Note: Descriptions are shown in the official language in which they were submitted.
CA 02032623 1998-04-01
LE~F CH~IN DR~VE ASSEMBLlr
~ield of the Invention
This invention relates to improvements in platform lifts of the type used in
buses and other vehicles, and more particularly to drive assemblies used in such5 lifts for causing the platform to raise and lower.
Background of the Invention
Platform-type wheelchair lifts of the type disclosed in U.S. Patent
No. 4,058,228 to Hall have been used extensively in passenger vehicles,
particularly urban buses. Platform lifts of the type disclosed in the Hall patent
10 are typically installed in one of the existing stairwells in a vehicle. These lifts
generally comprise a platform which is slidably mounted in the vehicle beneath
the stairwell so as to define the bottom step of the stairwell when in the retracted
position and so as to provide a platform which projects outwardly from the
stairwell when in the extended position. The platform is attached via a
15 parallelogram linkage assembly to a carriage. The latter is slidably mounted in a
pair of opposing channel members and is caused to move between retracted and
extended positions by a chain drive assembly which is actuated by a hydraulic orpneumatic linear actuator. The parallelogram linkage assembly is designed to
cause the platform to move between upper and lower positions relative to the
20 carriage when the platform is in the extended position. The parallelogram linkage
includes two pairs of parallel linkage arms, each arm having a proximal end which
is pivotally mounted to the carriage and a distal end which is pivotally mounted to
the platform.
The linkage arms are caused to pivot about their proximal ends, thereby
25 causing the platform to move between upper and lower positions, by a sprocketand chain drive assembly. This assembly includes a toothed driving sprocket
coupled with the proximal end of one of the parallel arms so as to rotate with the
arm and a driven toothed sprocket attached to the proximal end of the other arm
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so as to rotate with the arm. Rotational drive is ll~n~ ed from the driving sprocket
to the driven sprocket by a chain drive assembly comprising a single length of roller
chain. One end of the roller chain is wrapped partially around and is attached to the
driving sprocket and the other end of the roller chain is wrapped partially around and
5 is ~tt~ched to the driven sprocket. The driving sprocket is driven by a separate chain
and sprocket drive assembly which is ~ctu~ted by a linear actuator. In an alternative
embodiment of the above-described chain drive assembly, two lengths of roller chain
are employed, each of which is wrapped around a respective one of the sprockets.Two turnbuckles are provided for coupling the ends of the roller chains together so as
10 to form a continuous fiexible drive member. A preload can easily be applied to chain
assembly ofthe ~lt~ tive embodiment by appropliate adjllstment ofthe turnbuckles.
Platform lifts of the type described above have been used extensively in urban
buses with very favorable results. Recently, the need has arisen (a) to increase the
length of the parallel linkage arms of known platform lifts, (b) to increase the size and
15 hence weight of the platform of known platform li~s, and (c) to accommodate heavier
loads on known platform lifts. However, due to space limit~tions inherent in thedesign of the above-described platform lifts, it has not been possible to accommodate
roller chain in the chain and sprocket drive assembly of such lifts of a size and hence
strength sufficient to permit the lift to be modified in the manner described in the
20 preceding sentence.
Under conventional operation, the parallel arms of platform lifts of the type
disclosed in U.S. Patent No. 4,058,228 are caused to travel between a dowllw~dlyprojecting position, through a holi;Gon~al position, to an upwardly projecting p osition.
As the arms travel through the horizontal position, the tendency exists for the
25 platform of the li~ "buck" or bounce. Such "bucking" is believed to occur due to
stretching of the inherently flexible roller chains used in the chain and sprocket drive
assembly which arises when the load applied to the roller chains is shifted from the
linkage assembly to the chains as the parallel arms move from the upwardly projecting
to the dowllw~dly projecting positions, or vice versa. As those who have
30 experienced anomalous vibration or bouncing of elevators can appreciate, such"bucking" of the platform lift can be very disconcerting to a whePlch~ir occupant
positioned on the lift.
