Piston for use in a compressor

Details Piston for use in a compressor Piston for use in a compressor

1999-05-02

2001-07-17

Look, Edward K. (Department: 3745)

Expansible chamber devices

Relatively movable working members

Interconnected with common rotatable shaft

C092S172000

Reexamination Certificate

active

06260469

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a piston for use in a compressor, and more particularly, to a piston suitable for use in an automotive air conditioning compressor in which there is provided a piston having a construction to minimize a bending moment exerted thereon and a mechanism in response to such piston.
BACKGROUND OF THE INVENTION
Generally, a piston type compressor for use in an automotive air conditioning system comprises a cylinder block having a plurality of cylinder bores. A plurality of pistons are slidably disposed in the respective cylinder bores and reciprocated by, for example, a swash plate or wobble plate in the cylinder bores. In a variable capacity swash plate type compressor with a mechanism varying an inclination angle of the swash plate, a single-headed piston is generally used. The single-headed piston includes a body with a head, and a support portion for receiving shoes which convert rotation of the swash plate into reciprocation of the pistons. However, a bending moment acts on the pistons due to a component of the force that is exerted normal to the direction of motion of the pistons during operation of the compressor. Accordingly, the bending moment causes the deformation of pistons, and thus, a contact portion between the pistons and the cylinder bores is abraded.
In order to clarify the problems occurring in a typical swash plate type compressor with a variable displacement mechanism, description will be made with reference to FIG.
1
. The compressor
1
of this type has a cylinder block
2
, with a plurality of cylinder bores
4
, and with front and rear ends of the cylinder block
2
sealingly closed by front and rear housing portions
6
and
8
, respectively. The cylinder block
2
and the front housing
6
define an air-tight sealed crank chamber
10
. A valve plate
12
is mounted between the rear end of the cylinder block
2
and the rear housing
8
. The rear housing
8
has formed therein inlet and outlet ports
14
and
16
for input and output of a refrigerant gas, a suction chamber
18
, and a discharge chamber
20
. The suction and discharge chambers
18
and
20
are in communication with the respective cylinder bores
4
via suction and discharge valve mechanisms. A drive shaft
22
is centrally arranged to extend through the front housing
6
to the cylinder block
2
and rotatably supported by bearings mounted in the front housing
6
and the cylinder block
2
. The cylinder block
2
and the front and rear housing
6
and
8
are combined by screws
25
. A rotor
26
is mounted on the drive shaft
22
in the crank chamber
10
to be rotatable with the drive shaft
22
, and is supported by a thrust bearing
28
seated on an inner end of the front housing
6
. A spherical sleeve
30
, having an outer spherical surface formed as a support surface, is slidably supported by the drive shaft
22
. A spring
32
, mounted around the drive shaft
22
, is interposed between the rotor
26
and the spherical sleeve
30
, and biases the spherical sleeve
30
toward the rear housing
8
.
A swash plate
34
is rotatably supported on the outer surface of the spherical sleeve
30
. The swash plate
34
is connected to the rotor
26
via a hinge mechanism so as to be rotated with the rotor
26
. The hinge mechanism includes a support arm
36
that protrudes axially outwardly from one side surface of the rotor
26
, and an arm
38
that protrudes from one side surface of the swash plate
34
toward the support arm
36
of the rotor
26
. The support arm
36
and the arm
38
overlap each other and are connected to each other by a pin
40
. The pin
40
extends into a pin hole
42
formed through the support arm
36
of the rotor
26
and a rectangular shaped hole
43
formed through the arm
38
of the swash plate
34
. In this manner, the rotor
26
and the swash plate
34
are hinged to each other, and the sliding motion of the pin
40
within the rectangular hole
43
changes the inclination angle of the swash plate
34
so as to change the capacity of the compressor.
Pistons
44
are slidably disposed in the respective cylinder bores
4
. Each piston
44
has a body
46
with a head portion which is slidably disposed in the corresponding cylinder bore
4
, and a bridge portion
48
which has formed therein a recess
50
. Semi-spherical shoes
52
are disposed in shoe pockets
54
formed in the bridge portion of the piston
44
and slidably engaged with a peripheral portion of the swash plate
34
. Therefore, the swash plate
34
is rotated together with the rotation of the drive shaft
22
, and the rotation of the swash plate
34
is converted into the reciprocation of the pistons
44
.
A cutout portion
56
is formed at a lower front end portion of the piston
44
to prevent contact between a side surface of the swash plate
34
and the body
46
of the piston
44
when the piston is in its bottom dead center position.
A control valve means
60
is provided with the compressor to adjust a pressure level in the crank chamber
10
.
In the above-described type of compressor, a bending moment generated from various forces acting on the pistons
44
causes a deformation of the pistons
44
and potentially excessive abrasion about a contact portion between the pistons
44
and their corresponding cylinder bores
4
.
FIG. 2
illustrates an enlarged partial view of
FIG. 1
, showing the various forces acting on a piston. During the compression stroke of the piston
44
, the pressure P
c
in the crank chamber
10
acts on the forward end of the piston
44
while a compression reaction force P
d
acts on the other end of the piston
44
. The pressure P
c
in the crank chamber
10
and the compression reaction force P
d
act on the swash plate from the piston via the shoes
52
creating an action force on the swash plate
34
, with obviously a reaction force that is equal in magnitude and oppositely directed to the action force. That is, when the piston
44
is in its compression stroke, the force F exerted from the swash plate
34
on the piston
44
acts in a direction that is perpendicular to surfaces of the swash plate
34
at a contact location where the semi-spherical outer surface of the shoe adjacent to the body of the piston
44
comes into contact with the semi-spherical inner surface of the shoe pocket
54
. This location is at an apex of the shoe pocket
54
lying on the central axis O of the piston
44
. If the force F exerted from the swash plate
34
on the piston
44
is decomposed into two components, a horizontal and a vertical component, there will be a horizontal component F
x
lying on the central axis O of the piston
44
and a vertical component F
y
being perpendicular to the central axis O of the piston
44
. Let “m” be the mass of the piston
44
, “a” the acceleration of piston during the compression stroke, and “A” the surface area against which the pressure acts. Thus,
&Sgr;F
x
=ma
&Sgr;F
x
=AP
c
−AP
d
+F
x
By combining the above equations, we can write,
F
x
=ma+A(P
d
−P
c
)=ma+(&pgr;/4)*d
2
(P
d
−P
c
)
and
F
y
=F
x
tan &thgr;=tan &thgr;[ma+(&pgr;/4)*d
2
(P
d
−P
c
)]
which d is a diameter of piston.
The vertical component F
y
, then, will act on the piston
44
to create a bending moment which is maximum at the lower back edge designated by “p”. As stated above, the cutout portion
56
is provided to prevent the piston
44
from coming into contact with the rear surface of the swash plate
34
when the piston
44
approaches its bottom dead center position. However, the cutout portion
56
creates a horizontal distance x between the operating point of the force F acting on the piston and the location of the reaction force acting on the cutout portion
56
at p. This distance x creates a bending moment which acts on the piston
44
. The maximum bending moment M
max
acting on the piston is given by
M
max
=xF
y
=xtan &thgr;[ma+(&pgr;/4)*d
2
(P
d
−P
c
)]
Therefore, due to the

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