TITLE OF INVENTION
[0001] Hydraulic Drive System And Improved Filter Sub-System Therefor.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] This application is a continuation-in-part (CIP) application of co-
pending application U.S. Serial No. 10/828,590, filed April 21, 2004, in the
name of Rodney V. Singh for a "Hydraulic Drive System And Improved Filter
Sub-System Therefor" which is a continuation-in-part (CIP) application of co-
pending application U.S. Serial No. 10/624,805, filed July 22. 2003, in the name
of Rodney V. Singh for a "Hydraulic Drive System And Improved Filter Sub-
System Therefor".
BACKGROUND OF THE DISCLOSURE
[0003] The present invention relates to hydraulic drive systems of the type
including a pump-motor unit which operates as a pump during a portion of the
vehicle operating cycle, and as a motor during another portion of the vehicle
operating cycle. Even more particularly, the present invention relates to an
improved control circuit for controlling the drive system, and a filter sub-system
for use in such a hydraulic drive system.
[0004] Although the control circuit and the filter sub-system of the present
invention may be utilized in hydraulic drive systems of various types including
such drive systems which effectively serve as the primary vehicle transmission
during at least most of the vehicle operating cycle, the present invention is
especially advantageous when used on a hydraulic drive system which
comprises part of a vehicle hydraulic regenerative braking system, and will be
described in connection therewith.
[0005] In a vehicle hydraulic drive system having regenerative braking
capability, and assuming, by way of example only, that the vehicle is of the rear
wheel drive type, the primary drive torque is transmitted from the engine

through the conventional mechanical transmission, and then by means of a
conventional drive line to the rear drive wheels. During braking (i.e. during the
braking portion of a "deceleration-acceleration" cycle,) the kinetic energy of the
moving vehicle is converted by the hydrostatic pump-motor unit, which is
commanded to operate in its pumping mode, and the pump-motor unit charges
a high pressure accumulator. When the vehicle is subsequently accelerated,
the hydrostatic pump-motor unit is commanded to operate in its motoring mode,
and the high pressure stored in the high pressure accumulator is communicated
to the pump-motor unit. The resulting output torque of the pump-motor unit,
now operating as a motor, is transmitted to the vehicle drive line.
[0006] It will be understood by those skilled in the art that there are several
reasons why the present invention is especially suited for use in a drive system
of the type described above, and which has regenerative braking capability.
First, such a system typically includes not only the high pressure accumulator
referred to, but also a source of low pressure, including but not limited to an
open reservoir or a low pressure accumulator. However, the presence of the
high pressure accumulator and the source of low pressure in the drive system
complicates certain aspects of the configuration and the control of the drive
system. Secondly, the presence of a pump-motor unit, which operates in a
pumping mode for part of the vehicle cycle, and in a motoring mode for part of
the vehicle cycle, introduces certain additional requirements and complications
into the drive system and the controls therefor.
[0007] One of the complications which has been observed in a hydraulic
drive system of the type to which the present invention relates, and which is
used to accomplish regenerative braking, is the necessity to ensure proper
filtration of the oil. In a conventional closed-loop hydrostatic transmission, or
HST (i.e., a pump and motor combination), the pump almost always serves as a
pump, and the motor almost always serves as a motor, during the normal propel
operating cycle. In such a closed-loop HST system, it is conventional for some

portion of the case drain fluid to be directed through a parallel circuit including
elements such as a heat exchanger and a filter, after which that fluid is typically
returned to the closed-loop circuit by means of a charge pump
[0008] In the hydraulic drive system of the present invention, instead of a
separate pump unit and motor unit, there is the above-described pump-motor
unit. In view of the dual mode capability of the pump-motor unit of the type
used in the hydraulic drive system of the present invention, it is not feasible
simply to utilize the type of "parallel-path" filter circuit of the type typically
utilized in closed loop HST systems, and described previously. In addition,
whereas the "direction" of fluid flow in a typical closed-loop HST system
remains the same throughout its operating cycles, in a hydraulic drive system of
the type to which the present invention relates, many portions of the overall
hydraulic system "see" fluid flow in one direction during one operating mode
(e.g., deceleration) and "see" fluid flow in the opposite direction during the other
mode (e.g., acceleration). As is well known to those skilled in the hydraulic
circuit art, it is not feasible to utilize a conventional filter element in a circuit
which experiences reversal of flow as part of its normal operation.
[0009] By way of example only, in a hydraulic drive system of the type to
which the present invention relates, it is not advisable to locate a filter circuit or
filter element in series flow relationship with the inlet of the pump-motor unit.
When the pump-motor unit is operating in the pumping mode, the presence of a
filter element in series with the pump inlet restricts pump inlet flow (especially
after the filter element has collected a substantial amount of contaminant
particles), thus resulting in cavitation of the unit (in the pumping mode) and
excessive, undesirable noise emanating from the overall drive system. At the
same time, it is not advisable to locate a filter element in series with the outlet
of the unit (when it is operating in the motoring mode) because one result will
be an increase in the total pressure drop across the unit, thus reducing the
overall efficiency of the drive system. Another undesirable result would be

