Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ADJUSTABLE RACR FOR ELECTROSTATIC PAINTING AND THE LIKE
Background and Summary of the Invention
In a typical electrostatic paint spraying operation,
workpieces to be painted are suspended on a conveyor which
advances through a spray painting booth. The spray device
and the workpiece are oppositely charged at a high voltage,
so that charged paint particles issued from the spray
device are electrostatically attracted to the workpiece to
be painted, enabling the workpiece to be coated on surfaces
that are not directly visible to the spray device. In most
cases, special hangers or adaptors are provided for
suspending the workpiece from the conveyor at a suitable
location. Additionally, where the workpieces are
relatively small, it is common to employ special rack
devices, which are suspended from the conveyor and which in
turn support a plurality of workpieces.
For efficient production operations, it is beneficial
to provide for optimum density of the workpieces being
carried by the conveyor. Typically, this is provided for
by utilizing adjustable rack devices, which can be manually
configured to suspend a plurality of workpieces in a
desired arrangement, depending upon the size and shape of
the workpieces. For this purpose, the racks are provided
with a plurality of metal hooks upon which the workpieces
can be suspended in an arrangement and configuration
providing optimal density.
When workpieces are being conveyed through the spray
painting booth, the supporting rack is maintained at ground
potential by reason of its contact with the conveyor
mechanisms, and the workpieces themselves are maintained at
ground potential by reason of their contact with the hooks
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by which they are suspended on the rack. Because both the
rack and the workpieces are at ground potential, the highly
charged paint particles are attracted to the rack as well
as to the workpieces supported thereon. After a few cycles
S of use, there is a continuing buildup of paint on the rack,
which begins to present a progressively larger surface area
to the spray devices. Eventually, the efficiency of the
spray coating operation deteriorates. Accordingly, after
a certain number of cycles of use, the racks are placed in
a burn-off oven and subjected to high temperature for a
period of time sufficient to incinerate the layers of
coating accumulated thereon.
In order to minimize the inventory of racks required
in a typical operation, it has been proposed previously to
construct the racks to accommodate the use of workpiece-
supporting hooks in various configurations and of various
types. One example as such is represented by the Hines
U.S. Patent No. 4,628,859, wherein hook devices are formed
with spring clip means for mounting of the hooks on cross
bars of a rack. Previously known designs for such racks
have, however, suffered from significant shortcomings,
which are addressed by the present invention. Among those,
when the rack, or parts thereof, are subjected to extended
high temperature baking in the burn-off ovens, the temper
of the spring steel is lost, resulting, in some cases, in
the hooks being unsuitable for further use and, in other
cases, in an inefficient electrical contact. Where
electrical contact between the hook and the rack is
inefficient, the workpiece supported by the hook receives
a reduced amount of paint, and the rack itself tends to
become a primary target.
Pursuant to the present invention, a novel and
improved electrostatic paint spray rack is provided, in
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which a skeletal rack frame is arranged for removable
mounting thereon of a plurality of cross bar supports at
various horizontal levels over the vertical length of the
rack frame. To advantage, the mounting means for the cross
bar supports can be a plurality of threaded sockets for
receiving thumb screws or the like for securing of the
cross bars. When the rack is set up for a given operation,
one or a plurality of the cross bars is assembled to the
rack frame, depending upon the size and shape of the
workpieces to be painted.
Each of the cross bar supports is provided with a
plurality of workpiece-supporting hooks. In accordance
with the invention, the hooks are formed of stainless steel
wire, and are permanently secured to the cross bar supports
by welding. The stainless steel hooks are highly resistant
to the extended baking required in the burn-off ovens.
Additionally, unlike spring steel or cold rolled steel, the
stainless steel does not acquire an oxide coating during
the course of the burn-off operations and is thus better
able to maintain good electrical contact with workpieces to
be painted. In this respect, the welding of the stainless
steel hooks to the cross bar support provides for excellent
electrical contact between the hooks and the support, far
superior to that achieved by spring clip devices or the
like, especially after being subjected to burn-off cycles.
