Note: Descriptions are shown in the official language in which they were submitted.
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Description
Load Skidding Vehicle ~laving A
Positionally Biased Gra~e~
l~chnic~
This invention relates to load skidding
vehicles having a chassis and a load grasping grapple,
and, more particularly, to an apparatus for biasing the
grapple to an optimum skidding position relative to the
chassis during a load skidding operation.
Background Art
In certain classes of hauling operation such
as moving harvested trees from their felling point to a
collection point, there is often no feasible means of
transporting a load other than by dragging it behind a
vehicle. This is generally accomplished either by
attaching the load to a load skidding vehicle (skidder)
with a cable or by grasping the load with a grapple
suspended Erom an elevated support boom carried by the
vehicle chassis. Typical skidders have a front and
rear axle each of which is supported on at least two
wheels. Basic examples of grapple assemblies for use
in load skidding applications are set forth in U.S.
Patent 3,620,394 which issued to Symons, et al. on
November 16, 1971, and U.S. Patent 3,513,998 which
issued to Stone, et al. on May 26, 1970.
Previously, grapple equipped skidders had a
relatively smaller load capacity as compared to
otherwise equivalent cable skidders. Such relatively
smaller grapple skidder capacity resulted from the
skidding load being born by the skidding vehicle at a
comparatively high pivot point from which the grapple
was suspended. Exertion of the skidding load at the
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elevated pivot point caused a substantial overturning
moment to exist about the rearmost ground contacting
point (rear tires' contacting points) of the loaded
skiclding vehicle. Such overturning moment tended to
lift the Eront Oe the grapple skidder from the cJround
and thus reduce the traction of the grapple skidder's
front wheels.
U,S. Patent 4,140,233 which issued to
Muntjanoff, et al. on February 20, 1979, illustrates a
force application arm attached to the vehicle for
applying a downward force on the load at a point
located rearwardly from the grapple. Activating the
arm provided a reactive force tending to equalize the
load distribution on the front and rear axles of the
skidding vehicle. For most skidding applications the
optimum position for the grapple is one in which the
grapple is pivoted forwardly toward the vehicle chassis
so as to push the distal (dragged) end of the load
against the ground with a predetermined force. The
load element (often logs) tends to act as a structural
compression member of the skidding vehicle by extending
rearwardly from the chassis and into ground contact
behind the rear wheels to resist the overturning moment
exerted on the vehicle chassis. The load element must
then be pushed into the ground to permit the front axle
of the skidder to raise. Field tests of such equalizer
cylinder and cooperating linkage exhibited drawbar
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capability improvements of skidder vehicles of between
50 and 70% as compared with equivalent skidders
conventionally equipped.
While skidders having such pivotable, locking
grapples perform admirably on flat ground, signiicant
dif~iculties would be encountered when the skidders
utilizing the aforementioned pivotable grapple either
crestecl a ridge or forded the low point in a valley.
In the former case with the grapple locked in the
close~to-vehicle skidding position, the skidder would
tend to raise the entire load from the ground and thus
caused the skidder's front axle to rise from the
ground. In the latter case as the skidder's front axle
traversed the valley's low point and started climbing
the adjacent slope, the skidded load would be forced
against and into the ground so as to increase the
skidding load as the dragged object increasingly
penetrated the ground surface. Appropriate
manipulation oE the equalizing cylinder by the operator
during ridge cresting and valley traversing would
require quick reaction to changing ground conditions.
It may be impossible for the skidder operator to devote
the necessary level of attention to adjusting the
equalizing cylinder and cooperating linkage during such
valley traversal and ridge cresting due to his
encountering other problems at those times associated
with maneuvering the skidder. The present invention is
directed to overcoming one or more of the problems
previously set forth.
