Download - Manual Del Estudiante D8T - 12
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SISTEMA HIDRULICO DE LA TRANSMISIN
The three-section fixed displacement power train oil pump is mounted to the right front of
the main case and is driven by a drive shaft connected to the rear of the implement pump.
The transmission charging (C) section of the power train oil pump provides high pressureoil to the transmission main relief valve, which maintains a common top pressure for
operation of the transmission modulating valves and the brakes.With the common top pressure power train strategy, transmission clutch engagement
pressure calibrations and brake pressure adjustments no longer need to be performed.
(Clutch fill time calibrations and brake touch-up calibrations are still required.) When the
transmission main relief valve is properly adjusted, all of the pressures for the transmissionclutches and for the brakes are also properly adjusted.
The torque converter charging (B) section of the power train oil pump supplies oil to thetorque converter, through the priority valve. Oil from the transmission charging section that
flows past the main relief valve mixes with the lube oil from the power train oil cooler and
is used to lubricate and cool the transmission and the bevel gears.
The scavenge (A) section of the power train oil pump draws oil from the transmission and
bevel gear case and from the torque divider housing and directs it to the sump in the maincase.
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VLVULA DE PRIORIDAD
The illustration above shows the priority valve operating in the Priority Mode. The priority
valve operates in the Priority Mode under the following conditions:
- power train oil (main sump) temperature less than 40C (104F)
- engine speed below 1300 rpm- during transmission speed or directional changes
During operation in the Priority Mode, the solenoid coil of the priority valve is DE-
ENERGIZED. The solenoid valve then blocks torque converter charge oil from entering thespool cavity and allows the oil from the spool cavity to drain to tank. With only tank
pressure at the left end of the spool, the spring shifts the spool back to the left. With the
spool shifted to the left, only a small amount of torque converter charge oil can flow intothe passage going to the torque converter inlet relief valve. With the passage to the torque
converter mostly blocked, the pressure of the torque converter charge oil increases until it
overcomes the combined force of the check valve spring and the pressure of the oil fromthe transmission charging section of the power train oil pump. As a result, the check valve
opens and the flow of torque converter oil mixes with the transmission and brake oil. This
ensures that there is enough oil to safely operate the transmission and brakes.
The priority valve will default to Normal Mode when the parking brake is activated.
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VLVULA MODULADORA DE LA TRANSMISIN
The transmission clutches are hydraulically engaged and spring released. The transmission
modulating valve solenoids are energized to send transmission charge oil to the clutches, as
shown in the illustration above. As current is applied to the solenoid, the pin extends to the
right and moves the ball closer to the orifice. The ball begins to restrict the amount of oil todrain through the orifice. This restriction causes the pressure to increase at the left end of
the valve spool. As the pressure at the left end of the valve spool increases, the spool shiftsto the right, closing off the passage from the clutch to the drain. At the same time, the
movement of the valve spool to the right opens the passage from the pump supply to the
clutch. This causes the clutch pressure to increase.
De-energizing the solenoid decreases the force of the pin against the ball. This decreased
forc allows the pressure at the left end of the valve spool to unseat the ball, de-pressurizing
the chamber at the left end of the spool. With no pressure at the left end of the spool, thevalve spool shifts to the left due to the spring force plus the supply oil pressure. This
condition reduces the pressure to the clutch by closing off the supply passage to the clutch
and opening up the drain passage. When the pressure to the clutch falls below the clutchengagement pressure, the clutches will be released by spring force.
When the transmission is in FIRST SPEED FORWARD, the modulating valves that control
flow to the No. 2 and the No. 5 clutches receive a signal from the Power Train ECM. Thissignal energizes the solenoid which sends flow to engage the clutches.
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The proportional solenoid valve for the service brakes is controlled by the Power Train
ECM.The solenoid valve is ENERGIZED to release the brakes. The Power Train ECM
determines the amount of current to the solenoid by the position of the service brake pedal.
When the proportional solenoid (valve) is energized, the pilot valve is closed. This allowspump supply oil to pressurize the pilot pressure chambers at the proportional solenoid
valve, the parking brake valve and the secondary brake valve, and in the accumulator
chamber. As the accumulator chamber pressure increases, the reducing spool moves to the
right against the spring, closing off the drain passage. At the same time, the passage to thebrakes is opened to the passage from the pump supply oil. Pressure then builds in the
pressure feedback chamber and the passage to the brakes. As the pressure increases, the
spring applied brakes are released.
