An experimental study was performed for investigating the threshold of whirl instability for the rigid shaft supported by two externally pressurized air-lubricated journal bearings.
Three types of bearings, that is, circular, 3-lobe and 4-lobe bearings with 40mm diameter and 40mm length were tested with 3 rigid shafts whose masses are 0.307kg, 0.493kg and 1.413kg respectively. All shafts were rotated horizontally and air supply pressure varied from 1.0kgf/㎠ to 5.0kgf/㎠.
Whirl instability was observed when the rotational speed of the shaft exceeded speeds corresponding to approximately 2.3 to 3.5 times the lowest critical speed of the shaft-bearing system. Results indicated that whirl instability increases with increasing air supply pressure and decreasing shaft mass.
Also the whirl ratio at the threshold of whirl instability increased with increasing shaft mass but was relatively insensitive to air supply pressure. Finally, it was concluded that the 3-lobe bearing and the 4-lobe bearing are more stable than the circular bearing as the air supply pressure increases and shaft mass decreases.
