U.S. patent application number 11/955643 was filed with the patent office on 2008-06-26 for power transmission device of a compressor.
This patent application is currently assigned to Kabushiki Kaisha Toyota Jidoshokki. Invention is credited to Fuminobu Enokijima, Tetsuhiko Fukanuma, Naoya Yokomachi.
Application Number | 20080152511 11/955643 |
Document ID | / |
Family ID | 39256989 |
Filed Date | 2008-06-26 |
United States Patent
Application |
20080152511 |
Kind Code |
A1 |
Enokijima; Fuminobu ; et
al. |
June 26, 2008 |
POWER TRANSMISSION DEVICE OF A COMPRESSOR
Abstract
A power transmission device of a compressor includes a rotary
shaft, a pulley, a hub, a power shutoff member, a spacer and a
cylinder. The power shutoff member shuts off excessive torque
transmission between the pulley and the rotary shaft of the
compressor. The spacer is disposed on the rotary shaft. The
cylinder is disposed between the power shutoff member and the
rotary shaft. The cylinder has an external thread portion formed at
the outer periphery thereof for engagement with an internal thread
portion of the power shutoff member and an internal thread portion
formed at the inner periphery thereof for engagement with an
external thread portion of the rotary shaft. The cylinder has a
contacting surface for contact with a seating surface of the spacer
and a seating surface for contact with a contacting surface of the
hub at the axial end portion of the hub.
Inventors: |
Enokijima; Fuminobu;
(Kariya-shi, JP) ; Yokomachi; Naoya; (Kariya-shi,
JP) ; Fukanuma; Tetsuhiko; (Kariya-shi, JP) |
Correspondence
Address: |
WOODCOCK WASHBURN LLP
CIRA CENTRE, 12TH FLOOR, 2929 ARCH STREET
PHILADELPHIA
PA
19104-2891
US
|
Assignee: |
Kabushiki Kaisha Toyota
Jidoshokki
Kariya-shi
JP
|
Family ID: |
39256989 |
Appl. No.: |
11/955643 |
Filed: |
December 13, 2007 |
Current U.S.
Class: |
417/362 |
Current CPC
Class: |
F16H 35/10 20130101;
F16H 55/36 20130101; F16D 9/08 20130101 |
Class at
Publication: |
417/362 |
International
Class: |
F04B 35/00 20060101
F04B035/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2006 |
JP |
2006-337229 |
Claims
1. A power transmission device of a compressor comprising: a rotary
shaft rotatably supported by a casing of the compressor, the rotary
shaft having an external thread portion; a pulley rotatably mounted
on the casing; a hub connected to the pulley for rotation
therewith; a power shutoff member joined to the hub, the power
shutoff member being interposed between the hub and the rotary
shaft, the power shutoff member having an internal thread portion,
the power shutoff member shutting off excessive torque transmission
between the pulley and the rotary shaft; a spacer disposed on the
rotary shaft, the spacer having a seating surface; and a cylinder
disposed between the power shutoff member and the rotary shaft, the
cylinder having an external thread portion formed at the outer
periphery thereof for engagement with the internal thread portion
of the power shutoff member and an internal thread portion formed
at the inner periphery thereof for engagement with the external
thread portion of the rotary shaft, the cylinder having a
contacting surface for contact with the seating surface of the
spacer and a seating surface for contact with a contacting surface
of the hub at the axial end portion of the hub.
2. The power transmission device of the compressor according to the
claim 1, wherein the radial distances from the axis of the rotary
shaft to the contacting surface of the cylinder for contact with
the seating surface of the spacer and from the axis of the rotary
shaft to the contacting surface of the hub for contact with the
seating surface of the cylinder are different each other.
3. The power transmission device of the compressor according to the
claim 2, wherein the radial distance from the axis of the rotary
shaft to the contacting surface of the hub is smaller than that
from the axis of the rotary shaft to the contacting surface of the
cylinder.
4. The power transmission device of the compressor according to the
claim 1, wherein at least either the coefficients of friction
between the power shutoff member and the cylinder and between the
cylinder and the rotary shaft are different or the coefficients of
friction between the contacting surface of the hub and the seating
surface of the cylinder and between the contacting surface of the
cylinder and the seating surface of the spacer are different.
5. The power transmission device of the compressor according to the
claim 4, wherein the coefficient of friction between the power
shutoff member and the cylinder is smaller than the coefficient of
friction between the cylinder and the rotary shaft, and the
coefficient of friction between the contacting surface of the hub
and the seating surface of the cylinder is smaller than the
coefficient of friction between the contacting surface of the
cylinder and the seating surface of the spacer.
6. The power transmission device of the compressor according to the
claim 5, wherein lubricant is applied between the power shutoff
member, and the cylinder and between the contacting surface of the
hub and the seating surface of the cylinder.
7. The power transmission device of the compressor according to the
claim 4, wherein the coefficient of friction between the power
shutoff member and the cylinder is larger than the coefficient of
friction between the cylinder and the rotary shaft, and the
coefficient of friction between the contacting surface of the hub
and the seating surface of the cylinder is larger than the
coefficient of friction between the contacting surface of the
cylinder and the seating surface of the spacer.
8. The power transmission device of the compressor according to the
claim 7, wherein lubricant is applied between the cylinder and the
rotary shaft and between the contacting surface of the cylinder and
the seating surface of the spacer.
