Patent Description:
<CIT> describes a traveling system for a work machine. While construction machinery travels up a slope, speed steps of left and right travel motors may be changed from second speed to first speed. When pump pressures of first and second hydraulic pumps become higher than a predetermined first step shift pressure, the speed steps of the left and right travel motors may be shifted from the second speed to the first speed, while when the pump pressures become lower than a predetermined second step shift pressure, the speed steps of the left and right travel motors may be shifted from the first speed to the second speed.

However, in case that the construction machinery performs a pivot turn for change of direction while travelling in the second speed, if the average pressure of the pump pressures does not reach the predetermined first shift pressure, the speed steps may not be shifted from the second speed to the first speed. Thus, the lack of torque may cause the driver to feel lack of a rotation force and, in some cases, the machine to stall.

An object of the present invention provides a travelling speed control apparatus of construction machinery for stably shifting rotational speeds of left and right travel motors.

Another object of the present invention provides a travelling speed control method performed by the above travelling speed control apparatus.

The scope of the present invention is defined by the independent claims.

Accordingly, although construction machinery performs a pivot turn for change of direction while travelling in the second speed, speed steps of the first and second travel motors may be automatically changed from the second speed to the first speed to stably shift a driving speed. Thus, convenience may be provided to the driver and work efficiency may be improved.

However, the effect of the invention may not be limited thereto, and may be expanded without being deviated from the scope of the present invention.

Hereinafter, preferable embodiments of the present invention will be explained in detail with reference to the accompanying drawings.

In the drawings, the sizes and relative sizes of components or elements may be exaggerated for clarity.

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of example embodiments.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of example embodiments.

Example embodiments may, however, be embodied in many different forms and should not be construed as limited to example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of example embodiments to those skilled in the art.

<FIG> is a hydraulic circuit diagram illustrating a control system of construction machinery in accordance with example embodiments. <FIG> is a graph illustrating a controller of a travelling speed control apparatus in <FIG>.

Referring to <FIG> and <FIG>, a control system of construction machinery may include first and second hydraulic pumps <NUM>, <NUM>, first and second travel motors <NUM>, <NUM> connected to the first and second hydraulic pumps <NUM>, <NUM> and operable by hydraulic oil discharged from the first and second hydraulic pumps <NUM>, <NUM> respectively, and first and second control valves <NUM>, <NUM> installed in hydraulic lines between the first and second hydraulic pumps <NUM>, <NUM> and the first and second travel motors <NUM>, <NUM> and configured to control operations of the first and second travel motors <NUM>, <NUM> respectively.

In example embodiments, the construction machinery may include an excavator, a wheel loader, a forklift, etc. For example, the construction machinery may be the excavator including the first travel motor <NUM> as a left travel motor and the second travel motor <NUM> as a right travel motor. It will be understood that a travelling speed control apparatus of construction machinery may be applied to the construction machinery including the left travel motor and the right travel motor such as a variable displacement hydraulic motor which is capable of varying an amount of fluid pumped per revolution.

The first and second hydraulic pumps <NUM>, <NUM> may be connected to an engine <NUM> through a power take off such that a power of the engine may be transferred to the first and second hydraulic pumps <NUM>, <NUM> respectively. The first and second hydraulic pumps <NUM>, <NUM> may be connected to an output shaft of the engine <NUM>, and as the output shaft rotates, the first and second hydraulic pumps may be driven to discharge the hydraulic oil. The engine <NUM> may include a diesel engine as a driving source of the construction machinery such as the excavator. Additionally, a pilot pump <NUM> may be connected to the output axis of the engine, and as the output axis of the engine rotates, the pilot pump <NUM> may be driven to discharge a pilot working oil. For example, the pilot pump may include a gear pump. In this case, the hydraulic oil and the pilot working oil may include substantially the same material.

