U.S. patent application number 10/714729 was filed with the patent office on 2004-05-20 for safety toe-sensor for lift table.
Invention is credited to Stone, Robert M..
Application Number | 20040094077 10/714729 |
Document ID | / |
Family ID | 46300353 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040094077 |
Kind Code |
A1 |
Stone, Robert M. |
May 20, 2004 |
Safety toe-sensor for lift table
Abstract
A lift table includes a toe sensor responsive to contact around
the entire perimeter of the base. Multiple pressure switches are
installed over two sides of the base and capped by an overlapping
four-sided suspended guard. Multiple guides and springs of
appropriate stiffness support the guard suspended over the base, so
that any appreciable pressure exerted on the guard causes the
compression of the springs and the direct application of pressure
on an underlying switch which, in turn, de-energizes the mechanism
controlling the descent of the platform. The pressure switches are
incorporated into a control circuit that is activated only when the
platform is descending. The platform may be sized to extend over
the base enough to overlap the guard of the toe-sensor.
Inventors: |
Stone, Robert M.; (Tucson,
AZ) |
Correspondence
Address: |
QUARLES & BRADY STREICH LANG, LLP
ONE SOUTH CHURCH AVENUE
SUITE 1700
TUCSON
AZ
85701-1621
US
|
Family ID: |
46300353 |
Appl. No.: |
10/714729 |
Filed: |
November 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10714729 |
Nov 17, 2003 |
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09432525 |
Nov 3, 1999 |
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Current U.S.
Class: |
108/147 |
Current CPC
Class: |
B66F 17/00 20130101;
B66F 7/065 20130101 |
Class at
Publication: |
108/147 |
International
Class: |
A47B 009/00 |
Claims
I claim:
1. A lift table comprising: a base, a load platform, and a
vertically extendible means for raising and lowering the platform
relative to the base; a sensor mechanism including a plurality of
sensors mounted on said base of the table and affecting a perimeter
of the base; a guard operably connected to the base and mounted
over said sensor mechanism and adapted to activate the sensor
mechanism when pressure is exerted upon the guard; and a plurality
of springs between the base and the guard, such that the guard is
suspended over the base; thereby providing a means for interrupting
a descent of the platform in response to an object encroaching said
perimeter of the base and exerting pressure on the guard.
2. The lift table of claim 1, wherein said sensors comprise a
pressure switch.
3. The lift table of claim 1, wherein said sensors comprise a
Hall-effect switch.
4. The lift table of claim 1, wherein said sensors comprise an
optic-electronic fiber switch.
5. The lift table of claim 1, further comprising a valve actuated
by any one of said sensors to close and hydraulically interrupt the
descent of the platform when pressure is exerted on the guard.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part application of
Ser. No. 09/432,525, filed Nov. 3, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates generally to safety mechanisms for
load elevators; in particular, it relates to toe sensors for lift
tables.
[0004] 2. Description of the Prior Art
[0005] A typical lift-table configuration includes a load platform
extendibly supported by two pairs of legs. The two pairs are
located at opposite sides of the platform, and the legs of each
pair are pivotally connected to one another at the centers of the
legs. Each pair of legs thus forms a scissors-like assembly. The
upper end of one leg of each pair is hinged to the platform while
the upper end of the other leg has a roller movable along the
platform. The bottom ends of the legs are similarly mounted on a
base which may be movable or anchored to the floor of a warehouse
facility. An appropriate electric, pneumatic or hydraulic lifting
mechanism is used to raise and lower the platform of the lift
table.
[0006] Low-profile lift tables are favored in commercial
applications because of the need to minimize the height of ramps
used to place loads over them. In addition, because many loads
consist of palletized boxes stacked above shoulders' height, a low
profile advantageously facilitates their handling by workers while
reaching for loads placed on top. Therefore, it is very desirable
that a lift table be capable of compressing to the point where its
platform is only a few inches above ground floor. Currently, most
commercial lift tables are about 18 cm high (about 7 inches) when
completely lowered, some low-profile models being as low as 5 cm
(about 2 inches). Since the scissor assembly and the lifting
mechanism of a lift table are normally fully contained within the
base under the platform, a low profile can only be achieved by
having the platform overlap and protrude downward over the edges of
the base, so that the platform's structure becomes substantially
coextensive with the base. Thus, a safety hazard is created by the
platform closing in on the base as the lift table is lowered to its
lowest point.
[0007] Safety standards vary from country to country, but most
jurisdictions mandate that a predetermined clearance be maintained
between the platform and the base of a lift table in order to
prevent foot injuries. Since a lift-table user is likely to be
standing on its side loading and unloading packages, it would be
normal for him or her to extend a leg close to or under the
platform to gain more stability. Under such circumstances, it is
also likely that a foot may be rested against or over the edge of
the base, thereby placing the person at risk while the platform is
being lowered toward the base.