In an attempt to ,..;..;...i7e the "bucking" of the platform of conventional
platform lifts which occurs as the parallel arms travel through the horizontal position,
35 a significant preload has been applied to the roller chains. Although such preloading
does reduce the tendency of the platform to "buck" it .~imlllt~neously increases the
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load on the bearings and the strain on the chain, sprockets, and other components of
the chain and sprocket drive assembly. Also, the friction between movable
components of the chain and sprocket drive assembly is increased as a consequence of
such preloa~ling Such increases in load, strain, and friction result in added
m~intçn~nce costs and reduced product longevity.
In fields of technology completely unrelated to platform li~s, leaf chains have
been used for tr~nemitting motion from one movable elPmPnt to another. For
inet~ncP~, U.S. Patent No. 4,197,766 discloses a counterbal~nced pumping system
comprising a vertically movable pump member, a counterweight, and a leaf chain
supported on a plurality of pulleys for tr~n.emittitlg motion between the pumping
member and the counterweight. U.S. Patent No. 4,526,251 discloses a leaf chain
designed for use in a lift truck for l~ ;n~ motion from one end of a hydraulic
cylinder to the fork of the lift truck. In the chain drive assemblies of both U. S. Patent
Nos. 4,197,766 and 4,526,251, the ends of the leaf chains are secured to fixed or
movable members, as the case may be, and the length of the leaf chains are supported
by and pass back and forth over one or more sheaves or pulleys.
Although, in general, leaf chain is not designed to ll~1snl-l positive drive, it is
known to use leaf chain in this manner. U.S. Patent No. 4,058,021 comprises a drive
assembly compri~in~ a leaf chain having link edges which engage specially formedflutes on an associated pulley. As indicated in U.S. Patent No. 4,058,021, only a
limited quantity of torque may be tr~nemitted between the chain and pulley before the
chain will slip with respect to the pulley.
It is also known to use chain in a spring-biased counterweight assembly for
redll~ing the force required to raise a railroad p~eepnger car ~lai-w~y assembly. Such
use of a chain is disclosed in U.S. Patent No. 2,154,107, although the type of chain
employed, i.e., roller versus leaf chain, is not disclosed.
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Although the use of leaf chains in a wide range of mechanical contexts is
well known, as evidenced by the patents discussed above, platform lifts of the
type disclosed in U.S. Patent No. 4,058,228 have, since their inception, suffered
from the lack of chain strength and "bucking" problems discussed above. Such
5 drawbacks of these platform lifts have gone uncorrected for over ten years, in spite of extremely widespread use of such platform lifts.
Thus, a strong need exists for a sprocket and chain drive assembly for
causing the parallel arms of platform lifts of the type disclosed in U.S. Patent No.
4,058, 228 to Hall to move between the upper and lower positions which (a) is
10 stronger than the sprocket and chain drive assemblies currently employed,
(b) which does not cause the platform to "buck" as the parallel arms move through
the horizontal position, and (c) which does not require excessive preloading of the
roller chain links of the sprocket and chain drive assembly to avoid such "bucking"
of the platform.
lS Summary of the Invention
The present invention is a leaf chain drive assembly designed for use with a
platform lift of the type comprising a platform, a carriage assembly, and a linkage
assembly coupled with the platform and the carriage assembly for causing the
platform to move upwardly and downwardly along a circular arc relative to the
20 carriage. The linkage assembly comprises first and second linkage arms, with the
proximal end of the first linkage arm being pivotally mounted to the carriage so as
to be rotatable about a first axis and the proximal end of the second linkage arm
being pivotally mounted to the carriage so as to be rotatable about a second axis.
The leaf chain drive assembly comprises first sheave and second sheaves, each
25 comprising a radially projecting lug. The first sheave is coupleable with the first
linkage arm so as to rotate with the first linkage arm about the first axis, and the
second sheave is coupleable with the second linkage arm so as to rotate with thesecond linkage arm about the second axis. The leaf chain drive assembly also
comprises two leaf chains which are coupled together so as to surround and engage
30 the first and second sheaves. The leaf chains each comprise an extended
connecting link designed to engage the radially projecting lugs on the first andsecond sheaves so as to permit rotational drive to be transmitted between the the
first and second sheaves and the leaf chains.
