A company recently contacted Technymon GBT to resolve a frictional issue occurring on a sliding bearing (MU material). Initially, the company identified excessive friction and wear on approximately 10% of the assembled components, which was detected in the field during operation at the OEM end. The company was unable to analyze the root cause instantly. In the meantime, to deliver acceptable components to OEM, the company started to segregate the acceptable and unacceptable assembled components. This approach resulted in delay and stoppage in their assembly line, which further resulted in an increase in their processing and operating costs. The Technymon team was tasked with identifying the real root cause.
The technical team began by analyzing 50 assemblies provided by the company. They disassembled the sliding bearings and viewed them under the microscope. The excessive wear in about 10% of the inspected sliding bearings was seen on the PTFE layer, with this excessive wear reaching up to the intermediate layer of bronze. The team discovered the reason for excessive wear on the PTFE surface was due to extreme contact pressure and flash temperature. The actual contact pressure was exceeding the PTFE layer material bearing limits, leading to a plastic deformation of the bronze surface under the PTFE layer. Scoring and bronze debris could be seen. The metallic bronze debris was causing deep scratches and the PTFE layer was getting destroyed.
The deep scratches and excessive wear indicated that failure was originating during the sliding bearing mounting process because of the excessive outer diameter and roughness of the mounting pin. At this point, the team asked the company to elaborate on their mounting process and to share mounting components designs. After having a detailed discussion and analysis, the technical team was convinced that all operators at the company end were using the acceptable mounting procedures and mounting components.
After investigating all factors, the technical team determined that the problem was related to the assembly tolerance and fits. As such, the technical team measured the sliding bearings available in the company and Technymon GBT stock. Sliding bearings available in stock were meeting the tolerance and specifications completely. Also, 50 pieces of the company’s shafts and housings were taken and analyzed with the statistical variation.
Each parent component was examined and data recorded to include Cp and Cpk results. Every possible assembly scenario was considered to find the root cause. What stood out the most? The company was encountering the problem only in 10% of the assemblies.
After detailed theoretical calculations and practical examination, the problem was identified: the excessive wear was occurring when the shaft’s dimensional tolerance reached the high end of the tolerance limit and the housings, together with sliding bearings, were at the low end of the tolerance limit. In this operating scenario, excessive friction was generated and then converted into the temperature rise of the sliding bearing during actual operation. A thermal expansion on the shaft’s outer diameter was observed, which further converted the clearance fit into transition fit. As a result, the sliding bearing was experiencing the excessive contact pressure and, therefore, the excessive wear.
After identifying the issue, the technical team advised the company that the issue could be resolved by changing the sliding bearing’s internal clearance or shaft outer diameter.
By changing the thickness correlated to the theoretical calculation and practical examination, 50 sliding bearings were produced with new radial clearance. These sliding bearings were identified and numbered.
Before assembly of these dimensionally modified sliding bearings, the company’s components (i.e., housings and shafts) were also measured and numbered. The information was recorded to provide a statistical process control, with proper Cp and Cpk values. Additionally, these assemblies were tracked. Initial operational movement and testing on the customer end indicated the presence of radial clearance fit between the shaft outer diameter and sliding bearing internal diameter.
Over the next several weeks, the company monitored the assemblies’ performance and recorded results. Ultimately, the data proved that the modified assemblies’ higher radial clearance eliminated the excessive friction and wear. With the root cause and solution identified, the sliding bearing thickness was changed. Both parties were then able to update the sliding bearing drawing to procure the dimensionally modified sliding bearings for all future supplies.
Filed Under: Bearings