 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a steering axle for vehicles, being of a type
which comprises:
a beam arranged for conventional mounting to the load-bearing structure of
a vehicle;
a road wheel hub supported rotatably on each of the two opposite
longitudinal ends of the beam;
a steering actuator arranged to act on at least one of said road wheel
hubs; and
a steering drive interconnecting the two road wheel hubs to synchronize
their relative angular displacements in accordance with pre-determined
drive parameters, said steering drive comprising two shafts, synchronously
counter-rotating relatively to each other and being rotatively connected
to the corresponding one of the hubs through respective angle transfer
gear mechanisms, said transfer mechanisms including first and second gears
keyed respectively to the end of said shaft next to the corresponding hub
and being made rotatively rigid with said hub.
An axle with the above features is known from JP-A-60-1075.
The axle shown in that document is provided with a gear-type steering
system operated through a steering actuator for pivoting the road wheel
hubs fitted to the opposite ends of the beam.
That actuator is attached to the load-bearing structure of the vehicle and
has an output shaft which carries, keyed to its free end, one gear of the
steering system. The shaft extends through a bore in the load-bearing
structure of the vehicle and a corresponding tubular passageway formed in
the respective journal formation on the beam.
A steering axle to that design has a number of drawbacks.
First, the actuator is mounted at a location where it is subject to shocks
and may come to harm. This is more likely to occur when the axle is used
on such all-terrain vehicles as agricultural tractors, where the
probability of shocks is relatively high.
Secondly, with the above construction, relative rotation is allowed to take
place both between the actuator shaft and the corresponding journal
formation, as well as between the latter and the load-bearing structure of
the tractor whereto said actuator is, in turn, attached. The corresponding
journal formation of the beam intervenes, therefore, between the actuator
shaft and the load-bearing structure of the vehicle, while being mounted
to swing with respect to either. Accordingly, the machining tolerances for
the different component parts add algebraically to one another, to
establish operating conditions that make the required oil-tight fits
between the axle beam and the actuator shaft difficult to achieve. This
problem is made the more serious by that the standard admissible
tolerances for journal formations in the agricultural tractor industry are
comparatively large. Further, with that design, the axle tends to steer,
even with the steering wheel held still, every time that it is caused to
perform swinging movements relatively to the vehicle structure, and this
on account of the steering system gears being forced to roll about the
actuator gear whenever such a situation of relative oscillation is
encountered. Thus, the structure proposed in said document is unsuited to
such applications.
It should be also noted that in the axle according to the Japanese Patent
the drive which connects the axle shaft to the road wheel hub to drive the
latter is of a type comprising a bevel gear pair. This drive arrangement
increases the vertical dimension of the axle in the area of swivel
connection to the road wheel hubs.
A further consequence of this prior structure is that, as a torque is
transferred between the axle shaft and the wheel hub, radial thrust forces
appear between the bevel gear pair. The radial thrust is discharged, on
the axle shaft side, to a specially provided support which carries, at a
location proximate to the corresponding gear, the axle shaft and the
corresponding coaxial tubular shaft from the axle steering drive. The
structure shown has a first bearing between the axle shaft and the tubular
shaft of the steering drive, and a second bearing between the tubular
shaft and a supporting flange rigid with the axle beam. It follows that
the aforesaid radial thrust will be discharged to the support through the
two bearings and the tubular shaft. Due to the frictional resistances that
arise unavoidably from the above situation of radial loading, the torque
being transferred between the axle shaft and the wheel hub makes an
undesirable effect felt on the steering drive. This effect is the more
apparent the higher is the torque being transferred through the axle
shaft.
Other known axles to more traditional designs are equipped with a steering
drive which includes a quadrilateral linkage effective under steer to
cause the centers of rotation of the steering axle wheels, respectively on
the turn inside and outside, to be coincident. This arrangement has
substantially the shape of an isosceles trapezoid whose major base is
formed by the axle section included between the two stub axles for the
wheel hubs, oblique sides are the steering arms rigid with the respective
hubs, and minor base is a rod tying the steering arms together.