9. The power transmission device of the compressor according to the
claim 4, wherein the power shutoff member and the hub are made of a
material which has a different coefficient of friction from the
coefficient of friction of the rotary shaft and the cylinder.
10. The power transmission device of the compressor according to
the claim 1, wherein the carbon dioxide is used as a refrigerant
gas in the compressor.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No. 2006-337229
filed on Dec. 14, 2006, the entire contents of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to a power
transmission device having a torque limiting function, and more
particularly to a power transmission device of a compressor for a
vehicle air conditioner which is driven by an external drive source
such as an engine through a belt.
BACKGROUND OF THE INVENTION
[0003] A power transmission device for transmitting power to a
compressor has a torque limiter or a power interrupting device for
forestalling a trouble such as a breakage of a power transmission
belt in the event of a seizure in the compressor. A conventional
power transmission device which is disclosed, for example, in
Japanese Patent Application Publication No. 2006-266483, has a
pulley, a casing, a hub and a power shutoff member (as described in
pages 4-6 and FIGS. 1-4 of the publication). To be more specific,
the pulley is rotatably mounted to the casing, the hub is fitted in
the pulley and the power shutoff member as a torque limiter is
interposed between the hub and a rotary shaft. The front end
portion of the rotary shaft has a thread portion, a large-diameter
shaft portion and a shaft-side seating face. The thread portion has
a thread on the outer circumference thereof. The diameter of the
large-diameter shaft portion is larger than that of the thread
portion. A shaft-side seating face is formed as a stepped portion
between the thread portion and the large-diameter shaft portion. A
washer is provided in contact with the shaft-side seating face. The
casing and the rotary shaft are sealed by a shaft seal member (not
shown) to prevent refrigerant gas and oil from leaking out of the
compressor.
[0004] The power shutoff member functioning as a torque limiter is
formed by a cylindrical body which includes a large-cylindrical
portion and a small-cylindrical portion. The large-cylindrical
portion is engaged with a part of the hub. The small-cylindrical
portion has in the inner periphery thereof a thread portion which
is engaged with the thread portion of the rotary shaft. The inner
diameter of the large-cylindrical portion is slightly larger than
that of the small-cylindrical portion and a cutout is formed at the
transition between the inner peripheries of the large and
small-cylindrical portions so as to form a breakable portion.
[0005] A torque produced in accordance with the rotation of the
pulley is transmitted to the power shutoff member through the hub
and then to the rotary shaft. If the seizure is occurred in the
compressor, an excessively high axial force is applied to the
engagement portion between the thread portion of the power shutoff
member and the thread portion of the rotary shaft, so that the
breakable portion of the power shutoff member is broken. Therefore,
the torque transmission from the pulley is shut off, thereby
preventing a belt connecting the pulley to the power source from
being broken.
[0006] However, in the conventional power transmitting device
disclosed in Japanese Patent Application Publication 2006-266483,
heat is generated by sliding contact between the rotary shaft and
the shaft seal member which is mounted on the rotary shaft of the
compressor. The amount of the heat generated by the sliding contact
is proportional to the coefficient of friction of the slide contact
surfaces between the rotary shaft and the shaft seal member, load
acting on the rotary shaft from the shaft seal member and the
rotational speed of the rotary shaft. However, when the load is
increased to enhance the seal performance of the shaft seal member
or the load is increased due to the increase of the internal
pressure of the compressor, the peripheral speed of the rotary
shaft needs to be reduced so as to reduce the amount of the heat
generated by the slide contact between the rotary shaft and the
shaft seal member. The rotary shaft needs to be produced with a
reduced diameter for reducing its peripheral speed. However, if the
diameter of the rotary shaft is reduced, another problem may
result. That is, reducing the diameter of the rotary shaft in the
power transmission device having the power shutoff member will
decrease the strength of the rotary shaft, so that breakage of the
rotary shaft may occur before the breakable portion of the power
shutoff member is broken. In other words, the power shutoff member
may fail to function reliably.
[0007] The present invention is directed to a power transmission
device of a compressor which permits the power shutoff member to
function reliably.
SUMMARY OF THE INVENTION
[0008] In accordance with an aspect of the present invention, a
power transmission device of a compressor includes a rotary shaft,
a pulley, a hub, a power shutoff member, a spacer and a cylinder.
The rotary shaft is rotatably supported by a casing of the
compressor. The rotary shaft has an external thread portion. The
pulley is rotatably mounted on the casing. The hub is connected to
the pulley for rotation therewith. The power shutoff member is
joined to the hub. The power shutoff member is interposed between
the hub and the rotary shaft. The power shutoff member has an
internal thread portion and shuts off excessive torque transmission
between the pulley and the rotary shaft. The spacer disposed on the
rotary shaft. The spacer has a seating surface. The cylinder is
disposed between the power shutoff member and the rotary shaft. The
cylinder has an external thread portion formed at the outer
periphery thereof for engagement with the internal thread portion
of the power shutoff member and an internal thread portion formed
at the inner periphery thereof for engagement with the external
thread portion of the rotary shaft. The cylinder has a contacting
surface for contact with the seating surface of the spacer and a
seating surface for contact with a contacting surface of the hub at
the axial end portion of the hub.
[0009] Other aspects and advantages of the invention will become
apparent from the following description, taken in conjunction with
the accompanying drawings, illustrating by way of example the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The features of the present invention that are believed to
be novel are set forth with particularity in the appended claims.
