Abstract:
A fail-safe fluid actuator comprising a piston slidably disposed within a cylinder. In the event of a pressure loss, the piston is urged to a fail-safe position by a plurality of externally mounted constant force spring mechanisms. In one embodiment the mechanisms are located at one end of the cylinder with a telescoping cover for protecting them from exposure and damage. In another embodiment the mechanisms are located at the side of the cylinder whereby the overall length of the actuator is significantly shortened. The fail-safe force applied to the piston by the spring mechanisms is externally adjustable, and they are readily accessible fur replacement or repair.

Description:
RELATED U.S. APPLICATIONS 
     Applicant claims priority of Provisional Application Ser. No. 60/187,435 filed Mar. 7, 2000 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to fluid actuators, and more particularly the invention pertains to a piston type fluid actuator having an externally mounted constant force spring return mechanism for insuring fail-safe positioning in case in fluid pressure. 
     BACKGROUND OF THE INVENTION 
     Hydraulic or pneumatic actuators are employed to control operation of numerous types of devices such as valves, jacks, elevators, and heavy machinery. In simplest form, it comprises a cylinder with an open bore in which a piston moves axially when fluid pressure is applied to at least one end of the piston. In case of a failure in fluid pressure, mechanisms are provided which will cause the piston to move the device to a predetermined fail-safe position. 
     Such mechanisms typically include one or more mechanical coil or constant force springs acting against the piston to move it in the proper direction. A drawback of coil springs is that, due to its spring rate, the applied force against the piston diminishes as it reaches the critical fail-safe position, whereas a constant force spring is coiled in its relaxed state and as it is extended substantially the same force is applied throughout the travel of the piston. For instance U.S. Pat. No. 3,767,160 to Robert F. McCollum discloses a piston-operated regulator valve with constant force springs which urge the valve to a normally closed or open position when there is no fluid pressure. In one design the springs and piston are mounted to the valve body on opposite sides. The disadvantage of this design is it requires separate mounting flanges and seals on both sides of the valve body. In another design the springs are installed inside the piston cylinder. This requires the piston cylinder to be disassembled in order to inspect, adjust or replace the springs. In addition, the number of springs that can be installed in the cylinder is limited by size. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a fail-safe actuator with fail-safe spring return which can be easily mounted on a controlled device and adjusted for spring force. 
     Another object of the invention is to provide a piston-type fluid actuator having an external constant force spring return mechanism which is readily accessible for inspection, adjustment and replacement of the mechanism without disassembling the actuator or removing it from service. 
     Still another object of the invention is to provide a piston type fluid actuator in which the number of constant force springs required for fail-safe operation is not limited by the size of the piston cylinder. 
     A further object of the invention is to provide a lightweight self-adjusting enclosure for protecting a spring return mechanism mounted externally on a piston type fluid actuator. 
     A further object of the invention is to provide a fail-safe actuator which is less costly to manufacture and maintain than prior designs. 
     A still further object of the invention is to provide a fail-safe return mechanism utilizing constant-force springs which can be can be easily replaced. 
     These and other objects and advantages of the invention are accomplished by a fail-safe fluid actuator comprising a piston slidably disposed within a cylinder. In the event of a pressure loss, the piston is urged to a fail-safe position by a plurality of externally mounted constant force spring mechanism. In one embodiment the mechanism is located at one end of the cylinder with a telescoping cover for protecting it from exposure and damage. Another embodiment the mechanism is located at the side of the cylinder whereby the overall length of the actuator is significantly shortened. The fail-safe force applied to the piston by the springs is externally adjustable, and the springs are readily accessible for replacement or repair. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and novel features and advantages of the invention will become apparent from the following description when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 represents one embodiment according to the invention, in a longitudinal view partially in cross section, of a fail-safe fluid actuator containing a piston in the fail-safe position and in broken outline under operating fluid pressure; 
     FIG. 2 is a transverse view in cross section of the fluid actuator of FIG. 1 taken in a plane along the line  2 — 2  thereof; 
     FIG. 3 represents an another embodiment of a constant force spring mechanism for the fluid actuator of FIGS. 1 and 2; 
     FIG. 4 represents a longitudinal view partially in cross section of still another embodiment of a fail-safe fluid actuator according to the invention; 
     FIG. 5 is a transverse cross section of the actuator of FIG. 4 taken in a plane along the line  5 — 5  thereof; 
     FIG. 6 is fragmentary view of the actuator of FIG. 4 taken along the line  6 — 6  of FIG. 5 with a cover removed; and 
     FIG. 7 shows a portion of the actuator illustrating the manner of removing a constant force spring for repair or replacement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like reference numerals and characters designate like or corresponding parts throughout the several views, there is illustrated one embodiment of a fail-safe fluid actuator  10  in a fail-safe position when there is no operating fluid pressure being applied. 
