Reversible hydrostatic transmission with dump valve

A hydrostatic transmission is disclosed comprising a radial ball pump (11) and a radial ball motor (13) wherein the normal journal member (33) may be replaced by an alternative or modified journal member (33'), such that even with a reverse direction of actuation of the displacement control (49), the HST still operates in the forward direction, with no other changes required. The present invention also includes a simplified dump valve member (107) comprising a single, unitary member which may be merely inserted to its assembled position in which a pair of retention portions (123, 135) retain the dump valve, and permit it to pivot about the retention portions from an unactuated position to an actuated position.

BACKGROUND OF THE DISCLOSURE 
The present invention relates to hydrostatic transmissions (HSTs), and more 
particularly, to such transmissions which are used in connection with the 
propulsion of vehicles such as lawn and garden tractors. 
A typical hydrostatic transmission includes a pair of fluid displacement 
mechanisms, one of which functions as a pump and the other of which 
functions as a motor. Conventionally, the pump comprises a variable 
displacement hydrostatic unit, while the motor comprises a fixed 
displacement hydrostatic unit. Therefore, there are typically various 
controls associated with the pump, such as some form of displacement 
control, and perhaps other controls as well. One example of another 
control which would be present on a typical HST would be a "dump" control, 
which generally is well known in the art, and is used to interconnect the 
two sides of the hydrostatic loop (or connect both sides to the system 
reservoir), thus making it possible to push or tow the vehicle without 
having to overcome the dynamic braking effect of the hydrostatic 
transmission in its normal operating mode. 
Although the present invention may be used advantageously with hydrostatic 
transmissions including various types of fluid displacement mechanisms, it 
is especially advantageous when the fluid displacement mechanisms comprise 
radial piston devices, and the invention will be described in connection 
therewith. As used herein, the term "radial piston" will be understood to 
include devices in which the piston is either a true cylindrical piston 
which reciprocates in a cylinder, or a ball member which reciprocates in a 
cylinder. 
One of the problems associated with providing hydrostatic transmissions to 
vehicle original equipment manufacturers (OEMs) is that each OEM normally 
has its own preferred linkage arrangement already designed into its 
vehicle. For example, one OEM may utilize a pump displacement control in 
which depressing a foot pedal pushes or pulls a linkage member "forward" 
for the purpose of increasing pump displacement in a forward direction. 
Another vehicle OEM may have a pump displacement control in which 
depressing a foot pedal shifts a linkage member "rearward" to increase 
pump displacement in the forward direction. 
Typically, the manufacturers of HSTs have provided the pumps of their 
transmissions with pump displacement controls which move one direction to 
increase pump displacement moving forward and move in the opposite 
direction to increase pump displacement moving in reverse. If the 
potential OEM customer already has its control linkage set up for movement 
corresponding to what is already available on the HST, there are no 
linkage changes required. If, on the other hand, the HST manufacturer has 
its pump displacement control set up to move in a direction opposite to 
that of the linkage being used by the vehicle OEM, it is necessary for the 
HST manufacturer to request the vehicle OEM to redesign and reverse the 
vehicle linkage to accommodate the HST linkage. 
Accordingly, it is an object of the present invention to provide an 
improved hydrostatic transmission which can be used on a vehicle having 
either forward-moving linkage or rearward-moving linkage, without any 
substantial redesign or revision of the hydrostatic transmissions, and 
which eliminates the need for the vehicle OEM to redesign or reverse its 
linkage. 
The above and other objects of the invention are accomplished by the 
provision of an improved hydrostatic transmission comprising a housing 
assembly, a variable displacement radial piston pump, and a fluid 
pressure-operated motor disposed in the housing assembly. The radial 
piston pump includes displacement varying means operable to vary the 
displacement of the pump, including reversible control means operable to 
move the displacement varying means in first and second opposite 
directions, in response to movement of the control means in first and 
second opposite directions from a neutral position. The motor defines a 
fluid inlet and a fluid outlet, and the housing assembly defines high- and 
low-pressure passages in fluid communication with the fluid inlet and 
fluid outlet, respectively. The pump comprises a rotor assembly rotatably 
mounted on a journal member disposed within a journal bore defined by the 
housing assembly, the journal bore providing open fluid communication with 
both of the high- and low-pressure passages. The journal member defines 
high- and low-pressure pump slots in fluid communication with contracting 
and expanding chambers, respectively, of the pump, the journal member 
further defining high- and low-pressure housing slots in fluid 
communication with the high- and low-pressure passages, respectively, and 
the journal member defining a high-pressure axial bore interconnecting the 
high-pressure pump and housing slots, and a low-pressure axial bore 
interconnecting the low-pressure pump and housing slots. 
