Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BALF~NCED HYDRAULI C PROPULS ION S YSTEM
B k~round of the Inventio
1. Field of the Invention:
The invention is directed to a hydraulic propulsion system
for a large industrial machine such as an excavator. The
machine having at least two hydraulic propulsion motors which
are driven from the same source of hydraulic pressure.
2. Description of the Prior Art:
Large industrial machines are propelled many times by
1~ hydraulic motors. Typically, such machines are provided with
internal combustion engines that are used to drive hydraulic
pumps. The hydraulic pumps draw hydraulic fluid from a sump and
pump the hydraulic fluid into hydraulic lines where it is
directed tG the propulsion motors and other operating members.
Individual three-position directional control valves are used to
control the flow of hydraulic fluid to each of the mo~ors,
thereby controlling the propulsion motors and other hydraulic
motor6 used for driving the operating members.
In simple hydraulic systems, hydraulic fluid takes the path 20 of least resista~ce and flows to the area requiring the lowest
pressure. This is especially troublesome wherein two hydraulic
motors are being used to move a common load, for example two
crawler tracks of a crawler excavator, because the low pressure
motor will command more hydraulic fluid resulting in an uneven
operation of the two motors. To overcome this natural tendency
of the hydraulic fluid, compensator valve assemblies are
provided to better balance the flow between the two motors by
having the high pressure compensator valve assembly meter the
low pressure side to even the pressure between the two
assemblies.
Although c~mpensator systems work well in most instances,
another problem develops when the loads are equal or close to
being equal. This situation is noticeable when a crawler
operator wants to go in a straight line wherein the tracks need
to move equally to accomplish this task. The crawler operator
would notice that the crawler would tend to turn to one side or
the other as it moves. Therefore, the operator has to
continually adjust for this turning movement in the crawler.
This situation arises because one of the compensator valve
assemblies is dominating the other compensator valve assemb
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1 efectively reducing flow tllrough one of the hydraulic motors.
This typically happens because the directional control valves
are never opened simultaneously and the directional control
valve that is opened irst creates a dominating compensator
valve assembly as it becomes the high pressure compensator
valve. The compensator valve assembly associated with the
latter opening directional control valve becomes dominated by
earlier opening and now high pressure compensator valve assembly
and tends to reduce flow to the hydraulic motor to which it is
associated. Therefore, the hydraulic motor associated with the
firs~ opening directional control valve moves faster than the
motor associated with the latter opening directional control
valve resulting in a ~urning movement by the crawler.
Summary of the Invention
The present invention is designed to overcome this problem
with compensator valve assemblies by providing a small
communication hydraulic line between the downstream hydraulic
paths of the two compensator valve assemblies~ A source of
hydraulic fluid supplies hydraulic fluid to two directional
control valves each of which direct pressurized hydraulic fluid
to a pair of downstream compensator valve as~emblies. Each pair
of compensator valve assemblies is provided with a forward
compensator valve assembly for controlling forward movement of
the crawler and a backward compensator valve assembly for
controlling the backward movement of the crawler. The position
of the directional control valve determines which one of the
compensator valve assemblies in each pair of compensator valves
the hydraulic fluid is directed to, thereby controlling the
movement of the crawler. Two small communLcation hydraulic
lines are provided for transmitting hydraulic fluid between the
two forward compensator valve assemblies and the two backward
compensator valve assemblies.
~rief Descrintion of the Drawinas
.~ ~
FIG. 1 is a side view of a crawler excavator.
FIG. 2 is a hydraulic schematic of a hydraulic propulsion
system for an excavator crawler without the small communication
line.
FIG~ 3 is a hydraulic schematic of a hydraulic propulsion
system for an excavator crawler with the small communication
line.
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.
1Detailed Descrlption
FIG. 1 illustrates an excavator crawler to which the present
hydraulic propulsion is particularly well suited. Excavator 10
is provided with a movable boom 12, dipper 14 and bucket 16.
