Abstract:
An apparatus for bidirectionally translating and positioning loads under tension includes a linear hydraulic actuator suspendible from a hoist such as a crane and is useable to support, raise and lower heavy loads in precisely controllable small incremental distances. The apparatus utilizes a strain-gauge type load cell positioned between an upper end of the actuator and an upper anchor connector eye suspendible from a crane. A digital display device operatively coupled to the load cell provides an accurate indication of static weight of a load supported by a load connector eye attached to a lower end of a piston rod protruding from the actuator, and accurate and immediate indications of any load weight deviation resulting from contact of a load with an obstruction while being raised or lowered. A pressure relief valve is provided to enable flow of hydraulic fluid from the actuator cylinder to a fluid reservoir to prevent inadvertent over-pressuring of the cylinder.

Description:
BACKGROUND OF THE INVENTION 
   A. Field of the Invention 
   The present invention relates to machinery used for lifting and lowering heavy loads. More particularly, the invention relates to an auxiliary hoist control apparatus which is suspendible from a hoisting machine such as a crane and useable to support heavy loads. The invention relates specifically to a precision load positioner which utilizes a linear hydraulic actuator to raise and lower heavy loads incrementally with respect to a hoist, a load cell weight sensor and digital display which provides accurate and immediate indications of static-load weight as well as any load weight deviation resulting from a load contacting an obstruction while being raised or lowered, and a pressure relief mechanism for preventing over-pressurizing the actuator cylinder due to operator error. 
   B. Description of Background Art 
   Most hoisting machines, or hoists, such as fixed and mobile cranes or derricks used to manipulate heavy loads, are inherently limited in the precision with which they can vertically position a load. Thus, most hoists constructed to have a capacity for raising and lowering loads which weigh thousands of pounds are incapable of positioning such loads to a precision of much less than a few inches. However, there are many situations which require vertical positioning of a load with much greater precision. For example, proper placement of a horizontal beam with respect to a vertical column often requires greater positioning accuracy than attainable with existing hoists. Similarly, the lowering of an object such as a satellite or the upper stage of a rocket with respect to a lower stage of the rocket for mating the two, or lifting an upper object to de-mate it from another object, typically requires that the upper object be lowered or raised to a precision of much less than one inch, sometimes as small as a thousandth of an inch. 
   A partial solution to the problem of precise vertical control of heavy objects manipulatable by a hoist was disclosed by Hoover et al., in U.S. Pat. No. 2,500,459, Work Supporting Attachment For Hoists. The attachment disclosed in Hoover et al., includes an elongated cylinder which has an upwardly protruding connector eye, supportable from a crane hook, the cylinder being filled with hydraulic fluid above and below a piston fitted at the lower end thereof with a piston rod that protrudes through a lower end wall or bulkhead of the cylinder, and has at the lower end thereof a hook for suspending a load to be positioned. The attachment includes a hydraulic fluid storage area coupled at an upper end thereof by a port to the upper part of the interior of the cylinder and at a lower end thereof through a valve port and fluid passageway to the lower interior space of the cylinder below the piston. A spring located within the cylinder and disposed between the lower end of the piston and the lower bulkhead biases the piston to an upwardly retracted position. To lower a load incrementally with respect to the crane hook, the valve is opened for an interval sufficient to allow fluid beneath the piston to be expelled through the fluid reservoir and into the upper part at the cylinder above the piston in response to a downward tension force exerted by the load on the piston rod. The device disclosed in Hoover et al., was limited in usefulness because it included no means for raising a load in small, precise increments. 
   In U.S. Pat. No. 3,025,702, Merrill et al., Auxiliary Hoist Control, an apparatus was disclosed which could be suspended from a hoist hook, support a heavy load, and raise as well as lower the load incrementally relative to the hoist. The disclosed apparatus includes a body in which is located a vertically elongated inner cylinder containing a piston and piston rod which protrudes through a lower cylinder head that forms a bottom end wall of the body, and which has depending downwardly therefrom a load support eye. The upper end of the cylinder is closed by an upper cylinder head which has protruding upwardly therefrom an upper connector eye for suspension from a hoist hook. The portion of the cylinder below the piston is filled with hydraulic fluid which communicates through a fluid passageway, valve port and a down-valve to the lower portion of a vertically elongated annular volume formed between an outer wall of the inner cylinder and an inner wall of an outer coaxial cylinder. A hermetically sealing separator ring longitudinally slidably positioned in the annular volume between the inner and outer cylinders separates the annular volume into a lower storage chamber and an upper storage chamber. The lower storage chamber contains hydraulic fluid. The upper storage chamber contains a compressible fluid such as nitrogen gas which is introduced through a gas input port to pressurize the upper annular space above the separator ring, to a predetermined pressure. 
   In the Merrill apparatus, the down-valve provides for a controlled escape of a portion of the hydraulic fluid which supports the piston within the cylinder. When the down-valve is opened, hydraulic fluid pressurized by the weight of the piston, piston rod and load attached to the lower end of the piston rod escapes through the valve into the lower annular hydraulic fluid storage chamber. As hydraulic fluid escapes from the cylinder into the lower annular storage chamber, the separator ring is forced upwards, thereby compressing the gas stored in the upper chamber. The compressed gas thus functions like the spring of a hydraulic accumulator which retains the balance of pressure throughout the system, and which provides a return force to move the piston to its upwards, retracted position, when tension on the piston rod is reduced by removing the load. This return force function is enabled by the novel design of the down-valve, as will now be described. 
   The down-valve is so constructed as to permit passage of hydraulic fluid from the lower annular storage chamber back into the inner cylinder when the load is removed. In other words, when the load is removed, the down valve, which previously was manually actuated to allow passage of fluid from the cylinder to the lower annular storage chamber, now functions automatically as a dump-valve to allow passage of fluid from the lower annular storage chamber back into the inner cylinder. Downward pressure exerted on the separator ring by compressed gas in the upper chamber pressurizes hydraulic fluid in the lower annular storage chamber. Re-admission of hydraulic fluid thus pressurized into the inner cylinder forces the piston back upwards to a retracted position. 