The invention together with objects and advantages thereof, may
best be understood by reference to the following description of the
presently preferred embodiments together with the accompanying
drawings in which:
[0011] FIG. 1 is a longitudinal cross-sectional view showing a
power transmission device according to a first preferred embodiment
of the present invention;
[0012] FIG. 2 is an enlarged longitudinal cross-sectional view
showing the power transmission device of the compressor according
to the first preferred embodiment of the present invention;
[0013] FIG. 3 is an illustrative view showing the transmission of
torque and various radiuses of rotation of the power transmission
device according to the first preferred embodiment of the present
invention;
[0014] FIG. 4 is a graph showing the torque-axial force
characteristics of the power transmission device according to the
first preferred embodiment of the present invention;
[0015] FIG. 5 is a graph showing the relation between the radius of
rotation and the coefficient of friction in the torque-axial force
characteristics of the power transmission device according to the
first preferred embodiment of the present invention;
[0016] FIG. 6 is an enlarged longitudinal cross-sectional view
showing a power transmission device according to a second preferred
embodiment of the present invention; and
[0017] FIG. 7 is an enlarged longitudinal cross-sectional view
showing a power transmission device according to a third preferred
embodiment of the present invention.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0018] The following will describe a power transmission device of
the compressor according to the first embodiment of the present
invention with reference to FIG. 1 through FIG. 5. As shown in FIG.
1, the power transmission device of the compressor of the first
preferred embodiment is generally designated by reference numeral
10. The power transmission device 10 includes a pulley 11, a hub
12, a power shutoff member 13, a cylinder 14 and a casing 16 of the
compressor. The power transmission device 10 is used for
transmitting torque to the rotary shaft 15 from the pulley 11. The
pulley 11 is rotatably mounted on the casing 16 of the compressor
and driven by an engine and the like (not shown). The hub 12 is
connected to the pulley 11 for rotation therewith. The power
shutoff member 13 functioning as a torque limiter is interposed
between the hub 12 and the rotary shaft 15.
[0019] The pulley 11 is rotatably mounted through a bearing 17 on a
boss 16a which is provided at one end of the casing 16. A belt 18
is wound around the pulley 11, so that the pulley 11 is rotated by
way of the belt 18 which is also connected to an external source
such as an engine or a motor (not shown). The hub 12 is made of a
ferrous material and formed in a stepped cylindrical shape. That
is, the hub 12 includes a large-diameter portion 12a and a
small-diameter portion 12b which has a relatively smaller outer
diameter than the large-diameter portion 12a. The small-diameter
portion 12b has an axial end portion 12c extending toward the rear
side of the compressor. The axial end portion 12c includes a
hub-side contacting surface 12d which is contactable with a
cylinder-side seating surface 14e described later (refer to FIG.
2). In FIG. 1, the right side of the drawing corresponds to the
rear side of the power transmission device 10 and the left side to
the front side of the power transmission device 10. The pulley 11
is connected to the hub 12 through a connecting member 19 thereby
to transmit torque from the pulley 11 to the hub 12.
[0020] In FIG. 2, the power transmission device 10 further has a
power shutoff member 13 which functions as a torque limiter. The
power shutoff member 13 is of a cylindrical shape including a
large-diameter portion 13a and a small-diameter portion 13b whose
outer diameter is smaller than that of the large-diameter portion
13a. The large-diameter portion 13a of the power shutoff member 13
is connected to the hub 12. The large-diameter portion 13a of the
power shutoff member 13 with a polygonal shape is fitted into a
corresponding polygonal hole which is formed by the inner
peripheral surface of the large-diameter portion 12a of the hub 12.
By this method, the power shutoff member 13 and the hub 12 are
joined together securely. The small-diameter portion 13b is formed
at the inner periphery thereof with an internal thread portion 13c.
The cylinder 14 is formed at the outer periphery thereof with an
external thread portion 14b which is engaged with the internal
thread portion 13c of the small-diameter portion 13b of the power
shutoff member 13. The inner diameter of the large-diameter portion
13a is slightly greater than that of the small-diameter portion
13b. A cutout as a breakable portion 13d is formed at the
transition between the large-diameter portion 13a and the
small-diameter portion 13b. Thus, the power shutoff member 13 is
easily broken at the breakable portion 13d when an excessively high
axial tension is applied to the power shutoff member 13. In other
words, the power shutoff member 13 shuts off excessive torque
transmission between the rotary shaft 15 and the pulley 11.
[0021] Meanwhile, the rotary shaft 15 is made of a ferrous material
and rotatably supported in the casing 16. The rotary shaft 15 is
formed so as to include a small-diameter portion 15a, a
medium-diameter portion 15b and a large-diameter portion 15c which
are disposed in this order as viewed from the front side of the
power transmission device 10, as clearly shown in FIG. 2. A shaft
seal device 20 in the form of a lip seal is provided between the
casing 16 and the rotary shaft 15 for preventing refrigerant gas
and oil from leaking out of the compressor and maintaining the
airtightness of the compressor. In operation of the compressor, the
shaft seal device 20 is kept in slide contact with the
medium-diameter portion 15b of the rotary shaft 15. Because the
medium-diameter portion 15b of the rotary shaft 15 is formed with a
smaller diameter than the large-diameter portion 15c, the
peripheral speed of the medium-diameter portion 15b in sliding
contact with the shaft seal device 20 is lower than the peripheral
speed of the large-diameter portion 15c, so that the heat
generation between the medium-diameter portion 15b and the shaft
seal device 20 is reduced and durability of the shaft seal device
20 is improved, accordingly.
