WO1990014528A1

WO1990014528A1 - Double piston load cell

Info

Publication number: WO1990014528A1
Application number: PCT/US1990/003107
Authority: WO
Inventors: John M. Montague
Original assignee: Regal International, Inc.
Priority date: 1989-05-26
Filing date: 1990-05-25
Publication date: 1990-11-29

Abstract

Load cells used with high hydraulic presssures on their piston seals often develop the problems of wear and leaks. By bonding a rubber cup between the piston and associated cylinder, the problems are reduced. The structure includes a double piston load cell (10) having a cylinder (12), a pair of pistons (20, 30) and a resilient compressible cup (50) bonded to the inner wall of the cylinder (12) and to the outer wall (22, 32) of the piston (20, 30) to form an enclosed chamber (15) in the cylinder (12). A quantity of incompressible liquid (70) in the enclosed chamber (15) occupies a first volume, and a quantity of compressible gas in the chamber occupies a second volume. A change in the volume of incompressible liquid (70) relative to the second volume of compressible gas controls piston displacement when a predetermined force is applied to the pistons (20, 30). The load cell finds use as a bumper or marine landing structure.

Description

DOUBLE PISTON LOAD CELL

TECHNICAL FIELD The invention relates to a double piston load cell in which a resilient cup is bonded to the inner wall of a cylinder and outer walls of the pistons to form a chamber charged with pressurized fluid to ad¬ just the spring rate of the load cell.

BACKGROUND OF INVENTION

Hydraulic cylinders have been connected to a hydraulic accumulator and used as a spring for various applications. However, sliding seals between the pis¬ ton and cylinder wall may leak at high hydraulic pres¬ sures and may wear as a result of oscillating loads being applied to the piston. Patent No. 4,109,474, entitled "BUMPER ASSEMBLY

SHOCK CELL SYSTEM", and U.S. Patent No. 4,337,004, entitled "MARINE LANDING STRUCTURE WITH OMNI-DIREC- TIONAL ENERGY ABSORBING CHARACTERISTICS" disclose shock cells mounted between a bumper and an offshore platform to absorb shock transmitted to the platform during boat landings and the like. The shock cells incorporate a cylindrical rubber body adhered to the inner surface of a stationary supporting cylinder and to the outer surface of a movable cylinder telescopi- cally disposed in the stationary cylinder such that the cylindrical rubber body dissipates major compo¬ nents of shock applied to the movable inner cylinder. The cylinders were sealed to prevent entry of sea water and other corrosive material.

Shock cells of the type disclosed in the afore¬ mentioned patents offered significant improvements over structures previously used and have enjoyed sig¬ nificant commercial success for the disclosed applica¬ tion. However, the stiffness or "spring rate" of the shock cell was fixed when the shock cell was con¬ structed and could not be readily changed without extensive modification of the structure.

SUMMARY OF INVENTION The double piston load cell disclosed herein includes at least one piston telescopically disposed in a cylinder. A rubber cup is bonded to the outer surface of the piston and to the inner surface of the cylinder to form an enclosed chamber. A source of pressurized compressible fluid such as air or nitrogen communicates with the enclosed chamber. Piston dis¬ placement, resulting from loads applied to the piston, deforms the rubber cup and compresses the compressible fluid. Adjustment of the volume of the compressible fluid adjusts the "spring rate" of the load cell.

The chamber in the load cell is preferably charged with a quantity of compressible fluid and a quantity of incompressible material, the volume of incompressible material being adjustable to control the volume and pressure of the compressible fluid over a predetermined range of piston displacement when predetermined forces are applied to the piston. The pressurized fluid may be injected directly into the chamber or may be enclosed in an external hydraulic accumulator.

Physical dimensions of the load cell depends on the magnitude of force to be applied. Cells for some applications may require a piston displacement of from one to four inches when forces of from 1,000 to 2,000 pounds are applied.