   The auxiliary hoist control apparatus includes an up-pump for raising the piston and an attached heavy load in small, precisely controlled increments, e.g., one one-thousandths of an inch for a 12-inch travel piston supporting a 5-ton weight. The up-pump functions by withdrawing hydraulic fluid from the lower fluid storage chamber and injecting the fluid into the lower part of the inner cylinder, below the piston. 
   Loads in excess of twenty tons are accurately positionable using the auxiliary hoist control disclosed in Merrill et al., U.S. Pat. No. 3,025,702. That hoist control includes a return force gauge consisting of a pressure gauge which has an input port and gas passageway which communicates with the annular compressible gas storage area above the separator ring. The return force gauge is calibrated in pounds force exerted downwardly by the compressed gas on the separator ring. In a typical application of the auxiliary hoist control, nitrogen gas is introduced into an inlet port of the upper annular gas storage area at a pressure sufficient to provide a return force which is a fraction of the weight of a load to be positioned using the apparatus. For example, if an auxiliary hoist control of the type described has a capacity of 10,000 pounds and is being used to position a 5,000-lb. load, the upper annular gas storage chamber may be pressurized with nitrogen gas to provide a downward return force on the separator ring of 100 pounds force, as indicated on the return force gauge. In effect, adjusting the initial pressure within the upper annular gas storage chamber to produce a particular return force simulates the effect of having an adjustable spring constant compression spring disposed longitudinally within the upper annular gas storage chamber, between the upper surface of the separator ring and the lower, inner surface of the upper cylinder head. 
   The auxiliary hoist control disclosed in the &#39;702 patent also has a hydraulic fluid pressure gauge mounted on the body of the apparatus. The gauge has a relatively large face which is readable from a substantial distance, is coupled through a fluid passageway to the lower end of the inner hydraulic fluid cylinder below the piston, and is calibrated in pounds force exerted by a load suspended from the lower end of the piston rod. Thus, a 5,000-lb. load which is hung from the support eye at the lower end of the piston rod exerts a downward tension load on the rod which in turn is exerted on the piston and causes the piston to exert downward pressure on hydraulic fluid between the piston and lower cylinder head of the apparatus, producing a weight reading of 5,000 lbs. on the load weight gauge. 
   Rather than merely indicating to an operator the static weight of a load suspended from the auxiliary hoist control, the load weight gauge serves an additional important function. Thus, if a load is being lowered in a mating operation towards an object with which the load is to be mated, by operation of the down valve, for example, the reading on the load force gauge should remain constant. However, if the load is subjected to any sort of a resisting force in its downward travel, such as colliding with an underlying obstruction or sticking within the bore of a pipe, etc., the load gauge reading will decrease. This negative load deviation indication provides an immediate indication to an operator of a problem in the positioning of the load which requires corrective action. 
   Similarly, when a load is being raised, as in a de-mating operation, an increase in the load gauge reading provides an immediate indication to an operator of an overlying obstruction problem which must be corrected. Thus it can be appreciated that load deviation indication by the load force gauge is an important function of the auxiliary hoist control. However, the use of a hydraulic pressure gauge in the auxiliary hoist control of the &#39;702 patent is problematic for the following reason. When the up-pump is operated to raise a load, the injection of hydraulic fluid by the pump into the lower portion of the inner cylinder below the piston causes a transient dynamic over-pressure in the hydraulic fluid. The magnitude of the transient dynamic over-pressure can be a significant fraction of total force reading on the load force gauge, e.g., 200 lbs. For a 5,000 lb. Load. The transient pressure increase indication gives a false load deviation indication to an operator, thus requiring him to interrupt what was a normal lifting operation to search for a non-existent problem. The present invention was conceived to provide a precision load positioner apparatus which incorporates desirable features of the auxiliary hoist control disclosed in the &#39;702 patent, but which also provides an essentially error-free, immediate indication of load deviation and a mechanism to prevent over-pressuring the apparatus. 
   OBJECTS OF THE INVENTION 
   An object of the present invention is to provide a bi-directional linear hydraulic actuator apparatus which is capable of exerting tensional forces of several tons or more over a distance range of a foot or more, and capable of moving multi-ton loads in precisely controllable increments of one-thousandth of an inch or less. 
   Another object of the invention is to provide a precision load positioner apparatus for placement between a hoist such as a crane or derrick and a heavy load, and precisely raising and lowering the load incrementally with respect to the hoist. 
   Another object of the invention is to provide a precision load positioner apparatus which provides an accurate, immediate indication of weight of a load suspended from a hydraulic actuator piston of the apparatus, independent of transient variations of fluid pressure within the cylinder. 
   Another object of the invention is to provide a precision load positioner apparatus which provides accurate and immediate indication of static load weight, and of deviations in indicated weight of a suspended load being elevated or lowered by operation of the positioner which encounters an obstruction, while avoiding false indications of load deviation as a result of transient pressure variations within a hydraulic actuator of the apparatus used to raise and lower loads. 
   Another object of the invention is to provide a precision load positioner apparatus which utilizes a tensionable load cell between an upper hydraulic cylinder head and an upper anchor connector eye of the apparatus to measure load weight exerted by a load suspended from a lower load support connector eye fixed to the lower end of a piston rod protruding through a lower cylinder head of the hydraulic cylinder. 
   Another object of the invention is to provide a precision load positioner apparatus which utilizes a tensionable load cell electrically coupled to a visual digital display device to indicate the weight of a load suspended from the positioner. 
   Another object of the invention is to provide a precision load positioner apparatus which uses a tensionable load cell positioned between the upper head of an actuator cylinder of the apparatus and an upper anchor connector eye, the load cell producing an output voltage proportional to tensional force exerted between the upper anchor connector eye and a lower load-support connector eye, which is input to a visual digital display to indicate load weight, and an over-pressure relief valve which has an inlet port and fluid passageway connected to an inner hydraulic actuator cylinder and an outlet port and fluid passageway connected to a hydraulic fluid reservoir, to limit pressure within the inner cylinder to a predetermined maximum value. 
   Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims. 
   It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, we do not intend that the scope of our exclusive rights and privileges in the invention be limited to details of the embodiments described. We do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims. 
   SUMMARY OF THE INVENTION 
   Briefly stated, the present invention comprehends an accessory apparatus for use with hoisting machinery and comprises a precision load positioner which includes an upper anchor connector eye for suspending the apparatus from a hoist hook and a lower load support connector eye for suspending a load. The precision load positioner apparatus according to the present invention includes a body in which is located a vertically elongated inner hydraulic cylinder containing a piston and piston rod that protrudes through a lower cylinder head which forms a bottom end wall of the body, and which has depending downwardly therefrom a load support connector eye. The upper end of the cylinder is closed by an upper cylinder head which has protruding upwardly therefrom an upper anchor connector eye for suspension from a hoist hook. The portion of the cylinder below the piston is filled with hydraulic fluid which communicates through a fluid passageway, valve port and a down-valve to the lower portion of a vertically elongated annular volume formed between an outer wall of the inner cylinder and an inner wall of an outer coaxial cylinder. A hermetically sealing separator ring longitudinally slidably positioned in the annular volume between the inner and outer cylinders separates the space into a lower hydraulic fluid storage reservoir chamber and an upper gas storage chamber for a compressible fluid such as nitrogen gas which is introduced through a gas input port to pressurize an upper annular space above the separator ring to a predetermined pressure. 
   The down-valve of the apparatus provides for a controlled escape of a portion of the hydraulic fluid which supports the piston within the cylinder. When the down-valve is opened, hydraulic fluid pressurized by the weight of the piston, piston rod and load attached to the lower end of the piston rod escapes through the valve into the lower annular hydraulic fluid storage chamber. As hydraulic fluid escapes from the cylinder into the lower annular storage chamber, the separator ring is forced upwards, thereby compressing the gas stored in the upper chamber. The compressed gas thus functions as a spring which retains the balance of pressure throughout the system, and which provides a return force to the piston to its upwards, retracted position, when tension on the piston rod is reduced by removing the load. This return function is enabled by the design of the down-valve, as will now be described. 
   The down-valve is so constructed as to permit passage of hydraulic fluid from the lower annular storage chamber back into the inner cylinder when a load which exerts tension on the piston rod is removed. In other words, when the load is removed, the down valve, which previously was manually actuated to allow passage of fluid from the inner cylinder to the lower annular storage chamber between the inner and outer cylinders, now functions automatically as a dump-valve to allow passage of fluid from the lower annular storage area back into the inner cylinder. Downward pressure exerted on the separator ring by compressed gas in the upper chamber pressurizes hydraulic fluid in the lower annular storage chamber. Re-admission of hydraulic fluid thus pressurized into the inner cylinder forces the piston back upwards to a retracted position. 
   The auxiliary hoist control apparatus includes an up-pump for raising the piston and an attached heavy load in small, precisely controlled increments, e.g., one one-thousandths of an inch for a 12-inch travel piston supporting a 5-ton weight. The up-pump functions by withdrawing hydraulic fluid from the lower fluid storage chamber and injecting the fluid into the lower part of the inner cylinder, below the piston. 
   The precision load positioner according to the present invention includes a strain-gauge type load cell force-sensing transducer positioned longitudinally between the upper cylinder head of the actuator cylinder assembly, and the upper, anchor connector eye. The strain gauge transducer is electrically coupled through a cable to a digital display device, preferably a light emitting diode (LED) display which is readily viewable by an operator who may be located a substantial distance away from the apparatus. A tensional force exerted on the load connector eye at the lower end of the piston rod by the weight of a suspended load is transmitted through the piston rod, piston, immobile hydraulic fluid below the piston, through the device housing and strain gauge to the upper anchor connector eye and hoist. Thus, the LED display provides an accurate, immediate reading of load weight, not only when the load is static, but also when the up-pump or down-valve is being used to raise or lower a load. 
   Replacement of a hydraulic pressure gauge to indicate load weight by a load cell and digital force display device solves the problem of false load deviation readings caused by dynamic pressure transients within the hydraulic cylinder when the up-pump is operated. However, elimination of the hydraulic pressure gauge presents a problem. Thus, if the up-pump handle is manipulated to retract the piston, no indication is provided that the pump handle has been operated through a sufficient number of cycles to pressurize hydraulic fluid below the piston to a value which exceeds design limits. Thus, when the hydraulic pressure gauge is replaced by a load cell and display device, no indication of over-pressure within the hydraulic cylinder is provided; therefore, an operator may inadvertently pressurize the hydraulic fluid to a value great enough to rupture piston O-rings, thus causing serious damage to the apparatus. Over-pressurization of the hydraulic cylinder can occur in normal operation when the piston within the inner cylinder is at the upward limit of its travel, abutting the upper cylinder head, and the operator continues to operate the up-pump. According to the present invention, this problem is solved by providing the precision load positioner with load cell weight load sensor and digital indicator with a pressure relief valve. That valve has an input port which communicates through a fluid inlet passageway with hydraulic fluid below the piston in the inner cylinder, and an output port which communicates through a fluid output passageway with the lower annular hydraulic fluid reservoir. The relief valve is constructed to have an actuation, “cracking” or opening pressure which is close to the design pressure limit of the hydraulic actuator cylinder, e.g., 2000 psi gauge for a design limit of 1800 psig. With this arrangement, maximum pressure within the inner cylinder is limited to a predetermined maximum pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a prior art auxiliary hoist control,  FIGS. 2-8  illustrate features common to both the prior art auxiliary hoist control and the precision load positioner apparatus of the present invention and  FIGS. 9-18  illustrate the present invention. 
