Patent Description:
A gear pump includes: a pair of spur gears housed in a state of meshing with each other in a hole portion formed in a body; a driving shaft and a driven shaft for respectively fixing the spur gears; sliding contact members such as a pair of side plates in sliding contact with the side surfaces of the spur gears; a suction passage provided in a low-pressure region where the spur gears gradually separate from each other and is used for supplying hydraulic oil as a hydraulic fluid to the hole portion; and a discharge passage provided in a high-pressure region where the spur gears come into mesh and is used for discharging the hydraulic fluid from the hole portion. In place of the spur gears, a helical gear pump using helical gears has also been proposed because of their continuous tooth contact without creating closed cavity and low-noise quality due to small pulsation.

In such a helical gear pump, a large force is exerted in the thrust direction particularly on the helical gear on the driving side due to a force in the thrust direction caused by meshing of helical gears and a force in the thrust direction caused by hydraulic pressure distributed on a gear surface. In order to cope with such a force in the thrust direction, there has been proposed a gear pump or a motor, in which a hydraulic mechanism having a hydraulic chamber for pressing the shaft supporting the helical gear in a direction opposite to the direction in which the force in the thrust direction is exerted is provided on an end surface of the shaft, and hydraulic oil on a high pressure side is guided to the hydraulic chamber, so that the hydraulic mechanism presses the helical gear in the direction opposite to the direction in which the thrust force is exerted via the shaft (see Patent Literature <NUM>).

However, in order to provide the hydraulic mechanism as described in Patent Literature <NUM>, additional components are required, and the device configuration becomes complicated.

The present invention has been made to solve the above problem, and an object of the present invention is to provide a helical gear pump or a motor capable of reducing the magnitude of the force by which a driving-side helical gear is pressed against the sliding contact member with a simple configuration.

The present invention is defined by the features described in the independent claim.

Additional embodiments are defined in the dependent claims.

According to the invention of claims <NUM> to <NUM>, the action of the hydraulic fluid in the high-pressure hydraulic fluid groove formed on the sliding contact member allows the helical gear on the driving side to be pressed in the direction opposite to the direction in which the force in the thrust direction is exerted. By making the distance between the tooth bottom circle of the driving-side helical gear and the bearing hole of the driving shaft larger than the distance between the tooth bottom circle of the driven-side helical gear and the bearing hole of the driven shaft, it is possible to suppress a leakage flow of the hydraulic fluid.

According to the embodiment of claim <NUM>, by making the number of teeth of the driving-side helical gear larger than the number of teeth of the driven-side helical gear, the tooth bottom seal region of the driving-side helical gear can be made large, and a leakage flow of the hydraulic fluid can be suppressed. At this time, by increasing the number of teeth of the driving-side helical gear and setting the number of teeth of the driven-side helical gear to be the same as that in the conventional art, it is possible to prevent an increase in the force in the thrust direction due to the meshing torque transmission between the driving-side helical gear and the driven-side helical gear and to prevent the entire device from becoming excessively large.

According to the embodiment of claim <NUM>, by making the outer diameter of the driving shaft in the region penetrating the sliding contact member on which the driving-side helical gear is pressed among the pair of sliding contact members smaller than the outer diameter of the driven shaft, the tooth bottom seal region of the driving-side helical gear can be made large, and a leakage flow of the hydraulic fluid can be suppressed.

First, as a comparative example, a configuration of a helical gear pump in which a high-pressure hydraulic oil groove communicating with a high-pressure region of hydraulic oil in a casing is formed in an abutment region with a driving-side helical gear in the sliding contact member receiving a force in the thrust direction in order to press the helical gear on the driving side in a direction opposite to a direction in which the force in the thrust direction is exerted, and the driving-side helical gear is pressed in the direction opposite to the direction in which the force in the thrust direction is exerted due to the action of the hydraulic oil in the high-pressure hydraulic oil groove will be described.

<FIG> is a longitudinal cross-sectional view of a helical gear pump as a comparative example having such a configuration, and <FIG> is an A-A cross-sectional arrow view of the helical gear pump.

The helical gear pump is a helical gear pump that feeds hydraulic oil by the action of a pair of helical gears <NUM> and <NUM>, and includes a casing including a body <NUM>, a front cover <NUM>, and a rear cover <NUM>, the pair of the helical gears <NUM> and <NUM> that mesh with each other housed in a hole portion <NUM> referred to as a spectacle hole or the like formed on the body <NUM>, and a pair of bearing cases <NUM> and <NUM> that sandwich the pair of the helical gears <NUM> and <NUM> in the hole portion <NUM>.

