Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to a swivelling lift
mechanism for raising ~nd lowering a load platform
and has a particularly useful but not exclusive
application in a mechanism for moving a vehicle tail-
boar~ from its initial vertlcal position at the rear ofthe vehicle body into a horiæontal position and then in
sub~tantially the same horizontal attitude to ground
level and back again, whereby the tail~board c~n be
used to carry loads between the floor of the vehicle
and a lower level.
According to this invention there is provided a
lift mechanism for a load-carrying platform comprising
at least. one parallelogram linkage constituted by two
main levers each having three non-colinear pivots
and the pivots on the two levers.defining substantially
ide~tical ~riangles, and the levers being horizontally
o~fset from each other i~ a direction towards and away
from the platform, one pair of corresponding pivots on
the two levers being respectively connected to the
plat~orm at two fixed points which are spaced apart
a predetermined distance in the direction of the
length of the platform, the pivots constituting the .
seco~d pair being spaced apart by said predetermined
di~tance and interconnected by a synchronising l~nk,
and the pivots constituting the third pair being
~paced apart by said predetermined di~tance and inter-
connected by a tilting lever~ one of the pivots of the
thlrd pair having ~ fixed mounting on the vehicle 9
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means for rotating the main levers about their respective pivots
constituting the third pair, and means for selectively moving the
tilting lever about the fixed pivot point into a first position in
which the pivots constituting the third pair are disposed one vertically
above the other and into a second position in which the said pivots
constituting the third pair are substantially in horizontal alignment.
According to a preferred feature of the invention at least the
main lever nearest the platform is cranked and/or cut away in the region
of said first pivot thereof in such a manner that the parts of the lever
which overlap the platform do not protrude above the load carrying surface
of the platform in any position of the platform within its range of movement.
According to another preferred feature of the invention the pivot
having the fixed mounting is the pivot of the third pair furthest from
the platform when the latter is in a horizontal attitude.
In a preferred construction according to the invention a stop
operative to support the tilting lever in a first position in which the
pivots constituting the third pair are horizontally aligned with each other.
Preferably the first said stop is removable and a second stop is provided
which operates, on removal of the first stop, to permit the platform to
rotate the tilt lever about said fixed pivot beyond the first said position
towards a second position which is determined by the second
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stop, ~nd in which the platform is i.nclined downwardly ;
from its edge nearer said fixed mounting.
According to yet another preferred feature of the
invention the main levers are connected to the platform
adjacent the widthwise edge of the platform, and the
platform has pivotal.ly connected to its opposite
width~ise edge a ramp member, and a rod is connected `~
between ~he ramp member and the lin~age in such a manner
that movement of the linkage to raise the platform moves
the rod in a sense to swivel the ramp upward about its
pivot connection to the platform.
The invention will now be descrlbed in more detail
with reference to the accompanying diagrammatic drawings
in which:
Figure 1 shows a side elevatlon of a lift me hanism
according to the present invention a~ applied to a
vehicle taLl~board, and . .
Figure 2 illustrates a preferred feature of the
tail~board show~ in Figure l.
Referri~g to Figure 1 of the drawings, the tail-
board 10 is supported by two identical parallelogram
linkages 11 which are spaced apart widthwise of the
tail--board (i.e. transversely of the vehicle)~ These
linkages are disposed underneath the floor of the
veh~cle body, and are connected to the ~ail-board at
locations well inboard of the sides of the tail-board.
Each of the ~inkages includes two mAi~ levers 12,
13 each of which provides three non-colinear pivots, and
the triangles defined by these pivots are identical.
The levers are however of somewhat complex shape for
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reasons which will become clear presently. Levers 12, 13are spaced ~part in a direction lengthwise of the vehicle
and are slightly offset laterally from one another to
enable them to swing in respective vertical planes.
The pivots of the two levers form three pairs 12a and 13a,
12b and 13b, and 12c and 13c respectively and the two
pivots of each pair are linked together. Pivots 12a and
13a are linked by one arm of a tilt lever 14, pivots 12b
and 13b are linked by a synchronising link 16, and
pivots 12c and 13c are linked together by being connected
to the tail-board at positions spaced apart lengthwise
of the tail-board. The pivot 12a is mounted on a
bracket 18 fixed to the chassis 20 of the vehicle and
thus provides the only fixed support o~ the linkage.
The tilting lever 14 has a crank arm 21, and a
hydraulic jack 22 is cormected between the crank arm
and a lug o~ the bracket I8. A second hydraulic
~ack 23 is pivotally connected at 25 to a further lug 24
on the bracket 18 and to a pin 26 on the main lever 12, ~ -
the position of the pin being selected so that it does
not come into line with point 25 and pivot 12a in the
range of movement of the linkage.
