Patent GB-577912-A (GB)
Title: Improvements in servo motors
IPC: F15B11/048
Filed: 1944-01-01
Abstract: 577,912. Fluid-pressure servomotorcontrol systems. HOBSON (AIRCRAFT & MOTOR) COMPONENTS, Ltd., H. M., and MADDOX, S. V. Jan. 1, 1944, No. 12. [Class 135] In an hydraulic servomotor in which the power device (i.e. piston or rotary vane) is moved to one or other of two end positions under the control of an operating valve actuated by a pressure sensitive device, means under the control of, and operated by, the power device is provided to allow unrestricted pressure fluid to act on the latter after the normal (slow) initial movement in either direction to enable the power device to travel at high speed for a portion of the remainder of its stroke. As applied to a vane-type servomotor 21, Fig. 1, for changing the gear ratio of a blower on an aircraft engine, the operation of the servomotor is controlled by atmospheric pressure acting on a capsule stack 10 whose longitudinal movements are communicated to a piston valve 12. The latter may have one or two lands, and the lands are wider than the corresponding ports to allow a suitable lag in the operation of the vane and so prevent hunting at the gearchange altitude. The valve controls the flow of pressure fluid to one side or the other of the vane 21 and, at the same time, connects the other side to exhaust. The spindle 22 of the vane is provided with two longitudinal grooves 36, 37 and is surrounded by a rotatable sleeve 24-shown in developed plan in Fig. 2- through which the vane projects. When the valve 12 is actuated, pressure fluid from a passage 15 flows through the valve and out through port 18 or 19 to one side of the vane, the other side being connected to exhaust through the valve. After the vane has made an initial movement, -its spindle will have turned to allow unrestricted pressure fluid from the passage 15 to flow to the side of the vane then under pressure through a groove 47 in the vane casing, a port 38 or 39 in the sleeve, one of the grooves 36, 37, and the slot 25 in sleeve in which the vane works. The other side of the vane will be placed by similar means in unrestricted communication with exhaust. When the vane is nearing the end of its stroke, it will contact one of the edges 45 of the sleeve and carry the latter along with it. This movement will cut off the unrestricted supply of pressure fluid so that the vane will finish its stroke slowly under the pressure fluid passing through the valve 12. A spring-pressed plunger engages in one of two recesses in the sleeve to position the latter in the appropriate end position. The datum of the capsule stack 10 is altered by movement of the pilot's control lever connected to a lever 30 which is also connected to means to select the boost pressure as described in Specification 577,910. Movement of the lever 30 rotates a shaft carrying a cam 32 which acts through a follower 33 on a shaft carrying a stirrup 35 pressing on the end of the capsule stack. A lever 95 connected to the shaft 34 provides an overriding manual control for altering the blower gear ratio whilst the plane is on the ground. During flight, the gear can be changed by a further manual control lever acting on the valve 12.