The helical gear <NUM> is fixed to a driving shaft <NUM> that is rotated by driving of a motor (not illustrated). The helical gear <NUM> is fixed to a driven shaft <NUM>. One ends of the driving shaft <NUM> and the driven shaft <NUM> are each pivotally supported by the bearing hole <NUM> formed on the bearing case <NUM> via a bush <NUM>, and the other ends of the driving shaft <NUM> and the driven shaft <NUM> are each pivotally supported by the bearing hole <NUM> formed in the bearing case <NUM> via a bush <NUM>. The helical gears <NUM> and <NUM> rotate in directions of arrows illustrated in <FIG> in a state of being meshed with each other by driving of the driving shaft <NUM>.

A suction passage <NUM> for supplying hydraulic oil to the hole portion <NUM> is formed on the low-pressure region side where teeth of the pair of the helical gears <NUM> and <NUM> gradually separate in the hole portion <NUM> formed on the body <NUM>. Further, a discharge passage <NUM> for discharging the hydraulic oil from the hole portion <NUM> is formed on the high-pressure region side where the teeth of the pair of the helical gears <NUM> and <NUM> gradually mesh with each other in the hole portion <NUM> formed on the body <NUM>.

Of the pair of the bearing cases <NUM> and <NUM> sandwiching the pair of the helical gears <NUM> and <NUM>, in an outer region of the driving shaft <NUM> in the bearing case <NUM> on the rear cover <NUM> side, a high-pressure hydraulic oil groove <NUM> communicating with a high-pressure region of hydraulic fluid in the casing composed of the body <NUM>, the front cover <NUM>, and the rear cover <NUM> is formed. In <FIG>, the high-pressure hydraulic oil groove <NUM> on the back side of the helical gear <NUM> is illustrated by a solid line.

<FIG> is an explanatory view illustrating a force in the thrust direction acting on the pair of the helical gears <NUM> and <NUM> forming an external gear pair.

As shown in the diagram, the force in the thrust direction acting on the pair of the helical gears <NUM> and <NUM> in the helical gear pump is roughly divided into forces 101A and 101B in the thrust direction by the meshing torque transmission of the pair of the helical gears <NUM> and <NUM> and forces 102A and 102B in the thrust direction by the action of the hydraulic oil fed by the pair of the helical gears <NUM> and <NUM>. In the helical gear <NUM>, the forces 101B and 102B in the thrust direction are directed in opposite directions, whereas in the helical gear <NUM>, the forces 101A and 102A in the thrust direction are directed in the same direction. For this reason, the helical gear <NUM> is pressed against the bearing case <NUM> with a large force.

Therefore, in the outer region of the driving shaft <NUM> in the bearing case <NUM> on the rear cover <NUM> side, the high-pressure hydraulic oil groove <NUM> communicating with the high-pressure region of the hydraulic fluid in the casing including the body <NUM>, the front cover <NUM>, and the rear cover <NUM> is formed, and high-pressure hydraulic oil is supplied from the high-pressure hydraulic oil groove <NUM> toward the side surface of the helical gear <NUM>. In this manner, the helical gear <NUM> is prevented from being pressed against the bearing case <NUM> with a large force.

<FIG> is an enlarged view illustrating an arrangement relationship between the high-pressure hydraulic oil groove <NUM> formed in the outer region of the driving shaft <NUM> in the bearing case <NUM>, the helical gear <NUM>, and the driving shaft <NUM>. Also in this diagram, the high-pressure hydraulic oil groove <NUM> on the back side of the helical gear <NUM> is illustrated by a solid line.

As hatched in <FIG>, a region on the side where the pair of the helical gears <NUM> and <NUM> start to mesh on the side surface of the pair of the helical gears <NUM> and <NUM> is the high-pressure region. In contrast, a region of an outer peripheral portion of the driving shaft <NUM> and the driven shaft <NUM> on a side surface of the pair of the helical gears <NUM> and <NUM> is a low-pressure region. The high-pressure region and the low-pressure region are sealed by the tooth bottom seal region of the pair of the helical gears <NUM> and <NUM>.