[0008] In order to prevent this hazardous condition, U.S. ANSI
Standards prescribe the use of toe sensors mounted along the
underside of the platform's perimeter such that, when an
obstruction is met during descent, the lift will be brought to rest
before entrapment can occur, whether the lift is loaded or empty,
and movement will not be restored until the obstruction is removed.
Any unguarded horizontal clearance between the toe sensor and the
underside perimeter of the platform must be limited to 22
millimeters (about {fraction (7/8)} inches). Similarly, European
Community standards require a vertical clearance of the platform
over the floor of 120 mm (about 5 inches) and a horizontal
extension of the platform beyond the base of 150 mm (about six
inches) if no toe sensors or barriers are used. When a lift is
equipped with a contact-activated device fitted to the outer edge
of the platform, this cannot be more than 8 mm (about {fraction
(3/8)} inches) from the outer surface of the device.
[0009] As a result of these safety standards, lift-table
manufacturers have focused on equipping their products with
perimeter toe sensors mounted along the outer edge of the platform.
Most products utilize switches actuated by vertical guards that
slide upward and de-energize the mechanism controlling the descent
of the platform upon contact with an underlying obstacle. Some
models utilize pressure-sensitive switches that are actuated by
contact with the obstacle. Such switches are available in a variety
of forms, such as the Controflex.RTM. electric tape switch sold by
Tapeswitch Corporation of Farmingdale, N.Y., and the
optic-electronic fiber Lightspeed.RTM. available from Southworth
Products Corporation of Portland, Me. This solution to the problem
is undesirable because it still makes it possible for a lift
operator to rest his/her foot over the edge of the base with no
safety response until the toe guard (or other similar device)
attached under the descending platform contacts the foot and
disables the system. Because of the response time involved, by then
it may be too late to prevent some degree of injury.
[0010] Another approach, based primarily on compliance with
European standards, has been to build larger than necessary
platforms to provide the prescribed clearances. This alternative is
also undesirable because it requires a structural configuration
that is unrelated to functional design criteria, which in some
cases may be unacceptable for a particular application. For
example, it may important to be able to distribute the load
uniformly over the platform of a lift table, but that may be
difficult to achieve with a relatively large platform that extends
six inches beyond the base on all sides. Similarly, the lever arm
resulting from the six-inch extension tends to render the lift
unstable. Moreover, the vertical clearance requirement limits the
minimum height of an overhanging platform to about five inches,
which is higher than desirable and otherwise attainable for many
applications.
[0011] Simpler but less effective solutions have been implemented
by using disabling pressure-sensitive mats and/or protective guards
around the perimeter of the lift table to prevent encroachment by
the operator. The former approach also prevents productive
utilization of valuable space around the lift table; and the latter
requires the addition of often cumbersome and expensive guard
structure to the base of the lift table. Therefore, neither
solution is desirable. Accordingly, there is still a need for a
better solution in preventing injuries caused by the descending
platform of a lift table.
SUMMARY OF THE INVENTION
[0012] A goal of this invention is a lift table that includes a
safety device to prevent a user's foot from protruding under the
platform and suffering injuries as the platform is lowered to its
ground position over the base.
[0013] Another objective of the invention is a safety mechanism
that is actuated by a dangerous condition prior to contact with the
descending platform of the lift table.
[0014] Another goal of the invention is a safety mechanism that can
be implemented within the requirements of all current safety
standards for lift tables.
[0015] Still another objective is a safety mechanism in the form of
a toe sensor that can be implemented in the base of the lift table,
rather than its platform, thereby facilitating the realization of
an overall very low profile configuration.
[0016] A final objective is the realization of the above mentioned
goals in an economical and commercially viable manner. This is done
by utilizing simple components that are either already available
commercially or that can be produced competitively during the
manufacture of the lift table.
[0017] Therefore, according to these and other objectives, the
present invention consists of a lift table that includes a
disabling toe sensor responsive to contact around the entire
perimeter of the base. Multiple pressure switches are installed
over two sides of the base and capped by an overlapping four-sided
suspended cover. Multiple guides and springs of appropriate
stiffness support the cover in free-floating arrangement over the
base, so that any detectable pressure exerted on the cover causes
the compression of the springs and the direct application of
pressure on an underlying switch which, in turn, de-energizes the
mechanism powering the descent of the platform. The pressure
switches are incorporated into a control circuit that is activated
only when the platform is descending. The platform may be sized to
match the base or extend over the base and overlap the cover of the
toe-sensor mechanism.
[0018] As a result of this design, the descending motion of the
platform is automatically stopped as soon as pressure is exerted on
the cover of the invention by a foot or other obstacle that is
either resting on top of the cover or leaning against its side.