Disclosure of the Invention
In one aspect of the present invention a load
skidding vehicle having a chassis which is supported by
ground engaging members such as wheel assemblies is
provided with a boom pivotally mounted to the chassis
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and a load grasping device pivotally connected to the
boom. Apparatus is also provided for biasing the load
grasping device toward a predetermined pivotal position
relative to the chassis so as to optimize the load's
position relative to the vehicle and optimumly
distribute the load among the vehicle's ground engaging
member s ~
Brief Description of the Drawings
Fig. 1 is a side elevation view oE an
exemplary load skidding vehicle;
Fig. 2 is a partial side elevation view of the
load skidding vehicle of Fig. 1 illustrating a forward
pivotal position of a grasping apparatus attached
thereto;
Fig. 3 illustrates the apparatus of Fig. 2 but
illustrates the load grasping apparatus pivoted to a
rearward position;
Fig. 4 is an elevational view of the load
grasping apparatus as viewed from the vantage point
indicated by the arrow Y of Fig. l;
Fig. 5 is a schematic view of an open center
hydraulic system used to pivotally move and pivotally
bias the grasping means and open and close the grasping
means;
Fig. 6 is a schematic representation of an
electrical system used to actuate various solenoid
actuated valves of the hydraulic system of Fig. 5;
Fig. 7 is a schematic view of a closed center
hydraulic system used to pivotally move and pivotally
bias the grasping apparatus and open and close the
grasping means;
Fig. 8 is a schematic representation of an
electrical system used to actuate various solenoid
actuated valves of the hydraulic system of Fig. 7;
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Fig. 9 is a schematic view of an alternate
embodiment to the closed center hydraulic system of
Fig. 7; and
Fig~ 10 is a schematic representation of an
electrical system for actuating a valve of the
hydraulic system of Fig~ 9
Best Mode Eor Carryin~ Out the Invention
Referring now to Fig. 1, an articulated load
skidding vehicle 10 having a chassis 12 supported by a
front and a rear axle 14 and 16, respectively, is
illustrated. The chassis 12 has front and rear chassis
portions 12a and 12b, respectively, which are
articulated at a center joint 18 which, for purposes of
the present invention, may permit single axis pivoting
or multiple axis pivoting. The front axle 14 is
supported by the front chassis 12a and has mounted
thereon a pair of front wheel assemblies 20 while the
rear axle 16 is supported by the rear chassis 12b and
has mounted thereon a pair of rear wheel assemblies
22. While a wheel supported load skidding vehicle is
illustrated, it is to be understood that a track
supported, load skidding vehicle could utilize the
present invention with equal facility. A boom 24 is
pivotally mounted at a joint 26 to the rear chassis
12b, extends generally rearwardly, and is preferably of
the illustrated "A frame" type.
Load grasping means such as the illustrated
grapple assembly 28 is pivotally mounted to the support
boom 24 at a pivotal connection 30. Means 32 are
provided for pivoting the grapple assembly 28 about the
pivotal connection 30 along the path X and may be seen
to include a cooperating linkage 34 and a hydraulic
equalizer ram 36. The linkage 34 is pivotally
connected at a joint 38 to the grapple assembly 28, to
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the boom 24 at a pivotal connection 40, and to one end
of the hydraulic ram 36 5a piston portion 36a is
illustrated) at a pivotal connection 42. The hydraulic
equalizer ram 36 includes the piston 36a and a cylinder
36b which is preferably pivotally mounted to the
chassis l2b at a pivotal connection 42~ A hydraulic
booJI) ram 44 includes a piston 44a and a cylinder 44b
which are respectively pivotally mounted to the boom 24
at a pinned joint 46 and to the rear chassis 12b at a
pinned connection 48.
The grapple assembly 28 includes a grapple 50,
a rotator 52, and a vertical extender 54. The vertical
extender 54 is preferably attached to the rotator 52
and preferably constitutes the sites for the pivotal
connections 30 and 38 with the boom 24 and linkage 34
respectively. The rotator 52 rotatably positions the
grapple 50 to facilitate load pick up and manipulation
thereof by the grapple 50. A pair oE grapple tongs 56
(better illustrated in Fig. 4) and means such as a
hydraulic grapple ram 58 for opening and closing the
grapple tongs 56 together constitute the grapple 50.