[0022] As shown in FIG. 2, a front end 15d of the small-diameter
portion 15a of the rotary shaft 15 is formed so as to be held by a
tool. The small-diameter portion 15a of the rotary shaft 15 is
formed at the outer periphery thereof with an external thread
portion 15e. A cylindrical spacer 21 is press-fitted on the
medium-diameter portion 15b as a screw seat member at a border
between the small-diameter portion 15a and the medium-diameter
portion 15b. The spacer 21 is provided as a radially expanded
portion of the rotary shaft 15. The spacer 21 is pressed against
the region adjacent to the step between the small-diameter portion
15a and the medium-diameter portion 15b so as to be fixedly
positioned. As shown in FIG. 2, the spacer 21 has at the front end
face thereof a shaft-side seating surface 21a in the form of an
annular surface extending perpendicular to the axis M of the rotary
shaft 15. The shaft-side seating surface 21a is in contact with the
cylinder-side contacting surface 14d of the cylinder 14. The spacer
21 has a hook portion 21b used for extracting the spacer 21 from
the rotary shaft 15.
[0023] The cylinder 14 which is made of ferrous material has a
cylindrical shape and is disposed between the rotary shaft 15 and
the power shutoff member 13. The cylinder 14 includes at the inner
periphery thereof with an internal thread portion 14a for
engagement with the external thread portion 15e of the rotary shaft
15. The cylinder 14 further includes at the outer periphery thereof
with an external thread portion 14b for engagement with the
internal thread portion 13c of the power shutoff member 13. The
cylinder 14 has a flange 14c extending rearward and a cylinder-side
contacting surface 14d at the rear end face of the flange 14c for
contact with the shaft-side seating surface 21a of the spacer 21.
The aforementioned cylinder-side seating surface 14e is provided at
the front end surface of the flange 14c for contact with the
hub-side contacting surface 12d of the axial end portion 12c of the
hub 12.
[0024] The large-diameter portion 13a is fitted into the
large-diameter portion 12a, so that the hub 12 and the power
shutoff member 13 are integrated. Then, the internal thread portion
14a is threaded on the external thread portion 15e until the
cylinder-side contacting surface 14d of the cylinder 14 is pressed
against the shaft-side seating surface 21a of the spacer 21, so
that the cylinder 14 is securely connected or fastened to the
rotary shaft 15. Subsequently, the internal thread portion 13c is
threaded on the external thread portion 14b until the hub-side
contacting surface 12d is brought into pressing contact with
cylinder-side seating surface 14e of the cylinder 14 at the outer
periphery, so that the power shutoff member 13 is securely
connected or fastened to the cylinder 14.
[0025] Meanwhile, a lubricant agent is provided between the
external thread portion 14b of the cylinder 14 and the internal
thread portion 13c of the power shutoff member 13 and between the
cylinder-side seating surface 14e and the hub-side contacting
surface 12d. Carbon dioxide is used as a refrigerant gas in the
compressor.
[0026] The following will describe the operation of the power
transmission device 10 as constructed above. Referring to FIG. 3, P
represents the thread engagement portion of the internal thread
portion 14a of the cylinder 14 and the external thread portion 15e
of the rotary shaft 15. Similarly, Q represents the thread
engagement portion of the internal thread portion 13c of the power
shutoff member 13 and the external thread portion 14b of the
cylinder 14. A represents the contacting portion between the
cylinder-side contacting surface 14d of the cylinder 14 and the
shaft-side seating surface 21a of the spacer 21 which is
press-fitted on the rotary shaft 15. B represents the contacting
portion between the hub-side contacting surface 12d of the hub 12
and the cylinder-side seating surface 14e of the cylinder 14. In
FIG. 4, hatchings of the figure are partially omitted because of
easy understanding.
[0027] In transmitting torque from the hub 12 to the cylinder 14
through the thread engagement portion Q and the contacting portion
B, firstly the torque is transmitted from the hub 12 and the power
shutoff member 13 to the cylinder 14. Torque Ts, or the torque
transmitted from the pulley 11 to the hub 12, is transmitted
through the thread engagement portion Q and the contacting portion
B to the cylinder 14 as indicated by the arrows in FIG. 3. An axial
force is generated in the axial direction in the thread engagement
portion Q in response to the transmitted torque. The axial forces
represented by Lq are generated in the power shutoff member 13 and
the cylinder 14 respectively, and the two axial forces are
substantially equal in strength.
[0028] The torque Ts transmitted to the cylinder 14 is further
transmitted through the thread engagement portion P and the
contacting portion A to the rotary shaft 15. An axial force is also
generated in the thread engagement portion P. The axial forces
designated by Lp are generated in both of the cylinder 14 and the
rotary shaft 15, respectively, and these two axial forces Lp are
substantially equal in strength.
[0029] FIG. 4 is a graph showing the torque-axial force
characteristics in the thread engagement portions P, Q. The
horizontal axis indicates the torque and the vertical axis
indicates the axial force. The characteristics at the thread
engagement portion P is represented by the line E and the
characteristics at the thread engagement portion Q is represented
by the line F. As is obvious from the comparison between the
characteristics lines E and F in the graph, the axial force at the
thread engagement portion P is smaller than that of in the thread
engagement portion Q at any given torque. That is, the axial force
acting on the rotary shaft 15 is smaller than that on the power
shutoff member 13. In addition, the axial force L1 for a torque Ts1
at the thread engagement portion P is smaller than the axial force
L2 for the same torque Ts1 at the thread engagement portion Q.
