Industrial Wheel Design Optimization

Despitethe established physical and dynamic properties of specific materials (rubber,nylon and polyurethane), there are a number of design application issues toconsider in generating optimum wheel performance for demanding applications. Manychanges can be made to improve performance and extend the usable life ofindustrial wheels that are experiencing frequent failures (such as dynamicfailures known as blowouts, delaminations or fatigue life). Some of thesechanges include the use of alternative polyurethane chemistries, wheel designchanges or even altering manufacturing parameters.

Thereare many aspects of an industrial application to consider to determine theappropriate wheel design, material and manufacturing parameters to optimizewheel performance. Some of these considerations include:

Thefollowing case study describes a solution provided for a major retailer that improvedthe operating performance of the drive wheels for a large automated materialshandling crane located in the company's largest warehouse/distribution facilityin the U.S. In this case, the existingsolid wheels were constructed from another polyurethane material.

OnsiteEvaluation and Testing
Followingthe initial project review with engineers at the distribution facility, Uremet (asupplier of high-performance polyurethane wheels) conducted an onsiteevaluation of the automated materials handling crane system and its drivewheels. In addition to generalobservations relating to the operating environment and conditions, multipletests were conducted and data gathered during the initial evaluation, includingmeasuring the operating temperature of the existing wheel treads and theambient temperature of the facility. Details regarding the load, acceleration,maximum speed, operating cycle, crane weight and wear requirements were noted.The existing wheels were examined for wear, fatigue and delamination.

Ageneral review of the historical operating performance, life span and observedfailure modes for the existing wheel tread was also discussed with the facility'sengineers and maintenance technicians. The average tread life of the existingwheels was estimated to be less than three months, which caused significantdowntime issues for the system. The existing wheel treads typically failed dueto either a form of delamination known as bondline fatigue or severe fatiguecracking of the tread material. Bondline fatigue is a failure generally causedby overloading, fatigue or overheating of the bondline resulting in areas ofseparation between the hub and the tread. Fatigue cracking is a failuretypically caused by high-stress concentrations during cyclic loading. In bothinstances, the vibration induced by these failures was believed to be causingfatigue to the crane frame structure and premature failure of the drivetraingearboxes. In addition, the existing wheels were only available as new completewheel assemblies sourced from Europe, resulting in long lead times and highreplacement costs.

Analysis/Recommendation
Additionalanalysis was performed including the examination of the customer supplied wheelsamples. The wheel tread material was confirmed to be an NDI/polyester-basedpolyurethane with a hardness of 96 Shore A. Using the data gathered during the onsiteevaluation, the application was analyzed using computer models for targeteddeflections, traction, bondline stresses and load capacity based on historicalexperience and previous testing results from Uremet's application simulationtester. It was determined that the configuration of the existing wheel designwas satisfactory and that an adjustment to the formulation of the polyurethanetread would provide enhanced performance.

Uremet'sU3000 polyurethane, with a hardness of 78 Shore B, was selected as the optimumtread for this application. Compared to the original material, the U3000material has a lower compressive modulus, lower hysteresis, higher tensile andtear strengths and a slightly higher coefficient of friction.

Thelower compressive modulus of the material results in greater deflection underthe operating preload of the system. The increased deflection greatly reducesthe bondline fatigue of the wheel system by distributing the stress from theoperating loads over a larger area of the bondline. However, the key to the success of the U3000material is its outstanding dynamic properties, specifically its hysteretic propertieswhich, as measured by tan delta, are approximately 35 percent lower than theoriginal NDI-based polyurethane. Hysteretic properties are effectively a measureof the efficiency of a material in handling high loads and strains. The lowerthe tan delta of a material, the less heat is generated within the polyurethaneduring use, and therefore, the higher the load bearing capability of thematerial. Due to the increased deflection of the material resulting from itslower compressive modulus, the improved dynamic properties are required toprevent dynamic failures within the wheel tread. Another enhanced physicalproperty of the U3000 material is its higher tear strength (in excess of 625pliin ASTM D-1938 testing), which effectively resists the severe fatigue crackingthat typically plagues solid elastomeric drive wheels.

Results
Samplewheels with the alternative polyurethane material were manufactured using thecustomer supplied wheels stripped of their NDI tread and recoated. With theU3000 polyurethane chemistry, the retreaded wheels were installed on the systemand have generated significant tangible results, such as:

Greg Stevens, P.E., is head of research & developmentat Uremet Corporation.

For more information, visit:www.uremet.com