COUNTERBALANCING SYSTEM FOR OILFIELD PUMP JACKS
United States Patent 3646833
A counterbalancing system for an oilfield pump jack including a walking beam reciprocable by power means to operate the pump, said counterbalancing system comprising a hydraulic ram interconnected between said walking beam and a fixed support, said ram being hydraulically interconnected with an accumulator vessel of larger diameter than said ram and means for maintaining a counterbalancing air pressure in that portion of said accumulator vessel not occupied by hydraulic field.
US Patent References:
Pneumatic-hydraulic system for well pumping or drilling units
White - July 1951 - 2560676
Hydraulic-pneumatic pumping system
Bays - December 1938 - 2141703
Pumping apparatus
O'Leary - October 1940 - 2218214
Oil well pump and counterbalance
O'Leary - June 1941 - 2244428
Hydraulic pumping jack
Kyle et al. - July 1943 - 2325138
Pneumatic-hydraulic system for well pumping or drilling units
White - July 1951 - 2560676
Hydraulic-pneumatic pumping system
Bays - December 1938 - 2141703
Pumping apparatus
O'Leary - October 1940 - 2218214
Oil well pump and counterbalance
O'Leary - June 1941 - 2244428
Hydraulic pumping jack
Kyle et al. - July 1943 - 2325138
Application Number:
04/871463
Publication Date:
03/07/1972
Filing Date:
08/28/1969
View Patent Images:
Export Citation:
Assignee:
Bowers Jr., Raymond S. (Osawatomie, KS)
Primary Class:
Other Classes:
60/593, 60/431, 60/533
International Classes:
F04B47/14; F04B47/00; G05G1/24
Field of Search:
74/589 267/1G,1A 60/52PV
US Patent References:
Primary Examiner:
Atonakas, Manuel A.
Claims:
I claim
1. A counterbalancing system for an oil well pump including a reciprocable pump stem and power means for reciprocating said stem, said system comprising:
2. A system as recited in claim 1 wherein the upper and lower operating limits of said pressure-sensitive means are maintained respectively somewhat above and below the upper and lower limits of fluid pressure variation produced by operation of said ram when said system is sealed.
3. A system as recited in claim 1 wherein said accumulator vessel constitutes a cylinder and with the addition of a floating piston operably movable in said cylinder, said relatively compressible and incompressible fluids being disposed in said accumulator cylinder at respectively opposite sides of said floating piston.
1. A counterbalancing system for an oil well pump including a reciprocable pump stem and power means for reciprocating said stem, said system comprising:
2. A system as recited in claim 1 wherein the upper and lower operating limits of said pressure-sensitive means are maintained respectively somewhat above and below the upper and lower limits of fluid pressure variation produced by operation of said ram when said system is sealed.
3. A system as recited in claim 1 wherein said accumulator vessel constitutes a cylinder and with the addition of a floating piston operably movable in said cylinder, said relatively compressible and incompressible fluids being disposed in said accumulator cylinder at respectively opposite sides of said floating piston.
Description:
This invention relates to new and useful improvements in oilfield pumping apparatus, and has particular reference to means for counterbalancing the weight supported by the walking beams of pumping jacks.
In the pumping of oil wells, there is usually provided a pump stem consisting of small diameter pipe or rod extending the full depth of the well, and pumping action is provided by vertical reciprocation of said stem. The stem is commonly driven by a pump jack at the ground surface and consisting of a frame on which a walking beam is pivoted for vertical pivotal movement, and being connected at its free end to the upper end of the pump stem, and a prime mover for driving said walking beam.
