WO2002029166A1

WO2002029166A1 - Two-way, self-level valve

Info

Publication number: WO2002029166A1
Application number: PCT/US2001/030550
Authority: WO
Inventors: Jason P. Fance; Harish Narotham
Original assignee: Husco International, Inc.
Priority date: 2000-10-03
Filing date: 2001-09-28
Publication date: 2002-04-11
Also published as: AU2001294901A1

Abstract

A self-leveling hydraulic system (10) controls at least two functions including a boom (14) and a bucket (16). Double acting cylinders (18, 20) are provided for both the boom and the bucket, the cylinders being controlled by separate boom and bucket directional control valves (26, 28), and source of pressure (22) supplying the control valves. A self-leveling valve (34) is in fluid communication with the rod end of the cylinder, whereby flow from the rod end of the boom cylinder (18) is split into first and second flows. A first passage communicates the first of the flows to the head end of the bucket cylinder (20) to supply a sufficient flow to maintain the bucket at a level position as the boom is raised, and a second passage communicating the second of the flows back through one of the control valves to a tank. The self-leveling valve (34) includes a spool structure (36, 38) for controlling the rate of flow being delivered to and returned from the boom (18) and bucket (20) cylinders, and a pressure-compensated valve structure (40, 42) connected to the spool structure for maintaining the ratio of hydraulic flow between the rate being delivered into the head side of the boom cylinder and the rate being exhausted from a rod side of the boom cylinder (18) during raising and lowering cycles of the boom (14).

Description

TWO-WAY, SELF-LEVEL VALVE FIELD OF THE INVENTION This invention relates broadly to controls for hydraulic loaders, and more particularly, pertains to valves which efficiently maintain the rotational attitude of a boom-mounted bucket during raising and lowering of the boom.

BACKGROUND OF THE INVENTION It is well known to provide a hydraulic cylinder and separate control valve for manipulating the bucket of a front end loader, and a second hydraulic cylinder and associated control valve for assisting in selectively raising and lowering the boom of a loader. The hydraulic power system of the loader also typically includes a self-leveling valve which enables the operator of the loader to operate both valves simultaneously in a centralized control in an effort to maintain the bucket in a level position during up and down movement of the boom.

One common method of controlling the bucket position is explained in U.S. Patent No. 3,251,277 to Stacey et al. In this device, fluid displaced from the boom cylinder is directed to the bucket cylinder by actuation of the boom spool. This type of system requires a matching of volumes so the volume displacement from the boom cylinder will extend the bucket cylinder the precise distance to hold the bucket level as the boom is raised. This system is expensive and requires an unduly large bucket cylinder.

Another self-leveling system is shown in U.S. Patent No. 3,563,157 to Graber wherein the flow exiting from the boom cylinder passes through a proportional flow divider dumping a portion to drain while directing the remaining portion to the bucket cylinder to maintain a level condition while raising the boom. The excess oil is removed from the self-leveling circuit by the flow divider in a parallel circuit which provides a source of pump pressure to each valve spool. In this design, the self-leveling behavior takes place in the main control valve. This system does not rely on matched volumes of flow in the boom and bucket cylinders. Another known self-leveling valve is disclosed in U.S. Patent No. 4,408,518 to Diel et al. In this patent, boom pump flow is directed to the head side of the boom cylinder. A flow divider proportions the flow exhausting from the rod side of the boom cylinder into two paths. One path is directed to the head side of the bucket cylinder and is used to tip the bucket cylinder. The other path is dumped via a separate line to tank through the main control valve. An unloader spool prevents the bucket from excessive tipping and ensures that the bucket cylinder cannot cavitate. This valve allows self-level only during the "Raise" cycle. In addition, this design is limited for use in loader systems where the main control valve is of a series-type configuration. Also, this system is limited to use where the boom valve is located upstream of the bucket control valve.