The tooth bottom seal region is a region between the tooth bottom circle of the driving-side helical gear <NUM> and the bearing hole <NUM> of the driving shaft <NUM> on a side surface of the helical gear <NUM> on the driving side. The high-pressure hydraulic oil groove <NUM> communicating with the high-pressure region is formed in the tooth bottom seal region. For this reason, the distance L1 (seal length) between the high-pressure region formed by the high-pressure hydraulic oil groove <NUM> and the low-pressure region formed by an outer peripheral portion of the driving shaft <NUM> becomes extremely small. In this manner, a leakage flow rate of hydraulic oil from the high-pressure region to the low-pressure region on the side surface of the pair of the helical gears <NUM> and <NUM> becomes large, which causes a problem that the feeding performance of the hydraulic oil is deteriorated.

Next, a configuration of a helical gear pump that solves the problem of the above-described comparative example will be described. <FIG> is a longitudinal cross-sectional view of a helical gear pump according to an embodiment of the present invention, and <FIG> is a cross-sectional arrow view taken along line A-A of <FIG>.

The helical gear pump is a hydraulic helical gear pump that uses hydraulic oil as hydraulic fluid and feeds the hydraulic oil by the action of a pair of helical gears <NUM> and <NUM>. The helical gear pump includes a casing including a body <NUM>, a front cover <NUM>, and a rear cover <NUM>, a pair of the helical gears <NUM> and <NUM> that mesh with each other housed in a hole portion <NUM> referred to as a spectacle hole or the like formed on the body <NUM>, and a pair of bearing cases <NUM> and <NUM>, as sliding contact members, that sandwich the pair of the helical gears <NUM> and <NUM> in the hole portion <NUM>. Of the pair of the helical gears <NUM> and <NUM>, the number of teeth of the helical gear <NUM> is larger than the number of teeth of the helical gear <NUM>.

The fact that the number of teeth of the helical gear <NUM> is larger than the number of teeth of the helical gear <NUM> means that the tooth diameter of the helical gear <NUM> is larger than the tooth diameter of the helical gear <NUM>. That is, in a case where the helical gear <NUM> and the helical gear <NUM> mesh with each other and modules of them are the same, the tooth diameter increases as the number of teeth increases. The tooth diameter means, for example, a base circle diameter in a case where the helical gear <NUM> and the helical gear <NUM> are an involute gear. In this case, in the helical gear <NUM> and the helical gear <NUM>, values obtained by dividing the base circle diameter by the number of teeth are the same.

Sliding contact means contact in a relatively movable state. That is, the sliding contact member means a member that comes into contact with the pair of the helical gears <NUM> and <NUM> in a state where the pair of the helical gears <NUM> and <NUM> are rotatable.

The helical gear <NUM> and the driving shaft <NUM>, or the helical gear <NUM> and the driven shaft <NUM> are formed by executing cutting, polishing, quenching, and the like on a single metal member, and the helical gear <NUM> and the driving shaft <NUM>, or the helical gear <NUM> and the driven shaft <NUM> are integrated. In this description, a helical gear region in these integrally formed members is referred to as the helical gear <NUM> or the helical gear <NUM>, and a shaft region is referred to as the driving shaft <NUM> or the driven shaft <NUM>.

A suction passage <NUM> for supplying hydraulic oil to the hole portion <NUM> is formed on the low-pressure region side where teeth of the pair of the helical gears <NUM> and <NUM> gradually separate in the hole portion <NUM> formed on the body <NUM>. Further, a discharge passage <NUM> for discharging the hydraulic oil from the hole portion <NUM> is formed on the high-pressure region side where the teeth of the pair of the helical gears <NUM> and <NUM> gradually mesh with each other in the hole portion <NUM> formed on the body <NUM>. Either one or both of the suction passage <NUM> and the discharge passage <NUM> may be formed in an X direction (direction perpendicular to the surface of the diagram in <FIG>) which is the axial direction of the driving shaft <NUM> and the driven shaft <NUM>.

In an outer region of the driving shaft <NUM> in the bearing case <NUM> on the rear cover <NUM> side, that is, the bearing case <NUM> on which the driving-side helical gear <NUM> is pressed among the pair of the bearing cases <NUM> and <NUM> sandwiching the pair of the helical gears <NUM> and <NUM>, a high-pressure hydraulic oil groove <NUM> communicating with a high-pressure region of hydraulic fluid in the casing composed of the body <NUM>, the front cover <NUM>, and the rear cover <NUM> is formed. In <FIG>, the high-pressure hydraulic oil groove <NUM> on the back side of the helical gear <NUM> is illustrated by a solid line.