The improved hydrostatic transmission is characterized by operating in a 
forward direction in response to movement of the control means in the 
first direction from the neutral position in the presence of a first 
journal member disposed in the journal bore wherein the axial bores are 
oriented generally perpendicular to the pump slots, and the housing slots 
are oriented generally perpendicular to the axial bores. The hydrostatic 
transmission is further characterized by operating in the forward 
direction in response to movement of the control means in the second 
direction from the neutral position in the presence of a second journal 
member disposed in the journal bore wherein the axial bores are oriented 
at an acute angle relative to the pump slots, and the housing slots are 
oriented at an acute angle relative to the axial bores. 
In the typical prior art dump valve (unloading valve) arrangement, such as 
is shown in U.S. Pat. No. Re 28,953, the hydrostatic loop is "unloaded" by 
simultaneously unseating both of the anti-cavitation ball check valves. In 
the prior art, the mechanism which unseats the ball check valves has 
typically comprised structure which added substantially to the cost and 
complexity of manufacture and assembly of the pump. Furthermore, the 
typical prior art dump valve mechanism added substantially to the size of 
the pump and quite often involved members both internal and external to 
the pump housing, such that a fairly complex arrangement of seals was 
required. 
Accordingly, it is an object of the present invention to provide an 
improved hydrostatic transmission including a dump valve (unloading valve) 
arrangement which overcomes the above-described disadvantages of the prior 
art. 
The above and other objects of the invention are accomplished by the 
provision of a hydrostatic transmission comprising a housing assembly, a 
fluid pressure-operated pump, and a fluid pressure-operated motor disposed 
in the housing assembly. The pump defines a fluid inlet and a fluid outlet 
and the motor defines a fluid inlet and a fluid outlet. The housing 
assembly defines a case chamber, a first passage providing fluid 
communication from the pump fluid outlet to the motor fluid inlet, and a 
second passage providing fluid communication from the motor fluid outlet 
to the pump fluid inlet. A first check valve means is operable to permit 
fluid communication from the case chamber to the first passage in the 
absence of fluid pressure in the first passage, and a second check valve 
means is operable to permit fluid communication from the case chamber to 
the second passage in the absence of fluid pressure in the second passage. 
A dump mechanism is operably associated with the first and second check 
valve means to open both of said check valve means simultaneously in 
response to an input. 
The improved hydrostatic transmission is characterized by the housing 
assembly defining an insertion chamber adjacent said case chamber and in 
open communication therewith, and further defining retention means, the 
insertion chamber being disposed between the case chamber and the 
retention means. The dump mechanism comprises a unitary dump member 
including a dump portion including first and second projecting portions 
disposed for unseating engagement with the first and second check valve 
means, respectively, in response to movement of the dump portion toward 
the check valve means. The unitary dump member further includes a pair of 
resiliently deflectable mounting portions, each including a retention 
member, the mounting portions being deflectable during insertion thereof 
through the insertion chamber until the retention members engage the 
retention means, the unitary dump member thereafter being pivotable about 
the retention members. The dump mechanism includes means biasing the dump 
portion away from the check valve means, and the dump mechanism includes 
means operable in response to the input to cause the dump member to pivot 
about the retention members and move the projecting portions toward the 
check valve means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, which are not intended to limit the 
invention, FIG. 1 is an axial cross-section of a hydrostatic transmission 
of the type with which the present invention may be utilized. The 
hydrostatic transmission may be made generally in accordance with the 
teachings of U.S. Pat. No. 5,234,321, assigned to the assignee of the 
present invention and incorporated herein by reference. A hydrostatic 
transmission comprises a hydrostatic pump, generally designated 11, and a 
hydrostatic motor, generally designated 13. The pump 11 includes a pump 
cover 15, while the motor 13 includes a motor cover 17, and the pump 11 
and motor 13 share a common manifold body 19. The pump cover 15, motor 
cover 17, and manifold body 19 are joined together by a plurality of bolts 
not shown herein, to comprise a housing assembly, and to define therein a 
hydrostatic transmission case 21 (also referred to hereinafter as a "case 
chamber"). Preferably, the hydrostatic transmission, as shown in FIG. 1, 
comprises a self-contained, stand alone unit. 