The first hydraulic pump <NUM> may be connected to the first control valve <NUM> through a first hydraulic line <NUM>. The second hydraulic pump <NUM> may be connected to the second control valve <NUM> through a second hydraulic line <NUM>. The hydraulic oil discharged from the first hydraulic pump <NUM> may be supplied to the first travel motor <NUM> via the first control valve <NUM>. The hydraulic oil discharged from the second hydraulic pump <NUM> may be supplied to the second travel motor <NUM> via the second control valve <NUM>.

Although it is not illustrated in the figures, another control valve may be installed in the first and second hydraulic lines <NUM>, <NUM> to control operations of another actuator. The actuator may include a boom cylinder, an arm cylinder, a bucket cylinder, a swing motor, etc. Additionally, the control system of construction machinery may include a main control valve (MCV) as an assembly including the control valves. The main control valve may be an electro-hydraulic main control valve including an electro proportional pressure reducing valve (EPPRV) which controls the pilot working oil supplied to a spool of the control valve according to an inputted electrical signal.

In example embodiments, the control system of construction machinery may further include electro proportional pressure reducing valves (not illustrated) which output a secondary pressure corresponding to the inputted electrical signal to the first and second control valves <NUM>, <NUM> respectively. The electro proportional pressure reducing valves may be provided in both sides of a spool of each of the first and second control valves <NUM>, <NUM>, to selectively supply the pilot working oil for controlling a position of the spool.

The pilot working oil discharged from the pilot pump <NUM> may be supplied to the spools of the first and second control valves <NUM>, <NUM> via the electro proportional pressure reducing valves respectively. The electro proportional pressure reducing valves may supply a pilot signal pressure in proportion to the inputted control signal. The movement of the spool of each of the first and second control valves <NUM>, <NUM> may be controlled by the pilot signal pressure. A movement direction of the spool may be determined by a supply direction of the pilot signal pressure, and a displacement amount of the spool may be determined by a magnitude of the pilot signal pressure.

In example embodiments, the first travel motor <NUM> may be a left travel motor, and the second travel motor <NUM> may be a right travel motor.

The first control valve <NUM> may be connected to the first travel motor <NUM>, that is, A, B ports of the first travel motor, through a first left travelling hydraulic line <NUM> and a second left travelling hydraulic line <NUM> respectively. Accordingly, the first control valve <NUM> may be switched to selectively supply the hydraulic oil discharged from the first hydraulic pump <NUM> to the A, B ports of the first travel motor, to thereby control a rotation direction and a rotational speed of the first travel motor. The hydraulic oil discharged from the A, B ports of the first travel motor <NUM> may be drained to a drain tank T through the first left travelling hydraulic line <NUM> and the second left travelling hydraulic line <NUM> via the first control valve <NUM>.

The second control valve <NUM> may be connected to the second travel motor <NUM>, that is, A, B ports of the second travel motor, through a first right travelling hydraulic line <NUM> and a second right travelling hydraulic line <NUM> respectively. Accordingly, the second control valve <NUM> may be switched to selectively supply the hydraulic oil discharged from the second hydraulic pump <NUM> to the A, B ports of the second travel motor, to thereby control a rotation direction and a rotational speed of the second travel motor. The hydraulic oil discharged from the A, B ports of the second travel motor <NUM> may be drained to the drain tank T through the first right travelling hydraulic line <NUM> and the second right travelling hydraulic line <NUM> via the second control valve <NUM>.

In example embodiments, the first travel motor <NUM> may include a first oblique angle adjusting cylinder <NUM> operable to adjust an angle of a swash plate of the first travel motor and a first speed shift valve <NUM> switchable by a pilot signal pressure to drive the first oblique angle adjusting cylinder <NUM>. The first speed shift valve <NUM> may be switched by the pilot signal pressure to shift rotational speed steps of the first travel motor <NUM> between first speed and second speed.

For example, when the pilot signal pressure is supplied to the first speed shift valve <NUM>, the first oblique angle adjusting cylinder <NUM> may be driven to move forward to decrease the angle of the swash plate of the first travel motor <NUM> such that the speed step of the first travel motor <NUM> may be changed from the first speed to the second speed. When the supply of the pilot signal pressure to the first speed shift valve <NUM> may be stopped, the first oblique angle adjusting cylinder <NUM> may be driven to move backward to increase the angle of the swash plate of the first travel motor <NUM> such that the speed step of the first travel motor <NUM> may be changed from the second speed to the first speed.

