US3606929A

US3606929A - Hydraulic control system for earth moving implement

Info

Publication number: US3606929A
Application number: US38996A
Authority: US
Inventors: Claude M Frisbee; Carl O Pedersen
Original assignee: JI Case Co
Current assignee: Case LLC
Priority date: 1970-05-20
Filing date: 1970-05-20
Publication date: 1971-09-21
Anticipated expiration: 1988-09-21

Abstract

A HYDRAULIC CONTROL SYSTEM FOR AN EARTH MOVING IMPLEMENT WHICH HAS A BLADE ADJUSTABLY SUPPORTED ON A USHAPED FRAME WITH THE FREE ENDS OF ITS LEGS PIVOTALLY CONNECTED TO A VEHICLE AND THE HYDRAULIC CONTROL SYSTEM INCLUDES FLUID RAMS FOR (1) RAISING AND LOWERING THE FRAME WITH RESPECT TO THE VEHICLE, (2) ANGLING THE BLADE WITH RESPECT TO THE U-SHAPED FRAME ABOUT A VERTICAL AXIS, AND (3) PREVENT TILTING OF THE BLADE WITH RESPECT TO THE FRAME ABOUT A TRANSVERSE HORIZONTAL AXIS. THE HYDRAULIC CONTROL SYSTEM INCORPORATES AN INTERCONNECTION BETWEEN THE ROD ENDS OF THE ANTI-TILT RAMS WITH MEANS FOR SUPPLYING A CONSTANT CHARGING PRESSURE TO THE ROD ENDS OF THE ANTITILT RAMS. THE CHARGING MEANS MAY INCLUDE (1) A CONNECTION FROM THE ANGLING RAMS THROUGH A REGULATOR VALVE TO THE ROD ENDS OF THE ANTI-TILT RAMS, (2) A CONNECTION BETWEEN ONE END OF THE LIFT CYLINDERS THROUGH A REGULATOR VALVE TO THE ROD END OF THE ANTI-TILT RAMS, (3) A CONNECTION BETWEEN THE ANTI-TILT RAMS AND THE CONVENTIONAL PUMP OF THE TRANSMISSION HYDRAULIC SYSTEM OF THE VEHICLE, OR (4) A SEPARATE PUMP CONNECTED TO THE ROD ENDS OF THE ANTI-TILT RAMS, ALL ALTERNATES.

Description

Sept. 21, 1971 c, FRlSBEE EIAL 3,606,929

HYDRAULIC CONTROL SYSTEM FOR EARTH MOVING IMPLEMENT Original Filed Oct. 1'7, 1968 3 Sheets-Sheet 1 VB 1 NW w INVENTORS CLAUDE M. FRISBEE CARL 0. PEDERSEN.

ATT'YS.

BY: CULLE N, SE T.TL E SLOMAN 8 CANTO? Sept. 21, 1971 c. M. FRISBEE ETAL 3,606,929

HYDRAULIC CONTRQL SYSTEM FOR EARTH MOVING IMPLEMENT Original Filed 001;. 17. 1968 s Sheets-Sheet 2 a; w L

l l A w no J . V INvENmRs.

62 j Y CLAUDE M.FRISBEE CARL o. PEDERSEN.

BY: CULLEMSETTLE, AN aCANTm.

Sept. 21, 9 I c. M. FRISBEE ETAL 3,606,929

HYDRAULIC CONTROL SYSTEM FOR EARTH MOVING IMPLEMENT Original Filed Oct. 17, 1968 s SheetsSheet :s

IN VE N TORS.

- CLAUDE M-FRISBEE CARL O. PEDERSEN.

CULLEN, SETTLE, SLWAN 8 CANTOR.

ATT' YS.

United States Patent O Int. Cl. E02f 3 76 US. Cl. 172--804 13 Claims ABSTRACT OF THE DISCLOSURE A hydraulic control system for an earth moving implement which has a blade adjustably supported on a U- shaped frame with the free ends of its legs pivotally connected to a vehicle and the hydraulic control system includes fluid rams for (1) raising and lowering the frame with respect to the vehicle, -(2) angling the blade with respect to the U-shaped frame about a vertical axis, and (3) prevent tilting of the blade with respect to the frame about a transverse horizontal axis. The hydraulic control system incorporates an interconnection between the rod ends of the anti-tilt rams with means for supplying a constant charging pressure to the rod ends of the antitilt rams. The charging means may include (1) a connection from the angling rams through a regulator valve to the rod ends of the anti-tilt rams; (2) a connection between one end of the lift cylinders through a regulator valve to the rod end of the anti-tilt rams; (3) a connection between the anti-tilt rams and the conventional pump of the transmission hydraulic system of the vehicle, or (4) a separate pump connected to the rod ends of the anti-tilt rams, all alternates.