U.S. Patent No. 5,579,675 issued to Wilke describes a hydraulic fluid control valve for preventing uncontrolled load movement in the event of a fluid loss, such as from the bursting of a hose and further for allowing an operator to move the load in a controlled way despite fluid loss. The control valve is situated between a conventional main spool valve and a piston/cylinder hydraulic actuator, which powers the load. Fluid can flow to the actuator only when the control valve spool is in a position that provides a fluid path. The amount of spool movement, and therefore the amount of spool metering opening, is controlled proportionally by the rate of flow from the main spool valve, which is directly in the control of the operator.

A further self-level valve is set forth in U.S. Patent No. 5,797,310, issued to Casey et al. This self-level valve is used for loader applications which require self-level valves when raising and lowering the boom of a loader. Oil flow during "Raise" and "Lower" cycles are similar, in that pump flow is directed to the boom cylinder. A flow divider splits the flow from the return line of the boom cylinder by a preset ratio. Some of this return flow is used to operate the bucket cylinder, while the excess oil is dumped to tank through the main control valve. Unloader spools prevent the bucket from tipping out of control, and a relief valve and unloader spool are used to allow the boom to continue operating when the bucket has become fully tipped or crowded. This valve requires using an unloader spool when lifting the boom which also has the function of a relief valve to allow the operator to continue lifting the boom when the bucket cylinder is fully tipped. The use of a relief valve will result in the reduction of the lift capacity when the bucket cylinder is fully tipped.

Despite the attempts of the prior art, there remains a need to provide a self-level valve which will split flow in an improved manner so as to maintain the level position of the bucket during raising and lowering of the boom.

SUMMARY OF THE INVENTION The two-way self-level valve is designed predominantly for use on loader systems, which have one or more hydraulic cylinders for positioning the loader boom and one or more hydraulic cylinders for positioning the loader bucket. The two-way self-level valve will maintain the rotational level of the bucket relative to the start angle whilst the boom is being lowered or raised without the loader operator requiring to operate two services simultaneously.

It is a general object of the present invention to provide a two-way, self-level valve which splits the flow from the return line of a boom cylinder with some of the flow being directed to the bucket cylinder while the excess flow is directed to the tank through the main control valve. It is another object of the present invention to provide an independent self-level valve which requires pilot-operated spools, a lower counterbalance valve and pressure compensators to allow bucket leveling during another lowering or lifting of the boom.

It is also an object of the present invention to provide a self-level valve wherein a spool will control the rate of flow going to and from the boom and bucket cylinders.

It is an additional object of the present invention to provide a self- level valve incorporating a raise self-level spool and a raise pressure compensator. The spool and compensator control the ratio between the rate of oil being delivered to the head side of the boom cylinder and the rate of oil leaving the rod side of the bucket cylinder. This ratio will be maintained regardless of the machine loading conditions and the rate of flow to the boom cylinder.

It is a further object of the present invention to provide a self-level valve incorporating a lower self-level spool, lower counterbalance valve and a lower pressure compensator. The spool and the compensator control the ratio between the rate of oil being delivered to the rod side of the boom cylinder and the rate of oil leaving the head side of the bucket cylinder. This ratio will be maintained regardless of the machine loading conditions and the rate of flow to the boom cylinder. The counterbalance valve prevents cavitation of the boom cylinder rod side when lowering the boom.

It is a further object of the present invention to provide a self-level valve which does not require an unloader valve in direct communication with the bucket cylinder, or a relief valve incorporated into the self-level valve.