Similarly to the first travel motor <NUM>, the second travel motor <NUM> may include a second oblique angle adjusting cylinder <NUM> operable to adjust an angle of a swash plate of the second travel motor and a second speed shift valve <NUM> switchable by a pilot signal pressure to drive the second oblique angle adjusting cylinder <NUM>. The second speed shift valve <NUM> may be switched by the pilot signal pressure to shift the rotational speed of the second travel motor <NUM> between first speed and second speed.

For example, when the pilot signal pressure is supplied to the second speed shift valve <NUM>, the second oblique angle adjusting cylinder <NUM> may be driven to move forward to decrease the angle of the swash plate of the second travel motor <NUM> such that the speed step of the second travel motor <NUM> may be changed from the first speed to the second speed. When the supply of the pilot signal pressure to the second speed shift valve <NUM> may be stopped, the second oblique angle adjusting cylinder <NUM> may be driven to move backward to increase the angle of the swash plate of the second travel motor <NUM> such that the speed step of the second travel motor <NUM> may be changed from the second speed to the first speed.

According to the invention, a travelling speed control apparatus of the control system, includes first and second pressure sensors <NUM>, <NUM> installed in the hydraulic lines <NUM>, <NUM> to detect first and second pump pressures acting on the first and second travel motors <NUM>, <NUM> respectively, at least one travelling speed control valve <NUM> switchable by an inputted travelling speed control signal to adjust a swash plate angle of each of the first and second travel motors <NUM>, <NUM>, and a travelling speed controller <NUM> configured to compare a difference value between the first and second pump pressures with a predetermined value in case that a travelling speed auto control mode is selected and output the travelling speed control signal to the travelling speed control valve <NUM> to decrease the rotational speeds of the first and second travel motors <NUM>, <NUM> when the difference value is greater than the predetermined value. The travelling speed control apparatus further includes a selection portion <NUM> through which an operator selects the travelling speed auto control mode.

The first pressure sensor <NUM> is installed in the first hydraulic line <NUM> to detect a pressure of the hydraulic oil discharged from the first hydraulic pump <NUM> (that is, first pump pressure). The second pressure sensor <NUM> is installed in the second hydraulic line <NUM> to detect a pressure of the hydraulic oil discharged from the second hydraulic pump <NUM> (that is, first pump pressure).

The travelling speed control valve <NUM> may be installed in a control line <NUM> between the pilot pump <NUM> and the first and second travel motors <NUM>, <NUM>. The travelling speed control valve <NUM> is switched by the travelling speed control signal to adjust the swash plate angle of each of the first and second travel motors <NUM>, <NUM>. For example, the travelling speed control valve <NUM> may include a solenoid valve.

The control line <NUM> may be connected to the first and second speed shift valves <NUM> and <NUM> respectively. Accordingly, the pilot working oil discharged from the pilot pump <NUM> may be supplied to spools of the first and second speed shift valves <NUM>, <NUM> via the travelling speed control valve <NUM> respectively.

When the travelling speed control valve <NUM> is turned ON by the travelling speed control signal, the pilot working oil may be supplied to the first and second speed shift valves <NUM>, <NUM> to change the rotational speed steps of the first and second travel motors <NUM>, <NUM> from the first speed to the second speed. When the travelling speed control valve <NUM> is turned OFF by the travelling speed control signal, the supply of the pilot signal pressure to the first and second speed shift valves <NUM>, <NUM> may be stopped to change the rotational speed steps of the first and second travel motors <NUM>, <NUM> from the second speed to the first speed.

