US3747194A

US3747194A - Rivet squeezer

Info

Publication number: US3747194A
Application number: US00186048A
Authority: US
Inventors: M Christensen
Original assignee: McDonnell Douglas Corp
Current assignee: McDonnell Douglas Corp
Priority date: 1971-10-04
Filing date: 1971-10-04
Publication date: 1973-07-24
Anticipated expiration: 1990-07-24

Abstract

A tool to preload and upset rivets being installed in structure including a preload piston with a connected preload sleeve to establish a predetermined preload between the structure and the rivet, an anvil, and a squeeze piston with a connected head set for upsetting the rivet against the anvil once the preload has been established.

Description

United States Patent 11 1 1111 3,747,194 Christensen July 24, 1973 RIVET SQUEEZER 2,797,596 7 1957 Seely 29 24354 3,557,442 1 1971 Speller 29 24353 [751 lnvemor- Chmlensen, Long Beach, 3,581,373 6/l97l Murdoch 29 243.54

[73] Assignee: McDonnell Douglas Corporation, Primary Examiner-Robert C. Riordon Santa M nica, Ca if- Assistant Examiner-J. C. Peters [22] Filed. Oct 4 1971 Attorney-Walter Jason, George W. Finch et ul.

. I [2]] Appl. No.: 186,048

[57] ABSTRACT 52 U.S. c1 29/243.s4 72/391 72/465 A to Preload and "P rivets being installed in 511 1m. 01 nzi /22 Structure including P Piston with a connected [58] Field of Search 29/243.52 243.53, Preload sleeve to establish a predetermined Preload 29 241354. 72 3 391 39 2160 4 5 tween the structure and the rivet, an anvil, and a squeeze piston with a connected head set for upsetting 56] References Cited the rivet against the anvil once the preload has been es- UNITED STATES PATENTS tabl'shed 2,539,419 1/1951 Harcourt 29/243.54 7 Claims, 5 Drawing Figures RIVET SQUEEZER BACKGROUND OF THE INVENTION As larger high stress structures have been built, new fasteners have been required to improve the fatigue life of such structures. One such fastener, covered in part by U. S. Pat. No. 3,426,641, is the Rivbolt developed by the Marquardt Corporation and available'from Voi- Shan Company, Culver City, Calif. Rivbolts are capable of producing quality riveted joints of the highest shear strengths while producing significant improvements in the fatigue life of the structure into which they are inserted. These Rivbolts can be formed of high strength materials which are ordinarily difficult to squeeze form without cracking, yet the Rivbolts can be squeeze driven crack-free with precision quality because of a controlled upset of the driven Rivbolt and the utilization of a special collar and rivet set adaptor tooling. Examples of some of the materials that can be now used in Rivbolts include solution treated and fully aged alloys such as 6-4 and Beta III titanium, A-286 Waspaloy and 2024 aluminum. The controlled upset of the Rivbolt prevents structural deformation in the area of the driven head thereof and produces uniform and substantially equal Rivbolt swelling in the hole through which the Rivbolt is driven. The result ofusing Rivbolts is an optimum strength, riveted joint with high fatigue capability and minimum bowing or built in stress along the length of the riveted structure. These latter advantages enable substantialweight reduction and increased payload along a joint.

The Rivbolts have heretofore been installed with large, stationary or crane suspended riveting tools. Such tools usually include a single hydraulic assembly which first establishes a controlled preload from the head of the Rivbolt across the structure to a collar placed about the tail of the Rivbolt. Establishing the proper preload before the Rivbolt is upset is critical since too little or too much preload results in an inferior joint. Once the preloadin the structure has been established, the hydraulic assembly is used to apply force to the tail of the Rivbolt. The force causes the body of the Rivbolt to swell within the hole to expand the diameter of the hole to a predetermined degree, thereby stress coining" the walls of the hole. As the force is increased to a predetermined terminal value, the tail of the Rivbolt is mushroomed out over its collar, securely lockingthe Rivbolt in place. The prior art tools usually employ large mechanical springs or complex Hydra-Spring devices in conjunction ,with their hydraulic assemblies to establish the preload. Such spring.

devices have manydisadvantages in p reloading applications because they must be relativelylarge tosatisfy high force requirements, they can provide only a limited stroke, and the preload force. they establish is undesirably variable through the stroke thereof. Furthermore, a number of different spring devices must be stocked in order to provide the various preload forces and strokes which are required for installing Rivbolts of different lengths or different diameters. Foreach Rivbolt change, the prior art tools must usuallybe disas-v sembled to install the appropriate preload spring device.

