US2588588A - Pump and motor hydraulic system and control valve therefor - Google Patents

Description

March 11, 1952 w. E. SWENSON ETAL 2,588,588

PUMP AND MOTOR HYDRAULIC SYSTEM AND CONTROL VALVE THEREFOR Filed Dec. 21, 1946 4 Sheets-Sheet l FIE J.

swam wow HENRY 'AfHALG/PEN W/LLZAM E. SWENSON March 11, 1952 w. E. SWENSON EIAL 2,588,583

I PUMP AND MOTOR HYDRAULIC SYSTEM AND CONTROL VALVE THEREFOR Filed Dec. 21, 1946 4 Sheets-Sheet 2 l i I Ki 29 4/v I (f 37 l L i 30 gwue/wtow HEN/H ,4. HALGREN W/L LIAM E. SWE/VSON CW? Ma /L March 11, 1952 w. E. SWENSON ETAL 2,588,588

PUMP AND MOTOR HYDRAULIC SYSTEM AND CONTROL VALVE THEREFOR Filed Dec. 21, 1946 4 Sheets-Sheet 3 3 FIE E HENRY A. HALGREN W/LL/AM E. SWENSON March 11, 1952 w. E. SWENSON ET AL 2,588,588

PUMP AND MOTOR HYDRAULIC SYSTEM AND CONTROL VALVE THEREFOR 4 Sheets-Sheet 4 Filed Dec. 21, 1946 HEN/P) A. HALGREN W/LL/AM 5. SWENSON @Hiomwg Patented Mar. 11, 1952 PUMP AND Moron HYD AUL C YSTEM AND CONTROL VALVE THEREFOR William E. Swensoii, St. Paul, and Henry ArHalgi en, Minneapolis, Minn., assig nors, by mesne assignments, to Minneapolis-Moline Company,

Hopkins;.Minn., a corporation of Minnesota Application December 21, 1946, Serial N 717,790

12' Claims; (01. 6052) This invention relates to-improvements in hydraulic' control valves.

The valve is particularly adapted to control the flow of fluid under pressure'to and froma lift cylinder or jack in order to control the operation thereof, and is employed in connection. with a complete hydraulicsystem including a pump for supplying fluid under pressure, and a reservoir for supplying fluid to the suction side of the pump and for receiving displaced fluid from the jack. As one example of the use of suchsystems may be cited the usualt'ractor-implement combination wherein the implement is raised and lowered by a power lift jack which is fluid actuated in upand down directions to: correspondingly adjust the implement.

The primary object of the present invention is to provide a control valve which may be manually positioned to cause the flow of fluid to either end of the jack cylinder while allowing the re-' turn flow of fluid, displaced by movement of the jack, to the reservoir, andwhichembodies means operating automatically and responsive to the buildup of pressure as the jack reaches its. limit of travel in: either direction to restore the valve to neutral position, while leaving the jack. at its adjusted position. The possibility of damage to the parts of the system by' the increased fluid pressure as the jack reaches its limit of. travel: is thus prevented, wholly without attention on: the part of the operator.

Another-object is to provide. a valve of. this nature in a comparatively simple form, which is; desirable and positive in opera'tiomand. has other advantages in operation as will appear hereinafter.

These and other more detailed and specific objects will be disclosed in the course of the following speciflcation, reference being. had to the: accompanying drawings, in which- Fig. 1 is a diagrammatical view showing a complete hydraulic jack or liftsystem with which is embodied our improved control valve.

Fig; 2 isan enlarged sectional view along the line 2--2' in Fig. 1 showing the reservoir and the control valve and parts of the hydraulic lines connecting these parts into the system.

Fig. 3 is a sectional view along the line 3'3 in Fig. 1 showing the control valve in section and in its neutral. or normal. position.

Fig. 4 is aview similar to Fig. 3 but showing a control valve adjusted to a position for lifting the valve adjusted to a position for causing the:

jack to lower.

