Abstract:
A fluid actuator where an outside cylinder envelopes an inside cylinder producing four chambers, two for the power stroke and two for synchronization, for application where two or more cylinders are to drive a single load such that all cylinders advance at the same rate by cross plumbing synchronizing chambers. The combination is provided with valve means which opens at the end of each stroke to realign the actuators.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to the problem of advancing two or more hydraulic cylinders to move a single load, such that all cylinders advance at the same rate. 
     Synchronized movement of pairs of hydraulic cylinders is critical in certain aircraft applications. For example, aircraft flap systems are generally segmented and each segment is driven by two or more hydraulic actuators, and if the actuators are not synchronized in advance and retract the surfaces are subjected to undue stress. In some cases non-synchronization may cause the system to bind. Also the segments of the flap system must advance at the same rate. The same is true of large cargo doors. Thrust reversers in jet aircraft which retard the forward momentum of the aircraft after landing are generally operated by multiple hydraulic actuators. Thrust reverser movement must be synchronized although the loads in the individual actuators may vary. The problem, of course, is not limited to aircraft. 
     Synchronized movement of multiple hydraulic cylinders may be achieved by metered control of fluid flow, e.g. flow dividers and split delivery pumps. These methods must be made very complex in order to meet synchronization accuracies required. A more novel approach which inherently lends itself to the accuracies required was taught in U.S. Pat. No. 3,855,794 to Meyer et al and in an article which appeared in a periodical entitled HYDRAULICS AND PNEUMATICS published by Penton/IPC, Inc., Cleveland, Ohio in the February 1974 issue at page 39. Both of these disclosures teach using the rearward piston and that portion of the cylinder which, of course, produces two cavities which are used for the power stroke to extend and retract the piston while the front two chambers are used to synchronize the two cylinders. The two cavities of the respective synchronization cylinders are cross plumbed i.e. the front cavity of the first cylinder is plumbed to the rear cavity of the second cylinder and the second cavity of the first cylinder is plumbed to the first cavity of the second cylinder. The two cylinders must advance and retract together because of their interconnection. Of course, both cylinders must have the same bore and rod size so as to displace equal volumes of fluid for equal stroke. The problem with the apparatus of this teaching is that it requires a very long actuator and in many aircraft applications this length is simply not available. 
     Another article appeared in the same periodical, HYDRAULICS AND PNEUMATICS, in the July 1979 issue, at page 14, which disclosed a novel method of reducing the length of the actuator. However, one cylinder extends while the other cylinder retracts, both at the same speed, of course, and then teaches connecting the rod ends to different mechanical linkages with reversed motion. This approach may also create a space problem at the actuator end. 
     Another approach for synchronizing cylinders was taught in U.S. Pat. No. 4,241,581 to Chase which discloses a synchronizer which consists of a two piston pump. While this teaching does permit the two-piston pump to be located remotely from the actuator it requires a much more complex circuit, it does not inherently lend itself to dimensional control, and does not provide for any means to realign the actuator in case of internal leakage. 
     In summary, the teachings of the four references discussed in detail have their advantages and disadvantages. However, none of the art alone or in combination teaches how to solve the undue length of the actuator as taught in the first two references while retaining the inherent ease of maintaining the critical dimensions. None of the art teaches adequate means for adjusting the synchronization of the two cylinders because of leakage. 
     SUMMARY OF THE INVENTION 
     The present invention is two enveloping fluid cylinders enclosed in a single actuator which produces four fluid chambers. Two or more actuators are combined to drive a single load such that all cylinders advance at the same rate even though the individual cylinder loads may vary. Two of the four chambers provide the power stroke to extend and retract the cylinder piston rod. The two remaining chambers are cross-plumbed to corresponding chambers on the second actuator so that fluid actuation to the extend or retract chambers actuates the respective synchronizing chambers. Fluid is expelled from one synchronizing chamber to another synchronizing chamber and since synchronizing chambers are exactly equal in area and stroke one actuator cannot advance without the other actuator advancing as there would be no receptacle to receive the expelled fluid. 
