System and method for restricting gear shift operation in timber harvesting tractors

A system for automatically preventing gear shift operation in a timber harvesting tractor when the tractor is moving. The system includes a drive train powered by a transmission. The drive train includes a transfer case which is shifted by the operator of the tractor through manipulation of an electrical control circuit. The electrical control circuit is rendered ineffective to shift the transfer case when the drive train is being driven by the transmission and the tractor is moving.

FIELD OF THE INVENTION 
This invention relates generally to gear shift operation in a drive train 
for a tractor or the like. It relates particularly to a system and method 
for restricting gear shift operation in timber harvesting tractors. 
BACKGROUND OF THE INVENTION 
A typical timber harvesting machine is a feller buncher which includes a 
four-wheel drive tractor and an attachment. The attachment may be bunching 
shear. It may also be a saw, a brush cutter, a stump grinder or other 
forestry or industrial attachment. An example may be seen in the Model 
511EX feller buncher manufactured and sold by the forestry and industrial 
equipment division of Blount, Inc. in Owatonna, Minn., assignee of the 
present application. 
The Model 511EX tractor normally is powered by a 185 horsepower diesel 
engine. The engine supplies motive power to each of the four wheels 
through a hydrostatic transmission or hydrostat. The hydrostat includes a 
variable displacement hydraulic pump which drives a variable displacement 
hydraulic motor. The hydraulic motor in this tractor is a bent axis motor 
manufactured and sold by the Sauer-Sundstrand Company of Ames, Iowa, but 
other similar motors may be used. 
The bent axis motor has two settings, a minimum angle setting providing 
lower displacement and a maximum angle setting providing higher 
displacement. The output or drive shaft from the motor rotates at a speed 
determined by the angle setting of the motor and the volume of hydraulic 
fluid supplied by the pump. The direction of rotation of the motor and the 
forward or reverse travel of the tractor is dictated by the direction of 
pump output. The operator controls this from the cab through foot pedals 
and a hydraulic control circuit. 
The bent axis motor drives the tractor's front and rear axle drive shafts 
through a two-speed, helical gear transfer case. Shifting gears in the 
transfer case between a low gear and a high gear is effected by the 
operator through an electrical control circuit and a solenoid operated 
valve which directs hydraulic fluid to and from an actuator cylinder. The 
cylinder dictates the position of a gear shift fork. A three-way switch in 
the cab permits the operator to move the three-way, two-position valve 
between a low gear position, a high gear position and a neutral position. 
Standard operating procedure for such feller buncher machines involves 
stopping the tractor before shifting gears. This procedure is recommended 
by the manufacturer because, it has been found, shifting the machine while 
it is being driven frequently results in damage to the transfer case, 
including the gears and/or the shifting forks, and it consistently results 
in excess wear of these components. In addition, when the machine is 
shifted while it is moving, the synchronizing shaft in the hydrostat's 
bent axis motor is suddenly subjected to additional stress due to torque 
changes and this, it has been found, may result in damage to the shaft. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a system for 
restricting gear shift operation in a vehicle. 
It is another object of the present invention to provide a system for 
restricting gear shift operation in a timber harvesting tractor. 
It is still another object to provide a system for automatically preventing 
gear shift operation in a timber harvesting tractor when the tractor is 
moving. 
It is yet another object to provide a method for preventing gear shift 
operation in a vehicle. 
The foregoing and other objects are realized in a system wherein the 
transfer case is shifted by a hydraulic cylinder actuator. A solenoid 
actuated, three-position, two-way valve controls the flow of hydraulic 
fluid to the linear actuator cylinder. In its center position the valve 
supplies no pressurized fluid to the cylinder, leaving it in neutral and 
relieving the transfer case fork of any force. In its low position it 
supplies pressurized fluid to one end of the cylinder to drive the 
transfer case forks into low gear orientation. In its high position it 
supplies pressurized fluid to the other end of the cylinder to drive the 
transfer fork into high gear orientation. The fork moves a shifting collar 
in the transfer case to change gears. 
The system operator has a three-position gear shift switch in the cab with 
a zero position, a one position and a two position. The switch is an 
element in a control circuit and opens the circuit when the switch is in 
its one position. When the operator moves the switch to its two position, 
the circuit is conditioned to move the solenoid actuated valve into its 
low position. When the operator moves the switch to its zero position, the 
circuit is conditioned to move the solenoid actuated valve into its high 
position. The switch is "momentary" in that it returns to its one position 
automatically when released. 
The tractor is driven by the output shaft of the bent axis motor in the 
hydrostat. The output shaft from the motor drives the tractor forward or 
backward in the gear which has been selected using the gear shift switch. 