This helical gear pump, in which, similarly to the conventional helical gear pump shown in <FIG>, the helical gear <NUM> is pressed against the bearing case <NUM> with a large force, employs a configuration in which, in the bearing case <NUM> on the rear cover <NUM> side, the high-pressure hydraulic oil groove <NUM> communicating with the high-pressure region of the hydraulic fluid in the casing including the body <NUM>, the front cover <NUM>, and the rear cover <NUM> is formed, and high-pressure hydraulic oil is supplied from the high-pressure hydraulic oil groove <NUM> toward a side surface of the helical gear <NUM>.

As hatched in <FIG>, a region on the side where the pair of the helical gears <NUM> and <NUM> start to mesh on a side surface of the pair of the helical gears <NUM> and <NUM> is the high-pressure region. In contrast, a region of an outer peripheral portion of the driving shaft <NUM> and the driven shaft <NUM> on a side surface of the pair of the helical gears <NUM> and <NUM> is a low-pressure region. The high-pressure region and the low-pressure region are sealed by the tooth bottom seal region of the pair of the helical gears <NUM> and <NUM>. The high-pressure hydraulic oil groove <NUM> is formed in the tooth bottom seal region of the helical gear <NUM> on the driving side.

Here, the helical gear <NUM> on the driving side has a larger number of teeth than the helical gear <NUM> on the driven side. The modules of the helical gear <NUM> on the driving side and the helical gear <NUM> on the driven side equally mesh with each other. In this manner, the tooth bottom seal region of the helical gear <NUM> on the driving side (a region between the tooth bottom circle of the driving-side helical gear <NUM> and the bearing hole <NUM> of the driving shaft <NUM>) is an extremely large region as compared with that in the conventional helical gear pump shown in <FIG>. For this reason, even in a case where the high-pressure hydraulic oil groove <NUM> is formed in the tooth bottom seal region, the distance L2 (seal length) between the high-pressure region by the high-pressure hydraulic oil groove <NUM> and the low-pressure region by the outer peripheral portion of the driving shaft <NUM> can be set large. In this manner, a leakage flow rate of hydraulic oil from the high-pressure region to the low-pressure region on the side surface of the pair of the helical gears <NUM> and <NUM> can be suppressed. In this manner, an oil groove region of the high-pressure hydraulic oil groove <NUM> can be set large, and the force by which the helical gear <NUM> on the driving side is pressed against the bearing case <NUM> can be easily canceled by the pressure of the hydraulic oil.

As described above, the force in the thrust direction acting on the pair of the helical gears <NUM> and <NUM> in the helical gear pump is roughly divided into forces in the thrust direction by the meshing torque transmission of the pair of the helical gears <NUM> and <NUM> and forces in the thrust direction by the action of the hydraulic oil fed by the pair of the helical gears <NUM> and <NUM>. The force in the thrust direction by the meshing torque transmission does not depend on the number of teeth of the helical gear <NUM> on the driving side. For this reason, an increase in the force in the thrust direction due to an increase in the number of teeth of the helical gear <NUM> on the driving side is only due to an increase in a pressure receiving region of the hydraulic oil, and the increase in the force in the thrust direction can be sufficiently coped with by increasing the oil groove region of the high-pressure hydraulic oil groove <NUM>.

As described above, according to the helical gear pump of the embodiment of the present invention, by making the number of teeth of the helical gear <NUM> on the driving side larger than the number of teeth of the helical gear <NUM> on the driven side, the tooth bottom seal region of the helical gear <NUM> on the driving side can be made large, and the leakage flow rate of the hydraulic oil can be suppressed. At this time, by increasing the number of teeth of the helical gear <NUM> on the driving side and setting the number of teeth of the helical gear <NUM> on the driven side to be the same as that in the conventional art, it is possible to prevent an increase in the force in the thrust direction due to the meshing torque transmission between the helical gear <NUM> on the driving side and the helical gear <NUM> on the driven side and to prevent the entire device from becoming excessively large.

In the above-described embodiment, the high-pressure hydraulic oil groove <NUM> is formed in the outer region of the driving shaft <NUM> in the bearing case <NUM> on the rear cover <NUM> side of the pair of the bearing cases <NUM> and <NUM>. However, the high-pressure hydraulic oil groove may also be formed in an outer region of the driven shaft <NUM>.

