US3466010A - Apparatus to compensate for off-center loading on a material handling device having inflatable fluid pads - Google Patents

Description

Sept. 9, 1969 R. R. JUNG 3,466,010

APPARATUS T0 COMPENSATE FOR OFF-CENTER LOADING ON A MATERIAL HANDLING DEVICE HAVING INFLATABLE FLUID PADS Filed Dec. 16, 1966 4 Sheets-Sheet L9 i! E? E s 3 u.

INVENTOR RICHARD R. JUNG ATTOR NEY Sept. 9, 1969 R. R. JUNG 3,466,010

APPARATUS T0 COMPENSATE FOR OFF-CENTER LOADING ON A MATERIAL HANDLING DEVICE HAVING INFLATABLE FLUID PADS Filed Dec. 16. 1966 4 Sheets-Sheet 5 FIG. 5

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INVENTOR RICHARD R. JUNG ATTORNEY R- R. JUNG Sept. 9, 1969 APPARATUS T0 COMPENSATE FOR OFF-CENTER LOADING ON A MATERIAL HANDLING DEVICE HAVING INFLATABLE FLUID PADS Filed Dec. 16. 1966 4 Sheets-Sheet 4 FIG. 7

I N VEN TOR RIQHARD R. JUNG ATTORNEY United States Patent APPARATUS T0 COMPENSATE FOR OFF-CENTER LOADING ON A MATERIAL HANDLING DE- VICE HAVING INFLATABLE FLUID PADS Richard R. Jung, Battle Creek, Mich., assignor to Clark Equipment Company, a corporation of Delaware Filed Dec. 16, 1966, Ser. No. 602,269 Int. Cl. B66f 3/24; B60v 1/00; B25j 3/00 US. Cl. 25493 8 Claims ABSTRACT OF THE DISCLOSURE Apparatus to compensate for off-center loading of a device having a load supporting platform supported by three or more inflatable fluid pads. The apparatus is responsive to fluid pressure in the individual pads and operates to maintain the fluid flow to each pad directly proportional to the loading on the respective pad.

Material handling devices that utilize a plurality of inflatable air pads characteristically are unstable when the loading of the device is off center to any appreciable extent. A further drawback of such devices is that under ofl-center loading conditions the pad which is carrying the most load also requires the highest fluid pressure in the pad and the greatest amount of fluid flow; however, unless some compensating apparatus is built into the device the pad which has the greatest demand for pressurized fluid flow will receive the least fluid flow since fluids take the line of least resistance. If this occurs the pad with the greatest proportion of the load thereon may very well loose the fluid film between the pad and the supporting surface with the result that the pad will no longer slide over the supporting surface. Therefore, it is a principal object of my invention to provide apparatus to compensate for off-center loading of a device having inflatable fluid pads so that the pad carrying the greatest part of the load will have the greatest volume of fluid flowing to it.

In carrying out my invention in one embodiment I provide a load supporting platform to which four inflatable fluid pads are connected. Fluid is supplied to a plenum chamber to which four separate throttling valves are connected and which in turn are connected to individual pads. Each throttling valve has a pair of piston and cylinder assemblies connected to it so that actuation of the assemblies causes actuation of the associated valve. The cylinder of each assembly is divided into two adjacent chambers, a pair of corresponding chambers of each pair of assemblies being connected to the interior of a single pad. The chamber adjacent one of the corresponding chambers is connected to the interior of a pad adjacent the one pad and the other chamber adjacent the corresponding chambers is connected to the interior of another pad adjacent the one pad.

In another embodiment of my invention there are only three inflatable fluid pads connected to a supporting platform.

The above and other objects, features and advantages of my invention will be readily understood by persons skilled in the art when the following detailed description is taken in conjunction with the accompanying drawing wherein:

FIGURE 1 is a plan view of a material handling device embodying my invention,

FIGURE 2 is a side elevation of the device shown in FIG. 1,

FIGURE 3 shows schematically part of the fluid circuitry of my invention,

FIGURE 4 is a fragmentary cross section taken along line 4-4 of FIG. 2,

FIGURE 5 is a section taken along line 55 of FIG. 6 of a throttling valve and the assemblies connected thereto,

FIGURE 6 is a section taken along the line 66 of FIG. 5,

FIGURE 7 is a plan view of a device having three inflatable fluid pads and embodying my invention, and

FIGURE 8 shows schematically part of the fluid circuitry of the device shown in FIG. 7.

