IE20100557U1 - Hydraulic drive systems - Google Patents

Abstract

ABSTRACT A hydraulic drive system for a vehicle has series-connected first and second hydraulic motors on one side of the vehicle and series-connected third and fourth hydraulic motors disposed on another side of the vehicle, the vehicle also having hydraulic pump means for driving the motors on each side. In a first mode of operation, a pair of bridging connections allow communication between the pump and the respective hydraulic lines connecting the motors along either side, to provide four wheel drive. In a second mode of operation, the bridging connections are closed to gang together the first and second motors, and to gang together the third and fourth motors. In a third‘ mode of operation a» clutch is used to gang together the drives of the first to fourth motors, such that choosing the first second or third motors allows control of traction on different surfaces.

Description

Hydraulic drive systems ‘W AS was is seats: zesrtrtses echnilcaggcgs , arses: rest as This invention relates to hydraulic drive systems for vehicles.

Background Art Many hydraulic drive systems are known for vehicles. The main types are independent four-wheel drive systems in which each wheel is driven by a hydraulic pump. The hydraulic fluid follows a path of least resistance to thereby drive each of the four wheels as required.

Such systems work well in controlled environments, such as when employed in a -forklift in a warehouse, but are not suitable for surfaces in which different wheels encounter different resistances, such as for vehicles which might be driven on grass or on mixed surfaces. In those environments, one wheel might hit a slippery part of the surface, allowing it to fi'eely rotate, so that all of the power is transmitted through that one wheel.

The main alternative is a synchronous system in which all wheels are driven in series with one another, so that each rotates at the same speed as all others. Such systems are not suitable for large turning angles, because the differential turning speed of the outside and inside wheels is not accommodated, resulting in skidding and excessive tyre wear.

Disclosure of the Invention There is provided a hydraulic drive system for a vehicle having first and second hydraulic motors disposed on one side of the vehicle and third and fourth hydraulic motors disposed on another side of the vehicle, the vehicle also having hydraulic pump means for driving the motors, the hydraulic drive system comprising: (a) a first hydraulic drive circuit connecting, in series, the pump means with the first motor, the first motor with the second motor, and the second motor with the pump means; (b) a second hydraulic drive circuit connecting, in series, the pump means with the third motor, the third motor with the fourth motor, and the fourth motor with the pump (c) a first bridging connection from the pump means to the first hydraulic drive circuit between the first and second motors; (d) a second bridging connection from the pump means to the first hydraulic drive circuit between the first and second motors; (e) valve means for selectively opening and closing the bridging connections; and (f) a clutch for selectively engaging and disengaging a mechanical linkage between one of the first and second motors and one of the third and fourth motors.

The hydraulic system is advantageous in that it provides bridging connections which can be selectively opened and closed, such that when they are open, the pump can drive each motor independently. When closed, the two motors on the left side are connected in series, and the two motors on the right’ side are also connected in series, so that the two sides are being driven by independent, parallel hydraulic drive circuits. When the clutch is engaged and the valves are closed, then the two parallel circuits are linked to one another and the four motors are synchronously driven.

Thus, the drive system allows different modes of operation to be selected according to the conditions in which the vehicle is operating.

Preferably, the pump means is operable to receive hydraulic fluid firom a first port at a lower pressure and to pump the fluid out through a second port at a higher pressure.

Preferably the pump is switchable to a reverse mode of operation in which the pump receives hydraulic fluid from the second port at a lower pressure and pumps the fluid out through the first port at a higher pressure.

Preferably the first hydraulic drive circuit connects the second pump port to the first motor and connects the second motor to the first pump port, and the first bridging connection connects the second port to the first hydraulic drive circuit between the first and second motors.

Further, preferably, the second hydraulic drive circuit connects the second pump port to the third motor and connects the fourth motor to the first pump port, and the second bridging connection connects the second pump port to the second hydraulic drive circuit between the third and fourth motors.