The drawing shows the tail-bpard in its lowermost
position. The tilt lever 14 is prevented from
rotating in a clockwise directlon by a displaceable
stop 28 disposed between the arm 21 and secondF fixed,
stop 29 carried by the bracket 18. Exte~sion of the
jack 23 causes the ~ail board to be raised in a horizontal
attitude to the position shown in broken lines substant~
ially level with the floor of the vehicle body. In order
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now to swing the tail-board into its vertical positi.on,
jack 22 is extended and causes the tilting lever 14 to
swivel through 90 about the pivot 12a, which in turn
causes lever 13 to move bodily in an arcuate path~ :
about the pivots 12a, 12b and 12c, moving pivot 13c
into a position vertically above pivot 12c and hence
moving the tail-board into its vertical posltionO
Contractlon of the jacks in the reverse sequence
returns the tail-board first into a horizontal attitude
and then in the same attitude to ground level.
It will be seen that the attitude of the tail-board
corresponds to that of the arm 14 at all times.
Displacement of the displaceable stop 28 allows
the arm 21 to swlvel in a clockwise direction until-it
lS engages the fixed stop 29, causing the platform to tilt,
~ay 10 into a downwardly inclined position, and permit- :
ting the rearward end of the tail-board to rest on a
loading dock or other surfaceO
As will be seen in Figure 1, the part of lever 13
which extends along the tail~board does not project
above the surface of the tail-board anywhere along its
length~ and the arrangement therefore permits a load
to be moved on to the tail-board from either side as
well a~ from the end. By careful design, the resulting
elbow portion of the lever 13 can be arranged not to
hit the ground during the final stages of lowering the
tail~boa~d. In.~rac~ice, the design of the said part
of lever 13 may be such that a tolerance of 20 of
rot~ion o~ the lever 13 can be obtained in this
respect at the lower eud of the movement o~ the tail~
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board, which can be utilised to compensate for
differences in the floor height between one vehicle and
another and for the deflection of the springs of a
vehicle which occurs with variation of the load on the
S vehicleO Also the levers 12 and 13 are cut away
locally so that the~ clear the rearmost transverse
bearer 30 of the floor of the vehicle body as the tail-
board is raised to that level.
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The tail-board has a ramp 35 hinged to it at its
rear edge~ the hinge 36 being disposed at the top
edges of these components as shown so that the ramp
can pivot upward if it strikes an obstruction such as
the toes of one of the attendant personnel. The
weight of the ramp maintains it in contact with a
push-rod 38 which is slidably mounted in a guide
recess in one end edge of the tail-board and which is
resiliently ~rged forward by a spring (not shown). At
its forward end, the push-rod engages a cam surface
formed by a lobe 39 on the outer periphery of the boss
on the main lever 13 which boss encircle~ the pivot 13c.
It will be seen that as the link-plates 12 and 13 swing
to hoist the tail-board the lobe 39 cause~ the push-
rod to move rearward and swivel the ramp into a raisedposition, so that it prevents a load which is being
pushed rom the load platorm on to the tail~~oard
from over-~unning the rear edge o the tail-boardO
Subsequently~ during the tilt movement to bring the
tail board into it~ vertical position, the lobe and
push-rod are restored to the relative positions shown
in Figure 1, so that the ramp fonms a vertical
extension o the tail-board. The ramp liting device
may, if desired, be rendered inoperative, for example
by permitting the ramp to be pushed lengthwise of its
hinge connection to disengage the ramp rom the push-
rod, and a spring 37 may be provided which retur~s the
ramp to its former position when the platform is next
lowered to the grou~d. The ramp therefore performs
three useful functions~ Firstly lt operates as a
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ramp during loading and unloading, secondly, being in
a raised attitude during lifting and lowering o the
tail-board, it prevents the load from rolling or
sliding off the tail~board~ and thirdly, when the
tail-board is in a vertlcal position, the ramp
increases the effective height of the tail-boardc
It will be clear that the ramp operating
mechanism can take many alternative forms. The ramp
may for example be raised by a hydraulic jack operated
by low pressure fluid. Such jack could be connected
in circuit with the main lifting jack 23 so as to
raise the ramp be~ore li~ting commences. In another
alternative arrangement the front end of the push-rod
may be linked to a crank arm ln the place of lobe 39.
In still another alternative arrangement the ramp
may be operated by the main hydraulic jack ~3 through
a lost-motion device, permitting one end of the jack,
say pivot 2~to move relative to its carrying member
~ . 24~ duri~lg the first part of the jack movement,
and employing tnis movement to operate the ramp. This
arrangement too ensures that the ramp is raised before
lifting co~mences~
The illustrated lift mechanism as applied to a
tail-board provides a tail which is relatively thin,
f~cilitating movement of a load onto and off the tail-
board~ and furthermore enables all the required move-
ments of the tail board to be motor-driven. Additîon-
ally the lift mechanism enables these movements to be
carried out without any necessity for altering the
part of the standard supporting structure for the floor
of the vehicle adjacent the tail board.
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Referring now to Figure 2, the underneath
(rearward) face of the tail-board is preerably
recessed at its side edges so that a safety gap is
left between the tail-board and the ground at these
S edges when the tail~board ls fully lowered. The
thickness of the tail-board over the remainder o~
its area may if desired be such as to enclose the
structural members to which the main 1~vers 12 and 13
are connected at 12c and 13c. The '~skin" member 41
which defines part of the recess may be of corrugated
or grooved form as shown and may be curved upward at
its outer side edges wherein it jolns the load-
bearing surface 42 of the tail-board.