Fig. 6 is a view similar to Fig. 4 but: illustrating the operation of the valve to-relieve the pressure when the jack: reaches the limit of its movement in one direction. Fig. 7' is" an enlarged and elevational. view taken substantially along. the line l- -l in Fig. 1 and illustrating particularly the openings' by -which fluid is admitted to.- and. taken froimthe and those by which the lines are. connected to.

jack and pump are not here'shown, and. the linesthemselves are shown. only diagrammatically-.- The valve unit D' controls fluid flow through-thelines G, H and I while the line J communicates with the reservoir directly to" supply fluid there from to the suction side of the pump B The lines G- and H lead, respectively, to'theupper and lower ends of the jack A, whilethe line I isthe pressure line by which fluid, placedunder pres sure by and discharged from the-p1i'mp,.is-de1iivered to; the valve.

The valve. D functions tor-normally or inits neutral position, connect the pressure-line- I to the reservoir Cv so that the fluid circulates ireely between pump and reservoir. In one. adjusted position, then,v the valve connects the pressure" line I to the line G leading to the. top of the jack, while simultaneously connecting. line H1 tothe reservoir C, so-that thejack will be lowered; or rather itsplunger A will be pushed/down by the downward pressure of the fluid,and the fluid displaced from the lower end of the jackwill' re'-,

turn to reservoir.- I-n another adjusted. position these connections will be reversedand fluid'will.

flow'into the bottom of I the jack, while displaced fluid-will return from its upper'end' to reservoir,;

' ed on and secured to the inside of'the'reservoir end plate E. The side of the housing mnexti to this plate has fouropenings or ports lr; 1'2; t3 and t4 (see Fig. 7 particularly) which come municate respectively with the lines G, I-RYandf J- through-- the coupling casting. F secured to. r g

outside of the plate. The port l4 passes straight through a lug portion 15 of the valve housing thus connects the suction line J directly with the interior of the reservoir C, while the remaining ports ll, 12 and I3 communicate, as will presently appear, with the interior of the housing.

The housing In is provided with two spaced valve cylinders, designated generally at 16 and I1, which open at one end through an end of the housing, and into which are slidably inserted valve pistons or plungers l8 and IS. The cylinder I6 has three spaced enlargements or chambers 20, 2| and 22, while the.cylinder l1 has corresponding chambers 23, 24 and 25, and the end chambers 26-23 and 22-25 are connected together by passages 26 and 21. The aforesaid port [3 in the housing communicates centrally with the passage 21 so that fluid under pressure from the pump B may enter the several chambers just described except when excluded therefrom by the valve plungers l8 and I9 as will be described. The passage 26 communicates with the reservoir through a duct 28 in the housing which opens at one end into the passage 26 and leads therefrom into the interior of a boss 29 formed on the housing on the side facing the interior of the reservoir. Held by a bolt 30 against this boss is a conventional de-aerator unit 3| (Fig. 1) which may be a wire coil having closely spaced convolutions, so that fluid returning through the de-aerator to the reservoir will be prevented from spurting or gushing and forming air bubbles.

The valve plungers l8 and I9 are reduced in diameter intermediate their ends and thus are spool-shaped, having spindle portions lB -IQ, and the comparatively enlarged ends l8", I8, I9 and 19. The ends of the plungers nicely, slidably fit the cylinders 16 and I1 while the reduced spindle portions of the valves clear the walls of the cylinders, thus in the normal, or neutral, position of the valve (Fig. 3) providing communication between the chambers 26, 2|, and 22 and 23, 24 and 25.

' From what may be termed the outer ends of the valve plungers l8 and I9, or from the enlarged end portions 18 and l9 thereof, and which ends project from the open ends of the respective cylinders 16 and 11, there are extended stems 32 and 33, and the extremities thereof have surrounding, circular gear teeth 34-35. An actuating pinion 36 is positioned between and in mesh with the gear teeth 34-35, and this pinion is secured upon a shaft 31 which, as best shown in Fig. 1, is journaled in the reservoir and carried in bearings 38 affixed to the opposite ends thereof. At one end the shaft 31 projects through the end of the reservoir and a handle 29 is secured to this exposed end of the shaft by means of which handle the shaft may be turned by hand.

Adjacent the pinion 36 a cam 40 is secured to the shaft 31 and this cam has a fiat side 4| as seen in Fig. 2. A pressure arm 42 is pivotally mounted at one end upon a pin 43 carried by the reservoir end plate E, and traverses the cam, the free end of the arm having attached thereto a retraetile coil spring 44 stretched to a lug 45 on the valve housing 16. The arm is thus spring biased toward the cam and is caused to normally bear on the flat surface 4| thereof in such manner as to yieldably resist turning movement of the cam, shaft 31 and pinion 36. The cam is so positioned that, when thus engaged by the arm 42, the parts will be centered with the valve plungers l8 and [9 in the neutral positions of Fig. 3 as will be readily understood.

