Hydrostatic transmission system

An object of the present invention is to provide a hydrostatic transmission system integrally constructed without being large-sized while incorporating a variable displacement motor therein. In order to accomplish the object, a casing 11 has a variable displacement pump 1 and a variable displacement motor 3 incorporated therein in a manner to be juxtaposed to each other. The pump 1 is constituted by a cam plate type axial plunger pump which is constructed so as to render a tilting angle of a cam plate 18 variable and render a discharge rate of hydraulic fluid zero when the tilting angle is within a range set near 0 degree. The motor 3 is constituted by a cam plate type axial plunger motor including a cam plate 33 of which a tilting angle is variable. A cover 48 is provided with a pair of passages 2a and 2b, relief valves 9a and 9b, check valves 10a and 10b, and a change-over valve 5.

BACKGROUND OF INVENTION 
This invention relates to a hydrostatic transmission system, and more 
particularly to a hydrostatic transmission system including a variable 
displacement pump and a variable displacement motor. 
PRIOR ART 
A vehicle such as a tractor, a combined harvester and thresher, or the like 
conventionally employs a structure for varying a reduction ratio of a gear 
mission to expand a velocity range thereof by way of example. 
Unfortunately, utilization of the gear mission fails to vary a velocity of 
the vehicle during traveling thereof and causes complication in structure 
of the vehicle and large-sizing thereof. 
A hydrostatic transmission system of the separation type wherein a variable 
displacement pump and a hydraulic motor are arranged separately from each 
other is also used for this purpose. The hydrostatic transmission system 
permits a velocity of the vehicle to be varied in a stepless manner by 
controlling the variable displacement pump during traveling of the 
vehicle. However, this requires to arrange the variable displacement pump 
and hydraulic motor separately from each other, to thereby fail to solve 
the above-described problem of large-sizing of the vehicle. 
In view of the foregoing, a hydrostatic transmission system wherein a 
variable displacement pump and a hydraulic motor are arranged integrally 
with each other is proposed. Such a hydrostatic transmission system of the 
integral type is constructed in such a manner as shown in FIGS. 6 to 8 by 
way of example. 
The hydrostatic transmission system, as shown in FIG. 6, includes a 
variable displacement pump 101 and a fixed displacement motor 103 
connected to the pump 101 through a pair of passages 102a and 102b. 
The variable displacement pump 101 is constituted by a cam plate type axial 
plunger pump, wherein a tilting angle of a cam plate is varied to control 
forward movement of a vehicle, rearward movement thereof and stop thereof. 
More specifically, the variable displacement 101 is adapted to stop 
discharge of hydraulic fluid to stop of the fixed displacement motor 103 
when an operation lever (not shown) is kept at a neutral position to 
render a tilting angle of the cam plate zero. Then, when the cam plate of 
which a tilting angle is zero is inclined in any direction, the pump 
discharges hydraulic fluid depending on the tilting angle to drive the 
fixed displacement motor 103, to thereby carry out forward movement of the 
vehicle or rearward movement thereof. 
In this instance, when a neutral point of the operation lever for 
determining a tilting angle of the variable displacement pump 101 is 
single, it is highly difficult to ensure setting of the neutral point, so 
that handling of the operation lever is highly troublesome. Also, there 
occurs vibration in the vehicle, so that an operator is highly hard to set 
the operation lever at the neutral point even when he is skilled. 
Then, when the operation lever is deviated from the neutral point to cause 
the cam plate of the variable displacement pump 101 to be even slightly 
moved from the tilting angle of 0 degree, the pump discharges hydraulic 
oil, to thereby fail to keep the fixed displacement motor 103 stopped. 
In view of the foregoing, neutral valves 104a and 104b are connected to the 
passages 102a and 102b to provide the operation lever with a neutral width 
to a certain degree, respectively. 
The neutral valves 104a and 104b permit the passages 102a and 102b to 
communicate to tanks through constrictions 105 when they are in a normal 
state. Thus, when the tilting angle is within a range set near 0 degree to 
restrict a discharge rate of hydraulic fluid from the pump to a reduced 
level, all the hydraulic fluid is returned to the tanks, to thereby permit 
stop of the fixed displacement motor 103. 