The leaf chain drive assembly further comprises a third sheave coupled to
35 rotate with the first sheave, and a third leaf chain wrapped around the thirdsheave. One end of the third leaf chain is attached to the third sheave and the
other end of the third leaf chain is attached to a linear actuator. As the linear
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CA 02032623 1998-04-01
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actuator causes the other end of the third leaf chain to reciprocate, the third leaf
chain will cause the third sheave to rotate, thereby imparting rotational drive to
the first sheave which is coupled with the third sheave.
Brief Description of the Drawings
FIGURE 1 is a fragmented, perspective view of a portion of a platform lift
incorporating the chain and sprocket drive assembly of the present invention, with
the parallel arms of the lift being shown in the upwardly projecting position;
FIGURE 2 is an enlarged perspective view of the chain and sprocket drive
assembly illustrated in FIGURE 1, with associated portions of the platform lift
being removed for clarity of illustration;
FIGURE 3 is a side elevational view of the chain and sprocket drive assembly
with the parallel arms of the platform lift which are driven by the chain and
sprocket drive assembly being shown in solid view in an upwardly projecting
position and in phantom view in a downwardly projecting position;
FIGURE 4 is a top view of one of the lengths of leaf chain used in the chain
and sprocket drive assembly, with the chain being spread out flat for clarity ofil~ustration;
FIGURE 5 is a side elevation view of one of the sprockets used in the leaf
and chain drive assembly; and
FIGURE 6 is a front elevational view of the sprocket illustrated in
FIGURE 5.
Detailed Description of the Preferred Embodiment
The present invention is a leaf chain drive assembly 20 which is designed to
replace roller chain drive assemblies of the type used in platform lifts discIosed in
U.S. Patent No. 4,058,228 to Hall, which lifts shall be referred to hereinafter as
"platform lifts." FIGURE l illustrates a conventional platform lift and one
embodiment of the leaf chain drive assembly of the present invention incorporated
in such lift. As described in detail in U.S. Patent No. 4,058,228,
and as illustrated in FIGURE 1, conventional
platform lifts include a platform 22 for supporting a passenger such as a
wheelchair occupant, and a carriage, one portion of which is identified at 24, for
causing the platform to move horizontally between the retracted and extended
positions. Platform lifts also typically include a barrier 26 which is pivotallymounted to the outboard end of platform 22. Barrier 26 forms the bottom step in
the stairwell in which the platform lift is mounted when the lift is in the retracted
position. When the lift is in the extended position, barrier 26 iS typically
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CA 02032623 1998-04-01
maintained in the position illustrated in FIGURE 1 so as to prevent a wheelchairpositioned on platform 22 from rolling off the platform.
- Platform lifts further comprise a parallelogram linkage assembly for causing
platform 22 to move upwardly and downwardly along a circular arc when the
S platform is in the e~ctended position. This linkage assembly comprises two pairs of
parallel arms, one pair of which is illustrated in FIGURE 1 and comprises linkage
arms 30 and 32. The proximal ends (i.e., the lower ends as illustrated in
FIGURE 1) of linkage arms 30 and 32 are keyed to pivot shafts 36 and 38,
respectively, which shafts are pivotally mounted in spaced relation to carriage
10 bar 34. As a consequence of this arrangement, the proximal ends of linkage arms
30 and 32 are coupled to carriage 34 SO as to rotate about the pivot axes of pivot
shafts 36 and 38, respectively. The distal ends (i.e., the upper ends as illustrated
in FIGURE 1) of linkage arms 30 and 32 are pivotally mounted to platform 22 SO
that the spacing between the distal ends of the arms is identical to the spacing15 between the proximal ends of the arms.
As described in detail below, leaf chain drive assembly 20 iS designed to
cause linkage arms 30 and 32 to pivot with pivot shafts 36 and 38, respectively,about the pivot axes of the shafts, thereby causing platform 22 attached to
arms 30 and 32 to raise and lower. As described below, leaf chain drive
20 assembly 20 includes slave chain drive assembly 40 and lift chain drive
assembly 200 for causing arms 30 and 32 to pivot in this manner.
Referring to FIGURES 1-6, slave chain drive assembly 40 comprises a driven
sheave or pulley 50 having identically sized and shaped lugs 52 and 54 which arespaced apart from one another a predetermined distance and project along a
25 common radius of the sheave a predetermined distance from the outer
circumferential edge of the sheave 50. The specific size, configuration, and
relative spacing of projecting lugs 52 and 54 will be described in greater detail
below in connection with the description of leaf chain 120. Sheave 50 is keyed to
pivot shaft 36 so as to rotate with the pivot shaft.