In one aspect of the invention, a self-leveling hydraulic system controls at least two functions including a boom and a bucket . Double acting cylinders are provided for both the boom and bucket, the cylinders being controlled by separate boom and bucket directional control valves, and a source of pressure supplying the control valves. A self-leveling valve is in fluid communication with the rod end of the boom cylinder whereby flow from the rod end of the boom cylinder is divided into first and second flows. A first passage communicates the first of the flows to the head end of the bucket cylinder to supply a sufficient flow to maintain the bucket at a level position as the boom is raised, and a second passage communicates the second of the flows back through one of the control valves to a tank. The improvement resides in the self-leveling valve including a spool structure for controlling the rate of flow being delivered to and returned from the boom and bucket cylinders, and a pressure-compensated valve structure connected to the spool structure for maintaining the ratio of hydraulic flow between the rate being delivered into the head of the boom cylinder and the rate being exhausted from the rod side of the boom cylinder during raising and lowering cycles of the boom. The spool structure is constructed and arranged to constantly maintain hydraulic pressure on the head side of the bucket so as to prevent cavitation therein. The self-leveling valve permits continuous lifting of the boom regardless of the position of the bucket cylinder. In the preferred embodiment, the self-leveling valve is comprised of a first spool having one end in communication with the boom cylinder and the other end in communication with the first pressure compensated valve which, in turn, is in communication with one end of a second spool having another end in communication with a second pressure-compensated valve in communication with the other end of the first spool. The one end of the second spool is also in communication with a counterbalance valve. The first and second pressure-compensated valves are in communication with the bucket cylinder. A bleed orifice is connected between the self-leveling valve and the tank. The bucket cylinder directional control valve is in communication with the second pressure-compensated valve and the second spool. The hydraulic flow from the rod side of the bucket cylinder is directed to the second pressure-compensated valve through the first spool and the boom directional control valve and to the tank. The pressure-compensated valve maintains a predetermined pressure drop across the spool regardless of the load imposed by the bucket.

In another aspect of the invention, a self-leveling valve is interposed between boom and bucket cylinder directional control valves and boom and bucket cylinders used to raise and lower a boom of a loader and maintain a level position of a boom-mounted bucket. The valve includes a first spool valve interconnected with a first pressure-compensated valve used for raising of the boom. A second spool valve is interconnected with a second pressure-compensated valve and counterbalance valve used for lowering of the boom. The self-level spools and pressure-compensated valves maintain a predetermined ratio of hydraulic flow across selective ports of the spool valves so as to regulate balanced hydraulic flow into and out of the boom and bucket cylinders. Each of the self-level valves and counterbalance valve are used to prevent cavitation of the boom and bucket cylinders. Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings:

Fig. 1 is a hydraulic schematic of self-leveling hydraulic system constructed in accordance with the present invention showing the system in neutral;

Fig. 2 is a schematic of the present invention illustrating a raise cycle; and

Fig. 3 is a schematic of the present invention illustrating a lower cycle.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 illustrates the neutral condition of the overall hydraulic system which is generally identified by the reference numeral 10. The system includes a vehicle frame 12 of a loader to which is pivotally mounted a boom 14. At the forward end of the boom 14 is attached to a bucket 16 with movement of the boom 14 being controlled by a boom cylinder 18 and movement of the bucket 16 controlled by a bucket cylinder 20. The overall system 10 receives pressurized fluid from a pump 22, the output of which is communicated to a main control valve 24 comprised of a first three-position, directional control valve 26 which controls the boom cylinder 18, and a second, three-position, directional control valve 28 which controls the bucket cylinder 20. As is well known, the main control valve 24 includes a conventional relief valve 30 used to dump fluid into a reservoir or tank 32 when the working system pressure is exceeded.

Positioned between the control valves 26,28 and the boom and bucket cylinders 18,20 respectively, is a two-way, self-level valve 34 having a spring-biased, "Raise" spool 36 and a spring-biased "Lower" spool 38. It is the purpose of the self-level valve 34 to maintain the rotational angle of the bucket 16 during lifting and lowering of the boom 14 without the operator having to operate two sets of controls simultaneously. Spool 36 has a spring chamber and is provided with a plurality of ports A2 to X2, B2 to Y2, C2 to Z2, and D2 to W2. Spool 38 has a spring chamber and is provided with a plurality of ports Al to XI, Bl to Yl, CI to Zl, and Dl to WI. Also included in the self-level valve 34 is a "Raise" spring-biased, pressure-compensated valve 40 and a "Lower" spring-biased, pressure-compensated valve 42. Interconnected with the boom cylinder 18, and the spools 36,38 is a lower counterbalance valve 44 having a damping orifice 46. The counterbalance valve 44 improves system operation when the boom 14 is fully loaded. A bleed orifice 48 is included between the spool 36 and the relief valve 30 to prevent opening of the spools 36,38 when not necessary such as during operation of the bucket 16 alone.