With that quadrilateral linkage configuration, the centers of rotation are
brought to coincide when the extension lines from the steering arms
intersect approximately on the centerline of the rear axle. It follows
that this alignment will be dependent on several factors, such as the
vehicle wheelbase, tread (as a function of the stub axle spacing), etc. In
many cases, however, an axle is manufactured without all the parameters of
its ultimate utilization being known, or is intended for installation on
vehicles having different parameters, or is of an adjustable type whose
adjustments affect the steering geometry. Such axles cannot ensure,
therefore, an appropriate steering geometry for every installation and/or
adjustment, e.g. every tread adjustment.
In addition, axles of that kind are comparatively bulky constructions due
both to the steering arms and the tie rod provided. The intrusive steering
arms, by interfering with structural members of the vehicle, may become a
limiting factor of the maximum steering angle afforded. Further, the
steering arms and tie rod are both subject to shocks and the actions of
such environmental agents as dirt, mud, and dust, all apt to deteriorate
them over time.
To obviate in part the problems posed by the space requirements, axles are
employed which have steering actuators of the double-acting, two-rod
hydraulic jack type mounted close against the box-type structure of the
axle. The jack rods are swivel-associated with their respective steering
arms through struts. This construction, however, makes for inaccurate
steering geometry, which frequently results in a small, but perceivable,
amount of tyre slip in the radial direction on a turn.
SUMMARY OF THE INVENTION
It is the invention object to provide a steering power axle whose
construction can overcome the drawbacks of the aforementioned known axles.
This object is achieved, in a first embodiment of the invention, by an axle
as indicated in the preamble being characterized in that said actuator
comprises a stationary part which is rigid with the beam and independent
of the load-bearing structure of the vehicle, and a movable part
relatively to said stationary part which is connected to said road wheel
hubs through said steering drive.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the invention will become apparent from the
following detailed description of some exemplary embodiments thereof, as
shown by way of illustration and not of limitation in the accompanying
drawings, wherein:
FIG. 1 is a perspective view showing schematically a steering power axle
embodying this invention;
FIG. 2 is a fragmentary longitudinal section view of the axle shown in FIG.
1;
FIG. 3 is a sectional view taken along line III--III in FIG. 2;
FIG. 4 is a fragmentary longitudinal section view taken at one road wheel
hub of the axle in FIG. 1;
FIG. 5 is a perspective view of an angle transfer mechanism in the axle of
the previous FIGS.;
FIG. 6 is a fragmentary longitudinal section view corresponding to that of
FIG. 4 and showing a modified embodiment of this invention;
FIG. 7 diagramatically illustrates developed pitch lines for one embodiment
of the gears which make up the angle transfer mechanism in the axles shown
in the previous FIGS.;
FIG. 8 is a fragmentary longitudinal section view showing another
embodiment of a steering axle according to the invention; and
FIG. 9 is a sectional view taken along line IX--IX of the axle in FIG. 8,
as assembled to a load-bearing structure of a vehicle, with said structure
being only partially shown.
DETAILED DESCRIPTION OF THE INVENTION
Shown in FIG. 1 is an axle 10 which interconnects the driving road wheels
11 of a tractor.
This axle 10 comprises a beam of closed box-type construction, generally
shown at 12, which is formed of a middle part 13, two tubular parts 14
extending along opposite directions from the middle part 13, and two
cup-shaped bodies 15 located at the ends of the respective tubular parts
14. Dependent on the type of installation for which the axle is designed,
the beam 12 may be provided with conventional journal formations, not
shown, for swing mounting to the load-bearing structure of a vehicle (not
shown), as would be typical in the instance of a front steering axle of an
agricultural tractor, or conventionally attached to said load-bearing
structure.
Each cup-shaped body 15 is conventionally coupled for rotation about pivot
axis X--X (FIG. 4) to a corresponding cup-shaped body 16 rigid with the
hub 17 of a road wheel 11. The coupling is made pivotable by the provision
of a bearing 22 between the cup-shaped bodies 15, 16.
The closed box-type construction of the beam 12 accommodates such
transmission members as a differential mechanism 18, two axle shafts 19,
and two equal-velocity couplings 20, of which only one is shown in FIG. 4.
