GB2465572A

GB2465572A - Switchable source hydraulic supply system

Info

Publication number: GB2465572A
Application number: GB0821228A
Authority: GB
Inventors: Bruno Bozo; Vincent Chauvel; Thomas Deschodt
Original assignee: AGCO SA
Current assignee: AGCO SA
Priority date: 2008-11-20
Filing date: 2008-11-20
Publication date: 2010-05-26
Also published as: GB0821228D0

Abstract

A utility vehicle hydraulic supply system 10 comprises a fixed displacement pump 12 e.g. a gear pump and a variable displacement pump 14, e.g. a load sensing swash pump, and is operable between a first mode where the variable pump supplies a hydraulic circuit 20 which may include brake, steering and implement control circuits and a second mode where the fixed pump supplies the hydraulic circuit and the variable pump supplements the supply to the hydraulic circuit when the flow demanded is greater than the maximum flow which the fixed pump can supply. Mode switching is by solenoid 56 shifting a second, piloting directional valve 48 to apply or remove pilot pressure in a spring biased first directional valve 38 to control pressure balance 30, 32, 34, 36 across a regulator valve 20 which when open supplies fixed pump output to a filter and cooling circuit 26.

Description

DESCRIPTION

HYDRALIC SUPPLY SYSTEM

The invention relates to a hydraulic supply system for use in a utility vehicle comprising a fixed displacement pump and a variable displacement pump.

Utility vehicles such as agricultural tractors and plant machinery typically comprise a hydraulic supply system for supplying fluid pressure to a number of hydraulically operated functions. By way of example, most modern utility vehicles include power steering which employs hydraulic cylinders. Also agricultural tractors employ hydraulically actuated lift arms for the attachment of various implements such as ploughs and cultivation equipment.

The supply systems comprise at least one hydraulic pump which is powered in most situations by the internal combustion engine of the vehicle.

It is known to employ both a fixed displacement pump and variable displacement pump to source the fluid pressure to the various hydraulic functions supplied. The variable displacement pump often of the swash plate axial piston design typically supplies the fluid flow to attached implements which comprise their own hydraulic actuators. With the current trend of farm machinery to increase in size so too does the size and number of the hydraulic actuators to be powered.

Therefore there is a demand to increase and/or optimise the hydraulic flow that can be supplied by the system. However, an increase in the size of the variable displacement pump is accompanied by a significant rise in the cost of manufacture together with the space occupied thereby.

Therefore it is an object of the invention to provide a hydraulic supply system which can deliver an improved flow rate in a relatively cheap manner and without consuming any significant extra space.

In accordance with the present invention there is provided a hydraulic supply system for a utility vehicle comprising a fixed displacement pump and a variable displacement pump, the system being selectively operable between a first mode in which the variable displacement pump supplies a hydraulic circuit, and a second mode in which the fixed displacement pump supplies said hydraulic circuit, wherein the variable displacement pump supplements the supply to the hydraulic circuit in the second mode when the flow demanded is greater than the maximum flow which the fixed displacement pump can supply.

Preferably the fixed displacement pump is connected to the hydraulic circuit via check valve which is held in a closed position by a pressure created by the variable displacement pump.

Therefore, in the first mode when the pressure on the output side of the variable displacement pump is normally higher than that on the output side of the fixed displacement pump, the output of the former keeps the check valve iii a closed position thus isolating the fixed displacement pump from the hydraulic circuit.

The variable displacement pump may also be connected to the hydraulic circuit by a check valve which is held in a closed position by a pressure created by the fixed displacement pump.

Advantageously this isolates the variable displacement pump from the hydraulic circuit in the event of failure of that pump when in the first mode.

Preferably the output of the fixed displacement pump is directed to a low load circuit when not demanded by the hydraulic circuit. Therefore, when in the first mode the output from the fixed displacement pump is simply diverted to the low load circuit such as a cooling and filtration circuit. The power consumed for most of the time by the fixed displacement pump is relatively low which is particularly advantageous since the fixed displacement pump typically operates continuously and the output of which cannot be varied.