In case that the travelling speed auto control mode is selected by an operator, the travelling speed controller <NUM> analyses the difference value between the first and second pump pressures and output the travelling speed control signal to the travelling speed control valve <NUM> to control the rotational speeds of the first and second travel motors <NUM>, <NUM>. As illustrated in <FIG>, the travelling speed controller <NUM> may include a data receiver <NUM> to receive the first and second pump pressures, a calculator <NUM> to compare the difference value between the first and second pump pressures with the predetermined value, and a control signal output portion <NUM> to output the travelling speed control signal determined based on the comparison result to the travelling speed control valve <NUM>.

For example, in case that the construction machinery performs a pivot turn for change of direction while travelling in the second speed, the detected first pump pressure is <NUM> bar and the detected second pump pressure may be <NUM> bar. The predetermined value may be preset <NUM> bar. In this case, because the difference value between the first and second pump pressures is greater than the predetermined value (<NUM> bar), the travelling speed controller <NUM> outputs a travelling speed control signal for shifting the rotational speed steps of the first and second travel motors <NUM>, <NUM> from the second speed to the first speed (second speed → first speed shift control signal) to the travelling speed control valve <NUM>.

When the travelling speed control signal (second speed → first speed shift control signal) is inputted to the travelling speed control valve <NUM>, the travelling speed control valve <NUM> may be turned OFF, and thus, the supply of the pilot working oil to the first and second speed shift valves <NUM>, <NUM> may be stopped. As the supply of the pilot working oil to the first and second speed shift valves <NUM>, <NUM> may be stopped, the first and second oblique angle adjusting cylinders <NUM>, <NUM> may be driven to move backward to increase the angles of the swash plates of the first and second travel motors <NUM>, <NUM> such that the speed steps of the first and second travel motors <NUM>, <NUM> may be changed from the second speed to the first speed.

According to a comparative embodiment, an average value of the first and second pump pressures may be compared with the predetermined value, and when the average value is greater than the predetermined value, the speed steps of the first and second travel motors may be controlled to be changed. In this case, because the average value of the first and second pump pressures is less than the predetermined value (<NUM> bar), the speed steps may not be shifted. Thus, the lack of torque may cause the driver to feel lack of a rotation force and, in some cases, the machine to stall.

According to the invention, when it is determined that the first and second pump pressures are less than a predetermined value (second step shift pressure), the travelling speed controller <NUM> outputs a travelling speed control signal for shifting the rotational speed steps of the first and second travel motors <NUM>, <NUM> from the first speed to the second speed (first speed → second speed shift control signal) to the travelling speed control valve <NUM>.

When the travelling speed control signal (first speed → second speed shift control signal) is inputted to the travelling speed control valve <NUM>, the travelling speed control valve <NUM> may be turned ON, and thus, the pilot working oil may be supplied to the first and second speed shift valves <NUM>, <NUM>. As the pilot working oil is supplied to the first and second speed shift valves <NUM>, <NUM>, the first and second oblique angle adjusting cylinders <NUM>, <NUM> may be driven to move forward to decrease the angles of the swash plates of the first and second travel motors <NUM>, <NUM> such that the speed steps of the first and second travel motors <NUM>, <NUM> may be changed from the first speed to the second speed.

As mentioned above, in case that the travelling speed auto control mode is selected, the travelling speed control apparatus may detect and compare first and second pump pressures of the hydraulic oil discharged from the first and second hydraulic pumps for driving the first and second travel motors, and shift the rotational speeds of the first and second travel motors between first speed and second speed.

Accordingly, although the construction machinery performs a pivot turn for change of direction while travelling in the second speed, the speed steps of the first and second travel motors may be automatically changed from the second speed to the first speed to stably shift a driving speed, thereby providing convenience to the driver.

<FIG> is a hydraulic circuit diagram illustrating a control system of construction machinery in accordance with example embodiments. The control system may be substantially the same as or similar to the control system described with reference to <FIG> and <FIG>, except a configuration of a travelling speed control valve of a travelling speed control apparatus. Thus, same reference numerals will be used to refer to the same or like elements and any further repetitive explanation concerning the above elements will be omitted.