The inside ends l8-l9 of the valve plungers extend into long enlargements 46-41 of the cylinders 16-11, the opposite ends of which are closed by plugs 48-49 having small relief openings 56-5]. These enlargements 46-41 form back pressure chambers into which fluid may flow under control of valve sleeves 52-53 positioned in the respective valve plungers 16-19. For the accommodation of the valve sleeves 52-53 the valve plungers have axial recesses 54-55 which extend into the spindle portions Mi -l9 and open out into the chambers 46-41. The valve sleeves 52-53 slidably fit into these recesses and slide also upon long studs 56-51 screwed into the valve plungers and projecting therefrom out into the chambers 46-41. The valve sleeves 52-53, due to their sleeve-shape, have axial bores as clearly shown to slide on these studs. The recesses 54-55 have enlarged outer ends forming annular shoulders 58-59 against which may seat enlarged rim or flange portions 66-61 of the valve sleeves. The valves are yieldably held in such positions by expansion coil springs 62-63, coiled around the studs 56-51 and braced between the valves and nuts 64 screwed on the studs. The effective tension of the springs may,

of course, be adjusted by the nuts 64.

The valve sleeves 52-53 are further reduced in diameter, inwardly of the rims -6! to fit correspondingly reduced inner ends of the chambers 54-55 thus forming annular shoulders 65- 66 about the sleeves, and adjacent thereto are ports 61-68 opening out through the walls of the valve plunger spindle portions l8 -l9 into the chambers 22-25 in the valve housing.

In operation, assuming the valve to be connected with the various hydraulic lines G, H, I and J as has been previously described, it will be understood upon reference to Fig. 3 that in the normal position of the valve the fluid under pressure from the pump B will enter through the opening l3 into the passage 21 connecting the chambers 22-25. From these chambers the fluid may flow about the spindle portions of the valve plungers Iii-I9 through the chambers 20-23 from which the fluid may then enter the passage 26 and pass through the duct 28 out into the deaerator 3|. The fluid will thus return freely to the reservoir C and the pump B may operate continuously without interference from back pressure due to the free re-circulation path provided through the valve in its normal position. It will be understood that since there is no pressure differential at this time in the two chambers 2 l-24 the fluid will not flow to the jack A and any re- I turn flow from the jack is, of course, prevented by the use of the usual check valves or lock-out valves (not here shown) generally used between the control valve and the jack in hydraulic systems of this nature.

Assuming that it is now desired to lift the jack plunger A, the actuatin handle 39 is moved in one direction so as to turn the pinion 36 in the direction of the arrow in Fig. 4 whereupon the pinion is caused to move the valve plunger 18 outwardly or to the left and to move the plunger 19 in the opposite direction by an equal amount. Such movement of the valve plungers 18-19 is of course caused by the meshing relation between the pinion 36 and the gear teeth 34-35 upon the plungers and the aforesaid movement is further accompanied by a corresponding movement of the cam 40 to an extent such as to moveits fiat surface ll outof contact with the pressure am 42 when the cam 40 -isthus positioned the sprin biasedpressure of thearm 42 thereon loses its centering effect uponthe cam so that the cam and the pin-ion 36 and the valve as a whole, will stay in such adjusted position when the actuating handle 39' is released. Referring now particularly to Fig. 4 it will be observed therein that-the positions now assumed by the valve plungers l'8 -I'9 will allow the: fluid under' pressure entering the passage 21 toflow into the chamber 2 4 and thence out through the opening [2 into the line H leading to the lower end of the jack A At the sametime the chamber 23 is isolated from this chamber 24 by the valve plunger end I9 and therefore this fluid can not now return through the duct 28* to the'reservoir C A't-the same time the fluid entering from: the pump is prevented from. entering the chamber 2-l by theend: f8 of thevalve plunger [-8 but this chamberfl remains in communication with the chamber leading to the duct 28. Thus as indicated by the direction-"- arrows in- Fig. 4' fluid under pressure may now flow from the pump through the passage 21, chamber 25, chamber 24',- opening [2, and line H tothe lowerend of the jack so that the pressure of the fluid will elevate the jack plunger A. The fluid displaced from the-upper end of the jack by this upward movement of its plunger may then return tothe line G, the opening ll, chambers 24-20, passage 26-, and duct 28 to the reservoir C.