In certain marine applications, the cylinder of the load cell may be 36 inches in diameter and nine to ten feet long. Loading on the pistons may be in a range of several hundred tons while pressure in the cylinder is in a range of, for example, three hundred to one thousand pounds per square inch. The combined displacement of the two pistons may be, for example, 40 inches.

The resilient cups bonded between the piston and the cylinder body of the load cell permit limited movement of the pistons relative to the cylindrical load cell body. However, sliding seals typically employed in hydraulic cylinders have been eliminated. Incompressible liquid or solid material can be pumped into the cylinder for adjusting the volume occupied by the compressible gas to thereby adjust the spring rate of the load cell. The spring rate of the load cell is determined by the length of the resilient cups bonded between the pistons and the cylinder, the pressure of the gas in the chamber between the pistons and the relative volumes occupied by the pressurized gas at various piston displacements. DESCRIPTION OF DRAWINGS

Drawings of two preferred embodiments of the invention are annexed hereto so that the invention may be better and more fully understood, in which:

Figure 1 is a partially sectionalized view of a first embodiment of the load cell, the pistons being in an extended position;

Figure 2 is a partially sectionalized view simi¬ lar to Figure 1, the pistons being displaced inwardly of the cylinder; Figure 3 is a partially sectionalized view of a second embodiment of the load cell diagrammatically connected to a hydraulic system for adjusting pressure of fluid in the load cell;

Figure 4 is a partially sectionalized view of the second embodiment of the load cell, pistons being displaced inwardly of the load cell body; and

Figure 5 is a graph illustrating magnitudes of force exerted on pistons in the load cell resulting from distortion of material in the cup walls and com¬ pression of a controlled volume of gas. Numeral references are employed to designate like parts throughout the various figures of the drawing. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figure 1 of the drawing, the numeral 10 generally designates a double piston load cell com- prising a cylindrical body 12 having an internal cham¬ ber 15, and a pair of pistons 20 and 30 extending into opposite ends of chamber 15. The cylindrical body 12 has an inner cylinder wall 14 and an outer cylinder wall 16 encircling chamber 15. Pistons 20 and 30 have outer piston walls 22 and

32, respectively, and inner piston walls 24 and 34. Plates 26 and 36 are welded or otherwise secured to outer ends of pistons 20 and 30 while face plates 27 and 37 are secured to inner ends of pistons 20 and 30 forming chambers 25 and 35, respectively, in pistons 20 and 30. In the illustrated embodiment connectors 29 and 39 are welded or otherwise secured to plates 26 and 36, respectively, for connecting pistons 20 and 30 between any suitable members (not shown) for control- ling the relative movement of the members.

Face plate 27 on piston 20 has a passage 28 ex¬ tending therethrough such that chamber 15 and cavity 25 are in fluid communication. In the illustrated embodiment face plate 37 on piston 30 is not perfo- rated. However, a passage similar to passage 28 in face plate 27 may be formed through face plate 37, if it is deemed expedient to do so such that cavity 15 and chambers 25 and 35 are in fluid communication.

As illustrated in Figure 1 of the drawing, cups 50, having outer cylindrical surfaces 52 and inner cylindrical surfaces 54 are mounted in the annulus between the outer surface 22 of piston 20, the outer surface 32 of piston 30 and the inner wall 16 of cy¬ lindrical body 12.

The inner cylinder wall 14 adjacent opposite ends of cylindrical body 12 as well as outer surfaces 22 and 32 of pistons 20 and 30 are preferably shot blasted and cleaned with a solvent and then bonded to cup 50, as illustrated in the drawing. The inner cylindrical surface 54 of cup 50 is bonded to the outer surface 22 of the tube forming piston 20 while the outer cylindrical surface of cup member 50 is bonded to the inner cylinder wall 14. Cup 50 is simi¬ larly mounted on piston 30.

