Off highway tyre and rim assemblies are often subjected to loads and inflation pressures at their maximum capabilities. Construction and mining operations by their very nature can result in variations of the terrain, where higher than normal loading of individual wheel positions occur. Haulage vehicles loaded unevenly, downhill hauls with high speed turns, running a vehicle with one tyre of a dual assembly under inflated, or even flat, and others are examples of conditions that produce periodic high stresses in rim assemblies subsequently reducing fatigue life. Also, the effects of corrosion and fretting between mating parts of a rim assembly can produce surface irregularities that may result in crack initiation sites and ultimate fracture of a rim part.
Inflated tyre/rim assemblies are pressure vessels with high potential energy, it is extremely important to exercise great caution in servicing these assemblies. When this energy is released instantaneously, via a tyre blowout or fracture of a rim section, the results can be fatal. Given the serious nature of such an occurrence, the variabilities
of operating conditions and potential for premature fracture of rim parts, it is important to include a rim inspection program in a planned maintenance and inspection program. The following is part of a procedure to minimize the risks associated with off-highway tyre/rim assemblies and at the same time, produce the least cost of operation
through maximum tyre and rim life. The first step in the inspection process, following tyre removal, is to visually inspect the rim base and rim components (flanges, bead seat band and lock ring). Inspect for cracks, bending or distortion, pitting and excessive wear. The key areas of concern are the “high-stress” areas and areas of weld
(both circumferential and transverse butt welds). If nothing is found visually, the rim and rim components should then be cleaned. This is best accomplished by abrasive blasting per AS1627.4 and then visually re-inspected. This cleaning is necessary in all areas of the wheel assembly. Following cleaning, the parts should be inspected by one or both of the following techniques:
- Magnetic Particle Inspection – per AS1171
- Ultrasonic Testing – per AS2207
Experience has shown magnetic particle inspection to be the preferred method for locating fractures, as most fractures originate at the surfaces. Examination by the magnetic particle methods shall be made in the high stress areas and the welds (circumferential and transverse). Ultrasonic testing is useful in determining fracture depth.
This inspection can identify true fractures, as well as surface imperfections, or “crack-like” indications that, for example, may be simple rolling defects that are in an area of low stress and pose no problem with continued use. It is therefore necessary to properly interpret inspection findings and to follow with a proper disposition. To aid in this analysis, Titan have three defined types of imperfections, or findings, as follows:
Beyond Economical Repair – A tyre of defect that is not repairable and must not be reused.
Repairable – A type of defect that can be repaired and made serviceable.
Cosmetic – A type of imperfection that is acceptable, requires no repair.
In summary, the highlights of a good inspection and maintenance program are:
Frequent visual examination of the tyre/rim assembly on the machine.
Complete examination of all rim components at specific intervals.
Repair of those fractures only where approved, practical and safe.
Repainting after inspection and/or repair to prevent corrosion.