Referring to <FIG>, a travelling speed control apparatus of construction machinery may include at least one travelling speed control valve <NUM> switchable by an inputted travelling speed control signal to adjust a swash plate angle of each of first and second travel motors <NUM>, <NUM>. For example, the travelling speed control valve <NUM> may include an electro proportional pressure reducing valve (EPPRV). The electro proportional pressure reducing valve may be installed in a control line <NUM> between a pilot pump <NUM> and the first and second travel motors <NUM>, <NUM>.

The electro proportional pressure reducing valve may supply a pilot signal pressure in proportion to the travelling speed control signal from a travelling speed controller <NUM> to spools of first and second speed shift valves <NUM>, <NUM> to control displacement amounts of the spools of the first and second speed shift valves <NUM>, <NUM>.

In example embodiments, the travelling speed controller <NUM> may calculate an average value and/or a difference value of first and second pump pressures of hydraulic oil discharged from first and second hydraulic pumps <NUM>, <NUM> respectively, compare the average value and/or the difference value with a predetermined value and output the travelling speed control signal based on the comparison result to the travelling speed control valve <NUM> for controlling rotational speeds of the first and second travel motors <NUM>, <NUM>. The travelling speed control valve <NUM> may supply a pilot signal pressure in proportion to the inputted travelling speed control signal to the first and second speed shift valves <NUM>, <NUM>.

The movement of the spools of the first and second speed shift valves <NUM>, <NUM> may be controlled by the pilot signal pressure. That is, the displacement amounts of the spools of the first and second speed shift valves <NUM>, <NUM> may be determined by a magnitude of the pilot signal pressure. Thus, the swash plate angle of each of the first and second travel motors <NUM>, <NUM> may be adjusted (for example, decreased) in proportion to the displacement amount of each spool to thereby perform continuously variable transmission.

Referring to <FIG>, a travelling speed control apparatus of construction machinery may include first and second travelling speed control valves <NUM>, <NUM> switchable by an inputted travelling speed control signal to adjust swash plate angles of first and second travel motors <NUM>, <NUM> respectively. For example, the first and second travelling speed control valves <NUM>, <NUM> may include a solenoid valve.

In particular, the first travelling speed control valve <NUM> may be installed in a control line <NUM> between a pilot pump <NUM> and the first travel motor <NUM>, and may be switched by a first travelling speed control signal to adjust the swash plate angle of the first travel motor <NUM>. The second travelling speed control valve <NUM> may be installed in a control line <NUM> between the pilot pump <NUM> and the second travel motor <NUM>, and may be switched by a second travelling speed control signal to adjust the swash plate angle of the second travel motor <NUM>.

Accordingly, a pilot working oil discharged from the pilot pump <NUM> may be supplied to a spool of a first speed shift valve <NUM> via the first travelling speed control valve <NUM>, and the pilot working oil discharged from the pilot pump <NUM> may be supplied to a spool of a second speed shift valve <NUM> via the second travelling speed control valve <NUM>.

In example embodiments, a travelling speed controller <NUM> may calculate and analyze an average value and/or a difference value of first and second pump pressures of hydraulic oil discharged from first and second hydraulic pumps <NUM>, <NUM> respectively, and output a first travelling speed control signal to the first travelling speed control valve <NUM> for controlling a rotational speed of the first travel motor <NUM> and output a second travelling speed control signal to the second travelling speed valve <NUM> for controlling a rotational speed of the second travel motor <NUM>.

When the first travelling speed control valve <NUM> is turned ON by the first travelling speed control signal, the pilot working oil may be supplied to the first speed shift valve <NUM> to change a rotational speed step of the first travel motor <NUM> from first speed to second speed. When the second travelling speed control valve <NUM> is turned ON by the second travelling speed control signal, the pilot working oil may be supplied to the second speed shift valve <NUM> to change a rotational speed step of the second travel motor <NUM> from first speed to second speed.