       FIG. 1  is a front elevation of a prior art auxiliary hoist control and of an actuator portion of the present invention. 
       FIG. 2  is a front elevation in section of the auxiliary hoist control of  FIG. 1 . 
       FIG. 3  is a sectional view taken along lines  3 - 3  of  FIG. 1  with the down valve assembly and up-pump assembly removed. 
       FIG. 4  is a fragmentary elevation taken along lines  4 - 4  of  FIG. 1 , partially in section. 
       FIG. 5  is a sectional view of the up-pump of the auxiliary hoist control. 
       FIG. 6  is a section view of the down-valve of the auxiliary hoist control. 
       FIG. 7  is an enlarged partial sectional view of the down valve and piston, illustrated in  FIG. 6 . 
       FIG. 8  is a further enlarged partial sectional view of the down valve and piston illustrated in  FIG. 6 . 
       FIG. 9  is a front elevation view of a precision load positioner apparatus with positive over-pressure protection and weight deviation indication according to the present invention. 
       FIG. 10  is a side elevation view of the apparatus of  FIG. 9 . 
       FIG. 11  is an upper transverse sectional view of the apparatus of  FIG. 9 , taken in the direction of line  11 - 11 . 
       FIG. 12  is an intermediate transverse sectional view of the apparatus of  FIG. 9 , taken in the direction of line  12 - 12 . 
       FIG. 13  is a longitudinal sectional view of a cylinder head space air breather cap of the apparatus of  FIG. 9 , on an enlarged scale and taken in the direction of the double arrowhead arc A. 
       FIG. 14  is a longitudinal sectional view of a compressible fluid inlet port of the apparatus of  FIG. 9 , on an enlarged scale and taken in the direction of the double arrowhead arc B. 
       FIG. 15  is a fragmentary longitudinal sectional view of the apparatus of  FIG. 9 , taken in the direction of the double arrowhead arc C and showing a gas passageway between an upper annular compressible fluid storage area and a return force gauge of the apparatus. 
       FIG. 16  is a fragmentary longitudinal sectional view of the apparatus of  FIG. 9 , taken in the direction of the double arrowhead arc D and showing details of a hydraulic fluid addition passageway, piston rod wiper ring, O-ring and retainer ring, and separator ring and sealing O-rings. 
       FIG. 17  is a fragmentary side elevation view, partly in section, of the apparatus of  FIG. 10 . 
       FIG. 18  is a lower transverse sectional view of the apparatus of  FIG. 17 , taken in the direction of line  18 - 18 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The precision load positioner according to the present invention utilizes components which are similar in structure and function to the prior art auxiliary hoist control in Merrill et al., U.S. Pat. No. 3,025,702, assigned to the assignee/applicant of the present invention. Accordingly, an understanding of the novel and advantageous advancements of the present invention over the prior art may be facilitated by reviewing the following description referring to  FIG. 1 , which illustrate a prior art auxiliary hoist control, and  FIGS. 2-9 , which illustrate structural features common to both the prior art auxiliary hoist control and the precision load positioner according to the present invention. 
   Referring to  FIG. 1 , there is shown an auxiliary hoist control  11  which consists principally of a body portion  12 , and upper head  13 , to which a top eye  14  is connected, and a lower head  15 . A rotatable socket having a lower eye  17  is connected to a shaft extending through the lower head  15 . The lower head  15  has a down valve assembly  18  and an up-pump assembly  19  extending therethrough. A hydraulic fluid pressure gauge  21  and a compressible fluid pressure gauge  22  are located on the body portion  12  of the auxiliary hoist control. A compressible fluid filler plug  25  closes a compressible fluid addition inlet (see  FIG. 2 ). A breather cap  26  vents the space above the piston to the atmosphere through a passage  27  (see  FIG. 2 ) in the upper head. 
     FIG. 2  shows a sectional elevation of the auxiliary hoist control  11  of  FIG. 1 . A piston  30  is connected to a piston rod  31 , the lower end of which is attached to the lower eye  17 . The piston is inserted in an inner cylinder  32  having a wall  33 . Concentric about the inner cylinder  32  there is positioned a second, outer cylinder  34  so as to form a concentric annular volume with respect to the inner cylinder  32 . This annulus has a lower portion  35  which is divided by a solid brass separator ring  36  from an upper portion  37 . The lower portion  35  is used as, and hereinafter referred to as, the hydraulic fluid storage area. The upper portion  37  is used as, and hereinafter referred to as, the compressible fluid storage area. The separator ring  36  has an inner O-ring  38  and outer O-ring  39  which assist in forming a seal between the two storage areas. 
   A down-valve assembly bore  40  and an up-pump assembly bore  41  are located in the lower head assembly  15 . 
     FIG. 3  is a sectional view of the lower head  15 . Two bores  40  and  41  contain the down valve assembly  18  and the up-pump assembly  19  respectively, which assemblies are not shown in  FIG. 3  for purposes of clarity. Partial sections of these assemblies are shown in  FIGS. 5 , and  6 . A down valve assembly inlet hole  55  and outlet hole  56  provide apertures for by-passing hydraulic fluid from the inner cylinder into the hydraulic fluid storage area by means of the down valve assembly. Up-pump inlet and outlet holes  59  and  60  provide apertures for withdrawing hydraulic fluid from the storage area and injecting the fluid into the inner cylinder in conjunction with the up-pump assembly  19 . A gauge passage  61  connects the inner cylinder to the hydraulic fluid pressure gauge  21 . A hydraulic fluid addition passage  62  is closed by a cap  63 . 
   Hydraulic fluid is contained in the inner cylinder  32 . When a tensioning load is applied between the top eye  14  and the lower eye  17 , the hydraulic pressure exerted by the hydraulic fluid in the inner cylinder  32  increases. Through the action of the down valve assembly, as will subsequently be described, this hydraulic fluid is selectively passed from the inner cylinder  32  into the hydraulic fluid storage area  35 . A decrease in volume of hydraulic fluid contained in the inner cylinder  32  due to the movement of the piston  30  in response to the tensioning load, will result in the movement of the piston rod  31  out of the lower head assembly  15  in proportion to the amount of hydraulic fluid passed into the hydraulic fluid storage area  35 . 
   An increase in volume of the hydraulic fluid stored in the hydraulic fluid storage area  35  will move the separator ring  36  in a direction toward the upper head  13 . Air, nitrogen or other compressible fluid is normally stored in the compressible fluid storage area  37 . The movement upward of the separator ring  36  will compress the fluid stored in the compressible fluid storage area  37  in proportion to the amount of movement of the separator ring  36  which occurs, and therefore in proportion to the amount of hydraulic fluid transferred from the cylinder  32  to the hydraulic fluid storage area  35 . As will become apparent from the description below, the foregoing components of hoist control function as an accumulator which provides a return force to move piston  30  to the upward, retracted position when tension on the piston rod is reduced by removing a load from the lower load support connector eye. 