Preferably, the system further comprises a controller for controlling the operation of the valve means such that the controller can select a first mode of operation of the hydraulic system in which the first and second bridging connections are open and the first to fourth motors can turn independently of one another, or a second mode of operation in which the first and second bridging connections are closed and the first and second motors are driven in synchronicity with one another and the third and fourth motors are driven in synchronicity with one another.

It will be appreciated that engaging the clutch when the bridging connections are closed provides a third mode of operation in which the first to fourth motors are driven in synchronicity with one another.

Preferably, therefore, the controller is operable to control the clutch as well as the valve means and to thereby permit selection of any of the first, second or third modes of operation.

While the switching between modes may be automatic, e. g. if undue slippage of a wheel or loss of traction is detected, more preferably the controller operates under control of a user.

There is also provided a hydraulic differential for a hydraulic drive circuit, comprising: (a) a body having first, second, third and fourth hydraulic ports for connection to first, second, third and fourth motors, respectively; (b) a first main conduit providing fluid communication through said body between the first and second ports; (c) a second main conduit providing fluid communication through said body between the third and fourth ports; (d) at least one pump port in said body for connection to a hydraulic pump; (e) a first bridging connection providing fluid communication through said body between one of said at least one pump ports and said first main conduit; (f) a second bridging connection providing fluid communication through said body between one of said at least one pump ports and said second main conduit; (h) (i) (In) valve means disposed in said body to selectively open and close said first and second bridging connections; at least one charging pump port in said body for connection to a charging pump; a first charging conduit providing fluid communication through said body between one of said at least one charging pump ports and said first main conduit; a first non-retum valve disposed in said body along said first charging conduit to prevent reverse fluid flow from said first conduit towards said charging pump port; a second charging conduit providing fluid communication through said body between one of said at least one charging pump ports and said second main conduit; a second non-return valve disposed in said body along said second charging conduit to prevent reverse fluid flow from said second conduit towards said charging pump Port; a relief port provided in said body for permitting fluid to exit the body; a first relief conduit providing fluid communication through said body between said first main conduit and said relief port; a first relief valve disposed in said body along said first relief conduit to open said first relief conduit when the pressure in said first main conduit exceeds a predetermined level; a second relief conduit providing fluid communication through said body between said second main conduit and said relief port; a second relief valve disposed in said body along said second relief conduit to open said second relief conduit when the pressure in said second main conduit exceeds a predetermined level.

There is further provided a hydraulic drive system comprising the hydraulic differential defined above.

There is also provided a vehicle comprising any hydraulic drive system defined above, and further comprising a respective wheel connected to each of the first to fourth motors.

There is also provided a method of controlling a hydraulic drive system for a vehicle having first and second hydraulic motors disposed on one side of the vehicle and third and fourth hydraulic motors disposed on another side of the vehicle, the vehicle also having hydraulic pump means for driving the motors, the hydraulic drive system, the method comprising the steps of: lE1oo557 (a) in a first mode of operation, pumping fluid from the hydraulic pump means through each of: (i) a first hydraulic drive circuit connecting, in series, the pump means with the first motor, the first motor with the second motor, and the second motor with the pump means; (ii) a second hydraulic drive circuit connecting, in series, the pump means with the third motor, the third motor with the fourth motor, and the fourth motor with the pump means; (iii) a first bridging connection fiom the pump means to the first hydraulic drive circuit between the first and second motors; and (iv) a second bridging connection from the pump means to the first hydraulic drive circuit between the first and second motors; (b) in a second mode of operation, closing the first and second bridging connections to pump fluid from the hydraulic pump means through each of the first and second hydraulic drive circuits only; (0) in a third mode of operation, closing the first and second bridging connections to pump fluid from the hydraulic pump means through each of the first and second hydraulic drive circuits only, and engaging a mechanical linkage between one of the first and second motors and one of the third and fourth motors.

