Note: Descriptions are shown in the official language in which they were submitted.
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LIFT, TILT AND STEERING CONTROL FOR A LIFT TRUCK
This invention relates to a hydraulic control
system for the lift, tilt and steering functions of a lift
truck and, particularly, to such a control~system wherein
a pair of hydraulic pump5 for supplying pressure fluid are
driven by separately controlled power means.
BACKGROUND OF TH~ INVENTION
Heretofore others have suggested and used
hydraulic control systems for lift trucks wherein a pair
o~ hydraulic pumps are used to supply pressure fluid. One
: such system is shown in U.S. patent 3,443,380 wherein one
of two pumps supplies fluid to the lift cylinder by way of
passage byp~ssing~the tilt and lift control valves~ U.S.
:~ 15 patents 4,002,220 and 4,070,857 show lift truck hydraulic
systems in which a single pump supplies pressure fluid for
the lift,~til~ and steering functions and in ~hich a
priority valve is employed to provide priority flow of
pressure fluid to the power steering mechanism.
RIEF DESCRIPTION OF THÉ INVENTION
The hydraulic control s:ystem for the lift
cylinder, the tilt cylinder and the steering motor of a
lift truck includes first and second hydraulic pumps
: :driven, respectively, by~first and~ second electric motors
and lift, tilt and steering valves~ The first pump
: supplies fluid pressure to a priority valve which has a
high priority output connected to a steering valve and a
~: low priority connec~i~on to the lift and tilt valves. A
fluid delivery circuit independent of the priority valve
30 connects the output of the second pump in fluid delivery
relation to the lift control valve. Control means for the
second electric motor includes a switch operated in
response to movement of the lift control valve to and from
a raise position whereby the second pump is driven by the
second electric motor only when said lift control:valve is
moved to a raise position.
In a second embodiment of the invention, the
lift control valve may be moved to a first "raise1'
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position in which low speed lift is achieved through use
of flow from the first pump and also to a second "raise"
position in which the second pump is driven to supply
pressure fluid to the lift control valve for high speed
lift. In a third embodiment of the invention, the first
pump is also operated on "demand," that is, the first pump
is driven to deliver pressure fluid when the operator
steers the vehicle, when the operator moves the lift
control valve to a "raise" position or moves the tilt
control valve from its neutral, hold position to either of
i~s operating positions. In both the second and third
embodiments o~ the invention, the second pump is not
placed in operation for high speed lift unless the lift
control valve is moved o a second predetermined position.
BRIE~ DESCRIPTION OF THE DRAWINGS
Three different embodiments of the invention are
illustrated in the drawings in which:
Fig. 1 is a schematic showing a first embodiment
of a hydraulic control ystem for a lift truck;
: 20 Fig. 2 illustrates a second embodiment of the
invention in which the lift cylinder control valve must be
fully stroked in the direction of its "raise" position
be ore the second pump is driven to provide high speed
lift; and
Fig. 3 is a schematic showing of a third embodi-
ment of a hydraulic control system for a lift truck
wherein the pumps are operated only on "demand".
DETAILED DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic illustration of a lift
truck hydraulic control system which has particular
utility in an electric lift truck. The hydraulic control
s~stem includes a steering motor or cylinder 11 which is
controlled by a conventional steering valve 12 through
movement of the manually operated steering wheel 13. The
load carriage 14 of the lift truck is raised and lowered
on the mast, not shown, by lift cylinder 15 controlled by
lift control valve 19 and the mast is tilted about a
transverse horizontal axis by a pair of tilt cylinders
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16, 17 which are connected in parallel to a tilt control
valve 18. A pair of hydraulic pumps 21, 22 are driven by
electric motors 23, 24, respectively, through independ-
ently controlled circuits 26, 27. When the switch 28 at
the operator's station is closed, power from a source of
elect;ricity, in the form of a battery 31, is connected to
the motor 23, whereupon the motor 23 drives the pump 21
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to deliver pressure fluid to a steering control circuit
or conduit 31 interconnecting the pump 21 and a priority
valve 32. A fluid passageway or circuit, in the form of
conduit 33, is connected at one end in fluid receiving
relation to a high priority output 34 of priority valve
32 and at its other end to the steering control valve 12
The priority valve 32 has a low priority output 36
delivering fluid not reguired by the steering mechanism
to a pressure fluid delivery circuit 37 which includes a
main conduit 41 connected to the output 36 and a pair of
branch conduits 42, 43 connected in parallel to the main
conduit 41 and in fluid delivery relationship to lift
control valve 19 and tilt control valve 18. ~ fluid
delivery circuit 38 interconnects the output of pump 22
with the lift cylinder 15 by conduit segments 44, 45, 42,
53 with the lift control valve 19 operatively interposed
in the circuit 38. A relief valve 50 is provided for the
fluid delivery circuit 38, which acts as a relief valve
for both pumps.
The manually operated lift control valve 19
carries a control element in the form of an electrical
contactor 46~ on its lower end, as illustrated in Fig. 1,
which, upon downward movement to a predetermined "raise"
position, engages contacts 47 to complete an electric
circuit. When the circuit 27 of electric motor 24 is
thus energized, it drives pump 22 to deliver pressure
fluid to the fluid delivery circuit 38.