Referring now to FIGS. 1 through 4, the reference numeral 10 denotes generally a material handling device having a load-supporting platform 12 to which four inflatable fluid pads 14, 16, 18 and 20 are connected. At this point it should be mentioned that while the pads normally are supplied with pressurized air that various other fluids, for example, an inert gas, such as nitrogen, or even a liquid, such as water, also would work. The pads are disposed substantially rectangularly to each other. That is, the centers of the individual pads fall at what would be the corners of a rectangle. In the context of the rectangular arrangement of four pads, when a pad is referred to as adjacent another pad the relationship of pads 16 or 20 to pad 14, for example, is intended, rather than the relationship of pad 18 to 14, for example, which would be considered diagonally opposite each other.

Located adjacent one end of platform 12 is an internal combustion engine 22 which is supplied with fuel from a fuel tank 24. Engine 22 is connected to a compressor 26 by means of a suitable coupling 28. Engine 22 serves to drive compressor 26 which preferably draws air from the atmosphere, compressing it and then delivering the compressed air via a conduit 30 to a transverse conduit or plenum chamber 32. Connecting with plenum chamber 32 through throttling valves as will be explained in detail later is a series of individual ducts 34, 36, 38 and 40 which communicate with pads 20, 14, 18 and 16, respectively.

Device 10 is described as a self-contained unit, but it should be understood that my invention would work equally well where conduit 30 .is connectible to an outside source of pressurized air such as is generally available in industrial buildings.

Referring now specifically to FIG. 4, pad 20 will be described in detail. The other pads are identical with pad 20, and so this description will apply equally to them. Pad 20 includes a backing plate 42 to which a flexible bag 44 is sealingly connected at the outer periphery 46 of plate 42. Bag 44 also is connected to the center of plate 42 by means of a connector 48. Pad 20 preferably is connected to platform 12 by means of a suitable fastener 50, such as a screw, which passes through connector 48. Pad 20 preferably is connected to platform 12 by means of a suitable fastener 50, such as a screw, which passes through connector 48. Located in bag 44 adjacent the center thereof is a plurality of openings 52 which communicate with a chamber 54 defined by plate 42 and bag 44 and which is sometimes referred to hereinafter as the interior of the pad. When pad 20 is supplied with pressurized fluid the interior or chamber 54 is filled with pressurized fluid until bag 44 assumes substantially the shape shown in FIG. 4. After chamber 54 is filled with pressurized fluid, and as long as fluid continues to be supplied thereto, there is a flow of fluid through chamber 54 and out through openings 52 to the underside of the pad. The fluid then flows radially outwardly past the underside of the pad forming a film between the pad and the supporting surface.

So long as the load being carried by device 10 is substantially centered it will operate satisfactorily; however, when the load is located off center, the loading on one or more of the pads will be increased while the loading on one or more of the other pads will be decreased. As a result, the fluid flow to the pad having the decreased loading will increase and the fluid flow to the pad having the 3 increased loading will decrease. if the off-center loading is suflicient, the result will be that the air film beneath the pad having the increased loading will be eliminated because of insufficient fluid flow with the result that the device will no longer slide easily across the supporting surface and in efl ect become inoperative.

My invention is directed to this problem and involves apparatus to supply pressurized fluid to the inflatable pads and compensate for off-center loading on the pads so that the fluid flow to the pads is directly proportional to the loading on the pads, rather than inversely proportional as would otherwise be the case since fluids tend to follow the path of least resistance. The apparatus for accomplishing this includes throttling valves 56, 58, 60 and 62 which connect ducts 34, 36, 38 and 40, respectively, to plenum chamber 32. Connected to valve 56 for actuating it is a pair of piston and cylinder assemblies 64 and 66 which serve to actuate the valve as will be explained in more detail hereinafter. Similarly, a pair of piston and cylinder assemblies 68 and 70 are connected to valve 58, a pair of piston and cylinder assemblies 72 and 74 are connected to valve 60 and a pair of piston and cylinder assemblies 76 and 78 are connected to valve 62.