It is not economical that the prime mover be of sufficient power to lift the entire weight of the pump stem, plus the load imposed thereon by the pumping action, and it is therefore common practice to counterbalance this weight in some manner. In shallow wells, wherein the pump stem load is not excessive, counterbalancing of the stem load can be accomplished by extending the walking beam from its pivot oppositely from the end thereof from which the pump stem is suspended, and applying dead weight to said opposite end. The prime mover is obviously then required to do much less work. However, in deep wells, depths of 2 miles or more not being uncommon, the deadweight method is not practical due to the tremendous weight of the pump stem. In deep wells, therefore, it has become accepted practice to interpose an air cylinder having a piston movable therein between the walking beam and a fixed support such as the pump jack base, and to maintain air pressure in said cylinder tending to counterbalance the pump stem load. However, this method is also subject to certain disadvantages. It is not economically practical to use air pumps and controls for supplying air to and bleeding air from the cylinder at the appropriate portions of each cycle of the pump stem, whereby the air cylinder would comprise to some extent a substitute for the prime mover which operates the walking beam mechanically. On the contrary, accepted practice is that the air cylinder be effectually sealed, so that the air pressure therein is alternately increased and relieved as the walking beam reciprocates. Therefore, since the stroke length of the air cylinder is often very long, strokes of 20 feet being common, and due to limitations on the space available in which to mount the cylinder conveniently, the variations in air compression ratios therein which occur between the limits of a stroke become excessive. At one end of the stroke, the air pressure is so reduced as to offer little aid to the prime mover, and at the opposite end of the stroke the pressure becomes so high that the overcoming thereof may overload the prime mover. The practical result is that a counterbalancing system using only a sealed air cylinder can practically be employed to counterbalance only a portion of the pump stem load. Most suppliers of such air cylinder systems, can promise the balancing of about half of the pump stem load under maximum load conditions, even when employing cylinders specially designed to possess very large volumes in proportion to the displacement of their pistons.
Accordingly, the principal object of the present invention is the provision of a counterbalancing system for oil field pump jacks which overcomes the above described shortcomings of previous balancing systems, in that while still employing a fluid pressure cylinder yieldably supporting the pump jack, it possesses additional means which, while permitting the long stroke required of said cylinder, does so without excessive changes in fluid pressure therein, but with very small pressure changes so that the cylinder can counterbalance substantially the entire weight of the pump stem load. This permits the use of a less powerful, and hence cheaper prime mover than has heretofore been possible with systems inherently capable of supporting only a fraction of the pump stem load. Stated in summary, the invention consists in substituting a hydraulic ram for the air cylinder heretofore used for supporting the walking beam of the pump jack and connecting it hydraulically to one end of a larger accumulator cylinder in which is mounted a floating piston, said accumulator cylinder containing air at the opposite side of said floating piston.
Other objects are simplicity and economy of construction, efficiency and dependability of operation, and the provision of means for maintaining any desired fluid pressure in the system.
With these objects in view, as well as other objects which will appear in the course of the specification, reference will be had to the accompanying drawing, wherein the single view is a schematic layout of a counterbalancing system for oil field pump jacks embodying the present invention, a pump jack being indicated in dotted lines.
Referring in detail to said drawing, a pump jack indicated generally by the numeral 2 is shown therein, said pump jack including a base 4 rigidly mounted on the ground adjacent the head 6 of an oil well. Fixed on said base is an upright standard 8, braced by members 10, to the upper end of which one end of a walking beam 12 is pivoted on a horizontal axis, as at 14. To the free end of said walking beam is affixed a head 16 having flexible means 18 depending therefrom and attached at 20 to the pump stem 22 of the oil well. Said stem extends of course to the full depth of the well, and in deep wells represents many tons of weight. The pumping action is generated by vertical reciprocation of said pump stem. Walking beam 12 is pivoted vertically, whereby to reciprocate the pump stem, by an engine 24 or other prime mover mounted on base 4, and having a crank 26 mounted on the output shaft 28 thereof. A connecting rod 30 has one end thereof pivoted to the free end of crank 26, as at 32, and its opposite end pivoted to walking beam 12 at 34, whereby rotation of said crank produces vertical reciprocation of the walking beam, and of pump stem 22.
The counterbalancing system contemplated by the present invention is intended primarily to relieve prime mover 24 of the necessity of lifting the entire load imposed on connecting rod 30, principally by the tremendous weight of pump stem 22, and includes a hydraulic ram 36 interposed between walking beam 12 and base 4. Said ram includes a cylinder 38 pivoted at its lower end, as at 40 to base 4, a piston 42 movable in said cylinder, and a piston end 44 affixed to said piston and extending upwardly above said cylinder and pivoted at its extended free end to walking beam 12 adjacent head 16, as at 46. Above piston 42, cylinder 38 contains only air, and is vented to the atmosphere, as around piston rod 44 at its entry in the cylinder, to prevent buildup of air pressure therein due to reciprocation of said piston. Below piston 42, cylinder 38 is filled with a substantially incompressible fluid 48 such as oil.