[0030] L3 and L4 represent the axial forces developed at the thread
engagement portions P, Q when the torque Tmax is applied thereto,
respectively. The breaking strength of the rotary shaft 15 is
represented by L5 and the breaking strength of the power shutoff
member 13 as a torque limiter is represented by L6. The power
transmission device 10 is so arranged that the breaking strength L5
is greater than the axial force L3 and also the axial force L4 is
substantially the same as the breaking strength L6. If the seizure
is occurred in the compressor, the torque Tmax is applied to the
thread engagement portions P, Q and the axial force L3 is generated
in the rotary shaft 15 and the axial force L4 is generated in the
power shutoff member 13, accordingly. Because the axial force L4 is
set substantially the same as the breaking strength L6, the power
shutoff member 13 is broken at the breakable portion 13d. Because
the breaking strength L5 is set larger than the axial force L3,
breakage of the rotary shaft 15 does not occur.
[0031] In case the cylinder 14 is not provided in the power
transmission device 10 and the power shutoff member 13 is directly
mounted to the rotary shaft 15, the torque-axial force
characteristics would be represented by the single line F and,
therefore, axial forces of substantially the same magnitude would
be developed in the power shutoff member 13 and the rotary shaft
15. In such case the breaking strength of rotary shaft which is
represented by L7 in FIG. 4 would have to be set larger than the
breaking strength L6 as indicated by the chain double-dot line in
FIG. 4. According to the present invention, however, the provision
of the cylinder 14 and the thread engagement portions P, Q having
different torque-axial force characteristics E and F makes it
possible to set the axial force acting on the rotary shaft 15
smaller than the axial force acting on the power shutoff member 13,
with the result that the breaking strength for the rotary shaft 15
may be reduced from the level 7 to the level 5. In other words, the
diameter of the rotary shaft 15 may be reduced.
[0032] Transmission torque of a rotary component is proportional to
radius of rotation, coefficient of friction and axial force. As
shown in FIG. 3, the radius of rotation of the thread engagement
portion P is represented by Rp and the radius of rotation of the
thread engagement portion Q is represented by Rq, respectively. The
radius of rotation of the contacting portion A is represented by Ra
and the radius of rotation of the contacting portion B is
represented by Rb, respectively. The radiuses of rotation Rp, Rq,
Ra and Rb correspond to the radial distances from the axis M of the
rotary shaft 15 to P, Q, A and B. The coefficients of friction
between the cylinder 14 and rotary shaft 15 and between the
cylinder 14 and the spacer 21 (or between the flange 14c as a
contacting portion and the shaft-side seating surface 21a) are
represented by .mu.p. The coefficients of friction between the
power shutoff member 13 and the cylinder 14 and between the hub 12
and the cylinder 14 (or between the axial end portion 12c and the
flange 14c as a contacting portion) are represented by .mu.q. The
coefficients of friction at the thread engagement portion P and the
contacting portion A are .mu.p and the coefficients of friction at
the thread engagement portion Q and the contacting portion B are
.mu.q. The above coefficients of friction are provided on the
assumption that the rotary shaft 15 and the spacer 21 are made of
the same material, and that the power shutoff member 13 and the hub
12 are made of the same material.
[0033] FIG. 5 is a graph showing the influence of radius of
rotation R and coefficient of friction .mu.on the torque-axial
force characteristics. Referring to the torque-axial force
characteristics line E1 at the thread engagement portion P in the
graph, when the radius of rotation Ra or the coefficient of
friction .mu.p is decreased, the characteristics line E1 is changed
in such a way that the inclination angle of the line E1 relative to
the horizontal axis is increased or the line E1 is shifted toward
the line E2. When the radius of rotation Ra or the coefficient of
friction .mu.p is increased, on the other hand, the line E1 for the
thread engagement portion P is changed in such a way that the
inclination angle of the line E1 relative to the horizontal axis is
decreased or the line E1 is shifted toward the line E3. Now
referring to the torque-axial force characteristics line F1 at the
thread engagement portion Q in the graph, when the radius of
rotation Rb or the coefficient of friction .mu.q is decreased, the
line F1 for the thread engagement portion Q is changed in such a
way that the inclination angle of the line F1 relative to the
horizontal axis is increased or the line F1 is shifted toward the
line F2. When the radius of rotation Rb or the coefficient of
friction .mu.q is increased, the characteristics line F1 for the
thread engagement portion Q is changed in such a way that the
inclination angle of the line F1 relative to the horizontal axis is
decreased or the line F1 is shifted toward the line F3. Thus, the
torque-axial force characteristics are varied due to a change of
the radiuses of rotation Rp, Rq. The difference between the
radiuses of rotation Rp, Rq is relatively small, and, therefore,
the influence of the radiuses of rotation Rp, Rq on the
torque-axial force characteristics is small as compared to the
influence of difference between the coefficients of friction .mu.p,
.mu.q on the characteristics. Thus, the description of the
influence of the difference between the radiuses of rotation Rp, Rq
on the torque-axial force characteristics will be omitted. It is
noted that as coefficients of friction .mu.p, .mu.q are increased,
friction losses at the thread engagement portions P, Q are
increased and the axial forces are decreased. As the coefficients
of friction .mu.p, .mu.q are decreased, on the other hand, friction
losses at the thread engagement portions P, Q are decreased and the
axial forces are increased.
