Hydraulic power control circuit and construction vehicle comprising such circuit

In a load sensing hydraulic control circuit, a pressure compensator elaborates both a load sensing pressure signal to control a pump unit and a pressure compensated power flow for actuators, the power flow being either split or alternatively directed to at least a first and a second actuator control valves so that a differential pressure across the first and second control valves is controlled by the pressure compensator. In one embodiment, the circuit is onboard a construction vehicle to power actuators such is travel left, travel right, swing, boom, arm, bucket and the like.

The present invention relates to an hydraulic power control circuit for operation of a plurality of actuators, in particular for construction vehicles, such as loaders, excavators and the like.

BACKGROUND OF THE INVENTION

A construction vehicle is provided with a plurality of actuators that are controlled by an operator. It is known to provide a cost effective control circuit for a construction vehicle using an open center control circuit. However, a proportional control of actuator with an open center technology is not possible. This requires a particularly skilled operator for the construction equipment.

It is also know to provide a construction vehicle with a load sensing control circuit and a relative pump unit. The load sensing technology ensures a proportional control of the actuators, which can be operated simultaneously in order to increase efficiency of the construction vehicle. However a load sensing circuit requires a relatively large number of components because each control spool valve is associated to a pressure compensator. Furthermore a pressure compensator is a relatively expensive hydraulic component.

US2013/220425 discloses a hydraulic circuit with a single pressure compensated orifice controlling flow to two control valves.

It is therefore the scope of the present invention to provide a control circuit that is less expensive than a load sensing one and, at the same time, provide a comparable performance to obtain a relatively easy operation of the actuators.

SUMMARY OF THE INVENTION

The scope of the present invention is achieved with a load sensing hydraulic control circuit wherein a pressure compensator elaborates both a load sensing pressure signal to control a pump unit and an output power flow that is either split or alternatively directed to at least a first and a second actuator control valves so that a differential pressure across the first and second control valves is controlled by the pressure compensator. This provides a sharing of the compensator between first and second control valves.

A construction equipment vehicle may be provided with the control circuit cited above.

Additional features of the invention are comprised in the dependent claims.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1refers, as a whole, to a load sensing circuit1suitable for connection to a load sensing pump unit (not shown) having either a variable displacement and an adjustable spring to set a preferred differential pressure upon a load sensing pressure signal; or a fixed displacement pump and a pump load sensing circuit having a regulating valve to deliver to a tank an excess flow generated by the fixed displacement pump.

Load sensing circuit1is connectable to a first actuator and a second actuator, e.g. actuators of a construction vehicle, embodiments of which will be discussed later. In particular circuit1comprises a first actuator line A1, B1, and a second actuator line A2, B2, each of which is connectable to a respective actuator.

Circuit1comprises pump line PL that is connectable to the load sensing pump unit (not shown) and provides a power flow to circuit1in order to control actuators through actuator lines A1, B1, A2, B2.

Circuit1also comprises a load sensing line LS to collect a pressure pilot signal from actuator lines A1, B1, A2, B2, and deliver such pilot signal to the load sensing pump unit.

Circuit1comprises a tank line TL connectable to a hydraulic tank or sump (not shown) and normally kept at environment or at a selected and low pressure in order to provide a reference low pressure signal.

Circuit1is embodied in a control block2that is schematically shown inFIG. 1. Block2delimits ports that are connected to components not shown inFIG. 1. In particular, block2comprises a pump port PP connectable to the pump unit to feed pump line PL, a load sensing port LSP connectable to the load sensing circuit of the pump unit and a tank port TP to connect tank line TL to the tank. Ports of block2are preferably disconnectable ports so that block2can be mounted/demounted as a whole or in part from a construction vehicle e.g. for inspection and/or maintenance purposes.

According to the embodiment shown inFIG. 1, circuit1further comprises a first spool control valve V1and a second spool control valve V2to control connection of first and second actuator lines A1, B1, A2, B2respectively, to pump line PL and tank line TL. In particular first and second control valves V1, V2control the power flow and, in a working position, move first and second actuators through first and second actuator lines A1, B1, A2, B2respectively. First and second control valves V1, V2have a neutral position interrupting flow from pump line PL, i.e. a closed neutral position.

