to both the first pressure passage 53 and the second pressure passage 55 and the solenoid driven valve 51 allows to be supplied to the larger diameter bore 44 of the actuator 40 an equal pneumatic pressure is provided through the pressure passage 55 via junction 59 and applied to the smaller diameter bore d of the actuator body 40 with the valve 51 in the first position the equal pressure at either end results in a force differential generated by the larger surface area of the piston end 46 and therefore the larger force causes the piston to be moved to the right overcoming the force generated at the smaller diameter end 48 it is to be appreciated that where the first end 46 of the piston 47 is twice the area of the second end 48 the force generated by the larger diameter end 46 is twice that of the second smaller diameter end 48 and the piston is moved to the right turning now to fig3 b and again having the pressure supplied at junction 59 the valve 51 is the second position in which exhausts the second end 43 of the actuator 40 through the valve 51 the pressure p supplied to the smaller diameter end d of the bore 44 and the second end of the piston 47 urges the second end 48 of the piston 47 to the left this is possible with the valve 41 in the second position because there is no pressure supplied to the larger diameter end d therefore the piston 47 is returned to the left hand side and rotates the pinion 49 respectively the importance of generating equal and opposite forces to urge the reciprocating piston 47 to one side or the other is of particular importance where.