A single axis vibration test system includes a vibration force generator or "shaker" which can be used with a head expander, and separate pairs of hydrostatic self-aligning sliding bearings connecting two large reaction masses to the head expander. Preload forces are applied by separate preload pistons supported on a preload ring surrounding the shaker. Preload forces are transmitted by the preload ring to the sliding bearings to clamp the head expander directly to the reaction masses, which resists lateral and rotational motions during vertical axis testing. The reaction masses are supported on resilient supports carried on a sub-structure rigidly affixed to ground, for avoiding a rigid connection between the preload ring and ground which in turn avoids unwanted mechanical resonances within the operational frequency range of the system during use.