Circuit1also comprises a pressure compensator C1input connected to a T branched compensator inlet line TBIL1. Inlet line TBIL1is attached to respective outputs of first and second control valves V1, V2and has an input node IN defining the starting point of a main branch adducting to a compensator input CI the sum of flows coming upstream of input node IN. In view of this, a maximum flow corresponding to the cumulative flow directed to first and second actuator line A1, B1, A2, B2from pump line PL is elaborated by pressure compensator C1, which is therefore located downstream of first and second control valves V1, V2along compensator inlet line TBIL1. The latter is connected, through a compensator output CO1, to a T branched compensator output line TBCL1having a compensator output node CN defining the end of a main branch connected to output CO1. In output node CN the cumulative flow splits into a first flow directed to power the first actuator line A1, B1for moving the relative actuator and a second flow directed to power the second actuator line A2, B2for moving the relative actuator. To this regard, compensator output line TBCL1is attached to first and second control valves V1, V2. In the preferred embodiment ofFIG. 1, control valves V1and V2meter the flows directed to compensator C1. In particular, flow metering is operated by control valves V1, V2through a respective calibrated notch of the spool that feeds the inlet line TBIL1. Instead actuator lines A1, B1, A2, B2are fed by a respective on-off flow adduction, i.e. without calibrated notches.

As shown inFIG. 1, input node IN is where flows coming in parallel through control valves V1and V2merge upstream of pressure compensator input C1. To this regard, input node IN is connectable to pump line PL through a first line L1of inlet line TBIL1and through a second line L2of inlet line TBIL1. First and second lines L1, L2converge into input node IN. Input line TBIL1is connected to pump line PL through first and second control valves V1, V2when either first or second or both control valves V1, V2are in a working position.

Preferably, in order to avoid backflows, first and second lines L1, L2comprise a respective non-return valve NR1, NR2that stop flow directed from node IN to the relative control valve V1, V2. The provision of non-return valves NR1, NR2stabilizes the functioning of circuit1.

Additionally, in order to safeguard pressure compensator C1from an excessive flow, a calibrated restrictor R1processing the power flow entering in pressure compensator C1is placed between input node IN and compensator input CI.

In use, an operator can either at the same time or alternatively operate first or second control valve V1, V2. When the first or second control valves V1, V2are operated alternatively, e.g. control valve V1is operated, compensator C1is open and the differential pressure across control valve V1equals the setting of compensator C1. Compensator C1is shared by first and second control valves in that a single compensator serves two valves operated alternatively. In such a condition, control of an actuator attached to circuit1according toFIG. 1is proportional to the opening of the control valves V1, V2.

When both control valves V1and V2are simultaneously operated, the predefined differential pressure is applied to both the control valves, but the greatest part of the power flow directs to actuator having the lower load e.g. control valve V1and first actuator. Actuator controlled by valve V2moves slowly until the relative working pressure for actuation becomes, for example equal or lower than that of first actuator. In case of simultaneous operation of first and second control valves V1, V2, flow splits in output node CN depending on the load on first and second actuators, i.e. in case of higher load on the first actuator the higher share of flow will direct towards the second actuator. Therefore a proportional control of actuators can only be achieved when first and second control valves are non-simultaneously operated.

FIG. 2shows a circuit10and control block20that represent a second embodiment of the present invention. The description of embodiment inFIG. 2will be such that elements functionally identical to those of embodiment inFIG. 1will be indicated below using the same reference numerals adopted in the preceding paragraphs. In particular, embodiment ofFIG. 2differs from the embodiment ofFIG. 1in the following.

First control valve V1′ further comprises, with respect to control valve V1, a first and a second neutral through passage along respective first and second valve center through lines TL1, TL2that are open in a neutral position of first control valve V1′ and that, in working positions of first control valve V1′, are closed. First valve center through line TL1is connected to output node CN and second actuator line A2, B2when first control valve V1′ is in neutral position and second control valve V2is in a working position; second valve center through line TL2is the connection through which second line L2of compensator inlet line TBIL1is connected to pump line PL when first control valve V1′ is in neutral position. First and second valve center through lines TL1, TL2are closed when second control valve V2is in neutral position.

In use, action of compensator C1is shared alternatively by first and second control valves V1′, V2, namely compensator C1feeds alternatively valves V1′ or V2. Furthermore, first control valve V1′ is fed by compensator C1with an absolute priority, i.e. regardless the position of second control valve V2or the pressure on first and second actuator lines A1, B1, A2, B2. In particular, when control valve V1′ is operated, second actuator line A2, B2is blocked. In general, according to absolute priority, a control valve always meters the inlet flow to one and only one compensator and in case such compensator is receiving metered flow from other control valves, when the absolute priority valve is operated, flow from other control valves will be stopped and the compensator will receive metered flow from the absolute priority valve.