that, as the filter element collects contamination particles, the pressure drop
across the unit would vary, and therefore, the total system performance would
also vary. If the filter element is located in series with the outlet of the unit (in
the motoring mode) the filter element could rupture, and catastrophically
contaminate the entire system. Moreover, because of the large flow rates
involved, the filter element would have to be larger than is considered
desirable, especially for mobile applications.
BRIEF SUMMARY OF THE INVENTION
[0010] Accordingly, it is an object of the present invention to provide an
improved hydraulic drive system, and a control circuit therefor of the type which
may be utilized in connection with a vehicle hydraulic regenerative braking
system, which overcomes the above disadvantages of the prior art.
[0011] It is another object of the present invention to provide such an
improved hydraulic drive system which includes a filter sub-system capable of
meeting the needs of the system, and of the pump-motor unit, both when the
unit is in the pumping mode, and when the unit is in the motoring mode, with no
substantial change in system performance as the filter element collects
contamination particles.
[0012] It is yet another object of the present invention to provide such an
improved filter sub-system which achieves the above-stated objects, and which
defines two different flow paths, the first designed to provide relatively little flow
restriction when the pump-motor unit is pumping, and the second to accomplish
controlled filtration when the unit is motoring.
[0013] The above and other objects of the invention are accomplished by the
provision of an improved hydraulic drive system adapted for use on a vehicle
having an engine and a drive line operable to transmit driving torque from the
engine to a drive axle. The drive system includes a hydrostatic pump-motor
unit operable, in a pumping mode, to receive drive torque from the drive line

and operable, in a motoring mode, to transmit drive torque to the drive line. A
high-pressure accumulator is in fluid communication with a first port of the
pump-motor unit through a mode valve means whereby, when the pump-motor
unit is in the pumping mode, pressurized fluid is communicated from the pump-
motor unit to the high pressure accumulator. When the pump-motor unit is in
the motoring mode, pressurized fluid is communicated from the high pressure
accumulator to the pump-motor unit. A source of low pressure is in fluid
communication with a second port of the pump-motor unit.
[0014] The improved hydraulic drive system is characterized by a filter circuit
disposed between the source of low pressure and the pump-motor unit. The
filter circuit defines a relatively unrestricted first flow path from the source of low
pressure to the second port when the pump-motor unit is in the pumping mode.
The filter circuit defines a second flow path from the second port to the source
of low pressure when the pump-motor unit is in the motoring mode The second
flow path comprises one path portion through a filter shut-off valve and a filter
element in series, and in parallel therewith, another path portion through a
controlled flow restriction, whereby one portion of the fluid flow from the second
port flows through the filter element, and the remainder of the fluid flow from the
second port flows through the controlled flow restriction.
[0015] In accordance with a more limited aspect of the invention, the
hydraulic drive system is characterized by the relatively unrestricted first flow
path defined by the filter circuit excluding the filter shut-off valve and the filter
element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic view of an entire vehicle drive system of the
type with which the hydraulic drive system of the present invention is especially
well suited.

[0017] FIG. 2 is a hydraulic schematic of the hydraulic drive system of the
present invention, including both the control circuit and the filter sub-system of
the present invention, with the filter sub-system being shown only in schematic,
block form.
[0018] FIG. 3 is a detailed hydraulic schematic illustrating a preferred
embodiment of the filter sub-system which comprises one important aspect of
the present invention.
[0019] FIG. 4 is a view, partly in cross-section, and partly pictorial, of a
preferred embodiment of the filter sub-system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] Referring now to the drawings, which are not intended to limit the
invention, FIG. 1 illustrates a vehicle drive system of the type for which the
hydraulic drive system of the present invention is especially well suited. The
vehicle system shown schematically in FIG. 1 has four drive wheels W,
although it should be understood that the present invention is not. limited to a
vehicle having four-wheel drive (or even four drive wheels), but could also be
used with a vehicle having only two-wheel drive, and in that case, the two drive
wheels could be either rear drive wheels or front drive wheels. Operably
associated with each of the drive wheels W is a conventional type of wheel
brake B, the details of which form no part of the present invention, and the
wheel brakes B will be referred to only briefly hereinafter Preferably, the
wheel brakes B are part of an overall EHB (electro-hydraulic brake) system, of
the type which is just now becoming well known to those skilled in the art, and
commercially available.
[0021] The vehicle includes a vehicle drive system, generally designated 11,
which includes a vehicle engine 13 and a transmission 15. It should be
understood that the particular type of engine 13 and transmission 15 and the
construction details thereof, as well as the drive system arrangement, etc., form

