Steerable vehicles

A steerable vehicle has at least one rigid axle carrying steerable wheels, a steering box secured to the axle, and an input rotatable relative to the housing of the steering box with steering control means having first and second relatively rotatable parts, a connection between the first part of the steering control means and the housing of the box having a first element arranged to take up an average position between the first part and the housing during relative rotation therebetween, and a differential gearing between the second part of the control means and the input to the steering box, the differential gearing having a second element which takes up an average position between the second part and input during relative rotation, the first and second elements being connected for simultaneous movement. This prevents steering of the wheels due to relative roll occurring between the steering box and the control means.

BACKGROUND TO THE INVENTION 
Field of the Invention 
This invention relates to steerable vehicles including those having prime 
movers and those in the form of trailers or semi-trailers. 
It would be convenient in a vehicle having a rigid axle carrying steerable 
wheels to mount the steering box on the axle and to connect the box by 
some type of linkage to a steering control means, e.g. a steering column, 
the linkage allowing for relative movement between the axle and the 
steering control means. The difficulty is that, in use, the axle pivots 
about axes extending generally fore and aft of the vehicle due to road 
conditions thus causing relative rotation or roll between the steering 
control means and the steering box and thus steering the wheels even 
though the driver has not given any input to the steering control means. 
It is an object of the invention to provide steering means in which such 
relative rotation will affect the steering. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention we provide a steerable vehicle 
comprising steering control means having first and second relatively 
rotatable parts; a rigid axle carrying steerable wheels; a steering box 
having a housing secured to the axle, an input and an output rotatable 
relative to the housing; means connecting said output to the steerable 
wheels; a connection between the first part of the steering control means 
and said housing and having a first element arranged to take up an average 
position between the first part and the housing during relative rotation 
therebetween; and a differential gearing between the second part of the 
steering control means and said input and having a second element which 
takes up an average position between said second part and said input 
during relative rotation therebetween, said first and second elements 
being connected for simultaneous movement. 
If, for example, the first part of the steering control means is mounted on 
the vehicle chassis and there is relative rotation between the axle and 
the chassis due to roll under road conditions, the first element will take 
up an average position between said first part and the steering box 
housing. The second element will also take up an average position and, 
since there will have been no relative rotation between the first and 
second parts of the steering control means, the steering box output will 
have been moved by the second element taking up its average position to a 
position corresponding to that taken up by the axle and there will thus 
have been no relative movement between the steering box housing or axle 
and the steering box input. It follows, therefore, that relative 
rotational movement between the axle and the chassis during roll will not 
affect the steering. When steering is required, there will be relative 
movement between the first and second parts of the steering control means 
and thus relative rotation between the steering box input and the housing 
providing the desired output from the steering box. 
The connection between the first part of the steering control means and the 
housing may be by way of a second differential gearing. Each gearing may 
be in the form of a bevel differential gearing with the planet gears 
forming said first and second elements and being carried on a common 
shaft. The bevel differential gearing could be replaced by epicyclic 
gearing of the spur gear type. 
Alternatively, the first element may be provided by links capable of 
pivotal movement and between which the second element is carried. The use 
of such links is possible because the amplitude of the relative rotation 
between the first part of the steering control means and the housing is 
low. 
Preferably the first and second parts of the steering control means are 
arranged coaxially with the second part within the first part. Each of 
said parts may comprise a number of portions connected by universal 
joints. The universal joints between the inner portions may be aligned 
with the universal joints between the outer portions. 
The invention is applicable to a steerable vehicle having a number of 
steerable axles. A heavy vehicle may have two or more steerable axles and 
a trailer may, in some cases, have two or more steerable axles. 
According to this aspect of the invention we provide a steerable vehicle 
comprising steering control means having first and second relatively 
rotatable parts; two or more rigid axles each carrying steerable wheels; a 
steering box for each axle, each steering box having a housing secured to 
the axle and an input and an output both rotatable relative to the 
housing; means connecting the output of each steering box to the steerable 
wheels of the axle on which said box is mounted; a first connection 
between the first part of the steering control means and the housing of 
one of said steering boxes and having a first element arranged to take up 
an average position between the first part and the housing during relative 
rotation therebetween; a first differential gearing between the second 
part of the steering control means and the input of said one steering box 
and having a second element which takes up an average position between 
said second part and said input during said relative rotation 
therebetween; said first and second elements being interconnected for 
simultaneous movement; a second connection between the housing of said one 
steering box and the housing of another of said steering boxes, said 
connection having a third element arranged to take up an average position 
between said steering boxes during relative rotation therebetween; and a 
second different gearing between the input of said one steering box and 
the input of said other steering box and having a fourth element which 
takes up an average position between said inputs during relative rotation 
therebetween, said third and fourth elements being interconnected for 
simultaneous movement. 