REFERENCE TO RELATED APPLICATIONS The present application is a division of co-pending application Ser. No. 768,435, filed Oct. 17, 1968 for Hydraulic Control for Earth Moving Implement.

BACKGROUND OF THE INVENTION The present invention relates generally to earth moving implements and more particularly to an improved control system for a dozer blade supported on a vehicle.

In the construction and operation of earth moving implements, such as those used for moving dirt and grading roads, it has become customary to mount a scraper blade in a manner allowing considerable relative movement between the blade and the vehicle. Thus, in most commercial vehicles, the blade is mounted on a frame which is adapted to be raised and lowered with respect to the vehicle. This is normally accomplished by supporting the frame on transversely extending pivot axes on opposite sides of the vehicle with hydraulic lift cylinders or rams interposed between the vehicle and the frame.

Also, in most commercial vehicles of this type presently available, means are provided for angling the blade with respect to the normal transverse position at the forward end of the vehicle. This is normally accomplished by pivoting the blade on a vertical axis disposed at the front end of the frame. Angle cylinders are interposed between the frame and the blade to accommodate pivotal movement about the above-mentioned vertical axis.

Further, in some commercial implements of this type, it has been found desirable to provide a method for preventing tilting of the blade forwardly during dozing operation. Again, some prior art type of bulldozer constructions incorporate hydraulic means for changing the angular disposition of the cutting edge of the blade rel- 3,606,929 Patented Sept. 21, 1971 ative to the transverse axis of the vehicle, such as the type shown in Pat. No. 2,681,518, issued to C. C. Troop.

SUMMARY OF THE INVENTION The present invention provides an improved hydraulic control system for controlling the flow of fluid to, from and between the various cylinders normally incorporated in a bulldozer construction which is capable of having its blade raised and lowered with respect to the vehicle, angled with respect to the longitudinal axis of the vehicle and prevent forward tilting of the blade during dozing.

This is accomplished by providing a hydraulic control system in which the forward ends of a pair of antitilt cylinder assemblies are interconnected by a single conduit and in which pressure is at all times maintained in the conduits to insure that any leakage past the piston rods of the anti-tilt cylinders will immediately and automatically be replaced so that always maximum pressure is available at the rod end of the anti-tilt cylinders during dozing operation. Thus the control system of the present invention maintains the dozer blade in an upright position at all times by maintaining a positive pressure on the rod ends of the pistons normally slidably disposed in the cylinders of the convention type fluid rams.

Thus, the primary object of the present invention is to provide an improved hydraulic control system for cylinder assemblies normally associated with a scraper blade of an earth moving implement.

Another object is to provide a control system which automatically maintains a positive pressure in the forward or rod ends of tilt cylinders normally associated with a scraper blade and pivotally mounted upon a frame supported on the vehicle.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of one type of earth moving implement which may be modified to incorporate the control system of the present invention;

FIG. 2 is a schematic illustration of a hydraulic control system constructed in accordance with one embodiment of the present invention;

FIG. 3 is a schematic illustration showing a further embodiment of the control system of the present invention;

FIG. 4 is a schematic illustration showing another embodiment of the control system of the present invention; and

FIG. 5 is another schematic illustration showing still a further embodiment of the present invention.

DETAILED DESCRIPTION Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description only and not for limitation.

Referring in greater detail to the drawings, the invention is illustratively shown as being applied to an earth moving implement or dozer which includes a crawler tractor 10 having ground engaging means 12 (only one being shown) on opposite sides thereof supported on transversely extending axles 14. The dozer 10 further includes a scraper blade 20 supported adjacent the forward end of the tractor by a substantially U-shaped frame 22 having legs 24 extending along both sides of the tractor and which are pivotally supported at 26 to the main frame of the tractor 10.