The necessary burn-off operations are greatly
facilitated by the rack design of the present invention,
because the individual cross bar supports may be quickly
removed from the skeletal frame struc'.ure and placed in the
oven separately from the frame structures. This enables
far greater efficiency in the loading of the burn-off oven,
and minimizes tangling of the parts etc.
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In general, the frame structure of the invention
provides for significantly greater efficiencies in the
painting operations, while at the same time providing for
low cost inventory of the workpiece-supporting racks.
For a more complete understandlng of the above and
other features and advantages of the invention, reference
should be made to the following detailed description of
preferred embodiments of the invention and to the
accompanying drawings.
Description of the Drawin~s
Fig. 1 is a partially exploded perspective view of a
typical form of electrostatic paint spray rack constructed
in accordance with the principles of the invention.
Fig. 2 is an enlarged, fragmentary view illustrating
details of the mounting of cross bar supports on the
skeletal frame structure.
Fig. 3 is an enlarged, cross sectional view as taken
generally on line 3-3 of Fig. 2.
Fig. 4 is a cross sectional view as taken generally
along line 4-4 of Fig. 1.
Figs. 5-7 are fragmentary perspective illustrations
of various representative forms of workpiece-supporting
hooks utilized in the rack structure of Fig. 1.
Description of Preferred Embodiments
Referring now to the drawing, and initially to Fig.
1, the reference numeral 10 designates generally a skeletal
frame structure of an adjustable rack. The frame includes
a pair of spaced-apart vertically extending rod-like frame
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elements 11, 12, which are rigidly fixed in spaced-apart
relation to a pair of horizontally extending frame elements
13, 14. Additional frame elements 15, 16 are secured to
the respective vertical elements 11, 12 and extend
convergently upward, joining at the center. The several
frame elements 11-16 are welded together to form a rigid
structure. In a typical construction, the frame elements
11-16 may be of cold rolled steel rod of, for example, 3/16
- 1/4 quarter inch diameter.
At the region where the upwardly convergent frame
elements 15, 16 join, a washer 17 is secured by welding.
Directly below the washer 17, a similar washer 18 is welded
to the horizontal rods 13, 14.
A removable hanger element 19, preferably formed of
stainless steel rod, is provided at spaced-apart points,
corresponding to openings in the respective washers 17, 18,
with threaded nuts 20, 21. Advantageously, the nuts 20, 21
have a somewhat greater than normal axial length. By way
of example, they may be one quarter - 20 SAE standard high
nuts. These are secured to the hanger element 19 by
welding.
As shown in Fig. 4, the hanger element 19 is
removably secured to the rack frame by means such as thumb
screws 22, threaded portions 23 of which extend through the
respective washers 17, 18 and into the nuts 20, 21.
The hanger element 19 may be as long as desired, and
is formed with a hook portion 24 at its upper end, for
engagement with a paint system conveyor ~not shown). To
advantage, the hanger element 19 may be easily replaced
with one longer or shorter, by merely removing and
replacing the thumb screws 22, so that the rack frame 10,
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and any workpieces carried thereby, may be optimally
positioned, heightwise, within the spray booth.
In accordance with the invention, each of the
vertical rods 11, 12 has secured thereto by welding, at
S spaced points, nuts 25, which are similar to the nuts 20,
21 previously described. In a preferred construction, the
nuts may be spaced apart vertically approximately two
inches, and are arranged in cooperating pairs along the
spaced apart vertical rods 11, 12.
In accordance with the invention, the rack is
provided with removable cross bar elements 26, typically
comprising straight, horizontal rods of cold rolled steel
formed into a mounting eye 27 (Fig. 2) at each end, of a
diameter suitable for receiving the threaded portion 23 of
a thumb screw 22. Typically, the thumb screw 22 is
provided with a circular flange 28 adjacent the end of the
threaded portion 23. A cross bar 26 is rigidly mounted to
the rack frame structure by securing the mounting eyes 27
at each end to an appropriate pair of the welded-on nuts 25
using thumb screws at each end. The thumb screws are
tightened firmly, so that excellent electrical contact is
established between the cross bar 26 and the vertical frame
rods 11, 12.
