Slant plate type compressor with variable displacement mechanism

A slant plate type compressor with a variable displacement mechanism is disclosed. The compressor includes a drive mechanism having a drive shaft rotatably supported in a compressor housing and a coupling mechanism for drivingly coupling the drive shaft to pistons such that rotary motion of the drive shaft is converted into reciprocating motion of the pistons. The coupling mechanism includes a slant plate having an inclined surface. The slant angle changes in response to a change in pressure in the crank chamber and, thus, changes the capacity of the compressor. The drive shaft includes an inner end portion which has a diameter that is smaller than a diameter of the remainder of the drive shaft. A bias spring which has an outer diameter that is greater than a diameter of the remainder of the drive shaft is resiliently mounted on the inner end portion of the drive shaft between the slant plate and the cylinder block. The bias spring restores the slant plate back to its maximum slant angle when the slant angle is decreased below a predetermined angle without the bias spring interfering with the free pivoting motion of the slant plate between various inclination angles. Thereby, the impact forces which act on the internal component parts of the compressor when the compressor is started can be reduced, while at the same time the bias spring still can sufficiently urge the slant plate toward its maximum slant angle if the slant angle decreases below the predetermined slant angle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a refrigerant compressor and, 
more particularly, to a slant plate type compressor, such as a wobble 
plate type compressor, with a variable displacement mechanism suitable for 
use in an automotive air conditioning system. 
2. Description of the Prior Art 
A wobble plate type compressor with a variable displacement mechanism 
suitable for use in an automotive air conditioning system is disclosed in 
U.S. Pat. No. 4,960,366 to Higuchi, the disclosure of which is hereby 
incorporated by reference. As disclosed therein, the compression ratio of 
the compressor may be controlled by changing the slant angle of the 
inclined surface of the wobble plate. The slant angle of the inclined 
surface of the wobble plate and the slant plate on which it is disposed 
changes in response to a change in the crank chamber pressure relative to 
the suction chamber pressure. Changes in the crank chamber pressure are 
generated by a valve control mechanism which controls communication 
between the suction chamber and the crank chamber. 
The relevant part of the above-mentioned wobble plate type compressor is 
shown in FIGS. 1-3. Drive shaft 260 includes inner end portion 260a and 
intermediate portion 260b. Inner end portion 260a is rotatably supported 
by cylinder block 21 through bearing 31. The diameter of inner end portion 
260a is smaller than the diameter of intermediate portion 260b. Tapered 
ridge portion 260c is formed at the boundary between inner end portion 
260a and intermediate portion 260b of integrally formed drive shaft 260. 
Slant plate 50 includes opening 53 through which drive shaft 260 is 
disposed. Opening 53 of slant plate 50 has a configuration as disclosed in 
U.S. Pat. No. 4,846,049 to Terauchi, the disclosure of which is hereby 
incorporated by reference. Wobble plate 60 is nutatably mounted on hub 501 
of slant plate 50 such that slant plate 50 rotates with respect to wobble 
plate 60. Balance weight ring 80 which has a substantial mass is disposed 
on a nose of hub 501 of slant plate 50 in order to balance slant plate 50 
under dynamic operating conditions. Annular groove 502 is formed at an 
outer peripheral surface of the nose of hub 501. Balance weight ring 80 is 
held in place by means of retaining ring 81 which is firmly fixed in 
annular groove 502. 
Snap ring 330 is attached to inner end portion 260a, and is adjacent to 
intermediate portion 260b. Bias spring 340 is mounted on intermediate 
portion 260b, at a position between slant plate 50 and snap ring 330. One 
end (to the right in FIG. 1) of bias spring 340 is disposed about inner 
end portion 260a, adjacent to tapered ridge portion 260c. The inner 
diameter of the right end of bias spring 340 is smaller than the diameter 
of intermediate portion 260b. This right end of bias spring 340 is 
contained or sandwiched between tapered ridge portion 260c and snap ring 
330. Accordingly, axial movement of bias spring 340 along drive shaft 260 
is prevented. 