As best illustrated in Figures 1 and 3 of the drawing, end surfaces 56 and 58 on cup members 50 are inclined, arcuate end surface 56 being generally coni¬ cal in shape to form a projection extending outwardly from one end of cup 50. End surface 58 on the oppo¬ site end of cup 50 is conical shaped to form a tapered recess in the inner end of cup 50.

As best illustrated in Figures 2 and 4 of the drawing, an axially applied load "F" to pistons 20 and 30 deforms the wall of each cup 50 as the pistons move from the position illustrated in Figure 1 of the draw- ing to the position illustrated in Figure 2.

A second embodiment of the load cell is generally designated by the numeral 80 in Figures 3 and 4 of the drawing. The second embodiment is substantially iden¬ tical to the structure hereinbefore described in ref- erence to the first embodiment except that a guide sleeve 82, having perforations 84 formed in the wall thereof, extends into opposite ends of the tubular members forming pistons 20 and 30. It will be noted that face plates 27 and 37 have been omitted and that guide sleeve 82 is telescopically disposed in cavities 25 and 35 of pistons 20 and 30. Guide sleeve 82 main¬ tains pistons 20 and 30 axially aligned even under lateral loading. Chamber 15 is in fluid communication with cavities 25 and 35 through perforations 84 and the central passage 85 through guide sleeve 82. The stiffness of load cells 10 and 80 may be adjusted by selecting cups 50 having varying lengths which results in varying the volume of rubber or other resilient material which is compressed when pistons 20 and 30 are urged inwardly. To further adjust the stiffness of load cell 10, a volume of pressurized compressible fluid 70 may be deposited in chamber 15 such that the pressure of fluid 70 is increased as the volume of chamber 15 is reduced as pistons 20 and 30 are displaced inwardly. The volume of fluid in load cell 10 can be increased by forming a passage 28 in face plate 27 on the end of piston 20 and further increased by forming a second passage through face plate 37 on piston 30.

Fill pipes 65 and 75 having passages extending therethrough are welded or otherwise secured to the wall of body member 12 and to the wall of tube 20. Passages through fill pipes 65 and 75 may be closed with threaded plugs or equipped with suitable valves 62 and 72 through which pressurized gas, solid materi- al or liquid may be introduced to provide a desired initial pressure in chamber 15 and to adjust the vol¬ ume in the chamber occupied by the gas.

The stiffness of load cell 10 is further adjust¬ able by depositing a quantity of incompressible mate- rial, such as liquid 60, having a surface 61, in cham¬ ber 15 in addition to the volume 70 of compressible fluid. The incompressible fluid 60 preferably com¬ prises mineral oil or other inert material while the compressible fluid 70 preferably comprises nitrogen gas. It should be readily apparent that when incom- pressible liquid 60 is introduced into chamber 15, the volume of the chamber filled with compressible fluid 70 is reduced, compressible liquid 60 displacing the compressible gas 70. It should be appreciated that shot pellets or other solid material may be deposited in chamber 15 to reduce the volume which is occupied by gas.

For certain installations, an external accumula¬ tor may be employed in combination with a pump for delivering pressurized fluid into cavity 15. The pressurized fluid may be delivered through fill pipe

75 into cavity 25 or through fill pipe 65 into chamber 15. It should be readily apparent that either incom¬ pressible liquid 60 or compressible gas 70 may be introduced into chamber 15 or removed therefrom for adjusting pressure in chamber 15 and consequently the force required for movement of pistons 20 and 30. As illustrated in Figure 3 of the drawing, a hydraulic line 100 is connected between fill pipe 65 and hydraulic pump 105. Pump 105 is connected through suitable hydraulic lines to deliver incompressible liquid, such as mineral oil, from a reservoir 110 into chamber 15 in the cylindrical body 12. A conventional external bag-type hydraulic accumulator 120 is con¬ nected to hydraulic line 100. The bag type accumula- tor 120 includes a closed cylinder 122 containing a rubber bag 124 filled with air or gas. Hydraulic fluid 125 is pumped into cylinder 122 under pressure, compressing the gas in the bag 124. A release valve 126 is connected to hydraulic line 100. When the release valve 126 is opened, gas pressure in bag 124 forces hydraulic fluid 125 from the cylinder into hydraulic line 100. Valve 130 in line 132 which is connected to hydraulic line 100, can be selectively opened for reducing pressure in the load cell.