no part of the present invention, except to the extent specifically recited in the
appended claims, and therefore, will not be described further herein.
Furthermore, the present invention is not even limited specifically to use with
what is normally thought of as an "engine", and therefore, it will be understood
that, within the scope of the invention, references to an "engine" will mean and
include any type of power source or other prime mover.
[0022] Extending rearwardly from the transmission 15 is a drive line,
generally designated 17. In the subject embodiment, and by way of example
only, the drive line 17 includes a forward drive shaft 19, an intermediate drive
shaft (not visible in FIG. 1), a rearward drive shaft 23, an inter-wheel
differential 25 and left and right rear axle shafts 27 and 29 Those skilled in
the art will understand, from a subsequent reading and understanding of the
present specification, that the drive line 17 has been illustrated and described
as comprising the shafts 19 and 23 primarily to facilitate understanding of the
overall vehicle drive system 11, and not by way of limitation.
[0023] The drive system 11, in the subject embodiment, also includes left
and right forward axle shafts 31 and 33, respectively. Referring still primarily to
FIG. 1, in addition to the "mechanical" elements already described and which
are fairly conventional, the drive system 11 also includes a hydrostatic pump-
motor unit, generally designated 35, and disposed forwardly of the pump-motor
unit 35 is a valve manifold 37. Attached to a forward portion of the valve
manifold 37 is a source of low pressure 39, shown in FIGS. 1 and 2 as a low
pressure accumulator, and attached to a rear portion of the valve manifold 37 is
a high pressure accumulator 41, although the particular arrangement could be
reversed, or changed, or rearranged in some other manner. It should be
understood that the particular design and details of the valve manifold 37
(except to the extent noted hereinafter) and the accumulators 39 and 41 are not
essential features of the present invention, and therefore, the construction
details of each is not illustrated or described herein. Instead, the general

function and operation of each will be described briefly, in connection with the
system schematic of FIG. 2, but then only to the extent necessary to describe
the several operating modes of the hydraulic drive system as "environment" for
the explanation of the control circuit and the filter sub-system of the present
invention. It should also be understood by those skilled in the art that the
source of low pressure 39 shown in FIGS. 1 and 2 as a low pressure
accumulator for use in a closed-loop circuit could alternatively be an open
reservoir for use in an open-loop circuit. Therefore, all references made to the
low pressure accumulator hereinafter in the present specification are merely for
ease of description and are not intended to limit the present invention in any
way.
[0024] Referring still primarily to FIG. 1 the pump-motor unit 35 will be
described in slightly more detail, to facilitate an understanding of the overall
hydraulic drive system shown in FIG. 1. The pump-motor unit 35 includes a
clutch assembly portion, generally designated 43, and a pump-motor portion,
generally designated 45. It may be seen that the intermediate drive shaft
extends completely through the hydrostatic pump-motor unit 35 and would
preferably have, at its forward end, a universal joint coupling (not shown
herein), for connection to the forward drive shaft 19. Similarly, the intermediate
drive shaft would preferably have, at its rearward end, a universal pint
coupling(also not shown herein), for connection to the rearward drive shaft 23,
although, within the scope of the invention, the particular arrangement shown
and described could be reversed or changed in some other manner
[0025] Referring now primarily to FIG. 2, it should be understood that, other
than the pump-motor unit 35 and the two accumulators 39 and 41, everything
else shown in the hydraulic schematic of FIG. 2 would typically be included
within the valve manifold 37, seen in FIG. 1 or attached to the valve manifold
37. It should also be understood that, whenever the pump-motor unit 35 is in its
neutral (zero displacement) condition (which is the case whenever the vehicle is

