.
If this process is done right, your wheel refinishing line will continue to provide your company with a great revenue stream, extend the life of your customers' assets and make their operations safe and cost-efficient.