Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
106~)23Z
HYDROMECHANICAL TRANSMISSION WITH
OVERSPEED LIMITED VARIABLE DRIVE
This application is related in part to Canadian
application Serial No. 247,594, filed March 10, 1976,
(M-l - Cl. 74 - Sub. Cl. 105).
BACKGROU~D OF THE INVENTION
This invention relates to a variable speed drive
of the hydromechanical type in which the drive ratio is
varied by means of ~luid pump-motor units.
~ydromechanical transmissions wherein two power
paths are combined by a planetary gear unit to obtain a
drive ratio that is varied through a hydrostatic pump-motor
assembly in one of the power paths are well known. ~xamples
of such hydromechanical transmissions are disclosed in
United States Patent Nos. 3,090,~53, 3,736,813 and 3,733,931.
The foregoing type of hydromechanical drive is
often used in combination with a plural change speed gear
box of the shiftable type because of the relatively small
~06()Z3Z
variation in drive ratio associated with the hydromechanical
drive and the relatively low torque capacity of the fluid
pump-motor units. The shift type of change speed trans-
mission will therefore provide a plurality of speed range
steps while the fluid drive units are operative to effect
an infinite variation in drive ratio within each speed
- range. Transmission combinations of the foregoing type are
particularly suited for heavy duty vehicles such as earth
moving vehicles and agricultural tractors.
A serious problem arises in heavy duty instal-
lations employing the aforementioned types of transmission ~-
assemblies because of a destructive overspeed condition
that occurs whenever a relatively large input torque is
suddenly applied as would occur, for example, during starting
or sudden clùtch engasement. The overspeed condition arises
because of the low numerical drive ratio of the planetary
gear unit and is accompanied by excessive torque loading
of the fluid drive unit. It is therefore an important
object of the present invention to provide a hydromechanical
transmission that will automatically prevent the aforementioned
destructlve overspeed condition for the f luid drive unit.
SUMMARY OF THE INVENTION
In accordance with the present invention, a
hydromechanical transmission is interconnected in series -
with a three-speed and reverse shiftable type of gear
transmission between the prime mover or engine and the .
differential of a heavy duty type of vehicle in order to
provide an infinitely variable change in drive ratio within
'. " '
-2- ~
1~60232
each of the forward and reverse drive ranges of the shift
type transmission. The hydromechanical transmission includes
a planetary gear unit, havin~ an output planet gear carrier,
and sun and orbit gears respectively associated with
separate power paths from the engine through an input
clutch. One power path is established through countershaft
- gearing at a fixed drive ratio for positive and con~inuous
transmission of power to the orbit gear while the other
power path is estaklished through a pair of fluid drive
units that are hydraulically interconnected for yieldable
t_ansmittal of torque in both directions. One of the fluid
drive units is coupled to the sun gear of the planetary
gear unit while the other fluid drive gear unit is drivingly
connected to the engine upon engagement of the input clutch.
The fluid drive unit driven by the engine may be of the
variable displacement type and is operative to provide an
infinite variation in the drive ratio between the engine
and the output shaft connected to the car~ier of the
planetary gear unit. The other fluid drive unit may be of
the fixed displacement type that would ordinarily be subject
to the destructive overspeed condition when serving as a
torque reaction device for the planetary gear unit during
a regenerative power circulation mode of operation in the
hydromechanical transmission. However, overspeed of the
fixed displacement drive unit is prevented by automa~ic
engagement of a one-way clutch associated with countershaft
gearirg that drivingly interconnects the sun gear with the
engine in one direction to establish a speed ratio limit
between the fluid drive units. Below this limiting speed
ratio and during transmission of torque from the engine to
the planetary gear unit through both power paths, the one-
way clutch freewheels.