In view of the fact that compensator C1processes flow alternatively for actuator valves V1′ or V2, power flow in output node CN is not split but more simply directed either to second control valve V2when first control valve V1′ is in neutral position or to first actuator line A1, B1when first control valve V1′ is operated.

Differential pressure across first and second control valves V1′, V2is constant and predefined by the load sensing control unit and compensator C1. Actuators attached to control circuit10ofFIG. 2are always proportionally controlled with respect to the opening of the relevant control valve.

According to the embodiment ofFIG. 3it is possible to expand load sensing circuit10by adding one or more third spool control valves V3identical to first control valve V1′ ofFIG. 2and placed between first and second control valves V1′, V2. In particular, control valve V3has a spool identical to that of first control valve V1′. Spool control valves may comprise a valve body providing a number of ports for connection with conduits or pipes that are connected, i.e. welded, threaded or the like, to the valve body. As an alternative, the valve body defines portions of respective ducts so that, in order to assemble block20, valve bodies are fluidically connected without provision of dedicated intermediate tubes or pipes connected to the valve body. Neutral through passages of third valve V3are in series to corresponding neutral through passages of first valve V1′ by means of valve center through lines TL1, TL2respectively.

Furthermore, third control valve V3is such to selectively connect a third actuator line A3, B3to pump line PL and tank line TL in order to power the motion of a third actuator (not shown).

In particular, third control valve V3is connected to compensator input node IN through a third line L3. Third line L3comprises a non return valve NR3having the same function as NR1and connected by a T-junction T1to input node IN. This makes compensator input line TBIL1of circuit10a multi T-branched compensator input line. In general, each additional control valve used to expand circuit10according to the teaching ofFIG. 3adds an additional branch with the relative non-return valve to multi T-branched compensator inlet line TBIL1. To this regard, an expansion module EM of circuit10comprises a module through conduit11as a section of compensator output line TBCL1's main branch, module through conduit12as a section of valve center through line TL1intersecting third control valve V3, module through conduit13as a section of valve center through line TL2intersecting third control valve V3, module through conduit14as a section of pump line PL and module through conduit15as a section of load sensing line LS. A module through conduit of module EM is such to fluidically connect two opposing connection faces F1, F2of the module, e.g. of a valve body slidingly housing a control spool and defining the through conduits, so that the block20can be assembled comprising a stacking pack of modules EM.

Furthermore, expansion module EM comprises a bypass intercepted by third control valve V3for connection of conduit13to a section of input line TBIL1through line L3. A T-junction T1is provided for connection of line L3to input line TBIL1and a T-junction T2is provided for connection of the bypass to conduit13across third control valve V3; a T-junction T3for connection of a through section of tank line TL with third actuator line A3, B3; a T-junction T4for connection of third actuator line A3, B3to conduit12across third control valve V3; and conduits A3, B3.

In use, an absolute priority to meter power flow for compensator C1and move first actuator is given to first control valve V1′ with respect to the third control valve V3, which is located immediately downstream of first control valve V1′ along valve center through lines TL1, TL2with respect to second control valve V2. Furthermore, third control valve V3has a higher non-absolute priority to meter power flow for compensator C1and move third actuator with respect to second control valve V2. More in general, according to the expansion of circuit10shown inFIG. 3, each expansion module EM has a priority to receive power flow from compensator C1over the next downstream expansion module EM along valve center through lines TL1, TL2.

Also in circuit10ofFIG. 3the velocity of each actuator is proportional to the opening of the respective control valve V1′, V3, V2and, when necessary, compensator C1alternatively elaborates the power flow directed to the relative actuator. Therefore, first control valve V1′ of circuit10is an example of an absolute priority control valve to meter power flow to compensator C1and thus ensure proportional control of the relative actuator regardless simultaneous switch of either second or third control valve V2, V3. Furthermore, third control valve V3has a non-absolute priority over second control valve V2to meter flow to compensator C1. This ensures proportional control of the third actuator regardless the switch of second control valve V2and subject to switch of first control valve V1′, which enjoys absolute priority over compensator C1.