Brief Description of the Drawings The invention will now be further illustrated by the following description of embodiments thereof, given by way of example only with reference to the accompanying drawings, in which: F ig. 1 is a schematic circuit diagram of a hydraulic transmission system for a vehicle; Fig. 2 is a simplified version of the circuit diagram of Fig. 1; Fig. 3 is a representation of the circuit of Fig. 2 when the system is in a first mode of operation; Fig. 4 is a representation of the circuit of Fig. 2 when the system is in a second mode of operation; Fig. 5 is a representation of the circuit of Fig. 2 when the system is in a third mode of operation; Fig. 6 is an articulated vehicle shown in a straight ahead configuration; IE1oo557, 6 Fig. 7 is an articulated vehicle shown in a turning left configuration; and Fig. 8 is an articulated vehicle shown in a turning right configuration.

Detailed Description of Preferred Embodiments In Fig. 1 there is shown a closed—loop hydrostatic transmission system for a vehicle, in which a variable displacement hydraulic pump 10 drives a set of four hydraulic motors, namely a first motor 12 and second motor 14 located on a left-hand side of the vehicle (not shown) and a third motor 16 and fourth motor 18 located on a right-hand side of the vehicle. Each motor drives a respective shaft 20 which is an axle for a respective wheel, so that the pump 10 and motors 12,l4,16,18 form a transmission system to drive the vehic1e’s wheels.

Each node in the circuit, where a pair of hydraulic lines meet one another, is shown as a dot at the respective junction, the junctions being labelled with a reference numeral, while ' lines which cross one another in the circuit schematic of F ig.- 1 but which are unconnected, have no such dot and are unlabelled.

Thus, one can follow a first hydraulic drive circuit for driving the left-hand motors 12,14 as a series connection between pump 10, node 22, first motor 12, nodes 24, 26 and 28, second motor 14, node 30, and return line 32 back to pump 10.

Similarly one can follow a second hydraulic drive circuit for driving the right-hand motors 16,18 as a series connection between pump 10, node 22, third motor 16, nodes 34, 36 and , fourth motor 18, return line 40, node 30, and return line 32 back to pump 10.

A first bridging connection 42 exists from the pump 10 to meet the first hydraulic drive circuit at node 24 between the first and second motors 12,14. First bridging connection 42 passes through a valve 44 which is shown in an open position but which can be driven closed by a solenoid 46 under control of an electronic control circuit (not shown) which may be as simple as a power source having a switch in the driVer’s cab, or which may be implemented in a more sophisticated control interface in known manner.

Similarly, a second bridging connection 48 runs from pump 10 to meet the second hydraulic drive circuit at node 34 between the third and fourth motors 16,18. Second lE1oo557, bridging connection 48 also passes through a valve 50 controlled by a solenoid 52 in a similar manner.

It will be appreciated that the embodiment of Fig. 1 shows the first and second bridging connections 44,48 as sharing a common hydraulic line until they split at node 54, but the two connections can be entirely separate. However, the shared portion of line allows a variation on the embodiment, in which the two valves 44,50 can be replaced by a single valve located on the common part of the hydraulic line between nodes 22 and 54.

When valves 44 and 50 are shut off, the first and second hydraulic drive circuits are driven separately in parallel with one another. This locks the rotation of first and second motors ,14 to one another, and locks the rotation of third and fourth motors 16,18 to one A another. When the valves are opened, the pump has a direct connection to drive the second and fourth motors 14,18 as well as to first and third motors 12,16. Inpractice one may add a restrictor to the bridging connections so that there is a pressure differential across the first and third motors (i.e. so that nodes 22 and 54 are not at the same pressure). The extent to which the load is split on either side between the first and second motors, or between the third and fourth motors will depend on the resistance experienced by each Wheel, the degree of restriction, if any, on the bridging connections, and so on. In some cases, the front wheels may provide the majority of the driving force.

A charging system is provided to maintain the pressure in the circuit, comprising a charging pump 56 connected via a charging line 58 to node 60, where the line 58 splits into left-hand and right-hand charging lines 62,64 which run towards nodes 26 and 36, respectively, on the first and second hydraulic drive circuits.

