A self-aligning piston assembly has a piston member having a sidewall for slidable displacement within a bore, the piston member having an inner curved surface, the piston assembly having an inner race member having means to attach to a connecting member, the inner race member having a curved surface complimentary to the inner curved surface and, an elastomer member disposed therebetween substantially in the form of a sphere to provide a spherical type mounting of the piston to the connecting member. Utilizing a spherical type mounting, axial rigidity is maintained yet any side loading which acts through the connecting member or manufacturing tolerances are absorbed through the spherical elastomer to prevent misalignment or cocking of the piston within the bore during stroking. Utilizing the invention, excessive wear and fluid leakage by the piston is avoided since misalignment is absorbed by the elastomer member.

TECHNICAL FIELD 
This invention relates to a piston assembly and more particularly to a 
piston having a spherical bearing type mounting to a connecting member for 
accommodating self alignment of the piston within a bore. 
BACKGROUND 
There are many different piston designs used, for example, in air 
compressors, combustion engines, or in hydraulic actuators, among others. 
For example, in a hydraulic actuator, hydraulic fluid pushes against one 
side of a piston located within a bore to cause translation of a rod 
attached thereto. Typically the rod is connected to a member which is 
displaced in accordance with the translation of the piston within the 
bore. 
In FIG. 1, a conventional arrangement is shown where a piston (A) resides 
in a bore (B) having means for applying fluid pressure to one side of the 
piston which results in translation of a rod (C) attached thereto. This 
rod may be used for example to position the rotor blades of a helicopter 
aircraft for blade fold storage. Of course, such actuators have many 
different applications. 
One problem with these and other piston arrangements is that there is a 
rigid connection of the piston to the rod. Such a rigid connection locks 
the piston to the rod such that when bending loads are applied to the rod, 
they are transmitted to the piston which may cock within the bore. For 
example, if the actuator were used on an aircraft, flexing of the 
associated aircraft structures may cause such cocking of the piston and 
thereby result in excessive wear of the bore which in turn leads to 
leakage of the hydraulic fluid. Also, misalignment or eccentricities do 
occur due to tolerance stack-ups in manufacturing and assembly. Even in 
assemblies where the piston is pinned to a rod, such as a connecting rod 
in a combustion engine, such pinned type connections allow misalignment to 
be absorbed only in one direction, the direction about which the 
connecting rod is rotatable. In all other directions, it is a rigid 
connection and thus side loads could cause cocking of the piston and 
result in excessive bore wear. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a piston flexibly connected to 
a connecting member. It is yet another object of the present invention to 
provide a piston which is self-aligning in a cylinder bore. 
It is a further object to provide a piston assembly which includes a self 
aligning piston mounted by spherical bearing means to a connecting member. 
These and other objects of the present invention are achieved by a piston 
assembly comprising a piston having an outer race portion with a curved 
surface, an inner race member with a complimentary curved surface and an 
elastomer member located between the inner race member and the outer race 
portion, the elastomer member substantially in the form of a sphere. 
Instead of a rigid or pinned connection, the piston is mounted to a rod 
with a spherical bearing type mount which allows side loads acting on the 
rod in virtually any direction to be absorbed by the elastomer member. The 
piston with the outer race portion is also free to adapt to the bore to 
accommodate manufacturing tolerances. Consequently, the piston is centered 
and sealed within the bore, preventing excessive bore wear and leakage.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 2, a hydraulic damper 10 is shown. While a hydraulic 
damper is exemplary of the invention, it will be understood that any 
piston incorporating device could benefit from the invention. The damper 
includes a housing 11 which has a bore 12 within which a piston assembly 
13 is reciprocally movable. The piston assembly is attached to a rod 14 
which has a lug 15 at an end thereof for connection to an associated 
structure. Similarly, the housing 11 has a lug 16 for attachment to a 
corresponding structure. Translation of the piston within the bore in 
response to fluid pressure acting against the piston thus causes a 
movement of a structure connected to one of the lugs. 
Referring to FIG. 2a, an enlarged cross-sectional view of the piston 
assembly utilizing the present invention is shown. The piston assembly 13 
has a surface 18 for slidable engagement with a wall 19 of the bore 12. A 
resilient seal 20 is provided in this embodiment for engagement with the 
wall. The piston 13 also has an inner chamber 21 which has an inner curved 
surface 22, to form an outer bearing race portion. 
The piston assembly 13 also-includes an inner race portion 23 which has an 
inner bore 24, within which a stub 25 of the rod 14 is located. The inner 
race portion 23 has means for attachment to the stub of the rod. In this 
instance, the stub is threaded and a nut 26 is used to attach the inner 
race portion to the rod. Also, an O-ring 27 is provided in a groove 28 to 
prevent leakage of any fluid by the joint between the inner race portion 
and the rod. The inner race portion has a curved surface 29 complimentary 
to the curved surface 22, with a space therebetween. 
Located between the pair of curved surfaces is an elastomer member 30. The 
inner chamber forming the outer race portion has a substantially spherical 
type curved surface, while the inner race has a complimentary 
substantially spherical curved surface to, in effect, transform the piston 
assembly into a spherical elastomeric bearing. The elastomer member is 
bonded to the complimentary curved surfaces to provide a unitary piston 
assembly. 