   The auxiliary hoist control  11  is so constructed that there is an appreciable difference between the cross sectional area of the storage areas  35  and  37  and the cross sectional area of the cylinder  32 . The proportioning of these cross sectional areas permits the ultimate capacity of the unit to be widely varied so long as the structural limitations of the unit are not exceeded. 
   For example, assuming that there is a 1:2 ratio between the storage cross section and the cylinder cross section areas, the force which the compressible fluid will be required to exert on the separator ring, and consequently, on the hydraulic fluid, in order to exactly counterbalance a 20,000 pound tensioning force applied across the auxiliary hoist  11  will be only 10,000 pounds. If the cross section area of the cylinder  32  is 50 square inches, when the compressible fluid has been compressed to a pressure of 400 pounds per square inch, the system will be in equilibrium. 
   Assuming that the piston and piston rod are in their fully retracted position, the position shown in  FIG. 2 , and the compressible fluid in the upper annular area is at atmospheric pressure, when the piston is subsequently moved toward the lower head  15  by a tensioning force of 20,000 pounds, the system will be in equilibrium when the compressible fluid is compressed to approximately one twenty-fifth of its original volume. 
   However, if the pressure existing in the compressible fluid area is appreciably greater than ambient pressure when the piston  30  and piston rod  31  are in their fully retracted position, the application of a tensioning load of 20,000 pounds will cause the required 10,000 pounds force to be exerted by the compressible fluid upon the separator ring prior to the piston travel required for equilibrium in the preceding case. Thus, by pre-pressuring the upper annular storage area, it is possible to limit the ultimate extension of the auxiliary hoist in accordance both with the tension load applied and with the pre-pressuring used. 
   Pre-pressuring of the compressible fluid storage area may be accomplished through a compressible fluid inlet  45  ( FIG. 2 ). By means of this pre-pressuring facility, the auxiliary hoist control may be also utilized as a tension measuring device. Thus, knowing the pressure initially existing in the compressible fluid area, the tension exerted may be measured by the amount of extension of the piston rod. 
     FIG. 4  is an elevation, partially in section, showing the upper head  13 . A compressible fluid gauge outlet passage  65  connects the compressible fluid gauge  22  to the upper annular storage area. 
     FIG. 5  is a sectional view of the up-pump assembly  19 . The up-pump assembly consists of a hollow body portion  80  to which is connected an extension body  81  at one end and a piston  82  at the other. A pump handle  83  having a knob  84  extends into the body of the piston  82  and is held in position by a set screw  85 . A handle bearing  86  holds the handle  83  generally in position in the up-pump body  80  and reduces friction due to handle movement. The up-pump body  80  has a canted slot  87  indicated by the dotted line along which the handle  83  may be moved. A torsion spring  88  is connected between the piston and the pump body to rotatably return the piston to the position shown after it has been moved along the canted slot. Adjacent one end of the torsion spring  88  are a pair of flanges  89  which contain an O-ring  90 . The hollow pump body  80  narrows adjacent the flanges  89  so that the flanges  89  and the O-ring  90  provide a seal. The hollow pump body  80  has a pair of hydraulic fluid inlets  91  extending therethrough. The portion of the piston  82  adjacent the hydraulic fluid inlets  91  is of smaller diameter than the inner diameter of the pump body  80  at that point, thereby providing an annular hydraulic fluid containing space  92 . A second concentric hydraulic fluid containing space  92   a  obviates the necessity for aligning the inlets  91  with the inlet  59  (see  FIG. 3 ) of the lower head. 
   In the annular hydraulic fluid containing space  92 , the piston has a pair of hydraulic fluid inlet passages  93  which open into a longitudinal storage passage  94  within the piston  82  so as to form a small hydraulic fluid storage space. The longitudinal passage  94  opens onto a larger diameter ball check valve passage  95 . In the ball section valve passage  95  there is contained a ball  96  held in position by means of a ball check spring  97  so as to close the longitudinal storage passage  94 . The ball check spring  97  is held in compression by means of a washer  98  positioned against a snap ring  99  which engages the outer surface of the ball check valve passage  95 . 
   The extension body  81  has a hollow cylindrical central portion  100  and contains a ball  101  which is held against a check valve seat  102  in the form of a ring by a check valve spring  103 . The ball  101  and check valve spring  103  are contained within the hollow central body portion  100  of the body extension  81  when the up-pump  19  is assembled. Two hydraulic fluid outlet holes  105  extend from the outer surface of the extension body  81  into the hollow central portion  100 . A first O-ring  106 , in cooperation with the cylindrical bore  41  of the lower head and a shoulder on the pump body  80 , seals the hydraulic fluid contained in the annular storage area in one direction. A second O-ring  107  provides a hydraulic fluid seal between the inlet holes  91  and the outlet holes  105 . A third O-ring  108  provides a seal for the hydraulic fluid contained adjacent the extension body  81 . 
   An O-ring  109  seals the surface between the piston and body next to the inlet holes  93  in the direction of the extension body  81 . An O-ring  110  seals the junction of the check valve  102 , the extension body  81 , and the pump body  80 . 
   The up-pump is operated by rotating the up-pump handle  83 . Due to the canted construction of the slot  87  which contains the handle  83 , the piston  82  is driven toward the extension body  81  when the pump handle  83  is so rotated. Hydraulic fluid from the annular storage cylinder fills the inlet holes  91  and annular volume  92  associated therewith, together with the check valve inlet holes  93  and longitudinal storage passage  94 . The hollow volume extending between the first ball  96  and the second ball  101  is filled with hydraulic fluid. The movement of the piston  82  towards the extension body  81  compresses this latter volume of hydraulic fluid to a pressure which exceeds the pressure existing in the cylinder  32 . When the pressure exerted on this compressed volume between the check balls  96  and  101  exceeds the combined pressure existing in the cylinder  32  and the pressure exerted on the ball  101  by the check valve spring  103 , the ball  101  moves against the check valve spring  103  to the extent required to compress the spring  103  to equalize for the excess in pressure existing in the fluid between the trapped check balls. However, the movement of the check ball  101  against the check ball spring  103  moves the check ball  101  away from the check valve seat  102  which the check ball  101  formerly sealed. Thereupon, the fluid trapped between the two check balls escapes through the outlet holes  105  into the annular volume existing between the up-pump assembly and the cylindrical bore  41  of the lower head  15  and then into the cylinder  32  through the up-pump outlet hole  60  (see  FIG. 3 ). Hydraulic fluid will continue to so flow until the pressure existing in the fluid between the two check balls and the pressure existing in the fluid between the two check balls and the pressure existing in the cylinder is equalized. Thereupon, the ball  101  will be forced against the check valve seat  102  by the check valve spring  103 , again sealing hydraulic fluid between the two check balls. 