The speed at which the tractor is driven (within that gear range) is 
determined by the angle of the bent axis motor and the volumetric output 
of the system's hydraulic pump. 
The volume and direction of flow of hydraulic fluid to the motor is 
controlled by the operator through a hydraulic control circuit with foot 
pedals in the cab. By manipulating the foot pedals, the tractor is driven 
forward in low gear, for example. According to the invention, if the 
operator wishes to shift into high gear, the tractor must be stopped 
before the gear shift actuator cylinder will be actuated. 
If the operator presses the three way switch into its zero position, for 
example, while the tractor is moving, control pressure to the hydrostat is 
cut off and the tractor comes to a stop. Only when it has stopped (or 
nearly stopped), and servo-pressure in the hydrostat drops below 50-60 
psi, does the electrical control circuit transmit a signal to the transfer 
case actuator cylinder. The transfer case is then shifted into high gear.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, and particularly to FIG. 1, a timber feller 
buncher machine incorporating a drive train control system embodying 
features of the present invention is seen generally at 10. The feller 
buncher machine 10 is, in the present illustration, a HYDRO-AX 511EX 
rubber-tired feller buncher tractor 11 manufactured by the Forestry and 
Industrial Equipment Division of Blount, Inc., with a twenty inch bunching 
shear attachment 12, also manufactured by Blount, Inc. 
Referring now also to FIG. 2, a diagrammatic illustration of the driving 
components for the machine 10 is illustrated. The tractor 11 is driven by 
a diesel engine 15 through a drive train which includes a two-speed, 
helical gear transfer case 16 and a hydrostat 17. The hydrostat 17 
includes a bent axis, variable displacement motor 18 which is driven by a 
variable displacement pump 19. 
The engine 15 is a six-cylinder Cummins Diesel rated at 185 hp at a 
governed, full load RPM of 2200. It drives the hydrostat pump 19 which, in 
turn, supplies hydraulic fluid under pressure to drive the bent axis motor 
18. 
The output shaft 21 of the hydrostat is connected to the transfer case 16 
which, in the present illustration, is a conventional two-speed, helical 
gear unit. The transfer case 16 contains a two-position, gear shifting 
collar 23 which is movable between its high and low gear positions by a 
shifting fork 24. The shifting fork 24 is, in turn, movable by a hydraulic 
actuator cylinder 25. 
The transfer case 16 is designed to maintain the tractor 11 in either low 
or high gear. The low gear speed range in the machine illustrated is 0-4.5 
mph. The high gear speed range is 0-13.5 mph. 
The speed at which the tractor 11 moves within each gear range is 
determined by the angle of the bent axis motor 18 and the rate at which 
hydraulic fluid is supplied by the pump 19 in the hydrostat 17. The 
operator in his cab controls the pump 19 through a hydraulic control 
circuit 27. 
The hydraulic control circuit 27 includes two control pedals 28 through 
which the operator controls the output, both as to volume and direction, 
of the pump 19. The angle of the cylinder block 31 relative to the crank 
shaft drum 32 in the motor adjusts automatically. Rotation of the block 31 
and drum 32 is synchronized by a synchronizing shaft 33 between them. 
Gear shifting is effected by the operator through an electrical control 
circuit 40 which controls the operation of the hydraulic actuator cylinder 
25. FIG. 3 is a schematic illustration of this electrical control circuit 
40. According to the invention, the electrical control circuit 40 permits 
gear shifting to take place only when the tractor 11 is stationary or 
moving so slowly that servo-pressure in the pump 19 is below 50-60 psi, so 
that neither the motor nor the transfer case is damaged. 
Referring to FIG. 3, the control circuit 40 includes a three-position, gear 
selector switch 44. The switch 44 is on the operators control panel and 
permits the operator to select high gear or low gear by moving the switch 
from a one position to a zero position, or from a one position to a two 
position, respectively. The switch 44 is a momentary switch which always 
returns to its one position when released. In the one position, no current 
can flow to actuator solenoids 45 and 46 which control the operation of 
the hydraulic actuator cylinder 25, and no fluid pressure is exerted in 
the cylinder. This assures that no force acts on the shifting fork 24. As 
a result, wear on the fork 24 is reduced. Meanwhile, the tractor 11 is 
held in either previously selected gear by the detents in the transfer 
case. 
Assume now that the tractor 11 is in low gear and moving at 3-4 miles per 
hour, for example. If the operator attempts to switch to high gear by 
moving the switch 44 to the zero position (without stopping the tractor) 
the circuit 40 closes a pilot valve 47 and vents control pressure in the 
circuit 27 controlled by the foot pedals 28. The pump 19 operation is 
interrupted and the tractor stops. 