Referring now to FIG. 2, in conjunction with FIG. 1, the pump cover 15 
defines an input hub portion 23, within which is disposed an input shaft 
25, rotatably supported within the hub portion 23 by means of a bearing 
set 27. Typically, the input shaft 25 has an input pulley (not shown in 
FIG. 1) keyed to the input shaft, or attached in some other suitable 
means, by means of which engine torque is transmitted to the hydrostatic 
pump 11. At its inner end (bottom end in FIG. 1), the input shaft 25 
includes a terminal portion 29 through which a drive pin (not shown in 
FIG. 1) extends, transverse to the axis of rotation of the input shaft 25, 
the function of the drive pin being to engage a cut out portion of a pump 
rotor 31. Therefore, rotation of the input shaft 25 drives the pump rotor 
31. 
The pump rotor 31 is disposed about a cylindrical support member 33 (also 
referred to hereinafter as a journal member), which is press-fit into a 
cylindrical journal bore 34 in the manifold body 19. The support member 33 
defines a pair of axially-extending bores 35 and 37 (see also FIG. 2). The 
bore 35 comprises a low pressure inlet passage, and the bore 37 comprises 
a high pressure outlet passage, assuming rotation of the pump rotor 31 in 
a clockwise direction as viewed in FIG. 2. As is well known to those 
skilled in the art, if the direction of rotation of the pump rotor 31 were 
reversed, the bore 35 would be the high-pressure outlet passage, and the 
bore 37 would be the low pressure inlet passage. 
The pump rotor 31 defines a plurality of cylinder bores 39, and disposed 
within each bore 39 is a piston or ball 41, it being understood that the 
present invention is equally applicable to a radial piston or a radial 
ball type of device. The balls 41 are in engagement with, and restrained 
in their radial travel, by a concave surface on a race member 43, which is 
disposed within a cam ring 45. The cam ring 45 is disposed to pivot about 
the axis of a cam pivot pin 47 (shown only in FIG. 2), the pin 47 being 
received within a pair of aligned bores defined by the pump cover 15 and 
the manifold body 19. As is generally well-known to those skilled in the 
art, pivotal movement of the cam ring 45 varies the displacement and 
output fluid flow of the pump 11, for any given input speed. Pivotal 
movement of the cam ring 45 about the axis of the pivot pin 47 is 
accomplished by means of a displacement control assembly, generally 
designated 49 (shown only in FIG. 2). Such displacement controls are well 
known to those skilled in the art, are not an essential feature of the 
invention, and therefore, will be described only briefly herein. The 
displacement control assembly 49 includes a control shaft 51, which 
extends through the pump cover 15 for connection to a linkage arrangement 
(not shown herein). Appropriate movement of the linkage rotates the 
control shaft 51 about its axis of rotation, thereby causing angular 
displacement of a pin 53 which is received in an opening 55 defined by the 
cam ring 45. Thus, rotation of the control shaft 51 results in pivotal 
movement of the cam ring 45 about the cam pivot pin 47 in a known manner, 
to vary the displacement of the pump 11 from its neutral (zero 
displacement) position shown in FIG. 2. 
Assuming the control shaft 51 is rotated clockwise (thus pivoting the cam 
ring 45 counter-clockwise about the pivot pin 47), the operation of the 
pump 11 will now be described. As the input shaft 25 rotates, the pump 
rotor 31 rotates, with some of the balls 41 moving radially outward in 
their respective cylinder bores 39, drawing inlet fluid from the axial 
bore 35. At the same time, certain of the balls 41 are being displaced 
radially inwardly, thus pumping pressurized fluid from their respective 
cylinder bores 39 into the axial bore 37. Fluid communication from the 
axial bore 35 to the outwardly moving balls 41 is by means of a pump slot 
57 defined by the journal member 33. Similarly, communication of 
pressurized fluid from the inwardly moving balls 41 to the axial bores 37 
is by means of a pump slot 59, defined by the journal member 33. 
Referring now to FIG. 3, in conjunction with FIG. 1, the hydrostatic motor 
13 is a fixed-displacement motor, but is otherwise of the same general 
construction as the pump 11. Thus, the motor 13 includes a cylindrical 
support member 61 (also referred to hereinafter as a journal member), 
which is press-fit into a cylindrical journal bore 62 defined by the 
manifold body 19. The support member 61 defines a pair of axially 
extending bores 63 and 65, and a pair of housing slots 64 and 66 (labelled 
only in FIG. 6) communicating with the axial bores 63 and 65, 
respectively. As is well known to those skilled in the art, the 
construction of the motor support member 61 may be substantially the same 
as that of the pump support member 33. 