A first one-way valve 66 is located on left-hand charging line 62 to ensure that hydraulic fluid passed in one direction only to keep node 26 at a minimum pressure equal to that provided by the charging pump. Similarly a second one~way valve 68 is provided on the right-hand charging line 64, keeping node 36 at the same minimum pressure.

A left-hand relief line 70 and right-hand relief line 72 run respectively from nodes 28 and 38 to a common drain 74. Each relief line passes through a respective overpressure relief valve 76,78, such that pressure is relieved from the first hydraulic drive circuit at node 30 lE100557 if the pressure exceeds a threshold set by valve 76, and pressure is relieved from the second hydraulic drive circuit at node 38 if the pressure exceeds a threshold set by valve In combination, the charging circuits 56,58,60,62,64,66,68, and the relief circuits 70,72,74,76,'/'8, provide a mechanism for maintaining the pressure between the first and second motors 12,14 and the pressure between the third and fourth motors16,18, at a working range, avoiding the problems associated with over- or under-pressure arising from - a turning vehicle, particularly when the bridging connections are closed and there is a fixed volume of hydraulic fluid between the two left-hand wheels and between the two right- hand wheels, as discussed further below.

Also shown in Fig. 1 is a clutch 80 allowing engagement and disengagement of a mechanical linkage between a pair of shafts 82,84 respectively driven by the second and fourth motors 14,18. When the clutch is engaged to lock the rotation of the two shafts, the front wheels of -the vehicle have no differential available to them and rotation in synchronicity. If this engagement of the clutch occurs while the bridging connections ,48 are also closed off, then the four wheels must rotate in synchronicity. .

Finally, as regards Fig. 1, it can be seen that there is indicated in broken outline a box 86 which encloses a set of components all of which are implemented in a single hydraulic differential body in a preferred embodiment of the invention. The body is provided with a number of ports, indicated at each intersection of a hydraulic line with the boundary of the differential body 86, as follows.

The first to fourth motors are connected to the differential at respective first to fourth hydraulic ports 88,90,92,94. Within the differential body, the first hydraulic drive circuit is carried within a first main conduit running from first port 88 to second port 90, while the second hydraulic drive circuit is carried within a second main conduit running from third port 92 to fourth port 94.

A pump port 96 provides the shared connection (though individual connections can also be provided) between pump 10 and the bridging connections 42,48, which are carried by i£1oo557 ,, respective bridging conduits within the body and which terminate at the respective hydraulic drive circuits.

A charging pump port 98 provides the shared connection (though again individual connections can also be provided) between charging pump 56 and the charging lines 62,64 which are carried by respective charging conduits within the body and which terminate at the respective hydraulic drive circuits.

Finally, a relief port, which is not shown as an intersection with the box 86, is also provided in the differential body in the preferred embodiment, and this relief port allows excess, overpressure fluid to be relieved and drained, typically to a tank. This port is indicated symbolically by drain 74.

Fig. 2 is a simplified version of the full hydraulic circuit diagram of Fig’. 1, in which the same reference numerals are used, with the charging and relief circuits omitted for . simplicity, with the outline of the differential body omitted, and with the valves 44,50 on ' the bridging circuits represented as simple switches. Depending on the state_;of these valves and of theclutch 80, the system may assume one of a number of different modes of operation.

Fig. 3 shows a first such mode of operation, in which the valves (not shown) have been closed to complete the bridging connections 42,48, so that the pump 10 can drive each of the four motors 12,14,16,18, independently. The clutch (not shown) has been disengaged so that there is no linkage between the left and right sides.

In Fig. 4 a second mode of operation has been selected, in which the valves (not shown) are opened to break the bridging connections, (which accordingly are no longer shown).

The transmission system therefore simplifies into a pair of parallel drive circuits running between nodes 22 and 30, with both left wheels 12,14 being driven in synchronicity with each other, and both right wheels 16,18 being driven in synchronicity with each other.