not in a deceleration-acceleration cycle), there is no substantial flow within the
hydraulic system shown in FIG. 2, between the pump-motor unit 35 and the two
accumulators 39 and 41. However, as is well known to those skilled in the art
of such systems, because of the pre-charge on each of the accumulators 39
and 41, as will be discussed in greater detail subsequently, the system remains
"pressurized" even while the pump-motor unit 35 is in its neutral condition.
[0026] The hydraulic system (as shown in FIG. 2), which is included within
the valve manifold 37, includes a mode control valve 81, and operably
associated therewith, a step-orifice control valve 83 and a solenoid-type mode
pilot valve 85. The function and operation of the valves 81, 83 and 85 will be
described in somewhat greater detail subsequently, although what will be said
hereinafter about the valves 81, 83 and 85 will be by way of illustration and
enablement of the present invention, and not by way of limitation of the present
invention.
[0027] The pump-motor unit 35 is of the variable displacement type, and
therefore, includes some sort of displacement-varying means, such as a pair of
fluid pressure servo actuators of the type shown in FIG. 2 and designated 87
and 89. The servo actuators 87 and 89 are connected, hydraulically. to the
outlets of a typical electro-hydraulic controller 91. The function of the controller
91 is to communicate pressurized fluid from a conduit 93 to one of the servo
actuators 87 or 89, as appropriate to achieve the desired angle and
displacement of a swashplate 95, all of which is generally well known to those
skilled in the pump and motor art, and especially the axial piston pump art.
Those skilled in the art of hydraulic drive systems of the type to which the
invention relates will understand that, like typical HST systems, there can be
mechanical feedback from the swashplate 95 of the pump-motor unit 35 to the
controller 91. Preferably, however, feedback to the controller 91 is acnieved
electronically, even the indication of the position of the swashplate 95 It
should be understood that any type of feedback is within the scope of the

present invention.
[0028] Disposed in series between the high pressure accumulator 41 and
the electro-hydraulic controller 91 is an isolation valve 97 which, as shown in
FIG. 2, is preferably a poppet-type valve which is solenoid operated. Whenever
the hydraulic drive system 11 is operating, the isolation valve 97 is "ON", i.e.,
high pressure is freely communicated from the high pressure accumulator 41 to
the controller 91. Whenever the hydraulic drive system 11 is "OFF", the
isolation valve 97 is spring biased to the position shown in FIG. 2 in which it
keeps the pump-motor unit 35 and the controller 91 "isolated" hydraulically from
the high pressure accumulator 41, so that the accumulator 41 does not "leak
down" through the controller 91, while the system is not operating References
to the drive system being "OFF" will be understood to mean and include both
that portion of the vehicle operating cycle when the vehicle is not in a
deceleration-acceleration cycle, and those times when the vehicle is not
operating at all (engine "off conditions).
[0029] Referring still primarily to FIG. 2, the drive system 11 includes a
bypass valve assembly, generally designated 99, which may also be referred to
as an "unloading" valve or as a "dump" valve, as those terms are well
understood in the valve art. Thus, the bypass valve assembly 99 will "unload"
the pump-motor unit 35 whenever the engine is "off' (no driving pressure
present in the conduits 93, 109 and 111), so that there is no unintended torque
transmitted to the drive line 17. As is well known to those skilled in the art of
hydraulic circuits, the bypass valve assembly 99 would typically be included in
such a circuit to "unload" the pump-motor unit 35. It is believed to be within the
ability of those skilled in the art to determine the specific design and operation
of a particular sub-system, such as the bypass valve assembly 99.
[0030] The hydraulic drive system 11 also includes a relief valve, generally
designated 101 which, as is shown in FIG. 2, is spring biased to a closed
position. An inlet of the relief valve 101 is in communication with a conduit 103,