Next, another embodiment of the present invention will be described. <FIG> is a longitudinal cross-sectional view of a helical gear pump according to another embodiment of the present invention. A member similar to that in the embodiment illustrated in <FIG> is denoted by the same reference numeral, and omitted from detailed description.

In the embodiment described above, the bearing case <NUM> that houses the bush <NUM> and the bearing case <NUM> that houses the bush <NUM> are used as the pair of sliding contact members that sandwich an external gear pair including the helical gear <NUM> and the helical gear <NUM> from both sides. A configuration in which, in the bearing case <NUM> on the rear cover <NUM> side, the high-pressure hydraulic oil groove <NUM> communicating with the high-pressure region of the hydraulic fluid in the casing including the body <NUM>, the front cover <NUM>, and the rear cover <NUM> is formed, and the high-pressure hydraulic oil is supplied from the high-pressure hydraulic oil groove <NUM> toward the side surface of the helical gear <NUM> is employed.

In contrast, in the helical gear pump according to the present embodiment, a pair of side plates (side plates) <NUM> and <NUM> are used as a pair of sliding contact members that sandwich an external gear pair including the helical gear <NUM> and the helical gear <NUM> from both sides. A configuration in which, on the side plate <NUM> on the rear cover <NUM> side, the high-pressure hydraulic oil groove <NUM> similar to that in <FIG> and <FIG> communicating with the high-pressure region of the hydraulic fluid in the casing including the body <NUM>, the front cover <NUM>, and the rear cover <NUM> is formed, and the high-pressure hydraulic oil is supplied from the high-pressure hydraulic oil groove <NUM> toward the side surface of the helical gear <NUM> is employed.

In a case where a pair of the side plates <NUM> and <NUM> are used, one ends of the driving shaft <NUM> and the driven shaft <NUM> are each pivotally supported in the bearing hole <NUM> formed on the front cover <NUM> via the bush <NUM>, and the other ends of the driving shaft <NUM> and the driven shaft <NUM> are each pivotally supported in the bearing hole <NUM> formed on the rear cover <NUM> via the bush <NUM>.

In the above-described embodiment, the pair of the bearing cases <NUM> and <NUM> or the pair of the side plates <NUM> and <NUM> are used as the sliding contact members. However, the configuration may be such that the pair of the bearing cases <NUM> and <NUM> or the pair of the side plates <NUM> and <NUM> are omitted, and the front cover <NUM> and the rear cover <NUM> are used as the sliding contact members. In this case, on the rear cover <NUM>, the high-pressure hydraulic oil groove <NUM> similar to that is <FIG> and <FIG> communicating with the high-pressure region of the hydraulic fluid in the casing including the body <NUM>, the front cover <NUM>, and the rear cover <NUM> is formed. However, in a case where the pair of the bearing cases <NUM> and <NUM> or the pair of the side plates <NUM> and <NUM> are used, there are advantages that leakage of the hydraulic oil from a side surface region of the external gear pair including the helical gear <NUM> and the helical gear <NUM> can be reduced, and durability of the pump can be improved.

The configuration may be such that, as the sliding contact member, one of the bearing case <NUM>, the side plate <NUM>, and the front cover <NUM> is used on one side surface of the external gear pair including the helical gear <NUM> and the helical gear <NUM>, and one that is not used on the one side surface among the bearing case <NUM>, the side plate <NUM>, and the front cover <NUM> is used on the other side surface, so that they are used in a mixed manner.

Next, still another embodiment of the present invention will be described. <FIG> is a longitudinal cross-sectional view of a helical gear pump according to still another embodiment of the present invention, and <FIG> is a cross-sectional arrow view taken along line A-A of <FIG>. <FIG> is an enlarged view illustrating an arrangement relationship between the high-pressure hydraulic oil groove <NUM> formed in the outer region of the driving shaft <NUM> in the bearing case <NUM>, the helical gear <NUM>, and the driving shaft <NUM>. In <FIG> and <FIG>, the high-pressure hydraulic oil groove <NUM> on the back side of the helical gear <NUM> is illustrated by a solid line. A member similar to that in the embodiment illustrated in <FIG> is denoted by the same reference numeral, and omitted from detailed description.