The grapple ram 58 includes a piston 58a and a cylinder
58b which are pivotally connected between the grapple
tongs 56.
Figs. 2 and 3 respectively illustrate
forwardly and rearwardly pivoted positions of the
grapple 50. The grapple positions illustrated in Figs.
2 and 3 are two of the infinite number of positions
assumable along the illustrated path X of Fig. 1.
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Movement of the grapple 50 along the path X is provided
by suitable actuation of the equalizer cylinder 36 and
associated linkage 34. The grapple tongs 56 of the
grapple 50 may be moved between a closed, load captured
S confi~uration of mlnimum opening (illustrated in full
in Fig~ 4) and an open, load released configuration
(lllus~rated in phantom in Fig. 4) by suitable
actuation of the grapple ram 58.
An open center hydraulic system for actuating
the equaliziny ram 36 and the grapple ram 58 is
schematically illustrated in Fig. 5. The equalizing
piston 36a divides the interior of the equalizing
cylinder 36b into two fluid tight portions, 36c and 36d
while the grapple piston 58a divides the grapple
cylinder 58b into two fluid tight portions 58c and
58d. Pressurized fluid provided by a pressurized fluid
supply means such as a pump 60 is received by a three
position valve 62 which is capable of providing fluid
communication to a three position grapple valve 64 or
providing fluid communication to either portion (36c or
36d) of the equalizing cylinder 36 and providing fluid
communication from the other cylinder portion (36c or
36d) to a reservoir 66 from which the pump 60 draws
fluid. The three position grapple valve 64 provides
fluid communication from the pump 60 directly to the
reservoir 66 or provides fluid communication from the
pump 60 to either portion (58c or 58d) of the grapple
cylinder 58 as well as providing fluid communication
from the other grapple cylinder portion (58c or 58d) to
the reservoir 66. A second pressurized fluid supply
means such as pump 68 draws fluid from the reservoir 66
and supplies it to the valves 70, 72, and 74 as well as
to an accumulator 76. The open center pump 68 has a
substantially lower flow rate than the pump 60 for
reasons to be discussed hereinafter.
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The valve 70 either provides or obstructs
Eluid communication between the pump 68 and the
equalizing cylinder portion 36c. The valve 72 either
provides fluid communication or obstructs ~luid
communication between the pump 68 and the reservoir
66. ~r~e valve 74 either provides Eluid communication
between the pump 68 and the grapple cylinder portion
58c or obstructs Eluid communication therebetween. A
valve 78 selectively provides fluid communication
between the equalizer cylinder portion 36d and the
reservoir 66 or obstructs $1uid communication
therebetween. A valve 80 selectively provides fluid
communication between the equalizer cylinder portions
36c and 36d or obstructs fluid communication
therebetween. When the valve 80 provides fluid
communication between the equalizer cylinder portions
36c and 36d, it also provides fluid communication among
the equalizer cylinder portions 36c and 36d and the
reservoir 66. The valves 70, 72, 74, 78, and 80 are
schematically illustrated, and preferably constitute
valves which are respectively actuated by energizing
solenoids A, D, E, B, and C, but it is to be understood
that any suitable actuation method may be used for
regulating the valve modes.
Pressure relief-makeup valves 82, 84, 86, and
88 are in open fluid communication with the equalizer
and grapple cylinder portions 36d~ 36c, 58c, and 58d
respectively. A pressure relief valve 90 is in fluid
communication with the discharge side of the pump 68
and a pressure reducing valve 92 regulates the pressure
of the fluid provided to the valves 70 and 72 by the
pump 68 to a predetermined pressure. A check valve 94
insures that any fluid flow between the valve 74 and
the grapple cylinder portion 58c is from the valve 74
to the grapple cylinder portion 58c. An orifice 96
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g
provides open, but regulated fluid communication
between the grapple cylinder portion 58d and the
reservoir 66. The schematically illustrated hydraulic
system of Fig. 5 is reEerred to as an open center
S system since the pumps 60 and 68 supply a substantially
constant fluid flow which must be transmitted either to
a fluid utili~ing apparatus such as a hydraulic ram or
to the reservoir 66.