which interconnects the inlet with the port of the high pressure accumulator 41
and with the inlet of the mode control valve 81. Whenever the pressure in the
conduit 103 exceeds a predetermined maximum, the relief valve 101 is biased
("downward" in FIG. 2) to a position which permits communication from the
conduit 103 to a conduit 105 (which may be considered the "low pressure" side
of the system, as will become more apparent subsequently). Finally, referring
still to FIG. 2, the hydraulic drive system 11 includes a filter circuit, generally
designated 107 which will be described in greater detail subsequently.
[0031] Referring now to FIGS. 2 and 3 together, it may be seen that the
pump-motor unit 35 includes a port designated A which is connected by means
of a conduit 109 to the mode control valve 81. The unit 35 also includes a port
designated B which, by means of a conduit 111 is in fluid communication with
the filter circuit 107, and also with the conduit 105, such that the conduits 105
and 111 comprise the "low pressure" side of the system, as was mentioned
previously. As will be seen from the subsequent description, when the pump-
motor unit 35 is in the pumping mode, the port A is the outlet port (see arrows in
pump symbol in FIGS. 2 and 3), and when the unit 35 is in the motoring mode,
the port A is the pressurized inlet port and the port B is the exhaust outlet port.
[0032] Referring again primarily to FIG. 2, the general operation of the
hydraulic drive system 11 will be described briefly. As was mentioned
previously, when the vehicle is neither decelerating or accelerating the pump-
motor unit 35 (pump-motor portion 45 of FIG. 1) is de-clutched from the
intermediate drive shaft, and the overall vehicle drive system shown in FIG. 1
operates in the same manner as if the hydraulic drive system 11 were not
present.
[0033] When the vehicle operator begins to perform a braking operation, one
result is that the clutch assembly portion 43 is actuated, such that the pump-
motor unit 35 is now clutched to the drive shaft, and an appropriate command is

provided to the electro-hydraulic controller 91, displacing the swashplate 95 in
a direction such that the rotation of the drive line 17 (with the vehicle moving in
a forward direction) causes the pump-motor unit 35 to pump pressurized fluid
from the port A to the conduit 109. As is now well known to those skilled in the
art of hydraulic regenerative braking systems, the displacement of the
swashplate 95 (and therefore, the fluid output per rotation of the drive line 17) is
typically proportional to the extent to which the vehicle operator depresses the
brake pedal. It is now known to those skilled in the art how to set the
displacement of the swashplate 95 proportional to the brake torque applied by
the operator, or to the displacement of the brake pedal, although the particular
means, or criteria, selected for setting the displacement of the swashplate 95 is
not essential to the present invention.
[0034] With the pump-motor unit 35 in the pumping mode, pressurized fluid
communicated through the conduit 109 unseats a poppet member 113 in the
mode control valve 81, such that the pressurized fluid flows into the conduit
103, and from there, pressurizes the high pressure accumulator 41. tn the
subject embodiment, and by way of example only, the high pressure
accumulator 41 is of the gas-charge type. A hydraulic pressure is necessarily
maintained such that a minimum amount of oil is always retained in the high
pressure accumulator 41 (such that there is always a minimum charge of both
of the conduits 93 and 103). At the end of a typical deceleration cycle the high
pressure accumulator 41 is charged up to the maximum system pressure,
typically about 5000 psi.
[0035] At the completion of the deceleration portion of the braking cycle,
when the vehicle operator releases the brake pedal and begins to depress the
accelerator, an appropriate signal is communicated to the electro-magnetic
controller 91 which commands the pump-motor unit 35 to transition from the
pumping mode (described previously), to the motoring mode. In the motoring
mode, the swashplate 95 is disposed at an inclination opposite that which

existed when the unit was in the pumping mode (i.e., the swashplate 95 goes
"over-center"). When the pump-motor unit 35 is in the motoring mode, the
swashplate 95 is displaced such that flow through the pump-motor unit 35 (from
port A to port B) will cause the pump-motor unit 35 to transmit torque to the
drive line 17, tending to drive the drive line 17 in a direction corresponding to
forward movement of the vehicle. In the subject embodiment, and by way of
example only, the mode control valve 81 is constructed such that pressurized
fluid can always flow from the conduit 109 to the conduit 103 (i.e., the pumping
mode). However, only when the mode pilot valve 85 receives an appropriate
input signal to its solenoid is there an appropriate pilot signal 115 which assists
in the opening of the poppet member 113, to permit relatively unrestricted flow
of high pressure fluid from the accumulator 41 through the conduit 103 and then
through the conduit 109 to the port A of the pump-motor unit 35.
[0036] In the subject embodiment, and by way of example only the low
pressure accumulator 39 is also of the gas-charge type, and always maintains a
minimum inlet charge pressure at the pump-motor inlet port B of about 50 psi.,
in the subject embodiment, and by way of example only. This is true even
toward the end of the deceleration portion of the cycle, after the unit 35 has
pumped up the high pressure accumulator 41. After the completion of the
acceleration portion of the cycle, when the low pressure accumulator 39
contains almost all of the oil, the pressure in the low pressure accumulator 39
rises to about 150 psi, in the subject embodiment, and by way of example only.
[0037] Referring now primarily to FIG. 3, the filter circuit 107 will be
described. Although it was mentioned previously that the conduits 105 and 111
comprise the low pressure side of the system, it should be understood that,
because of the presence of the low pressure accumulator 39, the pressure in
the conduit 111 would never, during normal operation of the system, be at
essentially zero or reservoir pressure, as is the case in many hydraulic systems.
Instead, as was mentioned previously, but by way of example only, the low