SUMMARY OF THE INVENTION The present rivet squeezer tool isprimarilydesigned for installing rivets of the Rivbolttype. The tool is usually hydraulic powered and it may be a hand tool shaped in the general form of a C-clamp with separate preload and squeeze cylinders at the opposite ends. After the Rivbolt has been inserted through a hole in the structure stack-up to be joined and the collar has been positioned about the Rivbolt tail, a preload sleeve is slipped over the tail of the Rivbolt and is pressed firmly against the collar to surround it and restrain it within the tool. The preload sleeve is normally biased in an extended position by a connected preload piston in the pressurized preload cylinder. The preload pressure is set at a predetermined value to match the particular Rivbolts specified preload requirement.

Controls for the squeeze cylinder are then energized so a squeeze piston therein advances to extend a connected head set into contact with the head of the Rivbolt. As the squeeze piston continues to advance, the head set gradually builds up force through the structure stack-up until the specified preload as controlled by the pressure in the preload cylinder acting on the preload piston has been achieved. At this point in the riveting cycle, the preload force on the preload piston is overcome and the preload sleeve and the preload piston are driven back against the constant preload pressure. When the tail of the Rivbolt comes in contact with a post anvil which extends through the preload sleeve, a very rapid force build up occurs until the elastic limit of the Rivbolt is exceeded. This causes the Rivbolt to flow and swell within the hole in the structure, actually expanding the walls of the holeto increase the fatigue strength of the structure. With continuing build up of force, the tail'gradually mushrooms, increasing in diameter until it achieves a shape commensurate with the terminal upset force designed into the Rivbolt. The present tool is usually constructed so that it can be regulated to apply no more than the desired terminal upset force.

After upset has been accomplished, the tool is deenergized and the pressure in the squeeze cylinder is relieved so the head set moves back to its retracted position which enables the preload sleeve to return to its initial extended position. The tool is then removed from about the completed Rivbolt connection. The assistance of optional stripping means may be employed to force the preload sleeve off of the mushroom deformed tail of the Rivbolt although this is usually not necessary.

The riveting portion of the present tool is designed to be relatively lightweight for handling ease and portability and also to be relatively small in size so it can be used in tight quarters. The two opposite operating cylinders are used to handle the separate functional requirements of a Rivbolt tool without resort to the unneeded complication of employing co-axial pistons. The use of two separate cylinders also enables the con struction-of'a tool having a relatively long stroke to facili tate engagement into difficult structural configurationsand'to accommodate wide variations in structure stack thicknesses and .in tail lengths of the Rivbolts.

The .means to supply the preload pressure to the preload cylinder are usually a device such as a gas-fluid accumulator. Such accumulators include a gas chamber and a.fluid chamber which are separated from each otherby, a.flexible diaphragm. Chosen settings of the gas pressure charge in the accumulator correspondingly set the fluid pressure therein which is conducted to the rivetingportion of the tool to establish the controllable preload force. The force is made relatively constant by providing an accumulator whose gas volume is much larger than the volume of the preload cylinder. The squeeze cylinder, on the other hand, can be provided with relatively high operating pressures by means of such as a controllable air-oil pump. Both the accumulator and the pump can be relatively small, portable devices. This is desirable so they can accompany a tiveter using the tool into tight places, such as inside an aircraft wing, so that only short high pressure hoses are required to supply the desired pressures from the pump and accumulator to the riveting portion of the tool.

It is therefore an object of the present invention to provide a.tool for installing Rivbolt type fasteners.

Another object is to provide a rivet squeezer with adjustable preload force controlling means that do not vary through the stroke range of the squeezer.

Another object is to provide a rivet squeezer with a relatively long stroke capacity.

Another object is to provide a portable rivetsqueezer of small size and weight.

Another object is to provide a rivet squeezer which is relatively economical to build and maintain.

Another object is to provide a rivet squeezer which is safe and foolproof.