Annular depression 503 is formed at a rearward (to the right in FIG. 1), 
radially inner peripheral region of hub 501 of slant plate 50 so as to be 
able to receive bias spring 340 therewithin. Pillared hollow portion 504, 
which has a crescent-shaped lateral cross section, is formed at a rear (to 
the right in FIG. 1) end surface of one peripheral region of hub 501 of 
slant plate 50. An axis of pillared hollow portion 504 diagonally 
intersects with an axis of annular depression 503 so that the rear end 
surface of one peripheral region of hub 501 of slant plate 50 is archedly 
cut out as shown in FIG. 2. 
The non-tensioned length of bias spring 340 when no force acts thereon is 
selected such that the non-secured end of bias spring 340 does not contact 
any portion of the bottom surface of annular depression 503, so long as 
the slant angle of slant plate 50 is in a range between the maximum slant 
angle and a selected intermediate slant angle. However, slant plate 50 is 
urged towards the maximum slant angle by the restoring force of bias 
spring 340 if the slant angle of slant plate 50 decreases below the 
selected intermediate slant angle due to contact of the slant plate with 
the spring. When the slant angle of slant plate 50 is at a maximum, the 
compressor operates with maximum displacement. 
In operation, when the compressor is started, impact forces which act on 
the internal component parts of the compressor are generated. The 
magnitude of the impact forces is proportional to the slant angle of slant 
plate 50. Since slant plate 50 will very likely stay at or close to the 
selected intermediate slant angle when the compressor is stopped, the 
intermediate slant angle is selected to be a small percentage of the 
maximum slant angle, that is, the non-tensioned length of bias spring 340 
is selected to be small in order to reduce the magnitude of the impact 
forces which are generated when the compressor is restarted. 
However, the vacant space between the drive shaft and annular depression 
503 in which bias spring 340 is disposed, around intermediate portion 
260b, is limited to a small region because the diameter of intermediate 
portion 260b of drive shaft 260 is large. Therefore, the diameter of the 
body of bias spring 340 is limited to a small value and, thus, the modulus 
of elasticity of bias spring 340 is limited to a small value because the 
diameter of the body of bias spring 340 raised to the fourth power is 
proportional to the modulus of elasticity of bias spring 340. Accordingly, 
if the slant angle of slant plate 50 decreases below the selected 
intermediate slant angle, the restoring force of bias spring 340 may not 
sufficiently urge slant plate 50 back towards the maximum slant angle. 
Furthermore, pillared hollow portion 504 prevents bias spring 340 from 
interfering with hub 501 of slant plate 50 during the inclining motion of 
slant plate 50. However, the provision of pillared hollow portion 504 
decreases the mechanical strength of hub 501 because the thickness of hub 
501 is decreased in the one peripheral region where the hollow portion 504 
is located. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a variable 
capacity slant plate type compressor having a bias spring secured to the 
drive shaft which can sufficiently urge the slant plate back toward its 
maximum slant angle if the slant angle of the slant plate decreases below 
a selected intermediate slant angle, while at the same time providing for 
a reduction of the impact forces acting on the internal component parts of 
the compressor at the time when the compressor is started. 
It is another object of the present invention to provide a variable 
capacity slant plate type compressor having a bias spring secured to the 
drive shaft to urge the slant plate back towards its maximum slant angle 
without decreasing the strength of the hub of the slant plate, while at 
the same time eliminating any interference the bias spring may cause with 
the free pivoting motion of the slant plate between various inclination 
angles. 
