GB2055977A

GB2055977A - Hydraulic control systems

Info

Publication number: GB2055977A
Application number: GB8024266A
Authority: GB
Original assignee: Fiat Allis Construction Machinery Inc
Current assignee: Fiat Allis Construction Machinery Inc
Priority date: 1979-07-26
Filing date: 1980-07-24
Publication date: 1981-03-11
Also published as: IT8023638A0; DE3027563A1; IT1131979B; KR830003667A; SE8005276L; AU6035880A; BR8004571A; JPS5620240A

Abstract

The system comprises normally closed pilot-operated holding valves 52, 52A which are physically located on or near the lift cylinders and are operable in response to pilot fluid controlled by a pilot control valve 48. A pressure equalization conduit 100 is connected between the pair of lift cylinders to prevent overloading of one cylinder. Fluid flow orifices 101, 101A are located in the pressure equalization conduit to prevent rapid fluid loss from the lift cylinders and uncontrolled load dropping in the event of fluid loss caused by equalization line breakage. In one embodiment, the pilot control valve for the lift cylinders is actuable in response to a directional control valve 47 for the lift cylinders, and pilot fluid is supplied to the holding valves from the raise side of the cylinders as the pilot control valve opens. In another embodiment, a manually operable pilot control valve supplies pilot fluid from an auxiliary pump to operate the directional control valve and the holding valves. In this latter embodiment, in the event of auxiliary pump failure, pilot fluid is supplied from the raise side of the lift cylinders. <IMAGE>

Description

SPECIFICATION Hydraulic control systems for load-handling hydraulic motors Background of the Invention This invention relates in general to hydraulic control systems for load-lifting hydraulic motors, such as lift cylinders used to raise and lower the loading bucket on self-propelled loaders or the like. In particular, it relates to a hydraulic control system which employs load-holding valves for the hydraulic motors and means to operate the loadholding valves to prevent rapid lowering or dropping of the load in the event of a system failure.

Machine or apparatus such as self-propelled loaders or the like have boom-mounted loading buckets which are raised and lowered by doubleacting hydraulic motors, such as lift cylinders, connected to the booms. Various types of hydraulic control systems are provided to raise and lower the booms and the bucket mounted thereon. However, all such systems include a source of pressurized hydraulic fluid and a selectively-operable, multiposition, directional control valve for supplying the pressurized fluid to the lift cylinders. In some instances, the control systems employ double-pilot check valves to lock the hydraulic lift cylinders when it is necessary to hold the load in a raised position. However, such double-pilot check valves typically depend on high pressure in a fluid supply line from the directional control valve to the hydraulic lift cylinders for operation.Therefore, float-down, or intentional rapid lowering of the bucket, is not possible because the lift cylinders are only actuated by fluid pressure in the lines from the directional control valve. Furthermore, mechanical failure of a hydraulic fluid hose, conduit, or other hydraulic system component in some other types of systems, can result in an undesirable uncontrolled lowering of the load.

Summary of the Invention In accordance with the present invention a hydraulic control system is provided to operate a pair of hydraulic lift cylinders to equalize fluid pressure in the pair of lift cylinders and to prevent rapid unintentional boom lowering and dropping of the bucket in the event of loss of fluid pressure in the system.

The system comprises, generally, a pair of normally closed, two-position, pilot-operated, load-holding valves mounted on or near the lift cylinders and connected in fluid communication with the cylinder raise or lift chambers to eliminate the need for long and easily damaged fluid lines; a multi-position (neutral, raise, lower, float) directional control valve located remotely from the cylinders for operating the lift cylinders by controlling fluid flow between a pressurized fluid source and the cylinders; and a pilot-pressure control valve for effecting operation of the holding valves and located remotely therefrom. The system further includes a pressure equalizing fluid conduit having flow restricting orifices provided therein and connected between ports of the cylinder raise or lift chambers.The pressure equalizing conduit prevents excessive hydraulic pressure from occurring in one of the pair of hydraulic lift cylinders, such as might occur in a load shift resulting from travel of the wheel loader over uneven ground. The flow restricting orifices prevent the load from dropping faster than an acceptably safe speed in the event of fluid pressure loss resulting from breakage of the equalizer conduit or other fluid lines connected thereto. Check valves are provided to by-pass the holding valves when fluid is being supplied from the directional control valve to raise the lift booms.