Self-Level Raise Cycle (See Fig. 2 The loader operator raises the loader boom 14 by operating the main control valve boom spool 26. As the boom spool 26 is moved from neutral to the raise position, the open center in the valve 24 starts to close and raises the pressure in the main control valve bridge, and subsequently the main control valve workport 1A. This pressure build-up is proportional to the amount of movement of the boom spool 26 in the main control valve 24. Workport 1 A is directly connected to port A of the self-level valve assembly 34 which, in turn, is connected to the pilot chamber of the raise self-level spool 36. The boom cylinder head side is directly connected to port C on the self-level valve 34 which, in turn, is connected to the spring chamber of the raise self-level spool 36. The spool areas at both ends of the self- level spools 36,38 are equal. The raise self-level spool 36 will remain closed if the force in the spring chamber of the spool (generated by the oil pressure in the boom cylinder head side), plus the force from the raise spring, is greater than the force in pilot chamber of the raise self-level spool 36. The raise self-level spool 36 will open when the pressure in port A in the self-level spool increases, to such an extent that the force in the pilot chamber of the spool 36 is greater than the combined force of the spring and the pressure in the spring chamber of the spool. When the raise self-level spool 36 opens, flow will take place across the raise self-level spool ports of A2 to X2. This flow is known as "raise meter-in" flow. The rate of "raise meter-in" fluid flow is determined by the force balancing on the spool 36 induced by the pressures in the pilot chamber and the spring force and pressure in the spring chamber of the raise self-level spool 36.

The resulting "raise meter-in" oil flow across the raise self-level spool 36 will be directed to the head side of the boom cylinder to raise the boom 14.

Part of the return oil from the rod side of the loader boom cylinder(s) 18 is used to tip the bucket 16 forward by way of the open spool 36 between ports C2 and Z2, which opens a direct flow path to the head side of the bucket cylinder 20. This boom return oil is directed to the raise self-level spool via port D of the self-level valve 34, which is directly connected to port C2 of the raise self-level spool 36. Ports C2 and Z2 are timed to open before the "raise meter-in" ports open. This ensures that before the boom cylinder 18 raises, there is a connection open between these two ports. Port Z2 is connected directly to the head side of the bucket cylinder 20 and maintains fluid in the bucket cylinder, therefore preventing cylinder cavitation.

The excess return oil from the boom cylinder 18 is directed to workport IB of the main control valve 24 by way of the open spool 36 between ports W2 and D2. The excess oil is returned into port D of the self-level valve 34 which communicates to port W2 of the raise self-level spool 36. Ports W2 and D2 are timed to open before the "raise meter-in" ports open. This ensures a free passage for the excess oil to flow into port B of self-level valve 34. Because of the main control valve boom spool configuration, the excess oil returning to the main control valve 24 via port B of the self-level valve 34 and workport IB of the main control valve 24 will be directed to tank 32 (parallel circuit). Modifying the boom spool 36 to a series circuit will allow this return oil to be directed back to the open center of the main control valve 24 for use of this oil in the second spool (series circuit). When the "raise meter-in" flow starts to take place, the raise self-level spool 36 will also open ports Y2 and B2. This flow between ports Y2 and B2 is now identified as the "raise meter-out" flow. The raise pressure compensator 40 controls the pressure compensation across the raise self-level spool ports of Y2 and B2, this works as follows. Pressurized oil from the rod side of the bucket cylinder 20 communicates directly with the inlet port of the raise pressure compensator 40 via port F of the self-level valve 34. This control pressure is sensed across the normally open pressure compensator 40 and into the compensator pilot chamber. The pressure in the pilot chamber of the raise pressure compensator 40 will begin to close the opening across the pressure compensator 40 until the pressure drop across it stabilizes and controls the pressure drop across ports Y2 and B2 of the raise self- level spool 36. The "raise meter-out" oil from the bucket cylinder 20 will enter port B of the self-level valve 34 and add to the excess oil from the boom cylinder rod side. Therefore, the oil will be directed to tank 32 through the main control valve 24 via workport 1 B .