Referring to Figure 5 of the drawing, the end points A and B on the upper straight line curve graph¬ ically illustrate design criteria for a typical load cell. The lower curve containing points C and D graphically represents force exerted by a cup 50 to resist displacement of piston 20 relative to cylinder body 12.

If specifications require that a force of 1,000 pounds will displace piston 20 a distance of one inch and a force of 2,000 pounds will displace piston 20 four inches, the load cell is preferably tested to determine whether or not the specified loading and displacement requirements are met.

The graphically illustrated data indicates that when chamber 15 is vented to atmosphere a force of only 400 pounds displaced piston 20 one inch. Thus, pressurized air, nitrogen or other gas is delivered through valve 17 and fill pipe 75 to exert sufficient force on piston 20 to support an applied load of 1,000 pounds at a deflection of one inch. Thus, 400 pounds of force would be required to deform cup 50 while 600 pounds of force would be exerted by the compressed gas 70 to support the applied load of 1,000 pounds.

Since the area of the piston, normal to the di¬ rection of movement of piston 20 remains constant, while the volume of chamber 15 is reduced as the ap- plied load F is increased to 2,000 pounds the volume of chamber 25 occupied by pressurized gas 70 will decrease as the applied load F is increased to 2,000 pounds as required at point B on the curve illustrated in Figure 5. Since the pressure required to meet the specifications at point A on the curve is known, pres¬ surized liquid or other incompressible material is delivered through valve 62 in fill pipe 65 to cause load cell 10 to support the applied load F of 2,000 pounds at a deflection of four inches. In the illus¬ trated embodiment, cup 50 exerts a force of approxi- mately 800 pounds on piston 20 while the pressurized gas 70 exerts a force of approximately 1200 pounds.

It should be readily apparent that the increase in pressure of gas 70 in chamber 15 is related to the volume of air occupying all or a portion of chamber 15. When the volume of air compressed by movement of the piston is increased, the force required for dis¬ placing the piston one inch is less than the force required to displace the piston one inch to compress a smaller volume of gas. It should be readily apparent that solid material, such as metallic spheres may be introduced through fill pipe 65 for adjusting the volume of chamber 15 in lieu of liquid, if it is deem¬ ed expedient to do so.

From the foregoing it should be readily apparent that the volume of chamber 15 may be adjusted by in¬ troducing incompressible material into the chamber either before or after pressurized gas is delivered into chamber 15 for adjusting the deflection of the load cell to reach point A on the curve illustrated in Figure 5 when the specified load is applied.

It should be readily apparent that a family of curves AB may be generated by adjusting the proportion of the volumes of incompressible material and com¬ pressible material in load cells 10 and 80 to adjust the stiffness or "spring rate" of the load cells.

Although cups 50 are constructed of impervious material, under cyclic loading at high pressures, a small volume of pressurized gas 70 may be lost from the system. Periodic testing or constant monitoring the performance of the load cell will indicate when it is necessary to deliver an additional charge of gas or incompressible material into chamber 15 through valves 62 and 72. It should be noted that valves 62 and 72 may also function as bleed valves for removing gas or liquid from the system through one valve while gas or liquid is introduced through the other valve to adjust the proportion of the volume of incompressible materi¬ al to the volume of gas. The ratio in the change in volumes of gas compressed as piston 20 moves from the position illustrated in Figure 1 of the drawing^' to the position illustrated in Figure 2 of the drawing can be precisely controlled and adjusted.