When the operator depresses the service brake pedal, the PWM sensor attached to the
service brake pedal sends a signal to the Power Train ECM. The Power Train ECM thendecreases the current to the proportional solenoid at a rate that is directly proportional to the
movement of the pedal. As the solenoid is DE-ENERGIZED, the pilot valve opens and
allows the pump supply oil in the pilot pressure chamber to drain to tank. This reduces thepressure in the pilot pressure chamber at the solenoid valve. The accumulator chamber and
the parking/secondary brake valve pilot chamber are also reduced by draining through the
holes in the shutoff spool.
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As the shutoff spool moves back to the right, the holes in the spool are covered again by the
right end of the shutoff valve. This reduces the rate of reduction in pilot pressure, allowingthe brakes to be slowly applied. The pilot oil can then only escape by flowing between the
outer diameter of the shutoff spool and the inner diameter of the shutoff valve, and then
through the holes in the shutoff spool. As the pilot pressure slowly decreases, the spring
moves the shutoff spool further to the right until the holes in the spool are uncovered againat the right end of the shutoff valve. The remainder of the pilot pressure then completely
drains to tank through the shutoff spool.
As the pilot pressure decreases, the combined force of the reducing spool spring and the
pressure in the feedback chamber moves the reducing spool to the left. The accumulator
piston acts as a cushion and aids in preventing the reducing spool from moving too rapidly.As the reducing spool moves to the left, the pump oil supply passage to the reducing spool
is closed off. At the same time, the tank passage to the reducing spool is opened, allowing
the pressure oil in the brakes to drain to tank. As the pressure to the brakes decreases, theBelville springs begin to engage the brakes.
If the operator depresses the service brake pedal completely, the secondary brake switch is
activated. The secondary brake switch makes a direct connection between the battery andthe secondary brake valve solenoid, which ENERGIZES the secondary brake solenoid.
Also, when the parking brake switch is set to the ON position, the parking brake valve
solenoid is connected directly to the battery, which ENERGIZES the parking brakesolenoid. As a backup measure, the secondary brake solenoid is also ENERGIZED when
the parking brake switch is set to the ON position.
Energizing either the parking brake or the secondary brake solenoids completely drains all
pilot pressure oil, resulting in all the oil being drained from the brakes, resulting in full
engagement of the brakes.
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Illustration 80 shows the electronic brake valve when the brakes are fully engaged. When
the operator depresses the service brake pedal, the PWM rotary position sensor (connectedto the pedal) sends a signal to the Power Train ECM. The Power Train ECM then decreases
the current to the proportional (service) brake solenoid. The amount of current sent to the
solenoid is directly proportional to the position of the service brake pedal.The decreased current to the solenoid opens the poppet in the solenoid valve and opens the
flow of pump supply oil to drain. The result is decreased pilot pressure to both pressure
reducing spools. This decreased pressure allows the springs below the reducing spools to
move the reducing spools upward. As the spools move upward, the passage from the brakesis connected to the drain passage, which decreases the pressure to the brakes. This
decreased pressure allows the brake (Belville) springs to begin engaging the brakes.
When the operator completely depresses the service brake pedal, the secondary brake
switch is activated. The secondary brake switch then connects the battery to the secondary
brake solenoid. The ENERGIZED secondary brake solenoid valve completely dumps thepilot pressure to tank, which causes the reducing spools to move upward. As the spools
move upward, the passage from the brakes is connected to the drain passage, which
decreases the pressure to the brakes and the brakes are fully engaged.
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The steering system for the D8T Track-type Tractor has been upgraded to an electronically
controlled differential steering system. These upgrades include:
- solenoid controlled over-center bi-directional piston pump
- steering control lever using three rotary position sensors (triple redundant signal)- bent axis steering motor with speed and direction sensor
- steering system controlled by the power train ECM
Shown above is a schematic of the steering hydraulic system for the D8T in the NO TURN(NO STEER) condition. The gear-type charge pump and the over-center bi-directional
steering pump operate similar to the steering pump on the current D8R Series II Track-type
Tractor, except that the steering pump is controlled by two solenoid valves. (The D8RSeries II used a pilot operated pump control valve to control the steering pump.) Also, the
D8T steering control lever (tiller) uses three rotary position sensors to send a signal to the
pump control solenoid valves through the Power Train ECM, instead of the mechanicallyoperated pilot valve used in the D8R Series II machine. The steering motor is similar to that
used on the current D8R Series II, but now it utilizes a dual Hall Effect sensor in order to
provide speed and direction output information to the Power Train ECM.
