50 years ago ...
From the June 1954 issue of Applied Hydraulics (the original title of H&P).
To make mobile handling equipment maneuverable in narrow aisles, power units must be small. Therefore, size becomes an important factor in the selection of components.
Designers of the hydraulic power unit used on the Lewis-Shepard Master JackStacker solved one of its space problems by selecting rotary solenoids to operate the system's two-way valves.
Each rotary solenoid provides the mechanical power to shift the valve plunger. The rotary solenoid drive has a short linear stroke solenoid with the moving member supported from the stationary member on three ball bearings that travel in inclined ball races, or grooves. When the coil is energized, the moving member, or armature, is drawn into the coil; at the same time, the inclined ball races cause the armature to rotate. To use this rotary motion to push the linear moving valve plunger, a small eccentric cam is placed over the solenoid shaft. The cam acts against the valve plunger. The photograph above shows the valve's compact design.