The blade .20 is pivotally supported on a pin 28 defining a vertical axis adjacent the forwardmost end of the U-shaped frame 22. The connection between the vertical axis defined by the pin 28 and the blade 20 further includes means for pivotally supporting the blade for limited twisting movement about an axis extending normal to the vertical pivot axis 28. For this purpose, a ring 30 defining a bearing retainer is fixedly secured to the rear surface of the blade 20 which receives and rotatably cooperates with an integral bearing plate 32 having a leg portion 34 extending rearwardly which is pivotally attached to the front end of the frame 22 by means of the pin 28. Since the connection between the blade and the U-shaped frame as well as the frame and the tractor forms no part of the present invention no further detailed description thereof will be presented at this time. However, for a further detailed description of the frame and blade connections, reference may be had to Pat. No. 2,943,407 issued to E. B. Long on July 5, 1960 and assigned to the assignee of the present application.

As Was indicated hereinabove, the blade 20 is adapted to be raised and lowered with respect to the tractor about the transverse frame pivots 26, pivoted about a vertical axis defined by the pin 28 relative to the frame 22 and to twist or rotate about a longitudinal center axis of the frame 22. The movement of the blade in the various directions is accomplished by a plurality of fluid rams having cylinders and piston rods slidable within the cylinders. Each degree of movement is accomplished by a pair of cooperating cylinders. However, since the cylinders on opposite sides of the longitudinal center of the vehicle are duplicates, only one side will be described hereafter with respect to the illustration in FIG. 1 it being understood that a duplicate arrangement is present at the other side. If a further detailed description of the cylinder assemblies and the pivotal interconnection between the cylinder assemblies and the frame and the blade is desired, reference may again be had to the above-mentioned patent.

The means for changing the angle of the blade with respect to the longitudinal axis of the frame 22 or tractor 10 includes a pair of angle cylinder assemblies or fluid rams 40 each having a piston rod 42 connected by a pin 44 to a bracket 46 supported on the leg 24 of the frame 22. The cylinder 48 of the assembly 40 has a bracket 50 pivotally connected thereto, for a purpose which will become apparent hereinafter.

The bracket 50 is connected by a fixed length strut 52 to the lower outermost edge of the blade by a pivot joint 54. The bracket 50 is likewise connected to the upper outer corner of the blade by a tilt cylinder assembly or fluid ram 56 which includes a cylinder 60 and a piston rod 62 which is universally connected at 64 to the upper outer edge of the blade 20. The cylinder assembly 56 is utilized for twisting or rotating the blade about the longitudinal axis of the frame 22 by means of the bearing assembly 30, 32 as well as for maintaining the blade in a position normal to the longitudinal axis of the frame 22.

The means for raising and lowering the blade with respect to the ground surface includes lift cylinder assembly or fluid ram 70 having a cylinder 76 and piston rod 72 which is pivotally connected by a bracket 74 to the bight portion of the U-shaped frame 22. The cylinder 76 of the cylinder assembly 70 is pivotally mounted in a trunnion 78 fixedly secured to opposite sides of the tractor 10.

One embodiment of the hydraulic control system of the present invention is schematically illustrated in more detail in FIG. 2 and includes a pressured fluid source, such as a pump 80, connected by conduit 82 to a fluid reservoir 84 for selectively supplying pressured fluid to control valve means which are illustratively shown as three manually controlled three-

Way valves

86, 88 and 90. The control valve 86 is connected through conduit 92 to the pressured fluid source and through a return conduit 94 to the fluid reservoir 84. The two outlets of the three-way valve 86 diagonally connected through conduits 9,6 and 98 to the respective opposite ends of the cylinders 48 forming part of the angle cylinder assemblies 40. Thus, pressured fluid supplied through line 96 will extend piston rod 42 on the right side of the vehicle while simultaneously retracting piston rod 42 on the left side of the vehicle thereby angularly pivoting the blade 20 about pin 28 in one direction and fluid supplied through line 98 by changing the position of the valve 86 will extend piston rod 42 on the left side and retract piston rod 42 on the right side thereby pivoting the blade in the opposite direction.

Pressurized fluid is also supplied through control valve 88 and conduits 100 and 102 to the respective opposite ends of the pair of cylinders 76, forming part of the lift ram and in which line 100 is connected to the rod end of the cylinders 76 and line 102 to the piston end so that upon selective fluid introduction at either end the two piston rods of the cylinder assemblies 70 are simultaneously extended and retracted, to thereby raise or lower the blade 20 relative to the ground by pivoting the blade supporting frame 22 around the transverse pivot 26.

