Shift control system for continuously variable traction roller transmission

In a shift control system for a continuously variable traction roller transmission, a shift control valve includes a stepper motor which is driven in response to a commanded speed ration. The stepper motor drives a pinion which engages a rack. The rack is connected to a spool disposed in a sleeve. The relative position between the spool and sleeve determines the hydraulic pressure applied to change the speed ratio of the transmission. The sleeve is moved by shift members while the spool is moved by the stepper motor. The force necessary to move the spool is less than in other designs, thereby allowing for the use of a smaller stepper motor.

BACKGROUND OF THE INVENTION 
The present invention relates to a shift control system for a continuously 
variable traction roller transmission. 
A shift control system for a continuously variable traction roller 
transmission is disclosed, for example, in JP 2-131945. This shift control 
system comprises a shift valve including a stepper motor having a 
rotational position determined in accordance with a commanded speed ratio, 
a sleeve axially movable by the stepper motor, and a spool slidably fitted 
in the sleeve. The spool is axially driven to correspond to an actual 
speed ratio via a cam, a link, etc. When the actual speed ratio 
corresponds to the speed ratio commanded to the stepper motor, the 
relative positional relationship between the sleeve and the spool falls in 
a normal state, supplying hydraulic pressure to a hydraulic cylinder 
apparatus so as to maintain the speed ratio at that moment. On the other 
hand, when the relative positional relationship between the sleeve and the 
spool is out of the normal state, hydraulic pressure is supplied to the 
hydraulic cylinder apparatus so that the actual speed ratio corresponds to 
the commanded speed ratio. 
With the aforementioned shift control system for a continuously variable 
traction roller transmission, however, there is a disadvantage. The 
disadvantage is due to generation of friction resistance on both outer and 
inner peripheries of the sleeve when moving, thereby requiring a great 
operating force to displace the sleeve. Particularly, since the viscosity 
of hydraulic fluid increases when the temperature is extremely low, this 
disadvantage becomes striking at low temperature. Thus, the stepper motor 
for moving the sleeve must have great torque, resulting in an increased 
size of the stepper motor. 
It is, therefore, an object of the present invention to provide a shift 
control system for a continuously variable traction roller transmission 
which can be operated by a stepper motor having small torque. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, there is provided a shift 
control system for a continuously variable traction roller transmission, 
the continuously variable traction roller transmission including input and 
output disks, a pair of roller support members for the pair of traction 
rollers, each having rotation shanks and being rotatable therewith and 
movable in an axial direction thereof, and a hydraulic cylinder apparatus 
arranged to move each of the pair of roller support members in the axial 
direction of the rotation shanks, the shift control system comprising: 
a cam mounted to one of the pair of roller support members for unitary 
rotation thereof; 
a link arranged to be swingable with a rotation of said cam, said link 
having one end being in contact with said cam; and 
control valve means for adjusting a hydraulic pressure to be supplied to 
the hydraulic cylinder apparatus, said control valve means including a 
sleeve, a spool fitted therein, a stepper motor having a rotational 
position determined in response to a speed ratio as commanded, and a drive 
member driven by said stepper motor in an axial direction of said shift 
control valve means, 
said sleeve being connected to the other end of said link so as to be 
movable in accordance with a rotational displacement of the rotation 
shanks in said axial direction of said shift control means, said spool 
being connected to said drive member so as to be movable therewith in said 
axial direction of said shift control valve means, 
wherein when said rotational displacement of the rotation shanks 
corresponds to said speed ratio, a relative positional relationship 
between said sleeve and said spool falls in a normal state wherein said 
hydraulic pressure is adjusted to provide a force to the rotation shanks 
in the axial direction of the rotation shanks so as to maintain said 
rotational displacement of the rotation shanks, 
wherein when said relative positional relationship is out of said normal 
state, said hydraulic pressure is adjusted to change said force so that 
said rotational displacement of the rotation shanks corresponds to said 
speed ratio. 