pressure accumulator 39 insures that the conduits 117 and 111 are maintained
at a pressure of at least about 50 psi, in this embodiment of the invention. As
may also be seen in FIG. 2, the port of the low pressure accumulator 39 is in
communication with the filter circuit 107 by means of the conduit 117 (partially
shown also in FIG. 3).
[0038] Alternatively, if an open reservoir were used in the hydraulic drive
system as the source of low pressure 39 instead of the low pressure
accumulator 39 as previously described and shown in FIGS. 1 and 2, a charge
pump (not shown) would need to be incorporated into the system. The charge
pump (not shown) would provide charge pressure to the inlet of the pump-motor
unit 35 to prevent cavitation and insure that the conduits 117 and 111 are
maintained at a minimum pressure.
[0039] Referring now to FIG. 3, in conjunction with FIG 4, the filter circuit
107 would typically be disposed within a filter manifold, shown only
schematically in FIG. 3, but shown as a valve housing in FIG. 4 and generally
designated 119. Within the filter manifold 119 there is disposed a two-position,
two-way filter shut-off valve 121, which is spring biased to an open position (the
flow position "F" shown in FIG. 3), but the shut-off valve 121 may be manually
displaced by any suitable means, such as a handle 123, to a position blocking
flow therethrough (the isolation position "I" in FIG. 3). With the filter shut-off
valve 121 in the open position shown in FIG. 3, low pressure fluid may flow from
the conduit 111 to a conduit 125, which is shown in FIG. 3 as extending outside
of the filter manifold 119 for reasons which will be described subsequently. The
conduit 125 is in fluid communication with an "inlet" side of a filter element 127,
with an "outlet" of the filter element 127 being connected by means of a conduit
129 to the inlet of a check valve 131 (which prevents back-flow through the filter
element 127), and from there to the conduit 117.
[0040] The conduit 111 is also in communication with one port of an orifice
and valve assembly, generally designated 133, the other port of the assembly

133 being in open communication with the conduit 117. Within the orifice and
valve assembly 133 is a parallel path arrangement including a fixed flow orifice
135 and a check valve 137, the function of which will be described
subsequently.
[0041] In accordance with one important aspect of the present invention, and
as will be described in greater detail subsequently, one of the objects of the
present invention is met by the provision of the filter circuit 107, as shown in
FIG. 3, wherein flow passes through the filter element 127 while the pump-
motor unit 35 is in its motoring mode only, but when the pump-motor unit 35 is
in its pumping mode, the filter circuit 107 provides relatively little restriction to
fluid flow from the low pressure accumulator 39 to the inlet port (port B) of the
pump-motor unit 35.
[0042] The operation of the filter circuit 107 of the present invention will now
be described in somewhat greater detail. When the pump-motor unit 35 is in its
pumping mode, low pressure fluid (from about 150 psi. initially, down to about
50 psi., in the subject embodiment) from the low pressure accumulator 39 flows
through the conduit 117 but is blocked by the check valve 131 from flowing
through the filter element 127. Therefore, in this "first flow path" through the
filter circuit 107, all of the flow from the low pressure accumulator 39 flows
through the conduit 117 and then through the orifice and valve assembly 133.
The arrangement of the assembly 133 provides a relatively unrestricted flow
path through the assembly 133 (by unseating the check valve 137), and then
through the conduit 111 to the inlet port (port B) of the pump-motor unit 35. In
the above-described first flow path, some of the flow is through the fixed flow
orifice 135, but typically, the majority of the flow in the pumping mode, would be
through the unseated check valve 137
[0043] When the pump-motor unit 35 is switched to the motoring mode, such
that the port B is now the outlet port of the pump-motor unit 35, flow through the
conduit 111 flows through a "second flow path" by means of which fluid returns

