Diagnostic Instruments SMS20 Ball Bearing Stand. A boom stand designed for support of large research grade stereo microscopes, while providing them with fluid motion in a horizontal plane. The horizontal arm is mounted on linear ball bearing units in the solid aluminum cross slider. Locking the arm in position is quickly accomplished with secure soft-grip knobs. Features a large 1.72" diameter, 24â€ tall stainless steel vertical pole and 27.75" long horizontal arm w/ 22" of travel to support.