According to another aspect of the present invention, there is provided, in 
a continuously variable traction roller transmission: 
input and output disks; 
a pair of traction rollers arranged between a toroidal concavity defined by 
said input and output discs for frictional engagement therewith; 
a pair of roller support members for said pair of traction rollers, each 
having rotation shanks and being rotatable therewith and movable in an 
axial direction thereof; 
a hydraulic cylinder apparatus arranged to move each of said pair of roller 
support members in said axial direction of said rotation shanks; 
a cam mounted to one of said pair of roller support members for unitary 
rotation thereof; 
a link arranged to be swingable with a rotation of said cam, said link 
having one end being in contact with said cam; and 
control valve means for adjusting a hydraulic pressure to be supplied to 
said hydraulic cylinder apparatus, said control valve means including a 
sleeve, a spool fitted therein, a stepper motor having a rotational 
position determined in response to a speed ratio as commanded, and a drive 
member driven by said stepper motor in an axial direction of said shift 
control valve means, 
said sleeve being connected to the other end of said link so as to be 
movable in accordance with a rotational displacement of said rotation 
shanks in said axial direction of said shift control means, said spool 
being connected to said drive member so as to be movable therewith in said 
axial direction of said shift control valve means, 
wherein when said rotational displacement of said rotation shanks 
corresponds to said speed ratio, a relative positional relationship 
between said sleeve and said spool falls in a normal state wherein said 
hydraulic pressure is adjusted to provide a force to said rotation shanks 
in said axial direction of said rotation shanks so as to maintain said 
rotational displacement of said rotation shanks, 
wherein when said relative positional relationship is out of said normal 
state, said hydraulic pressure is adjusted to change said force so that 
said rotational displacement of said rotation shanks corresponds to said 
speed ratio.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 show a preferred embodiment of the present invention. 
Referring particularly to FIG. 1, first and second continuously variable 
transmission units 22, 24 are arranged in a casing 10. The first 
continuously variable transmission unit 22 comprises an input disk 26, an 
output disk 28, and a pair of traction rollers 30, 30' for transmitting 
torque between the two. The input and output disks 26, 28 have toroidal 
faces, respectively, which serve as contact faces with the pair of 
traction rollers 30, 30'. The ratio of rotational speed of the input disk 
26 to the output disk 28 can be continuously altered by changing the 
contact condition of the pair of traction rollers 30, 30' relative to the 
input and output disks 26, 28. In a manner similar to the first 
continuously variable transmission unit 22, the second continuously 
variable transmission unit 24 comprises an input disk 32, an output disk 
34, and a pair of traction rollers 36, 36'. It is to be noted that the 
arrangement of the input and output disks 32, 34 is opposite to that of 
the input and output disks 26, 28 of the first continuously variable 
transmission unit 22. That is, the output disks 28, 34 are arranged to be 
adjacent to each other. The input disk 26 is supported on an input shaft 
38 at the outer periphery thereof via a ball spline. The input shaft 38 is 
connected to a forward and reverse switching mechanism and a torque 
converter (not shown) so as to receive engine torque via the two. A cam 
flange 42 is disposed on the back side of the input disk 26. A cam roller 
46 is disposed on cam faces of the cam flange 42 and the input disk 26 
which face each other. The cam roller 46 is so shaped as to generate force 
to press the input disk 26 toward the output disk 28 upon relative 
rotation of the input disk 26 and the cam flange 42. Likewise, the input 
disk 32 of the second continuously variable transmission unit 24 is 
connected to the input shaft 38 via a ball spline. The input disk 32 
undergoes force in the direction of the output disk 34 from a dish plate 
51 which receives compression from a loading nut 50 engaged with the input 
shaft 38. The output disks 28, 34 of the first and second continuously 
variable transmission units 22, 24 are rotatably supported on the input 
shaft 38 via needle bearings, respectively. A drive gear 55 is arranged 
for unitary rotation with the output disks 28, 34. The drive gear 55 is 
engaged with a follower gear 60 connected, via a spline for unitary 
rotation, to a countershaft 59 at one end thereof which is disposed in 
parallel with the input shaft 38. Integrally formed with the countershaft 
59 at the other end thereof is a gear 61 which is engaged via an idler 
gear (not shown) with a gear 63 integrated with an output shaft 62. 
