This paper describes a device for the control of critical speeds consisting basically of a squeeze film oil damper between two non-rotating parts in parallel with a flexible bearing support. A mathematical model of the vibratory system is developed showing the existence of two different critical speeds as a function of damping. From the model, response of the system is predicted. A critical speed test rig was designed and fabricated and a test program was conducted. Results of the experimental investigation confirm the applicability of the model and the use of the device as a critical speed control. Further testing is reported on the effect of various design parameters on damping, and a simple method of varying the damping on the test stand during engine operation is shown. The results of the investigation were applied to the design of a similar device for a multistage compressor which was successfully operated with low amplitude throughout the speed range. Disassembly following 150 hours of compressor testing showed all parts to be in excellent condition.