Patent ID: 6935445

Claim:
A back-up steering system for a track laying vehicle comprising: a source of fluid pressure; a mode control valve; a main solenoid valve connected to the source of fluid pressure and the mode control valve; a pair of back-up solenoid valves each connected to the mode control valve and the source of fluid pressure; and, left and right service brakes connected to the mode control valve; wherein during normal operation the mode control valve is set so that the service brakes are in fluid communication with the mode control valve and the main solenoid valve so that when the main solenoid valve is actuated the source of fluid pressure is in fluid communication with both service brakes for simultaneous actuation thereof in response to operator induced braking, and during back-up or emergency operation the mode control valve is set such that the service brakes are in fluid communication with the mode control valve and the back-up solenoid valves so that the left and right service brakes are selectively brought into fluid communication with the source of fluid pressure via selective actuation of the back-up solenoid valves whereby the vehicle can be steered even though a primary steering system is not functioning properly.