In such an arrangement, steering will not be disturbed due to roll between 
the axles and the vehicle chassis or relative roll between the axles. 
A vehicle having multiple steered axles requires that the wheels on one of 
the axles be turned through different angles than the wheels on other of 
the axles during cornering and suitable gearing for this purpose can be 
introduced into the means which connect the steering box of one of the 
axles to the steering box of the other. It will be appreciated that one 
may have more than two steering boxes and they will be connected as 
described.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2 these will be used to describe the principle 
of the invention. There are outer members 10 and 11 and inner members 12 
and 13, the member 12 being rotatable within the member 10 and the member 
13 being rotatable within the member 11. The adjacent ends of the members 
12 and 13 are formed with bevel side gears 14 and 15 respectively; the 
side gears mesh with planet gears 16 and 17 which are rotatable on a pin 
18. The ends of the pin 18 are pivotally received in the bases of U-shaped 
links 19, the ends 20 of these links being rounded and being pivotally 
received in elongated guides 21 and 22 on the outer members 10 and 11 
respectively. 
If there is relative rotation between the outer members 10 and 11 the pin 
18 will take up an average position between the two members. Thus if there 
is a relative rotation of, say, 10.degree. between the members 10 and 11 
the pin 18 will take up a position at 5.degree. to each of the members 10 
and 11. The pin will take up this position because it is supported midway 
between the outer members 10 and 11 in the links 19 which can, due to 
their rounded ends 20, pivot in the guides 21 and 22 as said relative 
rotation takes place. Because the pin 18 also carries the planet gears 16 
and 17, the inner member 13 will have been moved and since the planet 
gears will have moved, in the foregoing example, through 5.degree. about 
the axis 23, the side gear 15 will have turned through 10.degree. about 
the axis 23 relative to the side gear 14. In the foregoing example, 
therefore, it follows that the outer member 11 will have turned 10.degree. 
relative to the outer member 10 and the inner member 13 will have turned 
through 10.degree. relative to the inner member 12 and the members 11 and 
13 will be in the same relative position as they were before said relative 
rotation took place between the outer members 10 and 11. 
If, therefore, the member 10 is connected to the chassis of the vehicle and 
the member 12 to a steering wheel and if the member 11 is mounted on the 
axle of a vehicle and the member 13 arranged to steer the wheels of that 
vehicle, if the axle rotates relative to the chassis about the axis 23 the 
steering will not be disturbed because there will be no relative movement 
between the members 11 and 13. 
If, however, there is relative rotation between the members 10 and 12 
without there being relative rotation between the members 10 and 11, this 
will produce a relative rotation between the members 11 and 13 by the 
differential gearing and in the example above steering can be obtained. If 
there is relative rotation between the members 10 and 12 and between the 
members 10 and 11 then the relative rotation between the members 10 and 11 
will not affect the relative rotation between the members 11 and 13 for 
foregoing reasons. This arrangement can be used to provide improved 
steering means for a vehicle as will be described below. 
In FIG. 3, the links 19 with their guides 21 and 22 have been replaced by a 
second differential gearing. Parts in FIG. 3 which are identical to parts 
in FIGS. 1 and 2 have been given the same reference numerals. 
In FIG. 3, the outer members 10 is provided with a side gear 24 and the 
outer members 13 is provided with a side gear 25. Planet gears 26 and 27 
mesh with the side gears 24 and 25 and are also carried by the pin 18. The 
differential gearing provided by the gears 24-27 acts in precisely the 
same manner as the links 19 so that relative rotation between the members 
10 and 11 will place the pin 18 in an average position due to the 
differential gearing and the inner differential gearing of the gears 14-17 
will ensure that the member 13 does not turn relative to the member 11. 
Referring now to FIGS. 4 and 5, a rigid axle for a vehicle is indicated at 
28 and carries steerable wheels 29. A steering box 30 is mounted on the 
axle 28 and has an output 31 which is connected to a trackrod 32 which in 
turn is connected to steering arms for the wheels 29, one of said arms 
being indicated at 33. A steering wheel 34 is rotatably mounted on a 
steering column 35 which is connected to the chassis (not shown) of the 
vehicle. The steering wheel is connected to a shaft 36 rotatable within 
the column and the column and the shaft are connected through universal 
joints 37 and 38 and coaxial shafts 39 to the steering box 30 as will now 
be described. 