Another object is to provide a rivet squeezer whose source of external power may be relatively low pressure air, low pressure hydraulic fluid and/or electricity.

Another object is to provide a rivet squeezer whose preload does not depend on a mechanical or an air-oil internal spring.

Another object is to provide a Rivbolt squeezer which can be made in practically any size and capacity so that it is adaptable over a wide range of size and length Rivbolt configurations.

These and other objects and advantages of the present invention will become apparent after considering the following detailed specification which covers preferred embodiments thereof in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a rivet squeezer portion of a tool constructed according to the present invention in position to start a Rivbolt fastening cycle;

FIG. 2 is a partial cross-sectional view of the present tool with relative positioning of the components therein being those at the start of the Rivbolt fastening cycle;

FIG. 3 is a partial cross-sectional view similar to FIG. 2 with a preload established by the tool;

FIG. 4 is a partial cross-sectional view similar to FIG. 2 with the tool components in final rivet upsetting positions;

FIG. 5 is a diagrammatic representation of the present squeezer tool including its pumps, accumulator, and controls.

DESCRIPTION OF THE PRESENT EMBODIMENTS Referring to the drawings more particularly by reference numbers, number in FIG. 1 refers to the rivet squeezer tool portion of a rivet squeezer tool 11 constructed according to the present invention. The tool 10 includes a generally C-shaped body portion 12 with a preload cylinder 14 connected to one end thereof and a squeeze cylinder 16 connected to the other. As can be seen, the tool 10 can be hand held in engagement with the workpiece 18 while rivets such as Rivbolts 20 are being installed to hold portions of the workpiece 18 together.

In FIG. 2 it can be seen how a Rivbolt 20 is positioned in a drilled hole 22 in the workpiece 23 with a collar 24 in place around the tail 26 of the Rivbolt 20. As the tool 10 is placed about the Rivbolt 20, a preload sleeve 28 including a collar mating annular groove 30 is placed in contact over the collar 24. The preload sleeve 28 is formed as either an integral or connected extension of a preload piston 32 which rides on the inner cylindrical surface 34 of the preload cylinder 14 and on the outer cylindrical surface 36 of a post anvil 38 whose function will be described hereinafter. The post anvil 38 extends from a plug member 40 which seals one end of the preload cylinder 14. The post anvil 38 may be integral with the plug member 40 or connected thereto as shown. Having a removable preload sleeve 28 and a removable post anvil 38 is advantageous as it allows widely varied types and sizes of Rivbolts 20 to be installed with the same basic tool 10.

Sealing means such as O-

rings

42 and 44 are included between the preload piston 32 and the

surfaces

34 and 36 respectively to form a sealed chamber 46. The chamber 46 is supplied with hydraulic fluid at a relatively constant and predetermined pressure through a port 48 by means described hereinafter. Therefore, the piston 32 is normally urged away from the plug 40 with a constant predetermined force no matter how large or small the chamber 46 becomes.

Once the Rivbolt 20 and its collar 24 are properly positioned within the preload sleeve 28, a head set 50 extending from a squeeze piston 52 and guided by a retainer ring 53 forming the inner end of the cylinder 16, is extended until it contacts the head 54 of the Rivbolt 20. The head set 50 may be replaceable as shown and it usually includes a shaped portion such as the depression 55 which is contoured to properly abut the head The squeeze piston rides on the inner cylindrical surface 56 of the squeeze cylinder 16 and includes high pressure sealing means such as a T-seal 58, available from Parker Seal Company of Los Angeles, Calif, therebetween. The cylinder 16 and the piston 52 define a sealed chamber 60 into which hydraulic fluid at high pressure is controllably admitted through a port 62. The squeeze piston 52 is normally biased away from the Rivbolt 20 by a spring 64 acting between the piston 52 and the retainer ring 53 so that the chamber 60 is usually of minimal size. When high pressure hydraulic fluid is admitted through the port 62, the piston 52 and its head set 50 are driven toward the Rivbolt 20 until the head set 50 engages therewith. From that point on as shown in FIG. 3, the extension of the head set 50 causes the preload sleeve 28 of the preload piston 32 to be forced back against the aforementioned constant pressure in the chamber 46 with the preload force being applied from the head 54, across the workpiece 23 to the preload sleeve 28. The extension of the head set 50 continues until the preload piston 32 is driven back far enough to allow the end 68 of the post anvil 38 to come in contact with the tail 26 of the Rivbolt 20. An abutment surface 69 is formed on the retainer ring 53 facing the piston 52 to act as a positive stop therefor in case the tool 10 is inadvertantly activated without a Rivbolt positioned in the working gap.