Slave chain drive assembly 40 includes a driving sheave 60 which preferably
has a size and configuration identical to that of sheave 50. Thus, sheave 60
includes a pair of axially spaced, radially projecting lugs, one of which is
identified in phantom at 62 in FIGURE 3, which are identical to lugs 52 and 54 of
sheave 50. Sheave 60 is keyed to pivot shaft 38 so as to rotate with the pivot
35 shaft.
Assembly 40 comprises an endless drive assembly 70 for transmitting
rotational drive from driving sheave 60 to driven sheave 50. In the preferred
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embodiment, drive assembly 70 comprises a leaf chain 72. Leaf chain 72 iS a so-
called 1'3-4" leaf chain comprising a plurality of rigid plates which are
interconnected by pins 74. More specifically, moving from the top to the bottom
of leaf chain 72 illustrated in FIGURE 4, the leaf chain comprises outer plates 76,
intermediate plates 78 and 80, central plates 82, intermediate plates 84 and 86,and outer plates 88. The ends of each outer plate 76 are aligned with the ends of
a corresponding respective inner plate 82 and a corresponding respective outer
plate 88. ~imilarly, the ends of a given set of intermediate plates 78, 80, 84, and
86 are aligned with one another. In addition, each set of intermediate plates 78,
80, 84, and 86 is offset one pitch from the associated set of outer, central, and
outer plates 76, 82, and 88. This design of leaf chain 72 iS preferred, although it is
to be appreciated that leaf chains having differing plate configurations may also
be satisfactorily employed, the only requirement being that the leaf chain have
sufficient strength and stiffness for the intended application, and the cross
sectional size of the leaf chain is such that it can be accomodated in the spaceprovided in the platform lift.
Leaf chain 72 also comprises an extended connecting link 100 which is
preferably, although not necessarily, positioned at or near the middle of the length
of the leaf chain. The extended connecting link comprises an outer plate lOOa
which is positioned along the plane of outer plates 76, an intermediate plate lOOb
which is positioned along the plane of central plates 82, and an outer plate 100c
which is positioned along the plane of the outer plates 88. With this design of
connecting link 100, apertures 102 and 104 are provided in leaf chain 72, which
apertures extend entirely through the thickness of the chain. Apertures 102 and
104 in leaf chain 72 are sized and configured, as are the radially projecting lugs
(one of which is identified at 62 in FIGURE 3) of sheave 60, so that the radially
projecting lugs may be received in apertures 102 and 104 with a close sliding fit,
whereby rotational motion may be transmitted from sheave 60 to leaf chain 72
without any lost motion.
Leaf chain 72 additionally comprises end fitting 106 which is attached to one
end of the leaf chain and end ~itting 108 which is attached to the other end of the
leaf chain. End fitting 106 comprises a threaded central bore 110 (FIGURE 4)
having a left hand thread, and end fitting 108 comprises a threaded central
bore 112 (FIGURE 4) having a right hand thread.
Endless drive belt assembly 70 additionally comprises a second leaf
chain 120. Leaf chain 120 is identical to leaf chain 72, and hence comprises an
extended connecting link 122 which is identical to connecting link 100, and end
CA 02032623 l998-04-Ol
fittings 124 and 126 which are identical to end fittings 106 and 108, respectively.
Thus, connecting link 122 iS designed to receive lugs 52 and 54 of sheave 50 such
that motion may be transmitted from the leaf chain to the sheave.
Drive belt assembly 70 further comprises connecting shafts 130 and 132.
Shaft 130 comprises a central portion 134 having a hexagonal cross-sectional
configuration and threaded ends 136 and 138. Threaded end 136 has a left hand
thread and the pitch of the thread is selected so that end 136 may be threadablyengaged in threaded bore 110 in end fitting 106 of leaf chain 72. End 138 has a
right hand thread and the thread pitch of the end is selected so that it may be
10 threadably engaged in the central bore in end fitting 126 of leaf chain 120.
Connecting shaft 132 has a similar construction with the end having a right handed
thread being threadably engaged in central bore 112 in end fitting 108 of leaf
chain 72, and the end having a left hand thread being threadably engaged in the
central bore of end fitting 124 of leaf chain 120.