Referring now specifically to FIGS. and 6, the construction of valve 56 and the construction of assemblies 64 and 66 and the way in which they are connected to valve 56 will be described in detail. Since all of the valves and assemblies and the manner in which they are connected are identical it is felt to be necessary only to describe valve 56 and assemblies 64 and 66 in detail. Valve 56 includes an elongated hollow body 80, the ends of which are closed by end plates 82 held in sealing relation with body 80 by means of nut and bolt assemblies 84 located at the corners of plates 82. Extending outwardly from opposite sides of body 80 is a pair of short tubes 86 and 88 which function as inlet and outlet ports, respectively. Extending longitudinally of body 80 and through end plates 82 is an operating rod 90 to which a block 92 is connected by means of a pin 94. Connected to block 92 by any suitable means, such as welding, is a movable member or blade 96 which can be moved from the position shown in FIGS. 5 and 6 toward the right, as viewed in FIGS. 5 and 6, to a position substantially blocking the opening from body 80 to outlet port 88. Thus, when member 96 is in the position shown in FIGS. 5 and 6 there is no restriction of fluid flowing through valve 56, but as member 96 is moved toward the right there is an increasing resistance to fluid flowing through the valve until member 96 reaches its rightmost position at which time the resistance to fluid flow will be at a maximum. Also connected to rod 90 is a pair of lock nuts 98 which serve to limit the rightward movement of member 96 and a bifurcated member or clevis 100 to which a cross arm 102 is pivotally connected by means of a pin 104.

As was pointed out earlier each of the valves is connected to plenum chamber 32 and ducts 34, 36, 38 and 40. Thus, valve 56, for example, has pressurized fluid supplied to inlet port 86 from plenum chamber 32, the fluid flowing through the valve and being supplied to duct 34 via outlet port 88. The other valves are similarly connected.

Assembly 64 includes a cylinder 106 disposed between end plates 82. Slidably disposed in cylinder 106 is a piston 108 which divides cylinder 106 into a pair of adjacent chambers 110 and 112. Connected to piston 108 is a piston rod 114 which extends outwardly through a pair of closure members 116 which are threadably connected to end plates 82. Located in one of closure members 116 is a port 118 which communicates via a fluid passage 120 with chamber 110 and located in the other closure member is a port 122 which communicates via a fluid passage 124 with chamber 112. Piston rod 114 has a pin 126 connected adjacent one end thereof which engages a slot 128 adjacent one end of cross arm 102. Also a compression spring 130 is carried by piston rod 114 and is disposed between closure member 116, as shown, and a washer 132 which is held in place by a pin 134 that passes through piston rod 114. Spring causes piston 108 and piston rod 114 to be biased to the position shown in FIG. 5.

Assembly 66 is identical to assembly 64 and includes a cylinder 136, a piston 138 slidably disposed in the cylinder and dividing it into adjacent chambers 140 and 142 and a piston rod 144 connected to piston 138 and extending outwardly through a pair of closure members 146. One of closure members 146 has a port 148 therein which communicates via a passage 150 with chamber 140 and the other of closure members 146 has a port 152 therein which communicates via fluid passage 154 with chamber 142. Connected to piston rod 144 is a pin 156 which engages a slot 158 in cross arm 102. Also, a compression spring 160 is carried by piston rod 144 and is disposed between closure member 146, as shown, and a washer 162 which is held in place by a pin 164 which passes through connecting rod 144.

At this point it will be noted that chambers 110 and 140 correspond to each other functionally, and, likewise, chambers 112 and 142 correspond to each other functionally. Further, when the pressure in all of the chambers of assemblies 64 and 66 is equal, movable member 96 of valve 56 will be disposed as shown in FIG. 5 due to the bias of springs 130 and 160. Now, if the pressure communicated to the various chambers is changed so that there is a differential between either or both of the adjacent chambers member 96 will be moved to a flow restricting position if the pressure in either chamber 112 or chamber 142 is greater than the pressure in the adjacent chamber.

As was pointed out hereinabove in the description of assemblies 64 and 66, each assembly includes a pair of adjacent chambers. Thus, assembly 68 includes a pair of chambers 166 and 168, assembly 70 includes a pair of chambers 170 and 172, assembly 72 includes a pair of chambers 174 and 176, assembly 74 includes a pair of chambers 178 and 180, assembly 76 includes a pair of chambers 182 and 184 and assembly 78 includes a pair of chambers 186 and 188.