The number of cross bars 26 employed in a completed
rack assembly is of course a function of the size and
character of the workpieces to be painted. It is to be
understood, of course, that the specific structure of a
frame may assume a rather wide variety of sizes and
configurations. By way of example only, a typical rack may
utilize vertical rods 11, 12 of sufficient length to
provide for 12-14 pairs of nuts 25 spaced about two inches
apart. Normally, only a few of these pairs, chosen to
provide appropriate spacing, would be employed in a given
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rack configuration.
Pursuant to the invention, each of the cross bars 26
is provided with a plurality of workpiece suspending hook
elements 29 (Fig. l). The individual hook elements are
formed of a stainless steel wire, for example, thirteen to
sixteen gauge wire, and may take a wide variety of specific
configurations, depending upon the workpieces to be
handled. Importantly, each of the hooks is permanently
secured by welding to the cross bar 26 to provide and
maintain optimum electrical conductivity between the cross
bar and the workpiece hook.
While it will be understood that the workpiece-
supporting hooks 29 may be of a wide variety of shapes and
sizes, representative examples are shown in Figs. 5, 6 and
7. In the modification shown in Fig. 5, which is the same
as in Fig. l, the hooks 29 are formed with vertically
extending shank portions 30, welded to the cross bar 26 and
terminating in a sharp "V" 31 at the lower end. In the
version of Fig. 6, the hook members 29' are provided with
horizontal shank portions 32, rigidly welded to the cross
bar 26 and terminating at their outer ends with a
relatively shallow, generally U-shaped portion 33 for
receiving the workpiece. In the modification of Fig. 7,
the hook elements 29'' have horizontally extending shank
portions 33 welded to the cross bar 26 and terminating at
their outer ends in a sharply defined V-shaped sections 34.
In the versions shown in both Figs. 6 and 7, the hook
members 29', 29 " extend alternately in opposite directions
from the cross bar 26 to assist in spacing of the
workpieces. The particular type and design and
arrangements of the hooks is relatively limitless, and does
not form a part of the present invention, apart from the
consideration that the hooks are to be of stainless steel
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permanently welded to the cross bar elements 26.
As reflected in Fig. 1, a typical rack configuration
may include, for example, two or three cross bars 26
carrying a plurality of hooks 29 of the desired
configuration. Desirably, the unused nuts 25 of the frame
are covered with masking caps 35 (Fig. 3). To advantage,
these may be of a style manufactured by Mocap, St. Louis,
MO. Particularly where the frames are to be used for
electrostatic powder coating, the masking caps should be
resistant to relatively high temperature, because the rack
will be exposed to a curing oven, immediately following
electrostatic powder coating, in order to fuse the powder
particles to the coated workpiece. The masking caps 35
keep the outer surfaces, and also the internal, threaded
portions of the nuts 25 free of any of the coating
material. This not only simplifies cleanup of the frame
portions of the rack, when that becomes necessary, but also
enables the rack to be reconfigured at any time without
baking of the frame, where that is appropriate.
In the normal use of a rack of the type shown in Fig.
1, the rack is initially configured according to the nature
of the workpieces to be painted, in order to achieve an
optimum workpiece density. Electrically conductive
workpieces are suspended from the hooks 29, and the entire
rack is suspended by its own hook 24 from the paint system
conveyor. The rack and its parts are then conveyed through
a spray booth containing one or more electrostatic spray
nozzles for applying a desired coating material to the
workpieces. The spray nozzles are typically charged to a
relatively high voltage in relation to the paint system
conveyor, which is at ground potential. The rack 10 is
also at ground potential, by reason of its contact with the
conveyor through the hook 24, as are the conductive
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workpieces (not shown) by reason of their contact with the
hooks 29. After the parts are spray coated, they are
typically passed through an oven, to drive off any liquid
vehicle, in the case of liquid coating materials, or to
fuse the powder particles, in the case of electrostatic
powder coating.