A slant plate compressor in accordance with the present invention includes 
a compressor housing having a cylinder block with a front end plate and a 
rear end plate attached thereto. The front end plate encloses a crank 
chamber within the cylinder block, and a plurality of cylinders are formed 
in the cylinder block. A piston is slidably fitted within each of the 
cylinders. A drive mechanism is coupled to the pistons to reciprocate the 
pistons within the cylinders. The drive mechanism includes a drive shaft 
rotatably supported in the compressor housing, a rotor coupled to the 
drive shaft and rotatable therewith, and a coupling mechanism for 
drivingly coupling the rotor to the pistons such that rotary motion of the 
rotor is converted into reciprocating motion of the pistons within the 
cylinders. The coupling mechanism includes a slant plate having a surface 
disposed at a slant angle relative to a plane perpendicular to the drive 
shaft. The capacity of the compressor is varied as the slant angle 
changes. 
The rear end plate includes a suction chamber and a discharge chamber 
defined therein. A communication path through the cylinder block links the 
crank chamber with the suction chamber. A valve control mechanism controls 
the opening and closing of the communication path, thereby generating a 
change in the pressure in the crank chamber. The slant angle of the slant 
plate changes in response to changes in the crank chamber pressure 
relative to the suction chamber pressure. 
The drive shaft includes an inner end portion which has a diameter that is 
smaller than a diameter of the remainder of the drive shaft. A bias 
spring, which has an outer diameter greater than the diameter of the 
remainder of the drive shaft, is resiliently mounted on the inner end 
portion of the drive shaft between the slant plate and the cylinder block. 
The bias spring restores the slant plate back to its maximum slant angle 
when the slant angle is decreased below a predetermined angle without 
interfering with the free pivoting motion of the slant plate between 
various inclination angles. Thereby, the impact forces which act on the 
internal component parts of the compressor at the time when the compressor 
is started can be reduced, while the bias spring can still sufficiently 
urge the slant plate toward the maximum slant angle if the slant angle 
decreases below a predetermined angle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In all of FIGS. 4-8, identical reference numerals are used to denote 
elements which are identical to the similarly numbered elements shown in 
the prior art FIGS. 1-3. Additionally, although the present invention is 
described below in terms of a wobble plate type compressor, it is not 
limited in this respect. The present invention is broadly applicable to 
slant plate type compressors. Furthermore, for purposes of explanation 
only, the left side of FIGS. 4, 5, 7 and 8 will be referenced as the 
forward end or front and the right side of the drawings will be referenced 
as the rearward end or rear. The term "axial" refers to a direction 
parallel to the longitudinal axis of the drive shaft, and the term 
"radial" refers to the perpendicular direction. Of course, all of the 
reference directions are made for the sake of convenience of description 
and are not intended to limit the invention in any. 
With reference to FIG. 4, compressor 10 includes cylindrical housing 
assembly 20 including cylinder block 21, front end plate 23 disposed at 
one end of cylinder block 21, crank chamber 22 enclosed within cylinder 
block 21 by front end plate 23, and rear end plate 24 attached to the 
other end of cylinder block 21. Front end plate 23 is secured to one end 
of cylinder block 21 by a plurality of bolts 101. Rear end plate 24 is 
secured to the opposite end of cylinder block 21 by a plurality of bolts 
102. Valve plate 25 is disposed between rear end plate 24 and cylinder 
block 21. Opening 231 is centrally formed in front end plate 23 for 
supporting drive shaft 26. Drive shaft 26 is supported by bearing 30 
disposed in opening 231. 
With additional reference to FIG. 5, drive shaft 26 includes inner end 
portion 26a and intermediate portion 26b which is adjacent to inner end 
portion 26a. The diameter of intermediate portion 26b is greater than the 
diameter of inner end portion 26a. Annular ridge 26c is formed at the 
boundary between inner end portion 26a and intermediate portion 26b. 