Referring to FIGURES 1 and 2, the leaf chain drive assembly 20 further
comprises a lift chain drive assembly 200 for causing sheave 60 to rotate. The lift
chain drive assembly comprises a sheave 202 which is keyed to pivot shaft 38 SO as
to rotate with the shaft. As illustrated in FIGURE 1, linkage arm 32 iS positioned
between sheave 202 and sheave 60, whereby sheave 202, linkage arm 32, and
20 sheave 60 rotate as a unit about the rotational axis of pivot shaft 38.
Lift chain assembly 200 further comprises leaf chain 208. The latter is
preferably a 1'3-4" leaf chain of the type used for leaf chains 72 and 120, as
described above, except that the size and strength of leaf chain 208 iS increased
as required to permit leaf chain 208 to withstand the substantially greater forces
25 to which it is subjected. End 210 of leaf chain 208 iS affixed to sheave 202 by
conventional means so that the end of the leaf chain will rotate with the sheave.
In one embodiment of the invention, sheave 202 comprises a pair of radially
projecting lugs (not shown) similar to lugs ~2 and 54 on sheave 50 as described
above. End 210 of leaf chain 208 iS then attached to these projecting lugs via a30 pin (not shown). The opposite end 212 of leaf chain 208 iS attached via fitting 214
to linear actuator 216. The latter is provided for causing leaf chain 208 to move
back and forth so as to wrap around and unwrap from the periphery of sheave 202,thereby causing the pivot sheave to rotate about its rotational axis. Linear
actuator 216 iS identical to the corresponding linear actuator used in the platform
35 lift described in U.S. Patent No. 4,058,228. Linear actuator 216 iS designed to
cause end 212 of leaf chain 208 to move back and forth along actuation axis 220.For a more detailed description of linear actuator 216, attention is directed to the
aforementioned patent.
B
CA 02032623 1998-04-01
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In connection with the following discussion of the operation of leaf chain
drive assembly 20, reference should be made to FIGURES 1-6. Initially, end 210
of leaf chain 208 is attached to sheave 202, the leaf chain is wrapped around
sheave 202 in the manner illustrated in FIGURES 1 and 2, and end 212 of leaf
5 chain 200 is attached via fitting 214 to linear actuator 216. Leaf chain 72 iswrapped around sheave 60 so that the radially projecting lugs of sheave 60 (one of
which is identified at 62 in FIGURE 3) are received in apertures 102 and 104 in the
leaf chain. Leaf chain 120 is wrapped around sheave 50 in a similar manner so
that radially projecting lugs 52 and 54 are received in the apertures of connecting
10 link 122. Connecting shaft 130 is then threadably engaged with end fittings 106
and 126, and connecting shaft 132 is threadably engaged with end fittings 108 and
124. The connecting shafts are then tightened just enough to remove all slack
from leaf chains 72 and 120. A significant preload does not have to be applied to
leaf chains 72 and 120, as is typically required with the two-chain alternative
15 embodiment (discussed above) of the chain drive assembly used with known
platform lifts.
In connection with the following discussion, it is assumed that parallel
linkage arms 30 and 32 are in the upwardly extending position illustrated in
FIGURE 1. When it is desired to move platform 22 to a more elevated position
20 with respect to carriage 24, linear actuator 216 is actuated so as to cause end 212
of leaf chain 200 to move inwardly (i.e., to the right as illustrated in FIGURE 1)
along the actuation axis 220 of the linear actuator 216. As leaf chain 200 movesin this direction, it causes sheave 202 to rotate in the clockwise direction as
illustrated in FIGURES 1 and 2. This clockwise rotation is transmitted via pivot25 shaft 38 to sheave 60 so as to cause the latter to also rotate in a clockwisedirection. Additionally, rotation of sheave 202 in a clockwise direction is
transmitted via pivot shaft 38 to linkage arm 32 so as to cause the pivot arm tomove upwardly and in a clockwise direction about the rotational axis of pivot
shaft 38.