FIG. 4shows a further embodiment of a load sensing circuit100and control block200. The description of embodiment inFIG. 4will be such that elements functionally identical to those of embodiments inFIGS. 1 to 3will be indicated below using the same reference numerals adopted in the preceding paragraphs. In particular, embodiment ofFIG. 4differs from the embodiment ofFIG. 3in the following.

Circuit100and block200ofFIG. 4comprise an additional pressure compensator C2having a compensator input CI2attached by means of a T-branched input line TBIL2to both second and third control valves V3′, V2. In particular, input line TBIL2comprises respective branches BC2and BC3connected to control valves V2and V3′ respectively through check valves CH2, CH3and parallel connected to CI2. Preferably input line TBIL2comprises a further branch for connection with an input port IP on block200. Such further branch is parallel connected to branches BC2, BC3and expands input line TBIL2into a multi T-branched feed line. Upstream of compensator input CI12, branch BC2extends across second control valve V2and ends attached to third control valve V3′ and branch L3extends across third control valve V3′ and ends attached to first control valve V1″. Therefore first, second and third control valves V1″, V2′, V3′ differ from the corresponding valves ofFIG. 3by the addition of ports to process fluid along branches BC2, BC3as defined above. Furthermore circuit100comprises a bridge BR to connect branch BC3between first and third control valves V1″, V3′ to branch BC2between second and third control valves V2, V3′ in order to bypass third control valve V3′. Downstream of both branches BC2, BC3, input line TBIL2comprises a restrictor R2to avoid input overflow to second compensator C2. Second compensator C2is shared by second and third control valve V2′, V3′ and not by first control valve V1″ because the latter is not attached to the output of compensator C2. Therefore compensator C2is downstream second and third control valve V2′, V3′ along input line TBIL2and, at the same time, disconnected from first control valve V1″.

In particular, a power output CO2of second pressure compensator C2is connected to a T-branched compensator output line TBCL2to feed second and third actuator lines A2, B2, A3, B3through second and third control valves V2′, V3′. Output line TBCL2preferably has a further branch connected to an output port OPon block200so that output line TBCL2, in some embodiments, is a multi T-branched output line of second compensator C2.

Preferably output line TBCL2has a output node CN2where flow coming from second compensator C2splits to reach the second and third actuator lines A2, B2, A3, B3. Downstream of output node CN2, each branch of compensator output line TBCL2is connected to a respective flow deflector FD2, FD3. Each flow deflector FD2, FD3feeds the relative actuator line A2, B2, A3, B3, with the flow from either second compensator C2or first valve center through line TL1to selectively feed second and third actuator lines A2, B2, A3, B3depending on the case.

In use, contrary to circuit10, when first control valve V1″ is operated in a working position, downstream control valves V3′ and V2′ remain parallel input connected to pump line PL through bridge BR and terminal section of third branch BC3in order to selectively feed input line TBIL2of second compensator C2when operated in a working position. In particular, bridge line BR is such to feed second control valve V2also when third control valve V3′ is in neutral position.

In case of simultaneous operation of first control valve V1″ with another control valve, compensator C1is prioritized to feed first actuator line A1, B1and neither second nor third actuator lines A2, B2, A3, B3. This is because first control valve V1″, when in a working position, closes second valve center through line TL2and feeds branch BC3input line TBIL2of second compensator C2.

Nevertheless, when alternatively operated, first, second and third control valves V1″, V2′, V3′ share compensator C1because V1″ is not connected to input line TBIL2of second compensator C2; and input line TBIL2is not fed when both first and third control valves V1″, V3′ are in neutral position.

When first control valve V1″ is neutral and second and third control valves V2′, V3′ are operated, both input lines TBIL1and TBIL2of respective compensators C1and C2are fed so that each control valve V2′, V3′ is assigned to a respective compensator C1, C2.

When all control valves are simultaneously operated, first control valve V1″ is prioritized to feed only compensator C1so that the first actuator can be controlled in velocity due to the predefined differential pressure regardless the conditions of second and third control valves V2′, V3′ (absolute priority); and second and third valve V2′, V3′ share second compensator C2so that second and third actuators can be controlled by predefined differential pressure in a flow saturation condition, i.e. the predefined differential pressure of C2is applied to the control valve feeding the actuator with the lower load, i.e. working pressure, first and, then to the other control valve. This is the same functioning of circuit1.

