Abstract: A hydro mechanical condition-responsive servo control valve for controlling a variable condition, such as position or rate of change of position, of a remote element, comprises a rotary fluid valve controlled by a differential gear train arranged to register differential movement between two input shafts, one of which is connected for movement according to a desired input condition and the other of which is alternatively connected either to register the actual element condition or a reference [datum] condition; the rotary valve including mating stationary and rotary valve plates each incorporating fluid flow ports cooperatively arranged on the mating plate surfaces thereby to control fluid flow from a fluid inlet to a fluid outlet in accordance with the relative position or movement of said two input shafts.

Abstract: An undercutting apparatus and a method for removing material from beneath railroad track. The apparatus comprises a rotary trench digging means and horizontal chain means to transport the material from beneath the railroad track to the trench digging means at an underground location. In operation the trench digging means digs a trench alongside of the track while at the same time transporting the material delivered by the horizontal chain means to a suitable location above the ground. Preferably the rotary trench digging means comprises a large hollow wheel and one end of the chain means is disposed inside of and near the bottom of the wheel during normal operation of the chain means. Means are provided to pivot the chain means about a vertical axis whereby the chain means can be pivoted from a first position wherein the chain means extends parallel to the track to a second position wherein the chain means extends perpendicular to the track.