Oldham coupling

An Oldham coupling for connecting two parallel structural parts such as shafts is disclosed wherein an intermediate ring is freely movable between the hubs of the two structural parts intended for connection. The ring carries on its flat sides two lands located at 90.degree. relative to each other and the lands engage corresponding grooves in the hubs. The lands are convex at their friction surfaces and the load bearing walls of the grooves are correspondingly concave to receive the lands. An Oldham coupling of this type is particularly suitable as both a guiding and a rotation inhibiting mechanism for the revolving displacement body of a rotating piston machine of the displacer type.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an Oldham coupling to connect two parallel 
structural parts, for example shafts, wherein the intermediate ring freely 
moving between the hubs of the two structural parts to be connected 
carries on its flat sides two lands located at 90.degree. relative to each 
other, said lands engaging corresponding grooves in the hubs. Oldham 
couplings are especially suitable as the coupling element for a device 
revolving on a path, in which two structural parts must be maintained in a 
predetermined angular relationships relative to each other. This is the 
case for example in apparatuses, such as rotating piston positive 
displacement machines. 
2. Discussion of Related Art 
Oldham couplings of the above-mentioned type are disclosed in EP 10930 Bl, 
U.S. Pat. No. 4,437,820 and DE 27 35 664. All of those disclosures relate 
to displacement machines for compressible media. They each comprise a 
working chamber defined by helical circumferential walls extending 
vertically from a side wall and leading from an inlet located outside the 
helices to an outlet inside the helices. They further contain a helical 
displacement body extending into the working chamber. The latter is 
supported rotatingly without rotation relative to the working chamber. Its 
center is eccentrically offset relative to the center of the 
circumferential walls, so that the displacement body is always in contact 
with both the outer and the inner circumferential wall of the working 
chamber along at least one advancing line. During the operation of the 
machine therefore a plurality of sickle shaped working spaces are 
enclosed. The working spaces move from the inlet to the outlet through the 
working chamber. Depending on the angle of contact of the helix, the 
volume of the working medium conveyed may be gradually reduced with a 
corresponding increase of the pressure of said medium. 
In the known machines, the relative rotating motion is always transmitted 
by a highly stressed and thus expensive ball bearing. Furthermore, no 
measure is provided to insure the operation without clearance of the 
machine in case of the wear of the material of the wobble rod or rods. In 
all of those known machines, Oldham (cross-keyed) couplings are the 
rotation inhibiting means for the displacer. Radial displacement is 
limited by the contact of the helical ribs with the walls of the working 
chambers. This limitation theoretically corresponds to a circle, in this 
case a translational circle. The displacer, which does not rotate relative 
to the working chamber, must be guided by means of the Oldham coupling in 
a manner such that the parallel guidance permits a larger diameter than 
that corresponding to the diameter of the translation circle. The reason 
for this is the fact that the radial displacement of the displacer is to 
be limited by the rib/chamber wall combination and not by the guiding 
Oldham coupling. Using this rule, the dimensions of the Oldham coupling 
are readily determined. 
It is generally believed that such Oldham couplings are not suitable for 
the transmission of large torques and high rpm in view of the bending 
fatigue exposure and frictional losses. As in operation the lands are 
constantly sliding back and forth in the grooves, the Oldham coupling 
should be running in an oil bath in order to reduce friction losses and 
wear. Thus for example, the lands of the Oldham coupling according to DE 
27 35 664 include, although they consist of a self-lubricating material 
such as polyimide, regular oil grooves in their lateral sliding surfaces, 
intended to promote the development of an effective and efficient oil film 
around the lands. 
In all of the known Oldham couplings the lands consist of rectangular 
blocks engaging correspondingly shaped grooves. The objections to the use 
of Oldham couplings is understandable to the extent that the lateral 
clearance in the grooves must be minimal for uniform guidance. However, 
this necessarily leads to frictional surfaces which tend to wear. In 
addition, dirt may penetrate the coupling and jam the parts, which 
interferes with the operation of the coupling. 
SUMMARY OF THE INVENTION 
It is therefore the object of the invention to develop an Oldham coupling 
which remains free of clearances even in the case of progressive abrasion 
of material due to wear. 
According to the invention, this object is attained in that the lands are 
convex on their frictional surfaces, that the load bearing walls of the 
grooves to receive the lands are correspondingly concave and that the 
bottom of the grooves are recessed to prevent contact with the land. 
The advantage of the invention is to be seen in that the novel 
configuration provides an Oldham coupling that is self-adjusting in 
operation and nearly free of maintenance. 
It is especially convenient to design the friction surfaces of the lands 
and the load bearing surfaces of the grooves with a circular cross 
section. This insures that the bearing zone is always sufficiently large, 
independently of the magnitude of the axial and horizontal forces acting 
on the coupling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The pump according to FIGS. 1 and 2 essentially consists of two housings 
halves 1, 2, connected in a suitable manner with each other, and a 
displacer inserted in between. In the left half of the housing 1 an 
annular working chamber 4 is located. It is divided by a web 16 extending 
over the entire depth of the chamber. On either side of the web, in the 
rear wall of the half 1 of the housing, the inlet 17 and the outlet 18 for 
the working medium to be transported, are located. This working chamber is 
engaged by the annular rib 5 of the displacer 3. The ring is slit at the 
location opposite to the web 16. In operation, the displacer performs an 
orbital motion. 
