Note: Descriptions are shown in the official language in which they were submitted.
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CASTER WITH DUAL OFFSET ORBITAL MOUNTING ASSEMBLY
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority benefit of the filing date of
Provisional Application no. 61/374,700, filed August 18, 2010.
FEDERALLY SPONSORED RESEARCH
[0002] Not applicable.
SEQUENCE LISTlNG, ETC ON CD
[0003] Not applicable.
BACKGROUND OF THE INVENTION
Field of the Invention
[0004] This invention relates to casters and, more particularly, to caster
assemblies that allow the caster to pivot easily to accommodate the direction
of
thrust applied to an object supported by the caster.
Description of Related Art
[0005] Caster designs tend to fall into one of two groups: the 'single wheel'
and the 'twin wheel'. The single wheel caster consists of one wheel with an
axle
through the center which is attached to a clevis-like bracket that supports
the
axle at opposed ends. The bracket is joined to a bearing assembly that allows
the
caster to pivot about a generally vertical axis while also permitting rotation
of the
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wheel about a generally horizontal axis, thereby allowing the caster to roll
in any
direction. The bracket is offset from vertical by a head angle, and the
bracket
may include a rake offset, so that the caster bracket will tend to rotate
about the
pivot axis and extend in a direction that is directly opposed to the direction
of
motion of the object supported by the caster. This arrangement, known in the
[0006] The main problem of the 'single wheel' caster is its inability to turn
easily
about the pivot. When changing direction, the wheel is required to rotate
about
the vertical axis of the bracket bearing assembly. Differing portions of the
contact area of the wheel describe arcs of differing radii, requiring
differing linear
velocities, thus necessitating that some of the contact area is in rolling
contact
while other portions are sliding and dragging ("scrubbing"). The result is
wear of
the wheel and greater rolling resistance at low speeds and tight turns, and
most
particularly high rolling resistance during startup from zero velocity. A
narrower
wheel minimizes this problem, but results in greater pressure (force per unit
area)
applied to the floor surface.
[0007] The 'twin wheel' caster offered an improvement over the 'single wheel'
in two important regards. The ability of the wheels to rotate at differing
rates or
in opposite directions at the same time greatly enhances the ability to turn
about
the vertical pivot axis, making a change in overall direction of the object
much
smoother. Also, the separation of the two wheels establishes a wide base for
stability. One problem germane to this caster type is the method of attachment
of
the wheel to the axle. Unlike the single wheel with the clevis bracket
supporting
the outer ends of the axle, the double wheel design typically attaches each
wheel
to its respective end of the axle, the vertical pivot shaft being disposed in
a plane
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between the two wheels. The wheels and the ends of the shaft are relatively
unprotected and vulnerable to impact, which can result in collision-damaged
wheels or a bent axle. This attachment configuration also creates some
inherent
stability inefficiencies. By not allowing the axle to extend through the
wheel, the
loading of the wheel on the axle is not symmetric. That is, the inside of the
wheel
is fully loading the axle while the outside is not loading the axle at all.
Also, the
limited space remaining in the device for the wheel thickness results in thin
wheels, which directly result in narrow annular (limited) bearing surfaces of
the
wheel on the axle. The twin wheel design is typically limited to use in office
furniture, where the loads are lighter and the objects are not often moved.
[0008] A problem that is common to single wheel and twin wheel casters is that
the casters can become "locked up" when at least two casters supporting a
wheeled object are pivoted to extend in directions that are substantially
misaligned with each other. For example, if a cart with four swiveling
(pivoting)
casters is pushed toward a wall and abutted against the wall surface, it may
then
become difficult to slide the cart along the wall to reposition it, due to the
fact
that the casters are misaligned with respect to the plane of the wall surface.
In
general, when casters, either single wheel or twin wheel, are forced to pivot
about
the contact area that they engage on a floor or carpet, their rotational
movement
creates a substantial frictional resistance due to the fact that the wheels
are not
primarily rolling but rather are "scrubbing" on the contact area. The result
is that
objects supported by casters may be difficult to start to roll in a desired
direction
if that direction does not align with the casters. And precision placement of
the
3,
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object may involve a great deal more maneuvering than would otherwise be
necessary if the casters were capable of easily tracking the direction of
thrust.