The boom, dipper and bucket are controlled by linear hydraulic
motors 18, 20, and 22, respectively. Excavator crawler 10 is a
self-propelled excavator being supported on two ground engaging
tracks 24 ~only one shown) which are used to drive and position
the excavator at a work site,
10The tracks are independently driven by rotary hydraulic
motors 26 and 28 which are coupled through compensator valve
assemblies 30, 32, 34 and 36 to directional control valves 38
and 40. ~ydraulic fluid is pumped to the directional control
valves 38 and 40 from 8ump 42 by hydraulic pump 44. The
hydraulic pump is driven by an internal combustion engine
mounted in the excavator. The operator in cab 46 can move or
position the excavator by manipulating the directional control
valves to propel the excavator forward or backward, or turning
the e~cavator by operating hydraulic motors 26 and 28 in
different directions.
It should be noted that although the invention is being
described with regards to an excavator crawler propulsion
~ystem, the present invention could be utilized in a number of
hydraulic applications wherein two independently controlled
hydraulic motors drive a common load from a single source of
pressurized hydraulic fluid.
FIG. 2 is the hydraulic schematic of the hydraulic
propulsion system without the small balancing communication line
between the downstream output of the compensator valve
assemblies. Each compensator valve assembly is provided with a
metering compensator spool 48, 50, 52 and 54, a shuttle spool
56, 58, 60 and 62, and a return flow check valve 64, 66, 68 and
70. For forwardly driving motor 26 hydraulic pump 44 pumps
hydraulic fluid into hydraulic pumping line 72 to directional
control valve 38. The directional control valve 38 directs the
fluid to forward compensator valve assembly 30 and specifically
to metering two-position compensator ~pool 48 having a
restricted orifice position and a checked position. SPOG1 48 is
spring biased into a closed position by spring 74 which is
overcome by hydraulic pressure in sensing line 76 which pushes
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1 the valve into the open position. Hydraulic pressure Erom line
72 is also directed through hydraulic line 77 to shuttle spool
56 into compensation communication line 78. Shuttle spool 56 is
hydraulically balanced by the hydraulic pressure in line 78 and
the pressure downstream of compensator spool 48 as transmitted
through line 80. The hydraulic fluid in line 80 is used both
for balancing spool 56 and for flowing thrvugh spool 56 to line
82 to ~alance spool 48 by adding to the biasing force of spring
~ .
10Hydraulic fluid passing through valve 48 into line B4 i5
directed to motor 26 driving one of the crawler tracks of ~he
excavator. The exhausted hydraulic fluid then passes into line
86 where it is directed to backward compensator valve assembly
32. As shuttle spool 58 is shifted into the closed position by
the hydraulic pressure in compensator communication line 78, and
: spool 50 is closed by the biasing force of spring 88 and the
hydraulic pressure in line 90 which i5 fluidically coupled to
compensator communication line 78 by the closed position of
spool 58; the exhausted fluid passes through check valve 66 and
into exhaust hydraulic line 92 wherein it is directed into sump
42, Hydraulic fluid does not pass through check valve 64 of
compensator valve assembly 30 because of the pressure drop
across the restricted orifice of spool 480
In FIG. 2, both motors are being driven in the same forward
direction as determined by directional control valves 38 and
40. However, compensator valve assembly 30 has become dominant,
either because it was triggered first by the operator or because
of shorter hydraulic line connections when compared with
compensator valve assembly 34. Compensator valve assembly 34
works in an identical manner to that of compensator valve
assembly 30 except that because of the hydraulic pressure in
compensation communication line 78 shuttle spool 60 tends to be
biased into a closed position which in turn directs hydraulic
pressure from line 78 through shuttle spool 60 and hydraulic
line 94 to aid spring 96 in biasing compensator spool 52
closed.