Rotatably supported on the support member 61 is a motor rotor 67 which 
defines a plurality of cylinder bores 69, with a piston or ball 71 being 
disposed reciprocally within each of the bores 69. The balls 71 are 
restrained in their radial travel by a concave surface of a race member 
73, the race member 73 being seated within a recess in the manifold body 
19, and restrained axially therein by the motor cover 17. The motor rotor 
67 includes an output portion 75, by means of which the rotor 67 can drive 
an output shaft 77 (shown in FIG. 3, but omitted from FIG. 1) by any 
suitable means, which is well known to those skilled in the art and will 
not be illustrated or described herein. 
Referring still primarily to FIGS. 1 and 3, the lower portion of the 
journal member 33 defines a housing slot 79 in open communication with the 
axial bore 35, and a housing slot 81 in open communication with the axial 
bore 37. The housing slots 79 and 81 could also be referred to as "motor 
slots" but are not so referenced herein because they are not in direct 
communication with the motor 13. By means of the opposite ends of a cross 
bore 83, which is plugged at its upper end in FIG. 3 by means of a 
threaded plug 85, the bore 35 and housing slot 79 are in communication 
with a passage 87, while the axial bore 37 and housing slot 81 are in 
fluid communication with a passage 89. 
Referring now also to FIG. 6, the passage 87 is in fluid communication with 
the axial bore 63 of the support member 61 by means of a vertical bore 91, 
and similarly, the passage 89 is in fluid communication with the axial 
bore 65 by means of a vertical bore 93. Thus, with the input shaft 29 
rotating clockwise, as was described previously, pressurized fluid is 
communicated through the axial bore 37, then through the cross bore 83, 
the passage 89, the vertical bore 93, then through the axial bore 65 to 
those cylinders 69 of the motor 15 in which the ball 71 is moving radially 
outward, as is well known to those skilled in the art. At the same time, 
in the case of those cylinders 69 in which the ball 71 is moving radially 
inward, low pressure exhaust fluid is being communicated through the axial 
bore 63, then through the vertical bore 91, through the passage 87, then 
through the cross bore 83, the housing slot 79, and through the axial bore 
35 to the inlet of the pump 11, also in a manner well known to those 
skilled in the art. As a result, the output shaft 77 of the motor 13 is 
driven in what will be considered hereinafter the "forward" direction. 
Referring again primarily to FIGS. 2 and 3, it should be noted that the 
axial bores 35 and 37 are "straight" (as illustrated in FIG. 1), and that 
the bores 35 and 37 are oriented at approximately a 45.degree. angle, 
i.e., a plane interconnecting the axes of the bores 35 and 37 would be 
oriented at approximately 45.degree. to a transverse plane passing through 
the axis of the support member 33. Such a plane would be oriented 
vertically in both FIGS. 2 and 3. In FIGS. 2 and 3, it may be seen that 
both the pump slots 57 and 59 and the housing slots 79 and 81 are oriented 
approximately perpendicular to the axial bores 35 and 37. In other words, 
the housing slot 79 includes a bottom surface 95 (see FIG. 3) and that 
bottom surface 95, as well as the bottom surfaces of the other slots 57, 
59 and 81 are all oriented approximately perpendicular to the imaginary 
plane passing through the axial bores 35 and 37. 
The above-described relationship of the axial bores 35 and 37 and slots 57, 
59, 79, and 81 is the conventional relationship which has been in use 
commercially and has the following result. With the input shaft 29 
rotating clockwise in FIG. 2, rotation of the control shaft 51 in a 
clockwise direction will result in pivotal movement of the pin 53 as 
indicated by the arrow in FIG. 2, and fluid will flow as described 
previously, resulting in rotation of the output shaft 77 in the forward 
direction. 