   Release of the pump handle  83  allows the torsion spring  88  to return to the pump handle  83  to its normal position and retract the piston  82  from the advanced position resulting from the prior rotating movement of the pump handle. Retraction of the piston  82  reduces the pressure on the fluid trapped between the two check balls. Check ball  101  remains seated against the check valve seat  102  due to the pressure exerted by the fluid in the hollow central portion  100  of the extension body  81  against the ball  101 . The ball  96  which heretofore closed the longitudinal passage  94  by the action of the compressed fluid trapped between the two check balls and also by the action of the check valve spring  95 , is now moved away from the valve seat by the pressure exerted on the ball  96  by the fluid contained in the holes  91  and  93  and the longitudinal passage  94 . When the hydraulic fluid contained between the two check balls  96  and  101  is at a pressure equal to that of the hydraulic fluid storage area  35 , the ball  96  is moved by the check valve spring  97  to close the longitudinal passage  94 . 
   Thus, fluid is extracted from the annular storage area and passed through the holes  91 ,  93  and the passage  94  around the check ball  96  and into the volume contained between the check balls  96  and  101 . A subsequent movement of the pump handle, as previously described, will thereupon result in the repetition of the pumping cycle which was described above. 
     FIG. 6  is a sectional view of the down valve assembly  18 . The down valve assembly  18  consists of a body  120  and a body extension  121  which together contain the various parts of the valve. A down valve handle  123  having a knob  124  inserted through the body  120  into the hollow central portion thereof. A valve actuator  125  is contained in the hollow central portion  122  of the body  120  and engages the handle  123 . The handle  123  is held against the valve actuator  125  by means of a set screw  126 . A torsion spring  127  is contained within the hollow cylindrical portion of the body  120  and is operable to return the valve handle  123  to the position shown when it has been rotated. A canted slot illustrated by the dotted line  128  allows the valve handle  123  to be rotated. A handle bearing  86 ′ holds the handle  123  generally in position in the down valve assembly  120  and reduces friction due to handle movement. Rotation of the valve handle causes the actuator  125  to move toward the body extension  121 . The actuator has a stem portion  129  extending through the hollow central portion  122  of body  120 . A pair of outlet holes  130  extend through the body portion  120  and open into the hollow cylindrical central section  122 . A seal of the hollow cylindrical central portion  122  in the direction of the valve handle  123  is formed by a pair of flanges  131  and an O-ring  132 . 
   A valve seat  133  is located at the junction of the body  120  and the extension  121 . A check valve piston  134  is contained within the check valve seat  133 . The check valve piston  134  is of novel construction and illustrated in greater detail in  FIG. 7 . An O-ring  135  seals the junction between the body section  120 , the extension section  121  and the valve seat  133 . 
   The annular chamber formed by the hollow cylindrical central portion  122  of valve body  120  and the stem  129  preferably has dimensions such that its longitudinal cross section area is at least three times greater than its lateral cross sectional area with the valve handle in the position shown. The use of this chamber configuration provides the proper location of the inlet and outlet holes for the valve. A helper spring  136  located in the extension  121  holds the valve piston  134  against the valve seat  133 . An O-ring  138  seals the outlet holes  130  in the direction of the valve handle. An O-ring  139  seals the outlet holes in the opposite direction. A pair of inlet holes  140  open into a hollow central portion  141  of the extension  121  between the helper spring  136  and the valve seat  133 . An O-ring  142  provides a seal adjacent the inlet holes  140 . 
     FIG. 7  shows in detail the construction of the valve piston  134  and valve seat  133 . The valve piston  134  consists of a piston head  145  which is connected to the main body portion  146  by a shoulder  147 . A stem  148  extends from the main body portion  146  in the opposite direction from the piston portion  145 . The piston head  145  has a slight narrowing taper in the direction away from the main body portion  146 . 
   It should be noted that the valve piston consists of an integral unit contained within the valve seat  133 . The valve seat  133  has an annular portion  149  extending down the main body portion  146 . The main body portion  146  preferably is constructed of square stock having slightly rounded edges. With such a construction, the extended annular portion  149  of the valve seat  133  surrounding the body portion  146  serves to align the head portion  145  and shoulder portion  147  with the orifice of the valve seat  133 , while the stem projecting from the body portion  146  in the opposite direction from the head portion  145  serves to provide firm contact with the helper spring  136  contained in the extension  121 . 
   Referring to  FIG. 6 , the operation of the down valve assembly will now be described. The down valve handle is rotated along the canted slot  128 , driving the actuator  125  in the direction of the extension  121 . The stem of the actuator is in contact with the face of the valve piston head portion  145 . Prior to movement of the down valve handle  123 , the valve seat  133  and the valve piston shoulder  147  form a seal to prevent movement of fluid from the inlet holes  140  through the valve assembly toward outlet holes  130 . The movement of the piston  134  caused by the actuator stem  129  driving the piston stem  148  against the helper spring  136  opens the seal formed between the shoulder  147  and the valve seat  133 . However, the piston head  145  is contained within the orifice of the valve seat  133 . A small annular by-pass area between the piston head portion  145  and the valve seat  133  exists. This small annular volume allows the movement of hydraulic fluid from the inlet holes  140  to the outlet holes  130 . As the rotation of the valve handle  123  continues, the piston head portion  145  is moved further back within the valve seat orifice. After the portion of the valve head portion  145  adjacent the shoulder  147  passes completely through the orifice, further movement of the valve head portion in this direction will result in an increase in the annular cross section available for the passage of hydraulic fluid, due to the taper of the valve head portion  145 . Therefore, the rate of passage of fluid through the down valve assembly is proportional to the amount of rotation of the down valve handle after the constant rate displacement of the piston head has been exceeded. 