The differential mechanism 18 is received in the middle part 13 of the
box-type construction 12, the two axle shafts 19 are received in the two
tubular parts 14, and the two equal-velocity couplings 20 are received
inside the two cup-shaped body pairs 15 and 16. Said couplings 20 have
been omitted from FIG. 1 for clarity. The differential mechanism 18
receives its motion from a drive shaft 21 and distributes it to the two
axle shafts 19; the axle shafts 19 transfer then the motion in a manner
known per se to the road wheels 11 via the equal-velocity couplings 20.
The closed box-type construction 12 also houses a steering arrangement
which acts on the cup-shaped body pairs 15 and 16 to pivot the two
cup-shaped bodies 15 and 16 of each pair relatively to each other.
Said steering arrangement includes a hydraulic actuator, generally shown at
22, which is rigid with the middle body 13 and has two cylindrical
recesses 23, 24, parallel and spaced apart from each other, each recess
having a respective piston 29, 30 slidable tightly therein which defines
two supply chambers 25, 26 and 27, 28, respectively, in each recess for a
pressurized medium, typically a pressurized oil. Chambers 25 and 28,
opposite to each other, as well as chambers 26 and 27, are communicated to
each other by connection conduits, not shown, and are supplied through
ports 23a, 24a. A respective threaded rod 31, 32 projects in an adjustable
manner (and is locked by means of a nut 33)into the two chambers 26 and 28
to provide a travel limiter for the respective piston 29, 30, and hence, a
steering limiter for the axle, as explained hereinafter.
The two pistons 29, 30 have respective cutouts facing each other and being
formed with teeth, rack-fashion 34. Included between said racks, and
enmeshed therewith, is a gear wheel 35 which is keyed to or integral with
a shaft 36 carried rotatably on the axle body 13 and protruding thereinto.
Keyed to that shaft 36 is a bevel gear 37, referred to hereinafter as the
central gear. It should be noted that gear 37 is made rotatively rigid
with shaft 36 but allowed a limited sliding movement therealong and biased
elastically toward a stop washer 38 by a Belleville washer or some other
spring means 39.
In addition to the steering actuator, the steering arrangement includes a
steering drive which interlinks the two wheel hubs 17 to synchronize their
angular movements relative to the box-type construction of the axle in
conformity with pre-determined drive parameters.
This steering arrangement comprises two tubular shafts 40, one for each hub
17, which extend coaxially with and are supported rotatably around the
axle shafts 19, each tubular shaft having first and third gears 41 and 42
keyed to its opposite ends.
At their keyed end to the gear 42, the tubular shafts 40 are supported by
means of bearings 40a on the same supports 40b which carry the
differential mechanism 18 rotatably within the beam 12. They are supported
at the opposite end in a corresponding seat in the beam 12 by a bearing
40c. It should be noted that an additional bearing 40d intervenes between
each tubular shaft 40 and the corresponding axle shaft 19 at the end keyed
to gear 41, thereby the axle shaft is supported in the same seat of the
beam 12 by bearing 40d, bearing 40c, and the portion of tubular shaft 40
extending therebetween. It should be noted, however, that despite this
arrangement of the support for the steering drive and the axle shaft at
gear 41, the steering drive is substantially unaffected by the torque
transferred through the axle shaft because, with the equal-velocity
coupling 20 provided for transferring motion to the hub 17, the radial
loads on the bearings 40c,d from said torque are significantly reduced.
The first gear 41 enmeshes with a second gear 43 secured inside the
cup-shaped body 16 for the corresponding hub 17. Said first and second
gears are bevel gears and jointly form an angle transfer mechanism. The
second gear 43 is retained within the cup-shaped body 16 by a pin 44 and a
screw 45 passed through a slot 46. Said gear can therefore be rotated
relatively to the hub 17 so that the amount of toe-in of the road wheels
11 can be adjusted by loosening the screw 45.
Said first and second gears 41, 43 have preferably a pitch line 41a, (FIG.
7)designed to provide a drive ratio that varies with the steering angle.
The law for such variation can be obtained through mathematical
computations based on the geometric parameters of the axle, such as the
wheel base and the distance between the rotation axes of the hubs 17.