to the low pressure accumulator 39. This second flow path includes two path
portions in parallel. The one path portion flows through the filter shut-off valve
121, then through the conduit 125 and the filter element 127, then through the
conduit 129 and past the unseated check valve 131 to the conduit 117, The
other path portion flows through the orifice and valve assembly 133 but flow in
the direction now being described can pass only through the fixed flow orifice
135, and then to the conduit 117, recombining with the fluid which has passed
through the filter element 127.
[0044] Therefore, by appropriately selecting the filter element 127, and the
fixed flow orifice 135, which is believed to be well within the ability of those
skilled in the hydraulics art, it is possible to have approximately a
predetermined percentage of the flow pass through the filter element 127 in the
motoring mode. In the course of the development of the subject embodiment,
and by way of example only, approximately eighty (80%) percent of the flow in
the motoring mode passes through the fixed flow orifice 135, while the
remaining twenty (20%) percent (of the total flow from port B to the accumulator
39) passes through the filter element 127. As is also well known to tnose
skilled in the art, these relative percentages can be varied to achieve objectives
such as a greater degree of filtration, on the one hand, or a reduced pressure
drop through the filter circuit 107, on the other hand.
[0045] With flow through the filter element 127 occurring only during the
motoring mode of the pump-motor unit 35, and with the low pressure
accumulator 39 maintaining a relatively constant low pressure, the filter element
127 may be selected appropriately, with the system designer knowing that the
filter element 127 will be subjected to only known, relatively constant, relatively
low pressures at all times. If the filter element 127 were subjected, periodically,
to substantially higher pressure drops, there would be a requirement for a more
robust, and more expensive, filter arrangement, and filter element material.
[0046] As mentioned previously, the filter circuit 107 of the present invention

accomplishes one of the objects of the invention by providing a relatively
unrestricted flow path to the inlet (port B) of the pump-motor unit 35 whenever
the unit 35 is in its pumping mode. Such unrestricted, low pressure flow to the
inlet, in the pumping mode, is especially important to prevent cavitation during
the pumping mode, and the noise which would result, especially when the
hydraulic drive system 11 of the present invention is utilized as part of a
hydraulic regenerative braking system and/or when the drive system 11 is
utilized as part of an on-highway vehicle. As is well known to those skilled in
the vehicle art, it is almost essential to minimize noise on most vehicles, but
especially so for on-highway vehicles. As is also well known, cavitation could
damage various parts of the pump-motor unit, thus reducing the useful life of
the drive system.
[0047] Another benefit associated with the filter circuit 107 of the present
invention is that, if and when the filter element 127 ever becomes partly or even
totally plugged by contamination particles, there is still an available, separate
flow path (through the fixed flow orifice 135), and there is no condition under
which flow to or from the pump-motor unit 35 is totally blocked. Furthermore the
relation between the filter element 127 and the fixed flow orifice 135 pre-
determines how flow transitions from the filter element 127 fully to and through
the orifice 135, as the filter element 127 becomes progressively filled with
contamination particles. With a very minor increase in pressure drop, the full
flow from the port B returns to the low pressure accumulator 139 through the
orifice 135.
[0048] Furthermore, in regard to the issue of the filter element 127 becoming
sufficiently plugged with contamination particles, it may be seen in FIG. 3 that
the filter circuit 107 includes a pressure-actuated electrical relay device,
generally designated 139. The relay device 139 receives a pilot signal 141
from the conduit 117, and also receives a pilot signal 143 from the conduit 125.
If the pressure differential between the pilot signals 141 and 143 (143 should

always be higher than 141 in the motoring mode), is sufficient to overcome the
force of a biasing spring 145, the relay within the device 139 is closed, thus
transmitting an electrical signal 147 to an appropriate warning device, such as
an electronic controller, or a warning light or a buzzer in the operator's
compartment.
[0049] In accordance with another aspect of the invention, replacement of
the filter element 127 (when it becomes sufficiently plugged with contamination
particles) may be accomplished without the need for de-pressurizing and
draining the closed loop hydraulic drive system 11 shown in FIG. 2. As is
understood by those skilled in the art of such closed loop drive systems, that
provide long fluid life, the low pressure side is always pressurized. The
pressure swings from a low to a high depending on the amount of fluid in the
low pressure accumulator 39 (for example, between 50 psi. and 150 psi. in this
embodiment). This is true even when the vehicle engine is in an "off'
condition.
[0050] When it is desired to replace the filter element 127 with a new, clean
element, all that is required, in the subject embodiment, is to depress the
handle 123, moving the filter valve 121 to the left from the position shown, to a
position in which flow from the conduit 111 to the conduit 125 is blocked.
Within the scope of the invention, the spring biasing the filter shut-off valve 121
and the handle 123 could be reversed. Once that blocking of flow to the
conduit 125 has occurred, the rest of the hydraulic drive system 11 is isolated
from the path portion which includes the conduits 125 and 129 and the filter
element 127. Therefore, the filter element 127 may then be replaced, and to
the extent any fluid is drained from either of the conduits 125 or 129 as a result,
the filter path portion (conduit 125) can be refilled by means of an air bleed and
fill valve 149 (see FIG. 2), and also by pre-filling the new filter element before it
is installed in the circuit.
[0051] As should be apparent to those skilled in the art, another benefit of

