TECHNICAL FIELD

The present disclosure relates to a hydraulic driving apparatus used with a construction machine such as a hydraulic excavator with a hydraulic motor, for example.

BACKGROUND ART

A hydraulic excavator is generally configured, as a typical construction machine, to include an upper revolving structure mounted rotatably on a lower traveling structure and a working mechanism provided capable of moving upward/downward on the upper revolving structure. The lower traveling structure is configured to include an endless crawler belt and right and left traveling driving apparatuses with a hydraulic motor driving the crawler belt to circle, for example. Herein, the rotational speed R of a traveling driving apparatus can be expressed by the following Formula 1, using a delivery flow amount Qp of a hydraulic pump and a capacity Vm of a hydraulic motor. The index “p” denotes a pump, while the index “m” denotes a motor.

R=K·QpVm⁢⁢(K:proportionality⁢⁢coefficient)[Formula⁢⁢1]

Therefore, the rotational speed RL of the left traveling driving apparatus is determined by a delivery flow amount Qp1of one hydraulic pump supplying pressurized oil to the left traveling driving apparatus (hereafter also referred to as “pump delivery flow amount Qp1”) and a capacity VmL of a hydraulic motor in the left traveling driving apparatus (hereafter also referred to as “motor capacity VmL”). Also, the rotational speed RR of the right traveling driving apparatus is determined by a delivery flow amount Qp2of the other hydraulic pump supplying pressurized oil to the right traveling driving apparatus (hereafter also referred to as “pump delivery flow amount Qp2”) and a capacity VmR of a hydraulic motor in the right traveling driving apparatus (hereafter also referred to as “motor capacity VmR”). Thus, a difference between the rotational speed RL of the left traveling driving apparatus and the rotational speed RR of the right traveling driving apparatus may be generated by the pump delivery flow amounts Qp1, Qp2and the motor capacities VmL, VmR. In this specification and its appended drawings, suffixes “L” and “R” to a variable or symbol may denote “left” and “right”, respectively, to make a distinction between left and right components.

In fact, a hydraulic excavator can mistrack on the level ground when a difference between the rotational speeds RL, RR of the right and left traveling driving apparatuses occurs. A typical solution for preventing such a mistracking of a hydraulic excavator may be synchronization of the rotational speeds RL, RR of the left and right traveling driving apparatuses to make a straight traveling correction. For instance, Patent Document 1 discloses an art for making a straight traveling correction by installing a bleed-off circuit making a straight traveling correction having a variable throttle on the way of a circuit supplying pressurized oil to a traveling hydraulic motor.

PRIOR ART DOCUMENT

Patent Document

Patent Document 1: Japanese Patent Laid-Open No. 2005-119619 A

SUMMARY OF THE INVENTION

The art described in Patent Document 1 fails to provide a sufficient effect of making a straight traveling correction for a traveling driving apparatus because the apparatus is unsuccessfully controlled or adjusted to achieve a smaller orifice of the apparatus on the low revolution side during zigzag movement or the like due to its constant orifice opening setting in operation. According to the art in Patent Document 1, since inlet and outlet oil passages of the traveling driving apparatus are connected to a bleed-off circuit, a constant flow amount of pressurized oil normally returns to a tank irrespective of the state of traveling, and the efficiency can decline. Furthermore, the art in Patent Document 1 can generate reverse flow of oil liquids on the motor drain circuit side due to pressure loss in a drain pipe in the center joint when a bleed-off circuit is installed near the traveling driving apparatuses. In this case, the pressure in the motor drain can increase, leading to degraded durability of a motor seal. Moreover, throttle calibration is needed and it can be laborious. A bleed-off circuit and a communication pipe must additionally be installed, which can bring about higher operational costs. A constant oil flow can affect brake and starting performance when a communicating oil passage is provided inside of a traveling driving apparatus. Moreover, a traveling driving apparatus can be larger if additional parts are installed inside of the traveling driving apparatus.

One embodiment of the present invention has an object to provide a hydraulic driving apparatus capable of suppressing mistracking attributed to a difference in rotational speed between the left traveling driving apparatus (first hydraulic motor) and the right traveling driving apparatus (second hydraulic motor) with proper timing.

The embodiment of the present invention provides a hydraulic driving apparatus including: a first hydraulic pump and a second hydraulic pump; a first hydraulic motor rotatively driven by pressurized oil from the first hydraulic pump; a second hydraulic motor rotatively driven by pressurized oil from the second hydraulic pump; a pair of first and second supply/discharge passages connecting the first hydraulic pump, a hydraulic oil tank and the first hydraulic motor; a pair of third and fourth supply/discharge passages connecting the second hydraulic pump, the hydraulic oil tank and the second hydraulic motor; a first directional control valve provided on the way of the first and second supply/discharge passages and switching the direction of the pressurized oil supplied and discharged between the first hydraulic pump, the hydraulic oil tank and the first hydraulic motor; a second directional control valve provided on the way of the third and fourth supply/discharge passages and switching the direction of the pressurized oil supplied and discharged between the second hydraulic pump, the hydraulic oil tank and the second hydraulic motor; a first counter balance valve located between the first directional control valve and the first hydraulic motor, provided on the way of the first and second supply/discharge passages and allowing a first spool to be displaced axially based on a pressure difference between the first and second supply/discharge passages; and a second counter balance valve located between the second directional control valve and the second hydraulic motor, provided on the way of the third and fourth supply/discharge passages and allowing a second spool to be displaced axially based on a pressure difference between the third and fourth supply/discharge passages, in which the first counter balance valve includes a first communicating passage communicating the first and second supply/discharge passages when the displacement of the first spool exceeds a predetermined amount based on a pressure difference between the first and second supply/discharge passages, and the second counter balance valve includes a second communicating passage communicating the third and fourth supply/discharge passages when the displacement of the second spool exceeds a predetermined amount based on a pressure difference between the third and fourth supply/discharge passages.

The embodiment of the present invention can suppress mistracking based on a difference in rotational speed between a first hydraulic motor (for example, left traveling driving apparatus) and a second hydraulic motor (for example, right traveling driving apparatus) with proper timing.

