Aggman is on the money, but here's some more info to help explain the range of values:
Steel - steel
Dry Static 0.8
Dry Sliding 0.4
Lubricated Static 0.16
Lubricated Sliding 0.03
If there's a critical safety aspect to the connection or application, it would do to test the particular material, geometry, and/or lubricants.
Adios
- Backfill