The lower end of cylinder 38 is interconnected by conduit 50 with one end of an accumulator cylinder 52 which may be mounted at any convenient location, such as on base 4. Said accumulator cylinder is of larger diameter than ram cylinder 38, and has a floating or free piston 54 slidably movable therein. Hydraulic fluid 48 fills the end of the accumulator cylinder to which conduit 50 is interconnected, as well as filling said conduit and the lower end of ram cylinder 38. Cylinder 52, at the side of piston 54 opposite to that containing hydraulic fluid 48, contains only a compressible fluid 56 such as air, said air being supplied to said cylinder through a conduit 58 by a compressor 60 driven by an electric motor 62 having an operative connection 64 to said compressor. Interposed in conduit 58 are a check valve 66 permitting air flow only toward cylinder 52, a pressure gauge 68 positioned to indicate pressure in cylinder 52, and a pressure-sensitive electric switch 70 responsive to pressure in cylinder 52. Electrically, switch 70 is interposed in a pair of conductor wires 72 and 74 supplying electric current to motor 62 from line wires 76 and 78. A pair of wires 80 and 82 are arranged to bypass switch 70, and a manually operable switch 84 is interposed in said bypass wires. Switch 70 is operable to close whenever fluid pressure falls below a predetermined low level, and to open whenever fluid pressure exceeds a predetermined high level, these operating limits being adjustable. Control switches of this type are well known in the art, but as diagramed, it will be seen that the actual electric switch 86 thereof is provided with an operating lever 88 adopted to be moved to the left, as viewed, to close the switch and to the right to open the switch. Said lever is operated by a pair of lobes 90 and 92 mounted on the extended end of a piston rod 94, and rendered independently adjustable therealong by setscrews 96 and 98. Said piston rod is affixed to a piston 100 movable in an air cylinder 102 against a spring 104 disposed in one end of said cylinder, the opposite end of said cylinder being interconnected with conduit 58 by conduit 106.
To set the system in operation, the approximate upper and lower limits of the pressure which will occur in the counterbalancing system is roughly calculated, this calculation involving such factors as the weight of pump stem 22 and walking beam 12, the pumping pressure, and the diameters and lengths of cylinders 38 and 52. Then, with electric power supplied to line wires 76 and 78, manual switch 84 is closed to supply current to motor 62 to operate air compressor 60 until the average or mean of the calculated minimum and maximum pressures is indicated on gauge 68, whereupon switch 84 is opened. Then prime mover 24 is set in operation to oscillate walking beam 12, and to reciprocate piston 42 in hydraulic cylinder 38. This action alternately drives hydraulic fluid 48 from cylinder 38 through conduit 50 to the lower end of accumulator cylinder 52, compressing air in the latter cylinder above piston 54 (as piston 42 is lowered), and allows piston 54 to be lowered by air pressure to return fluid 48 to cylinder 38 (as piston 42 is raised). This action is of course accompanied by fluctuations of air pressure in cylinders 52 and 102, and causes reciprocation of piston 100 in the latter, to the left as pressure drops, and to the right as pressure increases. Setscrews 96 and 98 are then loosened, and lobes 90 and 92 are set so that lobe 90 does not engage switch lever 88 at the lower limit of the normal pressure variation, but only at a somewhat still lower pressure, and lobe 92 does not engage lever 88 at the upper limit of the normal pressure variation, but only at a somewhat still higher pressure. The system is then ready for normal operation.
