the position shown in fig8 roller 198 of trigger 102 is in engagement with cam 100 preventing the drive sprocket 38 and shaft 20 from rotating in a clockwise direction to open valve 10 pressurizing gas may then be admitted to cylinder 61 behind piston 84 to urge the piston forward and place piston stem 86 and traveler car 190 in tension when it is desired to operate the valve pressurizing fluid is supplied to control cylinder 110 behind piston 116 to urge the piston 116 forward causing ball 120 to strike trigger 102 and release the cam 100 whereupon the piston 84 and stem 86 in cylinder 81 may move forward responsive to the expansion of compressed gas behind the cylinder this causes piston stem 86 and traveler car 190 to move forward to the left as viewed in fig8 very rapidly thereby rotating chain 240 about sprockets 38 and 246 at the same time causing drive shaft 20 to rotate in a clockwise direction approximately 90 u00b0 to open valve 10 with the valve in full open position the parts are positioned as shown in fig9 the double sprocket and endless chain operator means of fig8 has certain advantages over that of the fig2 embodiment in that the chain coupling 50 with its cantilevered arm 54 of fig2 is eliminated and the two equal size sprockets provide for smoother operation with less whipping of the piston stem 86 by using the traveler car 190 with linear bearing 194 all forces between piston stem 86 and endless chain 240 are kept linear with the.