In accordance with one important aspect of the present invention, and as 
was discussed in the BACKGROUND OF THE DISCLOSURE, in certain vehicle 
applications, it is desirable for the control shaft 51 to be connected to 
a linkage which moves in a direction opposite to what is "normal", wherein 
the resulting rotation of the control shaft 51 in the counter-clockwise 
direction (as viewed in FIG. 2) will result in rotation of the output 
shaft 77 in the forward direction. In accordance with the present 
invention, such may be accomplished by "replacing" the support member 33, 
having the construction shown in FIGS. 2 and 3, with a modified support 
member or journal member 33' having the construction shown in the 
fragmentary views of FIGS. 4 and 5. As used herein, it will be understood 
that reference to the journal member 33 being "replaced" by the modified 
journal member 33' typically means that, during the assembly of 
hydrostatic transmissions to be used with the "opposite" linkage, the 
modified journal member 33' will be inserted in the manifold body 19, 
rather than the standard journal member 33. 
Referring now primarily to FIGS. 4 and 5, it should be noted that those 
figures are oriented the same as FIGS. 2 and 3, respectively, but are 
enlarged relative thereto. In FIG. 4 it may be seen that the modified 
journal member 33' defines pump slots 57' and 59', and by comparing FIG. 4 
to FIG. 2, it may be seen that the slots 57' and 59' are oriented the same 
as were the slots 57 and 59 in FIG. 2. However, in the modified or 
"reverse" journal member 33' of FIGS. 4 and 5, the axial bores 35' and 37' 
are not perpendicular to the pump slots 57' and 59', as was the case with 
the journal member 33. Instead, the plane containing the axes of the bores 
35' and 37' is oriented at an acute angle to the pump slots 57' and 59'. 
In the subject embodiment, the acute angle is approximately 45.degree.. 
Referring now primarily to FIG. 5, the reverse journal member 33 defines 
housing slots 79' and 81', which are displaced about 90.degree. from the 
position of the housing slots 79 and 81, as shown in FIG. 3. The pump 
slots 79' and 81' also define an acute angle relative to the plane 
containing the axes of the bores 35' and 37'. 
Viewing the reverse journal member 33', as shown in FIGS. 4 and 5 in the 
context of FIGS. 2 and 3, the operation of the present invention, 
including the reverse journal member 33' in place of the regular journal 
member 33, will be described. Assuming the control shaft 51 is rotated 
counterclockwise (thus pivoting the cam ring 45 clockwise about the pivot 
pin 47), the balls in the cylinders communicating with the pump slot 59' 
are now moving radially outwardly, drawing inlet fluid from the axial bore 
37'. At the same time, the remainder of the balls 41 are being displaced 
radially inwardly, thus pumping pressurized fluid from their respective 
cylinder bores 39 into the pump slot 57' and the axial bore 35'. 
The pressurized fluid flows from the axial bore 35' into the housing slot 
79', which is in fluid communication through the cross-bore 83, passage 
89, etc., with the cylinders in the motor 13 in which the balls 71 are 
moving radially outward. At the same time, fluid is exhausted from those 
cylinders 69 in which the ball 71 is moving radially inward, with the 
low-pressure exhaust fluid flowing back through the passage 87, through 
the upper end of the cross-bore 83, and into the housing slot 81'. From 
there, low-pressure fluid flows through the axial bore 37' and back up to 
the pump slot 59', ready to enter the expanding cylinders of the pump 11. 
Thus, it may be seen that the motor 13 is still driven in the forward 
direction, even with the direction of input to the displacement control 
assembly 49 being reversed, simply by inserting the reverse journal member 
33' in place of (instead of) the journal member 33. No other change or 
modification of any part of the hydrostatic transmission is required, in 
order to accommodate such reversed direction of input to the displacement 
control assembly 49. Furthermore, it has been determined in connection 
with the development of the present invention that the modified reverse 
journal member 33' does not detract in any way from the efficiency (either 
volumetric or mechanical) of the pump 11, or of the overall hydrostatic 
transmission. 
Although the present invention has been described in connection with a 
situation in which the vehicle OEM wishes to utilize a reversed input to 
the displacement control assembly 49, the reverse journal member 33' could 
also be used in a situation where the vehicle OEM wishes to use "normal" 
input to the displacement control assembly 49, but is utilizing a vehicle 
engine which provides an input to the input shaft 25 of the pump 11 which 
is "reversed" from what is normal. In other words, if the standard, 
commercially available engine provides a clockwise rotation of the input 
shaft 25 (looking down in FIG. 1), some vehicle OEM's may have an engine 
which provides a counterclockwise input. In that case, the reverse journal 
member 33' may be used in conjunction with the counterclockwise input to 
the pump 11, and the direction of rotation of the output shaft 77 will 
still correspond to what is considered the "normal" forward direction of 
operation. Therefore, references hereinafter and in the appended claims to 
a "forward input" or "reverse input" will be understood to refer to either 
the input to the displacement control assembly 49 or to the input shaft 25 
of the pump 11. 