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BRIEF SUMMARY OF THE INVENTION
[0009] The present invention generally comprises a caster assembly that
enables
the caster wheel to easily align itself with the direction of thrust applied
to a
caster-supported object. This advantageous feature is made possible by
providing a dual pivot assembly in the caster mounting that is laterally
offset,
whereby the caster wheels may not only pivot about a wheel pivot axis that
extends through the plane of the caster wheel, but also revolve orbitally
about a
mounting pivot axis that is laterally offset from the wheel axis. As a result,
the
caster assembly easily may assume the proper orientation for any thrust
applied to
the caster-supported object, whereby caster "lock-up" is eliminated.
[0010] The caster assembly is comprised of a transfer plate that is provided
with
a pair of cylindrical openings extending side-by-side in the plate and
laterally
offset. A pair of bearing assemblies are provided, each secured in a
respective one
of the openings. Joined in and extending through one of the bearing assemblies
is a mounting post that extends to a mounting plate adapted to be secured to
the
bottom surface of an object. The mounting plate is fixed to the object, and
the
transfer plate is free to rotate about the axis of the mounting tube on its
respective bearing assemblies.
[0011] The other bearing assembly supports a cylindrical head post that
extends
to a clevis-like bracket that straddles a wheel and supports at the clevis
ends an
axle for the wheel, as is known in the prior art. The clevis has a head angle
that is
typical for a caster wheel, and the wheel is free to rotate about the axis of
the
head post, which in turn is supported by the transfer plate, which itself is
rotatable
about the axis of the mounting tube. Thus the caster wheel may roll on its
axle
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on a floor surface, and revolve about the pivot axis of the head post, and
also
rotate orbitally about the axis of the mounting tube. These three axes of
movement enable the caster wheel easily to move into the proper alignment for
any thrust applied to the object supported by the caster. Indeed, if the
caster is
"locked up", that is, transverse to the thrust on the object, the object
appears to
pivot about the locked up wheel, using the orbital motion of the transfer
plate,
until the thrust vector more closely aligns with the wheel direction, after
which
the wheel may pivot into aligned orientation with the thrust vector.
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BRIEF DESCRIPTION OF THE DRAWING
[0012] Figure 1 is a perspective elevation of the caster assembly of the
present
invention.
[0013] Figure 2A and 2B are elevations depicting two similar embodiments of
the caster assembly of the invention.
[0014] Figure 3 is a side elevation of the caster assembly of the invention.
[0015] Figure 4 is a cross-sectional detail taken along line 4-4 of Figure 1.
[0016] Figures 5 and 6 are plan views of the caster assembly and its orbital
and
rotational movements.
[0017] Figure 7 is a plan view of two caster assemblies supporting an object,
the
caster wheel arrayed in conflicting directions.
[0018] Figure 8 is a plan view as in Figure 7, showing an object supported by
two prior art casters arrayed in similar conflicting orientations.
[0019] Figure 9 is an exploded view of the components of the caster assembly
of the present invention.
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. ,
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention generally comprises a caster assembly that
enables
the caster wheel to easily align itself with the direction of thrust applied
to a
caster-supported object. With regard to Figures 1-3 and 9, the caster assembly
21
is composed of a transfer plate 22, which is a high strength plate (steel or
the like)
having two cylindrical openings 23 and 24 extending along adjacent, generally
parallel axes A and B respectively. A mounting bearing assembly 26 includes a
bearing housing tube 27 that is pressed into the opening 24 and supports upper
and lower ball bearing races 28 and 29. A mounting post 32 has an annular
shoulder 33 that defines an upper end 34. The upper end 34 of the post 32 is
received in an opening 37 of a mounting plate 38. Note that the end 34 may be
pressed in the opening 37 or welded thereat, and that the annular shoulder 33
is
abutting the surface of the plate 38 to form a wide engagement that prevents
any
movement of the post 32. It may be appreciated that the transfer plate 22 is
freely
rotatable about the mounting post 32 on bearings 28 and 29.