Annular ridge 26c is located to the right of slant plate 50. Snap ring 33 
is firmly fixed in annular groove 26d formed at an outer peripheral 
surface of inner end portion 26a. Annular groove 26d is located at a 
position immediately to the left of the forward front surface of cylinder 
block 21. Inner end portion 26a of drive shaft 26 is divided into forward 
region 26a' and rearward region 26a" by snap ring 33. Bias spring 34, 
which has an inner diameter slightly greater than the diameter of inner 
end portion 26a and is smaller than the diameter of intermediate portion 
26b, is mounted on forward region 26a' of inner end portion 26a of drive 
shaft 26. Rearward region 26a" of inner end portion 26a of drive shaft 26 
is rotatably supported by bearing 31, disposed within central bore 210 of 
cylinder block 21. 
Bore 210 extends to a rear end surface of cylinder block 21 and houses 
valve control mechanism 19 which is described in detail in U.S. Pat. No. 
4,960,367 to Terauchi, the disclosure of which is hereby incorporated by 
reference. Bore 210 includes a threaded portion (not shown) formed at an 
inner peripheral surface of a central region thereof. Adjusting screw 220, 
having a hexagonal central hole 221, is screwed into the threaded portion 
of bore 210. Circular disc-shaped spacer 230 having central hole 231 is 
disposed between the inner end of drive shaft 26 and adjusting screw 220. 
Axial movement of adjusting screw 220 is transferred to drive shaft 26 
through spacer 230 so that all three elements move axially within bore 
210. The construction and functional manner of adjusting screw 220 and 
spacer 230 are described in detail in U.S. Pat. No. 4,948,343 to Shimizu, 
the disclosure of which is hereby incorporated by reference. 
Cam rotor 40 is fixed on drive shaft 26 by pin member 261 and rotates 
therewith. Thrust needle bearing 32 is disposed between the inner end 
surface of front end plate 23 and the adjacent axial end surface of cam 
rotor 40. Cam rotor 40 includes arm 41 having pin member 42 extending 
therefrom. Slant plate 50 is disposed adjacent cam rotor 40 and includes 
opening 53 through which drive shaft 26 passes. Slant plate 50 includes 
arm 51 having slot 52. Cam rotor 40 and slant plate 50 are coupled by pin 
member 42 which is inserted in slot 52 to form a hinged joint. Pin member 
42 slides within slot 52 to allow adjustment of the slant angle of slant 
plate 50, that is, the angle of the surface of slant plate 50 with respect 
to a plane perpendicular to the longitudinal axis of drive shaft 26. Slant 
plate 50 slides along drive shaft 26 in the direction towards rear end 
plate 24 as it pivots away from its shown maximum slant angle (in the 
direction of arrow "a" in FIG. 4). Thus, the pivot center of slant plate 
50 is shifted to the right along drive shaft 26 during pivoting from the 
maximum slant angle to a smaller slant angle. 
Wobble plate 60 is mounted on slant plate 50 through bearings 61 and 62 
such that slant plate 50 may rotate with respect thereto. Fork shaped 
slider 63 is attached to the outer peripheral end of wobble plate 60 and 
is slidably mounted on sliding rail 64 disposed between front end plate 23 
and cylinder block 21. Fork shaped slider 63 prevents rotation of wobble 
plate 60. Wobble plate 60 nutates along rail 64 when cam rotor 40 and 
slant plate 50 rotate. Cylinder block 21 includes a plurality of 
peripherally located cylinder chambers 70 in which pistons 71 reciprocate. 
Each piston 71 is coupled to wobble plate 60 by a corresponding connecting 
rod 72. 
Rear end plate 24 includes peripherally positioned annular suction chamber 
241 and centrally positioned discharge chamber 251. Valve plate 25 is 
located between cylinder block 21 and rear end plate 24 and includes a 
plurality of valved suction ports 242 linking suction chamber 241 with 
respective cylinders 70. Valve plate 25 also includes a plurality of 
valved discharge ports 252 linking discharge chamber 251 with respective 
cylinders 70. Suction ports 242 and discharge ports 252 are provided with 
suitable reed valves as described in U.S. Pat. No. 4,011,029 to Shimizu, 
the disclosure of which is hereby incorporated by reference. 