In Fig. 5 a third modeof operation has been selected, which differs from that of Fig. 4 in that the clutch has been engaged so that driven shafis 82,84 and clutch 80 form a mechanically unitary linkage between the front left and right motors 14,18, thereby lockin g all four motors into synchronicity.

Figs. 7-9 show an articulated four-wheel vehicle in which this system may be used, to better illustrate the operation of the charging and relief circuits of Fig. 1. The vehicle has a front half 100 and a rear half 102 connected at an articulation point 104. In this vehicle, the front half is indicated as carrying a cab and the rear half an engine (to drive the hydraulic pump 10) but this designation is of course arbitrary and immaterial to what follows. Also, the articulation highlights a problem which nevertheless would be present ' in a rigid vehicle when steering to left and right.

The vehicle is driven by a set of four wheels, namely a lefi front (LF) wheel 106, a right . - front (RF) wheel 108, a lefi rear (LR) wheel H0 and a right rear (RR) wheel 112. When . driving straight ahead as shown in Fig. 6, it is evident that all four wheels rotate at the same speed, assuming good traction for each wheel.

In order to assume the turning left position of Fig. 7, the arrows on each wheel show the relative rotation between the wheels on each side of the vehicle. Thus, the LF wheel rotates backwards relative to the RF wheel. By this is meant that although the vehicle may turn when driving forward, so that both the LF wheel and RF wheel tum forward relative to the vehicle, the LF wheel will needs fewer rotations than the RF wheel and thus has a relative backward rotation with respect to the RF wheel. Conversely, the LR wheel rotates forward relative to the RR wheel. This of course leads to tyre scrub if not corrected, but also leads to a hydraulic problem.

It can be seen that the lefi-side wheels exhibit an imbalance: the LR wheel needs to rotate faster than the LF wheel. However, it will be recalled that in the second and third modes of operation, these wheels are driven in synchronicity with one another. This means that the LR wheel is pushing hydraulic fluid into the LF wheel (or more accurately the associated motor) faster than the LF front wheel “wants” to tum. Conversely, the RR wheel is “trying” to turn at a slower rate than the RF wheel and therefore is not feeding enough hydraulic fuel to satisfy the demand of the RF motor.

IE 1005 Fig. 8 shows that when turning right the problems are reversed: in the second and third modes of operation the LF motor experiences an under-pressure while the RF motor CXpCI'lC1’lCCS an OV6I'~pI'CSS1Jl'€.

This is where the charging and relief circuits are most important. The demands of each motor for more or less pressure due to the differential turning speeds of different wheels can be accommodated by connecting a charging pump into the lefi-hand and right-hand drive circuits between the two wheels, and by connecting an overpressure relief valve on each side also. When the fi'ont wheel needs to turn relatively faster than the rear wheel on a given side, then the charging pump compensates for the drop in pressure by keeping the circuit charged to a minimum working pressure (which is below the overpressure at which the relief valve operates), thereby avoiding cavitation. When the front wheel needs to turn relatively slower than the rear wheel on a given side, then the increased pressure closes off the one-way valve on the charging circuit, preventing damage to the charging pump, but triggers the overpressure relief valve to open on that side, safely relieving the pressure.

In appropriate cases, the bridging connection leading from the pump may be shut off at 22‘ .

(Fig. 1) entirely, while leaving a valved connection between the first and second circuits at 24 and 34, which may be opened to allow for free fluid flow between the two sides (useful when turning while stationary) or shut-off to enforce a partial differential lock as described above in relation to the second mode of operation

Claims (5)

Claims:

1. A hydraulic drive system for a vehicle having first and second hydraulic motors disposed on one side of the vehicle and third and fourth hydraulic motors disposed on another side of the vehicle, the vehicle also having hydraulic pump means for driving the motors, the hydraulic drive system comprising: (a) a first hydraulic drive circuit connecting, in series, the pump means with the first motor, the first motor with the second motor, and the second motor with the pump means; (1)) a second hydraulic drive circuit connecting, in series, the pump means with the third, motor, the third motor with the fourth motor, and the fourth motor with the pump- means; (c) a first bridging connection from the pump means to the first hydraulic drive circuit’ - between the first and second motors; (d) a second bridging connection from the pump means to the second hydraulic drive circuit between the third and fourth motors; (e) valve means for selectively opening and closing the bridging connections; and (t) a clutch for selectively engaging and disengaging the drive between one of the first and second motors and one of the third and fourth motors.