Fig. 6 is an electrical schematic for
indicating which valve solenoids are to be actuated
together for various modes on the open center hydraulic
system of Fig. 5. Additional discussion directed
toward valve actuation during operation of the load
skidding vehicle will be provided hereinafter.
Fig. 7 is known in the art as a closed center
hydraulic system. It is to be understood that like
reference characters indicate like elements and that
each primed reference character indicates an element
having a similar function to, but different structure
from, the corresponding unprimed reference character.
As can be seen in Fig. 7, a variable displacement pump
68' supplies pressurized fluid to a valve 62', a valve
64', and a valve 70. Due to the variable displacement
feature of the pump 68', the valve 62' needs one fewer
port for each of its three possible positions as
compared with the valve 62. Likewise, valve 64'
requires one fewer port for each of the three possible
valve positions as compared with valve 64. Means such
as detents are provided for holding the valve 64' in
its rightmost configuration. During nonmovement or
nonactuation of the hydraulic pistons 36a and/or 58a,
the variable displacement pump 68' adjusts its output
flow rate accordingly. Fluid still flows from pump to
reservoir, but amount is considerably reduced under
load thus reducing horsepower requirements of the
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pump. Fluid supplied to the valve 62' can be
selectively communicated to either of the equalizer
cylinder portions 36c or 36d while providing fluid
communication from the other equalizer cylinder portion
to the reservoir 66. Likewise, pressurized fluid
supplied to the valve 6~' can be selectively
transmitted to either grapple cylinder portion 58c or
58d while simultaneously providing Eluid communication
between the remaining grapple cylinder portion and the
reservoir 66.
The valve 70 may be selectively actuated (by
energizing the solenoid A) to either provide fluid
communication between the pump 68' and the equalizer
cylinder portion 36c or obstruct such fluid
communication. The valve 78 is selectively actuatable
(by energizing the solenoid B) to provide or obstruct
fluid communication between the equalizer cylinder
portion 36d and the reservoir 66 while the valve 80 is
selectively actuatable (by energizing the solenoid C)
to provide or obstruct fluid communication among the
equalizer cylinder portions 36c and 36d and the
reservoir 66.
As compared with the open center hydraulic
system of Fig~ 5, use of the variable displacement pump
68' permits elimination of the valves 72 and 74
(illustrated in Fig. 5), and eliminates the need for
the accumulator 76. Also, when the pump 68 of the open
center system is increased in size, made variable
displacement, and rearranged as illustrated in Fig. 7
to assume the position indicated as 68', -then the main
pump 60 of Fig. 5 can be eliminated.
Fig. 8 is an electrical schematic which
indicates which valve solenoids are to be actuated
together for various modes on the closed center
hydraulic system of Fig. 7. Discussion of the same
will be presented hereinafter.
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Fig. 9 is a modified version of the closed
center system of Fig. 7 wherein the valves 70 and 78
and the pressure reducing valve 92 have been
eliminated. The valve 62' of Fig. 9 has been provided
with means such as detents for holding the valve 62' in
its leEtmost conEiguration~
li'ig. lO is an electrical schematic which
indicates a valve solenoid (C) which is actuatable to
provide a float mode for the equalizer ram 36.
Discussion of the same will be presented hereinafter.