Referring now also to FIG. 4, as the tractor 11 slows to a stop, a pressure 
switch 54 connected to the pump 19 in the hydrostat 17 senses when 
servo-pressure therein falls below 50-60 psi (the tractor 11 has stopped 
or nearly stopped) and closes the circuit 40. A shuttle valve 51 between 
the pump 19 and the switch 54 assures that the switch operates in both 
directions of tractor 11 travel. Closing the switch 54 permits current to 
flow to the relay 55, closing this relay. Relay 59 is closed because the 
switch 44 is in its zero position, supplying current to that relay. Relay 
58 is open because the switch 44 is not in its two position. A circuit is 
completed to the high gear actuator solenoid 46. 
The high gear solenoid 46 is energized by current flowing to it. As seen in 
FIG. 5, the high gear solenoid 46 is shown mounted on one end of a 
three-position, two-way valve 60. The low gear actuator solenoid 46 is 
shown mounted on the other end. Actually, they are both mounted on the 
same end of the valve but are shown this way for ease of illustration. The 
valve 60 controls the flow of hydraulic fluid to the actuator cylinder 25 
from a suitable source (not shown). 
With the switch 44 held in its zero position, the high gear solenoid 46 
moves the valve 60 and reverses fluid flow to the cylinder 25. The 
cylinder 25 moves the fork 24 in the transfer case 16. The transfer case 
16 shifts gears from low gear to high gear. When the switch 44 is 
released, the pilot valve 47 opens. Control pressure to the hydrostat 
control circuit 27 is restored. The operator can then drive the tractor 11 
in high gear. 
Referring again to FIG. 3, the actuator circuit 40 will now be described in 
greater detail. The circuit 40 includes first and second leads 61 and 62 
from the tractor 11 battery (not shown). The leads 61 and 62 are connected 
by lead 65 to the input side of the aforementioned pressure switch 54. The 
output side of the switch 54 is connected by lead 66 to the light 
indicator 67. The indicator 67 is connected by lead 68 to ground. 
Lead 61 is also connected to the input side of the three-position gear 
selector switch 44. That switch 44 has three output positions, as has been 
pointed out, a zero position, a one position and a two position. The one 
output position is dead. The zero output position has lead 69 connected 
thereto. The two output position has lead 70 connected thereto. 
The lead 69 supplies current to close relay 59 solenoid when the switch 44 
is in its zero position. The lead 70 supplies current to close the relay 
58 solenoid when the switch 44 is in its two position. 
Another lead 71 connects the output side of the two position pressure 
switch 54 to the relay 55 solenoid. When the pressure switch 54 is closed, 
the lead 71 supplies current to the solenoid and closes the relay 55. 
Another lead 72 connects the battery leads 61 and 62 (through the lead 65) 
to the input side of the relay 55. When the relay 55 is closed, current 
flows through the relay 55 to the leads 73 and 74 which are connected to 
the input sides of the relays 58 and 59, respectively. The output side of 
the relay 58 is connected by lead 75 to the low gear solenoid 45. The 
output side of the relay 59 is connected by lead 76 to the high gear 
solenoid 46. 
The lead 69 previously referred to also supplies current through lead 83 to 
a relay 82 solenoid when the lead 69 is hot. The same is true of lead 70 
through lead 81. The input side of this relay 82 is connected to battery 
lead 62. 
The output side of relay 82 is connected by lead 85 to a brake solenoid 86. 
The brake solenoid 86 is also connected to the brake switch 87 through 
lead 88. The brake solenoid 86 is effective to close the pilot valve 47 
when the brake switch is actuated or switch 44 is moved to positions zero 
or two. 
The function of the circuit 40 in the context of restricting gear shift 
operation has previously been discussed. The operator cannot shift gears 
while the tractor 11 is moving because the circuit 40 is rendered 
inoperative to energize the solenoids 45 and 46 which control the position 
of the three-way, two position valve 60. The switch 44 is a momentary 
switch, however, so once it is switched to a selected gear position, 
shifting will automatically take place when the tractor 11 stops. When the 
light indicator 67 is on, the operator knows conditions are favorable for 
shifting. 
It should be pointed out here, however, that when the tractor 11 has 
stopped, the gears in the transfer case 16 may not be properly aligned to 
accommodate shifting by the collar 23, In such case, the operator can 
readily move the gears into alignment for mesh by turning the front wheels 
of the tractor 11 back and forth using power steering. 
While a preferred embodiment of the invention has been described, it should 
be understood that the invention is not so limited, and modifications may 
be made without departing from the invention. The scope of the invention 
is defined by the appended claims, and all devices that come within the 
meaning of the claims, either literally or by equivalence, are intended to 
be embraced therein.