-3-
106023Z
The particular invention claimed herein comprehends
a transmission including an input ~embe;r, an output member,
a positi've dri~ve power train arIven by the input-member and a
yieldable power train driven by the inp-ut-member and having a
variable drive ratio aevice of limited torque transmitting
capacity relative to the positive drive power train. Power
path combining means drivingly connects the power trains to
the output member for transmitting torque through a variable
drive ratio range. Overspeed control means are interconnected
between the input member and the power combining means for
limiting the torque loading of the variable drive ratio device
without requiring a decrease in the variable drive ratio range.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIGURE 1 is a somewhat schematic illustration of
a vehicle power transmission arrangement with the hydromechanical
transmission of one embodiment of the invention disclosed,
shown somewhat diagrammatically in section and this embodiment
is the subject matter of Canadian application Serial No. 247,594,
filed, March 10, 1976;
FIGURE 2 is a transverse sectional view taken sub-
stantially through a plane indicated by section line 2-2 in
Figure l;
FIGURE 3 is a graphical illustration depicting
typical operating characteristics of the hydromechanical trans-
mission; and
FIGURE 4 is a somewhat schematic illustration of a
vehicle power transmission arrangement with the hydromechanical
transmission of an alternative embodiment of the invention
disclosed and shown somewhat diagrammatically in section and
is the embodiment to which the claims of this application are
directed.
--4--
1060Z3Z
DETAILED DESCRIPTION OF ~ PR~FER~ED EMBODI~ENT
Referring now to the drawing in detail, Figure 1
illustrates somewhat diagrammatically a transmission
arrangement for a Aeavy duty vehicle such as an agricultural
tractor. The prime mover or engine 10 propels the-vehicle
through its tractor wheels 12, power being transmitted to
the wheels from the engine by means of the transmission
arrangement which includes the engine driven input shart 14
that extends through the transmission arrangement to
drive power take-off device 16. The transmission arrangement
extending between the engine driven input shaft and the
1~60Z3Z
tractor wheels includes the hydromechanical transmission of
the present invention generally denoted by reference numeral
18 connected in series with a three speed and reverse step
change gear transmission 20 from which the differential gear
assembly 22 is driven to transmit power to the tractor
wheels. The step change gear transmission 20 establishes a
plurality of forward and reverse speed ranges. Between the
drive ratio limits of each speed range, an infinite variation
in drive ratio is effected by means of the hydromechanical
transmission 18.
Power is delivered to the vehicle wheels from the
engine through the hydromechanical transmission 18 upon
engagement of an input clutch 24 coupling the engine driven
input shaft 14 to a tubular power shaft 26. A drive gear 28
is connected to one end of the power shaft 26 opposite the
end to which the clutch 24 is connected. The drive gear 28
is in continuous meshing engagement with a countershaft gear
30 fixed to one end of a countershaft 32 that is rotatably
mounted about a fixed axis disposed in a parallel spaced
relationship to the rotatable axis for the coaxial shafts 14
and 26. The drive gear 28 is also in continuous meshing
engagement with a gear 34 through which a posi'ive, fixed
ratio drive train is established between the power shaft 26
and a power planetary gear unit generally referred to by
reference numeral 36.
The planetary gear unit 36 includes a planet gear
carrier 38 connected to an output shaft 40 which constitutes
the input to the plural speed step change gear transmission
20. The output shaft 40 is rotatably mounted about a fixed
axis in parallel spaced relation to the rotational axis of
1060Z3Z
the input shaft 14. Planet pinions 42 rotatably mounted by
the carrier 38 are in continuous meshing engagement with an
orbit ring gear 44 connected to the gear 34. The orbit gear
44 thus constitutes a drive member of the planetary gear
unit to which torque is continuously transmitted in one
direction from the engine along a positive power path established
by the meshing gears 28 and 34. The planet pinions 42 also
are in continuous meshing engagement with a sun gear 46
connected to one end of a countershaft 48 rotatably mounted
about the rotational axis of output shaft 40. The sun gear
46 constitutes a control member of the planetary gear uni~
connected to one end of a yieldable power path through which
torque is transmitted in both directions. The yieldable
power path is established by a pair of fluid drive units 50
and 52.
