Self-latching eccentric cam for dual stroke compressor or pump

For a dual capacity refrigerant compressor of the type which has an eccentric cam rotatable on a crankpin between two opposite positions which changes the total eccentricity of the crankpin and cam so as to obtain two different stroke lengths, the rotation of the cam on the crankpin being effected by a reversal of motor operation, the cam moves through an angle of about 270.degree. around the crankpin so that a centrifugal force torque tending to hold the cam in place is available at least in the reduced stroke length position of the cam, and by providing lightening cavities and eccentric weightings, the center of mass 74 of the cam can be shifted to obtain the centrifugal torque in the proper direction at both the maximum and reduced stroke positions.

BACKGROUND OF THE INVENTION 
This invention pertains to the art of dual capacity compressors or pumps in 
which the dual capacity is obtained through changing the connecting rod 
stroke length from one length to another through reversal of the driving 
motor. 
My U.S. Pat. No. 4,236,874 teaches a reciprocating refrigerant (or other 
gas) compressor which employs an eccentric cam rotatably mounted on the 
crankpin and in which the rotation of the cam on the crankpin is angularly 
limited by a key in a circumferentially extending recess at the interface 
of the cam and crankpin. On reversal of compressor rotation, the cam 
shifts from one angular extremity or end point to an opposite angular 
extremity or end point and by so doing changes the vector sum of cam 
eccentricity and crankpin throw so as to provide two different stroke 
lengths. A change in displacement and clearance ratio results and two 
refrigeration capacities are obtained depending on the direction of 
rotation of the motor. The cam is driven from one end point to the other 
by the fraction of compressor torque developed by the cam eccentricity. 
The arrangement works satisfactorily in a compressor of the character 
illustrated in my patent where the cylinders are arranged radially about 
the crankshaft, and where a single crankpin accommodates three connecting 
rods. It is considered marginal where two connecting rods are accommodated 
on a single crankpin. However, for a compressor with more limited capacity 
and requiring only a single piston difficulties could be experienced in at 
least the shorter length mode of operation. The same difficulties could be 
expected to be experienced if multiple cylinders were used, but in a 
so-called aligned arrangement in which each of the connecting rods is 
accommodated by a separate crankpin, as some compressors are arranged. 
The difficulty is that in at least the minimum stroke position of the cam 
relative to the crankpin, there is an oscillating load between the key and 
the stop against which the key is to be seated or "latched". In that 
connection, there are various changing forces which exert influence to 
either prevent, or to tend to cause, the oscillating load. Among the 
forces which generally tend to prevent the possibility of oscillation are 
friction forces, viscous drag loads, and cam inertia forces. Gas thrust 
and piston rod inertia forces will have different effects depending upon 
the part of the cycle. Finally, it is believed that one of the more 
important forces is a centrifugal force torque which, when the cam 
eccentricity is angled to the crankpin throw, will tend to return the cam 
toward a position in which the eccentricity is aligned with the crank 
throw. Thus, with a gas torque reversal, as is most severely experienced 
in a single cylinder compressor, the centrifugal force added to the 
reversed gas torque force will destabilize the cam positioning in the 
reduced stroke position. 
It is the aim of my invention to provide a system in which the centrifugal 
force vector or torque, in at least the short stroke mode of operation, 
will aid in holding the cam to its one end position and thus to obtain, in 
effect, a self-latching of the cam in that one end position. It is a 
further aim of the invention to provide another embodiment in which the 
centrifugal force torque is available to aid the latching irrespective of 
which of the stroke length modes is being used. 
SUMMARY OF THE INVENTION 
In accordance with the invention in its essentially simplest sense, the 
rotation limiting means which prevents rotation of the cam on the crankpin 
beyond the two opposite end points is provided with a circumferential 
extent requiring the rotation of the cam relative to the pin to be in 
excess of a half circle in the repositioning of the cam from the one end 
point to the other end point. 
In another embodiment of the invention, the cam includes means providing an 
imbalance on one side of the diametral line passing through the maximum 
and minimum thickness portions of the cam and of an extent to provide a 
centrifugal force torque of the cam in directions to aid in holding the 
key against the end points irrespective of the end point to which the cam 
has rotated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is considered applicable to compressors and pumps other than 
those used in the refrigeration art, but the invention will be explained 
in that connection. 
The description will proceed in connection with the prior art structures 
disclosed in the noted patent to provide an explanation of one environment 
in which the invention may be applied, and also to provide an explanation 
of the problems experienced with the prior art structure. For full details 
of that prior art structure and the variations to which the invention of 
the noted patent applies, reference should be had to the noted patent. 