In each of the above-described embodiments, by making the number of teeth of the driving-side helical gear <NUM> larger than the number of teeth of the driven-side helical gear <NUM>, the distance between the tooth bottom circle of the driving-side helical gear <NUM> and the bearing hole <NUM> of the driving shaft <NUM> is made larger than the distance between the tooth bottom circle of the driven-side helical gear <NUM> and the bearing hole <NUM> of the driven shaft <NUM>. In contrast, the helical gear pump according to the present embodiment employs a configuration in which the outer diameter of the driving shaft <NUM> in the region 21a penetrating the bearing case <NUM> on which the driving-side helical gear <NUM> is pressed among the bearing cases <NUM> and <NUM> as the pair of the sliding contact members is made smaller than the outer diameter of the driven shaft <NUM>, so that the distance between the tooth bottom circle of the driving-side helical gear <NUM> and the bearing hole <NUM> in the region 21a of the driving shaft is made larger than the distance between the tooth bottom circle of the driven-side helical gear <NUM> and the bearing hole <NUM> of the driven shaft <NUM>.

As indicated by hatching in <FIG>, similarly to the embodiment illustrated in <FIG>, the region on the side where the pair of the helical gears <NUM> and <NUM> start to mesh on the side surface of the pair of the helical gears <NUM> and <NUM> is the high-pressure region. In contrast, the region of the outer peripheral portion of the driving shaft <NUM> and the driven shaft <NUM> on the side surface of the pair of the helical gears <NUM> and <NUM> is the low-pressure region. The high-pressure region and the low-pressure region are sealed by the tooth bottom seal region of the pair of the helical gears <NUM> and <NUM>. The high-pressure hydraulic oil groove <NUM> is formed in the tooth bottom seal region of the helical gear <NUM> on the driving side.

Here, the outer diameter of the driving shaft in the region 21a penetrating the bearing case <NUM> on which the driving-side helical gear <NUM> is pressed is smaller than the outer diameter of the driven shaft <NUM>. For this reason, the distance between the tooth bottom circle of the driving-side helical gear <NUM> and the bearing hole <NUM> in the region 21a of the driving shaft can be made larger than the distance between the tooth bottom circle of the driven-side helical gear <NUM> and the bearing hole <NUM> of the driven shaft <NUM>. In this manner, the tooth bottom seal region of the helical gear <NUM> on the driving side (the region between the tooth bottom circle of the driving-side helical gear <NUM> and the bearing hole <NUM> in the region 21a of the driving shaft) is an extremely large region as compared with that in the conventional helical gear pump shown in <FIG>. For this reason, even in a case where the high-pressure hydraulic oil groove <NUM> is formed in the tooth bottom seal region, the distance L3 (seal length) between the high-pressure region by the high-pressure hydraulic oil groove <NUM> and the low-pressure region by the outer peripheral portion of the driving shaft <NUM> can be set large. In this manner, a leakage flow rate of hydraulic oil from the high-pressure region to the low-pressure region on the side surface of the pair of the helical gears <NUM> and <NUM> can be suppressed.

The embodiment illustrated in <FIG> employs the configuration in which the outer diameter of the driving shaft <NUM> in the region 21a penetrating the bearing case <NUM> on which the driving-side helical gear <NUM> is pressed is smaller than the outer diameter of the driven shaft <NUM>. However, the outer diameter of the driving shaft <NUM> may be smaller than the outer diameter of the driven shaft <NUM> in the entire region.

Each of the helical gear pumps according to the above-described embodiments can also function as a helical gear motor that exhibits a motor action of introducing high-pressure hydraulic oil from the discharge passage <NUM> so as to take out rotational torque from the driving shaft <NUM> to drive an external load, and discharging hydraulic oil having a constant pressure from the suction passage <NUM>. That is, the helical gear pump in each of the above-described embodiments is also a helical gear motor.

Claim 1:
A helical gear pump or motor comprising:
an external gear pair including a driving-side helical gear and a driven-side helical gear which mesh with each other;
a pair of sliding contact members on which bearing holes of a driving shaft connected to the driving-side helical gear and bearing holes of a driven shaft connected to the driven-side helical gear are formed, the pair of sliding contact members sandwiching the external gear pair from both sides;
a casing configured to house the external gear pair and the pair of sliding contact members; and
a high-pressure hydraulic fluid groove which is formed in an abutment region between the driving-side helical gear and a sliding contact member on which the driving-side helical gear is pressed among the pair of sliding contact members, the high-pressure hydraulic fluid groove communicating with a high-pressure region of hydraulic fluid in the casing, characterised in that
a distance between a tooth bottom circle of the driving-side helical gear and a bearing hole of the driving shaft is set larger than a distance between a tooth bottom circle of the driven-side helical gear and a bearing hole of the driven shaft.