Steering mechanism for an articulated vehicle
This invention relates t o steering mechanisms for articulated vehicles, and has particular
application to load bearing vehicles such as forklift trucks.
Background Art
Articulated vehicles such as forklift trucks used in narrow spaces benefit from being able to
operate in the smallest width possible, and steering mechanisms should be designed to minimise
the space required to manoeuvre.
An example of a steering mechanism is found in WO 02/20333, where a steering bogie at the
front end of the truck carries the lifting forks and is coupled to the rear section of the truck by a
vertical shaft which is rotatably mounted in a projection at the front of the chassis. A chain
wrapped around the shaft can be pulled in either direction by a pair of steering cylinders disposed
in the rear section of the truck, to steer the vehicle.
Disclosure of the invention
There is provided an articulated vehicle comprising:
a first body section;
a second body section;
an elongate projection projecting from an end of said first body section towards said
second body section;
a pair of steering cylinders mounted such that they are partially or wholly disposed in or
on said elongate projection;
a rotatable member provided towards the end of the elongate projection which is distal
from the first body section, the rotatable member being operatively connected to the second
body section such that rotation of the rotatable member causes the steering angle between the
first and second body sections to vary, and such that the first and second body sections articulate
relative to one another about said distal end of the elongate projection; and
at least one linkage connecting the rotatable member to the steering cylinders, such that
actuation of a steering cylinder causes the linkage to force the rotatable member to rotate;
wherein the pair of steering cylinders are disposed one above the other such that they
overlap vertically.
The vehicle of the invention provides a more compact and manoeuvrable steering arrangement,
due to the disposition of the steering cylinders one above the other in or on the elongate
projection. Because of this arrangement, the projection can be made very narrow - not much
wider than the steering cylinders themselves - and this allows the bogie or second body section
to be tucked tightly in towards the centreline of the truck when steered left or right.
Preferably, the steering cylinders are disposed towards the end of the elongate projection which
is proximal to the first body section.
Preferably, the external width of the elongate projection tapers from a wider width at the distal
end towards a narrower width where the cylinders are disposed one above the other.
The advantage of this arrangement is that having a narrower section behind the distal end of the
elongate projection accommodates the second body section better when in the extreme steering
positions. The advantage of this projection being wider at the distal end, where the rotatable
member is mounted, is that the torque available to steer the truck is greater with a larger
diameter rotatable member.
Preferably, the at least one linkage comprises a first linkage connecting a first of the pair of
steering cylinders to the rotatable member, and a separate second linkage connecting the second
of the pair of steering cylinders to the rotata ble member.
Further, preferably, the first and second linkages are each connected to the rotata ble member at
a respective mounting such that the linkages extend from the respective cylinder to the
respective mounting, wrapping around the rotatable member in opposite directions with a
circumferential overlap between them.
This has the advantage that the rotatable member can be pulled by either of the linkages through
more than 90 degrees, allowing the truck to be steerable through significantly more than 180
degrees, e.g. 210 degrees or more.
Preferably, the first and second linkages are spaced vertically on the rotatable member.
By spacing the linkages vertically, they do not interfere with one another or overlap, and
furthermore, the force exerted is entirely tangential to the rotatable member, and does not have
a vertical force component.
Preferably, the rotatable member is a shaft carried on a bearing, the shaft being vertically
disposed and extending through said distal end of said elongate projection.
Preferably, the or each linkage is wrapped partially around the circumference of the shaft and is
fastened to the shaft.
Preferably, the steering cylinders are disposed at an angle to one another, each cylinder being
aligned tangentially to opposite sides of a circumference of the rotatable member.
Preferably, the steering cylinders are selected from hydraulic cylinders and linear actuators. Most
preferably, they are hydraulic cylinders. The invention is applicable to any driving mechanism
which can be used to pull a linkage, if that driving mechanism has an elongated, narrow shape,
like a hydraulic cylinder or linear actuator.
Brief description of the Drawings
The invention will now be further illustrated by the following description of embodiments
thereof, given by way of example only with reference to the accompanying drawings, in which:
Fig. 1 is a perspective view of a vehicle according to the invention;
Figs. 2-4 are plan views of the vehicle in different steering positions;
Fig. 5 is a side elevation thereof;
Fig 6 is a top plan view thereof;
Fig 7 is a side elevation of the steering arrangement of the vehicle of Fig. 1
Fig 8 is a top plan view of the steering arrangement;
Fig. !9 is a cutaway perspective view of the vehicle revealing the steering mecha
situ;
Fig. 10 is a cutaway top plan view of the vehicle revealing the steering mechanism in situ;
Fig. 11 is a diagram showing the vehicle in different steering positions in use; and
Fig. 12 is a hydraulic circuit diagram.
Detailed description of preferred embodiments
In Fig. 1, indicated generally at 10, there is shown a forklift truck having a rear body section 12
and a front body section 14. Forks 16 are carried on a mast 18 on the front section, while a
driver's cab 20 is located in the rear section.
