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Trolley Shoe Carbon Inserts: Testing for Success

The method used by the San Francisco Municipal Railway (Muni) to evaluate the wear of trolley-shoe carbon inserts is presented. The trolley shoe is that part of the bus that is directly in contact with the overhead lines. To extend the life of the trolley shoes and allow for their reuse, the cheaper carbon inserts are used as sacrificial material to take care of the wear that occurs between the overhead line and the trolley shoe. For years, Muni operated its fleet of trolley buses using carbon inserts that were purchased solely based on the price of the inserts. Although it would appear that the cheapest carbon inserts provided the best deal for the company, in reality the company was spending a lot more in terms of (a) lost revenue due to equipment failure, (b) damage to equipment, and (c) unscheduled maintenance cost. Muni’s materials management system database had a list of six different grades of carbon inserts. These grades of carbon inserts were included in the database as per the recommendation of the vendors. However, these grades of carbon inserts were never evaluated for compatibility with the bus or overhead system. Consequently, some would wear after one day of service although others would last 5 days or more. This test was conceived to identify the most suitable grade of carbon insert for Muni’s trolley coaches. For the test, weight loss was used to measure carbon wear instead of the change of height or thickness of the insert. To limit the variables affecting the inserts during the test, different grades of inserts were paired during service. Based on the test results, the number of approved carbon grades for use in Muni’s trolley fleet was reduced from six to two and 972.6 km (600 mi) was established as an acceptable service life for the inserts.