On the other hand, when the first travelling speed control valve <NUM> is turned OFF by the first travelling speed control signal, the supply of the pilot signal pressure to the first speed shift valve <NUM> may be stopped to change the rotational speed step of the first travel motor <NUM> from the second speed to the first speed. When the second travelling speed control valve <NUM> is turned OFF by the second travelling speed control signal, the supply of the pilot signal pressure to the second speed shift valve <NUM> may be stopped to change the rotational speed step of the second travel motor <NUM> from the second speed to the first speed.

Referring to <FIG>, a travelling speed control apparatus of construction machinery may include first and second travelling speed control valves <NUM>, <NUM> switchable by an inputted travelling speed control signal to adjust swash plate angles of first and second travel motors <NUM>, <NUM> respectively. For example, the first and second travelling speed control valves <NUM>, <NUM> may include an electro proportional pressure reducing valve (EPPRV).

In particular, the first travelling speed control valve <NUM> may be installed in a control line <NUM> between a pilot pump <NUM> and the first travel motor <NUM>, and the second travelling speed control valve <NUM> may be installed in a control line <NUM> between the pilot pump <NUM> and the second travel motor <NUM>.

The first travelling speed control valve <NUM> may supply a first pilot signal pressure in proportion to a first travelling speed control signal from a travelling speed controller <NUM> to a spool of a first speed shift valve <NUM> to control a displacement amount of the spool of the first speed shift valve <NUM>. The second travelling speed control valve <NUM> may supply a second pilot signal pressure in proportion to a second travelling speed control signal from the travelling speed controller <NUM> to a spool of a second speed shift valve <NUM> to control a displacement amount of the spool of the second speed shift valve <NUM>.

In example embodiments, the travelling speed controller <NUM> may calculate an average value and/or a difference value of first and second pump pressures of hydraulic oil discharged from first and second hydraulic pumps <NUM>, <NUM> respectively, compare and analyze the average value and/or the difference value with a predetermined value and output the first travelling speed control signal to the first travelling speed control valve <NUM> for controlling a rotational speed of the first travel motor <NUM> and the second travelling speed control signal to the second travelling speed control valve <NUM> for controlling a rotational speed of the second travel motor <NUM> based on the comparison result. The first travelling speed control valve <NUM> may supply the first pilot signal pressure in proportion to the inputted first travelling speed control signal to the first speed shift valve <NUM>. The second travelling speed control valve <NUM> may supply the second pilot signal pressure in proportion to the inputted second travelling speed control signal to the second speed shift valve <NUM>.

The movement of the spool of the first speed shift valve <NUM> may be controlled by the first pilot signal pressure. That is, the displacement amount of the spool of the first speed shift valve <NUM> may be determined by a magnitude of the first pilot signal pressure. The movement of the spool of the second speed shift valve <NUM> may be controlled by the second pilot signal pressure. That is, the displacement amount of the spool of the second speed shift valve <NUM> may be determined by a magnitude of the second pilot signal pressure. Thus, the swash plate angle of each of the first and second travel motors <NUM>, <NUM> may be adjusted (for example, decreased) in proportion to the displacement amount of each spool to thereby perform continuously variable transmission.

Hereinafter, a method of controlling construction machinery using the control system in <FIG> will be explained.

<FIG> is a flow chart illustrating a control method of construction machinery in accordance with example embodiments.

Referring to <FIG>, <FIG> and <FIG>, hydraulic oil is supplied from first and second hydraulic pumps <NUM>, <NUM> to first and second travel motors <NUM>, <NUM> respectively, and a first pump pressure of the first hydraulic pump <NUM> and a second pump pressure of the second hydraulic pump <NUM> may be detected (S100, S110).

According to the invention, a travelling speed controller <NUM> of construction machinery outputs a travelling speed control signal for second speed step travelling, to a travelling speed control valve <NUM>. When the travelling speed control valve <NUM> is turned ON by the travelling speed control signal, a pilot working oil may be supplied to first and second speed shift valves <NUM>, <NUM> to change rotational speed steps of the first and second travel motors <NUM>, <NUM> from first speed to second speed. Accordingly, as the first and second hydraulic pumps <NUM>, <NUM> supply the hydraulic oil to the first and second travel motors <NUM>, <NUM>, the construction machinery may travel in the second speed step.