Referring now specifically to FIG. 3, it will be noted that the chambers of the various valves are connected to the interior of the various inflatable pads so that the pressures in the various chambers are the same as the pressures in the pads to which they are connected. Specifically, corresponding chambers 110 and 140, together with chamber 168 and chamber 176, are in fluid communication with the interior of pads 20 via fluid circuitry 190. Corresponding chambers 166 and 170, together with chambers 142 and 188, are in fluid communication with the interior of pad 14 via fluid circuitry 192. Similarly, corresponding chambers 174 and 178, together with chambers 112 and 184, are in fluid communication with the interior of pad 18 via fluid circuitry 194, and corresponding chambers 182 and 186, together with chambers 172 and 180, are in fluid communication with the interior of pad 16 via fluid circuitry 196.

In order to enable persons skilled in the art to better understand my invention, I will now explain the operation of it. It will be assumed that engine 22 is operating to drive compressor 26 so that pressurized air is being supplied to plenum chamber 32. Further, it will be assumed that device 10 is carrying a load which is centered upon platform 12 so that the weight of the load is distributed equally to the four pads, and that the weight of device 10 is distributed equally to the four pads. Under this condition of operation the fluid pressure in the interior of the pads is equal so that the pressure in each of the chambers of the various assemblies is equal. As a result the springs associated with the various assemblies will bias valves 56, 58, 60 and 62 to non-flow restricting position, as exemplified by valve 56 in FIG. 5.

Now it will be assumed that a load is disposed upon platform 12 in an off-center position so that the portion of the load carried by pad 20 is increased substantially while the portion of the load carried by pad 14 is increased a lesser amount and the portion of the load carried by pad 18 is increased by a still lesser amount, the portion of the load being carried by pad 16 being decreased. In this condition of operation the pressure communicated to chamber 110 and 140 from the interior of pad 20 is greater than the pressure communicated to chambers 112 and 142 from the interiors of pads 18 and 14, respectively, so that valve 56 is maintained in its non-flow restricting position as shown in FIG. 5. At the same time the pressure communicated to chamber 168 from pad 20 will be greater than the pressure communicated to chamber 166 from pad 14 and the pressure communicated to chamber 170 from pad 14 will be greater than the pressure communicated to chamber 172 from pad 16 with the result that valve 58 is actuated to restrict fluid flow to the interior of pad 14 somewhat. Similarly, the pressure in chamber 176 from pad 20 will be greater than the pressure in chamber -174 from pad 18 and the pressure in chamber 178 from pad 18 will be greater than the pressure in chamber 180 from pad 16 so that valve 60 will be actuated to retard fluid flowing to the interior of pad 18. However, the retardation will be somewhat more than the retardation of fluid flowing to pad 14 because the pressure differential between chambers 176 and 174 is greater than the pressure differential between chambers 168 and 166 since the loading on pad 14 is greater than the loading on pad 18. As a result valve 60 is actuated to a greater fluid flow retarding position than valve 58. Finally, the fluid pressure in chambers 182 and 186 from pad 16 is decreased while the pressures in chambers 184 and 188 from pads 18 and 14, respectively, is increased, thereby causing both assemblies 76 and 78 to retract actuate valve 62 to a greater fluid flow retarding position than the other three valves. The extent to which any of the throttling valves restricts fluid flow depends upon the decrease in the proportion of the load being carried by the associated pad. Thus, it will be clear from the foregoing that the fluid flowing to each pad is in direct proportion to the proportion of the load being carried by the respective pad.

Referring now to FIGS. 7 and 8, there is shown a material handling device 198 which embodies a modification of my invention. Device 198 is similar to device 10, and so like reference numerals will be used for like parts and reference is made to the description hereinabove for a description of such parts. Device 198 differs principally from device in that only three inflatable air pads 200, 202 and 204 are connected to the underside of platform 12, preferably at the vertices of an equilateral triangle. Consequently, only three ducts 206, 208 and 210 connect pads 200, 204 and 202, respectively, with plenum chamber 32 via throttling valves 212, 214 and 216, respectively. These valves regulate fluid flow to the respective pads in the same manner as has been pointed out hereinabove in regard to device 10. Further, valves 212, 214 and 216 are identical to valve 56, and so reference should be made to FIGS. 5 and 6 and the description of valve 56 hereinabove for a detailed description.