Pressurized fluid is also selectively supplied to the piston ends of the pair of tilt cylinders 60 forming part of the tilt cylinder assemblies 56 by

separate conduits

104 and 106 leading from the control valve 90 to the respective ends of the cylinders 60. The rod ends of the two tilt cylinders 60, forming part of the ram or tilt cylinder assemblies 56, are cross connected to each other by a conduit 110 so that pressurized fluid selectively delivered to either of the piston ends of the cylinders 60 through

line

104 or 106 will move the piston within the pressurized cylinder and thereby displace fluid from the opposite side of the actuated piston to the rod end of the opposite cylinder to simultaneously move the other piston in the opposite direction By this arrangement the blade 20 will be twisted or rotated around the bearing 30, 32 in either selected direction.

According to the present invention, means are provided for simultaneously maintaining a predetermined fluid pressure at the rod ends of each of the tilt cylinders 60. This particular feature is of extreme importance in preventing the upper edge of the blade 20 from tilting for wardly during dozing operation due to the extreme loading at the bottom edge of the blade and thus reducing the load stresses at the pivot supports between the blade and the frame 22.

In the embodiment illustrated in FIG. 2, the charging pressure fluid circuit includes a pressure reducing valve 122 having its inlet port or opening connected through a conduit 124 to the conduit 102 leading to the piston ends of the respective lift cylinders 76. The outlet opening of the pressure regulator valve 122 is connected through a conduit 126 to the conduit 110 interconnecting the rod ends of the tilt cylinders 60. The regulator valve also has a bypass port connected through conduit 129 to the reservoir 84. Preferably, a one-way check valve 128 of the conventional type is located in the conduit 126 to prevent any increase in pressure at the outlet-port of valve 122 as well as maintain a predetermined pressure within the cylinder assembly 60 by preventing backflow of fluid through line 126. The valve 122 may be a commercially available type of direct acting, pressure reducing valve such as manufactured and sold by Fluid Controls, Inc. Mentor, Ohio. Since the valve per se forms no part of the present invention other than in the combination, no detailed description thereof appears to be necessary at this point.

Operation It is believed that the operation of the hydraulic control system shown in FIG. 2 is readily apparent from the above description. It will be understood that, during normal operation of the earth moving implement, the fluid trapped in the piston ends of the cylinder 76 will at all times be pressurized by the upwardly directed forces exerted from the ground on the lower edge of the blade during dozing. This pressure is transferred by conduit 102 to conduit 124 leading to the pressure regulating valve 122 and from there into conduit 126 to maintain a constant predetermined charging pressure at the rod ends of the tilt cylinders 60 through connecting conduit 110 at a rate in direct proportion to the load exerted on the blade 20. This positive pressure at the inlet of the pressure regulator valve 122 is sufficient to replace any fluid which may leak past the seals between the pistons and cylinders of the tilt cylinder assemblies 56'. The charging pressure on the rod ends of the tilt cylinders primarily maintains the dozer blade in an upright position relative to the frame 22 and causes the tilt cylinder assemblies to support the load that would normally cause the top of the dozer blade to pitch forward ifduring dozing-no positive pressure were exerted on the rod ends of the pistons slidably disposed in the tilt cylinders 60.

FIG. 3 embodiment The embodiment schematically illustrated in FIG. 3 is similar in respect to the hydraulic control circuit for the angle cylinder assemblies 40 and lift cylinder assemblies 70 as shown in FIG. 2 with the exception that the charging pressure means or the means for maintaining a charging pressure on the rod ends of the tilt cylinders 60 is supplied by means from a conventional hydraulic transmission control normally forming part of the vehicle or tractor to which the dozer blade is attached. Thus, the conventional type of crawler tractor incorporates a hydraulic control system for controlling operation of the vehicle transmission. This control system includes a pressurized fluid source which again, as in the embodiment described hereinabove, incorporates a pump 140* connected to a fluid reservoir '84 by a conduit 142 with the outlet of the pump connected through conduit 144 to a conventional transmission control valve 146 which is selectively actuatable to supply fluid to the various fluid controlled devices forming part of the transmission. Normally, the conduit 144 has associated therewith a pressure relief valve 148- which maintains the pressure within the conduit at a substantially constant level.

In the system illustrated in FIG. 3, the cross conduit 110 connecting the rod ends of the tilt cylinders 60 by a conduit 147 to the outlet side of the pump 140 with a conventional one way check valve 149 again forming part of the conduit 147 and which is similar in all respects and purposed to the check valve 128 in FIG. 2.

Thus, positive pump pressure Within conduit 144, which is at all times available when the power plant of the vehicle 10 is in operation, will maintain a constant charging pressure through

lines

147 and 110 at the rod ends of the tilt cylinders '60 for the same purpose and in the same manner as described hereinabove.