Referring to FIG. 1, a generally cylindrical hermetically sealed shell 10 
has an inlet 12 through which the suction gas refrigerant is admitted to 
the shell, and one or more discharge gas tubes 14 through which the 
compressed gas exits from the shell. The upper part of the shell houses a 
reversible electric motor 16 whose rotor 18 is fixed to the upper end of 
the crankshaft 20 to rotate the crankshaft in one direction or the other 
depending upon the direction of rotation of the rotor. 
In the illustrated unit, the compressor has two cylinders 22 in which the 
two pistons 24 reciprocate as they are driven by the connecting rods 26 
which, of course, have their one ends connected to the pistons and their 
other strap ends rotatably coupled to that lower portion 28 of the 
crankshaft which is provided with the crankpin of the crankshaft. 
Lubrication of the compressor is accomplished in a conventional well-known 
way. 
Referring to prior art FIG. 2, the dash line circle 30 indicates the 
location of the part of the crankshaft 28 which is journaled in the main 
bearings, while the solid line circle 32 shows the location of the 
crankpin relative thereto. 30a and 32a indicate the center lines of the 
shaft and crankpin, respectively. An eccentric ring or cam 34, which 
derives its eccentricity from the progressively varying wall thickness of 
the cam in a radial direction, is mounted on the crankpin 32 in rotatable 
relation therewith. The cam is split as along line 36 and is held in place 
on the crankpin by the strap 38 of the connecting rod 26. 
In FIG. 2 the means limiting the rotation of the eccentric cam 34 relative 
to the crankpin 32 comprises means located at the interface of the ring 
inner circumference and the crankpin outer circumference in the form of a 
key 40 which extends axially in one relieved area 42 extending along an 
arcuate portion of the outer circumference of the crankpin, and another 
relieved area 44 extending along an arcuate portion of the inner 
circumference of the eccentric ring, the depth of the two relief areas 
each equaling half the diameter of the key. 
FIG. 3 shows the way in which the means limiting the rotation of the 
eccentric cam relative to the crankpin between one end point and an 
opposite angularly displaced end point results in the addition of the 
maximum eccentricity of the cam to the eccentricity of the crankpin at the 
one end point, and at the opposite end point adding only a part of the 
maximum eccentricity of the ring to the eccentricity of the crankpin to 
give the change in stroke length. In FIG. 3A, the crankpin and cam are 
shown in a top dead center position under a condition of the crankshaft 
and crankpin rotating clockwise as indicated by the arrow. FIG. 3B shows 
the parts in the bottom dead center position under the clockwise rotation 
mode. The dash line 46 projections to the center of the drawing represent 
the maximum stroke length achieved under the clockwise rotation. 
When the compressor has been stopped and restarted in the opposite 
direction by the reversible electric motor, which in FIGS. 3C and 3D is 
indicated as counterclockwise by the arrows, the crankshaft and crankpin 
will turn within the eccentric cam until limited in the relative rotation 
to the point where the two relieved area spaces have reversed their 
relationship as compared to that in FIGS. 3A and 3B. In FIG. 3C the pin 
and cam are shown in a bottom dead center position, while in FIG. 3D they 
are shown in the top dead center position. Again, the lines projected 
therefrom to the center of the page indicate the reduced stroke length 
achieved under the counterclockwise rotation. 
The approximate center of mass of the eccentric cam is indicated by the 
symbol 48. The centrifugal force acting on the eccentric cam will be in a 
line passing through the center of the crankshaft 30a and the center of 
mass 48 and is indicated in each of the FIGS. 3A-D by the dash line arrow 
50. It will be noted that in the maximum stroke length mode, the 
centrifugal force causes no moment because the line of action is also 
through the crankpin center 32a. In other words, the centrifugal force of 
the cam is tending to keep the cam in a position in which the eccentricity 
is aligned with the crank throw. 
In FIGS. 3C and 3D it will be seen that the centrifugal force line 50, 
which passes through the crankshaft center 30a and the center of mass of 
the eccentric cam, is offset from the crankpin center 32a by a length 
perpendicular to the line 50 indicated by the dash line 52. As a result, a 
moment or centrifugal force torque exists with its direction being 
indicated by the arrows 54 in these two views. It will be apparent that 
the centrifugal force torque is in a direction to shift the eccentric cam 
away from the end points in which the key 40 is shown in FIGS. 3C and 3D 
toward the other end points shown in FIGS. 3A and 3B. While during a large 
portion of one revolution of the crankshaft, the various forces noted 
before are sufficient to hold the cam at the end point, during a small 
portion of the stroke these holding forces can go sufficiently low that 
the centrifugal force torque and any gas thrust reversal can move the 
eccentric cam slightly back toward the other end point. Thus, in at least 
the reduced stroke length mode of operation, an oscillating load can be 
imposed upon the parts making up the rotation limiting means. 