[0034] The radiuses of rotation Ra, Rb are factors related to the
transmission torques at the contacting portions A, B, respectively.
That is, when the radiuses of rotation Ra, Rb are decreased and the
transmission torques at the contacting portions A, B are decreased,
the transmission torques at the thread engagement portions P, Q are
increased, and the axial forces are increased, accordingly. When
the radiuses of rotation Ra, Rb are increased and the transmission
torques at the contacting portions A, B are increased, the
transmission torques at the thread engagement portions P, Q are
decreased and the axial forces are decreased, accordingly.
[0035] In the first preferred embodiment, the radius of rotation Ra
is substantially the same as the radius of rotation Rb and the
values for the radiuses are relatively large. In addition, the
thread engagement portion Q and the contacting portion B are coated
with lubricant, but no lubricant is applied to the thread
engagement portion P and the contacting portion A. Therefore, the
coefficient of friction .mu.p is larger than .mu.q. In other words,
it is so arranged in the first preferred embodiment that the radius
of rotation Ra is substantially the same as Rb, but the
coefficients of friction .mu.p and .mu.q are set different.
Therefore, the friction losses at the thread engagement portion Q
and the contacting portion B are decreased, and the axial force in
the power shutoff member 13 is increased, the power shutoff member
13 can be broken reliably by application of the predetermined
torque. Meanwhile, because the friction losses at the thread
engagement portion P and the contacting portion A are increased,
the axial force in the rotary shaft 15 is decreased to prevent the
rotary shaft 15 from breakage.
[0036] As stated above, the torque-axial force characteristics E, F
at the thread engagement portions P, Q may be varied by adjusting
the radius of rotation R and the coefficient of friction .mu.pas
required. Thus, in accordance with the breaking strengths of the
rotary shaft and the power shutoff member, the radius of rotation R
and the coefficient of friction may be adjusted to an optimum
setting. Especially, the torque-axial force characteristics E, F
have variations in the individual compressors, and the like
inclination angle between the characteristics E, F also has
variations, on the premise of which, optimum setting may be
obtained.
[0037] The power transmission device 10 of the present embodiment
has the following advantageous effects.
[0038] (1) According to the first preferred embodiment, the
large-diameter portion 13a is fitted into the large-diameter
portion 12a, so that the hub 12 and the power shutoff member 13 are
integrated. Then, the internal thread portion 14a is threaded on
the external thread portion 15e until the cylinder-side contacting
surface 14d of the cylinder 14 is pressed against the shaft-side
seating surface 21a of the spacer 21, so that the cylinder 14 is
securely connected or fastened to the rotary shaft 15.
Subsequently, the internal thread portion 13c is threaded on the
external thread portion 14b until the hub-side contacting surface
12d is brought into pressing contact with cylinder-side seating
surface 14e of the cylinder 14 at the outer periphery, so that the
power shutoff member 13 is securely connected or fastened to the
cylinder 14. By virtue of such fastening, the torque transmitted
from the pulley 11 to the hub 12 is further transmitted to the
rotary shaft 15 through the power shutoff member 13 and the
cylinder 14. As the torque is thus transmitted, the axial forces
are generated in the axial direction in the thread engagement
portion Q between the power shutoff member 13 and the cylinder 14
and in the thread engagement portion P between the cylinder 14 and
the rotary shaft 15. According to the torque-axial force
characteristics of the thread engagement portions P, Q, the axial
force acting on the thread engagement portion P at a given torque
may be decreased relative to the axial force acting on the thread
engagement portion Q at the same torque. Thus, the axial force
acting on the rotary shaft 15 may be decreased relative to the
axial force acting on the power shutoff member 13, with the result
that the diameter of the rotary shaft 15 may be reduced.
[0039] (2) When the axial forces developed at the thread engagement
portions P, Q when the torque Tmax is applied thereto are
represented by L3, L4, respectively. Also, the breaking strengths
of the rotary shaft 15 and the power shutoff member 13 are
represented by L5, L6, respectively. L5 is greater than L3 and L4
is substantially the same as L6. The axial forces L3, L4 are
generated in the rotary shaft 15 and the power shutoff member 13,
respectively, when the torque Tmax is applied to the thread
engagement portions P, Q in the event of a seizure in the
compressor. However, because the axial force L4 is set
substantially the same as the breaking strength L6, the power
shutoff member 13 is broken at the breakable portion 13d, and
because the breaking strength L5 is set greater than the axial
force L3, breakage of the rotary shaft 15 does not occur. That is,
the breakable portion 13d is firstly broken when an excessive
torque Tmax is applied, thereby to block the torque transmission
from the hub 12 to the rotary shaft 15 and to prevent any trouble
such as a break of the belt 18.
[0040] (3) Due to the application of the power shutoff member 13,
the diameter of medium-diameter portion 15b of the rotary shaft 15
which is in slide contact with the shaft seal device 20 may be
reduced. As a result, the peripheral speed of the medium-diameter
portion 15b which is in slide contact with the shaft seal device 20
is reduced, so that heat generation in the shaft seal device 20 is
suppressed and the durability of the shaft seal device 20 is
improved.