     Actuator  10  includes a cylinder housing  12 , sealed between a cylinder end  14  having a square perimeter and a cylinder end  16  having a circular perimeter, by rods  18  and nuts  20 , and a piston  22  slidably disposed therein. One end of a piston rod  24  passes coaxially through piston  22  and is secured thereto by threadingly engaging on end  26   a  of a coaxial rod extension  26 . The other end of piston rod  24 , adapted for connecting to a controlled device such as a valve (not shown), slidably extends through a seal  28  held under compression in a bore  30  of cylinder end  14  and through an aperture  32  of an end plate  34  secured to cylinder end  14  by tie rods  18 . 
     A port  36  in cylinder end  14  provides for admission and release of operating fluid pressure in cylinder housing  12  acting against the facing surface of piston  22 . 
     A fail-safe spring return mechanism  38  is externally mounted on cylinder end  16  and operatively connected to piston rod  24  to urge piston  22  toward cylinder end  14 . 
     Mechanism  38  comprises a sleeve  40  with four mutually orthogonal sides slidably receiving the other end of rod extension  26 . A reduced circular end portion  40   a  is inserted through an aligned bore in cylinder end  16 . An adjusting bolt  42  coaxially received in a threaded end  40   b  of sleeve  40  contacts an end face  26   b  of rod extension  26  for fine tuning the spring return force applied to the end of rod extension  26 . 
     Four spools of constant force springs  44 , also referred to as spring motors, are each tightly coiled in its relaxed state and mounted on a drum  46 , preferably of nylon, by opening the spring coil and releasing it after the drum is inserted. Springs  44  are usually fabricated in laminated strips, the number depending on the return force needed. Also, spools of springs may be added as well, either or both in a radial array as shown in FIG. 2 or in tandem as shown in FIG. 3 . Each drum  46  and spring  44  are rotatably carried on a spindle  48  supported at its ends on an axis parallel to a side of sleeve  40  between parallel facing sides of upright angle brackets  50 . Each bracket  50  is removably secure at one end to cylinder end  16  by a bolt  52  passing through a triangular web  50   a  extending across the sides. 
     The free or outer end of each spring  44  is attached by fasteners  54  to a respective side of sleeve  40  and transmits a return force via sleeve  40  and rod extension  26  to piston  22  whenever fluid pressure, introduced through port  36  to cylinder housing  12 , causes piston  22  to move in the direction shown in phantom outline as spring  44  extends with sleeve  40  at a constant spring force. A loss of pressure allows spring  44  to return the piston to the fail-safe position adjacent to cylinder end  14 . 
     Actuator  10  is further provided with a telescopic enclosure  56  for protecting spring return mechanism  38  from harmful contaminant and accidental intrusion. Enclosure  56  comprises an inner tubular member  58  coaxial with cylinder housing  12  and surrounding spring return mechanism  38 . One end is fixed to cylinder  16  and the other end is open. An outer tubular member  60  is closed at one end and axially slidable over inner member  58 . Adjusting bolt  42  is secured to the closed end of member  60  by nuts  62  and  64  for enabling the outer member to slide axially along member  58  as sleeve  40  extends and retracts with piston  22 . 
     FIG. 3 shows an alternate embodiment of a constant force spring for use in a fluid actuator according to the invention. As briefly noted above, it comprises two springs  44   a  and  44   b  mounted between each pair of angle brackets. As applied to the radial configuration in FIG. 2 of a sleeve with four sides, a total of eight springs are employed to provide a commensurate increase in spring return force. 