As walking beam 12 oscillates under the influence of prime mover 24, piston 42 moves in a stroke of fixed length in cylinder 38, often as much as 20 feet in large pump jacks. As previously discussed, an air cylinder has previously been used in counterbalancing systems in place of cylinder 38, but inherent space limitations on the length of the air cylinder produced extreme variations in the air compression ratio in the cylinder as its piston reciprocated, to the point that reduced air pressure at the upper end of its stroke resulted in entirely inadequate counterbalancing force, and increased air pressure at the lower end of its stroke could in itself impose an overload on prime mover 24. As a result, with such air balancing systems, they were often designed to balance only about a half of the total load, the prime mover thus being required to lift the other half. In the present system, on the other hand, hydraulic fluid is transferred incompressibly to cylinder 52 through conduit 50, producing a fixed stroke of piston 54, the stroke of piston 54 being shorter than that of piston 42 since cylinder 52 is of larger diameter than cylinder 38. The strokes are inversely proportional to the squares of the piston diameters, that is, if piston 54 is three times the diameter of piston 42, piston 54 will move only one-ninth as far as piston 42. Thus since piston 54 moves through a lesser stroke than piston 42, it will produce less compression of air in cylinder 52 above piston 54 than would be the case in an air cylinder substituted for cylinder 38, provided only that the proportion between the length of the air end of cylinder 52 and the stroke length of piston 42 is greater by any degree than the ratio of the stroke of piston 54 to that of piston 42. For example, if the relative diameters of the two pistons were selected so that piston 54 moves one-ninth the stroke of piston 42, and if piston 42 had a stroke of 20 feet, the air end of cylinder 52 would have to be greater than one-ninth of 20 feet, or 21/3 feet. Actually, cylinder 52 would ordinarily be made much longer than this minimum, in order that the normal fluctuation of fluid pressure due to reciprocation of piston 42 would be still further reduced. In a system utilizing a cylinder 52 as shown, further reduction of the amplitude of the normal fluctuation of pressure resulting from reciprocation of piston 42 can be obtained either by increasing the diameter of said cylinder, or the length of the air end thereof.
By suitable selection of the diameter and length of cylinder 52, the fluctuation of fluid pressure in the system resulting from reciprocation of piston 42, with the system completely sealed, can be reduced to a very small fraction of the total load capable of being imposed by the jack on ram 36. Thus the fluid pressure can be adjusted to support virtually the entire load, rather than only a relatively small fraction thereof as in previous balancing systems, so that prime mover 24 need perform only a very small fraction of the work heretofore required. This permits economically valuable reduction in the size and power of the prime mover.
The fluid system is in normal operation completely sealed by check valve 66, and the system in no way constitutes a prime mover for driving the pump jack, since the operating lobes 90 and 92 of pressure-sensitive switch 70 are set to operate only at pressures respectively below and above the normal range of pressure fluctuation as previously discussed. In the normal reciprocation of piston 100 and piston rod 94, lobes 90 and 92 do not engage switch lever 88 at all. However, when due to gradual leakage of air or hydraulic fluid the pressure of the system drops more than a predetermined degree, lobe 90 will engage lever 88 to close switch 86, setting compressor 60 in operation to increase the pressure. Then, in the cyclical movement of piston rod 94, lobe 92 will eventually engage lever 88 to open switch 86. In this manner fluid pressure in the system can be maintained within very close limits to any set level. If the various air and hydraulic seals employed are of good quality and are well maintained, compressor 60 will become operative only very infrequently, perhaps once a week. For this reason, and since the compressor is never required to deliver air at the high rate which would be required for example if air were required to operate ram 26 directly, said compressor may be of a relatively low delivery rate, low power, and hence inexpensive type.
While I have shown and described a specific embodiment of my invention, it will be readily apparent that many minor modifications thereof could be made without departing from the spirit of the invention. For example, while floating piston 54 is desirable for some purposes, such that it permits the accumulator cylinder to be used in any position which may be desirable or convenient, and that it prevents any turbulence or "foaming" at the juncture of the gas-liquid fluids and hence promotes smoother operation, it could for most purposes be entirely omitted, member 52 then constituting merely a tank or other vessel sealed except for the air and liquid connections thereto. This would render member 52 much more economical, not requiring the fine machining necessary in a cylinder and piston, and would not materially affect its performance.
In the pumping of oil wells, there is usually provided a pump stem consisting of small diameter pipe or rod extending the full depth of the well, and pumping action is provided by vertical reciprocation of said stem. The stem is commonly driven by a pump jack at the ground surface and consisting of a frame on which a walking beam is pivoted for vertical pivotal movement, and being connected at its free end to the upper end of the pump stem, and a prime mover for driving said walking beam.