Suction chamber 241 includes inlet portion 241a which is connected to an 
evaporator of an external cooling circuit (not shown). Discharge chamber 
251 is provided with outlet portion 251a which is connected to a condenser 
of the cooling circuit (not shown). Gaskets 27 and 28 are positioned 
between cylinder block 21 and the inner surface of valve plate 25 and the 
outer surface of valve plate 25 and rear end plate 24, respectively. 
Gaskets 27 and 28 seal the mating surface of cylinder block 21, valve 
plate 25 and rear end plate 24. Gaskets 27 and 28 and valve plate 25 thus 
form valve plate assembly 200. 
Conduit 18 is axially bored through cylinder block 21 so as to link crank 
chamber 22 to discharge chamber 251 through hole 181 which is axially 
bored through valve plate assembly 200. A throttling device, such as 
orifice tube 182, is fixedly disposed within conduit 18. Filter member 183 
is disposed in conduit 18 at the rear of orifice tube 182. Accordingly, a 
portion of the discharged refrigerant gas in discharge chamber 251 always 
flows into crank chamber 22 at a reduced pressure generated by orifice 
tube 182. The above-mentioned construction and functional manner are 
described in detail in Japanese Patent Application Publication No. 
1-142277, the disclosure of which is hereby incorporated by reference. 
Communication path 400 links crank chamber 22 and suction chamber 241 and 
includes central bore 210 and passageway 150. Valve control mechanism 19 
controls the opening and closing of communication path 400 in order to 
vary the capacity of the compressor. 
During operation of compressor 10, drive shaft 26 is rotated by the engine 
of the vehicle (not shown) through electromagnetic clutch 300. Cam rotor 
40 rotates with drive shaft 26, causing slant plate 50 to rotate as well. 
The rotation of slant plate 50 causes wobble plate 60 to nutate. The 
nutating motion of wobble plate 60 reciprocates pistons 71 in their 
respective cylinders 70. As pistons 71 are reciprocated, refrigerant gas, 
introduced into suction chamber 241 through inlet portion 241a, is drawn 
into cylinders 70 through suction ports 242 and subsequently compressed. 
The compressed refrigerant gas is discharged from cylinders 70 into 
discharge chamber 251 through respective discharge ports 252 and then into 
the cooling circuit through outlet portion 251a. 
Some of the partially compressed refrigerant gas in cylinders 70 is blown 
into crank chamber 22 from cylinders 70 through gaps between respective 
pistons 71 and cylinders 70 during the compression stroke of pistons 71. 
This gas is known as blow-by gas. In addition, a portion of the discharged 
refrigerant gas in discharge chamber 251 always flows into crank chamber 
22 with a reduced pressure generated by orifice tube 182. Valve control 
mechanism 19 includes bellows 19a which expands or contracts in response 
to the crank chamber pressure. When the pressure in crank chamber 22 
exceeds a predetermined value, which is determined by appropriately 
designing valve control mechanism 19, communication path 400 is opened due 
to contraction of bellows 19a of valve control mechanism 19. Thereafter, 
crank chamber 22 is linked to suction chamber 241. Accordingly, the 
pressure in crank chamber 22 decreases to the pressure in suction chamber 
241. However, if the pressure in crank chamber 22 decreases below the 
predetermined value, communication path 400 is blocked by expansion of 
bellows 19a of valve control mechanism 19 so that the communication 
between crank chamber 22 and suction chamber 241 is prevented. 
Accordingly, the pressure in crank chamber 22 gradually increases due to 
the partially compressed (blow-by) refrigerant gas from cylinders 70. 
Thus, the pressure level in crank chamber 22 is controlled by valve 
control mechanism 19. 