On the contrary, when the tilting angle exceeds the set range described 
above, to thereby cause a discharge rate of hydraulic fluid from the pump 
to be increased, a pressure difference across each of the constrictions 
105 is increased. Then, when a pressure on an upstream side of the 
constriction 105 reaches a predetermined level, the neutral valve 104a or 
104b is changed over, to thereby interrupt communication between the 
passage 102a or 102b and the tank. This permits all hydraulic fluid 
discharged from the pump to be fed toward the fixed displacement motor 
103, so that the fixed displacement motor 103 may be driven. 
The passages 102a and 102b have a relief valve 106 connected thereto for 
protection of a circuit. For example, when a pressure in the passage 102a 
is excessively or abnormally increased, the pressure acts to open the 
relief valve 106. Such opening of the relief valve 106 permits the passage 
102a increased in pressure to communicate with the passage 102b reduced in 
pressure, to thereby protect the circuit. 
Also, the passages 102a and 102b are arranged so as to communicate with a 
charge pump 121 through check valves 107. The check valves 107 permit 
hydraulic fluid to flow from a side of the charge pump 121 to the passages 
102a and 102b. Of the passages 102a and 102b, the passage reduced in 
pressure is fed with hydraulic fluid discharged from the charge pump 121 
through the check valve 107. 
The conventional hydrostatic transmission system thus constructed may be 
realized in such a manner as shown in FIGS. 7 and 8. The following 
description will be made mainly on positional relationship among the 
variable displacement pump 101, fixed displacement motor 103, and valves 
104a, 104b, 106 and 107. 
The variable displacement pump 101 and fixed displacement motor 103, as 
shown in FIG. 7, are incorporated in a casing 108. 
The variable displacement pump 101 is so constructed that a cylinder block 
110 is rotated together with an input shaft 109, resulting in a plunger 
111 arranged therein being guided by a cam plate 112 to carry out stroke, 
to thereby discharge hydraulic fluid. A tilting angle of the cam plate 112 
is varied through an operation lever (not shown), so that a fluid 
discharge rate may be varied. 
Also, the fixed displacement motor 103 is basically constructed in 
substantially the same manner as the variable displacement pump 101 
described above. However, operation of the fixed displacement motor 103 is 
carried out in order contrary to the pump 101. More particularly, 
hydraulic fluid discharged from the variable displacement pump 101 acts on 
a plunger 113, to thereby ensure stroke of the plunger 113. At this time, 
the plunger 113 is guided by a cam plate 114, to thereby carry out stroke, 
so that a cylinder block 115 may be rotated to rotate an output shaft 116. 
The casing 108 is mounted thereon with a cover 117, which is formed with a 
pair of passages 102a and 102b which permit the variable displacement pump 
101 and fixed displacement motor 103 to communicate with each other 
therethrough, as shown in FIG. 8. 
The neutral valves 104a and 104b in a pair are arranged in the casing 108 
in a manner to be positioned above the input shaft 109. The neutral valves 
104a and 104b each function to permit selective communication between each 
of the passages 102a and 102b and the tank through a plunger 118 inserted 
into the casing 108 through the cover 117. 
More specifically, the neutral valves 104a and 104b each function to 
normally keep each of the constrictions 105 communicating with a tank port 
120 by means of a spring 119, to thereby permit each of the passages 102a 
and 102b to communicate with the tank. Then, when a pressure in each of 
the passages 102a and 102b reaches a predetermined level, the pressure 
acts to move the plunger 118 against the spring 119, to thereby deviate 
the constriction 105 from the tank port 120, resulting in interrupting 
communication between each of the 102a and 102b and the tank. 
The relief valves 106 and check valves 107 are arranged below the output 
shaft 116 of the fixed displacement motor 103. Construction of the relief 
valves 106 and check valves 107 will be described below. 
Unfortunately, substitution of a variable displacement motor for the fixed 
displacement motor 103 in the conventional hydrostatic transmission system 
thus constructed causes such problems as described below. 
More particularly, use of the variable displacement motor requires a 
change-over valve for changing over a tilting angle of the variable 
displacement motor. 
However, the casing 108 is provided therein with no space sufficient to 
receive such a change-over valve therein, as will be noted from FIG. 7. 