It is not economical that the prime mover be of sufficient power to lift the entire weight of the pump stem, plus the load imposed thereon by the pumping action, and it is therefore common practice to counterbalance this weight in some manner. In shallow wells, wherein the pump stem load is not excessive, counterbalancing of the stem load can be accomplished by extending the walking beam from its pivot oppositely from the end thereof from which the pump stem is suspended, and applying dead weight to said opposite end. The prime mover is obviously then required to do much less work. However, in deep wells, depths of 2 miles or more not being uncommon, the deadweight method is not practical due to the tremendous weight of the pump stem. In deep wells, therefore, it has become accepted practice to interpose an air cylinder having a piston movable therein between the walking beam and a fixed support such as the pump jack base, and to maintain air pressure in said cylinder tending to counterbalance the pump stem load. However, this method is also subject to certain disadvantages. It is not economically practical to use air pumps and controls for supplying air to and bleeding air from the cylinder at the appropriate portions of each cycle of the pump stem, whereby the air cylinder would comprise to some extent a substitute for the prime mover which operates the walking beam mechanically. On the contrary, accepted practice is that the air cylinder be effectually sealed, so that the air pressure therein is alternately increased and relieved as the walking beam reciprocates. Therefore, since the stroke length of the air cylinder is often very long, strokes of 20 feet being common, and due to limitations on the space available in which to mount the cylinder conveniently, the variations in air compression ratios therein which occur between the limits of a stroke become excessive. At one end of the stroke, the air pressure is so reduced as to offer little aid to the prime mover, and at the opposite end of the stroke the pressure becomes so high that the overcoming thereof may overload the prime mover. The practical result is that a counterbalancing system using only a sealed air cylinder can practically be employed to counterbalance only a portion of the pump stem load. Most suppliers of such air cylinder systems, can promise the balancing of about half of the pump stem load under maximum load conditions, even when employing cylinders specially designed to possess very large volumes in proportion to the displacement of their pistons.
Accordingly, the principal object of the present invention is the provision of a counterbalancing system for oil field pump jacks which overcomes the above described shortcomings of previous balancing systems, in that while still employing a fluid pressure cylinder yieldably supporting the pump jack, it possesses additional means which, while permitting the long stroke required of said cylinder, does so without excessive changes in fluid pressure therein, but with very small pressure changes so that the cylinder can counterbalance substantially the entire weight of the pump stem load. This permits the use of a less powerful, and hence cheaper prime mover than has heretofore been possible with systems inherently capable of supporting only a fraction of the pump stem load. Stated in summary, the invention consists in substituting a hydraulic ram for the air cylinder heretofore used for supporting the walking beam of the pump jack and connecting it hydraulically to one end of a larger accumulator cylinder in which is mounted a floating piston, said accumulator cylinder containing air at the opposite side of said floating piston.
Other objects are simplicity and economy of construction, efficiency and dependability of operation, and the provision of means for maintaining any desired fluid pressure in the system.
With these objects in view, as well as other objects which will appear in the course of the specification, reference will be had to the accompanying drawing, wherein the single view is a schematic layout of a counterbalancing system for oil field pump jacks embodying the present invention, a pump jack being indicated in dotted lines.
Referring in detail to said drawing, a pump jack indicated generally by the numeral 2 is shown therein, said pump jack including a base 4 rigidly mounted on the ground adjacent the head 6 of an oil well. Fixed on said base is an upright standard 8, braced by members 10, to the upper end of which one end of a walking beam 12 is pivoted on a horizontal axis, as at 14. To the free end of said walking beam is affixed a head 16 having flexible means 18 depending therefrom and attached at 20 to the pump stem 22 of the oil well. Said stem extends of course to the full depth of the well, and in deep wells represents many tons of weight. The pumping action is generated by vertical reciprocation of said pump stem. Walking beam 12 is pivoted vertically, whereby to reciprocate the pump stem, by an engine 24 or other prime mover mounted on base 4, and having a crank 26 mounted on the output shaft 28 thereof. A connecting rod 30 has one end thereof pivoted to the free end of crank 26, as at 32, and its opposite end pivoted to walking beam 12 at 34, whereby rotation of said crank produces vertical reciprocation of the walking beam, and of pump stem 22.