In a preferred arrangement the output of the fixed displacement pump is connected to the low load circuit via a pressure control valve. In the first mode in normal use the pressure control valve provides a low resistance path to the low load circuit for the output of the fixed displacement pump. The pressure control valve is preferably held open by a pilot pressure created by the variable displacement pump. Advantageously, in the event of failure of the variable displacement pump in the first mode, the directional control valve becomes a pressure relief valve which only opens when the flow demand from the hydraulic circuit is low. Conversely, when the demand from the hydraulic circuit is high, the closure of the pressure control valve causes the output from the fixed displacement pump to be routed to the hydraulic circuit.

In a further preferred arrangement the second mode is enabled by isolating the pilot pressure created by the variable displacement pump from the pressure control valve. Combined with appropriate control of the variable displacement pump the output flow from the fixed displacement pump takes priority in supplying the hydraulic circuit.

Preferably the pilot pressure created by the variable displacement pump is isolated from the pressure control valve by a first directional control valve. This may be activated by a pilot pressure sourced from the output of the fixed displacement pump and directed thereto by a second directional control valve.

The second directional control valve is preferably activated by a solenoid.

In yet another preferred arrangement an operator can switch the system between the first mode and the second mode thereby providing increased user control over the hydraulic system. This has been found to be favourable to operators of utility vehicles and allows them to manually select the second mode when a high flow rate is demanded.

Further advantages of the invention will become apparent from the following description of a preferred embodiment with reference to the appended drawing in which:-Figure 1 is a schematic view of a hydraulic supply system in accordance with an embodiment of the invention.

The hydraulic supply system illustrated by Figure 1 is fitted to a tractor (not shown) which is fitted with power steering, hydraulic brakes and hydraulically-actuated lift arms at the rear for the attachment of various implements. Furthermore the tractor is powered with an internal combustion engine although it should be appreciated that alternative propulsion means may be provided by an electric motor and generator for example.

The hydraulic supply system 10 comprises a fixed displacement pump 12 and a variable displacement pump 14 both powered, directly or indirectly, by the internal combustion engine.

Both pumps 12,14 are fed from an oil reservoir 16 via an oil filter 18.

The fixed displacement pump 12 is a gear pump of known construction and provides a constant flow of oil (at a constant engine speed) at its output 12a of around 40 litres/mm.

The variable displacement pump 14 is an axial piston pump of the swash plate type which is also of a known design. The flow delivered at output 14a is variable between zero and 150 litres/mm (at a constant cnginc spccd) and is controlled by a load sensing (LS) pump controller 58.

Both thc gear pump 12 and axial piston pump 14 supply oil to a working hydraulic circuit rcprcscntcd at 20. The working circuit 20 comprises a hydraulic stccring circuit, a hydraulic brake circuit and an auxiliary circuit for controlling the lift arms of the tractor. Each said component of the working circuit 20 is of a known construction and will not be described any further.

The gear pump 12 is connected to the working circuit 20 via check valve 22. In a similar manner the piston pump 14 is connected to the working circuit 20 by another check valve 24.

Each check valve 22,24 serves to isolate the respective output of the two pumps 12, 14 and their involvement in the operation of the supply system 10 will be described in more detail below.

The output of gear pump 12 is connected to a cooling and filtration circuit represented at 26 via pressure control valve 28. The cooling and filtration circuit 26 simply serves to cool and filter the oil passed therethrough and is of a known construction. The oil passed to the cooling and filtration circuit 26 is returned to the oil reservoir 16 by a return line (not shown).

The pressure control valve 28 serves to regulate oil flow from the gear pump 12 to the cooling and filtration circuit 26. A spring 30 and a pilot line 32 conveying a hydraulic load sensing signal from the working circuit 20 biases the pressure control valve 28 into a closed or intermediate position. The spring 30 is set at a pressure of 30 bar to ensure that valve 28 regulates flow to circuit 26 (in the second mode) with the pressure at the output of the gear pump 12 being 30 bar greater than the LS signal pressure.