To lower the jackthe actuating handle 39 is moved in the opposite direction from its neutral position whereupon the pinion 36 moves the valve plunger I8- inward or to the right and moves the valve. plunger l9 outward or inthe opposite direction by an equal amount, as seen in Fig. 5. As will-be readily understood the flow conditions just recited are now'reversed and fluid under pressure from the pump may flow from the chamber 22 into the chamber 2t and thence through the opening H and line G to the upper end of t e jack forcing its plunger A downwardly. At the sametime fluid displaced from the lower end'of the jack will return through theline H and opening 12 to the chamber 24 and thence through chamber. 23 and passage 23 into the duct 28le'ad ing back to the reservoir. Here again it will. also beundersto'od that the cam 40 will be moved so thatits flat surface 4 I is out of centering engagement' with the pressure arm 42 so that the valve Will maintain its adjusted. position for lowering the jack without tension.

As has: been previously described our control valve has the advantageous property of: automatically shutting off when the jack reachesthe limit of its travel in either direction so thatno damage to the jack or'otherparts of the system will result from the increased pressurewhich otherwise would result under such conditions. The manner in which this action takes place will now bedescribed and reference is particularly had to Fig. 6 wherein the valve is shown in position for raising the jack. In this position of the valve the fluid entering under pressure for the pump and into the passage 21 enters through the port-'68 in the spindle portion of the valve [9 and bears upon the shoulder 66 of the valve sleeve 53 which normally is positioned immediately adjacentsaid port as seen in Fig. 3. The effective tension of the spring 63 is so adjusted and selected' that under all normal operating pressures, the spring will resist any tendency of the valve sleeve 53 to-be moved by the pressure of the fluid entering through the port 68 However, the ten 6 slon of the sprin 63 is further so selectedithat the increase in fluid pressure in the passage 21 whichresults immediately upon the jack reaching the limit of its upward travel will become cf- 5 fecti-veupon the shoulder 66 to such an extent as to move the valve sleeve 53 outward or to the right as viewed in Fig. 6 againstthe tension of the spring. When the valve sleeve 53 is so moved the fluid is allowed to escape from-the passage 21 through the port 68 and around the small end of the valve sleeve 53 outwardly into the. sbackipressure chamber 41 as clearly indicated by the direction arrows in-Fig. 6. Immediate relief of. this increased fluid pressure is thus permitted and the valve. sleeve 53 will remain in this position. until the fluid. completely fills the back pressure chamber 41.. Itwillbe understood that while the relief opening 5| will allow some fluid to escape from the chamber 41 the fluid will enter faster than it can escape so that the chamber quickly becomes filled with high pressure fluid. As this occurs then the pressure of the fluid in the filled chamber is exerted endwise againstv the valve sleeve 53 and the valve plunger [9 in a direction such as to tend to move the sleeve and plunger to the left as viewed in Fig. 6 or return them. toward their normal positions; Due to the fact that the area exposed at the end of the valve sleeve 53 and plunger [9 is much greater than that of the shoulder 66 the pressure of the fluid in the chamber 47 immediately as it becomes filled will bear with suflicient force endwise upon the valve plunger [9 to move it endwise in thedirection of the arrow in Fig. 6. This back pressure on the valve plunger is then. sufficientto return the plunger toward. its normal or. neutral position whichhas the result: of. rotating thepinion 36 and cam 40 so that the arm 42 becomes effective upon the flat: surface. lil of the cam to assistin centering and. returning the valve to normal position. Of course, as this occurs the. fluid flow to thejack is immediately cut off and pressure conditions through. the system returnv to normalbefore any damage: can occur and wholly without any atten-' 45 tion upon the part of the operator. It will be further understood that the valve will operate in the same manner and responsive to theincreased' -or' back pressure in the same fashion to restore itself to neutral position. when. the jack reaches the limit of its downward travel. The back pressure chambers 46-4T will stand more or less completely filled with fluid normally but due to. the relief openings 50'5l this fluid will normally be under no pressure and will not interfere in any way with the adjustment of the valve.