Furthermore, third control valve V3′ enjoys a non-absolute priority to compensator C1with respect to second control valve V2′ so that, when first and third control valves V1″ and V3′ are neutral, second control valve V2′ is associated to compensator C1. However, in case third control valve V3′ and second control valve V2′ are simultaneously in a working condition, then third control valve is associated to compensator C1and second control valve V2′ meters power flow to compensator C2. This applies when first control valve V1″ remains neutral.

Circuit100is expandable through second expansion module EM′ (FIG. 5) that has a valve body defining conduits and comprising check or one way valves such to provide a module that serially expands block200in case a fourth or additional actuators are added to share first and second compensators C1, C2. Second expansion module EM′, additionally to expansion module EM, includes: a bridge to connect T-junction T4to a T-junction T5along a module through conduit16of second compensator output line TBCL2; the flow deflector FD3for connection of T-junctions T4, T5to actuator line A3, B3across third control valve V3′; a bridge to connect a module through conduit17of bridge BR to a module through conduit18of compensator inlet line TBIL2through branch BC3, such former bridge having a T-node TN for connection to an inlet port of expansion module EM′ and third control valve V3′ being across the main branch between T-node TN and module through conduit18; and an output conduit19attached between third control valve V3′ and an outlet of expansion module EM′ for accession to bridge BR and second control valve V2′ outside of expansion module EM′. In particular, suitable one-way valves W are placed along bridge BR in order to avoid backflow from conduit19when third control valve V3′ is in an operating position. Therefore flow from T-node TN bypasses third control valve V3′ to reach second control valve V2′ in a first direction and cannot backflow in the opposite direction due to one-way valves W.

A schematic view of flows when all three control valves are in a respective working conditions is provided inFIG. 6.

FIG. 7shows a circuit1000that is an expansion of circuit100and provided onboard of a construction vehicle to command power actuators. Actuators of construction vehicles are connected to circuit1000in order to best optimize the sharing of pressure compensators considering which function does not need to be simultaneous with other ones and which other function, instead, needs to be coupled simultaneously with other ones. In particular, circuit1000comprises a first and a second inner packs IP100, IP100′ preferably equal to one another and comprising respective first, second, third control valves V1″, V2′, V3′, compensator C1and multi T-branched input line TBIL1and T-branched compensator output line TBCL1. In particular inner packs IP100, IP100′ are aggregated sub-modules from circuit100ofFIG. 4.

Circuit1000further comprises a pack P having three spool control valves V5that differ from first and third control valves V1′, V3ofFIG. 3in that a third neutral through passage is present in neutral position. Third neutral through passage is such to connect flow from BC2and BC3parallel branches of inner packs IP100, IP100′ to a third compensator C3of pack P by means of a third valve center through line TL3. Therefore third valve center through line TL3is a main branch of a multi T-branched inlet line TBIL3that feeds compensator C3. In particular third valve center through line TL3converges into input node IN3that, excluding such additional connection, is functionally identical to input node IN of circuit10,FIG. 3.

As a last power control valve upstream of input node IN3along through line TL3, pack P comprises a control spool valve V6identical to control valves V1′, V3. In its neutral position, control valve V6closes pump line PL. Furthermore, first neutral through passage of control valve V6is part of a multi T-branched compensator output line TBCL3of compensator C3that when also control valves V5are in neutral position, reaches second and third actuator lines A2, B2, A3, B3of inner packs IP100, IP100′ (seeFIG. 8). This is because compensator output line TBCL3comprises, downstream of its output node CN3, which functionally corresponds to output nodes CN, CN2, T-junctions TT for connection to actuator lines attached to control valves V5, V6and T-junctions, e.g. T-junctions T5, for connection with second and third actuator lines of circuits IP100, IP100′. At last, second neutral through passage of valve V6connects in neutral position through line TL3of compensator inlet line TBIL3to input node IN3. According to the connections described above, when control valves of all packs are alternatively operated, they share the compensator C1, C3of the relative pack. Control valves V5, V6of pack P cannot be actuated to have simultaneous respective working positions. This functioning is in common with that of circuit10. Consistently with circuit10, there is a single control valve V5with an absolute priority over compensator C3with respect to other control valves V5and V6of pack P as well as with respect to packs IP100, IP100′. Furthermore, remaining control valves V5and V6enjoy a non absolute priority over compensator C3, such non-absolute priority prevailing on that of packs IP100, IP100′, i.e. in case a control valve from pack IP100, IP100′ meters power flow to compensator C3and one of remaining control valves V5, V6is operated, the flow to the control valve of pack IP100, IP100′ is interrupted.