Associated with valve 212 is a pair of piston and cylinder assemblies 218 and 220. Similarly, a pair of piston and cylinder assemblies 222 and 224 are associated with .valve 214 and a pair of piston and cylinder assemblies 216 and 228 are associated with valve 216. These assemblies are all constructed and connected to their respective valves in the same manner as was described hereinabove in detail in regard to assemblies 64 and 66. Also, assembly 218 is divided into a pair of adjacent chambers 230 and 232, assembly 220 is divided into a pair of adjacent chambers 234 and 236, assembly 222 is divided into a pair of adjacent chambers 238 and 240, assembly 224 is divided into a pair of adjacent chambers 242 and 244, assembly 226 is divided into a pair of adjacent chambers 246 and 248 and assembly 228 is divided into a pair of adjacent chambers 250 and 252. These chambers are all connected by fluid circuitry to the various pads so that pressure from the various pads is communicated to the respective chambers. Specifically, corresponding chambers 230 and 234, together with chambers 240 and 248, are in fluid communication with the interior of pad 200 via fluid circuitry 254; corresponding chambers 238 and 242, together with chambers 232 and 252, are in fluid communication with the interior of pad 204 via fluid circuitry 256 and corresponding chambers 246 and 250, together with chambers 236 and 244, are in fluid communication with the interior of pad 202 via fluid circuitry 258.

The apparatus for compensating for off-center loading of device 198 functions in the same manner as the apparatus used with device 10, and so the operation of device 198 is felt to be obvious in view of the description of operation of device 10.

The above-detailed description of two embodiments of my invention is intended to be illustrative only, and so should not be considered as limiting the scope of my in vention. Various modifications and changes which are Within the scope and spirit of my invention will be apparent to persons skilled in the art. For example, my invention is applicable to devices having more than four inflatable pads. Further, the throttling valves can be arranged so that they are normally in a flow-restricting position rather than in a non-flow restricting position. Consequently, the limits of my invention should be determined from the following appended claims.

I claim:

1. For use with a material handling device having a load supporting platform and at least three inflatable fluid pads connected to the platform, apparatus for supplying fluid to the pads and compensating for off-center loading on the platform comprising means connectible to a source of pressurized fluid, a plenum chamber connected to the said means, a separate fluid distribution duct connected to each pad and the said plenum chamber for supplying pressurized fluid to the pads, a throttling valve disposed in each of the said ducts, and means responsive to changes in fluid pressure in the pads for actuating the said valves so that the amount of fluid flowing to each pad is directly proportional to the load being supported by the pad.

2. Apparatus'as set forth in claim 1 wherein the said actuating means includes a separate pair of piston and cylinder assemblies operatively connected to each of the said valves, each cylinder being divided into two chambers by the respective piston and each chamber being connected to the interior of one of the pads so that the pressure in the pad is communicated to the chamber.

3. Apparatus as set forth in claim 1 wherein the said actuating means includes a separate pair of piston and cylinder assemblies operatively connected to each of the said valves, the cylinder of each assembly being divided into two adjacent chambers by the respective piston, two corresponding ones of the said chambers being connected to the interior of one of the pads, the chamber adjacent one of the corresponding chambers being connected to the interior of the first pad adjacent the one pad and the chamber adjacent the other of the said corresponding chambers being connected to the interior of a second pad adjacent the one pad.

4. Apparatus as set forth in claim 1 wherein each valve includes. a first member and a second member movable relative to the said first member between a flow restricting position and a non-flow restricting position and the said actuating means includes a separate pair of piston and cylinder assemblies connected to one of the said members of each valve, a separate cross arm pivotally connected to the other member of each valve and slidably and pivotally connected to the associated pair of assemblies and separate spring means tending to bias the said second member of each valve to one of the said positions.

5. Apparatus as set forth in claim 4 wherein the cylinder of each assembly is divided into two adjacent chambers by the respective piston and each of the said pairs of assemblies has a pair of corresponding chambers connected to the interior of one of the pads, the chamber adjacent one of the said corresponding chambers connected to the interior of a first pad adjacent the one pad and the chamber adjacent the other of the said corresponding chambers connected to the interior of a second pad adjacent the one pad.

6. Apparatus as set forth in claim 3 wherein each valve is actuatable between a non-flow restricting position and a flow-restricting position and the said actuating means includes spring means operatively connected to each valve to bias the respective valve toward the said nonflow restricting position.