     The system is further refined by further plumbing the cross plumbed synchronizing chamber through a check and relief valve means to system return so that any over pressure in the synchronizing system may be vented to return and any leakage may be replenished if the pressure in the synchronizing chamber should fall below the system return pressure. 
     A further valve means is provided which is actuated by the moving piston by opening the valve means to permit fluid flow between either the extend or retract chamber and one of the synchronizing chambers which realigns the two actuators at the end of each full stoke. The alignment may occur at either end of the stroke, or at both ends. 
     An object of the invention is to provide two or more fluid actuators to drive a single load and maintain synchronization even though the loads may vary. It is a further object of the invention to provide an actuator for the noted purpose with an end-to-end length approaching one half of that of a tandem hydraulic cylinder. 
     Another objective of the invention is to provide valve means to realign the two actuators at one end of each stroke and to provide means to accommodate thermal expansion of the fluid in the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     With reference to the drawings, like reference numerals designate like portions of the inventions: 
     FIG. 1 is a sectional view through both actuators, shown schematically, which shows the connecting hydraulic plumbing; and 
     FIG. 2 is a detailed view of an actual embodiment of the acutator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring in detail to the drawing of FIG. 1, a pair of actuators 10 and 11 are shown which are essentially identical. One actuator consists, essentially, of an outside cylinder, 9, which envelopes an inside cylinder 8. The actuator consists of an outside cylinder barrel 12 having an internal tail rod 14 which is integrally connected to the tail cap end 15 of the outside cylinder barrel 12. Moving piston 16 rides in the inside diameter of the outside cylinder barrel 12 and circumscribes the internal tail rod 14 to form an extend chamber C 1 . Attached to the moving piston 16 is the inside cylinder barrel 18 which also functions as the piston rod for piston 16 and circumscribes the fixed piston 20. Fixed piston 20 in turn attaches to the end of the internal tail rod 14 to also function as a fixed piston rod for the fixed piston 20 and form a chamber, C 2  formed between the inside diameter of the inside cylinder barrel 18 and the outside surface of the internal tail rod 14 and the two opposing faces of the moving and fixed pistons 16 and 20. Another chamber is formed between the fixed piston 20 and the inside cylinder barrel closed end 21 which is identified as S 2 . The head end cap 22 closes the bore in the outside cylinder barrel 12 and circumscribes, the inside cylinder barrel 18, which functions as the, piston rod for piston 16 to form the fourth chamber identified as S 1 . The chambers C 1  and C 2  are the power stroke chambers to extend and retract the piston rod while the chambers S 1  and S 2  form the synchronization chambers which are equal in piston area and stroke. 
     Now, the pressurized fluid power supply is connected to first ports 24 via conduit 42 for external or alternately to second ports 25 via conduit 43 for retract through appropriate valving (not shown) to supply the power stroke for both cylinders. One set of the ports, of course, is alternately connected to return. Correspondingly, the third port 26 of the actuator 10 is connected to the third port 28 of the actuator 11 by conduit 44, while the fourth port 26 of the actuator 11 is connected to the third port 28 of actuator 10 by its conduit 44. In other words, the synchronization chambers are cross-plumbed in the paired actuators 10 and 11, i.e. synchronization chamber S 1  for acutator 10 is connected to synchronization chamber S 2  of actuator 11, while synchronization chamber S 1  of actuator 11 is connected to synchronization chamber S 2  of actuator 10. In order for the piston 16 to extend the barrel 18, the fluid in the chamber S 1  must expel into the synchronization chamber S 2  of the opposing actuator and in order for the piston 16 to retract, the synchronization chamber S 2  must expel into the synchronization chamber S 1  of the opposing actuator thereby synchronizing the motion of the two cylinders. 