The counterbalancing system contemplated by the present invention is intended primarily to relieve prime mover 24 of the necessity of lifting the entire load imposed on connecting rod 30, principally by the tremendous weight of pump stem 22, and includes a hydraulic ram 36 interposed between walking beam 12 and base 4. Said ram includes a cylinder 38 pivoted at its lower end, as at 40 to base 4, a piston 42 movable in said cylinder, and a piston end 44 affixed to said piston and extending upwardly above said cylinder and pivoted at its extended free end to walking beam 12 adjacent head 16, as at 46. Above piston 42, cylinder 38 contains only air, and is vented to the atmosphere, as around piston rod 44 at its entry in the cylinder, to prevent buildup of air pressure therein due to reciprocation of said piston. Below piston 42, cylinder 38 is filled with a substantially incompressible fluid 48 such as oil.
The lower end of cylinder 38 is interconnected by conduit 50 with one end of an accumulator cylinder 52 which may be mounted at any convenient location, such as on base 4. Said accumulator cylinder is of larger diameter than ram cylinder 38, and has a floating or free piston 54 slidably movable therein. Hydraulic fluid 48 fills the end of the accumulator cylinder to which conduit 50 is interconnected, as well as filling said conduit and the lower end of ram cylinder 38. Cylinder 52, at the side of piston 54 opposite to that containing hydraulic fluid 48, contains only a compressible fluid 56 such as air, said air being supplied to said cylinder through a conduit 58 by a compressor 60 driven by an electric motor 62 having an operative connection 64 to said compressor. Interposed in conduit 58 are a check valve 66 permitting air flow only toward cylinder 52, a pressure gauge 68 positioned to indicate pressure in cylinder 52, and a pressure-sensitive electric switch 70 responsive to pressure in cylinder 52. Electrically, switch 70 is interposed in a pair of conductor wires 72 and 74 supplying electric current to motor 62 from line wires 76 and 78. A pair of wires 80 and 82 are arranged to bypass switch 70, and a manually operable switch 84 is interposed in said bypass wires. Switch 70 is operable to close whenever fluid pressure falls below a predetermined low level, and to open whenever fluid pressure exceeds a predetermined high level, these operating limits being adjustable. Control switches of this type are well known in the art, but as diagramed, it will be seen that the actual electric switch 86 thereof is provided with an operating lever 88 adopted to be moved to the left, as viewed, to close the switch and to the right to open the switch. Said lever is operated by a pair of lobes 90 and 92 mounted on the extended end of a piston rod 94, and rendered independently adjustable therealong by setscrews 96 and 98. Said piston rod is affixed to a piston 100 movable in an air cylinder 102 against a spring 104 disposed in one end of said cylinder, the opposite end of said cylinder being interconnected with conduit 58 by conduit 106.
To set the system in operation, the approximate upper and lower limits of the pressure which will occur in the counterbalancing system is roughly calculated, this calculation involving such factors as the weight of pump stem 22 and walking beam 12, the pumping pressure, and the diameters and lengths of cylinders 38 and 52. Then, with electric power supplied to line wires 76 and 78, manual switch 84 is closed to supply current to motor 62 to operate air compressor 60 until the average or mean of the calculated minimum and maximum pressures is indicated on gauge 68, whereupon switch 84 is opened. Then prime mover 24 is set in operation to oscillate walking beam 12, and to reciprocate piston 42 in hydraulic cylinder 38. This action alternately drives hydraulic fluid 48 from cylinder 38 through conduit 50 to the lower end of accumulator cylinder 52, compressing air in the latter cylinder above piston 54 (as piston 42 is lowered), and allows piston 54 to be lowered by air pressure to return fluid 48 to cylinder 38 (as piston 42 is raised). This action is of course accompanied by fluctuations of air pressure in cylinders 52 and 102, and causes reciprocation of piston 100 in the latter, to the left as pressure drops, and to the right as pressure increases. Setscrews 96 and 98 are then loosened, and lobes 90 and 92 are set so that lobe 90 does not engage switch lever 88 at the lower limit of the normal pressure variation, but only at a somewhat still lower pressure, and lobe 92 does not engage lever 88 at the upper limit of the normal pressure variation, but only at a somewhat still higher pressure. The system is then ready for normal operation.
