HAZARD IDENTIFICATION
As indicated above, the first step in the FMEA process is to identify hazards potentially associated with the use of the product. Because of various factors including several design problems in the tire and wheel systems in widespread use in this country, the principle risks a tire mounter faces is the violent explosion of tire and wheel assemblies occurring during the inflation process. Warnings on most tire mounting machines demonstrate awareness of this risk on the part of those responsible for placing the machines in the market place. Further, they typically demonstrate an awareness that the machine enhance the risk by facilitating the projection of the exploding tire and wheel assembly upwards and outwards into the person of the operator. The question remains whether the design of the machine reflects a proper consideration of the hazard so as to minimize or if possible eliminate it to the fullest extent practical.
PROJECTION OF THE EXPLODING TIRE:
By means of very simple experiments it can readily be demonstrated that the behavior of a single-piece tire and wheel assembly, exploding as a result of a bead grommet or side wall failure, depends on the disposition of the bead in relation to adjacent fixed surfaces. Detailed observation of exploding tire and wheel assemblies, by means of slow motion videography, demonstrate that as the circumference of the fractured bead structure dilates and overrides the wheel flange it will interact with adjacent surfaces, propelling itself away. The process is analogous to that of a lap swimmer pushing away from the pool wall with his legs.
In 1966, the Coats company, which has historically dominated the US market for mounting machines, conducted tests which documented this effect which they called the "launching pad" effect. Later in 1997 Coats also conducted a series of tests to provide insight into the distance from which a tire/wheel assembly must be situated away from a solid surface in order to minimize the launching pad effect and thereby suppress the violent projection of the tire/wheel assembly.
Understanding the launching pad principle is critical, because it reveals a means to render potential explosion events relatively harmless through the proper design of tire mounting machines. All that is required to prevent violent tire/wheel assembly propulsion is to design the machine so as to deny the exploding assembly any surface to push against, making it relatively easy to restrain any exploding assembly. Responsible manufactures have drawn on this simple physical fact together with other proven design concepts of general applicability to create and sell functional yet safe mounting machines.
Unfortunately, stylish and esthetically sensitive polished aluminum wheels had become increasingly popular. Machines using hold down cones and traditional style combination tools, which had evolved to restrain an exploding tire/wheel assembly, are capable of marring the finish of such wheels. European mounting machine manufacturers developed the so-called rotary design or RC machines to avoid such damage. These rotary type machines can be designed to avoid substantial demounting and mounting tool contact with such cosmetically fragile wheel products because of the way the combination tool position can be indexed positional with respect to the wheel rim flanges.
However, as typically implemented the design provides an effective launching platform while providing no restraint, and serves to place the operator in the likely path of any exploding assembly. Such designs also obscures the operators ability to visualize the lower bead, making explosion events more likely. Thus these machine not only do nothing to reduce the hazard of concern they actually enhance it.
The hazard associated with the RC design, however, is wholly unnecessary. The FMC Model 8700 is a European rotary type machine with the above-described functional advantages. However, in the FMC modified design the flat work top has been reduced in size there by eliminating the launching pad, and rim clamping system has been eliminated and replaced by a traditional style threaded post and cone hold down system. The cone is padded so as to prevent cosmetic damage to alloy wheels.
The author has conducted a video demonstration in which the underbead of light truck tires is caused to fracture resulting in explosion of the tire while the tire/wheel assembly is in place on a platform such as is employed on the FMC 8700 machine. Even with the hold down cone removed the tire/wheel assembly is observed to remain on the tire-changing machine. This and other design demonstrate that the hazards associated with the RC design can be practically eliminated through the adoption of reasonable design changes.
CONCLUSION
RC style rim clamp tire changing machines as typically designed are unreasonably dangerous in relation to the machine's ordinary and anticipated use in tire mounting operations. The hazards associated with this design are unnecessary and the direct result of a failure to employ standard concepts of safe machine design. While the manufactures have identified the hazard, as evidenced by the warnings frequently employed, they have violated the FMEA hierarchy by, in effect, passing over the more desirable hazard remedies and proceeding to directly to the use of warnings As the warnings employed can be shown in several respects to have no practical application for the user, one is left with the impression that they are offered merely as a litigation defense strategy.
James Bruce McMath is a member of the firm McMath Woods PA., in Little Rock, Arkansas.
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