Itisunderstood that suitable modifications may be made in the structure as disclosed, provided such modificationscomewithin the spirit and scope-of the appended claims. Having now therefore fully illustrated and described our in-- vention, what we claim to be new and desire to protect by Letters Patent is:

1 In a valve of the character described for controlling fluid flow from a pressure source to a hydraulic jack motor, a valve housing having a cylinder and a chamber: at one end thereof, a plunger. slidable in the cylinder and operative upon movement therein toward the chamber to control: fluid. flow to the jack motor, and another valve'arran'gedin slidable association withthe plunger and operative in response to a rise Yin" pressure in thefluid controlled by said plunger above a predetermined point to admit the fluid to the chamber and cause the fluid'to urge the plunger endwise away from the chamber ends of the cylinder.

2. In a valve of the character described for controlling fluid flow from a pressure source to a hydraulic jack motor, a valve housing having a cylinder and a chamber at one end thereof, a plunger slidable in the cylinder and operative upon movement therein endwise toward and into the chamber to admit fluid from the pressure source to the jack motor, and another valve means slidable on the plunger and responsive to pressure of the fluid flowing to the jack motor to admit the fluid to the chamber when the fluid pressure rises above a predetermined point whereby the fluid in the chamber will bear on the end of the valve plunger and move the same back to a position for shutting off the fluid flow to the jack motor.

3. For a hydraulic system including a pump, a reservoir and a jack motor, the improvement comprising, a control valve of the character described comprising a housing having a cylinder and a back pressure chamber at oneend thereof, a valve plunger in the cylinder and operative in its neutral position to recirculate fluid between the pump and reservoir and upon movement toward and into the chamber to connect the pump and jack motor, and another valve means in the valve plunger operative by pressure of fluid flowing to the'jack motor to admit the fluid into the back pressure chamber and return the valve plunger to neutral position when the fluid pressure rises above a predetermined maximum.

4. For use in a hydraulic system including a pump, a reservoir and a jack motor, the improvement which consists of a control valve comprising a housing having a cylinder and a back pressure chamber at one end thereof, a valve plunger in the cylinder and operative upon movement toward and into the chamber to connect the pump and jack motor, the said valve plunger having a recess opening into the chamber, a valve member in said recess operative by pressure of fluid flowing to the jack motor to admit a partof the fluid to the chamber for urging the valve plunger back to neutral position,

and means for preselecting the pressure at which the fluid will so enter the back pressure chamher.

5. For use in a hydraulic system having a pump, a reservoir and a jack motor, the improvement which consists of a control valve comprising a housing having a cylinder and a back pressure chamber at one end thereof, a valve plunger in the cylinder and operative upon movement toward and into the chamber to connect the pump and jack motor, the said valve plunger having a recess opening into the chamber and a port for admitting fluid from the pump into the recess, and a spring biased valve in the recess operative upon a rise of fluid pressure above a predetermined maximum to open and allow the fluid to flow through the port and recess into the chamber and there become effective to urge the valve plunger back to neutral position to halt the further flow of fluid to the jack motor.

6. A valve of the character described for supplying fluid from a pressure source to either end of a jack motor and returning displaced fluid from the other end of the jack motor to a reservoir, comprising a valve housing having two cylinders and each cylinder having therein three spaced enlargements forming a center chamber and two end chambers at opposite sides of the center, the center chambers being connected respectively to opposite ends of the jack motor, the end chambers at one side of center being connected to the pressure source and the other end chambers connected to the reservoir, and oppositely movable valve plungers positioned in the cylinders and operative in oppositely adjusted positions to selectively isolate the center chambers from opposite adjacent end chambers.

7. For use in a hydraulic system having a fluid reservoir, a pump and a fluid motor, a control valve comprising a housing mounted in the reservoir and having a cylinder and a communicating back pressure chamber, said housing having a restricted relief opening from the chamber into the reservoir, and a plunger in the cylinder operative upon movement therein toward and into the chamber to connect the pump and'motor, and a pressure responsive valve operative by pressure of fluid flowing to the motor to open and admit said fluid to the chamber upon a predetermined rise in pressure faster than fluid can escape through the relief opening and accumulate in the chamber to act on the plunger and urge it toward its neutral position.