Furthermore, second and third valves V2′, V3′ of inner packs IP100, IP100′ can share third compensator C3, in case of simultaneous working position of the respective first valve V1″ and neutral position of control valves V5, V6of pack P. When one of control valves V5, V6is switched in working position, compensator C3feeds the relative actuator attached to pack P so that actuators attached to pack P take priority for use of compensator C3over actuators attached to first and second inner packs IP100, IP100′. This is because, through compensator inlet line TBIL3, compensator C3is downstream to second and third control valves V2′, V3′ of modules IP100, IP100′ and to control valves V5, V6of module P.

Preferably, the following actuators are onboard of the construction vehicle and attached to circuit1000: travel left, travel right, bucket, boom, arm, service I, service II, dozer blade, swing and boom swing. In particular swing refers to rotary motion of an upper frame of the construction vehicle with respect to a lower frame to which travel system of the vehicle is attached. Furthermore boom swing refers to an additional rotational degree of freedom of a boom with respect to the lower frame.

Preferably absolute priorities are associated to operation of:travel left within inner pack IP100;travel right within inner pack IP100′;swing within pack P.

According to a not-shown embodiment, where there are only two packs having one pressure compensator each, the absolute priority to the use of compensators is respectively assigned to travel left and travel right actuators.

FIG. 9is a further embodiment of the present invention comprising two inner packs identical to IP100, IP100′ of circuit1000and an additional pack P2that is an expanded circuit100, i.e. having two control valves V3′ and respective expansion modules EM′. In particular, compensator C2of pack P2is connected to all control valves but first control valves V1″ of the circuit as a whole by means of an extended multi T-branched compensator output line TBCL4. Compensator C2functions in case a fourth actuator fed by second and third control valves V2′, V3′ is simultaneously operated to other three actuators.

FIG. 10schematically shows the priorities associated to the actuators ofFIG. 9. In particular, absolute priority is associated to the following components:First control valve V1″ of travel left and compensator C1of IP100;First control valve V1″ of travel right and compensator C1of IP100′; andFirst control valve V1″ of swing and compensator C1of P2.

Other actuators are given a non-absolute priority over compensator C1of the respective pack and, in case of simultaneous operation with another control valve of the same pack, compensator C4takes over the control of the valve that has a lower priority.

The advantages of a hydraulic control circuit according to the present invention are the following.

Sharing of pressure compensators C1, C2, C3among actuators reduces costs, dimensions and weight of the hydraulic control block2,20,200.

Furthermore, different level of priorities are assignable to the control valves for interaction with the compensators in sharing, namely absolute priority (first control valves of circuits10,100,1000), and non-absolute priority.

The new system is modular providing expansion capabilities through expansion modules EM, EM′. In particular expansion modules EM, EM′ comprises valve bodies defining ducts and comprising check or one-way valves such to control additional actuators without requiring to be adapted to the specific actuator. Therefore a block20,200may comprise three or more identical expansion modules EM, EM′ depending on the number of actuator to be controlled and powered.

Provision of non-return or check valves in selected locations improves stability of the circuit.

In view of the priorities it is important to have the travel left and right functions to be independent from one another in order to drive the vehicle. Furthermore, when at least boom actuator, arm actuator, bucket actuator, swing actuator, service I actuator and service II actuator, the following groups are preferred in order to guarantee the simultaneous operation of the following functions:absolute priority: travel left, travel right and swing;non-absolute priority and in different circuits: service I and boom or bucket; arm and service II.

It was estimated that the following simultaneous operations rendered possible in view of the above combinations, are very common:

travel left and travel right and boom;

travel left and travel right and arm;

travel left and travel right and bucket;

boom, arm and swing;

service I and boom or bucket;

service I and service II.

Finally it is clear that modifications may be made to the control circuit disclosed and shown herein without departing from the scope of protection defined by the appended claims.

When only two circuits are used, the actuators can be grouped as follows:

Spool control valves V1, V1, V1″, V2, V2′, V3, V3′, V5and V6may be manually controllable (see the figures) or other types of controls such as hydraulic control or electromagnetic control are applicable.