Be Safe: Know Your Wheels


There are several modes of failure for polyurethane wheels used in demanding industrial applications. The following describes the common modes of failure and some of Uremet’s recommended inspection tips:


Blowout (Load at Failure) – A blowout of a solid polyurethane wheel occurs when the material’s critical temperature is exceeded causing it to melt. The heat generated during use in high speed and / or high load situations can become a significant issue because polyurethane has low thermal conductivity, resulting in the retention of heat in the material. In cases where the critical temperature of the elastomer is exceeded, the internal damage to the polyurethane decreases its physical properties and causes it to liquefy, resulting in a blowout.


Hysteresis is a dynamic property that measures the efficiency of a material. One of the virtues of polyurethane is that materials are available over a range of hysteretic properties. In many cases blowout issues can be corrected by switching to a lower hysteretic compound of polyurethane.


Fatigue Cracking – Fatigue cracking, a normal wear pattern that usually occurs over a long period of time, is typically caused by high stress concentrations during use. Fatigue cracks typically do not pose a threat to the wheel until they begin to connect with each other, spread open, or reach a depth close to the hub. Since the rate of propagation of fatigue cracks differs between materials, applications and even wheel position, it is important to develop an inspection criterion that takes into account the unique properties of the application. In many cases where fatigue cracks are significantly limiting wheel life, alterations can be made to the manufacturing parameters to slightly increase the tear strength of the material and effectively extend the fatigue life of the wheel. These alterations, however, can result in other problems, such as blowout, and therefore, would not be recommended for a wheel that is experiencing occasional blowout issues.


Delamination – Fatigued bond, a normal mode of failure that can lead to delamination, generally occurs overtime as the bond is weakened through overloading or overheating. A fatigued bond can be a result of many different factors, however, no matter the cause, is easily discovered through proper inspection. It is usually detected by a white powdery residue on the sidewall near the bondline, a high or hollow spot on the tread or the separation of the tread from the hub at the edge of the wheel.



Inspection Tips


Bondline Inspection – Bondlines should be visibly checked during each inspection. If a white powder and / or tread separation is apparent, gently insert a machinist scale or a blunt thin object between the hub and the tread to check the separation depth and length. Do not force any sharp object into the polyurethane or bondline as this may create an inclusion or additional stress where material and / or bondline fatigue may begin.


Color Inspection – Wheels should be visually checked for changes in the tread color during each inspection. Color changes can be caused by lubricants and track paints, but also can be indications of an overheated or blown out wheel.


Nicks and Cuts – The presence of nicks and cuts are usually caused by contact with the track or train structure. Unusual wear patterns could indicate a problem with track joints, supports or other surfaces.


Wheel Tracking – Maintaining an effective wheel tracking system will not only determine actual wheel usage and average wheel life for an application, but also will provide a helpful tool for wheel improvements in the future, such as potential changes in materials.