The "raise meter-in" flow will also have a controlled pressure drop. By controlling the pressure drops in the "raise meter-in" and "raise meter-out" flows, it is possible to control the rate of flow across the spool 36 for the meter-in and meter-out. Depending on the geometry and cylinder sizes of the loader system, there will be a particular ratio between the amount of "raise meter-out" required in relation to the "raise meter-in" when the loader boom 14 is being raised.

During the raise cycle, the lower self-level spool 38 will remain fully closed. Operation of the main control valve, bucket spool 28 is not necessary to maintain the bucket level orientation.

Self-Level Lower Cycle (See Fig. 3) The loader operator lowers the loader boom 14 by operating the main control valve boom spool. As the boom spool 26 is moved from neutral to the lower position, the open center in the valve 24 starts to close and raises the pressure in the main control valve bridge, and subsequently the main control valve workport IB. This pressure build-up is proportional to the amount of movement of the boom spool 26 in the main control valve 24. Workport ^Λ IB is directly connected to port B of the self-level valve assembly 34 which, in turn, is connected to the pilot chamber of the lower self-level spool 38. The boom cylinder rod side is directly connected to port D on the self-level valve 34 which, in turn, is connected to the spring chamber of the lower self-level spool 38. The spool areas at both ends of the self-level spools 36,38 are equal. The lower self-level spool 38 will remain closed if the force in the spring chamber chamber of the spool (generated by the oil pressure in the boom cylinder rod side), plus the force from the lower spring, is greater than the force in pilot chamber of the lower self-level spool 38. The lower self-level spool 38 will open when the pressure in port B of the self-level valve 34 increases, to such an extent that the force in the pilot chamber of the spool 38 is greater than the combined force of the spring and the pressure in the spring chamber of the spool. When the lower self-level spool 38 opens, flow will take place across the lower self-level spool ports of Al to XI. This flow is known as "lower meter-in" flow. The rate of "lower meter-in" fluid is determined by the force balancing on the spool 38 induced by the pressures in the pilot chamber and the spring force and pressure in the spring chamber of the lower self-level spool 38. The resulting "lower meter-in" oil flow across the lower self-level spool 38 will be directed to the rod side of the boom cylinder 18 to lower the boom 14.

Part of the return oil from the head side of the loader boom cylinder(s) 18 is used to crowd the bucket 16 back by way of the open spool 38 between ports CI and Zl, which opens a direct flow path to the rod side of the bucket cylinder 20. This boom return oil is directed to the lower self-level spool 38 view port C of the self-level valve 34, which is directly connected to port CI of the lower self-level spool 38. Ports CI to Zl are timed to open before the "lower meter-in" ports open. This ensures that before the boom cylinder 18 lowers, there is a connection open between these two ports. Port Zl is connected directly to the rod side of the bucket cylinder 20 and this fluid, which is directed to the rod side of the bucket cylinder 20 is used to crowd the bucket 16 back during the lower cycle.

The excess return oil from the boom cylinder 18 is directed to workport 1A of the main control valve 24 by way of the open spool 38 between ports WI and Dl and the lower counterbalance valve 44. The excess oil is returned into port C of the self-level valve 34 which communicates with the inlet port of the lower counterbalance valve 44. The counterbalance valve 44 remains closed until there is sufficient force in the pilot side of the counterbalance valve to shift the spool over. Both of the ends of the counterbalance valve 44 have the same area, so the spool 38 is balanced between the oil pressure in the pilot chamber on one side of the counterbalance valve 44 and the oil pressure in the spring chamber plus the spring force in the opposing side. A damping orifice 46 is included in the pilot chamber of the counterbalance valve 44 to damp its movement. This increases the stability of the counterbalance valve 44. This counterbalance valve 44 ensures that the boom 14, when loaded with a high load, cannot cavitate down and maintains fluid in the rod side of the boom cylinder. The excess return oil, after passing through the lower counterbalance valve 44, is then directed to port WI of the lower self-level spool 38. Ports WI and Dl are timed to open before the "lower meter-in" ports open This ensures a free passage for the excess oil to flow into port A of self- level valve 34. Because of the main control valve boom spool configuration, the excess oil returning to the main control valve 24 via port A of the self-level valve 34 and workport 1 A of the main control valve 24 will be directed to tank 32 (parallel circuit). Modifying the boom spool to a series circuit will allow this return oil to be directed back to the open center of the main control valve 24 for use of this oil in the second spool (series circuit).