Referring to Figs. 2-4, an elongate projection 22 extends from the front end 24 of the rear body
section, with the front body section being pivotally steerable about the distal end 26 of the
elongate projection 22 (see Figs. 3 and 4). In Fig. 2, the vehicle is being steered in the straightahead
direction, whereas in Fig. 3 it is being steered to the right and in Fig. 4 to the left.
Figs. 5 and 6 are side elevation and top plan views, respectively, of the vehicle 10 when being
steered to the left. As can be seen in Fig. 5, the rear section 12 of the vehicle has a pair of rear
wheels 28 (one of which is visible), which are not steerable. Steering is accomplished by rotating
the entire front body section 14, which forms a steering bogie supported on a pair of front wheels
30, about a vertical pivoting axis 32, indicated in the plan view of Fig. 6. A single front wheel could
equally be used.
Figs. 7 and 8 show the steering mechanism in side elevation and in top plan view, respectively. A
first steering cylinder 34 is mounted above a second cylinder 36 such that, when viewed from
above (Fig. 8), they overlap in the vertical direction. The steering cylinders 34,36 are mounted at
the rear proximal end) of the elongate projection 22. Alternatively they can be mounted to the
rear body section of the vehicle to extend into the elongate projection 22.
A small portion 38 of the front body section of the truck can be seen in Fig. 8, noting that the
truck is being steered fully to the left. Steering is accomplished by rotating a rotatable shaft 40
which is mounted in a bearing (not shown) and which extends vertically through the elongate
projection 22 at its distal end 26, and this rotation is effected by a pair of linkages or chains 42,44.
Chain 42 connects a piston rod 46 of a hydraulic piston (not visible) mounted in lower steering
cylinder 36 to a mounting point 48 on the shaft 40. Chain 44 similarly connects a piston rod 50 of
a hydraulic piston (not visible) mounted in the upper steering cylinder 34 to a mounting point 52
on the bearing 40. It will be seen that the chains, piston rods and mounting points are separated
vertically from one another so that they do not interfere with one another.
As shown in Figs. 7 and 8, the truck is steered fully to the left. The lower piston rod 46 and the
lower steering cylinder 36 is fully retracted and this has caused the upper piston rod 50 in the
upper steering cylinder 34 to be fully extended, with the upper chain 44 wrapped completely
around the shaft 40.
To steer the truck to the straight-ahead position, piston rod 50 would be hydraulically retracted
halfway into cylinder 34, drawing piston rod 46 halfway out of cylinder 36 and rotating the
bearing by about 95 degrees clockwise. Continued retraction of piston rod 50 causes the chain 44
to pull the mounting point 52 further clockwise, until piston rod 50 is fully retracted, piston rod 46
fully extended, and the truck is steered fully right.
Figs. 9 and 10 show the mechanism in situ in the truck, first in a perspective view (Fig. 9) and then
in a plan view (Fig. 10) both cut away t o show the mechanism. In Fig. 9, shaft 40 is indicated,
while in Fig. 10 the upper steering cylinder 34, its piston rod 50 and chain 44 are also indicated.
Fig. 11 shows the forklift truck 10 in operation within a narrow aisle 52 manoeuvring pallets 54.
The truck is shown in three positions, steered to the left at position 60, steered straight ahead at
position 62 and steered to the right at position 64. It will be appreciated that because of the
arrangement of the steering cylinders one above the other, the elongate projection 22 can be
made very narrow and thus the front end can be steered left or right to a more extreme angle
than with conventional forklift trucks.
Fig. 12 shows a hydraulic circuit used in steering the vehicle. A steering motor in the form of a
conventional steering orbital unit 70 has four ports P, T, Rand L. Ports Rand L are connected to
the piston rod side of the hydraulic cylinders 34 and 36 respectively, port Pis connected to a
source 72 of hydraulic oil under pressure, and port T t o a tank 74. A respective non-return valve
76 is connected in series in each hydraulic line connecting the unit 70 to the cylinders 34 and 36,
and a respect ive pressure relief valve 78 is connected in parallel with each valve 76. The unit 70 is
rotatable such that port Pmay be connected selectively to cylinder 34 via port R, or to cylinder 36
via port L, or to neither cylinder. When the port Pis connected to one of the ports or L, the
other port Lor Ris connected to port T.
Starting from the straight ahead position when the port Pis connected t o port R, oil under
pressure is forced into the piston rod side of the cylinder 34 via the respective non-return valve
76.
This retracts the piston rod 50 and thereby pulls the chain 44 away from the shaft 40. Due to the
attachment of the chain 44 to the shaft at 52, this movement of the chain rotates the shaft 40,
and hence the second body section of the truck, in a clockwise direction to steer the forklift truck
to the right.