Also, although the present invention has been described in connection with 
replacing the journal member 33 of the pump 11, those skilled in the art 
will appreciate that the invention is not so limited, and that the journal 
member 61 of the motor 13 could be replaced by a reverse journal member 
(made generally in accordance with the teachings of FIGS. 4 and 5, and the 
functional result would be the same. In other words, by utilizing a normal 
pump journal 33 but a reverse motor journal, if either the input to the 
displacement control assembly 49 or the input drive to the input shaft 25 
is reversed, the output shaft 77 will rotate in the "forward" direction. 
It will also be understood by those skilled in the art that if both inputs 
are reversed, then it is necessary to utilize both the normal pump journal 
and the normal motor journal in order for the output shaft 77 to operate 
in the forward direction. 
FIGS. 6 through 8 
Referring now primarily to FIGS. 6 and 7, in conjunction with FIG. 1, the 
improved dump valve (unloading valve) of the present invention will be 
described. Disposed within the lower end of the vertical bore 91 is a 
check valve assembly including a ball check valve 101, and similarly, 
disposed in the lower end of the vertical bore 93 is a check valve 
assembly including a ball check valve 103. As is explained in the 
BACKGROUND OF THE DISCLOSURE, and as is well known to those skilled in the 
art, the function of a dump valve is to unseat, simultaneously, both of 
the ball check valves 101 and 103, such that both sides of the hydrostatic 
loop (i.e., bores 63 and 65) are in communication with each other through 
the case chamber 21, which is also in communication with the system 
reservoir. 
Referring now primarily to FIG. 7, disposed axially between the lower end 
of the cylindrical journal bore 34 and the case chamber 21 is a narrow 
opening, which will be referred to hereinafter as an insertion chamber 105 
(see also FIG. 1). As may best be seen in FIG. 1, although not an 
essential feature of the invention, from the bottom of the bore 34, there 
is a step-down to the bottom of the insertion chamber 105, and from there 
a further step-down to the bottom surface of the case chamber 21. 
Disposed within the continuous open region defined by the bore 34 and the 
chambers 21 and 105 is a dump valve member generally designated 107, which 
is preferably formed as a single unitary member. For example, the dump 
valve member 107 may be an injection-molded plastic part or may be formed 
from some other suitable material, as long as the resulting part has the 
necessary resilience or "springiness" to perform the function to be 
subsequently described. 
The dump valve member 107 includes a main dump portion 109, including a 
pair of upstanding projections 111 and 113, illustrated herein as being 
generally cylindrical, the function of which will be subsequently 
described. The dump portion 109 defines a generally cylindrical recess 115 
in which, as is shown primarily in FIG. 1, there is disposed a coiled 
compression spring 117, the function of which is to normally bias the dump 
valve member 107 "downward" to the position shown in FIGS. 1 and 6. 
Formed integrally with the dump portion 109 is a pair of axially-extending 
legs 119 and 121, which pass through the insertion chamber 105, the legs 
119 and 121 comprising the means for mounting the dump valve member 107. 
The legs 119 and 121 terminate, at their forward end (left end in FIGS. 1 
and 7) in a pair of retention portions 123 and 125, respectively, shown 
only in FIG. 7. With the dump valve member 107 in the assembled position 
shown in FIG. 7, the retention portions 123 and 125 engage the journal 
bore 34 in such a way as to prevent movement of the member 107 to the 
right in FIG. 7. At the same time, the dump valve member 107 is prevented 
from moving any further "forwardly" (i.e., to the left in FIGS. 1 and 7) 
by the engagement of the forward surface of the dump portion 109 with a 
generally cylindrical actuator member 127. 
It is an important aspect of the present invention that the legs 119 and 
121 be "resiliently deflectable," which can mean either that the legs 
themselves are resilient or deflectable or, as is the case in the subject 
embodiment, the legs themselves remain relatively straight but the dump 
portion 109 is itself somewhat deformable as the legs 119 and 121 are 
moved closer together, as will be subsequently described. Another 
important aspect of the dump valve member 107 of the present invention is 
its simplicity in terms of the number of parts and the ease of assembly. 