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Steering System Components
Charge pump: The charge pump fills the system with oil during start-up and provides oilfor the drive loops and pilot oil for the steering control valve. Charge pressure is maintained
by the charge pressure relief valve and is set to approximately 2930 kPa (425 psi) at high
idle. (On machines equipped with dual tilt, charge oil is also used as pilot oil for theoperation of the dual tilt valve.)
Pressure override (cutoff) valve: When the pressure in either side of the steering loopreaches approximately 40160 kPa (5825 psi), the pressure override (POR) valve opens and
destrokes the pump by draining the charge pressure sent to the steering control valve, which
is used to move the pump actuator piston.
Charge pressure relief valve: The charge pressure relief valve limits the charge pressureto approximately 2850 kPa (413 psi) at 2000 rpm. Charge oil is then sent to the drive loop,the steering control valve, and the pump actuator piston.
Crossover relief and makeup valves: Each side of the drive loop has a crossover reliefand makeup valve that limits the pressure spikes in either side of the drive loop. Thesevalves also direct the charge pressure through an internal check valve that opens to fill the
low pressure side of the drive loop.
Pump control spool and pump actuator piston: The steering pump control spool iscontained in the pump control valve. The pump control spool is moved by the pump control
solenoids. The pump control spool directs charge pressure to the left or to the right end ofthe pump actuator piston. As the pump actuator piston moves, it changes the angle and/or
direction of the swashplate. The feedback lever in the pump control valve follows up to
move the pump control spool back against the pump control solenoid. This ensures that thecorrect pressure is metered to the pump actuator piston for the amount of steering flow
requested. In the NEUTRAL (or NO STEER) position, reduced charge pressure is present
at each end of the pump actuator piston.
Steering lever position sensors: Three (triple redundant) PWM rotary position sensors areattached to the shaft of the steering control lever (tiller). The position sensors send PWM
signals to the Power Train ECM. The PWM signals reflect the position of the steering tiller.The Power Train ECM then sends current to either the left or the right steering pump
control solenoid, which moves the pump control spool.
Steering pump control solenoids: The steering pump control solenoids (left and right) areinstalled in the steering control valve and are energized by the Power Train ECM. The
Power Train ECM determines the amount of current and which pump control solenoid to
energize based on the signals received from the steering lever position sensors. As the
solenoids are energized, the solenoid pin pushes against the end of the pump control spool
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SISTEMA DE IMPLEMENTOS
MULTIPLE PILOTO
The pilot manifold is mounted to the end cover on the valve stack. It supplies pilot oil to the
solenoid valves that are located on either end of each implement control valve. The pilotmanifold is supplied with oil from the implement pump, through the inlet manifold, the
valve stack, and then the end cover. The pilot manifold contains the implement pump
pressure sensor, the pressure reducing valve, the Hydraulic Pilot Accumulator Pressure
(HPAP) test port, and the Hydraulic Pilot Supply (HPS) pressure test port.
As the oil enters the pilot manifold, it passes through a screen before it reaches the pressure
reducing valve. The pressure reducing valve is infinitely variable, and meters the oil toprovide pilot oil pressure of approximately 3275 172 kPa (475 25 psi). After passing
through the pressure reducing valve, this oil becomes pilot oil.
The pilot oil then passes through the pilot filter. From the pilot filter, the pilot oil thenpasses through the accumulator check valve, where it is available to the accumulator and
the pilot relief valve.
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The pilot relief valve limits the pressure past the pressure reducing valve to approximately
6500 kPa (940 psi). In the event of pressure spikes in the pilot system, this valve opens to
dissipate the excess pressure. The accumulator stores energy (pilot pressure) so that the
implements may be lowered in a dead engine situation.
A check valve is positioned upstream of the accumulator which prevents back-flow in the
system in case of low pressure conditions. The check valve also prevents the accumulatorfrom discharging when the machine is shut down.
From the accumulator, the pilot oil then flows to the implement lockout valve. Theimplement lockout valve is solenoid operated and is ENERGIZED, when in the
UNLOCKED condition.
The implement lockout valve is controlled by the implement lockout switch, located on theright console, in the operator compartment. When this valve is in the LOCKED condition,
or DE-ENERGIZED, the pilot oil is blocked and the implements cannot be moved with the
implement controls.