During operation of a lift truck, such as an
electric lift truck, the switch 28 is normally closed
during operation of the truck thus causing the electric
motor 23 to drive pump 21 to deliver pressure fluid to
the priority valve 32. The priority valve 32 insures an
adequate supply of pressure fluid for the s~eering
function by way of conduit 33 between its high priority
output 34 and the steering valve 12. Low priority flow,
that is, fluid flow not utilized for steering purposes is
exhausted by way of a low priority port or output 36 to
the fluid delivery circuit 37. A one-way valve in the
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form of check valve 51 prevents flow from the low
priority side of the priority valve 32 from passing to
the pump 22 by way of conduit 41. Pressure fluid
delivered by the pump 21 to the lift and tilt cylinder
circuits 42, ~3 by way of conduit 41 is sufficient in
rate of flow for providing normal tilt operations and the
pump 21 and motor 23 may be sized to no larger than neces~
sary to provide steering and tilt functions. The volume
of fluid supplied by pump 21 is not sufficient to provide
lift speeds for the carriage 14 that are normally desired
in a lift truck operation. However, the delivery of pump
21 is sufficient to produce a low speed lift operation.
In the embodiment illustrated in Fig. 1, when the
operator desires to operate the lift cylinder 15, that
is, to expand it, the spool of the lift control valve 19
is moved downwardly to provide a fluid connection between
circuit 42 and conduit 53. This downward movement of the
spool of ~he valve 19 causes the contactor 46 to engage
the contacts 47 thereby causing the electric motor 24 to
drive the pump 22 to deliver sufficient pressure fluid to
the lift control valve 19 and lift cylinder 15 to provide
a high speed lift. During lifting, pressure fluid flows
through a check valve 56 of a conventional flow regulator
valve 57. During a lowering operation, the check valve
56 is closed and fluid is exhausted from the lift
cylinder lS through a restriction 58 in the flow
regulator valve 57 which regulates the lowering speed of
the carriage 14. The tilt cylinder control includes a
counterbalance valve 61 of conventional construction
which forms no part of the present invention.
Referring to FigO 2, a modified lift control
valve 119 is illustrated which provides a two-speed lift
for the lift truck in which the invention is used. The
spool of the lift control valve 119 may be shifted down-
wardly to a first "raise" position in which the contactor146 moves to the position shown by dash lines 146'. In
this first predetermined "raise" position the contactor
146 does not engage contacts 147, however, the lift valve
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119 does establish a fluid connection between circuit 42
and conduit 53 thereby effecting delivery of pressure
fluid from pump 21 to the lift cylinder 15 so as to
provide a low speed lift for the lift truck carriage 14.
If the operator moves the spool of the control valve 119
downwardly, a further distance to a second predetermined
"raise" position in which the contactor 146 engages the
contacts 147, the pump 22 will be driven, thus delivering
sufficient fluid to the lift cylinder to provide a high
speed carriage lift. Thus by using the embodiment of the
invention illustrated in Fig. 2, the operator has a
choice of low speed lifting without using energy in addi-
tion to that used to drive the pump 21. Pump 22 is
driven only when the operator desires to have a high
speed lift. It should be understood, of course, that
during a lowering operation, fluid is exhausted from the
lift cylinder 15 without the need for any pressure fluid.
A third embodiment of the invention is illus-
trated in Fig. 3. In the third embodiment, an electric
circuit 126 for the electric motor 23 for driving the
pump 21,~includes a switch 125 which is closed to drive
the pump 21 when the operator rotates the manually
operated steering wheel 13 in either direction. (The
switch 28 is closed when the lift truck is operated.)
Thus, the pump 21 is operated to supply pressure fluid
for steering only "on demand" of the lift truck operator.
Additionally, parallel electric circuits 127, 128 are
provided for the electric motor 23 for the pump 21,
whereby the pump 21 is~operated "on demand" of the lift
truck operator, as by the operator moving the spool of
the tilt control valve 118 up or down from its illus-
trated hold position to its operating positions in which
its contactors 130, 131 engage contacts 151 of circuit
128 or by the operator moving the spool of the lift
3s control valve 119 from its illustrated hold position to
its first predetermined "raise" position wherein its
contactor 129 engages contact 152 to energize circuit
127. If the operator desires a high speed lift, he moves
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the spool of the lift control valve 119 downwardly a
further distance to a second predetermined "raise"
position in which its contactor 132 engages contacts 133
thus energizing a circuit 154 causing the electric motor
24 to drive the pump 22. Thus, the embodiment of the
invention illustrated in Fig. 3 is a "demand" system
wherein the pumps for the hydraulic fluid control system
are driven only when the operator selects a particular
function to be performed by the opera~ing cylinders of
the system.
The invention as illustrated and described
provides a hydraulic control system particularly useful
in an electric lift truck wherein the conservation of
electric energy is extremeIy important. A relatively low
volume pump 21 may be utilized for the steering and tilt
cylinder functions (and for a slow speed lift function as
well) without requiring the high volume pump 22 to be
driven. The high volume pump 22 is selectively driven
only when the operator desires a high speed lift. This
system conserves considerab;e electrical energy as
compared to a system wherein high and low volume pumps
are continuously driven. In the Figs. 1 and 2 embodi-
ments, the low capacity pump 21 is driven whenever the
lift truck is being operated, whereas in the embodiment
of Fig. 3, the low capacity pump 21 is driven only "on
demand" of the operator as expressed by his operation of
the steering wheel 13, lift valve 19 or tilt valve 18.
The embodiments of the invention illustrated in Figs. 2
and 3 allow the operator to accomplish low speed carriage
lift without driving pump 22 thus saving energy when only
low speed lifting is required.