When the tail 26 of the Rivbolt 20 comes into contact with the anvil end 68, a very rapid force build-up occurs across the Rivbolt 20. The pressure in the chamber 60 is predetermined to apply sufficient force across the Rivbolt 20 to exceed the elastic limit thereof so the Rivbolt 20 swells within the hole 22 to expand the walls of the hole 22 thereby creating residual compressive stresses which increase the fatigue life of the workpiece 23. As the force continues to build-up, the tail 26 of the Rivbolt 20 gradually mushrooms as shown in FIG. 4, increasing in diameter until it achieves a shape commensurate with a terminal upset force specified for the Rivbolt 20 and controlled by the pressure supplied to the chamber 60. After the Rivbolt upset has been accomplished, the pressure in chamber 60 is relieved and the spring 64 moves the squeeze piston 52 backward in the squeeze cylinder 16 to retract the head set 50 thereby allowing extension of the preload sleeve 28 to the position shown in FIG. 2. It should be noted that the cross-sections of the cylinders 14 and 116 are thicker at the sides connecting to the C-shaped body portion 12 than at the opposite sides. This is done to optimize the strength of the tool while enabling it to be used in tight places such as in the cramped workpiece 23 of FIGS. 2, 3 and 4.

In some instances when the Rivbolt 20 is slightly too long, the mushroomed tail 26 will expand outwardly far enough to seize on the inner surface 71 of the preload sleeve 28. Optional stripping means can be provided to overcome this seizing by including a guide ring 72 which threadably engages withthe inner end of the preload cylinder 14 and which includes sealing means such as an O-ring 74 to form a seal against the outer surface 76 of the preload sleeve 28. This forms a sealed chamber 78 between the preload piston 32 and the guide ring 72. A port 80 can then be used to conduct pressurized medium into the chamber 78 to force the preload piston 32 back against the constant pressure in the chamber 46 until the piston 32 engages the inner surface 82 of the plug 40.

If the mushroomed tail 26 of an overly long Rivbolt 20 is seized to the preload sleeve 28, the action of pressurizing the chamber 78 causes the sleeve 28 to retract over the post anvil 38 which drives the mushroomed tail 26 out of the sleeve 28 and frees the tool 10 for its next cycle of operation. The stripping means can also be energized to increase the gap across the tool 10 when positioning the tool in a cramped location.

FIG. 5 shows suitable means by which it is possible to provide the preload pressure, the squeeze pressure and the stripping pressure to the tool 10. The constant pressure preload can be provided by any suitable means such as a gas-fluid accumulator 84 which is shown connected to the preload chamber 46 through port 48 by a line 86. The accumulator 84, as shown, has a gas chamber and a fluid chamber which are separated by a flexible diaphragm 87. The preload pressure on the fluid in the chamber 46 is set by charging the accumulator 84 to a predetermined pressure with gas, which pressure transfers to the fluid therein, thereby providing fluid at relatively constant pressure to the chamber 46. Typical design pressures applied to the chamber 46 are in the range of 800 to 1,000 psi. The volume of the gas chamber should be relatively large with respect to the volume of the preload chamber 46 so that movements of the sleeve piston 32 which change the volume of the chamber 46, do not appreciably change the volume of the gas chamber and hence the preload pressure extablished thereby.

High pressure, in the design range of 10,000 psi supplied to chamber can be provided by means such as an air-oil pump 88, which are connected to port 62 by a high pressure line 90. The air-oil pump 88 can be powered with relatively low pressure air in the range of 90 psi through line 92. In response to the actuation of a valve 94 thereon, the pump 88 pumps high pressure oil through the line 90 and the port 62 to the chamber 60. The'valve 94 as shown, includes a controlling pedal 95. Tilting of the pedal 95 in the direction shown by arrow 96 causes an increasing flow of pressurized oil to be pumped to the chamber 60 while tilting in the direction of arrow 98 causes dumping of the pressurized fluid. The pedal 95 is usually spring-loaded in the neutral position shown to maintain any pressure in line 90 and chamber 60, thereby maintaining the squeeze piston 52 at whatever position it is in. A pressure gauge 100 to indicate the pressure in line 90 and the squeeze chamber 60 can be included although it is normally not required since air-oil pumps such as pump 88 are easily controlled by conventional regulator or relief means 101 to provide a predetermined maximum pressure.

