Abstract:
A linear fluid actuator having two pistons and two cylinder chambers in tandem to provide redundancy when supplied from independent pressure sources. The actuator is balanced in that the actuator output force is equal in extend or retract positions. The two cylinder chambers are made from separate housings to prevent crack propagation between the chambers and dual piston rod construction is provided to enhance redundancy.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The Government has rights in this invention pursuant to Contract No. NAS-9-15550 awarded by the National Aeronautics and Space Administration. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to linear fluid actuators and more particularly to actuators employing multiple pistons on a single piston rod. 
     Many products, including aircraft and spacecraft, require actuators to move some device. In the case of aircraft and spacecraft, the device is often a control surface. One type of actuator is a linear actuator which is so named because it produces linear motion. Representative of linear actuators is the hydraulic or fluid cylinder. Fluid cylinders may be single acting in that fluid pressure is applied to only one side of the piston or double acting where pressure is applied to both sides of the piston. Typically, the double acting cylinder is unbalanced in that equal pressure applied to either side of the piston produces unequal forces because of the area of the piston rod which extends from one side of the piston through the cylinder housing. Conventionally, a double acting cylinder is balanced by providing an external tail rod which is nothing more than a piston rod on both sides of the piston protruding from both ends of the cylinder housing. Usually the external tail rod is covered by extending the cylinder housing and the cylinder attach point is provided at the distal end. This makes the cylinder substantially twice as long, as the total stroke is accommodated at both ends of the cylinder. The conventional method of coping with this problem is to provide an internal tail stock which is nothing more than another rod attached to the cylinder end plate, internal to the cylinder, with a bore through the piston and external rod which slips over the internal tail stock. 
     However, since the two rods are concentrically engaging, one must necessarily be smaller than the other and the effective piston areas are unbalanced by this differential area. 
     Frequently in air and space craft it is required that the actuator have a dual or second redundant means of actuation. This is generally provided by a single cylinder with a divider at the midpoint to form two compartments with a piston in each compartment and one common rod through both pistons. To balance this actuator an external tail rod is provided, as discussed above. Again, this makes the actuator very long. 
     A need arose in a critical space craft application to provide a redundant (dual) balanced double acting linear actuator having an overall length no longer than a single balanced double acting linear actuator and capable of installation within the same space envelope, and without change to the mounting structure. The object of this invention is to satisfy these requirements. 
     Failures in hydraulic cylinders which are highly stressed, other than packing failures, often occur as fatigue cracks in the cylinder housing. It is a further object of this invention to provide a cylinder housing in which a crack cannot propagate from the first cylinder compartment to the second cylinder compartment. 
     It is a further objective of the present invention to meet the above objectives within the structure of the actuator itself as opposed to complex and less reliable control means or combinations of electrical and hydraulic actuators as have been taught by others. 
     SUMMARY OF THE PRESENT INVENTION 
     In summary, the hydraulic actuator of this invention accomplishes the above objects and overcomes the disadvantages of alternate combined devices by providing a cylindrical housing having multiple chambers to accommodate multiple pistons and having a piston rod connecting the pistons for axial movement in their respective compartments between extend and retract positions. An internal tail rod is provided having an area exactly equal to the external or projecting portion of the piston rod to provide a balanced actuator. The cylinder housing is split to provide separate compartments or cylinder barrels to prevent crack propagation. Dual piston rod construction is provided. To provide further redundancy, two separate sources of hydraulic pressure may be applied to each of the dual chambers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With reference to the drawings, wherein like reference numerals designate like portions of the inventions: 
     The FIGURE is a sectional view, cut along the longitudinal axis, of the balanced tandem actuator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is shown in sectional view in the FIGURE. For purposes of simplicity and orientation, the end of the actuator through which the external piston rod protrudes is identified as the rod end. The opposing, closed end of the actuator, is identified as the head end. 
     The actuator is a fluid cylinder comprising a head housing 11 and a rod housing 13 having a through bore 15. The through bore 15 is divided by a pair of identical glands 17 which are duplicated simply for purposes of redundancy. The two housings 11 and 13 are joined in tandem, with the glands 17 between, by a series of suitable fasteners shown at 19 to divide the cylinder 9 into a pair of chambers. 
     The head end wall 21 is provided with an external attachment point 23 which is shown as a spherical self-aligning bearing. However, many alternative attachments are well known in the art. The opposing surface of the head end wall 21 is provided with an internal tail rod having a projected area A at the diameter so indicated. 
     A head piston 27 is located in the head chamber and has suitable dynamic sealing means 26 and 28 respectively, at the surface 29 and 31 to provide a sliding seal between the piston 27 and the cylinder bore 15 and the tail rod 25 and the piston bore 33. 
     The piston 27 terminates in a piston rod 35. Rod 35 has a first diameter 37 which extends to a point intermediate the length of the rod 35 sized to allow full stroke within the head chamber. At this intermediate point, the first diameter 37 steps to a reduced diameter 39 shown followed by a relief 40 and terminating in a thread at 41. Dynamic seals at 38 seal the rod diameter 37. 
     Circumscribing the reduced diameter 39 is the piston 43 attached to the head piston rod 35 via the piston rod 44 at the threaded connection 41. Viewing the assembly of pistons and rods, the piston rod 44 provides a second diameter indicated as A&#39; and having a projected area A&#39; equal to the projected area A of the internal tail rod 25 as previously discussed. 
     Housing 13 terminates in a rod end wall 45 which contains an insert 47 housing the dynamic seal at 49 to enclose the rod end chamber. Dynamic seal means are also located at 51 to seal between the piston 43 and the bore 15 of the housing 13. A spherical rod end 53 is shown threadably connected at 55 as the rod end mounting means. Static seals are shown at 54, 56, and 58. 
     The cylinder porting means have been identified as extend ports 16 and retract ports 18. Porting is provided to admit hydraulic fluid in both chambers on either side of the pistons 27 and 43 so that the two sides cooperate in moving the piston between its extend and retract positions while the fluid on the opposing surfaces of the pistons is ported to return. It should now be obvious that the sum of the area of the piston surface 60 of the head piston 27 plus the area of the surface 57 of the piston 43 is equal to the area of the surface 59 of the piston 43 plus the surface 61 of the head piston 27. Therefore the actuator is balanced generating equal force in both directions, retract and extend. 
     To take advantage of the redundancy available, one pressure source is ported to either surface 60 or 61 while an alternate source is ported to surface 57 or 59. Pressure at surfaces 59 and 61 retract the cylinder and, alternately, pressure at surfaces 60 and 57 extend the cylinder, with the non-pressurized surfaces ported to return. 
     While the actuator is balanced with both pressure sources functioning and both chambers operative if either one chamber is lost or one pressure source is lost the actuator is slightly unbalanced because of the differential area on the two opposing surfaces of a single piston, e.g. 57 and 59. In this mode the actuator performs as a single piston, double acting, internal tail rod cylinder which was discussed earlier as a near balanced actuator. In a six-inch diameter cylinder the near balance exceeds 90%. 
     It may thus be seen that the piston and chamber arrangement depicted in the preferred embodiment of this invention provides a balanced, redundant, tandem, linear actuator which may be installed in the same spaced envelope and end fittings that accommodate a conventional external rail rod, balanced, single linear hydraulic actuator.