When the "lower meter-in" flow starts to take place, the lower self- level spool 38 will also open ports Yl to Bl. This flow between ports Yl and Bl is now identified as the "lower meter-out" flow. The lower pressure compensator 42 controls the pressure compensation across the lower self-level spool ports of Yl to Bl, this works as follows. Pressurized oil from the head side of the bucket cylinder 20 communicates directly with the inlet port of the lower pressure compensator 42 via port E of the self-level valve 34. This control pressure is sensed across the normally open pressure compensator and into the compensator pilot chamber. The pressure in the pilot chamber of the lower pressure compensator 42 will begin to close the opening across the pressure compensator 42 until the pressure drop across it stabilizes and control the pressure drop across ports Yl to Bl of the lower self- level spool 38. The "lower meter-out" oil from the bucket cylinder 20 will enter port A of the self-level valve and add to the excess oil from the boom cylinder head side. Therefore, the oil will be directed to tank 32 through the main control valve 24 via workport 1A.

The "lower meter-in" flow will also have a controlled pressure drop. By controlling the pressure drops of the "lower meter-in" and "lower meter-out" flows, it is possible to control the rate of flow across the spool for the meter-in and meter-out. Depending on the geometry and cylinder sizes of the loader system, there will be a particular ratio between the amount of "lower meter-out" required in relation to the "lower meter-in" when the loader boom 14 is being lowered.

During the lower cycle, the raise self-level spool 36 will remain fully closed. Operation of the main control valve, bucket spool 28 is not necessary to maintain the bucket level orientation.

Independent Bucket Operation The two-way self-level valve 34 provides a direct connection from the bucket service ports of the main control valve 24 and the bucket cylinder 20. When only the bucket cylinder 20 is operated, without any operation of the boom service, then both the self-level spools 36,38 remain in the neutral position (closed) and operation of the bucket cylinder 20 is unaffected by the presence of the two- way self-level valve 34 in the system. The bleed orifice 48 connected between port B of the self-level valve 34 and system tank 32 prevents the raise or lower self- level spools 36 or 38 from opening during independent operation of the bucket service. This is accomplished by preventing any pressure build-up due to internal leakage across the self-level spools 36,38.

Operation of Dual Services (Boom and Bucket Services Boom raise and Bucket Tip Dual Operation Tipping the bucket 16 whilst raising the boom 14 will result in additional pump flow (or boom return flow if the boom main control valve spool 26 is series spool) coming from the main control valve port 2B and into the self-level valve 34. This oil flow will add to the boom return oil from port Z2, which normally tips the bucket 16 to keep the loader bucket level. The flow from the rod side of the bucket cylinder 20 will be diverted directly to tank 32 through the port 2A of the main control valve 24.

Boom Raise and Bucket Crowd Dual Operation Crowding the bucket 16 whilst raising the boom 14 will result in additional pump flow (or boom return flow if the boom main control valve spool is series spool) coming from the main control valve port 2A and into the self-level valve 34. This flow is directed to the rod side of the bucket cylinder 20. The boom return flow, which normally goes to head side of the bucket cylinder 20 from port Z2, will instead be directed to tank 32 through port 2B of the main control valve 24. The flow from the head side of the bucket cylinder 20 will add to the boom return flow from port Z2 and be diverted directly to tank 32 through port 2A of the main control valve 24.

Boom Lower and Bucket Crowd Dual Operation Crowding the bucket 16 while lowering the boom 14 will result in additional pump flow (or boom return flow if the boom main control valve spool 26 is series spool) coming from the main control valve port 2A and into the self-level valve 24. This oil flow will add to the boom return oil flow from port Zl which normally crowds the bucket 16 to keep the loader bucket level. The flow from the head side of the bucket cylinder 20 will be diverted directly to tank 32 through the port 2B of the main control valve 24. Boom Lower and Bucket Tip Dual Operation Tipping the bucket 16 whilst lowering the boom 14 will result in additional pump flow (or boom return flow if the boom main control valve spool 26 is series spool) coming from the main control valve port 2B and into the self-level valve 34. This flow is directed to the head side of the bucket cylinder 20. The boom return flow, which normally goes to rod side of the bucket cylinder 20 from port Zl, will instead be directed to tank 32 through port 2 A of the main control valve 24. The flow from the rod side of the bucket cylinder 20 will add to the boom return flow from port Zl and be diverted directly to tank 32 through port 2A of the main control valve 24.

