Unipart Porcelain Insulators - U (Unipart)
System overview
The U-Chart was developed by Jack Tod and published in 1971, Porcelain Insulators Guide Book for Collectors. His 3rd edition book was published in 1988. Mr. Tod died in 1989 and in 1991 his wife granted permission to Elton Gish to assign new U-numbers as new styles were reported. Most of the U-numbers were assigned using drawings from manufacturer's catalogs, so specimens can look slightly different. Dry process porcelain insulators are pressed out in a mold similar to glass insulators, so the profile should be fairly consistent for the same manufacturer. Very early dry process insulators did have the tie-wire groove trimmed in, which will result in profile differences. Since all wet process have the profile manually trimmed in on each insulator, profile differences can be great even for the same manufacturer. Additional changes in profiles and design often can be observed over time for the same manufacturer and some of these are considered different enough to have a different U-number. Actual insulator production can look quite different than the catalog drawings, so do not expect your insulator to match the U-number drawing or photograph exactly.
When selecting a U-number for a specific insulator in hand, start with the profile and try to find the closest U-number. Dimensions are part of the selection, but not as important as finding the right profile. Dimensions can vary as much as +/- 1/4" for small insulators and up to +/- 1/2" for large insulators. The U-Chart is laid out by type of insulator (ex. Pony, exchange, signal, transposition, cable-top, etc.) as well as the number of petticoats and if the inner skirt is extended. Start by looking in the area for the type of insulator you have and narrow down the search to approximate size, then look for the best profile match. If you can't find a profile that matches, but find one that is similar, note the U-number like this: Sim-U-300.
When selecting a U-number for a specific insulator in hand, start with the profile and try to find the closest U-number. Dimensions are part of the selection, but not as important as finding the right profile. Dimensions can vary as much as +/- 1/4" for small insulators and up to +/- 1/2" for large insulators. The U-Chart is laid out by type of insulator (ex. Pony, exchange, signal, transposition, cable-top, etc.) as well as the number of petticoats and if the inner skirt is extended. Start by looking in the area for the type of insulator you have and narrow down the search to approximate size, then look for the best profile match. If you can't find a profile that matches, but find one that is similar, note the U-number like this: Sim-U-300.
