Patent GB-1352844-A (GB)
Title: Screw actuator
IPC: B64C13/00
Filed: 1970-03-20
Abstract: 1352844 Screw actuators DOWTY BOULTON PAUL Ltd 19 April 1971 [20 March 1970 11 Dec 1970] 13476/70 and 58884/70 Heading B8J [Also in Division G3] A screw actuator for adjusting the tail plane of an aircraft comprises a rotary screw 1, a ball nut 3 and a pair of hydraulic motors 41, 241 respectively driving the screw 1 through gears 51, 50, 40 and 251, 250, 240, the gears 40, 240 being attached to the screw 1 for rotation therewith. The gears 51, 251 are interconnected by a transfer gear 81 so that torque provided by each motor 41, 241 acts jointly through the gears to drive the screw 1, either motor driving the screw 1 in the case of failure of the other. "No-lock" brakes 57, 257 prevent the load driving the screw 1. A pair of substantially identical servo systems is provided to control the screw 1. The right hand system (the left hand system has similar parts distinguished by the prefix "2") comprises an input regulator lever 11 pivoted centrally to a linkage 12 extending to a lever 24  and at one end 14 to a screw-threaded feedback device 15. The free end of the lever 13 is connected to a remote control source (not shown). The feed-back device 15 is energized by a gear train 16 from the screw 1. The central fulcrum 18 of the lever 24 is pivoted to a plunger 33 loaded by spring 34. A detent valve 35 is provided to respond to movement of the plunger 33 to take safety action. Hydraulic supply and return lines 36, 37 are connected to a valve 31 pivoted at 19 to the lever 24. Joint operation of the valve 31 with the corresponding valve 231 is ensured by inter-connection through a caged spring unit 84. Movement of the levers 11 thus feeds hydraulic pressure to the motors 41, 241 to rotate the screw 1, this rotation passes through the gear trains 16, 216 to the feedback devices 15, 215 to move the ends 14, 214 of the levers 11, 211 to bring the valves 31, 231 to their neutral positions. The nut 3 has a flange 169 with a pair of holes 171, 271 spaced to receive cables 163, 263 extending from a plate 162 at the free end of the screw 1 to a pair of spring boxes 164, 264 from which hollow shafts 165, 265 extend over part of the cables 163, 263. The shafts 165, 265 terminate in pin and slot lost motion connections 166, 266 with the ends of the levers 24, 224 opposite the connections 17, 217. A pin and slot connection 168 inter-connects the shafts 165, 265 through lugs 167, 267. The springs in the boxes 164, 264 hold the shafts 165, 265 fixed except on application of a suitable load either to the cables 163, 263 or to the shafts 165, 265. This load is supplied at the upper end of the run of the nut 3 by its flange 169 contacting overrun ferrules 174, 274 and at the lower end of the run of the nut 3 by the flange 169 contacting the shafts 165, 265 directly, as shown. This causes a reaction on the levers 24, 224 to move the valves 31, 231 to their neutral positions, stopping the motors 41, 241. In the unmounted state of the actuator the nut 3 is prevented from rotation by the cables 163, 263 thus maintaining any preset relation between the position of the nut 3 and the control levers 13, 213.