Referring to FIG. 4, the universal joint 38 is a double universal joint 
between the outer shaft 40 of the coaxial shafts 39 and a bell shaped 
member 41 rotatably mounted on bearings 42 on a spigot 43 of the steering 
box 30. The universal joint 38 also connects the inner shaft 44 of the 
coaxial shafts 39 to a shaft 45 connected to the input of the steering 
box. Thus the shaft 44 has a yoke 46 at its end which is connected to a 
yoke 47 on the end of the shaft 45 by the usual cruciform member 48. The 
bell shaped member 41 is pivotally connected to a hoop 49 which is 
pivotally connected to ears 50 on the outer shaft 40. The pivotal axes 
between the member 41 and hoop 49 on the one hand and between the hoop 49 
and the ears 50 on the other hand are perpendicular. 
The input shaft 45 carries a bevel side gear 51 which meshes with planet 
gears 52 carried on a pin 53. The planet gears 52 mesh with a further side 
gear 54 connected to an input shaft 55 which is connected to a rack, not 
shown, within the steering box. A pinion, not shown, engages the rack and 
is connected to the steering box output 31. Links 56 engage the ends of 
the pin 53 and have their ends formed spherically and engaged in spherical 
recesses 57 in the bell shaped member 41 and the end of the spigot 43 of 
the steering box. The arrangement acts in the manner described in detail 
in relation to FIGS. 1 and 2. Thus if there is relative rotation between 
the member 41 and the spigot 43 the links 56 will take up an average 
position and the differential gearing will ensure that the input shaft 55 
retains the same relative position relative to the housing as before the 
relative rotation. 
The outer and inner shafts 40 and 44 of the coaxial shafts 49 are connected 
through a universal joint 37 similar to the joint 38 to the steering 
column 35 and the shaft 36 respectively. It follows from what has been 
said that if there is any rotation of the axle 28 relative to the column 
35, this will not disturb the steering because of provision of the links 
56 and the differential gearing. The universal joints 37 and 38 and the 
coaxial shafts 39 are merely a connection between the steering column 35 
and the shaft 36 on the one hand and the steering box housing and the 
shaft 45 on the other so as to allow for relative movement between the 
chassis and the axle. When steering is required, relative rotation takes 
place between the column 35 and the shaft 36 thus causing relative 
rotation between the member 41 and the shaft 45 and thus producing 
relative rotation between the steering box housing and the shaft 55. 
The arrangement above described with reference to FIGS. 4 and 5 thus 
applies the principle of FIGS. 1 and 2 to the vehicle, but the 
modification of FIG. 3, i.e. utilising a second differential gearing in 
place of the links 56, may be utilised in this and the following example. 
FIGS. 6 and 7 shows an arrangement in which there are two steerable axles 
60 and 61 carrying steerable wheels 62 and 63 respectively. The wheels 62 
are connected by a track rod 64 to the output 65 of a first steering box 
66 and the wheels 63 are connected by a track rod 67 to the output 68 of a 
second steering box 69. A steering wheel 70 and column 71 are connected to 
the steering box 66 in the manner described in relation to FIGS. 4 and 5. 
The steering box 66 is connected to the steering box 69 by means 72 shown 
in FIG. 7. This means 72 comprises coaxial shafts 73, 74 having at their 
ends universal joints 75 identical to the universal joint 38 described 
above. The housing of the steering box 66 is connected to the housing of 
the steering box 69 via the shaft 73 and the outer differential gearing of 
a double differential of the construction shown in FIG. 3. The input shaft 
77 of the steering box 66 is connected to the input shaft of the steering 
box 69 via a gearing 78, the shaft 74 and the inner differential gearing 
of the differential 76. The gearing 78 is such that, during steering, 
there is a difference between the angle of rotation of the input shaft of 
the steering box 69 and the angle of rotation of the input shaft 77. This 
displacement is arranged so that the inclination of the wheels 63 relative 
to the wheels 62 is correct during steering of the vehicle around a 
corner. 
Various modifications may be made to the invention. Thus the universal 
joints such as 37, 38, and 73 may be replaced by plunging universal joints 
having capacity for longitudinal displacement between their parts. 
The bevel gear differential gearing described may be replaced by epicyclic 
differential gearing of the spur-gear type.