Also, the cover 117, as shown in FIG. 8, has the neutral valves 104a and 
104b already incorporated in a region A thereof. A region B of the cover 
117 has the relief valves 106 and check valves 107 already arranged 
therein. Further, the cover 117 has the passages 102a and 102b formed in a 
region C thereof. 
Thus, the casing 108 and cover 117 each are formed into a size which fails 
to permit the change-over valve for the variable displacement motor to be 
received therein. Therefore, actually it is required to mount the 
change-over valve on a vehicle while being separated from the variable 
displacement motor. 
OBJECT OF INVENTION 
Accordingly, it is an object of the present invention to provide a 
hydrostatic transmission system which is capable of being integrally 
constructed without causing large-sizing thereof while incorporating a 
variable displacement motor therein. 
Means of Solution of Problems 
In accordance with the present invention, a hydrostatic transmission system 
is provided. The hydrostatic transmission system includes a casing and a 
plate-like cover mounted on the casing. The casing is provided therein 
with a variable displacement pump and a variable displacement motor in a 
manner to be juxtaposed to each other. The variable displacement pump is 
constituted by a cam plate type axial plunger pump which includes a cam 
plate having a tilting angle variably set and is constructed so as to keep 
a discharge rate thereof zero when the tilting angle is within a range set 
near 0 degree. The variable displacement motor is constituted by a cam 
plate type axial plunger motor having a tilting angle variably set. The 
cover is formed with a pair of passages which permit the variable 
displacement pump and variable displacement motor to communicate with each 
other therethrough. The cover is provided therein with relief valves and 
check valves in a manner to be positioned at an end thereof on a side of 
the variable displacement pump and so as to communicate with said 
passages, respectively. The cover is provided therein with a change-over 
valve in a manner to be positioned at an end thereof on a side of the 
variable displacement motor, to thereby control a pressure for determining 
the tilting angle of the cam plate of the variable displacement motor. 
In a preferred embodiment of the present invention, the variable 
displacement pump includes a movable thrust plate provided on the cam 
plate and operatively associated with plungers and a rotation regulating 
mechanism for regulating rotation of the movable thrust plate with respect 
to the cam plate, so that the movable thrust plate may be returned to a 
neutral position by neutral return force when the cam plate is tilted 
within the range set near 0 degree. 
In a preferred embodiment of the present invention, the variable 
displacement motor includes a plurality of abutment surface sections 
defined at angles different from each other on a surface thereof opposite 
to a surface thereof on a side of plungers of the cam plate. Any of the 
abutment surface sections is abutted against the casing to vary the 
tilting angle of the cam plate.

BEST MODES FOR CARRYING OUT INVENTION 
Now, a hydrostatic transmission system according to the present invention 
will be described hereinafter with reference to FIGS. 1 to 5. 
Referring first to FIG. 1, an embodiment of a hydrostatic transmission 
system according to the present invention is generally illustrated. A 
hydrostatic transmission system of the illustrated embodiment includes a 
variable displacement pump 1 and a variable displacement motor 3 connected 
to the variable displacement pump 1 through a pair of passages 2a and 2b. 
The variable displacement pump 1 is so constructed that a discharge rate at 
which hydraulic fluid is discharged therefrom is kept zero when not only a 
tilting angle of the cam plate is exactly zero, but a cam plate is tilted 
within a range set near 0 degree, unlike the variable displacement pump 
101 in the prior art described above. 
The variable displacement pump 1 thus constructed keeps hydraulic fluid 
from being discharged therefrom, to thereby hold the variable displacement 
motor 3 stopped, even when the cam plate is tilted within the set range 
from 0 degree. This eliminates arrangement of the neutral valves 104a and 
104b in the prior art described above. 
Also, in the illustrated embodiment, the variable displacement motor 3 is 
constructed so as to be variable between two or high and low speed levels, 
unlike the fixed displacement motor 103 in the prior art described above. 
The cam plate of the variable displacement motor 3 is operatively 
associated with a pair of control plungers 4; so that when a change-over 
valve 5 is at a position shown in FIG. 1, the control plungers 4 may be 
permitted to communicate with a tank, to thereby keep a capacity of the 
variable displacement motor 3 reduced. Whereas, when the change-over valve 
5 is changed over, the control plungers 4 are permitted to communicate 
with the passages 2a and 2b, respectively. This results in a discharge 
pressure of the variable displacement pump 2 being applied or introduced 
through the passage 2a or 2b to the control plunger 4, leading to an 
increase in capacity of the variable displacement motor 3. 