In one embodiment, the pilot-pressure control valve is actuable, as by cam means, in response to actuation of a manually operable directional control valve. Pilot fluid is supplied from the raise side of the cylinders to open the holding valves as the pilot pressure control valve is opened by the cam means in response to actuation of the directional control valve to the retract (lower) position.

In another embodiment, a manually operable pilot-pressure control valve controls pilot fluid flow to a pilot-operated directional control valve and the pilot-operated holding valves. In this embodiment, pilot fluid is supplied from an auxiliary pump, or in the event of auxiliary pump failure, from the raise side of the lift cylinders to operate the holding valves.

A control system in accordance with the invention has several advantages. For example, location of the holding valves on or near the lift cylinders eliminates or reduces the risk of damage to fluid lines connected thereto. Spring bias of the holding valves in a closed position positively prevent unintentional release of fluid from the cylinder raise chambers during operations.

Pressure equalization between the raise sides of the pair of lift cylinders prevents excessive pressure conditions from occurring in one cylinder when lifting uneven loads. Use of the flowrestricting orifices in the equalization line prevents uncontrolled dropping of a raised load in the event of a failure in the equalizer line or fluid lines connected thereto. Supplying pilot fluid from the lift cylinders themselves to operate the holding valves, either as part of normal operations or during emergency operations, increases safety factors. Furthermore, it insures sufficient fluid pressure when the bucket and load are being lowered at a rate which exceeds the capacity of the main pump to deliver operating fluid to the lower ends of the lift cylinders. Other objects and advantages of the invention will hereinafter appear.

Brief Description of the Drawings Further objects of the invention together with additional features contributing thereto and advantages accruing therefrom will be apparent from the following description of a preferred embodiment of the invention which is shown in the accompanying drawings with like reference numerals indicating corresponding parts throughout, wherein: Fig. 1 is a perspective view of a self-propelled wheel loader employing a hydraulic control system in accordance with the invention; Fig. 2 is a schematic diagram of one embodiment of a hydraulic control system in accordance with the invention; and Fig. 3 is a schematic diagram of another embodiment of a hydraulic control system in accordance with the invention.

Description of Preferred Embodiment Referring to Fig. 1, the numeral 10 designates a self-propelled articulated wheel loader, of a type with which a hydraulic control system in accordance with the invention is advantageously employed. The loader 10 includes a rear axle frame 11 articulately coupled to a front axle frame 11 a and supported upon engine-driven groundengaging rear wheels 1 2 and front wheels 14, respectively. An internal combustion engine 1 5 is mounted on the rear axle frame 11 for supplying power to the wheels and other loader components. An operator's cab 1 6 is also supported on the rear axle frame 11, while loadhandling apparatus is connected to the front axle frame 11 a at the front of the loader 10.The loadhandling apparatus includes a pair of laterally spaced load booms 20 pivotally connected at their upper ends to the front axle frame 11 a by pins 21.

The load booms 20 are pivotally connected at their lower ends to a load-carrying bucket 23. The load-handling apparatus further includes a pair of laterally spaced double-acting load boom lift cylinders or hydraulic motors 25 and 25A each connected between one of the load booms 20 and the front axle frame 11 a. As Figs. 2 and 3 show, each hydraulic motor 25 and 25A has its cylinder head end pivotally connected to the front axle frame 11 a by a pin 26. At the opposite or rod end, each hydraulic motor has a piston rod 37 pivotally connected to the load boom 20 by a pin 27.

Operation of the hydraulic motors 25 and 25A effects raising and lowering of the load booms 20 and the bucket 23 attached thereto. It is to be understood that extension of the boom lift cylinders 25 and 25A raises the load booms 20 and the bucket 23 thereon. As shown in Fig. 1, a pair of hydraulically actuated bucket adjusting jacks or cylinders 28 are connected between the front axle frame 11 a and a linkage 30, connected between the load booms 20 and the bucket 23, to pivot the bucket about pins 22.