Referring now primarily to FIG. 7, prior to assembly, the entire dump valve 
member 107 is disposed in the case chamber 21, with the retention portions 
123 and 125 being disposed in engagement with the angled wall surfaces 129 
and 131, respectively. Prior to assembly, the compression spring 117 is 
already disposed within the cylindrical recess 115, and during the 
assembly process, the upper surface of the spring 117 merely slides along 
an undersurface 133 of the manifold body 119, the undersurface 133 being 
shown best in FIG. 1. 
In order to insert the dump valve member 107 to its "assembled" position 
shown in FIG. 7, the member 107 may be merely pushed or moved manually 
from its preassembled position to the left in FIG. 7, which will cause the 
legs 119 and 121 to be deflected "inward" (i.e., toward each other) far 
enough so that the outside surfaces of the retention portions 123 and 125 
engage the inside walls of the insertion chamber 105. The dump valve 
member 107 is moved further forward until it reaches the position shown in 
FIG. 7, at which point the resiliently deflectable legs move outward, 
returning to, or nearly to, their pre-assembly position. In other words, 
the entire assembly process merely requires manually inserting the member 
107 until it "snaps" into place. With the dump valve member 107 in the 
assembled position of FIG. 7, the retention portions 123 and 125 are now 
in engagement with the journal bore 34, providing the retention function 
described previously. 
Referring now primarily to FIGS. 1, 6 and 8, the actuation of the dump 
valve member 107 will be described. It should be noted that the retention 
portions 123 and 125 perform the additional function of providing a pivot 
point, about which the rest of the member 107 can pivot during actuation. 
FIGS. 1 and 6 represent the unactuated position of the dump valve 107, 
with the compression spring 117 biasing the dump valve downwardly and 
indirectly biasing the actuator member 127 forwardly (to the left in FIG. 
1). Actuation of the dump valve is accomplished simply by moving the 
actuator member 127 to the right in FIGS. 1 and 8, thus pivoting the dump 
valve in a counterclockwise direction about the pivot points and causing 
the dump valve to move from the position shown in FIG. 1 to that shown in 
FIG. 8. As that pivotal movement occurs, the upstanding projections 111 
and 113 move upward until the tips thereof engage the ball check valves 
101 and 103, respectively, lifting them from their respective seats to the 
position shown in FIG. 8. With the ball check valves 101 and 103 both in 
the position shown in FIG. 8, both sides of the hydrostatic loop are 
connected to each other and to the system reservoir through the case 
chamber 21, as was previously described and as is well known to those 
skilled in the art. With the dump valve member 107 in the actuated 
position of FIG. 8, the vehicle may be pushed or towed without having to 
overcome the dynamic braking effect of the pump 11 and motor 13, which 
would result if the ball check valves 101 and 103 were seated, thus 
closing the hydrostatic loop. 
After the vehicle has been towed or pushed and it is again desired to 
propel the vehicle by means of the hydrostatic transmission, the dump 
valve 107 may be returned to its unactuated position merely by releasing 
the actuator member 127 and permitting the compression spring 117 to again 
bias the dump valve 107 to the unactuated position shown in FIG. 1. This 
will permit the ball check valves 101 and 103 to return to engagement with 
their respective seats, and if the vehicle is then to be propelled in a 
forward direction, as previously described, pressurized fluid flows 
through the passage 89, then through the vertical bore 93 into the axial 
bore 65. The pressurized fluid causes the ball check valve 103 to tightly 
engage its seat, while on the other side of the loop, there is 
low-pressure fluid in the axial bore 63, the vertical bore 91, and the 
passage 87. Therefore, the ball check valve 101 is only lightly engaging 
its seat and if pressure on the low-pressure side of the loop drops below 
the case or reservoir pressure, make-up fluid can flow from the case 
chamber 21 into the vertical bore 91 to prevent cavitation within the 
closed loop, in a manner which is well known to those skilled in the art. 
Alternatively, it would be within the scope of the present invention for 
the dump valve member and the adjacent housing to be configured such that 
assembly of the dump valve member would initially involve resiliently 
deflecting the legs away from each other and then permitting them to be 
returned to their normal position, engaging a portion of the housing 
disposed between the legs, and pivoting thereabout. 
The invention has been described in great detail in the foregoing 
specification, and it is believed that various alterations and 
modifications of the invention will become apparent to those skilled in 
the art from a reading and understanding of the specification, It is 
intended that all such alterations and modifications are included in the 
invention, insofar as they come within the scope of the appended claims.