Patent attributes
A hydraulic motor is driven by a fixed displacement, three stage gear-type pump. The three equal displacement sections provide 3 equally-stepped flows so that the hydraulic motor can have three speeds. Valving and controls are provided for remote on-the-fly shifting of the speeds, automatic pump unloading in neutral, and adjustable maximum pressure. A closed loop system permits use of a relatively small reservoir. The contamination tolerance of the system is greater than that of variable displacement piston-type pumps typically used. Direct recirculation of hydraulic fluid is provided for unused speeds to permit hydraulic fluid to recirculate in the pump without doing work. A unique control circuit is provided to control the first speed valve in conjunction with a logic cartridge which controls hydraulic motor direction to permit fluid flow to the hydraulic motor to be stopped when coming to neutral without slamming the hydraulic motor and rotating drill pipe to a stop. The first speed valve also acts as a relief valve to provide a mechanism for adjusting maximum system pressure.