Figs. 2 and 3 are schematic diagrams of alternate embodiments of a hydraulic control system for operating the lift cylinders 25 and 25A in accordance with the invention. In Figs.1,2 and 3, corresponding components in the several figures are designated by the same reference numerals. Duplicate components in the same figure are designated by a reference numeral and the same reference numeral plus a suffix letter.

Except as otherwise noted, corresponding and duplicate components are constructed and operate in the same manner. A description of one component applies to the other and for convenience of illustration only one of the duplicate systems will be described in detail.

Reference to the duplicate companion system will only be made to describe the interconnection of the system.

Referring specifically to Fig. 2, the boom lift cylinder or hydraulic motor 25 comprises a cylinder 35 having a piston 36 slideably mounted within the cylinder. The piston rod 37 is connected to the piston and extends outwardly through an opening 38 in the rod end of the cylinder. A boom raise port 40 is located near the base end of cylinder 35 and communicates with a load lift or piston end chamber 41, A boom lower port 42 is located near the opposite or rod end of cylinder 35 and communicates with a load lower or rod end chamber 43.

In the embodiment of the control system shown in Fig. 2, an engine-driven main pump 44 pumps fluid from a fluid reservoir 45 to a multiposition directional control valve 47. A pressure relief valve 46 is connected to the output from pump 44 to limit the pressure to the system. A pilot control valve 48, which is actuated by movement of an operating lever 49 for directional control valve 47, through a cam mechanism 50, enables simultaneous operation of valves 47 and 48. A boom safety or load holding valve 52 is mounted in fluid communication on the lift cylinder 25 and connected to the boom raise port 40.A pressure equalizing fluid conduit 100 having a pair of flow restriction orifices 101 and 1 01A therein is connected between the raise ports 40 and 40A of the lift cylinders 25 and 25A, respectively, for a purpose to be hereinafter described in detail. A point of juncture 79 in a vent line 54, used to vent fluid to the reservoir 45 when lowering the bucket 23, is connected by a pilot fluid line 74 to an inlet port 75 of pilot control valve 48. An outlet port 76 of the pilot control valve 48 is connected by a pilot fluid exhaust line 87 to the reservoir 45.

Pump 44 is connected to reservoir 45 by a fluid line 60 and has its output connected to an inlet port 61 of directional control valve 47 by a fluid supply line 62. An exhaust port 63 of directional control valve 47 is connected by a fluid return line 64 to reservoir 45. Fluid supply line 62 from pump 44 is connected to pressure relief valve 46 by a branch line 65 and a pilot pressure line 66 so that when fluid pressure in supply line 62 exceeds a predetermined level, pressure relief valve 46 operates to dump fluid through line 65 and exhaust line 67 to reservoir 45.

Directional control valve 47 includes motor ports 67 and 68 which are connected by main supply lines 69 and 70, respectively, to branch lines 71 and 72, respectively. The branch line 72 is connected between the boom lower ports 42 and 42A, of the lift cylinders 25 and 25A, respectively. The branch line 71 is connectable to the boom raise ports 40 and 40A, respectively, through the holding valves 52 and 52A, respectively. More specifically, branch line 71 connects a first port 73 on one side of holding valve 52 with a first port 73A on one side of holding valve 52A. A second port 77 on the other side of the holding valve 52 is connected by a line 78 to raise port 40 of cylinder 25. Similarly, a second port 77A on the other side of holding valve 52A is connected by a line 78A to raise port 40A.

Directional control valve 47 is a four-position valve (neutral, raise, lower, float) and is selectively movable to any one of its four positions by the manually operated lever 49. Operating lever 49 is connected, as by a linkage 80, to a cam mechanism 50. The cam 50 is engaged by a follower rod 81 to operate the two-position (openclose) pilot control valve 48, which is shown spring-biased into a closed position in Fig. 2. The linkage 80, cam mechanism 50, and follower rod 81 operate so that the valve 48 is opened whenever the directional control valve 47 is in the lower and float positions. The valve 48 remains spring biased into a closed position whenever the directional control valve 47 is moved into the neutral or raise positions.

Referring to an illustrative half of the system, the load-holding valve 52 includes a valve spool -82 which is held closed by a spring 85 when the pilot control valve 48 is closed, i.e., when the main directional control valve 47 is in neutral or raise.