Industrial Applicability
Operation of the load skidding vehicle 10 and
the hydraulic systems illustrated in Figs. 5, 7, and 9
will be described in a manner reflecting the loading,
skidding, and unloading of a group of logs. The boom
ram 44 is manipulated to adjust the height of the
grapple SO above the log or logs to be loaded. At such
time the solenoid C is typically energized so as to
actuate the valve 80 for the open and the closed
hydraulic systems to equalize the pressure on both
sides of the equalizer ram 36 and thus cause the
grapple 50 to assume a vertical position as illustrated
in Fig. l. If, however, the logs are on uneven ground
or are disposed in a position other than horizontal,
the solenoid C is deactivated and the valves 62 and 62'
are actuated by the operator to adjust the position of
the grapple 50 to a desired position along the path X.
When a desired grapple position is attained, the
valves 62 and 62' are returned to the illustrated,
nonactuated position wherein the grapple position
relative to the boom 24 is locked in place.
The grapple tongs 56 are then moved to the
open configuration as illustrated in phantom in Fig. 4
by actuating the valves 64 and 64' to their leftmost
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position so as to provide fluid flow from the pumps 60
and 68' to the grapple cylinder portion 58d. The
hydraulic boom ram 44 is then further manipulated to
properly position the grapple 50. When it is desired
to close the grapple tongs 56, the valves 6~ and 64'
are operator actuated to their rightmost position in
which fluid communication is provided from the pumps 60
and 68' to the grapple cylinder portion 58c and from
the grapple cylinder portion 58d to the reservoir 66.
When the grapple tongs 56 have encompassed a group of
logs and occupy a closed configuration similar to that
illustrated in full in Fig. 4, the valve 64 is returned
to the illustrated, nonactuated position (preferably by
the schematicized spring) in which the grapple tongs 56
are locked into place. The valve 64' of Figs. 7 and 9
is detented in position so as to provide a continuing
biasing force tending to close the tongs 56 to their
minimum opening.
The hydraulic boom ram 44 is manipulated to
raise the grasped end of the log(s) from the ground to
the desired dragging height. The valves 62 and 62' are
then operator actuated to their leftmost position and
fluid communication is provided from the pump 60 and
the pump 68' to the equalizer cylinder portion 36c and
from the equalizer cylinder portion 36d to the
reservoir 66. When the desired log skidding position
such as that illustrated in Fig. 2 is obtained, the
valves 62 and 62' are returned to their illustrated
nonactuated postion in which the equalizer ram 36 (and
thus grapple 50) is locked in position relative to the
boom 24 and chassis 12b. Providing the illustrated
(Fig. 2) relative position between the grapple 50 and
the boom 24 enables realization of a 50 to 90~
improvement over conventional grapple equipped log
skidders having no equalizer cylinder 36 and associated
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linkage 3~. However, when rapidly changing ground
slopes are encountered, the positionally locked mode
for the grapple 50 relative to the chassis 12b has
disadvantages previou.sly alluded to. To avoid such
disadvantages the hydraulic equalizer ram 36 is
un:loclced and placed in a biasing mode. This also
r~suLt~ in a reactive Eorce on the log(s) which tends
to Eurther equalize the load on the vehicle's ground
engaging members (track or wheels).
For the open center system of Fig. 5 biasing
the equalizer cylinder 36 toward the skidding position
illustrated in Fig. 2 is initiated by actuating the
valves 70, 72, and 78 from their illustrated,
nonactuated position preferably by energizing the
solenoids A, D, and B as indicated in Fig. 6. Upon
such valve actua-tion pressurized fluid from the pump 68
is fluidly communicated to the equalizer cylinder
portion 36c by the valve 70 while the valve 72
obstructs fluid communication from the pump 68 to the
reservoir 66. The valve 78, when actuated by
energizing the solenoid B, provides fluid communication
hetween the equalizer cylinder portion 36d and the
reservoir 66. When in the biasing mode, the equalizer
ram 36 tends to move the grapple 50 toward the position
illustrated in Fig. 2, but will permit pivotal movement
thereof about the pin connection 30 to a relatively
rearward position along the path X when the forces
exerted on the grapple 50 by the skidded load surpass
the forces exerted by the equalizer ram 36. The
biasing force of the equalizer ram 36 on the grapple 50
is substantially independent of the position of the
grapple 50 and has an essentially constant magnitude.