With reference to FIGS. 5 and 6, a first embodiment of the present 
invention will be described in detail. The non-tensioned length of bias 
spring 34 when no force acts thereon is greater than the axial length of 
forward region 26a' of inner end portion 26a of drive shaft 26. Therefore, 
bias spring 34 is resiliently sandwiched between snap ring 33 and annular 
ridge 26c. The axial length of forward region 26a' of inner end portion 
26a of drive shaft 26 is selected such that the left side of bias spring 
34 does not contact any portion of the bottom surface of annular 
depression 503, so long as the slant angle of slant plate 50 is in a range 
between the maximum slant angle and a selected intermediate slant angle. 
However, if the slant angle of slant plate 50 decreases below the selected 
intermediate slant angle with a corresponding sliding of slant plate 50 to 
the right along drive shaft 26, the bottom surface of annular depression 
503 contacts and compresses bias spring 34. Therefore, slant plate 50 is 
urged back toward its maximum slant angle by the restoring force of bias 
spring 34. The configuration and material of snap ring 33 are selected so 
as to sufficiently resist the reaction force generated by the compression 
of bias spring 34 by slant plate 50 when slant plate 50 assumes its 
minimum slant angle. 
The radius of the body of bias spring 34 is designed to be generally equal 
to the height of annular ridge 26c. Therefore, the overall outer diameter 
of bias spring 34 is greater than the diameter of intermediate portion 26b 
of drive shaft 26 by the approximate length of the diameter of the body of 
bias spring 34. Accordingly, an outer half of the body of bias spring 34 
protrudes from the outer periphery of intermediate portion 26b of drive 
shaft 26. 
The assembly process of the first embodiment is as follows. Inner end 
portion 26a of drive shaft 26 is held adjacent to the left end of bias 
spring 34, and drive shaft 26 is inserted through bias spring 34 until the 
left end of bias spring 34 contacts annular ridge 26c of drive shaft 26. 
Snap ring 33 is firmly fixed in annular groove 26d while bias spring 34 is 
compressed so that bias spring 34 is resiliently sandwiched in between 
annular ridge 26c and snap ring 33. 
In operation, the pressure in crank chamber 22 gradually increases due to 
the partially compressed (blow-by) refrigerant gas from cylinders 70. A 
change in the pressure in crank chamber 22 relative to suction chamber 24, 
generates a corresponding change in the slant angle of both slant plate 50 
and wobble plate 60 so as to change the stroke length of pistons 71 in 
cylinders 70 and, thus, vary the displacement of compressor 10. If the 
slant angle of slant plate 50 decreases below the selected intermediate 
slant angle with a corresponding sliding of slant plate 50 to the right 
along drive shaft 26, slant plate 50 compresses spring 34. Thus slant 
plate 50 is urged back towards the maximum slant angle by the restoring 
force of bias spring 34. 
As described above, in the present invention, the vacant space for 
disposing bias spring 34 around drive shaft 26 can be increased in 
comparison with the prior art by disposing bias spring 34 around forward 
region 26a' of inner end portion 26a which has a diameter smaller than the 
diameter of intermediate portion 26b. Therefore, even though an 
intermediate slant angle is selected that is smaller than prior art 
intermediate slant angles so that the magnitude of the impact forces 
generated when the compressor is started is reduced, slant plate 50 can 
still be sufficiently urged toward its maximum slant angle by the 
restoring force of bias spring 34 when the slant angle of slant plate 50 
decreases below the selected intermediate slant angle. In addition, since 
bias spring 34 is initially compressed, slant plate 50 can be sufficiently 
urged back to its maximum slant angle at the initial contact between the 
left side of bias spring 34 and slant plate 50. 
Furthermore, the decrease in the mechanical strength of hub 501 of slant 
plate 50 can be prevented because the pillared hollow portion as described 
in the prior art is not required to prevent the bias spring from 
interfering with the free pivoting motion of slant plate 50 between 
various inclination angles. 
With reference to FIG. 7, a second embodiment of this invention is shown. 