The valve spool 82 is pilot pressure responsive and is provided with a first pilot pressure port 84, which is connected by a first pilot line 83 in fluid communication with the vent line 54 at a juncture point 86. Point 86 is located in the vent line 54 between flow restrictor 56 and the juncture point 79 of the pilot fluid line 74 to provide a pressure differential for actuating the spool 82. The valve spool 82 is further pilot pressure responsive at the end opposite the spring 85, and opposite the first pilot pressure port 84 by a second pilot pressure port 89. Pilot pressure line 90 connects the second pilot pressure port 89 to vent line 54 at juncture point 91 which is also in fluid communication with pressure equalization line 100.

It is to be noted that point 91 is located between the flow restrictors 56 in the vent line 54 and the boom raise port 40. This location may be further described as locating the flow restriction orifice 56 between the first pilot pressure port 84 and the second pilot pressure port 89 in the holding valve 52. The location of the flow restrictor orifice 56 is such that release of pressure in the pilot fluid line 74, through the pilot control valve 48 to the fluid reservoir 45, causes a pressure drop across the orifices 56. This pressure drop causes a pressure differential between the first pilot line 83, and the second pilot line 90. A predetermined size of the orifice 56 in combination with a pre-selected bias designed into the spring 85 is effective to cause the valve spool 82 to shift.This action allows fluid in the raise end 41 of lift cylinder 25 to pass through load-holding valve 52, the main fluid supply line 69, the directional control valve 47 and the fluid return line 64 to the fluid reservoir 45.

As Fig. 2 shows, the load-holding valve 52, which includes the valve spool 82 is mounted directly at the raise port 40 of the lift cylinder 25.

If preferred, the valve 52 could be connected to the raise port 40 with piping having a large factor of safety. When the directional control valve 47 is in the neutral or raise positions, the pilot control valve 48 is closed. Fluid pressure in the raise end chamber 41 of the lift cylinder 25 is applied equally at the pilot ports 84 and 89 on the spool 82 through the lines 83 and 90, respectively, because no fluid is flowing through the orifices 56.

With the pressure force balanced between port 84 and the port 89, the spring 85 for valve spool 82 holds the spool 82 in the closed position. As previously described, opening the pilot control valve 48 connects the spring seat end of the valve spool 82 with the tank or reservoir 45 while a small quantity of fluid flows from ports 40 of lift cylinder 25 to tank through orifice 56. The pressure drop in line 83, resulting from the location of orifice 56, allows higher fluid pressure upstream of the orifice 56 to act on the spool 82, to shift them to their open position against spring 85. Therefore, flow out of the cylinder port 40 occurs through lines 78, 71, 69 and directional control valve 47.

Returning the directional control valve 47 to neutral (or hold) returns the pilot control valve 48 to the closed position thereby stopping flow through orifice 56 to line 74. This removes the force resulting from the pressurize differential on the spool 82, and the spring 85 will move the spool 82 to the closed position blocking flow at the valve port 77 and at the cylinder port 40. A failure of the lift cylinder conduits with the directional control valve 47 in neutral would, therefore, not allow the lift cylinders 25 and 25A to empty. As illustrated in Fig. 2, both load-holding valves 52 and 52A are simultaneously operated by means of the directional control valve 47 and pilot control valve 48.

Check valves 88 and 88A are connected between lines 54 and 71 to provide a means to pressurize the raise ends (chambers 41 and 41A) of the lift cylinders 25 and 25A, respectively, when the directional control valve 47 is in the raise position. Check valves 55 and 55A in the vent line prevent flow from one lift cylinder to the other through the vent line. These valves prevent the spool 82 of one cylinder from opening if the other cylinder should fail.

If during a lowering operation of bucket 23 there is a lack of pressure in the raise or piston ends 41 and 41 A of the lift cylinders 25 and 25A, respectively, that condition is corrected by pressurizing the lower or rod ends 43 and 43A, which would move the pistons 36 and 36A and pressurize the raise end of the cylinder. This situation would occur only when lowering the bucket 23 below ground, or other obstruction, as pressure would always be avaiiable in the raise ends 41 and 41 A of the lift cylinders 25 and 25A whenever the bucket was raised off the ground or obstruction.

