They are very common on vertical turbine pumps and also on ship propeller shafts. In addition to metals such as bronze, rubber or wood (lignum vitae on ships) are not uncommon.
You might try contacting major manufacturers of vertical turbine pumps for further information. Many have helpful engineering data and practical experience. Some of the substantial manufacturers include IDP, Fairbanks Morse, Aurora, Byron Jackson, Johnston and Floway. There are many others.