FIG. 2 shows a fragmentary section of the first continuously variable 
transmission unit 22. It is to be noted that the structure of the second 
continuously variable transmission unit 24 is generally identical to that 
of the first continuously variable transmission unit 22 as shown in FIG. 
2. Referring to FIG. 2, particularly to a right half thereof, a roller 
support member 83 is rotatably and vertically movably supported on upper 
and lower rotation shanks 83a, 83b via spherical bearings 110, 112. The 
traction roller 30 is rotatably supported on the roller support member 83 
via an eccentric shaft 84. The spherical bearing 110 is supported by a 
link 114 which is in turn supported by a link post 116 fixed to the casing 
10. Likewise, the spherical bearing 112 is supported by a link 118 which 
is in turn supported by a link post 120. The roller support member 83 
comprises an extension shank 83c disposed to be concentric with the 
rotation shank 83b. The extension shank 83c is so constructed as to be 
rotatable with the rotation shank 83b. A piston 124 is arranged at the 
outer periphery of the extension shank 83c. The piston 124 is fitted in a 
piston insertion hole 304 which is formed in a main cylinder body 302a 
mounted to the casing 10 by a bolt 300. An auxiliary cylinder body 302b is 
mounted to the main cylinder body 302a on the lower side thereof by the 
bolt 300 via a separate plate 306. The main and auxiliary cylinder body 
302a, 302b constitute the cylinder body 302. Thus, hydraulic chambers 128, 
130 are formed above and below the piston 124. It is to be noted that 
hydraulic chambers 128', 130' on the left in FIG. 2 are vertically opposed 
to the hydraulic chambers 128, 130. The piston 124 is vertically movable 
by hydraulic pressure operating therein. The piston 124 and the piston 
insertion hole 304 of the main cylinder body 302a constitute a hydraulic 
cylinder apparatus. 
A valve body 310 is disposed below the cylinder body 302. The valve body 
310 comprises a main valve body 310a and an auxiliary valve body 310b 
mounted to the main valve body 310a on the upper side thereof via a 
separate plate 311. A shift control valve 410 is disposed on the main 
valve body 310a. The shift control valve 410 comprises a stepper motor 412 
driven in response to a speed ratio as commanded, a spool (drive member) 
414 with rack having teeth engaged with a pinion 412a which is driven by 
the stepper motor 412 and being axially movable by rotation thereof, a 
spool 416 having one end connected to the spool 414 and being axially 
movable therewith by rotation of the stepper motor 412, a sleeve 418 
arranged on the outer periphery of the spool 416, a spring 419 for biasing 
the sleeve 418 to the left as viewed in FIG. 2, and a retainer 420 fitted 
in the sleeve 418 at the outer end thereof. The main valve body 310a has 
hydraulic passages 422, 424. The hydraulic passage 422 is connected to the 
hydraulic chambers 128, 128', whereas the hydraulic passage 424 is 
connected to the hydraulic chambers 130, 130'. Line pressure within a 
hydraulic passage 423 serves as a hydraulic source, which is distributed 
to the hydraulic passages 422, 424 in accordance with the relative 
positional relationship between the spool 416 and the sleeve 418. 
Specifically, the relationship between a land of the spool 416 and an oil 
groove of the sleeve 418 is so established that hydraulic pressure within 
the hydraulic passages 422 is equal to hydraulic pressure within the 
hydraulic passages 424 in a normal state as shown in FIG. 2, and that 
hydraulic pressure within the hydraulic passage 424 is higher than that 
within the hydraulic passage 422 when the spool 416 is moved relatively to 
the left as viewed in FIG. 2, whereas hydraulic pressure within the 
hydraulic passage 424 is lower than that within the hydraulic passage 422 
when the spool 416 is moved relatively to the right as viewed in FIG. 2. 
Mounted to the extension shank 83c' at the lower end thereof is a cam 320 
which is rotatable with the extension shank 83c', and has a bevel with 
which a link 322 is in contact. Thus, with rotation of the cam 320, the 
link 322 is swung so that a point thereof can press the retainer 420. 