As previously described, such increased
skidding forces typically occur after cresting a hill
or when fording a valley. Movement of the grapple 50
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in a rearward direction causes the equalizer cylinder
portion 36c to diminish in size and the equalizer
cylinder portion 36d to increase in size. The
displaced volume of fluid from the equalizer cylinder
portion 36c is transmitted to the reservoir 66 through
~he pressure relief valve 90. When the load skiclding
fo~ces return to lower magnitudes (which are typical
for the greatest percentage oE skidding operations),
the equalizer piston 36a is displaced to the left as
illustrated in Figs. 5, 7, and 9 so as to return the
grapple 50 to the preferred skidding position relative
to the chassis 12b. When the equalizer ram 36 is in
the biasing mode and the volume of the cylinder portion
36c either remains constant or diminishes in size, the
flow volume at the discharge pressure from the pump 68
is throttled through the pressure relief valve 90 to
the reservoir 66. To minimize the energy expended in
such throttling the pump 68 is of minimum size
(approximately 10% as much flow) as compared to the
main pump 60. A discharge pressure from the pump 68 of
approximately 2500 psi and a biasing pressure of
approximately 1000 p5i downstream from the pressure
reducing valve 92 permit the use of an equalizer ram 36
of reasonable size as compared to the utilizing
skidding vehicle 10.
When the biasing mode of the equalizer ram 36
is desired for the closed center system of Fig. 7, the
valves 70 and 78 are actuated by energizing the
solenoids A and B to provide fluid communication
between the variable displacement pump 68' and the
equalizer cylinder portion 36c as well as between the
equalizer cylinder portion 36d and the reservoir 66.
The remaining elements associated with the equalizer
ram's biasing mode fluid circuit in Fig. 7 cooperate as
do the corresponding elements of Fig. 5.
5~i
When the biasing mode of the equalizer ram 36
is desired for the modified closed center system of
Fig. 9, the valve 62' is held in the leftmost position
by the valve's detents. In such de-tented position
fluid communication is provided rom the pump 68' to
the equalizer cylinder portion 36c and Erom the
eclualiæer cylincler portion 36d to the reservair 66.
Deletion oE the valve 70, pressure reducing valve 92,
and interconnecting conduit from the system of Fig. 7
subjects the equalizer cylinder portion 36c of Fig.
to Eull discharge pressure from the pump 68' rather
than the reduced pressure provided by the pressure
reducing valve 92. Providing a high pressure such as
the full discharge pressure to the equalizer ram 36 to
facilitate dislodgement of the skidding vehicle 10 from
a stuck condition is desireable, but transmission of
the same discharge pressure to the equalizer ram 36
during the biasing mode may inhibit, to an undesireable
extent, the grapple assembly's pivoting action during
valley fording and after hill cresting. For such
reason the embodiment of Fig. 7 is preferred over the
embodiment of Fig. 9.
When a log bunch has been grasped by the
grapple tongs 56, the logs therein may be somewhat
wedged together with voids of substantial size
occurring thereamong. If the log bunch shifts to fill
such voids, the cross-sectional area of the log bunch
may substantially reduce in size and often permit loss
of load from the grapple 50 in a direction along the
longitudinal axis of the skidding vehicle 10. To
prevent or at least minimize log loss it has been found
desirable to bias the grapple tong ram 58 toward its
closed, minimum opening as illustrated in full in Fig.
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The biasing mode for the open center system's
grapple tongs 56 is obtained by energiæing the
solenoids D and E to actuate the valves 72 and 74,
respectively, so as to obstruct fluid flow from the
pump 68 to the reservoir 66 and to provide 1uid flow
from the pu~p 68 to the grapple cylinder portion 58c.