In the arrangement according to the invention as shown in FIGS. 4-7, those 
parts which correspond to the prior art parts of FIGS. 1 and 3 are given 
identical numerals. Thus, the crankshaft 30 carries the crankpin 32 upon 
which the eccentric cam 34 is rotatably mounted. The key 40 is secured in 
the eccentric cam 34 and projects into a recess 56 provided in the outer 
circumferential face of the crankpin 32. The one and opposite end points 
58 and 60 of the recess are angularly displaced approximately 270.degree. 
in the embodiment shown. Thus, these end points are reversed in their 
angular sense from the end points of the prior art arrangement so that 
when the eccentric cam is pushed by the compressor torque from the maximum 
stroke length position of FIG. 4 to the reduced stroke length position of 
FIG. 5, the cam will have rotated on the crankpin through the long angle 
rather than the short angle route. 
The approximate center of mass of the eccentric cam 34 in FIGS. 4 and 5 is 
indicated by the symbol 62 and the centrifugal force line of the eccentric 
cam is indicated by the dash line 64, that line passing as in any such 
arrangement through the center of mass and the crankshaft center 30a. It 
will be observed in FIG. 4 that the centrifugal force line 64 also passes 
through the crankpin center 32a so that there is no moment or centrifugal 
force torque imposed upon the cam to rotate away from its position upon 
the crankpin. However, in FIG. 5, showing the reduced stroke length 
position, the dash line 66 which extends perpendicularly from the 
centrifugal force line 64 to the crankpin center 32a and represents a 
moment results in a centrifugal force torque indicated by the arrow 68 
which has a direction opposite to the direction of rotation of the 
crankshift and crankpin. Thus, with the arrangement of FIGS. 4 and 5, the 
cam effectively self-latches itself in at least the lesser stroke length 
position shown in FIG. 5 through the centrifugal force torque. 
While in most applications it would be expected that a centrifugal force 
torque would not be needed when the eccentric cam is in its maximum throw 
position, there may be instances in some applications, according to the 
structure and operating characteristics of the device, where a centrifugal 
force torque component would be desirable when operating in the maximum 
stroke length mode. In accordance with another aspect of my invention as 
shown in FIGS. 6 and 7, it is possible to obtain this torque by providing 
an imbalance on one side of the cam; the sides in this sense meaning those 
on the opposite sides of a diametral line passing through the maximum and 
minimum thickness portions of the cam. One way of obtaining this imbalance 
is to remove material from the cam on the one side as by way of the holes 
70 drilled in the cam and by loading other parts of the cam by inserting 
material (such as lead) 72, which is heavier than the cam material, in the 
holes on the other side of the cam. As a result, the center of mass 74 is 
shifted from its original position so that in the maximum stroke length 
position the centrifugal force line 76, which again passes through the 
crankshaft center 30a and the center of mass has a moment 78 and a 
centrifugal force torque indicated by the arrow 80 is in a direction 
opposite to the direction of rotation of the crankshaft and crankpin. 
Other combinations of weight addition and/or removal are obvious. 
It will be seen from FIG. 7 that even though the center of mass 74 has been 
shifted to provide the imbalance, a centrifugal force torque 80 is still 
available when the cam is positioned in its reduced stroke length 
position, and in a direction in opposition to the rotative direction of 
the crankshaft. 
It will be appreciated that since the centrifugal force torque is a product 
of the quantity of the mass and the moment, the relation of the torque 
forces in the opposite directions can be varied in accordance with the 
extent of lightening and loading to provide the imbalance. 
In the application of the invention to a refrigerant compressor useful in a 
heat pump especially adapted for northern climate operation, providing the 
two opposite end points 58 and 60 at around 90.degree. apart provides good 
results. Thus, the eccentric cam moves through about 270.degree. between 
its two rotative positions on the crankpin. However, the principle of the 
invention is considered to be applicable so long as the movement of the 
cam around the crankpin is at least in excess of 180.degree., or a half 
turn. In most applications of the invention, it is believed that providing 
the end points somewhere between about 70.degree. and 120.degree. apart 
will provide good results.