It should be appreciated that the present invention provides an improved self-leveling system which makes use of a unique spool, counterbalance valve and pressure compensator arrangement that maintains the level of the bucket throughout raising and lowering of the boom, and simultaneously tipping and crowding of the bucket. Unlike the prior art, such system functions to prevent cavitation of the bucket without an unloader valve because the self-level spools, counterbalance valve and pressure compensator valves always maintain pressure on the head side of the bucket cylinder. In further contrast with the prior art, there is no need to employ a relief valve to allow an operator to continue lifting the boom when the bucket cylinder is fully tipped. As a result, there is full lifting capacity throughout all the motion of the boom and the bucket.

Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.

Claims

CLAIMS We claim:

1. A self-leveling hydraulic system controlling at least two cylinders having a boom and a bucket, double acting cylinders for the boom and the bucket, the cylinders controlled by separate boom and bucket directional control valves, and a source of pressure supplying the control valves, and a self-leveling valve in fluid communication with the rod end of the boom cylinder whereby flow from the rod end of the boom cylinder is divided into first and second flows, a first passage communicating the first of the flows to the head end of the bucket cylinder to supply a sufficient flow to maintain the bucket at a level position as the boom is raised, and a second passage communicating the second of the flows back to one of the control valves to a tank, the improvement wherein: the self-leveling valve includes a spool structure for controlling the rate of flow being delivered to and returned from the boom and bucket cylinders and a pressure-compensated valve structure connected to the spool structure for maintaining the ratio of hydraulic flow between the rate being delivered into the head side of the boom cylinder and the rate being exhausted from a rod side of the boom cylinder during raising and lowering cycles of the boom.

2. The improvement of claim 1, wherein the spool structure is constructed and arranged to constantly maintain hydraulic pressure on the head side of the bucket so as to prevent cavitation therein.

3. The improvement of claim 1, wherein the self-leveling valve permits continuously lifting of the boom regardless of the position of the bucket cylinder.

4. The improvement of claim 1, wherein the self-leveling valve is comprised of a first spool having one end in communication with the boom cylinder and another end in communication with a first pressure-compensated valve which, in turn, is in communication with one end of a second spool having another end in communication with a second pressure-compensated valve in communication with the other end of the first spool, the one end of the second spool also being in communication with a counterbalance valve.

5. The improvement of claim 4, wherein the first and second pressure-compensated valves are in communication with the bucket cylinder.

6. The improvement of claim 4, including a bleed orifice connected between the self-leveling valve and the tank.

7. The improvement of claim 4, wherein the bucket cylinder directional control valve is in communication with the second pressure- compensated valve and the second spool.

8. The improvement of claim 4, wherein the hydraulic flow from the rod side of the bucket cylinder is directed to the second pressure-compensated valve through the first spool and the boom directional control valve and to the tank.

9. The improvement of claim 1 , wherein the pressure- compensated valve maintains a predetermined pressure drop across the spool regardless of the load imposed by the bucket.

10. A self-leveling valve interposed between boom and bucket directional control valves and boom and bucket cylinders used to raise and lower the boom of a loader and maintain a level portion of a boom-mounted bucket, the valve comprising: a first spool valve interconnected with a first pressure-compensated valve used for raising of the boom; a second spool valve interconnected with a second pressure- compensated valve and a counterbalance valve used for lowering of the boom; and the pressure-compensated valves maintaining a predetermined ratio of hydraulic flow across selective ports of the spool valves so as to regulate balanced hydraulic flow into and out of the boom cylinder, and each of the pressure- compensated valves is used to prevent cavitation of the bucket cylinder.