The hydrostatic transmission system of the illustrated embodiment also 
includes orifices 6 interposedly arranged between the control plungers 4 
and the change-over valve 5 to reduce shock. 
In the illustrated embodiment, a pilot pressure is utilized as a means for 
changing over the change-over valve 5. 
For this purpose, a charge pump 7 is connected through a pilot valve 8 to a 
pilot chamber 5a of the change-over valve 5. This permits the pilot 
chamber 5a of the change-over valve 5 to communicate with the tank, to 
thereby keep the change-over valve 5 at a position shown in FIG. 1, when 
the pilot valve 8 is at a position shown in FIG. 1. Then, when the pilot 
valve 8 is changed over, the pilot chamber 5a of the change-over valve 5 
is permitted to communicate with the charge pump 7, leading to 
changing-over of the change-over valve 5. 
The passages 2a and 2b are connected to relief valves 9a and 9b, as well as 
check valves 10a and 10b as in the prior art. 
Now, the hydrostatic transmission system of the illustrated embodiment will 
be more detailedly described with reference to FIG. 2. 
The variable displacement pump 1 and variable displacement motor 3, as 
shown in FIG. 2, are incorporated in a casing 11. 
First, the variable displacement pump 1 will be described hereinafter. 
As shown in FIG. 2, the casing 11 is provided therein with a receiving 
space 12, through which an input shaft 13 is inserted. The input shaft 13 
is rotatably supported in the casing 11 by means of a bearing 14. 
The input shaft 13 is mounted thereon with a cylinder block 15 by spline 
coupling. The cylinder block 15 is formed with a plurality of cylinders 
16, in each of which a plunger 17 is slidably received. 
The receiving space 12 is provided therein with a cam plate 18 formed with 
a through-hole 19, through which the above-described input shaft 13 is 
inserted. 
The cam plate 18 is provided on both sides thereof with a pair of trunnion 
pins, which are arranged in a direction vertical to the sheet of FIG. 2 
and pivotally supported on a side of the casing 11. Thus, inclination of 
the cam plate 18 about the trunnion pins in directions indicated at arrows 
k in FIG. 2 permits a tilting angle of the cam plate 18 to be varied. 
Also, a movable thrust plate 20 is arranged opposite to the cam plate 18. 
The movable thrust plate 20 has an opposite surface 20a positioned opposite 
to the cam plate 18, which is formed into a conical shape having an angle 
.theta. defined about a center o thereof, as shown in FIG. 3(a). 
Further, the opposite surface 20a of the movable thrust plate 20, as shown 
in FIG. 3(b), is formed with ball holes 21. Likewise, the cam plate 18 is 
formed on a surface thereof opposite to the surface 20a of the movable 
thrust plate 20 with ball holes (not shown) in a manner to correspond to 
the ball holes 21. The ball holes each have a ball fitted therein so as to 
serve as a rotation regulating mechanism. Such construction regulates 
rotation of the movable thrust plate 20 with respect to the cam plate 18. 
Further, as shown in FIG. 2, the plungers 17 incorporated in the cylinder 
block 15 each are operatively associated with a shoe 23 through a ball 22. 
The shoe 23 has an end surface abutted against the movable thrust plate 
20. 
The input shaft 13 is inserted through the cylinder block 15 so that a 
space may be defined between the input shaft 13 and the cylinder block 15. 
The space has a spring 24 received therein. The spring 24 is operatively 
associated at one end thereof with the cylinder block 15 through a stopper 
24. Also, the spring 24 is operatively associated at the other end thereof 
with the shoe 23 through a rod 26 and a sleeve 27 arranged through the 
cylinder block 15. Such construction permits elastic force of the spring 
24 to keep the cylinder block 15 abutted against a cover 48 described 
hereinafter and the shoe 23 abutted against the movable thrust plate 20. 
In the variable displacement pump 1 thus constructed, rotation of the input 
13 permits the cylinder block 15 to be likewise rotated through the spline 
coupling. 