In case that an operator manipulates that the construction machinery performs a pivot turn for change of direction while travelling in the second speed, first and second pressure sensors <NUM>, <NUM> detect the first and second pump pressures of the first and second hydraulic pumps <NUM>, <NUM>.

Then, a difference value between the first and second pump pressures is compared with a predetermined valve (S <NUM>), and rotational speeds of the first and second travel motors <NUM>, <NUM> are controlled based on the comparison result (S130).

When an operator selects a travelling speed auto control mode through a selection portion <NUM>, the travelling speed controller <NUM> receives the first and second pump pressures and compare the difference value with the predetermined value.

During the pivot turn the detected first pump pressure is <NUM> bar and the detected second pump pressure may be <NUM> bar. In this case, because the difference value between the first and second pump pressures is greater than the predetermined value (<NUM> bar), the travelling speed controller <NUM> outputs a travelling speed control signal for shifting the rotational speed steps of the first and second travel motors <NUM>, <NUM> from the second speed to the first speed (second speed → first speed shift control signal) to the travelling speed control valve <NUM>.

When the travelling speed control signal (second speed → first speed shift control signal) is inputted to the travelling speed control valve <NUM>, the travelling speed control valve <NUM> may be turned OFF, and thus, the supply of the pilot working oil to the first and second speed shift valves <NUM>, <NUM> may be stopped. As the supply of the pilot working oil to the first and second speed shift valves <NUM>, <NUM> is stopped, first and second oblique angle adjusting cylinders <NUM>, <NUM> may be driven to move backward to increase angles of swash plates of the first and second travel motors <NUM>, <NUM> such that the speed steps of the first and second travel motors <NUM>, <NUM> may be changed from the second speed to the first speed.

As mentioned above, in the travelling speed control method of construction machinery, in case that the travelling speed auto control mode is selected, the first and second pump pressures of the hydraulic oil discharged from the first and second hydraulic pumps <NUM>, <NUM> for driving the first and second travel motors <NUM>, <NUM> are detected, the difference value between the first and second pump pressures is compared with the predetermined value, and based on the comparison result, the rotational speeds of the first and second travel motors are shifted between the first speed and the second speed.

Accordingly, although the construction machinery performs a pivot turn for change of direction while travelling in the second speed, the speed steps of the first and second travel motors are automatically changed from the second speed to the first speed to stably shift a driving speed. Thus, convenience may be provided to the driver and work efficiency may be improved.

Claim 1:
A travelling speed control apparatus of construction machinery, comprising:
first and second hydraulic pumps (<NUM>, <NUM>) configured to supply hydraulic oil to first and second travel motors (<NUM>, <NUM>) respectively;
first and second pressure sensors (<NUM>, <NUM>) installed in hydraulic lines (<NUM>, <NUM>) of the first and second hydraulic pumps (<NUM>, <NUM>) configured to detect first and second pump pressures acting on the first and second travel motors (<NUM>, <NUM>) respectively;
a selection portion (<NUM>) through which an operator selects a travelling speed auto control mode,
at least one travelling speed control valve (<NUM>) switchable by an inputted travelling speed control signal configured to adjust a swash plate angle of each of the first and second travel motors (<NUM>, <NUM>);
a travelling speed controller (<NUM>) configured to compare a difference value between the first and second pump pressures with a predetermined value, output the travelling speed control signal to the travelling speed control valve (<NUM>) configured to shift rotational speed steps of the first and second travel motors (<NUM>, <NUM>) from second step to first step when the travelling speed controller (<NUM>) receives a selection signal of the travelling speed auto control mode from the selection portion (<NUM>), one of the first and second pump pressures is <NUM> bar, and the difference value is greater than the predetermined value, and, when it is determined that the first and second pump pressures are less than a predetermined value, output the travelling speed control signal to the travelling speed control valve (<NUM>) configured to shift the rotational speed steps of the first and second travel motors (<NUM>, <NUM>) from the first speed to the second speed to the travelling speed control valve (<NUM>).