Embodiment of FIG. 4

The system shown in FIG. 4 is in many respects similar to that described hereinabove with regard to FIG. 3 with the exception that the pump 140, relief valve 148 and check valve 149 form a separate fluid circuit independent from the transmission control circuit shown in FIG. 3 and specifically designed for supplying constant charging pressure to the cross conduit or interconnecting conduit 110' and into the rod ends of the tilt cylinders 60.

Embodiment of FIG. 5

The system shown in FIG. 5 discloses a further modification of a charging pressure control circuit for maintaining a predetermined charging pressure in the rod ends of the tilt cylinders 60'. In the system illustrated in FIG. 5, the inlet of the pressure regulator valve 122 is connected to both conduits 96- aud 98 leading diagonal to the opposite ends of the pair of angle cylinders 48 forming part of the cylinder assemblies 40. The means for connecting the inlet of the pressure regulating valve 122 to the

respective conduits

96 and 98 includes a pair of

conduits

170 and 172 each having a conventional check valve 174 incorporated therein with the outlet sides of the check valves connected to the inlet of the pressure regulator valve while the inlet sides are in communication with

respective conduits

96 and 98.

In normal operation of a conventional dozer, both of the conduits 96' and 98 transmit fluid pressure through

conduits

170 and 172 and conduit 124 into the pressure regulator valve 122 due to the forces being applied by the ground being worked on to the forward surface of the dozer or scraper blade which in turn will be transferred via lines 126 and into the rod ends of the tilt cylinders 60 sufficient to maintain a constant positive charging pressure therein. i

From the above description it will be appreciated that the present invention provides a simple and eflicient fluid system at all times maintaining a positive upright position of the blade with respect to the blade support frame 22 and counterbalancing the work load exerted along the bottom edge of the blade during dozing. The various means for maintaining a positive charging pressure on the rod ends of the tilt cylinders 60' will also automatically replace any hydraulic fluid which is lost due to leakage. Of course, the postive charging pressure also maintains a true relationship at the rod ends of the respective tilt cylinders to prevent a forward pitching or tilting of the blade and to maintain the blade in an upright position at all times. This is accomplished without affecting the conventional fluid cylinder actuated blade positioning arrangements to (1) raise or lower the blade relative to the ground; (2) angle the blade relative to a longitudinal vehicle axis and (3) to twist or rotate the blade around said longitudinal axis. These conventional arrangements per se form no part of the invention except that portions of the fluid systems forming part of the novel combination.

We claim:

1. In combination with a vehicle having an adjustable blade mounted thereon for pivotal movement about at least a pair of axes normal to each other, a hydraulic control system for pivoting said blade about said axes and comprising: A first and a second pair of fluid ram assemblies each having a cylinder and a piston rod reciprocable therein and interposed between said vehicle and said blade for pivoting said blade about said axes; at least one pressurized fluid source; first conduit means interconnecting said cylinders with a fluid source; a control valve in said first conduit means for selectively supplying pressurized fluid to said cylinders; second conduit means interconnecting one end of the cylinders of both of said second pair of fluid rams; pressure regulat ing means within said second conduit means; and a fluid line for connecting said pressure regulating means to a pressurized fluid source, to thereby maintain a constant pressure in said ends of said cylinders of said second pair of fluid rams.

2. The combination as defined in claim 1, wherein said control system further includes a third pair of fluid rams each having a cylinder and a piston rod reciprocable therein and interposed between said vehicle and said blade for raising and lowering said blade on said vehicle and said first conduit means includes a fluid line interconnecting an end of the cylinders of said third pair of fluid rams with said control valve in said first conduit means for selectively supplying pressurized fluid to said one end to lower said blade on said vehicle, said pressure regulator means comprising a pressure regulator valve having an inlet and an outlet; said outlet being connected to said second conduit means and said inlet being con nected to said last named fluid line to thereby maintain a constant pressure in said ends of said cylinders of said second pair of fluid rams, said pressure being generated at said one end of said cylinders of said third pair of fluid rams.

3. The combination as defined in claim 1, said vehicle having a transmission and a pump supplying pressurized fluid to a transmission control device, said pressure regulator means comprising a pressure regulator valve having an inlet connected to the output of said pump and an outlet connected to said second conduit means to thereby maintain a constant pressure generated by said transmission pump in said ends of said cylinders of said second pair of fluid rams and provide one of said pressurized fluid sources.