As will be understood, during the spray coating
procedure, the elements of the rack itself become coated,
as well as the workpieces. Accordingly, after a few round
trips through the painting system, a given rack accumulates
a heavy coating of the sprayed-on material, as successive
layers build up over previously applied layers. Moreover,
as the layers build up, an increasingly larger surface area
of the rack is presented to receive coating material from
further coating operations. As a result, the operation
becomes progressively less efficient, and at some point the
rack must be taken out of service and cleaned. Typically
this is done by placing the rack in a high temperature oven
for a sufficient period of time to burn off the accumulated
paint products.
One of the significantly advantageous features of the
present invention resides in the fact that the cross bar
elements 26 can be quickly separated from the rack frame,
by removal of the thumb screws 22, and placed separately in
the burn-off oven. Several advantages are derived from
this: For one, the rack frame may not require burn-off as
frequently as the hook-supporting cross bars 26. With the
structure of the present invention, the rack frame can be
reconfigured with ne~ cross bar elements and used again.
In this respect, the masking caps 35 keep the unused nuts
clean and free of coating material, so that a coated rack
frame may nevertheless be utilized with "new" cross bar
elements either in the same locations or in different
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locations, as may be desired.
Because the burn-off operations involve principally
the cross bar elements, smaller oven facilitates may be
employed, as a group of cross bar elements will nest
together in a highly compact manner. If rack frames are
also being burned off, it is also possible to place them in
the oven in flat, compact relation, after the cross bar
elements have been detached. In all cases, the burn-off
operation is simplified and expedited in comparison to
previous experience.
The structure of the applicant's rack, comprising
permanently mounted stainless steel hooks for suspending
the workpieces, and where desired a stainless steel hook
for suspending the rack itself from the system conveyor,
has very significant advantages over conventional racks.
With racks of conventional design, for example, using hooks
of spring steel material, the hooks generally loose their
temper after about two burn~off operations, after which
they must be discarded. Additionally, parts made of cold
rolled steel form a black oxide coating during the burn-off
operation, which substantially reduces the conductivity at
critical points, where the hooks are attached to the rack
frame, where the hooks engage the workpiece, and where the
rack engages the electrically grounded system conveyor.
Reduction of conductivity in any or all these areas leads
to serious inefficiencies, as the workpieces become less
attractive to the particles of coating material, and a
higher proportion of the coating material may be attracted
to the rack itself. With the structure of the invention,
on the other hand, the individual work-supporting hooks are
permanently welded to the cross bar, so that conductivity
between the hooks and the cross bar is totally unaffected
by burn-off or other activity. Additionally, the
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workpiece-supporting hooks do not acquire an oxide coating
during burn-off, so that there is no diminution of the
conductivity between workpiece and supporting hook.
Likewise, the hanger element by which the rack is supported
from the conveyor, if fcrm~d of stainless steel, will
maintain optimum electrical contact at that point as well.
In the rack design of the present invention, although
the cross bar elements 26 are formed of cold rolled steel
and will themselves acquire an oxide coating during burn-
off, good electrical contact is easily established betweenthe rack frame and the cross bars, by tightening of the
thumb screws 22, which clamp the mounting eyes 27 of the
support member tightly, and with some relative movement
between the circular flanges 28 and the surface of the
mounting eye 27 to provide a cleaner metal-to-metal
contact.
The rack structure of the invention, among other
advantages, has a greatly extended operating life as
compared to conventional rack designs. Because there is
essentially no deterioration of the hooks and/or of the
conductive association of the hooks with the cross bar 26,
the cross bar elements may be exposed to extensive burn-off
operations without significant degrading of performance.
It should be understood, of course, that the specific
forms of the invention herein illustrated and described are
intended to be representative only, as certain changes may
be made therein without departing from the clear teachings
of the disclosure. Accordingly, reference should be made
to the following appended claims in determining the full
scope of the invention.