[0041] (4) The thread engagement portion Q and the contacting
portion B are coated with lubricant, but no lubricant is applied to
the thread engagement portion P and the contacting portion A. Thus,
the coefficient of friction .mu.p is larger than .mu.q and,
therefore, the friction losses at the thread engagement portion Q
and the contacting portion B are less than those at the thread
engagement portion P and the contacting portion A. As a result, the
power shutoff member 13 can be broken reliably when the axial force
in the power shutoff member 13 is increased to reach a
predetermined torque. Meanwhile, because the friction losses at the
thread engagement portion P and the contacting portion A are larger
than the friction losses on the thread engagement portion Q and the
contacting portion B, the axial force in the rotary shaft 15 is
decreased and the rotary shaft 15 is certainly prevented from
breakage. In addition, because the coefficient of friction may be
decreased just by application of lubricant, selection of materials
for the respective parts for the power transmission device can be
done with ease. Also, the materials are not need to be changed and
the power transmission device 10 is easy to assemble.
[0042] (5) The shaft-side seating surface 21a is formed by
press-fitting the spacer 21 on the rotary shaft 15. Thus, the
number of manufacturing processes may be reduced as compared to the
case where the rotary shaft 15 is directly formed with the
shaft-side seating surface 21a.
[0043] (6) When carbon dioxide is used as a refrigerant for the
compressor, the load applied from the shaft seal device 20 to the
rotary shaft 15 is increased by the increase of the internal
pressure of the compressor. Thus, heat generation at the slide
contact portion between the medium-diameter portion 15b of the
rotary shaft 15 and the shaft seal device 20 needs to be
suppressed. This can be accomplished by reducing the diameter of
the medium-diameter portion 15b of the rotary shaft 15 and hence
the peripheral speed of the rotary shaft 15 at the slide contact
portion. Therefore, the compressor may be adapted to use of carbon
dioxide as a refrigerant for the compressor.
[0044] The following will describe a second preferred embodiment of
the present invention with reference to FIG. 6. The second
preferred embodiment differs from the first preferred embodiment in
that the shape of the axial end portion 12c of the first preferred
embodiment are modified. For the convenience of explanation, common
or similar elements or parts are designated by the same reference
numerals as those used in the first preferred embodiment and,
therefore, the description thereof will be omitted and only the
modifications will be described.
[0045] Referring to FIG. 6, showing a power transmission device 40
of the second preferred embodiment according to the present
invention, the power transmission device 40 includes a hub 41 of a
stepped cylindrical shape having a large-diameter portion 41a, a
medium-diameter portion 41b and a small-diameter portion 41c
arranged in this order as viewed from the front side of the
compressor. The small-diameter portion 41c extending rearward of
the compressor from the medium-diameter portion 41b has a
shaft-side end portion 41d having a hub-side contacting surface 41e
which is axially contacted to the cylinder-side seating surface
14e.
[0046] In the second preferred embodiment, the connected surfaces
between the cylinder-side contacting surface 14d of the cylinder 14
and the shaft-side seating surface 21a of the rotary shaft 15
correspond to the contacting portion A and the connected surfaces
between the hub-side contacting surface 41e of the hub 41 and the
cylinder-side seating surface 14e of the cylinder 14 correspond to
the contacting portion B. The radiuses of rotation Ra, Rb of the
contacting portions A, B are set such that Rb is smaller than Ra.
Thus, the present second embodiment differs from the first
embodiment also in that the radiuses of rotation Ra, Rb, as well as
the coefficients of friction .mu.p, .mu.q, are different from each
other. The variation of Rb means shifting the characteristics line
F1 for the thread engagement portion Q toward the line F2 in the
torque-axial force characteristics graph in FIG. 5.
[0047] As a result, the axial force in the thread engagement
portion Q or the axial force acting on the power shutoff member 13
may be increased as compared to the axial force acting on the
rotary shaft 15. For example, in consideration of the individual
compressors, the characteristics E1, F1 cannot be represented by
single lines due to exist of the fluctuation in the manufacture
process. Therefore, the design of the power transmission device 40
is made on the premise of such fluctuations. If the characteristics
E1 and F1 are overlapped each other, the breaking strengths of
rotary shaft and the power shutoff member may not set properly.
However, decreasing the radius of rotation Rb thereby to increase
the inclination angle of the characteristics line F1, the
characteristics lines E1, F1 will have no overlapped part.
[0048] It is readily understood by substituting the hub-side
contacting surface 12d of the first embodiment with the hub-side
contacting surface 41e of the second embodiment. The operation of
the power transmission device 40 of the second preferred embodiment
is substantially the same as the power transmission device 10 of
the first embodiment. Therefore, the detailed description of the
operation of the power transmission device 40 will be omitted.
[0049] According to the power transmission device 40 of the second
preferred embodiment, the same advantageous effects as mentioned in
the paragraphs (1) through (6) for the first embodiment are
obtained. In addition, the following advantageous effect is
obtained.
[0050] (7) Decreasing the radius of rotation Rb of the contacting
portion B between the hub-side contacting surface 41e of the hub 41
and the cylinder-side seating surface 14e of the cylinder 14
thereby to increase the inclination angle of the characteristics
line F for the thread engagement portion Q relative to the
horizontal axis, the axial force acting on the power shutoff member
13 is increased.
[0051] The following will describe a third preferred embodiment
according to the present invention with reference to FIG. 7. The
third preferred embodiment differs from the first preferred
embodiment in that the shape of the flange 14c of the first
preferred embodiment is modified. The other structure of the power
transmission device 50 is substantially the same as that of the
first preferred embodiment and, therefore, common or similar
elements or parts are designated by the same reference numerals as
those used in the first preferred embodiment and the description
thereof will be omitted.