4. The combination as defined in claim 1, in which one of said pressurized fluid sources comprises a first pump in said first conduit means supplying pressurized fluid to said control valve and a second source comprising a second pump supplying pressurized fluid to said pressure regulator means.

5. A hydraulic control system for an adjustable blade adapted to be vertically and angularly adjusted with respect to a vehicle, comprising first, second and third pairs of fluid rams connected between said blade and said vehicle and each having a cylinder having opposite fluid inlets, a piston slidably disposed in each of said cylinders between said inlets, said first and said second pairs of fluid rams adapted for vertical and angular adjustment of said blade on said vehicle, at least one source of fluid under pressure connected to control valve means selectively supplying pressurized fluid to said cylinders, the improvement of first conduit means cross connecting one end of the cylinders of one of said pairs of fluid rams, and pressure regulating valve means fluidly connected to said first conduit means and to a source of fluid pressure for maintaining a constant pressure charge in said one end of the cylinders of said one of said pairs of fluid rams.

*6. In the control system as defined in claim 5, the further improvement of said source of fluid pressure comprising a first pump supplying pressurized fluid to said control valve means and a second pump supplying pressurized fluid to said first conduit means and said pressure regulating valve means comprising a relief valve connected to said first conduit means between said second pump and said one end of the cylinders of the one of said pairs of fluid rams.

7. In the control system as defined in claim 5, second and third conduit means respectively connecting opposite ends of the cylinders of said other pairs of fluid rams to said control valve means to selectively supply pressurized fluid through said control valve means to opposite end of said cylinders, the further improvement of a fourth conduit communicating with one of said second or third conduit means, and said regulating valve means comprising a pressure reducing valve having an inlet connected to said fourth conduit means and an outlet connected to said first conduit means.

8. In the control system as defined in claim 5, wherein said blade is adapted to be pivoted about a first and a second axis substantially perpendicular to each other with said first pair of fluid rams adapted for pivoting said blade about said first axis and said third pair of fluid rams adapted for pivoting said blade about said second axis, the further improvement of second and third conduit means connecting said control valve means to opposite ends of the cylinders of said third pair of fluid rams, fourth and fifth conduit means each having a check valve therein and respectively connected to said second and third conduit means, said regulating valve means comprising a pressure reducing valve having an outlet connected to said first conduit means and an inlet connected to said fourth and fifth conduit means whereby pressurized fluid in either of said second and third conduit means will maintain a constant pressure on the fluid in said first conduit means.

9. A hydraulic system for a dozer having free ends of legs of a U-shaped frame pivoted thereto comprising, a dozer, a pair of lift cylinder and piston rod assemblies disposed between the frame and the dozer, a blade pivoted on said frame on a first axis and a second axis perpendicular thereto, a pair of angle cylinder and piston rod assemblies disposed between said frame and said blade for pivoting said blade about said first axis and a pair of tilt cylinder and piston rod assemblies disposed between said frame and said blade for pivoting said blade about said second axis, said control system comprising; a source of fluid pressure connected to control valve means selectively supplying pressurized fluid to each of said cylinders, and means for simultaneously maintaining a predetermined charging pressure on the blade ends of each of said tilt cylinders.

10. In the control system as defined in claim 9, first and second conduits connected to said control valve means and respectively communicating with one end of each of said angle cylinders, said means comprising a pressure regulating valve connected to said one end of each of said tilt cylinders and first and second check valves having upstream sides respectively connected to said conduits and downstream sides connected to said regulating valve whereby pressurized fluid in either of said first and second conduits will produce a charging pressure in said blade ends of each of said tilt cylinders.

11. In the control system as defined in claim 9, in which said means comprises a pressure regulating valve having an inlet connected to one end of each of said lift cylinders and an outlet connected to said one end of said tilt cylinders.

12. In the control system as defined in claim 9, in which the source of fluid pressure includes pump means, a first fluid line connecting said pump means to said control valve means, said means for simultaneously maintaining a predetermined charging pressure including a second fluid line connected to said pump means, and a pressure regulating means in said second fluid line.

13. In th control system as defined in claim. 9, in which said source of fluid pressure comprises first and second pumps with saidfirst pump supplying pressurized fluid to UNITED STATES PATENTS 2,943,407 7/1960 Long 172804 3,158,944 12/1964 Rehberg et al 172-803 3,232,184 2/1966 Ulrich 172- 803X ROBERT E. PULFR EY, Primary Examiner C. D. CROWDER, Assistant Examiner US. Cl. X.R. 172807