Further pilot lines 34,36 act upon the pressure control valve 28 against the spring 30 and pilot line 32. Pilot line 34 is connected to an output port of a first directional control valve 38 whereas pilot line 36 is connected to the output of gear pump 12. The involvement of each pilot line 32,34,36 and the operation of pressure control valve 28 will be discussed in more detail below.

The directional control valve 38 is switchable between a first position and a second position (shown in Figure 1). In the first position a pilot line 40 is connected to pilot line 34 so as to deliver a pilot pressure from axial piston pump 14 to the pressure control valve 28. In a second position pilot line 34 is connected to tank line 16 which relieves the pressure thereon.

A spring 44 biases the directional control valve 38 in to the second position. A pilot line 46 connected to an output port of a second directional control valve 48 biases the first directional control valve 38 in to the first position.

The second directional control valve 48 is switchable between two positions. In a first position (shown in Figure 1) a pilot line 50 is connected to pilot line 46 so as to deliver a pilot pressure froiii the gear pump 12 to pilot line 46 thus forcing the first directional control valve 38 in to the first position. In a second position of the second directional control valve 48 pilot line 46 is connected to tank line 16 so as to relieve the pressure thereon thus allowing spring 44 to force the directional control valve 38 in to the second position.

Directional control valve 48 is biased in to the first position by spring 54. Furthermore an electrically operated solenoid 56 forces directional control valve 48 in to the second position against the biasing force of the spring 54. Solenoid 56 can be manually activated by an operator so as to switch the system between a first mode and second mode of operation as described in more detail below.

The output flow of axial piston pump 14 is controlled by the LS pump controller 58 in response to a hydraulic LS signal which is conveyed from the working hydraulic circuit 20.

The output pressure of axial piston pump 14 is maintained at 20 bar above the pressure of the LS signal by the LS pump controller 58.

The principle of operation of the two modes in accordance with the invention will now be described in relation to the hydraulic supply system shown in Figure 1. In a first mode directional control valve 48 is in the first position which delivers a pilot pressure from gear pump 12 to directional control valve 38 via pilot lines 50 and 46. As a result directional control valve 38 is forced in to the first position against the force of spring 44 thus delivering a pilot pressure from axial piston pump 14 to pressure control valve 28 via pilot lines 40 and 34. The working pressure of axial piston pump 14 in normal working conditions is sufficient to open pressure control valve 28 against the force of spring 30.

Therefore, in the first mode the flow output of gear pump 12 is directed to the cooling and filtration circuit 26. Furthermore the pressure differential across check valve 22 maintains this in a closed position thus isolating the output of gear pump 12 from the hydraulic circuit 20.

The LS signal is communicated to the LS pump controller 58 in a continuous manner so as to vary the output of piston pump 14 to meet the demand from working circuit 20.

In the event of failure of the piston pump 14 when in the first mode the pilot pressure delivered by line 40 reduces to an extent resulting in a reduction in the biasing force on pressure control valve 28 at line 34. If there is demand from working circuit 20 a pressure differential occurs across check valve 22 so as to allow a flow of oil from gear pump 12 into working circuit 20. The absence of a working pressure from the failed piston pump 14 creates a pressure differential also across valve 24 thus isolating piston pump 14 from the working circuit 20.

In the event of such failure of piston pump 14 the pressure control valve 28 serves as a regulator valve wherein pilot line 36 forces valve 28 into an open or intermediate position when the demand from working circuit 20 is low. In short the excess flow from gear pump 12 is regulated through pressure control valve 28 to the cooling and filtration circuit 26.

Advantageously therefore even in the event of failure of the axial piston pump 14 the priority hydraulic circuits present in working circuit 20 can still function by the supply of oil from gear pump 12.

In a second mode selectable by the operator solenoid 56 is activated so as to force directional control valve 48 into the second position thus isolating pilot line 46 from the pilot pressure in line 50. As a result the directional control valve 38 is biased into the second position (shown in Figure 1) resulting in the isolation of the pilot pressure on line 40 from pilot line 34.