When stripping means are employed in the present invention, a second air-oil pump 102 similar to the pump 88 but restrained from such high pressure outputs is connected to the port 80 and hence the chamber 78 by means of an output line 104. The pump 102 is powered by low pressure air fed through line 106 from the source of low pressure air whose output is also fed to the pump 88 by line 92. The valve 108 on the pump 102 is similar to valve 94 but it is interconnected with the valve 94 to only apply stripping pressure to the chamber 78 when the pedal 95 of the valve 94 is moved toward the extreme in the dump direction as shown by arrow 98. This causes automatic stripping of the sleeve 28 from the Rivbolt 20 whenever the squeeze cylinder 60 is dumped to release the pressure therein. Although air powered pumps and accumulators are shown as the pressure generating means for the tool 11, any other device which can essentially duplicate their actions can be substituted as desired.

Thus there has been shown and described novel rivet squeezers which fulfill all of the objects and advantages sought therefor. Many changes, alterations, modifications and other uses and applications of the subject rivet squeezers will become apparent to those skilled in the art after considering this specification and the accompanying drawings. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.

What is claimed is:

l. A tool for preloading and upsetting a rivet in a structure, the rivet having a rivet head on one end, a tail on the other end and a collar which fits over the tail once the rivet has been inserted through the structure, said tool including:

a body portion having first and second ends;

an upset anvil connected to said first body portion end adapted to abut one end of the rivet;

preload means for establishing a relatively constant force against the structure connected to said first body portion end, said preload means including a first chamber, a piston forming one end of said first chamber, means to maintain a relatively constant pressure within said first chamber to act on said piston, and a preload sleeve connected to said piston and formed around said upset anvil, said preload sleeve being adapted to transmit force to the structure and including a ring shaped abutment surface facing said rivet drive member to abut the structure and an annular groove adjacent to the inner edge of said ring shaped abutment surface for engaging the collar;

a rivet drive member slidably connected to said second end of said body portion facing said upset anvil; and

means to controllably advance said rivet drive member toward said upset anvil and retract said rivet drive member from said upset anvil whereby said rivet drive member contacts one end of the rivet and drives the rivet and the structure back against said preload means so that the other end of the rivet is driven back into abutment with the upset anvil and is upset thereby.

2. The tool defined in claim 1 wherein said means to maintain a relatively constant pressure within said first chamber include:

a gas-fluid accumulator having a volume of pressurized gas substantially greater than the maximum volume of said first chamber; and

a line connecting pressurized fluid from said gas-fluid accumulator to said first chamber.

3. The tool defined in claim 1 wherein said preload sleeve includes:

an inner contoured surface adjacent to said annular groove, said inner contoured surface restraining the upset tail of the rivet to a predetermined shape.

4. The tool defined in claim 1 wherein said means to controllably advance and retract said rivet drive member include:

a hydraulic cylinder;

a drive piston in said hydraulic cylinder connected to said rivet drive member;

means for controllably applying pressurized fluid to said cylinder to act on said drive piston to extend said rivet drive member; and

a spring operatively connected to retract said rivet drive member when said pressurized fluid is being released from said cylinder.

5. A tool for preloading and upsetting a rivet in a structure, said tool including:

a body portion having first and second ends;

preload means for establishing a relatively constant force against the structure connected to said first body portion end;

means to controllably advance said rivet drive member toward said upset anvil and retract said rivet drive member from said upset anvil whereby said rivet drive member contacts one end of the rivet and drives the rivet and the structure back against said preload means so that the other end of the rivet is driven back into abutment with the upset anvil and is upset thereby, said means to controllably advance and retract said rivet drive member including a hydraulic cylinder including a cylinder housing connected on one side thereof to said body portion, said cylinder housing having a greater cross-sectional thickness at its side adjacent said body portion than its cross-sectional thickness at the side opposite therefrom, a drive piston in said hydraulic cylinder connected to said rivet drive member, means for controllably applying pressurized fluid to said cylinder to act on said drive piston to extend said rivet drive member, and a spring operatively connected to retract said rivet drive member when said pressurized fluid is being released from said cylinder.