As walking beam 12 oscillates under the influence of prime mover 24, piston 42 moves in a stroke of fixed length in cylinder 38, often as much as 20 feet in large pump jacks. As previously discussed, an air cylinder has previously been used in counterbalancing systems in place of cylinder 38, but inherent space limitations on the length of the air cylinder produced extreme variations in the air compression ratio in the cylinder as its piston reciprocated, to the point that reduced air pressure at the upper end of its stroke resulted in entirely inadequate counterbalancing force, and increased air pressure at the lower end of its stroke could in itself impose an overload on prime mover 24. As a result, with such air balancing systems, they were often designed to balance only about a half of the total load, the prime mover thus being required to lift the other half. In the present system, on the other hand, hydraulic fluid is transferred incompressibly to cylinder 52 through conduit 50, producing a fixed stroke of piston 54, the stroke of piston 54 being shorter than that of piston 42 since cylinder 52 is of larger diameter than cylinder 38. The strokes are inversely proportional to the squares of the piston diameters, that is, if piston 54 is three times the diameter of piston 42, piston 54 will move only one-ninth as far as piston 42. Thus since piston 54 moves through a lesser stroke than piston 42, it will produce less compression of air in cylinder 52 above piston 54 than would be the case in an air cylinder substituted for cylinder 38, provided only that the proportion between the length of the air end of cylinder 52 and the stroke length of piston 42 is greater by any degree than the ratio of the stroke of piston 54 to that of piston 42. For example, if the relative diameters of the two pistons were selected so that piston 54 moves one-ninth the stroke of piston 42, and if piston 42 had a stroke of 20 feet, the air end of cylinder 52 would have to be greater than one-ninth of 20 feet, or 21/3 feet. Actually, cylinder 52 would ordinarily be made much longer than this minimum, in order that the normal fluctuation of fluid pressure due to reciprocation of piston 42 would be still further reduced. In a system utilizing a cylinder 52 as shown, further reduction of the amplitude of the normal fluctuation of pressure resulting from reciprocation of piston 42 can be obtained either by increasing the diameter of said cylinder, or the length of the air end thereof.
By suitable selection of the diameter and length of cylinder 52, the fluctuation of fluid pressure in the system resulting from reciprocation of piston 42, with the system completely sealed, can be reduced to a very small fraction of the total load capable of being imposed by the jack on ram 36. Thus the fluid pressure can be adjusted to support virtually the entire load, rather than only a relatively small fraction thereof as in previous balancing systems, so that prime mover 24 need perform only a very small fraction of the work heretofore required. This permits economically valuable reduction in the size and power of the prime mover.
The fluid system is in normal operation completely sealed by check valve 66, and the system in no way constitutes a prime mover for driving the pump jack, since the operating lobes 90 and 92 of pressure-sensitive switch 70 are set to operate only at pressures respectively below and above the normal range of pressure fluctuation as previously discussed. In the normal reciprocation of piston 100 and piston rod 94, lobes 90 and 92 do not engage switch lever 88 at all. However, when due to gradual leakage of air or hydraulic fluid the pressure of the system drops more than a predetermined degree, lobe 90 will engage lever 88 to close switch 86, setting compressor 60 in operation to increase the pressure. Then, in the cyclical movement of piston rod 94, lobe 92 will eventually engage lever 88 to open switch 86. In this manner fluid pressure in the system can be maintained within very close limits to any set level. If the various air and hydraulic seals employed are of good quality and are well maintained, compressor 60 will become operative only very infrequently, perhaps once a week. For this reason, and since the compressor is never required to deliver air at the high rate which would be required for example if air were required to operate ram 26 directly, said compressor may be of a relatively low delivery rate, low power, and hence inexpensive type.
While I have shown and described a specific embodiment of my invention, it will be readily apparent that many minor modifications thereof could be made without departing from the spirit of the invention. For example, while floating piston 54 is desirable for some purposes, such that it permits the accumulator cylinder to be used in any position which may be desirable or convenient, and that it prevents any turbulence or "foaming" at the juncture of the gas-liquid fluids and hence promotes smoother operation, it could for most purposes be entirely omitted, member 52 then constituting merely a tank or other vessel sealed except for the air and liquid connections thereto. This would render member 52 much more economical, not requiring the fine machining necessary in a cylinder and piston, and would not materially affect its performance.