For this, a wobble rod drive is provided. A crank drive 6, not shown in 
detail, is equipped on the crank side with a spherical articulated socket, 
in which the spherical end 7 of a wobble rod 8 is supported rotatingly. By 
means of a spherical collar 9, the wobble rod is supported practically 
without radial clearance in the hub of the displacer 3 in a rotating and 
pivoting manner. On its side engaging the left half 1 of the housing the 
wobble rod is also supported by means of a spherical end 10. 
During its revolving motion the displacer is in constant contact with both 
the inner and the outer circumferential walls of the working chamber. By 
the displacement of its position, the working medium is suctioned in 
through the inlet 17 into the chamber 4 and discharged from the machine 
through the outlet 18. 
An Oldham coupling is provided for the guidance of the displacer without 
rotation. It essentially comprises an intermediate ring 11, provided on 
its flat sides with lands 12, 13. In the example shown, the lands 12 
facing the displacer 3 may be displaced relative to the displacer on a 
common vertical axis. The lands 12 engage the correspondingly shaped 
grooves 14 in the displacer 3. The lands 13, which are offset by 
90.degree. with respect to the lands 12, i.e., horizontal in this case and 
thus not shown in FIG. 1, are facing the stationary right half 2 of the 
housing and may be displaced relative to it on a common horizontal axis. 
The lands 13 slide in appropriately configured horizontal grooves 15 in 
the frontal side of the housing half 2. 
The principle may be seen in FIG. 3, in which the hubs of the structural 
parts to be coupled are shown as simple rings. With reference to the pump 
shown in FIG. 1, the reference number 2 is used for the stationary housing 
part and the reference numbers 3 and 5 for the revolving displacer. 
The proper geometry of the parts sliding on each other is shown in FIGS. 4a 
and 4b. The semi-cylindrical frictional surface 19 of the lands 12, 13 
should coincide with the semi-cylindrical curvature of the wall of the 
grooves 14, 15. A semi-cylindrical shape with a radius R is chosen for 
both parts. FIG. 4b shows an inserted coupling in which the wall of the 
grooves 14, 15 carries over the entire available surface and is thus 
clearance free. FIG. 4a however shows a coupling prior to extended use 
wherein because of manufacturing inaccuracies or even an intentional 
difference in the radii of "ball and socket", the land 12, 13 is not in 
contact with the groove 14, 15 over the entire surface of the wall of the 
grooves. The land 12, 13, however, remains carried over a considerable 
section on the upper edge of the groove 14, 15. Consequently, the coupling 
in FIG. 4a is still without clearance. It may also be seen that jamming as 
a result of material wear after extended use is not possible. The coupling 
is absolutely without clearance, regardless of the mutual position of the 
land and the groove. 
The bottom 21 of the groove 14, 15 is offset rearward so that even with a 
complete insertion of the land 12, 13 into the groove there is no contact 
with the bottom. The recessed bottom of the groove prevents in any case 
the location of the load bearing zone in the head of the land, i.e., in 
the bottom of the groove. In such a case, as shown by experiments, a 
lateral clearance may develop between the wall and the lands. 
The prevailing forces include firstly the contact pressure F.sub.s, which 
according to FIGS. 4a and 4b acts in the vertical direction, i.e., in the 
axial direction of the coupling. This force usually corresponds to a 
spring force. Therefore in view of the minimal spring path, the force 
F.sub.s is usually constant to some extent. Secondly, horizontal forces 
F.sub.t acts on the vertical lands 12, 13, which forces F.sub.t are 
variable relative to size and direction. Both of these are functions of 
the position and magnitude of the frictional forces between the annular 
displacer 3 and the wall of the working chamber 4. 
The normal force acting on the load bearing wall 20 of the grooves is the 
resultant of the forces F.sub.s and F.sub.t. It is therefore seen that the 
load along the bearing zone is not uniform. If F.sub.t is larger than 
F.sub.s, the load in the upper segment section of the groove is higher 
than in the lower segment. It may further occur that upon a reversal of 
the force ratios the center vector of the reaction force slowly turns 
downward. Any migration of the force vector into the groove bottom must be 
prevented. The solution is the recessed groove bottom. 
It is further seen in FIGS. 4a and 4b that the intermediate ring 11 and the 
lands 12, 13 are of a single piece. It may consist of a deep-drawn 
workpiece, which has a favorable effect on production costs. 
In the afore-described application in positive displacement machines it ma 
be advantageous to make the integral workpiece of a corrosion resistant 
spring steel. As shown in FIG. 5, it is then possible to prestress the 
intermediate ring so that abutment in the grooves without clearance is 
assured under all operating conditions. In addition, the element also 
applies the axial force to the displacer 3, that is necessary to maintain 
the sealing action between the frontal sides of the rib 5 of the displacer 
3 and the working chamber 4. 
Although only preferred embodiments are specifically illustrated and 
described herein, it will be appreciated that many modifications and 
variations of the present invention are possible in light of the above 
teachings and within the purview of the appended claims without departing 
from the spirit and intended scope of the invention.