Test rigs that replicate the conditions for thrust collars (TCs) used in an integrally geared compressor (IGC) are scarce. The test rig described here is based on a typical IGC and is the first rig specifically designed to measure the dynamic reaction force coefficients of the lubricated area of the TC. The test rig uses low-speed and high-speed shafts with independently controlled speed and a pneumatically pressurized thrust disk to apply an axial load ̅ to create the hydrodynamic wedge that balances the imposed axial load. The speed ratio between the low-speed shaft (LSS) and the pinion shaft is 11.67.The geometry of the shafts matches that of a typical IGC. Tests were conducted at pinion speeds of 5, 7.5, and 10 krpm and ̅ = 200, 300, and 400 N. The resulting range of applied pressures is smaller than those arising in practice.The author conducts static tests by applying an incrementally-increasing ̅ on the pinion shaft and measuring the relative displacement between the BG and the TC (Δ ̅ ). One test is conducted at each predetermined spin speed. Run-out on the TC as well as the BW obscures the data. Averaging works well to eliminate the effects of run-out.The author uses the averaged ̅ and Δ ̅ values to create a static, load/ relativedisplacement curve and the slope is the measured static stiffness coefficient ( ̅ ). The axial stiffness coefficient results are compared to predictions from a code based on a 2016 model due to Cable et. al. Their dynamic reaction-force model is = − Δ − Δ̇ iii where is the reaction force of the TC, and is the axial damping coefficient. The trends and the magnitudes of the measured ̅ values and the predicted values from San Andres code for agree very well, especially for the 5 krpm test case. The author then conducts dynamic tests involving an applied impulse load to the TC shaft. One hundred impulses are conducted at each spin speed ( ), ̅ test condition for averaging purposes. A one degree of freedom damped motion model uses Δ ( ) measurements to determine the damped natural frequency ( ) and damping factor ( ) for each test point. The thrust collar mass and the measured were then used to calculate and . The values obtained in this fashion were consistently (and markedly) smaller than the static ̅ values. Based on the results, the author uses the following model = − Δ − Δ̇− Δ̈ that includes the virtual-mass coefficient ( ). The Cable et al. model was based on the Reynolds equation and accordingly did not produce a virtual-mass term.The term is calculated for each test point using ̅ , , and . increases as a function and ̅ . It ranges from 0 to 19.5 kg; the mass of the pinion shaft is 12.8 kg.Both predictions and measurements show an increase in with increasing ̅ . The test rig produced damping coefficients that increased for increasing , while the predicted values decreased. The magnitude of was lower than the predicted damping by a factor of 2 -10.iv ACKNOWLEDGEMENTS