With a check valve 88 of sufficient capacity in each holding valve 52, the valve spool 82 in the holding valves need not open in a raise operation.

This provides the automatic protection of a check valve against a system failure occurring in the raise mode. However, the cam mechanism 50, which operates the pilot control valve 48, could be contoured such that the valve 48, and thus the spool valve 82 of the holding valve 52, would open in the raise mode of the directional control valve 47. During lifting operations, however, in the preferred embodiment, the valve spool 82 would not open, and all of the raise flow would go past the check valve 88 in the holding valve 52. The flow restriction orifices 56, in addition to providing pressure differential between lines 83 and 90, for control of valve spool 82, also prevents sudden discharge of fluid from raise chambers 41 in the event of failure of the vent line 54 or 54A or the pilot fluid line 74. The flow restrictors 101 and 101 A in equalization line 100 perform the same function.

The embodiment disclosed with reference to Fig. 3 differs from that described with reference to Fig. 2 in regard to the means for operating the directional control valve 47 and the pilot control valve 48, and also in regard to the manner in which the holding valves 52, 52A are operated in response to pilot pressure. The load-holding valves 52, 52A have their pilot pressure ports 84, 84A connected directly to the reservoir 45 through lines 83, 83A. In addition, the pilot pressure port 89 is connected by a pilot pressure line 110 for actuation by the same pilot pressure that is used to operate the directional control valve 47 to the load lowering position. The flow restriction orifice 56 is only used to prevent sudden discharge of fluid from lift chamber 41 in the event of failure of the equalization line 54 or the pilot fluid line 74.In Fig. 3, line 54 is the equalization line. The venting done in Fig. 2 is not done in Fig. 3. Other differences between the embodiments disclosed in Fig. 2 and Fig. 3 will be apparent as hereinafter described.

Pilot operation of the load-holding valve 52 of the Fig. 2 embodiment effects a pressure drop across the orifice 56 and shifts the valve spool 82 against the bias of spring 85. Substantially all of the fluid from chamber 41 then flows through the load-holding valve 52 and the directional control valve 47 to the reservoir 45. With regard to the embodiment of Fig. 3, the pilot operation of the load-holding valves 52 is as follows. As stated above, the orifice 56 in the Fig. 3 embodiment has no function in connection with the pilot operation of the load-holding valve 52. This orifice serves to prevent sudden escape of fluid from the raise of the cylinder 25 in the event of failure in the lines 54 or 74 in Fig. 3.The line 74 functions to provide a source of pilot pressure to a manually actuated pilot valve 48A in the event that auxiliary pump 44A does not provide pilot pressure with the boom raised.

A pilot pressure regulating valve 120 (diagrammatically shown in the upper left of Fig.

3) actually contains two valves; a first valve 122 for regulating the auxiliary pump pilot pressure to approximately 250 psi; and a second valve 124 for regulating pilot pressure originating at the load-carrying raise chambers 41, 41 A in the boom lift cylinders 25, 25A. The regulating pressure of valve 1 24 is set for approximately 230 psi and is arranged so that fluid pressure in the raise sides 41 of the cylinder 25 is not released to the tank 45 and wasted. A check valve 126 is also provided to prevent cylinder-originating-fluid from passing through the auxiliary pump pressure regulating valve 1 22. Pilot pressure from either source (line 74 or pump 44A) is therefore avaiiable to the manually actuated pilot control valve 48A.

As seen in Fig. 3 schematic, when pilot pressure fluid is directed to the loader boom directional control valve 47 for lowering the boom, or for float position, pilot pressure fluid is also directed to the load-holding valves 52, 52A at the ports 89, 89A. The spool 82 is shifted to open against the bias of spring 85. The bleed lines 83, 83A which are connected to the valves at ports 84, 84A merely prevent build-up of pressure or externai ieaks at the spring end of the spools 82, 82A and have no other influence on pilot operation. Check valves 88 and 88A provide means to pressurize the raise ends (chambers 41 and 41A) of the lift cylinders 25 and 25A when the directional control valve 47 is in the raise position such as was described with reference to Fig. 2.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without department from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include ail embodiments falling within the scope of the appended claims.