Next, the operation of this embodiment will be described. With an increase 
in rotation of the input shaft 38, the input disk 26 is rotated to follow 
the cam flange 42 due to operation of the cam roller 46, and generates at 
the same time thrust corresponding to input torque of the input shaft 38. 
Thus, the traction rollers 30, 30' are rotated without slippage in holding 
between the input and output disks 26, 28, transmitting power from the 
input disk 26 to the output disk 28. When altering the speed ratio on the 
large side, for example, the spool 414 with rack is moved to the left as 
viewed in FIG. 2 by the stepper motor 412 so as to move the spool 416 to 
the left as viewed in FIG. 2. Since the sleeve 418 is not moved 
immediately, the relative relationship between the spool 416 and the 
sleeve 418 is changed to increase hydraulic pressure within the hydraulic 
passage 424 and decrease pressure within the hydraulic passage 422. Since 
hydraulic pressure within the hydraulic passage 422 is supplied to the 
hydraulic chamber 128, and pressure within the hydraulic passage 424 is 
supplied to the hydraulic chamber 130, the piston 124 undergoes force for 
upward motion. On the other hand, since the hydraulic chambers 128', 130' 
are arranged to be vertically opposed to the hydraulic chambers 128, 130, 
the piston 124' undergoes force for downward motion. Thus, the roller 
support member 83 is urged to move upwardly, whereas the roller support 
member 83' is urged to move downwardly. Since this causes a change in the 
direction of force which operates on the traction rollers 30, 30' in the 
tangent direction thereof, the roller support members 83, 83' are rotated 
inversely with the rotation shanks 83a, 83b and 83a', 83b', respectively. 
Thus, the radius of each of the traction rollers 30, 30' at a contact 
point thereof with the input disk 26 is decreased, whereas the radius of 
each of the traction rollers 30, 30' at a contact point thereof with the 
output disk 28 is increased. That is, the speed ratio is altered on the 
large side. Rotation of the roller support member 83' is transmitted to 
the cam 320 via the extension shank 83c'. With rotation of the cam 320, 
the link 322 is swung so that the point thereof is moved to the left as 
viewed in FIG. 2. Thus, the sleeve 418 is urged to move to the left as 
viewed in FIG. 2 by the spring 419. As the sleeve 418 is moved to the left 
as viewed in FIG. 2, a level of hydraulic pressure within the hydraulic 
passage 422 and a level of hydraulic pressure within the hydraulic passage 
424 come close to each other. Finally, the sleeve 418 is stabilized in the 
normal state in which the two levels are the same. 
When altering the speed ratio on the small side, the operation is generally 
identical as described above, however, it is carried out inversely since 
the stepper motor 412 is rotated in the opposite direction. Accordingly, 
since the direction of torque transmitted to the cam 320 via the extension 
shank 83c' also becomes inverse, the point of the link 322 is urged to 
move to the right as viewed in FIG. 2 so as to press the retainer 420 to 
the right as viewed in FIG. 2. Thus, the sleeve 418 is moved to the right 
as viewed in FIG. 2, and stabilized when a level of hydraulic pressure 
within the hydraulic passage 422 becomes identical to a level of hydraulic 
pressure within the hydraulic passage 424. Having described the shift 
operation with regard to the first continuously variable transmission unit 
22, it is to be noted that the same shift operation is carried out with 
regard to the second continuously variable transmission unit 24. 
As described above, the spool 416 of the shift control valve 410 is driven 
by the stepper motor 412 via the spool 414 with rack. As to the spool 416, 
the outer diameter portion thereof is fitted in the inner diameter portion 
of the sleeve 418, generating friction resistance only on this outer 
diameter portion of the spool 416 when moving axially. On the other hand, 
as to the sleeve 418, not only the inner diameter portion thereof is 
received in the spool 416, but the outer diameter portion thereof is 
fitted in a hole of the main valve body 310a, generating friction 
resistance on both inner and outer diameter portions of the sleeve 418. 
Accordingly, the force necessary to drive the spool 416 axially is smaller 
than that necessary to drive the sleeve 418 axially, resulting in a 
decreased size of the stepper motor 412.