In a similar manner, when the port Pis connected to cylinder 36, oil under pressure is forced into
the cylinder 36 via the respective non-return valve 76 t o retract the piston rod 46 and thereby
rotate the shaft 40 in an anticlockwise direction to steer the forklift truck to the left.
In each case, in order to allow the piston rod 50 or 46 t o retract when oil under pressure is
supplied to the corresponding cylinder 34 or 36, it is necessary that oil be allowed to flow out of
the other cylinder 36 or 34 respectively to allow the piston rod of that other cylinder to extend.
This is because the total distance between the piston rods 46 and 50, as measured along the
chains 42 and 44 and around the shaft 40, is fixed. This is achieved in each case by the pressure
relief valves 78, which allow hydraulic oil to flow out of a cylinder to the tank 74 when the
cylinder's port Ror L is connected t o port T and the pressure of the oil therein exceeds a certain
threshold pressure set by the relief valve. This ensures that the chains 42 and 44 are kept under
tension at all times, even if the chains should increase in length over time, and provides precise
steering control.
The diameter of the shaft 40, the diameter of the hydraulic cylinders 34 and 36 and the
displacement of the steering motor 70 govern the steering speed of the mechanism. Although the
foregoing has used chains to couple the hydraulic cylinders t o the shaft, it is possible to use any
elongated substantially inelastic flexible linkage such as steel cable.
The advantages of the above embodiment are that steering is precise and controlled due to the
absence of slack in the system, even where the components are subject to wear over time.
Further, the steering speed is constant for all angles of the bogie 14 which allows the system to
be used over a full steering range of approximately 210 degrees. Also, the steering motor 70
provides a hydraulic brake, so that no separate steering brake is necessary.
In summary, the articulated vehicle has a first body section, a second body section, and an
elongate projection projecting from the first body section towards the second body section. A
pair of steering cylinders are mounted such that they are partially or wholly disposed in or on the
elongate projection, and are disposed one above the other so they overlap vertically, thereby
narrowing the projection at the end nearer the first body section and allowing the second body
section to be rotated closer to the centre line. A rotata ble shaft of the second body section is
mounted towards the far end of the elongate projection and is steered by a pair of linkages such
as chains, each of which is connected to the shaft and to a respective one of the steering
cylinders. The chains wrap around the shaft with a circumferential overlap, though they are
spaced vertically. This circumferential overlap allows steering angles significantly in excess of 180
degrees (i.e. from the extreme left position to the extreme right position, compare Figs. 3 and 4)
to be achieved.
The invention is not limited to the embodiment(s) described herein but can be amended or
modified without departing from the scope of the present invention.
Claims
1. An articulated vehicle comprising:
a first body section;
a second body section;
an elongate projection projecting from an end of said first body section towards said
second body section;
a pair of steering cylinders mounted such that they are partially or wholly disposed in or
on said elongate projection;
a rotatable member provided towards the end of the elongate projection which is distal
from the first body section, the rotatable member being operatively connected to the second
body section such that rotation of the rotatable member causes the steering angle between the
first and second body sections to vary, and such that the first and second body sections articulate
relative to one another about said distal end of the elongate projection; and
at least one linkage connecting the rotatable member to the steering cylinders, such that
actuation of a steering cylinder causes the linkage to force the rotatable member to rotate;
wherein the pair of steering cylinders are disposed one above the other such that they
overlap vertically.
2. An articulated vehicle as claimed in claim 1, wherein the steering cylinders are disposed
towards the end of the elongate projection which is proximal to the first body section.
3. An articulated vehicle as claimed in claim 1 or 2, wherein the external width of the
elongate projection tapers from a wider width at the distal end towards a narrower width where
the cylinders are disposed one above the other.
4. An articulated vehicle as claimed in any preceding claim, wherein the at least one linkage
comprises a first linkage connecting a first of the pair of steering cylinders to the rotatable
member, and a separate second linkage connecting a second of the pair of steering cylinders to
the rotatable member.
5. An articulated vehicle as claimed in claim 4, wherein the first and second linkages are
each connected to the rotatable member at a respective mounting such that the linkages extend
from the respective cylinder to the respective mounting, wrapping around the rotatable member
in opposite directions with a circumferential overlap between them.
6. An articulated vehicle as claimed in claim 5, wherein the first and second linkages are
spaced vertically on the rotatable member.
7. An articulated vehicle as claimed in any preceding claim, wherein the rotatable member is
a shaft carried on a bearing, the shaft being vertically disposed and extending through said distal
end of said elongate projection.
8. An articulated vehicle as claimed in claim 7, wherein the or each linkage is wrapped
partially around the circumference of the shaft and is fastened to the shaft.
9. An articulated vehicle as claimed in any preceding claim, wherein the steering cylinders
are disposed at an angle to one another, each cylinder being aligned tangentially to opposite
sides of a circumference of the rotatable member.