Claims

1. A load-handling apparatus comprising: a pair of hydraulic lift cylinders, each said lift cylinder having a load raise chamber and a load lower chamber, and a hydraulic control system for said lift cylinders including an equalizer conduit connected between said load raise chambers of said lift cylinders to equalize fluid pressure therebetween, said equalizer conduit including a pair of spaced apart flow-restricting orifices therein, one orifice for each said lift cylinder, for restricting fluid flow from said load raise chambers in the event of a loss of fluid pressure in said equalizer conduit between said orifices.

2. A hydraulic control system for a loadhandling apparatus including a pair of hydraulic lift cylinders, each said hydraulic lift cylinder having a load raise chamber and a load lower chamber in fluid communication with a source of pressurized hydraulic fluid, a hydraulic fluid directional control valve controlling the passage of pressurized hydraulic fluid to said load raise and load lower chambers for operating said lift cylinders, a pair of pilot pressure operated holding valves interposed between a reservoir for the hydraulic fluid and said load raise chambers for controlling fluid flow from said load raise chambers of said lift cylinders to said reservoir, a pilot pressure control valve operatively coupled to said pilot pressure operated holding valves for controlling the application of pilot pressure for operating said pilot pressure operated holding valves, an equalizer conduit connected between said load raise chambers of said lift cylinders to equalize fluid pressure therebetween, and a pressure reducer connected between said pilot pressure control valve and each said load raise chamber to effect operation of said pilot pressure operated holding valves in response to fluid pressure conditions in said load raise chambers when said pilot pressure control valve is operated.

3. Apparatus according to Claim 2 wherein said equalizer conduit includes a pair of spaced apart flow restricting orifices therein, one orifice for each lift cylinder, for restricting fluid flow from said load raise chambers in the event of a loss of fluid pressure in said equalizer conduit between said orifices.

4. Apparatus of Claim 2 further including means for simultaneously operating said directional control valve and said pilot pressure control valve.

5. Apparatus according to Claim 4 wherein said pressure reducer comprises a pair of spaced-apart flow restricting orifices coupled in a venting conduit to said fluid reservoir with one orifice for each lift cylinder to provide a fluid pressure drop across said pilot pressure operated holding valve for effecting fluid flow from said load raise chambers of said lift cylinders.

6. A hydraulic control system for a loadhandling apparatus including a pair of hydraulic lift cylinders, each said lift cylinder having a load raise chamber and a load lower chamber in fluid communication with a source of pressurized hydraulic fluid, a multi-position directional control valve controlling the passage of pressurized hydraulic fluid to said load raise and load lower chambers for operating said lift cylinders, a pair of normally closed pilot pressure operated holding valves operatively connected to said load raise chambers to control flow of pressurized hydraulic fluid from said raise chambers of said lift cylinders, a normally closed pilot pressure control valve operatively coupled to said pilot pressure operated holding valves to control the operation thereof, means operable to open said pilot pressure control valve, thereby opening said normally closed pilot pressure operated holding valves, when said multi-position directional control valve is positioned to supply pressurized hydraulic fluid to the load lower chambers of said lift cylinders, an equalizer conduit connected in fluid communication between said load raise chambers of said lift cylinders to equalize pressure therebetween, and a pressurized hydraulic fluid vent line connected between said normally closed pilot pressure operated holding valves and said normally closed pilot pressure control valve to effect opening of said pilot pressure operated holding valves when said pilot pressure control valve is open.