Rotation of sheave 60 in a clockwise direction is transmitted via its radially
projecting lugs, e.g., lug 62, to leaf chain 72 so as to cause the portion of the leaf
chain wrapped around shesve 60 to also rotate in the clockwise direction. Such
motion of leaf chain 72 is transmitted via connecting shafts 130 and 132 to leafchain 120, thereby causing the portion of leaf chain 120 wrapped around sheave 50
35 to rotate in a clockwise direction. Such movement of leaf chain 120 is
transmitted via its connecting link 122 to projecting lugs 52 and 54 and hence to
sheave 50 so as to cause sheave 50 to rotate in a clockwise direction. This
q~
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CA 02032623 1998-04-01
1 0--
rotation of sheave 50 is transmitted via pivot shaft 36 to linkage arm 30 so as to
cause the linkage arm to rotate about the rotational axis of pivot shaft 36. Thus,
when sheave 202 is driven in the clockwise direction, linkage arms 30 and 32 arecaused to move along a circular arc in a clockwise direction so as to cause
platform 22 to raise relative to carriage 24.
On the other hand, when it is desired to lower platform 22 with respect to
carriage 24 linear actuator 216 is operated so as to cause end 212 of leaf
chain 200 to move outwardly (to the left as illustrated in FIGURE 1) along
actuation axis 220. Such movement of sheave 202 causes linkage arm~ 30 and 32
10 to move in a counterclockwise direction along a circular arc so as to cause
platform 22 to move outwardly and downwardly with respect to carriage 24. As a
consequence of the mechanical interconnection provided by a slave chain drive
assembly 40, linkage arms 30 and 32 are permitted to move downwardly in a
parallel simultaneous fashion.
While the preferred method for connecting sheaves 50 and 60 with drive belt
assembly 70 comprises the connecting link and radially projecting lug arrangement
described above, it is to be appreciated that other approaches for achieving such
connection between the leaf chains and the sheaves also fall within the scope ofthe present invention.
As illustrated and described above, a leaf chain drive assembly 20 iS
typically provided on only one side of platform 22. However, in certain operating
environments, it may be desirable to provide an assembly 20 on each side of
platform 22.
The slave chain drive assembly 70 and lift chain drive assembly 200 of the
25 present invention possess several i mportant advantages over the corresponding
roller chain drive assemblies of the platform lift disclosed in U.S. Patent
No. 4,058,228. The cross-sectional size of leaf chains 72, 120, and 200 is similar
to that of the corresponding roller chains used in the platform lift described in
U.S. Patent No. 4,058,228 due to size constraints imposed by the environment in
30 which the leaf chains are used. However, leaf chains 72, 120, and 200 are
significantly stronger than their roller chain counterparts due to the inherently
greater tensile strength of leaf chains. Consequently, the slave chain drive
assembly 70 and leaf chain drive assembly 200 enjoy a significantly greater margin
of safety than their roller chain counterparts used in the platform lift described in
35 U.S. Patent No. 4,058,228.
Although it is fairly widely known by those of ordinary skill in the art that a
leaf chain of a given size has greater tensile strength than a comparably sized
,~r~ CA 02032623 1998-04-01
-11-
roller chain, few ordinary practitioners, even those specializing in the narrow art
of chain engineering, appreciate that leaf chains are also stiffer than roller chains
of corresponding cross-sectional size. Such additional stiffness is responsible for
the dramatic improvement in the operation of platform lifts incorporating the
5 slave and leaf chain drive assemblies of the present invention. More specifically,
by utilizing relatively stiff leaf chains in place of their relatively flexible roller
chain equivalents, the tendency of platform 22 to "buck" or bounce as parallel
linkage arms 30 and 32 are caused to rotate through the horizontal position is
eliminated. Elimination of such motion enhances significantly a user's sense of
10 security when riding up or down on platform 22. Moreover, because the leaf
chains of assemblies 70 and 200 do not have to be preloaded to the extent required
with their roller chain counterparts used in the platform lift of U.S. Patent
No. 4,058,228, the maintenance requirements for the linkage assembly of the
platform lift are reduced significantly and the longevity of the various
15 components of the platform lift is increased significantly.
Since certain changes may be made in the above apparatus without departing
from the scope of the invention herein involved, it is intended that all matter
contained in the above description or shown in the accompanying drawings shall be
interpreted in an illustrative and not in a limiting sense.