the filter circuit 107 of the present invention is the ease of "adjustability", i.e.,
the ease of changing, on a future model of the drive system 11, the percentage
of fluid flow which flows through the filter element 127'., versus the percentage of
fluid flow which flows through the fixed flow orifice 135. By way of example, the
fixed flow orifice 135 could comprise an orifice member, such that the entire
filter circuit 107 and filter manifold 119, etc. could remain the same, with the
only change for the prospective, future model of the drive system being the
replacement of one particular size of orifice member with another orifice
member providing a different size of fixed flow orifice 135, and therefore, a
different percentage of the total fluid flow (from the port B to the accumulator
39) passing through the filter element 127.
[0052] The invention has been described in great detail in the foregoing
specification, and it is believed that various alterations and modifications of the
invention will become apparent to those skilled in the art from a reading and
understanding of the specification. It is intended that all such alterations and
modifications are included in the invention, insofar as they come within the
scope of the appended claims.

What is claimed is:
1. A hydraulic drive system adapted for use on a vehicle having an engine
and a drive-line operable to transmit driving torque from said engine to a
drive axle, said drive system including a hydrostatic pumip-motor unit
operable, in a pumping mode, to receive drive torque from said drive-
line, and operable, in a motoring mode, to transmit drive torque to said
drive-line; a high pressure accumulator in fluid communication with a first
port of said pump-motor unit through a mode valve means whereby,
when said pump-motor unit is in said pumping mode, pressurized fluid is
communicated from said pump-motor unit to said high pressure
accumulator, and when said pump-motor unit is in said motoring mode,
pressurized fluid is communicated from said high pressure accumulator
to said pump-motor unit; a source of low pressure in fluid communication
with a second port of said pump-motor unit; characterized by
(a) a filter circuit disposed between said low pressure accumulator
and said pump-motor unit;
(b) said filter circuit defining a relatively unrestricted first flow path
from said source of low pressure to said second port when said
pump-motor unit is in said pumping mode;
(c) said filter circuit defining a second flow path from said second port
to said source of low pressure when said pump-motor unit is in
said motoring mode; and
(d) said second flow path comprising one path portion through a filter
shut-off valve and a filter element in series, and in parallel
therewith, another path portion through a controlled flow
restriction, whereby one portion of the fluid flow from said second
port flows through said filter element, and the remainder of said
fluid flow from said second port flows through said controlled flow

restriction.
2. A hydraulic drive system as claimed in claim 1, characterized by said
controlled flow restriction being selected and sized, relative to said filter
shut-off valve, such that said one portion of the fluid flow from said port
comprises approximately a predetermined percentage of the total fluid
flow from said port.
3. A hydraulic drive system as claimed in claim 1, characterized by said
filter shut-off valve comprises a two-position, two-way valve including a
flow position defining said one path portion, and an isolation position
blocking flow from said port through said filter element whereby, when
said filter shut-off valve is in said isolation position, said filter element
can be replaced without draining fluid from the rest of said hydraulic
drive system.
4. A hydraulic drive system as claimed in claim 1, characterized by said
relatively unrestricted first flow path defined by said filter circuit excludes
said filter valve and said filter element.

A hydraulic drive system (11) of the type including a hydrostatic pump-motor unit (35) having a pumping mode in which
the unit pressurizes, from its port (A), a high pressure accumulator (41), and a motoring mode, in which the unit is driven by pressurized fluid from the high pressures accumulator..
The system also includes a source of low pressure (39) in communication with the opposite port (B) of the pump-motor unit
(35), and a filter circuit (107) disposed therebetween. The filter circuit (107) defines an unrestricted first flow path
from the source of low pressure (39) to the port (?) when the unit is in the pumping mode, and a second flow path h from the port(B) to the source of low pressure (39) when the unit is in
the motoring mode. The second flow path comprises one path portion (125) through a filter shut-off valve (12:) and a filter
(127) in series, and in parallel therewith, another path portion through a controlled flow restriction (135).1 Thus, filtration
occurs during only the motoring mode, and the percentage of fluid being filtered can be predetermined.