Thus, the rotations of the wheel on the turn inside and that on the turn
outside can be accommodated to suit the steering angle.
Variations to the wheelbase of the vehicle can be accommodated in a
comparatively simple and economical way by exchanging the first and second
gears for other gears having a different pitch line. Practical
observations have shown that the optimum pitch line follows a pattern
which is either oval or near-elliptical (FIG. 7).
To have the shafts 40 counter-rotate to each other, thereby an impulse to
steer can be suitably transmitted to the two hubs 17, the third gear 42 of
each shaft 40 is in mesh engagement with the central gear 37 on
diametrically opposite sides thereof. Gears 42 are also bevel gears. Said
first, second and third gears 41-43 are in the form of gear segments.
To steer the axle 10, oil is supplied under pressure by means of a
conventional control valve operated through a steering wheel, not shown,
into one of the chamber pairs 25, 28 or 26, 27 of the steering actuator
22, while the other pair is communicated to the discharge end.
As a result of the pistons 29 and 30 being moved in opposite directions,
the gear wheel 35 and central gear 37 are rotated. It should be further
noted that the Belleville washer 39 keeps the central gear 37 urged
against the third gears 42, to take up any backlash therebetween. This
washer also produces, due to the bevel design of the central gear 37 and
third gears 42, an axial force on the shafts 40 tending to take up any
backlash between the first and second gears 41, 43.
Rotation of the central gear causes each shaft 40 and corresponding first
gear 41 of the angle transfer mechanism to perform a similar but
oppositely directed rotation at the hub. Owing to the particular pitch
line of the first and second gears, the rotational angle about the axis
X--X of the hub on the turn inside will be different (greater) than the
corresponding angle turned by the outside hub, thereby compensating for
the different paths travelled by the two road wheels 11. It should be
noted that, in quite a similar manner, instead of using a non-circular
pitch line for the angle transfer mechanisms at the wheel hubs 17, this
pitch line could be used for the third gears 42 and corresponding gear
segments enmeshed therewith at the central gear 37.
Shown in FIG. 6 is a second embodiment of an axle 50 according to the
invention. Similar parts are denoted by the same reference numerals as in
the previous embodiment.
In this variation, the axle has a forked formation 51 (only partially
shown, the omitted portion being know per se) at its opposite ends, to
which a similar forked part 53 of the hub 17 is pivoted by means of pins
52.
In the forked formation 51, there is defined a chamber 54 accessible
through a detachable cover 55, wherein the first and second gears 41, 43
are housed away from dust and weather. Seals 57 prevent the entry of dust
into and exit of lubricant from chamber 54.
The second gear 43 fits for rotation on pin 52 over a splined section 56 of
same; said pin is, in turn, rigid for rotation with the hub 17, or
preferably, attachable to the hub while being also adjustable in relative
angular setting through such conventional arrangements as by slotting the
holes through which the screws are passed for fastening to the forked part
53, so that the amount of axle wheel toe-in can be controlled. The
operation of this variation is quite similar to that of the previously
described embodiment.
Indicated at 100 in FIGS. 8 and 9 is a steering power axle, such as the
front axle of a four-wheel drive agricultural tractor whose load-bearing
structure is only partially shown in FIG. 9. This axle 100 comprises a
box-type beam 102 housing a steering mechanism 103 and transmission 104.
The transmission 104 comprises a differential mechanism 105 which includes
a pinion gear 106, and respective shaft 107, and a ring gear 108; the
differential mechanism 105 is designed to distribute the motion input to
the pinion gear 6 differentially to two axle shafts 110a,b being each
associated with a respective axle road wheel.
Said differential mechanism 105 is carried inside the beam 102 on supports
109 and conventional bearings 109a.
The steering mechanism 103 comprises a steering actuator generally
indicated at 111 for operating two tubular shafts 112a,b, one for each
road wheel, which fit inside the beam 102 coaxially with the axle shafts
110a,b.
On the differential mechanism 105 side, the shafts 112a,b are supported
rotatably on the same supports 109 as the differential mechanism 105 with
suitable bearings 113 therebetween, and carry a steering gear 114a,b each,
which gear is made rotatively rigid therewith by means of a splined
connection 115. On the remote side, said shafts and the axle as a whole
are configured similarly to the previous embodiments.
