<p> A slipper model of axial piston pumps and motors that includes the vibration of a swashplate and constraint by a retainer is developed. The effects of the vibration and constraint upon the dynamic behavior and tribological characteristics are numerically examined. The theoretical model is established in consideration of the vibration of the counter surface of the slipper and the suppression of the slipper by the additional load. The calculation is performed under unsteady mixed lubrication and the motion, flow rate, friction torque, power loss, and stiffness are shown. The effects of the swashplate vibration, the retainer loads, and the operating conditions on the tribological characteristics of the slipper are discussed. The retainer enables to suppress the jumping motion of the slipper and the appropriate loads contribute to reduction in leakage flow rate and power loss and to improvement of tribological characteristics under a wide range of operating conditions.</p>