The fluid drive units S0 and 52 may be of the
hydrostatic piston type such as disclosed in U. S. Patent
No. 3,736,813 to Kress et al., as aforementioned. The fluid
drive unit 50 is of the fixed displacement type and is
coupled to the sun gear 46 of the planetary gear unit
through the countershaft 48. The other fluid drive unit 52
is of the variable displacement type and is drivingly
connected to the fixed ratio power path established by
the gears 28 and 34, by means of the countershaft 32 and
gear 30 in mesh with gear 28. The fluid drive units are
hydraulically interconnected by means of conduits 54
in order to form a variable fluid drive assembly, the speed
ratio of which is infinitely varied by controls associated
- with the variable displacement fluid drive unit 52, in a
manner well known to those skilled in the art.
~ ,
~6~23Z
The hydromechanical transmission 18 operates in
two different modes within each drive range while the drive
ratio is being varied between drive ratio limits of the
speed range. At the lower end of the drive ratio range, the
planetary gear unit transmits power in a regenerative power
circulating mode wherein the control sun gear 46 rotating in
a direction opposite to that of the orbit gear 44 serves as
a reaction gear. Torque is accordingly transmitted from the
reaction sun gear 46 to the fluid drive unit 50 acting as a
pump to drive the fluid drive unit 52 acting as a motor.
During the upper end of the speed range, torque is trans-
mitted in parallel through both power paths to the orbit
gear and sun gear respectively so that the planetary gear
unit then operates in a non-regenerative, power combining
mode. In the latter operational mode, the variable displace-
ment drive unit 52 acts as a pump while the fixed displace-
ment fluid drive unit 50 acts as a motor. The power from
the engine, transmitted to the power shaft 26 by engagement
of the input clutch 24, is split between the two power paths
respectively established by the countershaft gearing and the
hydraulically interconnected fluid drive units.
Mounted within the gear 28 and in engagement
with the input engine shaft 14 is an anti-coast one-way
clutch 56 that will prevent free-wheeling of the vehicle
when input clutch 24 is disengaged. In accordance with the
present invention, a second overrunning one-way clutch 58 is
also associated with the hydromechanical transmission and
acts as an automatic overspeed control device. The one-way
clutch 58 is mounted within a countershaft gear 60 at an end
of the countershaft 32 opposite gear 30. The countersha~t
- "
~6~Z3Z
gear 60 is in continuous mesh with a countershaft gear 62
connected to one end of countershaft 48 opposite the sun
gear 46. Ordinarily, the one-way clutch 58 freewheels so
that no driving connection is established between the
countershafts 32 and 48 by the meshing gears 60 and 62. The
relative speeds of the countershafts 32 and 48 are determined
instead by operation of the fluid drive units. However,
under high input torque loading conditions, that occur for
example when the input clutch 24 is suddenly engaged during
start up of the vehicle, a relatively high speed ratio
condition is imposed on the fluid drive units ordinarily
accompanied by excessive torque loading of the fluid drive
unit 50 under the regenerative power circulation mode of
operation aforementioned. The increasing speed ratio between
the countershafts 32 and 48 that occurs during the regenerative
power mode of operation is therefore limited by engagement
of the one-way clutch 58 establishing through the gears 60
and 62 a driving connection between the engine 10 and the
sun gear 46 at the desired speed ratio limit. The speed
ratio of the fluid drive assembly is thereby also limited to
prevent any unsafe or destructive overspeeding of the fluid
drive units.
Figure 3 graphically depicts typical operating
characteristics of the hydromechanical transmission within
one of the forward speed ranges. Curve 64 depicts the
increasing speed of the output carrier. The speed of the
orbit gear 44 increases at a higher rate as shown by curve
66. The sun gear 46, on the other hand, increases its speed
in a reverse direction relative to .he carrier as depicted
by curve 68. The increasing speed of the sun gear is,
_ g _ ::
': ', . ' ' : ` '
~6~232
however, limited to the drive ratio limit 70 by engagement
of the one-way clutch 58 as aforementioned. Thus, as the
output carrier continues to increase in speed, the speed of
the sun gear in the reverse direction decreases as indicated
by curve 72. The direct drive connection between the engine
10 and the orbit gear limits the speed of the orbit gear to
a maximum value corresponding to the curve portion 74. The
speed characteristic of the orbit gear is followed by that
of the fluid drive unit 52 as indicated by curve 76 in
Figure 3. Curve 78 shows the variation in speed of the
countershaft gear 60 beginning with engagement of the
overspeed control clutch 58. The fluid drive motor 50 being
coupled to the sun gear 46 will accordingly be limited in
its rotational speed as indicated by the curves 68 and 72.