     FIGS. 4-6 represent another embodiment of a fail-safe fluid actuator  70 , according to the invention, operatively connected to a poppet valve  72  shown in solid lines in a fail-safe closed position with no operating fluid pressure present in actuator  70 , and in broken lines in an open position when fluid operating pressure is present. 
     Actuator  70  consists of a piston  74  reciprocative in a short-stroke cylinder  76  between cylinder heads  78  and  80 . A port  81  in head  80  provides communication between the chamber formed between piston  74  and head  80  and a source of operating fluid pressure source, not shown. A spring support member  82  is contiguously secured to head  78  by threaded fasteners  88 . A piston rod  90  is threadingly secured at one end to piston  74  by a bolt  92 ; the other end slidably extending through support member  82  and threadingly connected to a crosshead  86  for reciprocating with piston  74  and rod  90 . 
     Poppet valve  72  includes a valve body  94  mounted in axial alignment with actuator  70  by four parallel, spaced-apart yoke posts  96  threadingly secured between a flange  100  of valve body  94  and support member  82 . A valve stem  102  slidably extending through valve body  94  is threadingly connected at one end in coaxially alignment with the end of piston rod  90  abutting crosshead  86 . The other end of stem  102  includes a sealing disc  104  arranged to close an inlet port  106  of valve  72  under fail-safe conditions. Under safe conditions, sealing disc  104  opens inlet port  106  permitting pressurized fluid to flow to an outlet port  108  in valve  72 . 
     A pair of spring return mechanisms  110  are mounted on outwardly disposed flange portions  82   a  of support member  82  located on opposite sides of cylinder  76  for urging piston  74  and valve stem  102  toward the fail-safe position. Each mechanism  110  includes a constant force spring  112  rotatable on a spindle  114  between bifurcated ends of a bracket  116  and in a plane normal to the axis of cylinder  76 . Spindle  114  is held in place such as by a cotter pin. Bracket  116  is secured to flange portion  78  by an adjustable tensioning screw  117  and a locking nut  117   a . A spring capsule  118  fixed to an outer end  112   a  of spring  112  includes a hole which registers with a post  120  projecting from crosshead  86  for transmitting a spring return force to piston  74  and fail-safe position to valve  72 . A sleeve  119  extending from flange portion  82   a , and a cover  121 , removably secured to flange portion  82   a , shield component parts of the spring return mechanism  110 . Like the springs in actuator  10  supra,  112  are usually fabricated in laminated strips—the number depending on the return force needed. When an even greater return force is required, additional springs may be employed around the actuator or in tandem as shown in FIG. 3 . 
     In operation, when fluid pressure is introduced through port  81 , the force applied to piston  74  moves crosshead  86  in the direction shown in broken outline against the constant spring-return force of springs  114  whereby sealing disc  104  moves out of valve body  94  to open inlet port  106 . A loss of pressure allows disc  104  to return to the fail-safe position and close inlet port  106 . 
     FIG. 7 illustrates the manner by which a constant force spring  112  can be removed for repair or replacement. Tensioning screw  117  is turned to release the tension on spring  112  permitting the cotter pin and spindle  114  to be removed from bracket  116 . Spring capsule  118  lifted off of post  120  for replacement by a repaired or new capsule and spring assembly. The new or repaired spring may now be installed in bracket  116  with the spindle and cotter, and the tensioning screw turned to extend the spring. 
     Some of the many advantages of the invention should now be readily apparent. For instance, a fail-safe fluid actuator is provided which can be easily mounted on a controlled device such as on the body of a valve and tailored for the appropriate spring return force necessary for fail-safe operation. The spring return mechanism is readily accessible for inspection, fine adjustment, maintenance and repair, and can be completely removed for replacement without having to disassemble the piston and cylinder housing assembly. The number of constant force spring is not limited by the size of the cylinder housing. The spring return mechanism is completely shielded from contaminating materials and from possible mechanical abuse at all times of operation. The design particularly lends itself to simplicity of parts and manufacture with attendant lower costs. 
     It will be understood, of course, that various other changes in the details, materials, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principles and scope of the invention as expressed in the appended claims.