TM 10-3930-653-14&P The sector shaft, supported by the bushing fitted to the side cover and the needle bearing fitted to the gear box, is installed on the steering shaft at a right angle to the worm. At the center of the side cover is the adjusting screw to adjust engagement of the ball nut rack and sector gear. On the other side of the sector shaft is a tapered serration to which the pitman arm is installed with a nut. The  control  valve  consists  of  a  valve  housing,  spool,  spring  and  reaction  piston.  The  grooves  on  the  inner periphery of the valve housing and the outer periphery of the spool serve to control the oil flow. The spool is installed at th e lower part of the steering shaft by a thrust bearing with a nut. (1)  Hydraulic Operation Principle (a)   Steer handwheel at straight position. When the steer handwheel is in the straight position, the control valve is in the neutral position by operation of centering springs (8) and (36) as shown in Fig. 4-22 In this case, the oil discharged from  the  hydraulic  pump  flows  through  the  control  valve  into  the  tank,  resulting  in  no  pressure  to  be  applied  on  the power steering cylinder. (b)  When the steer handwheel is turned left:  When the steer handwheel is turned left, the pitman arm does not move if the steering force is small, due to resistance of the truck body applied on the pitman arm, (ball nut (41) is also locked,) so  that  worm  shaft  (40)  rotates  counterclockwise  moving  in  such  a  direction  that  the  steering  handwheel  sinks  (the centering spring is contracted.) With the movement of the worm shaft, the steering control valve is actuated to allow oil passages B and D to open and A and C to close so the oil flow route is determined. The pressure oil from the pump, thus,  flows  through  oil  passage  B  of  the  control  valve  to  the  power  steering  cylinder.  Meanwhile,  the  oil  discharged from  the  power  cylinder  passes  through  oil  passage  D  of  the  control  valve,  returning  to  the  tank.  When  the  steer handwheel  stops  to  turn,  the  worm  shaft  is  pushed  back  by  the  centering  spring,  placing  the  spool  in  the  neutral position so the control valve is put in "Straight Travel" position as shown in Fig. 4-422 (c)   When  the  steer  handwheel  is  turned  right:    The  steering  operation  principle  is  the  same  as  in  Step  (b):    As  the handwheel is turned right, the worm shaft is pushed upwards to close oil passages B and D and open oil passages A and C, thus allowing the truck to turn to the right. 4-211