Then, when the operation lever (not shown) is operated to tilt the cam 
plate 18 in the direction indicated at arrows k under such a state, the 
opposite surface of the cam plate 18 forces the movable thrust plate 20, 
to thereby tilt the movable thrust plate 20. Such tilting of the movable 
thrust plate 20 permits each of the plungers 17 to carry out stroke in the 
cylinder 16 while being guided by the movable thrust plate 20. This 
results in the cylinder 16 discharging hydraulic fluid in an amount 
corresponding to stroke of the plunger 17. 
The surface 20a of the movable thrust plate 20 opposite to the cam plate 
18, as described above, is formed into a conical shape having an angle 
.theta.. This permits the movable thrust plate 20 to be independently 
tilted within a range of the angle .theta. with respect to the cam plate 
18. 
In general, a cam plate type axial piston pump is constructed so as to 
generate neutral return force which acts to return a cam plate to a 
neutral position, when a tilting angle of the cam plate is about 0 degree. 
Thus, in the illustrated embodiment, even when the cam plate 18 is tilted 
within the set range of the angle .theta., the neutral return force keeps 
the movable thrust plate 20 at a straight state. Thus, even when the 
tilting angle of the cam plate 18 is not exactly 0 degree, the movable 
thrust plate 20 is kept parallel with respect to the cylinder block 15, to 
thereby maintain the discharge rate at a zero state. 
Now, the variable displacement motor 3 will be described. 
As shown in FIG. 2, an output shaft 28 is inserted through the receiving 
space 12 of the casing 11 while being positioned substantially in parallel 
to the input shaft 13 of the variable displacement pump 1. The output 
shaft 28 is rotatably supported in the casing 11 by a bearing 29. 
The output shaft 13 is mounted thereon with a cylinder block 30 by spline 
coupling. The cylinder block 30 is formed therein with a plurality of 
cylinders 31, in each of which a plunger 32 is slidably received. 
Also, the receiving space 12 of the casing 11 has a cam plate 33 received 
therein, which is formed with a through-hole 34, through which the output 
shaft 28 is inserted. 
The cam plate 33, as shown in FIG. 4(a), is formed into a donut-like or 
ring-like shape and has a tilting surface 35 arranged opposite to the 
cylinder block 30. The plungers 32 incorporated in the cylinder block 30 
each are operatively associated at a distal end thereof with a shoe 37 
through a ball 36. The shoe 37 is abutted at an end surface against the 
tilting surface 35. 
Further, the cam plate 33, as shown in FIG. 4(a), is formed on a 
casing-side surface thereof facing an inner surface of the casing 11 with 
a first abutment surface section 38 and a second abutment surface section 
39. The second abutment surface section 39 is formed in a manner to be 
contiguous to the first abutment surface section 38 and extend therefrom 
at an angle different from that of the first abutment surface section 38. 
A boundary portion between the first abutment surface section 38 and the 
second abutment surface section 39 on the casing-side surface of the cam 
plate 33 is formed with a pair of ball holes 40. Likewise, the inner 
surface of the casing 11 is formed with ball holes (not shown) in a manner 
to correspond to the ball holes 40 of the cam plate 33. The ball holes 
each are fitted therein with such a ball as indicated at reference numeral 
41. Such construction regulates rotation of the cam plate 33 with respect 
to the casing 11. 
Also, as shown in FIG. 2, the casing 11 is formed on the inner surface 
thereof with two control cylinder holes 42, although only one such control 
cylinder hole 42 is shown. The control cylinder holes 42 are arranged in 
juxtaposition to each other in a direction perpendicular to the sheet of 
FIG. 2. 
The control cylinder holes 42 each have a control plunger 43 slidably 
received therein. The control plunger 43 functions to permit thrust 
thereof obtained due to application of a pilot pressure to the control 
plunger 43 to act on an upper end of the first abutment surface section 38 
of the cam plate 33 as indicated at dotted lines in FIG. 4(a). 
The control cylinder hole 42 and control plunger 43 cooperate with each 
other to provide each of the control cylinders 4 shown in FIG. 1. 
The output shaft 28 is inserted through the cylinder block 30 with a space 
being defined therebetween, in which a spring 44 is arranged. The spring 
44 is operatively associated at one end thereof through a stopper 45 with 
the cylinder block 30. Also, the spring 44 is operatively associated at 
the other end thereof with the shoe 37 through a rod 46 and a sleeve 47 
arranged in the cylinder block 30. Such construction permits elastic force 
of the spring 44 to keep the cylinder block 30 abutted against a cover 48 
described below and the shoe 37 abutted against the slanting surface 35 of 
the cam plate 33. 
