A number of bucket bushes in a tin dredge move slowly and operate under a high load. If you are required to choose a surface rubbing material: between lead, polytetrafluoroethylene (PTFE), and polyethylene for the pin to be fitted to the steel bucket bush, which of the following materials will give the longest wear life? Given that, the hardness for lead, PTFE, and polyethylene are 30 MPa, 50 MPa, and 70 MPa, respectively. Additionally, the adhesive wear coefficient for the rubbing material: lead, PTFE, and polyethylene on steel are 2*10-5, 2*10-5 and 1*10-7, respectively. Comment on the suitability of your results for practical application.
From the calculations, it is obvious that polyethylene should give the longest life wear compared to PTFE and lead.
This example is included to illustrate the dangers of a theoretical study without knowing the actual operating conditions. The use of polymers has been attempted before by the very enterprising engineers at the then Malayan mining companies in the early 1970s, and all the experiments failed miserably. The problem was that the bearing stresses were calculated by just dividing the maximum tension of the bucket band by the projected area of the bushes. This method did not consider that the pin was relatively long and that the elastic bending caused during operation resulted in much higher localized stresses at the edges of the bush material, which exceeded its flow stress and caused it to squeeze out. It was found through extensive experience that no bush material except for austenitic manganese steel (AMS) could survive the service and this material is still used to this day.