It should be noted that the shuttle and compensating spools
are two-position metering spools which are hydraulically
balanced. As such, the spools are reciprocated between the each
of the two positions during operation and they do not normally
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1 maintain a fixed position. There~ore ;n viewing FIG. 2, it
should be noted that dominatiny compensa~ g spool 48 in
compensating valve assembly 30 is opened and transmits more
hydraulic fluid because of its higher pressure, if it is the
dominating valve assembly, and compensating spool 52, of
compensating valve assembly 34 transmits less hydraulic fluid
because of its lower hYdraulic pressure when compared to
dominating compensating valve assembly 30.
As with compensating valve assemblies 30 and 32, hydraulic
fluid from pump ~4 flows through pumping line 72 to directional
control valve 40 where it is transmitted to compensating spool
52. ~ydraulic fluid passes through the restricted orifice in
compensating spool 52 and is directed to pump 28 from which it
is exhausted to compensating valve assembly 36. As with
lS compensating valve assembly 32, hydraulic fluid is prevented
from passing through compensating spool 54 and instead passes
through check valve 70 and back to sump 42. The balancing
hydraulic lines for all of the compensating-spools and shuttles
spools of compensating valve assemblies 32, 34 and 36 are
identical to those explained with regards to compensating valve
assembly 30 and function in the same manner.
If the excavator crawler is to be reversed, directional
control valves 38 and 40 are moved to the left to direct pumping
fluid to backward compensating valve assemblies 32 and 36. In
this situation, the pumps exhaust hydraulic fluid through check
valves 64 and 68, respectively. ~o pivot the machine, one
hydraulic motor is operated in the forward direction and the
other in a reverse direction. The excavator itself can be
pivoted on the tracks which means that since the hydraulic
motors are adjacent to the tracks, the hydraulic lines leading
from the pump to the motors must pass through a hydraulic line
swivel (not shown) which is well known ln the art.
FIG. 3 illustrates the small communication hydraulic lines
used for overcoming the problem illustrated in FIG. 2.
Hydraulic lines 98 and 100 fluidically couple hydraulic line 84
to line 102, and line 86 to line 104, respectively. When the
excavator crawler is moving forward, line 98 tends to equalize
the hydraulic pressure between compensating valve assembly 30
and compensating valve assembly 34. As compensating valve
assembly 30 tries to dominate valve assembly 34, hydraulic fluid
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l pressure increases in line 84 increasing the pressure in line 98
and line 102 which in turn increases pressure in line 106
causing metering shuttle spool 60 to remain open for
transmitting pre~sure through line 108 ~o help bias compensating
spool 52 open, and better equalizing the hydraulic flow to both
motors During forward movement, exhaust lines 86 and 104 are
joined by line lO0, but this does not effect the operation of
the system because the hydraulic pressure in compensating li~e
78 serves to maintain compensating valve assemblies 32 and 36
closed except for the normal exhaust flow through check valves
66 and 70.
In reversing the excavator crawler, communication line lO0
would prevent either compensating valve assembly 32 or 36 from
dominating one another. As with the forward operation, exhaust
lines 84 and 102, even through coupled through line 98, would
; not effect operation of the compensating valve assemblies.
To prevent inexact operation~, lines 98 and lO0 must be
guite small when compared to hydraulic lines 84, 86~ 102 and 104
which are used to transfer hydraulic fluid to the pumps. For
example, li~es 84, 86, 102 and 104 can be 0.75 inches in
diameter and in accordance therewith communication lines 98 and
lO0 should be 0.~5 inches in diameter. In addition, lines 98
and lO0 should be provided with an orifice further restricting
flow. This orifice should be 0.004 inches in diameter to
further reduce the cross flow between the pumping lines.
Compensating communication line 78 serves an additional
function as indicated by arrow llO and that is to provide a
pressure sensing circuit with a hydraulic feedback to better
control the operation of the hydraulic pump.
The presen~ invention described above should not be limited
by the above described embodiments, but should be limited solely
by the claims that follow.
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