[0052] Referring to FIG. 7 showing a power transmission device 50
of the third embodiment according to the present invention, the
power transmission device 50 includes a cylinder 51 having formed
at the inner periphery thereof an internal thread portion 51a and
screwed over the rotary shaft 15 through engagement between the
internal thread portion 51a of the cylinder 51 and the external
thread portion 15e of the rotary shaft 15. The cylinder 51 is
further formed at the outer periphery thereof with an external
thread portion 51b for engagement with the internal thread portion
13c of the power shutoff member 13. In addition, the cylinder 51
has a flange 51c extending radially outward and a projection 51d
extending from a radially inner region of the flange 51c axially
rearward of the compressor. The projection 51d has at the rear end
thereof a cylinder-side contacting surface 51e for contact with the
shaft-side seating surface 21a of the rotary shaft 15. The flange
51c has at the front end face thereof a cylinder-side seating
surface 51f for contact with the hub-side contacting surface 12d of
the axial end portion 12c of the hub 12.
[0053] The connected surfaces between the cylinder-side contacting
surface 51e of the cylinder 51 and the shaft-side seating surface
21a of the rotary shaft 15 in the third preferred embodiment
corresponds to the contacting portion A of the first preferred
embodiment. The connected surfaces between the hub-side contacting
surface 12d of the hub 12 and the cylinder-side seating surface 51f
of the cylinder 51 corresponds to the contacting portion B of the
first embodiment. The radiuses of rotation Ra, Rb of the contacting
portions A, B are set such that Ra is smaller than Rb. Then,
lubricant is applied to the thread engagement portion P and the
contacting portion A, but no lubricant is applied to the thread
engagement portion Q and the contacting portion B. Thus, the
coefficient of friction .mu.p is smaller than .mu.q. That is, the
coefficients of friction .mu.p, .mu.q, as well as the radiuses of
rotation Ra, Rb, are different from each other in the third
preferred embodiment.
[0054] The variation of Ra and .mu.p means shifting the
characteristics line E1 for the thread engagement portion P toward
the line E2 or increase in the inclination angle relative to the
horizontal axis in the torque-axial force characteristics graph in
FIG. 5. On the other hand, the variation of Rb and .mu.q means
shifting the characteristics line F1 for the thread engagement
portion Q toward the line F3 or decrease in the inclination angle
relative to the horizontal axis in the torque-axial force
characteristics graph in FIG. 5. Therefore, the torque-axial force
characteristics E, F at the thread engagement portions P, Q may be
reversed. That is, the axial force in the thread engagement portion
P or the axial force acting on the rotary shaft 15 is increased,
while the axial force in the thread engagement portion Q or the
axial force acting on the power shutoff member 13 is decreased. In
this case, the rotary shaft 15 may be broken before the power
shutoff member 13 is broken.
[0055] According to the power transmission device 50 of the third
preferred embodiment, the following advantageous effects are
obtained.
[0056] (8) The radiuses of rotation Ra, Rb and the coefficients of
friction .mu.p, .mu.q are set such that Ra is smaller than Rb and
.mu.p is smaller than .mu.q. Therefore, the inclination angle of
the characteristics line E1 for the thread engagement portion P
relative to the horizontal axis in the graph of FIG. 5 can be
increased. On the other hand, the inclination angle of the
characteristics line F1 for the thread engagement portion Q
relative to the horizontal axis can be decreased. Thus, the
torque-axial force characteristics E, F of the thread engagement
portions P, Q may be reversed, and the rotary shaft 15 may be
broken before a breakage of the power shutoff member 13.
[0057] The present invention is not limited to the above-described
embodiments but may be modified into the following examples.
[0058] Though the coefficient of friction .mu.p is larger than
.mu.q in the first and second embodiments and .mu.p is smaller than
.mu.q in the third embodiment, the coefficients of friction .mu.p
and .mu.q may be set at substantially the same value. In this case,
lubricant is applied to the thread engagement portions P, Q and the
contacting portions A, B, thereby reducing the variations in the
torque-axial force characteristics E, F.
[0059] In the first through third embodiments, the coefficients of
friction at the thread engagement portion P and the contacting
portion A are set at .mu.p and the coefficients of friction at the
thread engagement portion Q and the contacting portion B at .mu.q.
Although the coefficients of friction at the thread engagement
portions P, Q and also at the contacting portions A, B are
different from each other, the power transmission device may be
designed such that there is a difference either one of between the
coefficients of friction at the thread engagement portions P, Q and
between the contacting portions A, B. Specifically, there may be a
difference between the coefficients of friction at the thread
engagement portions P, Q and no difference between the coefficients
of friction at the contacting portions A, B. Alternatively, there
may be a difference between the coefficients of friction at the
contacting portions A, B and no difference between the coefficients
of friction at the thread engagement portions P, Q.
[0060] Besides the use or non-use of lubricant, a material such as
rubber may be used appropriately as a means for adjusting the
coefficients of friction .mu.p, .mu.q. In other words, the power
shutoff member and the hub may be made of a material which has a
different coefficients of friction from that of the rotary shaft
and the cylinder.
[0061] Therefore, the present examples and embodiments are to be
considered as illustrative and not restrictive, and the invention
is not to be limited to the details given herein but may be
modified within the scope of the appended claims.
* * * * *