In the variable displacement motor 3 thus constructed, hydraulic fluid 
discharged from the variable displacement pump 1 is fed to each of the 
plungers 31 of the cylinder block 30. Then, when the plunger 32 in the 
cylinder 31 carries out stroke, it is guided by the slanting surface 35 of 
the cam plate 33, leading to rotation of the cylinder block 30. This 
causes rotation of the output shaft 28 through the spline coupling, which 
is then externally transmitted. 
When the control cylinder holes 42 each communicate with the tank to keep 
any driving force or thrust from being generated at the control plunger 
43, the cam plate 33, as shown in FIG. 2, keeps the first abutment surface 
section 38 abutted against the inner surface of the casing 11 by means of 
elastic force of the spring 44. 
Whereas, a pilot pressure is applied to any of the control cylinder holes 
42 to permit the control plunger 43 to generate thrust, the thrust acts on 
an upper end of the first abutment surface 38 of the cam plate 33. This 
causes the cam plate 33 to be tilted about the balls 41, resulting in the 
first abutment surface section 38 being separated from the inner surface 
of the casing 11 and the second abutment surface section 39 abutted 
against the inner surface of the casing 11. 
Thus, inclination of the cam plate 33 through the first and second abutment 
surface sections 38 and 39 permits an angle of the slanting surface 35 
with respect to the cylinder block 30 to be varied. This leads to a 
difference in stroke of the plunger 32, so that a rotational speed of the 
output shaft 20 may be varied between two or high and low speed levels. 
Turning force of the output shaft 20 is then externally transmitted. 
The illustrated embodiment, as described above, is so constructed that the 
cam plate 33 is arranged separately from the casing 11 and the first and 
second abutment surface sections 38 and 39 are abutted against the inner 
surface of the casing 11. Such construction eliminates arrangement of the 
cam plate 114 in the casing 108 in the prior art described above with 
reference to FIG. 7. Thus, incorporation of the variable displacement 
motor 3 in the casing 11 may be carried out with improved efficiency, 
because all parts including the bearing 29 and the like may be 
incorporated therein from a side of the cover 48. 
Also, as shown in FIG. 5, the plate-like cover 48 mounted on the casing 11 
is formed with the passages 2a and 2b in a pair through which the variable 
displacement pump 1 and variable displacement motor 3 communicate with 
each other. 
Further, the cover 48 is formed at an end thereof positioned on a side of 
the variable displacement pump 1 with a charge passage 49 connected to the 
charge pump 7. The charge passage 49 is arranged at a position between the 
passages 2a and 2b. 
In addition, the cover 48 is provided at the end thereof with the relief 
valves 9a and 9b and check valves 10a and 10b, which are arranged above 
the input shaft 13 of the variable displacement pump 1. Now, such 
arrangement will be more detailedly described hereinafter. In this 
respect, the following description will be made in connection with only 
the relief valve 9a and check valve 10a for the sake of brevity. 
The cover 48 is formed therein with a receiving hole or space 50 in a 
manner to inwardly extend from one of sides thereof. The receiving hole 50 
is arranged so as to communicate with the passage 2a and charge passage 
49. Also, the receiving hole 50 is formed at a portion thereof facing the 
charge passage 49 with a seat portion 51. 
The receiving hole 50 has a check member 52 of a semicylindrical shape 
slidably received therein and adapted to be seated on the seat portion 51. 
When the check member 52 is seated on the seat portion 51, the check 
member 52 is abutted at a distal end thereof against a distal end of a 
check member 52 arranged on a side of the passage 2b. 
The check member 52 is provided on an inner peripheral surface thereof with 
a support member 54 which is formed with a passage 53 so as to extend in 
an axial direction thereof. The support member 54 is fixed at a proximal 
end of an outer peripheral surface thereof on the inner peripheral surface 
of the check member 52. The check member 52 has a spring 55 and a poppet 
member 56 received therein. The spring 55 acts to force the poppet member 
56 against the support member 54 to close the passage 53. 