The dotted line extension 80 of curve 72 shows the higher
speed to which the fluid drive unit 50 wQuld be driven if
not for the overspeed control clutch 58.
The automatic engagement of the overspeed control
clutch 58 and its beneficial action may be appreciated by
considering what occurs when a vehicle is at a standstill
and the input clutch 24 is suddenly engaged to begin rotation
of the fluid drive unit 52 through the countershaft gears 28
and 30. The orbit gear 44 of the planetary gear unit will
also be rotated in a forward direction at that instant.
Since the vehicle is at a standstill, the planet carrier
output 38 is initially stationary. To begin rotation of the
planet output carrier, a relatively high reaction torque
must be applied to the sun gear 46. Since the fluid drive
unit S0 does not have sufficient torque capacity to provide
the initially high reaction for the sun gear 46, its speed
-- 10 --
~L~363:)232
rapidly increases toward an overspeed condition. When the
speed reaches the desired speed llmit, determined by the
relative number of teeth of the meshing gears 60 and 62, the
one-way clutch 58 engages in order to establish a drive
connection between the sun gear 46 and the orbit gear 44
determined by the relative number of teeth of the gears 60,
62, 30, 28 and 34. Once the load on the output carrier is
overcome and the tendency for the sun gear 46 to overspeed
no longer exists, then the drive ratio of the 'trans~ission
may be controlled by the fluid drive units with the over-
speed control clutch 58 free-wheeling. The overspeed control
clutch 58 will, of course, also engage during other types of
overspeeding situations caused by a suddenly applied input
torque that is excessive relative to the rotating inertia of
the output carrier. The input clutch 24 is provided between
the engine and the transmission 18 to insure input rotation
at a minimum speed because the transmission does not have a
speed ratio range beginning with zero ground speed.
The alternative embodiment of the invention as
disclosed and to which the claims herein are directed is
depicted in Figure 4, wherein the tor~ue transmitting and
overspeed control de,vice of the transmission are arranged
differently, In this embodiment the drive gear 82 is in
continuous meshing engagement with the countershaft~gear 84
which driv~ the fluid arive unit 52 through countershaft
86. The countershaft gear 84 is also in continuous meshing
engagement with a gear 88 connected to orbit gear'44 through
which the positive, fixed ratio drive train is established
between power shaft 26 and the power planetary gear unit , '
36. By hydraulically reversing the rotation of the fluid drive
unit 50 the rotation of the sun gear 46 of the alternative
,
.
106()Z32
embodiment is opposite to the rotation of the sun gear 44
of the preferred embodiment. Since t~e gear 88 is indirectly
driven through gear 84, the rotation of the orbit gear 44 of
the alternative embodiment is also opposite to the rotation
of the orbit gear 44 of the preferred embodiment. The
reversal of these rotations results in the reversal of
rotation of the carrier 38, thereby reversing the rotation
of the output shaft 40.
These reversals of rotation allow the sec~nd
overrunning one-way clutch 58 to selectively establish a
driving connection between gears 90 and countershaft gear 92
under the regenerative power circulation mode of the trans-
mission. In the alternative embodiment the one-wav clutch
is mounted within gear 90 and in engagement with collar 94
- of the clutch 24. The gear 90 is in continuous mesh with
countershaft gear 92 connected to one end of the countershaft
48 opposite the sun gear 46. Through rearranging the torque
transmitting and overspeed control device of the transmission,
the speed ratio of countershafts 86 and 48 is limited by
the engagement of the one-way clutch 58 establishing through
gears 90 and 92 a driving connection between engine 10 and
the sun gear 46 at the desired speed ratio limit.
- 12 -
- '