7. For use with a material handling device having a load supporting platform and first, second and third inflatable fluid pads connected to the platform, apparatus for supplying fluid to the pads and compensating for offcenter loading on the platform comprising means connectible to a source of pressurized fluid, a plenum chamber connected to the said means, first, second and third throttling valves connecting the said plenum chamber with the first, second and third pads, respectively, and means responsive to changes in fluid pressure in the pads for actuating the said valves so that the amount of fluid flowing to each pad is directly proportional to the load being supported by the pad, the said actuating means including first and second piston and cylinder assemblies operatively connected to the said first valve for actuating the said first valve, the cylinders of the said first and second assemblies being divided into first and second adjacent chambers by the respective pistons, the said first chambers of the said first and second assemblies corresponding to each other functionally and being connected to the interior of the first pad, the said second chamber of the first assembly being connected to the interior of the said third pad and the said second chamber of the said second assembly being connected to the interior of the said second pad, third and fourth piston and cylinder assemblies operatively connected to the said second valve for actuating the said second valve, the cylinders of the said third and fourth assemblies being divided into first and second adjacent chambers by the respective pistons, the said first chambers of the said first, third and fourth assemblies corresponding to each other functionally and being connected to the interior of the said second pad, the said second chamber of the said third assembly being connected to the interior of the first pad and the said second chamber of the said fourth assembly being connected to the interior of the third pad, and fifth and sixth piston and cylinder assemblies operatively connected to the said third valve for actuating the said third valve, the cylinders of the said fifth and sixth assemblies being divided into first and second adjacent chambers by the respective pistons, the said first chambers of the said fifth and sixth assemblies corresponding to each other functionally and being connected to the interior of the third pad, the said second chamber of the said fifth assem bly being connected to the interior of the second pad and the said second chamber of the said sixth assembly being connected to the interior of the said first'pad.

8. A material handling device comprising a load-supporting platform, first and second, third and fourth infiatable fluid pads connected to the platform in substantially rectangular relationship relative to each other, the said first pad being adjacent the said second and fourth pads, and apparatus for supplying pressurized fluid to the pads and compensating for off-center loading on the platform, the said apparatus including means connectible to a source of pressurized fluid, a plenum chamber connected to the said means, first, second, third and fourth throttling valves connecting the said plenum chamber with the first, second, third and fourth pads, respectively, each of the said valves being actuatable between a nonfiow restricting position and a flow-restricting position, and means responsive to changes in fluid pressure in the pads for actuating the said valves so that the amount of fluid flowing to each pad is directly proportional to the load being supported by the pad, the said actuating means including first and second piston and cylinder assemblies operatively connected to the said first valve for actuating the said first valve, the cylinders of the said first and second assemblies being divided into first and second adjacent chambers by the respective pistons, the said first chambers of the said first and second assemblies corresponding to each other functionally and being connected to the interior of the first pad, the second chamber of the said first assembly being connected to the interior of the said fourth pad and the said second chamber of the said second assembly being connected to the interior of the said second pad, third and fourth piston and cylinder assemblies operatively connected to the said third and fourth assemblies being divided into first and second adjacent chambers by the respective pistons, the said first chambers of the said third and fourth assemblies corresponding to each other functionally and being connected to the interior of the said second pad, the said second chamber of the said third assembly being connected to the interior of the said first pad and the said second chamber of the said fourth assembly being connected to the interior of the said third pad, fifth and sixth piston and cylinder assemblies operatively connected to the said third valve for actuating the said third valve, the cylinders of the said fifth and sixth assemblies being divided into first and second adjacent chambers by the said respective pistons, the said first chambers of the said fifth and sixth assemblies corresponding to each other functionally and being connected to the inteior of the said third pad, the said second chamber of the said fifth assembly being connected to the interior of the said second pad and the said second chamber of the said sixth assembly being connected to the interior of the said fourth pad, seventh and eighth piston and cylinder assemblies operatively connected to the said fourth valve for actuating the said fourth valve, the cylinders of the said seventh and eighth assemblies being divided into first and second adjacent chambers by the respective pistons, the said first chambers of the said seventh and eighth assemblies corresponding to each other functionally and being connected to the interior of the said fourth pad, the said second chamber of the said seventh assembly being connected to the interior of the said third pad and the said second chamber of the said eighth assembly being connected to the interior of the said first pad, and separate resilient means operatively connected to each valve for biasing the valve to one of the said positions.

References Cited UNITED STATES PATENTS 542,702 7/1895 Christensen 251-62 X 616,886 1/1899 Brinkmann 25131 3,038,692 6/ 1962 Holmes 251-31 X 3,233,692 2/1966 Guienne 180-121 X 3,251,432 5/1966 Fischer et al 180-121 3,332,508 7/1967 Bertin et al. 180-121 OTHELL M. SIMPSON, Primary Examiner US. Cl. X.R.

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