~:e course, i~ the equalizer ram 36 has already been
placed in the previously described biasing mode, the
valve 72 is already actuated and need not be reactuated
to provide the biasing mode for the grapple ram 58.
Since, unlike the equalizer ram's biasing mode, no
enlargement of the log bunch after the initial grasp is
to be permitted, fluid communication between the
grapple cylinder portion 58d and the reservoir 66 is
provided through the orifice 96. Additionally, the
check valve 9~ prevents fluid flow from the grapple
cylinder portion 58c toward the discharge of the pump
68. Since the pump 68 is sized to provide a relatively
small flow (approximately 10%) as compared to the main
pump 60 and thus substantially reduces the power
consumption thereof, the~grapple ram's piston 58a must
respond quickly to sudden shifts in the log bunch so as
to prevent loss thereof. Due to the relatively small
flow rate of the pump 68, the accumulator 76 is added
to provide the high fluid flow rate of typically short
duration which is required when a log bunch shift to a
smaller cross sectional size is encountered.
Placing the closed center hydraulic systems of
Figs. 7 and 9 into the grapple tong biasing mode is
accomplished by moving the valve 64' to its rightmost
position past detent so as to provide fluid
communication between the pump 68' and the grapple
cylinder portion 58c and between the grapple cylinder
portion 58d and the reservoir 66. The remaining
elements in the hydraulic circuit for the grapple
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biasing mode of the closed center system of Figs. 7 and
9 cooperate and react as do the corresponding elements
of the already described open center system of Fig. 5.
The accumulator 76 of Fig. 5 need not be included in
S the illustrated closed center systems since the
variable displacement pump 68' can respond rapidly to
sudden shifts in the log bunch's cross sectional area
to supply the high fluid flow rate required to quickly
move the grapple tongs 56.
When the load skidding vehicle 10 reaches the
log dump site, the biasing modes for the open center
system is deactivated by deenergizing the solenoids A,
D, B, and E which deactuate the valves 70, 72, 78, and
74, respectively. The equalizer and tong biasing modes
for the closed center system of Fig. 7 are respectively
deactivated by deenergizing -the solenoids A and B
(which deactuate the valves 70 and 78) and by moving
the valve 64' to the illustrated position by overcoming
the valve detents. The biasing modes for the system oE
Fig. 9 are deactivated by moviny the valves 62' and 64'
to the illustrated, deactuated positions by overcoming
the valves' detents. Thereafter, the valves 62, 64,
and 80 of the open center system and the valves 62',
64' and 80 of the closed center system are manipulated
to maneuver and unload the log bunch.
While electrically actuated valves
70,72,74,78,80 have been illustrated, it is to be
understood that other valve actuation means (including
manually) may be used with equal facility - especially
for the valve 80 since there is presently no
anticipated cooperation or need to be simultaneously
actuated with any other valve. It is to be further
understood that the use of any other means for biasing
the grapple assembly 28 toward the preferred skidding
position with substantially constant force such as a
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hydrostatically driven winch or appropriately designed
spring(s) system is considered to be equivalent to the
illustrated embodiments.
It should now be apparent that an improved
load skidding vehicle 10 has been provided which
utilizes means for biasing the skidded load to an
optimum skidding position and exerts a downward force
on the distal end of the log(s) which tends to equalize
the load on the vehicle's ground engaging members for
level terrain as well as peaks and valleys. Biasing
the skidded load with a predetermined force rather than
locking the load into position relative to the boom 24
and chassis 12b can provide a 20 to 45% improvement in
drawbar capability as compared with the locked
equalizer ram alone. Biasing the grapple 50 (and thus
the skidded load) permits the vehicle operator to
devote his entire attention to maneuvering the skidding
vehicle 10 rather than also having to manipulate the
hydraulic controls when the vehicle encounters rapidly
changing ground slopes. Additionally, the skidded load
size is increased, the skidding vehicle 10 is made more
productive, and the stability of the vehicle 10, when
loaded, is enhanced.