[0021] A head bearing assembly 46 includes a bearing housing tube 47 (similar
to tube 27) that is pressed into the opening 23 and supports upper and lower
ball
bearing races 48 and 49. A head post 42 has a radial flange 43 at the upper
end
thereof. The head post 42 extends through the bearing races 48 and 49, with
the
flange 43 abutting the upper bearing race 48. A clevis bracket 51 is comprised
of
a central web portion 52 extending transversely to the axis A, with an opening
54
extending coaxially through the web 52. A pair of arms 53 extend in parallel
fashion from opposite sides of the central web 52, and support a wheel 56 on
an
axle 57 as is well-known in the prior art. The lower end of head post 42 is
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dimensioned to be received and secured in the opening 54 of the clevis
bracket,
as by press fit, weldment, or the like. As shown best in Figures 1-3, the arms
53
are parallel to axis A, and equally spaced apart therefrom. In addition, the
arms 53
are oriented at a head angle that is typical for a caster wheel, so that the
wheel 56
is free to rotate about the axis A of the head post 42, as well as to revolve
about
the axle 57. The head angle enables the caster wheel to trail in the direction
opposite to the velocity vector of the caster's path, as is known in the prior
art.
[0022] The structure of the caster assembly provides two independent, parallel
rotating mechanisms between the caster wheel and the supported object (the
load), and these two mechanisms combine to enable greater freedom of movement
of the wheel. The caster wheel may roll on its axle on a floor surface, and
rotate
about the pivot axis of the head post, and also rotate orbitally (circularly)
about
the axis of the mounting tube. These three axes of movement enable the caster
wheel easily to move into the proper alignment for any thrust applied to the
object supported by the caster. For example, as shown in Figure 5, the clevis
bracket 51 and wheel 56 may rotate freely about the head post 42 to assume any
directional orientation. In addition, and at the same time the transfer plate
22 may
rotate about the mounting post 32 in orbital fashion, as shown in Figure 6.
[0023] To illustrate one example of the benefits of the double pivot of the
invention, Figure 8 depicts a situation in the prior art in which an object M
has
two typical swivel casters C mounted at adjacent corners of the object. The
caster wheels are oriented obliquely transversely to the thrust vector V that
is
applied to the object M. As a result, it is difficult to initiate motion of
the object
M until the wheels can pivot to align in a trailing direction relative to
vector V.
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And it is difficult to pivot and align the wheels without some object
movement.
This is the "lock up" situation referenced previously.
[0024] With regard to Figure 7, in the same situation the casters 21 of the
invention enable easy initial movement, due to the dual pivot design. Indeed,
if
the wheel misalignment tends to block movement of the object M in the thrust
direction, the object may urge the object M to rotate about the axes A or B,
where there is freedom of motion, as suggested by the arrow in Figure 7. It is
apparent that the orbital motion of the transfer plate 22 enables object
motion in
the zero velocity "lock-up" situation, thereby alleviating a notable problem
in
the prior art. Thereafter the wheels will orient themselves and rolling
movement of
object M in the direction V will proceed smoothly. The ability of the double
pivot
orbital caster assembly to overcome the "lock-up" problem is not limited to
the
geometries of the example of Figure 7; indeed, it is applicable to all caster
wheel
directional orientations that may be expected.
[0025] Note that Figure 2A depicts the caster assembly described above, except
that it is fitted with a split-tread single wheel, as described in US patent
6,880,203,
issued to Phillip Aubin on April 19, 2005. A side-by-side twin wheel
arrangement
secured in clevis bracket 51 is also within the scope of the invention.
[0026] The foregoing description of the preferred embodiments of the invention
has been presented for purposes of illustration and description. It is not
intended
to be exhaustive or to limit the invention to the precise form disclosed, and
many
modifications and variations are possible in light of the above teaching
without
deviating from the spirit and the scope of the invention. The embodiment
described is selected to best explain the principles of the invention and its
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practical application to thereby enable others skilled in the art to best
utilize the
invention in various embodiments and with various modifications as suited to
the
particular purpose contemplated. It is intended that the scope of the
invention be
defined by the claims appended hereto.
I 1,