6. The tool defined in claim 5 wherein said cylinder housing includes: abutment means for engaging said drive piston to restrict the distance said drive piston can move in the rivet drive member extending direction, said spring being a compression spring positioned between said drive piston and said cylinder housing, whereby said abutment means prevent the movement of said drive piston to a position in which said compression spring is compressed beyond a predetermined point.

7. The tool defined in claim 5 wherein said preload means include:

a preload chamber connected to said first body portion end;

a gas-fluid accumulator having a volume of pressurized gas substantially greater than the maximum volume of said preload chamber; and

a line connecting pressurized fluid from said gas-fluid accumulator to said preload chamber.

Claims

1. A tool for preloading and upsetting a rivet in a structure, the rivet having a rivet head on one end, a tail on the other end and a collar which fits over the tail once the rivet has been inserted through the structure, said tool including: a body portion having first and second ends; an upset anvil connected to said first body portion end adapted to abut one end of the rivet; preload means for establishing a relatively constant force against the structure connected to said first body portion end, said preload means including a first chamber, a piston forming one end of said first chamber, means to maintain a relatively constant pressure within said first chamber to act on said piston, and a preload sleeve connected to said piston and formed around said upset anvil, said preload sleeve being adapted to transmit force to the structure and including a ring shaped abutment surface facing said rivet drive member to abut the structure and an annular groove adjacent to the inner edge of said ring shaped abutment surface for engaging the collar; a rivet drive member slidably connected to said second end of said body portion facing said upset anvil; and means to controllably advance said rivet drive member toward said upset anvil and retract said rivet drive member from said upset anvil whereby said rivet drive member contacts one end of the rivet and drives the rivet and the structure back against said preload means so that the other end of the rivet is driven back into abutment with the upset anvil and is upset thereby.

2. The tool defined in claim 1 wherein said means to maintain a relatively constant pressure within said first chamber include: a gas-fluid accumulator having a volume of pressurized gas substantially greater than the maximum volume of said first chamber; and a line connecting pressurized fluid from said gas-fluid accumulator to said first chamber.

3. The tool defined in claim 1 wherein said preload sleeve includes: an inner contoured surface adjacent to said annular groove, said inner contoured surface restraining the upset tail of the rivet to a predetermined shape.

4. The tool defined in claim 1 wherein said means to controllably advance and retract said rivet drive member include: a hydraulic cylinder; a drive piston in said hydraulic cylinder connected to said rivet drive member; means for controllably applying pressurized fluid to said cylinder to act on said drive piston to extend said rivet drive member; and a spring operatively connected to retract said rivet drive member when said pressurized fluid is being released from said cylinder.

5. A tool for preloading and upsetting a rivet in a structure, said tool including: a body portion having first and second ends; preload means for establishing a relatively constant force against the structure conNected to said first body portion end; an upset anvil connected to said first body portion end adapted to abut one end of the rivet; a rivet drive member slidably connected to said second end of said body portion facing said upset anvil; and means to controllably advance said rivet drive member toward said upset anvil and retract said rivet drive member from said upset anvil whereby said rivet drive member contacts one end of the rivet and drives the rivet and the structure back against said preload means so that the other end of the rivet is driven back into abutment with the upset anvil and is upset thereby, said means to controllably advance and retract said rivet drive member including a hydraulic cylinder including a cylinder housing connected on one side thereof to said body portion, said cylinder housing having a greater cross-sectional thickness at its side adjacent said body portion than its cross-sectional thickness at the side opposite therefrom, a drive piston in said hydraulic cylinder connected to said rivet drive member, means for controllably applying pressurized fluid to said cylinder to act on said drive piston to extend said rivet drive member, and a spring operatively connected to retract said rivet drive member when said pressurized fluid is being released from said cylinder.

7. The tool defined in claim 5 wherein said preload means include: a preload chamber connected to said first body portion end; a gas-fluid accumulator having a volume of pressurized gas substantially greater than the maximum volume of said preload chamber; and a line connecting pressurized fluid from said gas-fluid accumulator to said preload chamber.