The elastomer member disposed between the inner and outer races may be 
composed of a single unitary elastomer layer, as shown in FIG. 2 or be 
produced as a plurality of bonded laminated plys as shown in FIG. 2a and 
be composed of an elastomer such as natural or synthetic rubber, or 
silicon elastomer, fluoro-elastomer or blends thereof. Of course, other 
elastomers may be chosen to provide specific performance characteristics 
related to fluid compatibility, temperature pressure limitations, fatigue 
life, etc. This choice can be made by one skilled in the art. For example, 
the unitary elastomer member may be composed of a blend of natural and 
synthetic rubbers, about 75 to 90 percent natural rubber, and 25-10 
percent synthetic rubber. Among these synthetic rubbers usable are 
polybutane, butanol, and butanol rubbers, among others. Additionally, 
silicon or fluoro-elastomers may be used. 
When a layered elastomer member is used, it is also possible to place plys 
of different elastomer materials together and to optionally include shims 
between the elastomer plys. The shims are made of a non-extensible 
material such as metal or a generally rigid plastic composite. Typically, 
metal shims are utilized between the plys. 
Referring to FIG. 3, an alternative embodiment of the invention is shown. 
In this embodiment, a piston assembly 31 has an outer race portion 32 and 
an inner race portion 33. A nut 34 secures the inner race portion 33 to a 
stub 35 of a rod 36. A laminated elastomer member 37 is located between a 
curved surface 38 on the inner race portion and a curved surface 39 of the 
outer race portion. The elastomer member has three elastomeric plys, 40a, 
b and c, separated by two non-extensible shims 41a and 41b. In this 
embodiment, three elements 42a, b and are disposed in three grooves 43 in 
a cylindrical sidewall 44 of the piston to assist in centering and sealing 
the piston assembly. The element 42a is a seal, and the elements 42b and c 
are guide rings which are commonly used to react to loads applied to the 
piston head. Also, spreading the elements across the sidewall assists in 
self-aligning the piston in a bore. 
The quantity, location and even provision of such seals or guide rings 
depends on the ultimate use of the piston, and no seals or guide rings are 
necessary to practice the invention. 
Preferably, the elastomer member and curved surfaces are shaped to act as a 
spherical type bearing mount. However, the degree of curvature and 
thickness of the elastomer can be tailored to accommodate known or 
anticipated loading conditions and the invention is not limited to a 
perfect sphere shape. The generally spherical shape is useful in piston 
assemblies because it limits axial distortion while providing sufficient 
flexibility to allow self-alignment within a bore. 
In an alternative embodiment of the present invention, shown in FIG. 4, a 
piston assembly 45 has an outer race portion 46, with a cylindrical 
sidewall 47, which has an inner curved surface 48. The degree of curvature 
and the thickness of an elastomer member 49 are tailored, by the curved 
surface 48, to accommodate expected design loads. An inner race portion 50 
has a curved surface 51 more closely resembling the ball shape, common to 
a spherical type mount. Regardless of these variations, a self-aligning 
piston is achieved. 
Referring to FIG. 5, an alternative embodiment is shown. A piston assembly 
52 has an outer race portion 53, having a cylindrical sidewall 54 and an 
inner curved surface 55. The outer race portion also has a piston head 
surface 56. A rod 57, has a forward end 58 which is shaped to have an 
outside curved surface 59 such that the it forms the inner race of the 
piston assembly. An elastomer member 60 is located between the curved 
surfaces. Thus, the piston head has an integral outer race and the rod has 
an integral inner race with the connection between the rod and piston 
accommodated through the generally spherical elastomer member 60 disposed 
and bonded therebetween. 
Yet another embodiment of the present is shown in FIG. 6 where a piston 
head 61 has a cylindrical sidewall 62 and an inner curved surface 63. An 
inner race portion 64 is provided which includes a pair of lugs 65 having 
a pair of co-axial passages 66 therethrough. A rod 67 has a complimentary 
passage 68 at a forward end thereof. A pin 69 is provided for attaching 
the rod to the piston portion via the lugs 65. 
As in the previous embodiments, an elastomer member 70 is disposed between 
a pair of curved surfaces 63 and 71 to form a spherical bearing type 
mount. Consequently, even where a pin type connection is used, the 
spherical bearing type mounting can provide self alignment of a piston to 
minimize the potential for wear and fluid leakage. 
Utilizing the invention, a self aligning piston is provided which is 
adaptable for absorbing various forces which may act to cock the piston 
during its stroking within a bore. The elastomer is fairly rigid with the 
physical shape limiting axial distortion yet the spherical type elastomer 
member provides sufficient flexibility to allow the piston to align itself 
properly within the bore even in the face of forces tending to cause 
cocking and distortion. In addition, the direction of these forces is 
irrelevant as the spherical type mounting is effective from all 
directions. 
While specific embodiments of the present invention have been shown and 
described, it will be understood by those skilled in the art that various 
changes and modifications could be made without varying from the scope of 
the present invention. For example, the inventive piston assembly may be 
disposed, rather than on an end of a connecting member, on a middle 
portion of the connecting member such as in mounting the piston (D) to the 
rod (C) in FIG. 1. Essentially, any piston could benefit from the 
spherical-type mounting arrangement of the present invention.