Because of the negligible horizontal clearance between the platform
and the cover of the toe sensor, any obstacle sufficiently removed
from the cover to prevent the actuation of the pressure sensors
would also be unaffected by the complete descent of the platform.
On the other hand, any obstacle within reach of the platform would
cause its arrest by contacting the toe sensor prior to connecting
with the platform. Moreover, this design makes it possible to lower
the profile of the lift table to the fullest extent allowed by the
size of the various components, with no extra space required for
arbitrary safety clearances.
[0019] Various other purposes and advantages of the invention will
become clear from its description in the specification that follows
and from the novel features particularly pointed out in the
appended claims. Therefore, to the accomplishment of the objectives
described above, this invention consists of the features
hereinafter illustrated in the drawings, fully described in the
detailed description of the preferred embodiment and particularly
pointed out in the claims. However, such drawings and description
disclose only one of the various ways in which the invention may be
practiced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a perspective view of a conventional lift table
modified according to the invention by the incorporation of a toe
sensor within the base of the unit.
[0021] FIG. 2 is a perspective view of the base of the lift table
of FIG. 1 prior to modification according to the invention.
[0022] FIG. 3 is a perspective view of the base of the lift table
of FIG. 1 including perimeter pressure switches and
toe-sensor-cover support springs according to the invention.
[0023] FIG. 4 is a cross-sectional view of the base of FIG. 3, as
seen from line 4-4 in that figure, showing a pressure switch of the
invention.
[0024] FIG. 5 is the cross-sectional view of FIG. 4, further
illustrating a support spring of the invention.
[0025] FIG. 6 is the cross-sectional view of FIG. 5, further
illustrating the cover of the invention, as would be seen in part
from line 6-6 in FIG. 1.
[0026] FIG. 7 is a perspective view of the base of the lift table
of FIG. 1 including the toe-sensor cover of the invention.
[0027] FIG. 8 is a perspective view of the toe-sensor cover of the
invention.
[0028] FIG. 9 is cross-sectional view of the cover of the invention
as seen from line 9-9 in FIG. 8.
[0029] FIG. 10 is a top view of the base of the lift table
according to the invention without the switch guard.
[0030] FIG. 11 is a top view of the base of the invention
illustrating a foot stepping on one of its ends.
[0031] FIG. 12 is a schematic diagram of the electrical circuit
including the safety switches of the invention.
[0032] FIG. 13 is a perspective view of another embodiment of the
invention, wherein the toe sensor is implemented with non-contact
perimeter beam sensors.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] This invention is based on the idea of providing a
toe-sensor mechanism in the fixed base of the lift table rather
than in the moving platform, as previously done in the art. The
approach simplifies the implementation of the toe-sensor function
and minimizes the additional space required for compliance with
safety standards.
[0034] Referring to the figures, wherein like parts are designated
throughout by like reference numerals and symbols, FIG. 1
illustrates in perspective view a load elevator or lift table 10
according to the invention. In conventional manner, the lift table
10 includes a horizontal platform 12 for supporting loads, a
scissor assembly 14, a ground base 16 (which may be anchored to the
floor), and a motor or equivalent mechanism 18 with appropriate
electrical and hydraulic hardware for actuating the scissor
assembly and elevating or lowering the platform. The inventive part
of the lift table 10 resides in a toe sensor 20 incorporated into
the base 16.
[0035] As illustrated in isolation in the perspective view in FIG.
2, the base 16 consists of a rectangular support structure defined
by two C-channel side beams 22 and two angle end bars 24. The
scissor assembly 14 and the power unit 18 (only shown in FIG. 1)
are anchored in conventional manner to the base 16. As shown in the
perspective and cross-sectional views of FIGS. 3 and 4, multiple
pressure switches 26 are mounted, preferably on a support plate 28,
along the length of the side beams 22 and are connected to the lift
table's control circuitry, as detailed below, such that any
appreciable degree of pressure exerted on any one switch 26 causes
the deactivation of the system that controls the descent of the
platform. Appropriate wiring 30 strung around the frame 16 is used
to connect the switches 26 and to link them to the control
circuitry of the lift table.
[0036] As seen in FIGS. 3 and 5, the toe sensor of the invention
further includes a plurality of conical springs 32 mounted on the
support plates 28 in line with the switches 26 and distributed over
the length of the side beams 22. The springs 32 are provided to
support a cover or guard 34 suspended substantially in
free-floating arrangement over the base, as illustrated in
cross-sectional and perspective views in FIGS. 6 and 7. The switch
guard 34 consists of a rectangular structure with L-shaped angle
sides 36 and straight vertical ends 38 sized to overlap and snugly
fit around the base 16 (see FIGS. 8 and 9). Thus, the guard 34 can
be seated over the base 16 resting on the springs 32 with side lips
40 slightly extending over and overlapping the side beams 22, as
seen in the cross-section of FIG. 6. It is important that the guard
34 be loosely coupled to the base 16, so that it can slide
vertically along its side beams 22 and end bars 24 when a force is
applied to it, but not be so large as to materially increase the
size of the base.