8. For use in a hydraulic system having a, reservoir, a pump and a double-action fluid motor, a control valve for selectively connecting the pump and reservoir to the opposite ends of the motor and comprising a housing having spaced valve cylinders, a valve plunger mounted in each cylinder, means for axially adjusting the plungers in the cylinders such that movement of the plungers from neutral position will connect the pump to one end of the motor and reservoir to the other end, the said housing having back pressure chambers communicating with the cylinders, and valves axially movable in the plungers and operative to open by a rise in pressure of fluid flowing to the motor to cause the fluid to flow into the chambers, the said chambers having connection to the reservoir.

9. For use in a hydraulic system including a pump, a reservoir and a fluid motor, a control valve for selectively connecting the motor to the pump and reservoir and having a neutral position, comprising a valve housing having a valve cylinder and a connected axially aligned back pressure chamber having a fluid flow restricting opening connected to the reservoir, a valve plunger reciprocable in the cylinder from a neutral position toward and away from the chamber and fluid connections controlled'by the plunger such as to connect the pump to the fluid motor on movement of the plunger into the chamber, the said plunger having an axial recess and a port admitting fluid flowing to the fluid motor into said recess, and a valve member in said recess normally isolating the port from the back pressure chamber but operative by a, predetermined rise in pressure of the fluid in the recess to allow the fluid to travel into the chamber wherein it will bear on the end of the plunger to move it to its neutral position.

10. For use in a hydraulic system including a pump, a reservoir and a fluid motor, a control valve for selectively connecting the motor to the pump and reservoir and havin a neutral position, comprising a valve housing having a valve cylinder and a connected axially aligned back pressure chamber having a fluid flow restricting opening connected to the reservoir, a valve plunger reciprocable in the cylinder from a neutral position toward and away from the chamber and fluid connections controlled by the plunger such as to connect the pump to the fluid motor on movement of the plunger into the chamber, the said plunger having an axial recess and a port admitting fluid flowing to the fluid motor into said recess, a valve member slidable in said recess and. having an enlarged end normally closing the recess at the chamber end of the plunger to isolate the recess from that chamber, and a spring biasing the valve member to this closed position but releasing the valve member for axial movement by a predetermined increase in pressure flowing to the fluid motor whereupon the fluid will escape into the chamber and there act against the plunger to return it to neutral position.

11. For use in a hydraulic system including a pump, a reservoir and a fluid motor, a control valve for selectively connecting the motor to the pump and reservoir and having a neutral position, comprising a valve housing having a valve cylinder and a connected axially aligned back pressure chamber having a fluid flow restricting opening connected to the reservoir, a valve plunger reciprocable in the cylinder from a neutral position toward and away from the chamber and fluid connections controlled by the plunger such as to connect the pump to the fluid motor on movement of the plunger into the chamber, the said plunger having an axial recess and a port admitting fluid flowing to the fluid motor into said recess, a valve member slidable in said recess and having an enlarged end normally closing the recess at the chamber end of the plunger to isolate the recess from that chamber, a stud secured to and extending axially from the plunger into the chamber and slidably supporting the valve member, and a spring on said stud biased against the enlarged end of the valve member to normally close that member but upon a predetermined rise in pressure of the fluid flowing to the fluid motor the valve member is opened and the fluid escapes into the chamber wherein it becomes effective on the ends of the valve plunger and valve member to return the plunger to neutral position.

12. A control valve for selectively connecting a high pressure fluid source to the opposite ends of a double-action fluid motor, comprising in combination, a housin having parallel valve cylinders and communicating back pressure chambers, a pair of valve plungers oppositely reciprocable in the cylinders and means connecting the plungers for moving them in opposite axial directions, fluid connections controlled by the plungers and such that movement of either from a neutral position toward the associated back pressure chamber will admit fluid to one end of the fluid motor, and means forming part of said plungers and operative upon a predetermined increase in pressure of the fluid flowing to the fluid motor to admit fluid into the back pressure 'chamber toward which the plunger has been moved to initiate said flow whereby the fluid will become effective on the end of that plunger to return both plungers to neutral position.

WILLIAM E. SWENSON. HENRY A. HALGREN.

' REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 385,377 Schoettes July 3, 1888 901,606 Fickert Oct. 20, 1908 1,069,725 Puskas Aug. 12, 1913 2,293,906 Kvavle et a1 Aug. 25, 1942 2,295,948 Henry Sept. 15, 1942 2,363,235 Ellinwood Nov. 21', 1944

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