     Bleed and fill valves 30 are provided for each synchronization cylinder as it is essential that all air be bled from the system as the systems are dead ended. 
     A synchronization valve 31 is provided inside the cylinder shown in the moving piston 16 comprised of a poppet 32, (See FIG. 2) a poppet seat 33, a biasing spring 34, a ball 35 and a ball seat 36. The synchronization vlave 31 is provided to realign the acutators after each actuation. As the piston 16 retracts and bottoms against the surface 38, the poppet 32 unseats by compressing the spring 34 and if the pressure in the S 1  chamber is higher than the C 1  chamber, fluid will flow from chamber S 1  to C 1  as the ball 35 will unseat because of differential pressure. An out of sync condition could occur if there were leakage past the seals. While the synchronization valve 31 is shown in the piston 16 so as to synchronize on the return stroke, it could be located in the piston 20 so as to synchronize on the extend stroke. 
     Further, the synchronization chambers S 1  and S 2  of actuators 10 and 11, respectively, have been plumbed to the return line 45 through a check and relief system 40 to assure that the synchronization system is always full of fluid and provide a thermal relief path to accommodate fluid expansion due to heat. The check valve function is conventional and if any synchronization chamber exceeds a predetermined pressure it will be relieved through the system 40. Additionally, should either of the synchronization chambers, for some reason, lose fluid and the chamber pressure drop below the return pressure, fluid will flow from the return line 45 to the synchronization chamber through the check portion of the check and relief system 40. In aircraft systems the return line is maintained at a positive pressure, usually in the range of 50 or 60 psi. 
     An embodiment of the actuator shown as 10 and 11 in FIG. 1 is shown in FIG. 2. Secured in the outside cylinder barrel 12 by the shoulder nut 13 is the internal tail rod 14 which is integral with the end cap 15. Moving piston, and the only moving piston, is shown as a gland 17 which combines with the hollow inside cylinder barrel 18 whose end portion terminates in a skirt to form the piston 16 which slideably engages the inside diameter of the outside cylinder barrel 12. Attached to the end of the internal tail rod 14 is the fixed piston 20 which is retained by the nut 19. Piston 20, while stationary, slideably engages the inside diameter of the outside cylinder piston rod or inside cylinder barrel 18 which terminates in the inside cylinder barrel closed rod end 21 which completes the inside cylinder. The head end cap 22 closes the open end of the outside cylinder barrel 12 to form a second fluid cylinder which envelopes the first fluid cylinder. The inside fluid cylinder has the cavities C 2  and S 2  while the outside fluid cylinder has the cavities C 1  and S 1 . 
     The cap end 15 terminates in a suitable actuator support means shown as a bore 23 and accommodates the extend and retract ports 24 and 25 and the synchronization or second extend port 28 along with the bleed valve 30 to bleed the chamber S 2 . The outside cylinder barrel 12 has an alternate synchronization or second retract port at 26 and a bleed valve at 30 for bleeding the chamber S 1 . These ports are shown as bosses on the cylinder 12. Piston rod 18 also terminates in a suitable attachment at the rod end shown as a bore 27. 
     Enclosed in the gland 17 portion of the first piston 16 is a synchronization valve 31. The valve consists of a poppet 32 which projects above the end surface 29 of the first piston 16, is biased against the seat 33 which threads into the gland 17 by the spring 34 which also biases the ball 35 against its seat 36. Again, the function of the synchronization valve 31 is to open when the first piston 16 reaches the end of its stroke against the surface 38 by unseating the poppet 32. At this point the chamber pressure C 1  is return system pressure and if the chamber S 1  exceeds return pressure fluid may flow from S 1  to C 1  to return and thereby synchronize the actuators when used as shown in FIG. 1. 
     It is not intended to limit this invention to the embodiment disclosed above, but all changes and modifications thereof not constituting deviation from the spirit and scope of the invention are intended to be included.