   When the pressures existing between the hydraulic fluid in the cylinder and the hydraulic fluid in the annular storage chamber are equal, no flow of fluid through the down valve assembly will occur. If the valve handle  123  is thereupon returned to the position shown in  FIG. 6 , the helper spring  136  will force the piston shoulder  147  against the valve seat  133 , thereby again sealing the annular storage chamber against a further introduction of fluid from the hydraulic fluid of the cylinder. As was previously stated, the upper portion of the annular storage chamber contains a compressible fluid in a confined volume. When the tension causing the extension of the auxiliary hoist is removed, thereby releasing the pressure on the hydraulic fluid in the cylinder, the compressed fluid in the compressible fluid storage area  37  exerts a pressure on the hydraulic fluid in the hydraulic fluid storage area  35  which is greater than the pressure existing on the hydraulic fluid in the cylinder  32 . The down valve assembly  18  thereupon commences to function as a dump-valve due to its unique construction. The hydraulic fluid under high pressure in the hydraulic fluid storage area  35  forces the piston head  145  to retract through the valve seat  133  orifice. Hydraulic fluid flows from the hydraulic fluid storage area  35 , through the outlet holes  130 , the valve seat  133  orifice, the inlet holes  140  and into the cylinder  32 . This flow of fluid continues until the piston and rod have been completely retracted or until the pressures exerted upon the separator ring by the compressible fluid and by the hydraulic fluid are equalized. 
   Turning now to  FIGS. 9-18 , a precision load positioner apparatus  150  according to the present invention is shown. Precision load positioner apparatus  150  according to the present invention shares many structural and functional characteristics in common with auxiliary hoist control  11  described above. Thus the foregoing description of auxiliary hoist control  11  should be considered as part of the description of the present invention. 
   In an embodiment of load positioner  150  shown in  FIGS. 9-18 , cylinder head  13  is rotated 180 degrees with respect to body  12  from the orientation shown in  FIG. 2 , thus interchanging the relative position of gas filler plug  25  and air breaker cap  26 . The orientation of cylinder head of course is a matter of ordinary design choice, and has no effect upon the function of apparatus  150 . 
   Referring to  FIGS. 9-11 , a precision load positioner apparatus  150  according to the present invention may be seen to include a spacer ring  151  which protrudes upwardly from the upper surface  152  of upper cylinder head  13  of apparatus body  12 . Spacer ring  151  has the shape of a right-circular annular ring. Apparatus  150  also includes a load cell  153  which has the shape of a squat right-circular cylinder which has a flat, horizontal lower surface  154  that seats on a flat upper surface  155  of the spacer ring  151 . Load cell  153  and spacer ring  151  are secured together and to the upper cylinder head  13  by a plurality of bolts  156  which are arranged in a circular pattern, the bolts having shanks  157  which protrude downward through clearance holes  158  disposed through spacer ring  151 , and which are tightened into threaded blind bores  159  disposed vertically downwards into upper cylinder head  13  from the upper surface  160  of the cylinder head. 
   Upper anchor connector eye  14  of apparatus  150  has a downwardly depending threaded shank which is screwed into a threaded bore  162  that extends downwardly into the center of upper surface  163  of the load cell. Load cell  153  produces at an electrical output port  164  thereof an output voltage which is proportional to tensional force exerted between upper connector eye  14  and the load cell. A particular device found suitable for the use as load cell  153  in the present invention is a Model 1200 Pressure Series universal load cell, manufactured by Interface Company, 7401 East Butherus Drive, Scottsdale, Ariz. 85260, which utilizes temperature compensated strain gages to produce an output voltage proportional to tensional forces exerted on the device. That output voltage is input to a digital display device  165  by means of an electrical cable  166 . Digital display device  165 , which is preferably a Light Emitting Diode (LED) type has an internal signal processor amplifier, powered by internal batteries, which scales voltages output from load cell  153 , and converts those voltages to weight force readings in pounds, kilograms, or decanewtons of force, selectable by a range selection switch. None of the foregoing components of display device  165  are shown nor need further description, since the design and construction of such features are commonly known to those skilled in the art. 
   As shown in  FIGS. 9 and 10 , digital display device  165  is physically positioned on body  12  of precision load positioner apparatus  150  in place of the hydraulic pressure gauge  21  used in auxiliary hoist control  11  shown in  FIG. 1 . And, as has been described above, without hydraulic pressure gauge  21 , there is no indication to an operator that he might be over-pressurizing inner cylinder  32  with hydraulic fluid. Thus, if an operator oscillates up-pump handle  83  to retract piston  30  upwards in inner cylinder  32 , the operator will have no indication of over-pressurizing cylinder  32 . Therefore, the operator may inadvertently operate up-pump  19  a sufficient number of cycles to increase the hydraulic pressure in cylinder  32  below piston  30  to a value which exceeds design limits of the apparatus, thus blowing O-ring seals and causing damage to the apparatus. Over-pressurization of the hydraulic cylinder can occur in normal operation when the piston within the inner cylinder is at the upward limit of its travel, abutting the upper cylinder head, and the operator continues to operate the up-pump. To prevent such an occurrence, precision load positioner apparatus  150  is provided with a pressure relief valve, as will now be described. 
   Referring to  FIGS. 9 ,  10  and  17 - 18 , precision load positioner apparatus  19  may be seen to include a pressure relief valve  170 . As shown in  FIG. 18 , pressure relief valve  170  has an inlet port  171  connected through a hydraulic fluid inlet tube  172  to inner cylinder  32 . Pressure relief valve  170  also has an outlet port  173  connected through a hydraulic fluid outlet tube  174  to the lower portion  175  of annular fluid reservoir storage chamber  35  below separator ring  36  and between inner cylinder wall  33  and outer cylinder wall  34 . 
   The design and construction of pressure relief valve  170  are selected so that the actuation or “cracking” pressure that opens the valve to permit pressurized flow of hydraulic fluid from inner cylinder  32  to fluid reservoir storage chamber  35  between the inner cylinder and outer cylinder  34  is slightly above the design limit for a particular size positioner apparatus  150 . Thus, for example, a “5-ton, Model C” version of apparatus  150  according to the present invention has the following parameters: 
   