In the second mode the flow from gear pump 12 is directed to working circuit 20 as required with any excess flow regulated through pressure control valve 28 in a similar manner to the situation described above with regard to the failed piston pump 14.

When the gear pump 12 cannot meet the demand of working circuit 20, the pressure on the output side 1 2a thereof drops until check valve 24 opens allowing the axial piston pump 14 to supplement the supply. Therefore the flow output from piston pump 14 is equal to the flow demanded by working circuit 20 minus the output flow of gear pump 12.

Advantageously the invention allows the operator to switch between the first mode and the second mode which gives increased control to the operator. The gear pump serves primarily as a back-up pump which is availablc on dcmand by thc opcrator. At othcr timcs thc gcar pump output is dircctcd to thc filtration circuit which consumcs rclativcly littlc encrgy.

From rcading thc prcscnt disclosurc. othcr modification will bc apparcnt to pcrsons skillcd in thc art. Such modifications may involve othcr fcaturcs which arc aircady known in thc field of hydraulic supply systcms and component parts therefore and which may be used instead of or in addition to features already described herein.

Claims

CLAIMS1. A hydraulic supply system for a utility vehicle comprising a fixed displacement pump and a variable displacement pump, the system being selectively operable between a first mode in which the variable displacement pump supplies a hydraulic circuit, and a second mode in which the fixed displacement pump supplies said hydraulic circuit, wherein the variable displacement pump supplements the supply to the hydraulic circuit in the second mode when the flow demanded is greater than the maximuiii flow which the fixed displacement pump can supply.
2. A hydraulic supply system according to Claim 1, wherein the fixed displacement pump is connected to the hydraulic circuit via a check valve which is held in a closed position by a pressure created by the variable displacement pump.
3. A hydraulic supply system according to Claim 1 or 2. wherein the variable displacement pump is connected to the hydraulic circuit via a check valve which is held in a closed position by a pressure created by the fixed displacement pump in the first mode when the variable displacement pump fails.
4. A hydraulic supply system according to any preceding claim, wherein the output of the fixed displacement pump is directed to a low load circuit when not demanded by the hydraulic circuit.
5. A hydraulic supply system according to Claim 4, wherein the low load circuit comprises fluid cooling and/or filtration means.
6. A hydraulic supply system according to Claim 4 or 5, wherein the output of the fixed displacement pump is connected to the low load circuit via a pressure control valve.
7. A hydraulic supply system according to Claim 6, wherein the pressure control valve is held open by a pilot pressure created by the variable displacement pump.
8. A hydraulic supply system according to Claim 7, wherein said pilot pressure created by the variable displacement pump is isolated from said pressure control valve in the second mode.
9. A hydraulic supply system according to Claim 8. wherein said pilot pressure created by the variable displacement pump is isolated from said pressure control valve by a first directional control valve.
10. A hydraulic supply system according to Claim 9, wherein the first directional control valve is activated by a pilot pressure sourced from the output of the fixed displacement pump and directed thereto by a second directional control valve.
11. A hydraulic supply system according to Claim 10, wherein the second directional control valve is activated by a solenoid.
12. A hydraulic supply system according to any one of Claims 6 to 11, when in the second mode the pressure control valve regulates the supply from the fixed displacement valve to the low load circuit in response to a load sensing signal from the hydraulic circuit.
13. A hydraulic supply system according to any preceding claim wherein an operator can switch the system between the first mode and the second mode.
14. A hydraulic supply system according to any preceding claim, wherein the hydraulic circuit comprises at least one of a steering circuit, a brake circuit, a loader circuit and an implement control circuit.
15. A hydraulic supply system according to any preceding claim, wherein the output of the variable displacement pump is controlled by a load sensing pump controller in response to a load sensing signal from the hydraulic circuit.
16. A hydraulic supply system constructed and arranged substantially as hereinbefore described with reference to, and as shown in the accompanying Figure.