[0037] In order to provide horizontal stability of the guard 34 in
relation to the base 16, the ends 38 of the guard contain vertical
slots 42 suitable for slidable connection with retaining cap screws
44 attached to the outer surface of the end bars 24 of the base 16
(FIGS. 3 and 7). In the preferred embodiment of the invention, I
found that four pressure switches 26 located near the corners of
the base 16, as illustrated clearly in the top view of the base in
FIG. 10, are sufficient to provide the sensitivity to pressure
necessary to de-energize the power system of the lift table
whenever the guard 34 is touched by an encroaching object. For
example, when the foot F of an operator steps over any part of the
guard 34, as illustrated in FIG. 11, the springs 32 readily
collapse to allow pressure to be exerted on at least one switch 26
and interrupt the descent of the platform 12.
[0038] The preferred switch used for the invention is marketed by
Tapeswitch Corporation under the name Ribbon Switch, which is
suitable for connection between a power source and a load. Ribbon
Switches are normally closed between the power source and the load
and are shorted by the application of pressure, thereby opening the
circuit connection to the load. As such, the control aspect of the
invention can be implemented simply by wiring the switches 26 in
parallel (although they appear installed in series on the base 16)
in the down-cycle circuitry of the lift table, as illustrated
schematically in FIG. 12. It is noted that the descent of the
platform 12 of a lift table is typically effected through an open
valve V that controls the release of hydraulic fluid from a support
ram. Whenever one of the switches 26 is short-circuited by the
application of pressure exerted through the guard 34, the
down-cycle circuit is opened, and the solenoid S is deenergized,
thereby closing the valve V which controls the descent of the
platform.
[0039] I also found that four conical springs 32, one placed
approximately next to each pressure switch 26, are suitable for
supporting the guard 34 of the invention about 6 mm ({fraction
(1/4)} inches) above the switches, so that minimal contact with the
guard is sufficient to trigger at least one of the switches. For a
guard 26 weighing about 10 kg (approximately 12 pounds), springs
with a spring rate of about 1.1 kg/cm (approximately 6 lb/inch)
have been found to be adequate. With these component
characteristics, a threshold force of about 1-2 Kg (approximately
2-4 lb) applied anywhere to the guard 34 has been found to suffice
to trigger the toe sensor of the invention (the exact force of
course depending on the location where the force is applied and the
corresponding lever arm resulting from the guard 26). The thickness
of the plate 28 can be used advantageously to adjust the gap
between the bottom of the guard 34 and the top of the switches 26
to obtain the desired pressure sensitivity. Four cap screws 44
placed on the end bars 24 near the corners of the base with
corresponding slots 42 in the guard have also been found to
adequately retain the guard in place while suspended over the
base.
[0040] Thus, the present invention provides an effective mechanism
for preventing injuries cause by the descending platform of a lift
table. The invention is implemented in the base of the lift table,
rather than in the platform, and without material modification to
the conventional configuration of the base. The toe sensor of the
invention meets all safety standards currently in effect without
noticeable increase in the dimensions of the lift-table's platform
and base. Moreover, it can be implemented with relatively
inexpensive additional components.
[0041] It is noted that the main aspect of the invention resides in
the incorporation of a contact toe-sensor in the base of the lift
table. Accordingly, while the preferred embodiment has been
described in terms of pressure switches mounted around the base,
the concept of the invention can be implemented in equivalent
fashion utilizing different kinds of switches, such as, without
limitation, Hall-effect, limit, and optic-electronic fiber
switches. The essence of the invention is to couple a sensor to the
base of the lift table so that any intruding object is detected and
the descent of the platform immediately arrested. To that end, as
illustrated in FIG. 13, non-contact perimeter beam-sensors 46 can
be used in equivalent fashion to practice the invention. A
photoelectric, infrared, ultrasound or other equivalent radiation
emitter 48 is utilized to provide a beam (illustrated as B in the
figure) that is appropriately reflected by mirrors 50 around the
perimeter of the base 16 toward a receiver 52. When the beam is
interrupted by an encroaching object, the descent of the platform
12 is stopped, as taught by this disclosure.
[0042] Thus, while the present invention is described with
reference to particular embodiments, those skilled in the art will
recognize that many variations and geometries may be employed
without departing from the spirit and scope of the invention as set
forth in the claims.
* * * * *