7. A hydraulic control system for a load handling apparatus including a pair of doubleacting hydraulic lift cylinders, each said doubleacting hydraulic cylinder having a load raise chamber and a load lower chamber with each such chamber having a fluid port formed therein in fluid communication with a source of pressurized hydraulic fluid, a source of pressurized fluid, a reservoir for hydraulic fluid, a multiposition directional control valve for operating said lift cylinders and having at least neutral, lower and raise positions, said multi-position directional control valve being in fluid communication with said source of pressurized hydraulic fluid and said reservoir for hydraulic fluid, first and second pressurized hydraulic fluid lines coupled in fluid communication with said multi-position directional control valve, said first pressurized hydraulic fluid line coupled to the fluid ports of said load lower chambers of said double-acting hydraulic lift cylinders, a pair of normally closed pilot pressure operated holding valves connected in fluid communication with said fluid port of said load raise chambers of said double-acting hydraulic lift cylinders to control fluid flow therefrom, each said pilot pressure operated holding valve being interposed between said fluid ports of said load raise chambers and said second pressurized hydraulic fluid line, each said pilot pressure operated holding valve having a pilot pressure port to effect opening of said pilot pressure operated holding valve in response to pilot pressure conditions at said pilot pressure port, a pilot pressure control valve in fluid communication with each of said pilot pressure operated holding valves for effecting operation thereof, actuator means for operating said multiposition directional control valve and said pilot pressure control valve to close said pilot pressure control valve when said multi-position directional control valve is is neutral and to open said pilot pressure control valve when said multi-position directional control valve is in said lower position, an equalizer conduit in fluid communication between said fluid ports of said raise chambers of said double-acting hydraulic lift cylinders to equalize the pressure therebetween, a pair of spaced-apart flow restricting orifices positioned in said equalizer conduit for restricting fluid flow from said load raise chambers, and pilot pressure line coupled between said reservoir of hydraulic fluid and said pilot pressure port to open said normally closed pilot pressure operated holding valves upon fluid flow through said pilot pressure line.

8. The hydraulic control system of Claim 7 further including a pair of check valves, each check valve interposed between said second fluid line and the fluid port of said raise chamber, each check valve being coupled to permit fluid flow into said raise chamber from said second fluid line.

9. The hydraulic control system of Claim 7 wherein said multi-position directional control valve is operated by pilot pressure and further including an auxiliary pump for supplying pilot pressure and a selectively operable control valve for directing pilot pressure from said auxiliary pump to said multi-position directional control valve and to the pilot pressure ports of said holding valves.

10. A hydraulic control system for a load handling apparatus including a pair of doubleacting hydraulic lift cylinders, each said doubleacting hydraulic cylinder having a load raise chamber and a load lower chamber with each such chamber having a fluid port formed therein in fluid communication with a source of pressurized hydraulic fluid, a source of pressurized fluid, a reservoir for hydraulic fluid, a multi-position directional control valve for operating said lift cylinders and having at least neutral, lower, and raise positions, said multi-position directional control valve being in fluid communication with said source of pressured hydraulic fluid and said reservoir for hydraulic fluid, first and second pressurized hydraulic fluid lines coupled in fluid communication with said multi-position directional control valve, said first pressurized hydraulic fluid line being coupled to the fluid ports of said load lower chambers of said double-acting hydraulic lift cylinders, a pair of normally closed pilot pressure operated holding valves connected in fluid communication with said fluid port of said load raise chambers of said double-acting hydraulic lift cylinders to control fluid flow therefrom, each said pilot pressure operated holding valve being interposed between said fluid ports of said load raise chambers and said second pressurized hydraulic fluid line, each said pilot pressure operated holding valve having a pilot pressure port to effect opening of said pilot pressure operated holding valve in response to pilot pressure conditions at said pilot pressure port, a pilot pressure control valve in fluid communication with each of said pilot pressure operated holding valves for effecting operation thereof, actuator means including an auxiliary pump for supplying pilot pressure and a control valve for directing said pilot pressure to operate said multi-position directional control valve and said pilot pressure operated holding valves so that said pilot pressure operated holding valves are closed when said multi-position direction control valve is in neutral and said pilot pressure operated holding valves are open when said multi-position directional control valve is in lower position, and equalizer conduit in fluid communication between said fluid ports of said raise chambers of said double-acting lift cylinders to equalize the pressure therebetween, a pair of spaced-apart flow restricting orifices positioned in said equalizer conduit for restricting fluid flow from said load raise chambers, and a pilot pressure conduit having one end connected to each pilot pressure port of said pilot pressure operated holding valves and having its other end operatively connected to said pilot control valve for supplying pilot pressure to operate said holding valves in the event of a failure of pressure from said auxiliary pump.

11. The hydraulic control system of Claim 10 wherein said pilot control valve is actuated to supply pilot pressure to said pilot pressure operated holding valves through said multiposition directional control valve.

12. A hydraulic control system for loadhandling hydraulic motors substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.