Patent GB-673377-A (GB)
Title: Improvements in hydraulic servo motors
IPC: F15B18/00
Filed: 1949-06-10
Abstract: 673,377. Fluid-pressure servomotor-control systems. HOBSON, Ltd., H. M., and MOTT, L. F. June 2, 1950 [June 10, 1949], No. 15501/49. Class 135. A hydraulic power unit comprises a pair of hydraulic servomotors 10, 10a, the movable members of which are connected to an output member 12 through a differential link 19 and the control elements 22, 22a of which are linked to a manually operable input member 23, the movable member of each servomotor having a locking device 52 and stops 44 to limit its movement in each direction, the failure of either servomotor rendering operative the locking device appertaining thereto and rendering ineffective the stops of the other servomotor. The system shown is applicable to the control surface of an aircraft but the Specification refers also to the control of ships and vehicles. The servomotors 10, 10a each comprise a moving cylinder 16 with an extension 17 pivoted to the differential link 19 and a piston 14 with a piston rod 13 connected to a part 9 of the aircraft structure. Each servomotor is controlled by a control valve 22. In some cases the control valves may be replaced by variable stroke hydraulic pumps. Each valve 22 has a housing 21 unitary with the servocylinder 16 and containing passages 42, 43 to the ends of the cylinder and connections to a pressure liquid inlet 11 and exhaust 41. The two pressure sources are separate. Each control valve 22 is connected through a linkage 26, 30 to a differential link 24 normally rigid with the input member 23, but adapted to pivot with respect to the member 23 on release of a collapsible ball catch 25 such as is described in Specification 673,369. The catch 25 collapses when a projection 32 on the link 30 engages either of the stops 33. This may arise due to the servocylinder seizing on its piston or due to the control valve 22 seizing in its housing. In the latter case, a collapsable member 29 such as described in Specification 673,369 is released and itself releases a trip lever 37 normally holding open a spring-closed valve 35 in the supply duct to the control valve 22. When the valve 35 closes, the pressure inlet 11 is connected through a passage 58 to the exhaust 41 as described in Specification 673,370. The stops 44 are formed as collars slidable on the piston rod 13 and connected to fixed points 15, 115 thereon by toggle links 45. The toggle links 45 are normally held straight by spring-loaded pistons 46, 46a, the pistons corresponding to one servomotor being subject through cross connecting passages 51, 51a to the liquid supply pressure of the other servomotor. Thus if the normally open valve say 35 in the liquid supply to one servomotor 10 is closed, the stops 44a of the other servomotor 10a are disabled. Normally, the stops 44, or 44a limit the travel of the cylinders 16, 16a to the central third of the maximum travel so that if one servomotor fails, the extended travel permitted to the other servomotor imparts full movement to the output member. Locking devices 52, 52a of the jamming roller type are provided to lock the servocylinders against movement on their pistons if the pressure supply thereto should fail, or the control valve or the servocylinder should seize. The locking devices comprise spring-loaded balls or rollers 55 coacting with inclined surfaces 57 and normally pushed out of jamming engagement with the piston rod 13 by pistons 54 exposed to pressure from the inlet 11 through a conduit 53.