To obtain the proper ratio of the actual change in volume upon displacement of piston 20, the total volume available to be occupied by gas can be adjusted to establish the correct mathematical ratio of pres¬ sure to volume required to cause load cell 10 to per¬ form in accordance with the specifications graphically illustrated in Figure 5.

From the foregoing it should be readily apparent that the double piston load cell 10 comprises a cy¬ lindrical body 12 and a pair of tubular pistons 20 and 30 having cups 50 bonded to outer cylindrical walls 22 and 32 of pistons 20 and 30 and to inner wall 24 of cylindrical body 12 to form an enclosed vessel having chamber 15 disposed therein. Force required for mov¬ ing pistons 20 and 30 into cylindrical body 12 can be adjusted by adjusting the pressure of compressible fluid 70 in chamber 15. The volume of chamber 15 occupied by compressible fluid 70 can be adjusted by introducing a volume of incompressible fluid 60 into chamber 15.

Gas 70 is bled from the chamber 15 and cavity 25 through fill pipe 75 to fill both the chamber 15 and cavity 25 with fluid 125 when accumulator 120 is em¬ ployed.

Claims

Having described the invention, it is claimed:

1. A double piston load cell comprising: a cyl¬ inder having inner and outer cylinder walls and a chamber; a pair of pistons, one end of one of said pistons extending into each end of said chamber, each of said pistons having an outer piston wall, said outer piston walls and said inner cylinder wall being positioned to form an annulus therebetween; a resil¬ ient compressible member in said annulus bonded to said inner cylinder wall and said outer piston walls to form an enclosed chamber in said cylinder; a quan¬ tity of compressible fluid in said chamber occupying a volume; and means to change pressure of said compress¬ ible fluid to control the rate of piston displacement when a predetermined force is applied to said pistons.

2. A double piston load cell according to Claim

1, said resilient compressible member comprising: a cylindrical cup having inner and outer cylindrical cup walls, said outer cup wall being bonded to said inner cylinder wall and said inner cup wall being bonded to said outer piston wall.

3. A double piston load cell according to Claim

2, said cup having conical end walls adjacent opposite ends thereof, said conical end adjacent one end of said cup projecting outwardly from said cup body, and said conical end wall on the other end of said cup body projecting inwardly from the end of said cup body forming a recess communicating with said chamber in said cylinder.

4. A double piston load cell according to Claim 3, said conical end surfaces on said cup being con¬ structed and arranged such that displacement of one of said pistons inwardly of said cylinder compresses material in said resilient cup wall in the annulus between said inner cylinder wall and said outer tube wall to resist displacement of said piston.

5. A double piston load cell according to Claim 1, said means to change pressure of said compressible fluid comprising: fill means communicating with said chamber.

6. A double piston load cell according to Claim 1, one of said pistons comprising: a hollow tubular member; means closing one end of said hollow tubular member; a face plate having a passage extending there¬ through secured to the other end of said tubular mem¬ ber forming a cavity in said tubular member, said cavity being in fluid communication with said chamber in said cylinder through said passage.

7. A double piston load cell according to Claim 6, said resilient compressible member comprising a cup body having inner and outer cylindrical walls, said cup body having inclined end walls intersecting said inner and outer surfaces, a flange extending inwardly from said inner surface, said passage extending through said flange, said inner cylindrical surface and said flange forming a socket in said cup body, said tube extending into said socket.

8. A double piston load cell according to Claim 6, with the addition of: means to maintain said pair of pistons axially aligned.

9. A double piston load cell according to Claim

8, each of said pistons having a passage, said means to maintain said pair of pistons axially aligned com¬ prising: a tube having opposite ends extending into said passages.

10. A double piston load cell according to Claim

9, said tube having perforations formed therein in fluid communication with said chamber.

11. A double piston load cell according to Claim

I, said means to change pressure of said compressible fluid comprising: a quantity of incompressible liquid in said chamber; and means to change the volume of incompressible liquid relative to the volume of com¬ pressible fluid.

