Motor vehicles – Power – Including traction motor of kind driven by noncompressible...
Reexamination Certificate
1999-01-11
2001-04-03
Vanaman, Frank (Department: 3611)
Motor vehicles
Power
Including traction motor of kind driven by noncompressible...
C180S165000
Reexamination Certificate
active
06209675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to a travel drive apparatus for a hydraulic drive work vehicle and a control method therefor, and more particularly to a travel drive apparatus for a hydraulic drive work vehicle, and a control method therefor, designed for a hydraulic drive vehicle that travels by driving a hydraulic motor with pressure oil from a variable delivery hydraulic pump for traveling that is driven by an engine.
2. Description of the Related Art
Variable delivery hydraulic pumps (hereinafter called hydraulic pumps) are conventionally used as pumps for generating oil under pressure for driving hydraulic shovels and other working machines or traveling equipment. These hydraulic pumps are equipped with power control mechanisms (hereinafter called regulators) to prevent the engine that drives the pump from stalling. Such a regulator controls the flow rate Q in response to the discharge pressure P, so that equipment is run with roughly constant torque (P×Q=constant). When the discharge pressure P is low, the force generated by a piston (not shown in the drawings) is smaller than the force of an opposing spring, wherefore the piston does not move, so that the cylinder block of the pump is in the position of maximum tilt angle, and the pump discharge quantity is also at maximum. When the load acting on the pump, that is, either the load of the working machine or the load when traveling, increases, the piston moves to a position that balances the force of the spring, decreasing the cylinder block tilt angle, and control is effected so that torque becomes constant. As described above, the spring used in the regulator pushes the cylinder block in the direction of maximum tilt angle. Alternatively, in another known example, the spring used in the regulator pushes the cylinder block in the direction wherein the tilt angle is minimized. Thus, when the engine is started, the load driving the pump becomes small, making engine startup easy.
Also known are hydraulic drive apparatuses, which use hydraulic pumps and hydraulic motors, for enabling hydraulic shovels and other construction equipment to travel. Among these known hydraulic drive apparatuses are those wherein the hydraulic pumps and hydraulic motors are connected in a closed circuit, and those wherein the hydraulic pumps and hydraulic motors are connected in an open circuit with changeover valves inserted between the hydraulic pumps and hydraulic motors. An example of such an open circuit is disclosed in Utility Model Registration No. 2543146, in gazette. According to this model, as diagrammed in
FIG. 9
, this hydraulic circuit comprises a hydraulic pump
201
for driving various actuators in the work vehicle, compound control valves
202
A,
202
B that are collection of control valves for controlling the supply of pressure oil from the hydraulic pump
201
to each of various actuators, and a travel hydraulic motor
204
that is drive-controlled by travel control valves
203
a
,
203
b
for the compound control valves
202
A,
202
B. To a main line
205
for the travel hydraulic motor
204
are connected crossover relief valves
206
, counterbalance valves
207
, and lower makeup valves
208
. The lower makeup valves
208
and an oil tank
210
are connected by a makeup circuit
212
, and an oil cooler
211
is provided in the return oil line of the compound control valve
202
A. A center joint CJ is also provided for circulating oil between an upper revolving structure and a lower revolving structure.
As based on the present invention, furthermore, while one end of the makeup circuit
212
A is connected to the main lines
205
A,
205
B via a lower makeup valve (second makeup valve)
208
, the other end thereof is connected to a line
222
upstream of a cooler relief valve
221
via the makeup circuit
212
B. The upstream line
222
is the return line for the compound control valves
202
A,
202
B. Also, upper main lines
205
C,
205
D between the center joint CJ and the travel control valves
203
a
,
203
b
built into the compound control valves
202
A,
202
B is connected to the makeup circuit
212
B via an upper makeup valve (first makeup valve)
223
, and makeup oil is replenished from various portions to the upper main lines
205
C,
205
D. A hydraulic pump
201
D is a steering hydraulic pump, connected to a hydraulic steering cylinder
225
via a steering valve
224
. The return oil from the steering valve
224
is connected to the upstream line
222
of the cooler relief valve
221
via the return line
226
and the makeup circuit
212
B. Thus makeup oil is introduced to the upper main lines
205
C,
205
D on the side of the upper revolving structure that connects the center joint CJ and the compound control valves
202
A,
202
B. Accordingly, if a large flow rate of makeup oil is introduced, a large flow rate of makeup oil will be conveyed all the way to the travel hydraulic motor
204
installed in a lower traveling body. Also, the return oil from the steering hydraulic pump
201
D is merged into the makeup circuit
212
, wherefore an adequate makeup flow rate can be secured. Accordingly, cavitation can be definitely prevented in the travel hydraulic motor
204
. In addition, makeup oil is also replenished directly to a lower main line from the makeup circuit
212
. The pressure of the makeup oil can be set by the cooler relief pressure, and it is stated that replenishment efficiency is improved.
With a hydraulic drive apparatus for effecting travel in a hydraulic shovel or other construction machine, however, there are problems. Namely, when decelerating, descending a slope, or changing either from forward travel to reverse travel or from reverse travel to forward travel, cavitation occurs which damages the hydraulic motor, and, when descending a slope, due to overrun, the vehicle ceases to be controllable. Therefore, when a closed circuit configuration is used, in order to prevent overruns, the inertial energy of the working vehicle (roughly 125% of the d vehicle speed) must absorb the inertial energy generated by the reverse drive torque of the engine. Hence, in order to absorb this with the travel hydraulic pump and travel hydraulic motor, the capacity (discharge volume per revolution, in cc/rev) has to be made large. With the closed circuit configuration, moreover, oil is supplied from a charge pump on the intake side of the closed circuit so that cavitation does not occur, and the supply volume of this charge pump must also be made large. Thus, the charge pump drive force must become large, and the engine output horsepower must be increased. The engine will become large, and wasted energy will be developed when traveling normally. The maximum traveling speed is determined by the capacities of the hydraulic pump and hydraulic motor, wherefore, at the very least, it is necessary to use a hydraulic pump having large capacity from the outset. In a large working vehicle, a hydraulic pump having larger discharge capacity will become necessary, and, together therewith, it will be necessary to increase the engine output horsepower, which is uneconomical. Next, according to Utility Model Registration No. 2543146, in gazette, configured with an open circuit, makeup oil is replenished from the makeup valve when normally moving forward, moving in reverse, accelerating, or descending a slope, and cavitation is prevented. However, when changing from forward travel to reverse travel or from reverse travel to forward travel, cavitation develops, damaging the hydraulic motor, and rendering control of the vehicle impossible. Suppose, for example, that an operator moves travel control valves
203
a
,
203
b
from the forward position (A) past neutral to the reverse position (C), thereby changing the vehicle from forward travel to reverse travel. While moving forward, pressure oil passes through the upper main line
205
A and reaches the intake
204
A in the travel hydraulic motor
204
, turning the travel hydraulic motor
204
, and moving the vehicle forward. When reve
Hayashi Seita
Kado Hideki
Nunotani Sadao
Greer Burns & Crain Ltd.
Komatsu Ltd.
Vanaman Frank
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