In FIG. 7, the same numerals are used to denote elements which are 
identical to the similarly numbered elements shown in FIG. 5 so that an 
explanation thereof is omitted. In this second embodiment, annular ring 
member 35 is disposed around forward region 26a' of inner end portion 26a 
of drive shaft 26 between annular ridge 26c and the left side of bias 
spring 34. An inner diameter of annular ring member 35 is slightly greater 
than the diameter of inner end portion 26a of drive shaft 26 so that 
annular ring member 35 may move axially along forward region 26a' of drive 
shaft 26. An outer diameter of annular ring member 35 is generally equal 
to the overall diameter of bias ring 34. Therefore, when the slant angle 
of slant plate 26 decreases below the selected intermediate slant angle 
and slant plate 50 slides to the right along drive shaft 50, the bottom 
surface of annular depression 503 compresses bias spring 34 through 
annular ring member 35. Accordingly, bias spring 34 is more effectively 
compressed by slant plate 50 when the slant angle of slant plate 50 
decreases below the selected intermediate slant angle because of contact 
between the plain surfaces. In addition, the left side of bias spring 34 
is more firmly received by annular ring member 35 is comparison with 
annular ridge 26c. 
The assembling process of the second embodiment is as follows. Inner end 
portion 26a of drive shaft 26 is held adjacent to annular ring member 35 
and the left end of bias spring 34. Drive shaft 26 is then inserted 
through annular ring member 35 and bias spring 34 until annular ring 
member 35 contacts annular ridge 26c of drive shaft 26. Snap ring 33 is 
then firmly fixed in annular groove 26d while bias spring 34 is compressed 
so that bias spring 34 is resiliently sandwiched in between annular ring 
member 35 and snap ring 33. 
With reference to FIG. 8, a third embodiment of this invention is shown. In 
FIG. 8, the same numerals are used to denote elements which are identical 
to similarly numbered elements shown in FIG. 5 so that explanation thereof 
is omitted. In this embodiment, bias spring 341 is disposed in an 
uncompressed state on inner portion 26a. Forward region 26a' of inner end 
portion 26a and annular ridge 26c are extended more towards slant plate 50 
than in the previous embodiments. Bias spring 341 has a non-tensioned 
length "d.sub.2 " which is equal to the length "d.sub.1 " of forward 
region 26a' in FIG. 5. Thus, bias spring 341 in FIG. 8 will urge slant 
plate 50 towards its maximum slant angle after the slant angle of slant 
plate 50 decreases below the selected intermediate slant angle and slant 
plate 50 has shifted to the right along drive shaft 26. Bias spring 341 
has an overall inside diameter along its right end that is slightly 
smaller than the diameter of inner end portion 26a, and the right end of 
bias spring 341 is located so as to be in contact with the left side 
surface of snap ring 33. Thus, bias spring 34 is prevented from axial 
movement along drive shaft 26. This embodiment allows the overall diameter 
of the body of bias spring 341 to be larger than the diameter of the body 
of prior art springs because of the increased space created above smaller 
diameter inner end portion 26a. Additionally, slant plate 50 is urged 
toward its maximum slant angle without bias spring 341 interfering with 
hub 501 of slant plate 50 when slant plate 50 pivots between various 
inclination angles. 
In the present invention, even though drive shaft 26 includes inner end 
portion 26a which has a diameter that is smaller than the diameter of 
intermediate portion 26b in order to allow bias spring 34 to be disposed 
around forward region 26a' of inner end portion 26a, the decrease in the 
mechanical strength of drive shaft 26 is negligible. 
This invention has been described in connection with the preferred 
embodiments. These embodiments, however, are merely for example only and 
the invention is not restricted thereto. For example, the terms right and 
left are used merely for convenience of description, and the invention is 
not restricted in this manner. It will be understood by those skilled in 
the art that other variations and modifications of this invention can 
easily be made within the scope of this invention as defined by the claims 
.