[0001] The invention is directed to a hydraulic propulsion system comprising first and second
hydraulic motors, a pump supplying pressurised hydraulic fluid to the motors, a first
compensating valve assembly between one side of the first motor and the pump, and
a second compensating valve assembly between the corresponding one side of the second
motor and the pump.
[0002] Large industrial or construction machines such as track-laying excavators are often
propelled by 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 to the propulsion motors for the tracks and to other operating members.
Individual three-position directional control valves are used to control the flow
of hydraulic fluid to each of the motors, thereby controlling the propulsion motors
and other hydraulic motors used for driving the operating members.
[0003] In simple hydraulic systems, hydraulic fluid takes the path of least resistance 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.
[0004] Although compensator systems work well in most instances, another problem develops
when the loads are equal or close to being equal. This situation is notaceable 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 assembly effectively reducing flow through 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 first creates a dominating compensator
valve assemby as it becomes the high pressure compensator valve. The compensator
valve assembly associated with the later opening directional control valve becomes
dominated by the earlier opening and now high pressure compensator valve assembly,
and tends to reduce flow to the hydraulic motor with which it is associated. Therefore,
the hydraulic motor associated with the first opening directional control valve moves
faster than the motor associated with the later opening directional control valve
resulting in a turning movement by the crawler.
[0005] The present invention overcomes or reduces this problem.
[0006] According to the present invention the connections between the first and second assemblies
and motors are coupled by a first communication hydraulic line.
[0007] By providing a small communication hydraulic line between the downstream hydraulic
paths of the two compensator valve assemblies domination of one over the other is
reduced or avoided. In the case of a crawler an arrangement is as follows. 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
assemblies. Each pair of compensator valve assemblies is provided with a forward compenstor
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 hyraulic fluid is directed to, thereby controlling
the movement of the crawler. Two small communication hydraulic lines are provided
for transmitting hydraulic fluid between the two forward compensator valve assemblies
and between the two backward compensator valve assemblies.
[0008] An embodiment of the invention will now be described with reference to the accompanying
diagrammatic drawings in which:
Fig. 1 is a side view of a crawler excavator;
Fig. 2 is a schematic of a hydraulic propulsion system for an excavator crawler without
the small communication line referred to above; and
Fig. 3 is a schematic of a hydraulic propulsion system for an excavator crawler with
the small communication line.
[0009] 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.
[0010] The 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. Hydraulic fluid is pumped to the directional control valves 38
and 40 from sump 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 excavator by operating hydraulic motors 26 and
28 in different directions and at different speeds.
[0011] It should be noted that although the invention is being described with regard to
an excavator crawler propulsion system, 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.
[0012] 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 spool 48 having a restricted orifice position
and a checked position. Spool 48 is spring biased into a closed position by spring
74 which is overcome by hydraulic pressure in sensing line 76 which pushes the valve
into the open position. Hydraulic pressure from line 72 is also directed through hydraulic
line 77 to shuttle spool 56 and 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 through spool 56 to
line 82 to balance spool 48 by adding to the biasing force of spring 74.
[0013] Hydraulic fluid passing through valve 48 into line 84 is directed to motor 26 driving
one of the crawler tracks of the 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 is 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.
Hydaulic fluid does not pass through check valve 64 of compensator valve assembly
30 because of the pressure drop across the restricted orifice of spool 48.
[0014] 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 communica
tion 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.
[0015] 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
each of the two positions during operation and they do not normally maintain a fixed
position. Therefore, in viewing Fig. 2, it should be noted that dominating compensating
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.
[0016] As with compensating valve assemblies 30 and 32, hydraulic fluid from pump 44 flows
through pumping line 72 to directional control valve 40 where it is transmitted to
compensating spool 52. Hydraulic 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 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 shuttle 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.
[0017] 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. To 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 in the art.
[0018] Fig. 3 illustrates the small communication hydraulic lines used for overcoming the
problem arising 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 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 pressure
through line 108 to 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 100, but this does not affect the operation of the system because the
hydraulic pressure in compensating line 78 serves to maintain compensating valve assemblies
32 and 36 closed except for the normal exhaust flow through check valves 66 and 70.
[0019] In reversing the excavator crawler, communication line 100 would prevent either compensating
valve assembly 32 or 36 from dominating one another. As with the forward operation,
exhaust lines 84 and 102, even though coupled through line 98, would not affect operation
of the compensating valve assemblies.
[0020] To prevent inexact operations, lines 98 and 100 must be quite small when compared
to hydraulic lines 84, 86, 102 and 104 which are used to transfer hydraulic fluid
to the motors. For example, lines 84, 86, 102 and 104 can be 0.75 inches in diameter
and in accordance therewith communication lines 98 and 100 should be 0.25 inches
in diameter. In addition, lines 98 and 100 should be provided with an orifice further
restricting flow. This orifice should be 0.004 inches in diameter to reduce further
the cross flow between the pumping lines.
[0021] Compensating communication line 78 serves an additional function as indicated by
arrow 110 and that is to provide a pressure sensing circuit with a hydraulic feedback
to better control the operation of the hydraulic pump.
1. An hydraulic system comprising first and second hydraulic motors (26, 28), a pump
(44) supplying pressurised hydraulic fluid to the motors (26, 28), a first compensating
valve assembly (30) between one side of the first motor (26) and the pump (44), and
a second compensating valve assembly (34) between the corresponding one side of the
second motor (28) and the pump (44), characterised in that the connections (84, 86)
between the first and second assemblies (30, 34) and motors (26, 28) are coupled by
a first communication hydraulic line (98).
2. A system according to claim 1 in which the system includes a third compensating
valve assembly (32) between the other side of the first motor (26) and the pump (44),
and a fourth compensating valve assembly (36) between the corresponding other side
of the second motor (28) and the pump (44) characterised in that the connections
between the third and fourth assemblies (32, 36) and the motors (26, 28) are coupled
by a second communication hydraulic line (100).
3. A system according to claim 1 or 2 in which the first, second, third and fourth
compensating valve assemblies (30, 34, 32, 36) are provided respectively with first,
second, third and fourth bypass hydraulic lines from the associated motors (26, 28)
to the pump (44), each line having a check valve (64, 68, 66, 70) therein permitting
flow only in the direction of the pump (44).
4. A system according to any preceding claim including first and second three-position
control valves (38, 40) between the pump (44) and the first and third (30, 32), and
second and fourth, compensating valve assemblies (34, 36).
5. A system according to any preceding claim in which the compensator valve assemblies
(30, 32, 34, 36) are fluidically coupled to one another by a compensating hydraulic
line (78).
6. A systen according to claim 5 in which each compensator valve assembly (30, 32,
34, 36) has a metering two-position compensator spool (48, 50, 52, 54) and a metering
two-position shuttle spool (56, 58, 60, 62).
7. A system according to any preceding claim characterised in that the diameter of
the first, or first and second, communication hydraulic line or lines (98, 100) is
smaller than the diameter of the connections (84, 102) between the first and second
assemblies (30, 34) and motors (26, 28).
8. A system according to claim 7 characterised in that the diameter of the said line
or lines (98, 100) is substantially one third of that of the said connections (84,
102).
9. A propulsion unit for an industrial machine characterised in that the unit includes
a system according to any preceding claim, the hydraulic motors (26, 28) being drivingly
connected to ground-engaging means for moving the machine.
10. A propulsion unit according to claim 9 in which the means for moving the machine
are tracks (24).
11. An industrial machine characterised in that it has a propulsion unit according
to claim 9 or 10.
12. An industrial machine according to claim 11 in which the machine is an excavator
(10).