Also, the support member 54 is provided on the inner peripheral surface 
thereof with a spring receiving member 58 on which elastic force of a 
spring 57 acts. Reference numeral 59 designates a filter which is 
interposedly arranged between a distal end of the spring receiving member 
58 and the inner peripheral surface of the support member 54. The spring 
exhibits elastic force which acts on the check member 52 from the spring 
receiving member 58 through the support member 54, so that the check 
member 52 may be forced against the seat member 51. 
The check member 52 is formed on a side surface thereof with a 
communication hole 60 which permits the passage 2a to communicate with the 
inner peripheral surface of the check member 52. This results in hydraulic 
fluid which is guided through the communication hole 60 acting on the 
poppet member 56 through a gap between the inner peripheral surface of the 
check member 52 and the support member 54, a small hole 61 formed through 
a side wall of the support member 54, a passage formed at the spring 
receiving member 58, the filter 59 and the passage 53 of the support 
member 54. Concurrently, hydraulic fluid in the passage 2a is fed through 
the passage of the spring receiving member 58 to a spring chamber in which 
the spring 57 is received, to thereby act as a back pressure on the check 
member 52. 
Supposing that the variable displacement pump 1 is driven to produce a high 
pressure in the passage 2a and a low pressure in the passage 2b, hydraulic 
fluid guided from the charge pump 7 to the charge passage 49 acts on the 
check member 52. The check member 52 arranged on the side of the passage 
2a is kept from being moved by elastic force of the spring 57 and a 
pressure in the passage 2a guided to the spring chamber, whereas the check 
member 52 on the side of the passage 2b is moved against the spring, to 
thereby be released or separated from the seat portion 51. Such separation 
of the check member 52 from the seat portion 51 causes hydraulic fluid in 
the charge passage 49 to be fed to the passage 2b reduced in pressure. 
Thus, the check member 52, spring 57 and the like cooperate with each other 
to provide the check valve 10a. 
When a pressure in the passage 2a is abnormally or excessively increased, 
the poppet member 56 is moved against the spring 55 to open the passage 53 
of the support member 54, because a pressure in the passage 2a acts on the 
poppet member 56. Opening of the passage 53 permits a part of hydraulic 
fluid in the passage 2a to escape through a passage 62 formed at a distal 
end of the check member 52 toward the passage 2b. 
Thus, the poppet member 56, spring 55 and the like cooperate with each 
other to constitute the relief valve 9a. 
The cover 48 has the change-over valve 5 incorporated in an end thereof 
facing the variable displacement motor 3 while being positioned below the 
output shaft 20 of the variable displacement motor 3. 
More particularly, the cover 48 is formed therein with a receiving hole 63 
in a manner to inwardly extend from one side thereof, in which a spool 64 
is slidably received. Also, the receiving hole 63 has a plug 66 received 
therein from a left-hand side thereof and a spring 67 and a plug 68 
received from a right-hand side thereof. The spring 67 is arranged in a 
spring chamber 69 formed in the receiving hole 63 so as to communicate 
with a tank. The plug 66 is formed therein with the pilot chamber 5a. 
Also, the cover 48 is formed with ports 70a and 70b communicating with the 
passages 2a and 2b, respectively. The cover 48 is further formed with 
pilot ports 72a and 72b so as to be arranged at positions deviated from 
the ports 70a and 70b, respectively. 
The pilot ports 72a and 72b are arranged so as to communicate with pilot 
passages 71a and 71b formed via the casing 11. The pilot passages 71a and 
71b, as shown in FIG. 2, each function to guide a pilot pressure to each 
of the control cylinder holes 43. The pilot passages 71a and 71b each are 
formed at an intermediate portion thereof with an orifice 6 for reducing 
shock. 
Communication between the pilot chamber 5a and the tank through the pilot 
valve 8 (FIG. 1) permits the spool 64 to be kept abutted against the plug 
66 as shown in FIG. 5. This causes the ports 70a and 70b to be isolated 
from the pilot ports 72a and 72b, respectively. Also, it causes the pilot 
ports 72a and 72b to communicate through the spring chamber 69 with the 
tank. 