2. A hydraulic drive system as claimed in claim 1, further comprising a controller for controlling the operation of the valve means such that the controller can select a first mode of operation of the hydraulic system in which the first and second bridging connections are open and the first to fourth motors can turn independently of one another, or a second mode of operation in which the first and second bridging connections are closed and the first and second motors are driven in synchronicity with one another and the third and fourth motors are driven in synchronicity with one another.

3. A hydraulic drive system as claimed in claim 2, wherein engaging the clutch when the bridging connections are closed provides a third mode of operation in which the first to fourth motors are driven in synchronicity with one another.

4. A hydraulic differential for a hydraulic drive circuit, comprising: a body having first, second, third and fourth hydraulic ports for connection to first, second, third and fourth motors, respectively; a first main conduit providing fluid communication through said body between the first and second ports; a second main conduit providing fluid communication through said body between the I third and fourth ports; at least one pump port in said body for connection to a hydraulic pump; a first bridging connection providing fluid communication through said body between one of said at least one pump ports and said first main conduit; a second bridging connection providing fluid communication through said body between one of said at least one pumpports and said second main conduit; valve means disposed in said body to selectively open and close said first and second bridging connections; at least one charging pump port in said body for connection to a charging pump; a first charging conduit providing fluid communication through said body between one of said at least one charging pump ports and said first main conduit; 3. first non-returii valve disposed in said body along said first charging conduit to prevent reverse fluid flow from said first conduit towards said charging pump port; a second charging conduit providing fluid communication through said body between one of said at least one charging pump ports and said second main conduit;

5. a second non-retum valve disposed in said body along said second charging conduit to prevent reverse fluid flow from said second conduit towards said charging pump 130“; a relief port provided in said body for permitting fluid to exit the body; a first relief conduit providing fluid communication through said body between said first main conduit and said relief port; a first relief valve disposed in said body along said first relief conduit to open said first relief conduit when the pressure in said first main conduit exceeds a predetermined level; a second relief conduit providing fluid communication through said body between said second main conduit and said relief port; l a second relief valve disposed in said body along said second relief conduit to open said second relief conduit when the pressure in said second main conduit exceeds a predetermined level. A method of controlling a hydraulic drive system for a vehicle having first and second hydraulic motors disposed on one side of the vehicle and third and fourth hydraulic motors disposed on another side of the vehicle, the vehicle also having hydraulic pump means for driving the motors, the hydraulic drive system, the method comprising the steps of: (a) in a first mode of operation, pumping fluid from the hydraulic pump means through each of: (i) a first hydraulic drive circuit connecting, in series, the pump means with the first motor, the first motor with the second motor, and the second motor with the pump means; (ii) a second hydraulic drive circuit connecting, in series, the pump means with the third motor, the third motor with the fourth motor, and the fourth motor with the pump means; a first bridging connection from the pump means to the first hydraulic drive (iii) circuit between the first and second motors; and a second bridging connection from the pump means to the first hydraulic drive (iv) circuit between the first and second motors; in a second mode of operation, closing the first and second bridging connections to pump fluid from the hydraulic pump means through each of the first and second hydraulic drive circuits only; in a third mode of operation, closing the first and second bridging connections to pump fluid from the hydraulic pump means through each of the first and second hydraulic drive circuits only, and engaging a clutch between one of the first and second motors and one of thethird and fourth motors.