12. A double piston load cell according to Claim

II, said means to change the volume of incompressible liquid comprising: a hydraulic accumulator; and means connecting said hydraulic accumulator to said chamber in said cylinder.

13. A double piston load cell according to Claim 11, said means to change the volume of incompressible liquid comprising: a pump; and means connecting said pump to said chamber in said cylinder.

14. A double piston load cell comprising: a cylinder having inner and outer cylinder walls and a chamber; a pair of pistons, one of said pistons ex¬ tending into opposite ends of said chamber, each of said pistons having an outer piston wall, said outer piston walls and said inner cylinder walls being posi¬ tioned to form an annulus therebetween; a resilient compressible member in said annulus bonded to said inner cylinder wall and said outer piston wall to form an enclosed chamber in said cylinder; a quantity of incompressible fluid in said enclosed chamber occupy¬ ing a first volume; a quantity of compressible fluid in said chamber occupying a second volume; and means to change said first volume relative to said second volume to control piston displacement when a predeter¬ mined force is applied to said pistons.

15. A double piston load cell according to Claim

14, each of said pistons comprising: a hollow tubular member; means closing one end of each of said hollow tubular members; a face plate having a passage extend- ing therethrough secured to the other end of each of said tubular members forming cavities in said tubular members, said cavities being in fluid communication with said chamber in said cylinder through said pas¬ sages.

16. A double piston load cell according to Claim

15, said means to change said first volume relative to said second volume comprising: a source of pressurized fluid communicating with said chamber.

17. A double piston load cell according to Claim 14, said means to change said first volume relative to said second volume comprising: a fill pipe communicat¬ ing with said cavity.

18. A double piston load cell according to Claim 14, said resilient compressible member comprising: a cylindrical cup body having inner and outer cylindri¬ cal surfaces on a resilient cup wall, said cup having conical end surfaces adjacent opposite ends thereof, said conical surface end adjacent one end of said cup body projecting outwardly from said cup body, and said conical end surface on the other end of said cup body projecting inwardly from the end of said cup body forming a recess communicating with said chamber in said cylinder, said outer surfaces on said cup wall being bonded to said inner cylinder wall and said inner surface on said cup wall being bonded to said outer tube wall; said conical end surfaces on said cup body being constructed and arranged such that dis¬ placement of one of said pistons inwardly of said cylinder compresses material in said resilient cup wall in the annulus between said inner cylinder wall and said outer tube wall to resist displacement of said piston.

19. A double piston load cell according to Claim 18, with the addition of: a hollow tube having perfo¬ rations formed in the wall of the tube, opposite ends of said tube extending through passages in said pis- tons and arranged to maintain said pistons axially aligned.

20. A load cell comprising: a cylinder having inner and outer cylinder walls and a chamber; a pis¬ ton, one end of said piston extending into said cham¬ ber, said piston having an outer piston wall, said outer piston wall and said inner cylinder wall being positioned to form an annulus therebetween; a resil¬ ient compressible member in said annulus bonded to said inner cylinder wall and said outer piston walls to form an enclosed chamber in said cylinder; a quan- tity of incompressible liquid in said chamber occupy¬ ing a volume; and accumulator means communicating with said chamber to change pressure of said incompressible liquid to control piston displacement when a prede¬ termined force is applied to said piston.

21. A load cell comprising: a cylinder having inner and outer cylinder walls and a chamber; a pis¬ ton, one end of said piston extending into said cham¬ ber, said piston having an outer piston wall, said outer piston wall and said inner cylinder wall being positioned to form an annulus therebetween; a resil¬ ient compressible member in said annulus bonded to said inner cylinder wall and said outer piston walls to form an enclosed chamber in said cylinder; a quan- tity of pressurized gas in said chamber occupying a volume; and means to change the volume of said pres¬ surized gas to control piston displacement when a predetermined force is applied to said piston.