Measurements of the operating characteristics of journal bearings up to high surface speeds have revealed abnormalities beyond a certain critical value. A rapid increase in bearing torque, power loss, and oil-film temperature occurs as speed is increased beyond the critical value, while oil flow decreases below normal. These phenomena are attributed to the onset of instability or turbulence in the bearing oil film with an accompanying increase in energy absorption within the film. The experiments were conducted in a newly designed test apparatus capable of driving large bearings at high speeds. Maximum speed attained was 14,700 rpm, or 30,800 fpm, for an 8-in-diam bearing, and 20,000 rpm, or 21,000 fpm, for a 4-in. bearing.