At this state, the pilot passages 71a and 71b are kept communicating with 
the tank, so that a pressure in the tank is applied to the control 
cylinder holes 42. Thus, any thrust is not generated at each of the 
control plungers 43, so that the first abutment surface section 38 of the 
cam plate 33 of the variable displacement motor 3 may be kept abutted 
against the inner surface of the casing 11. 
Whereas, when the pilot valve 8 (FIG. 1) is changed over to guide a 
discharge pressure of the charge pump 7 to the pilot chamber 5a, the 
pressure acts to change over the spool 64 against the spring 67. This 
permits the ports 70a and 70b to communicate through an annular groove of 
the spool 64 with the pilot ports 72a and 72b, respectively. Concurrently, 
the pilot ports 72a and 72b are isolated from the spring chamber 69 and 
therefore the tank. 
Under such conditions, a pressure increased in the passage 2a or 2b is 
introduced through the pilot port 72a or 72b to any one of the control 
cylinder holes 42. This causes thrust to occur at any one of the control 
plungers, so that the second abutment surface section 39 of the camp late 
33 of the variable displacement motor 3 is abutted against the inner 
surface of the casing 11, as described above. 
The spool 64 is formed with constriction holes 73a and 73b which permit an 
outer peripheral surface of the spool 64 to communicate with the spring 
chamber 69. Also, a clearance is defined between the receiving hole 63 and 
the outer peripheral surface of the spool 64 so as to permit the ports 70a 
and 70b to communicate with the constriction holes 73a and 73b, 
respectively, although it is not shown in FIG. 5. 
Therefore, hydraulic fluid in the passages 2a and 2b is returned in a 
slight amount from the ports 70a and 70b through the clearance, 
constriction holes 73a and 73b, and spring chamber 69 to the tank. Also, 
hydraulic fluid is supplemented from the charge pump 7 to the variable 
displacement pump 1, so that hydraulic fluid in a closed circuit provided 
by the variable displacement pump 1 and variable displacement motor 3 is 
replaced little by little, to thereby minimize a deterioration in 
hydraulic fluid in the closed circuit. 
Thus, the hydrostatic transmission system of the illustrated embodiment is 
so constructed that the variable displacement pump 1 is constituted by a 
cam plate type axial plunger pump adapted to keep a discharge rate thereof 
zero when a tilting angle is within a range set near 0 degree. Such 
construction eliminates arrangement of the neutral valves 104a and 104b in 
the prior art described above. Also, in the illustrated embodiment, the 
relief valves 9a and 9b, check valves 10a and 10b, and change-over valve 5 
are incorporated in the cover 48. Thus, the illustrated embodiment 
provides the hydrostatic transmission system integrated with the casing 11 
and cover 48 while being prevented from being large-sized, irrespective of 
incorporation of the variable displacement motor 3 therein. 
As can be seen from the foregoing, in the hydrostatic transmission system 
of the present invention, the variable displacement pump is constituted by 
a cam plate type axial plunger pump adapted to keep a discharge rate 
thereof zero when a tilting angle is within a range set near 0 degree. 
This ensures the neutral width without any additional valve such as a 
neutral valve. Concurrently, the relief valves, check valves and 
change-over valve are incorporated in the cover. Thus, the hydrostatic 
transmission system of the present invention may be integrated with the 
casing and cover while being prevented from being large-sized irrespective 
of use of the variable displacement motor. 
Also, the present invention may be constructed so that the movable thrust 
plate is incorporated in the variable displacement pump. Thus, the 
variable displacement pump permits a tilting angle of the cam plate to be 
varied without being substantially kept from being large-sized. Also, it 
keeps a discharge rate thereof zero when the tilting angle is in a range 
set near 0 degree. 
Further, in the hydrostatic transmission system of the present invention, 
the variable displacement motor may be constructed in the manner that the 
cam plate is provided with abutment surface sections. Thus, the variable 
displacement motor may be kept from being large-sized. 
______________________________________ 
1 Variable displacement pump 
2a, 2b Passage 
3 Variable displacement motor 
5 Change-over valve 
9a, 9b Relief valve 
10a, 10b Check valve 
11 Casing 
17 Plunger 
18 Cam plate 
20 Movable thrust plate 
20a Opposite surface 
22 Ball 
32 Plunger 
33 Cam plate 
38 First abutment surface section 
39 Second abutment surface section 
48 Cover 
52 Poppet member 
64 Spool 
______________________________________