     
       
             
             
           
             
             
             
             
           
         
             
                 
             
           
           
             
               Effective lower surface area of piston 30: 
               6.285 square inches 
             
             
               Maximum Pressure With Inner Cylinder 32: 
             
           
        
         
             
               Load 10,000 
               lbs 
               Pressure 1,590 
               psi 
             
             
               5,000 
               kg 
               1,753 
               psi 
             
             
               5,000 
               dn (Deca Newtons) 
               1,787 
               psi 
             
             
                 
             
           
        
       
     
   
   Accordingly, since the apparatus  150  according to the present invention is designed and constructed so as to operate safely at a certain margin above its maximum rated capacity of 5,000 decanewtons, relief valve  170  would be selected to have an actuation or cracking pressure of about 10% above 1,787 psi, i.e., about 2000 psi. Preferably, pressure relief valve  170  is of a throttling, or variable flow rate type. Thus, if the pressure is increased above the threshold actuation or cracking pressure, the flow rate increases. Therefore, if the up-pump inadvertently continues to be operated to over-pressure cylinder  32 , the flow rate of hydraulic fluid from inner cylinder  32  into fluid reservoir storage chamber  35  increases. This increased flow rate prevents over-pressurization of inner cylinder  32  in spite of continued over operation of up-pump  19 . A valve found suitable for use as pressure relief valve  170  is the Model 5120 Series Inline Relief Valve, manufactured by Circle Seal Controls, Inc., 2301 Wardlow Circle, Corona, Calif. 92880.