When the implement lockout valve is in the UNLOCKED condition, the pilot oil exits the
pilot manifold at the outlet and is directed through a passage in the end cover and thenthrough the pilot oil passages in the valve stack. Each implement valve then directs the pilot
oil to the solenoid valves located on either end of each implement control valve.
When the operator activates an implement, the appropriate solenoid valve directs the pilot
oil into the pilot chamber of the valve. The pilot pressure then shifts the implement valve
spool
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This schematic shows the components and conditions in the implement system with the
engine started and all the implements in HOLD. Oil is drawn from the hydraulic tank by theload sensing variable displacement, piston-type implement pump. Supply oil is directed to
the closed-center control valves by the pump. Return oil from the control valves and pump
case drain oil are sent to the tank.
When a control lever is moved, oil from the implement control valve is directed to the
doubl acting implement cylinders.
The signal network line is in series with each control valve and passes through each valvebody.
The signal network terminates at the pump control valve. When an implement is activated,
a signal is generated by the work port load. This signal is sent through the signal network.A resolver network inside the implement valves consists of a series of resolver valves
which compare the signals from the implements and send the highest signal to the pump
control valve.The major components in this system are: the implement pump, the inlet manifold, the
blade lift and tilt control valves, the ripper lift and ripper tip control valves, the pressure
reducing valve, the solenoid controlled pilot valves, the implement cylinders, and the
quick-drop valve.
When the operator moves the dozer control lever from HOLD to RAISE, the dozer controllever sends a signal to the Implement ECM. The Implement ECM then sends a
corresponding current to the solenoid controlled blade lift pilot valve. The pilot valve opens
to send pilot oil into the pilot chamber, which moves the main valve spool to the RAISE
position. This allows high pressure pump supply oil to flow to the rod end of the blade liftcylinders and the blade raises.
As the blade raises, oil from the head end of the lift cylinders returns through the blade liftcontrol valve. It flows past the main valve spool and then back to the hydraulic tank.
At the same time, the pressure in the rod end of the lift cylinders is felt in the blade lift
control valve. That pressure, or load sensing signal, is transmitted through the signalresolver network back to the pump compensator valve. The pump compensator valve is set
to command the pump to upstroke and increase pump flow to meet the demand.
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SISTEMA DEL VENTILADOR HIDRULICO
Standard on the D8T Track-type Tractor is the hydraulic demand fan. Although the fan is
part of the hydraulic system, it is controlled by the Engine ECM. The Engine ECMconsiders four inputs for controlling the fan. The hydraulic oil temperature sensor, the
engine intake air temperature sensor, and the engine coolant temperature sensor all providetemperatura information to the Engine ECM. The Engine ECM monitors all three of these
temperatura inputs. The highest temperature (relative to the percentage of its temperature
map) is the controlling temperature for fan speed. The fan pump discharge pressure sensor
is the fourth input to the Engine ECM. Fan pump discharge pressure is controlled by theEngine ECM to determine fan speed.
The Engine ECM monitors the temperature inputs and also considers fan pump dischargepressure to provide a signal to the (proportional) fan pump pressure control solenoid. When
the solenoid receives minimum current from the Engine ECM, maximum flow is sent to the
fan motor, causing the fan to turn at the maximum controlled rpm (about 1350 + 25 rpm),as shown above. Maximum mechanical pump pressure (high pressure cutoff - no current or
a failed solenoid) is set to approximately 15000 + 860 kPa (2175 + 125 psi). Maximum
pressure regulated by the Engine ECM software is approximately 13250 + 500 kPa (1922 +75 psi).
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The standard hydraulic demand fan with the fan at minimum speed:
If maximum fan speed is not required, the fan pump pressure control solenoid is energized,
causing the fan to turn at a slower speed. Minimum fan speed is attained when the fan
pump pressure control solenoid is completely energized (approximately 450 + 50 rpm). Fan
pump pressure at minimum speed should be set to approximately 1827 240 kPa (265 35psi).
If communication is lost between the Engine ECM and the fan pump pressure control
solenoid, the fan will default to the maximum mechanical pressure setting, which isapproximately 15000 + 860 kPa (2175 + 125 psi). This results in a fan speed of
approximately1369 rpm (as set at the factory).
NOTE:If the engine is in the overspeed condition, the Engine ECM will regulate the
fan toward minimum pressure in an effort to protect the fan hydraulic system