Gears 114a,b are both in mesh engagement with a main gear 116 keyed to a
shaft 118 of actuator 111 while being allowed to slide therealong by the
provision of a splined connection 117.
Said actuator 111 is typically configured internally as a conventional
hydraulic motor, and has been drawn schematically only, it being
understood that it has--for parts not shown--a conventional design well
known to a skilled person in the art, whereby the shaft 116 can be driven
rotatively about its axis in either directions when supplied a suitable
medium, such as a pressurized oil. It comprises a case, generally shown at
119, having two flanges 120, 121 clamped together on a sleeve 122 by means
of threaded tie rods 123. Flange 120 is formed peripherally with a
plurality of holes for screws 124 and pins 125 whereby the actuator can be
secured at an aperture 126 in the beam 102 so as to seal it off.
Shaft 118 is supported on the flanges 120, 121 through respective bearings
127, 128 and has seals 129, 130 effective to prevent pressurized oil from
leading out of the case 119 or contaminants from seeping into the
actuator. Between bearing 127 and the main gear 116 is an elastic member
such as a Belleville washer 131 tending to urge said gear 116 into mesh
engagement with the steering gears 114a,b. A snap ring 132 is arranged at
the corresponding free end of shaft 118 to prevent gear 116 from slipping
off. The ring 132 is positioned to allow of an axial play for the gear
116, such that the washer 131 can perform the function of a backlash
take-up device between the steering mechanism gears, as explained in
detail with reference to the previous embodiments. Alternatively or
additionally to washer 131, other elastic means could be provided for
taking up the backlash, such as coil springs 131a or Belleville washers
acting between each gear 114a,b and a shoulder on the corresponding
tubular shaft 112a,b, e.g. a snap ring 131b.
Formed in the flange 121 are two conduits 133a,b, respectively for delivery
and return of the working medium to/from the actuator 111. It is also
contemplated that, for enhanced protection of the supply piping to the
actuator, said conduits 133a,b be formed radially of the flange 120, as
illustrated by dash lines in FIG. 9, to communicate, for example,
proximate to that beam portion which encloses and supports shaft 107. The
last-mentioned approach has not been shown because well within the ability
of a person of ordinary skill in the art on the grounds of the directions
given hereinabove.
The actuator case 119 has a cylindrical outer surface 134 over which there
fit, at the sleeve 122 and part of flange 120, first and second washers
135, 136 having tubular and annular configurations, respectively, for a
radial and axial bearing conventionally provided on a support 138 which
belongs to the load-bearing structure of a vehicle equipped with the axle
100.
Thus the case 119 provides one of the two journal formations about which
the axle 100 is allowed to swing. The other journal formation, shown at
140, is defined on the exterior of a collar 141 extending from the beam
102 on the transversely remote side from the case 119 coaxially therewith.
Said second journal formation encircles shaft 107, providing support for
it, and also includes washers 142, 143 serving the same function as the
respective washers 135, 136 for a corresponding bearing provided on a
support 144, again belonging to the load-bearing structure of a vehicle
equipped with the axle 100.
Among the advantages afforded by the invention, and with reference in
particular to the last embodiment illustrated, is that it provides optimum
protection for the steering actuator while significantly reducing its
space requirements. Further, the standard machining tolerances for the
case of the hydraulic motor constituting the actuator are by themselves
adequate to fill the requirements for accuracy of the fit between the
journal formations on the beam and the supports on the load-bearing
structure of the vehicle.
Also, the axle is free to swing about the journal formations without
interfering with the operation of the steering system. Lastly, by having
the supply conduits arranged such that they communicate to an area
included between the supports on the load-bearing structure of the vehicle
from the actuator or the beam, the supply piping to the actuator can be
well guarded.
In addition, all external linkages can be eliminated from the box-type
construction of the axle, which affords increased steering angles thanks
to reduced interference with structural parts of the vehicle, elimination
of space allowances externally of the axle, and a more compact design for
the latter, not to mention improved protection for the steering system.
* * * * *
|
|
 |
|
 |
Description |
|
