Note: Descriptions are shown in the official language in which they were submitted.
~a~7ss
BACKGROUND OF THE INVENTION
This invention relates generally to a force transmit-
ting apparatus and more specifically to a force transmittlng
apparatus which is advantageously utilized to effect turning
movement of a steerable wheel of a vehicle.
A known variable ratio steering apparatus is disclosed
in Canadian Patent Application Serial No. 294,635 filed
January 10, 1978 by Frederick John Adams and Ralph Malcolm
Lehman and entitled "Improvements In Or Relating To Rack And
Pinion Assemblies". The steering apparatus disclosed in that
application includes a pinion having teeth which mesh with
helical gear teeth on a rack bar. The rack bar is displaced
longitudinally relative to a housing upon rotation of the pinion.
A cam track is disposed on the rack bar and is engaged by a
follower which is connected with the housing. The cam track
and follower cooperate to effect rotation of the rack bar upon
longitudinal movement of the rack bar. This rotation of the rack
bar causes the rack gear teeth and pinion gear teeth to interact
in such a manner as to vary the rate of axial movement of the
rack bar.
Although the steering apparatus disclosed in the
aforementioned application is believed to be generally satisfact-
ory in its construction and mode of operatlon, it is believed
that after extended usage, ball joints at opposite ends of the
rack bar may tend to wear. This wear of the ball joints is, to
some extent at the least, induced by the rotation of the rack bar.
In addition, the cam track of the steering apparatus
disclosed in the aforementioned application is axially offset
from the rack gear teeth. Therefore, the rack bar must have a
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minimum length which is at least twice as great as the length of the array
of rack gear teeth. It is believed that this relatively long length may be
objectionable if the steering apparatus is used with relatively small vehi-
cles. In addition, by having the cam track axially offset from the rack gear
teeth, the rack gear is subjected to both torsional and bending forces when
the rack gear is rotated by the cam follower.
The present invention provides a variable ratio gear assembly com-
prising a pinion rotatably mounted in a housing and in meshing engagement
with a rack of a longitudinally extending rack member, said rack member being
rotatably mounted with respect to the housing and being longitudinally dis-
placeable in response to rotation of the pinion; a yoke member engaging with
the rack member to support it with the rack in meshing engagement with the
pinion; control means for imparting controlled rotation to the rack member
relative to the housing during longitudinal displacement of the rack member
so that the ratio of the gear is varied, and wherein the rack teeth extend
part way only around the circumference of the rack member to an extent that
they maintain engagement with the pinion throughout the intended rotational
movement of the rack member during longitudinal displacement thereof and the
non-toothed portion of the rack member which is co-extensive longitudinally
with the rack provides a slideway which said yoke member straddles and slid-
ably engages to restrain displacement of the rack member in the direction of
the axis about which the pinion rotates.
According to another aspect of the invention there is provided an
apparatus for use in transmitting force with variable ratio gearing, said ap-
paratus comprising a housing, a rotatable drive member mounted in the housing,
said rotatable drive member being displaceable along its axis of rotation
and relative to the housing; a rack gear comprising an array of helically
formed teeth disposed on said drive member; a rotatable pinion gear mounted
in the housing and engaging with the rack gear to effect axial displacement
of said drive member upon rotation of the pinion gear; a cam track disposed
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on said drive member, at least a portion of said cam track being co-extensive
with said rack gear in the direction of the axis of rotation of said drive
member; follower means disposed in engagement with said cam track for effect-
ing rotational movement of said drive member relative to said pinion gear
upon axial displacement of said drive member; each of said rack gear teeth
extending only part way around the periphery of the drive member to an extent
that the rack gear maintains engagement with the pinion gear throughout the
intended rotational movement of the drive member; a slideway disposed on said
drive member to be substantially co-extensive with said rack gear and being
provided by the part periphery of the drive member in~o which said rack gear
teeth do not peripherally extend, and support means for at least partially
supporting said drive member, said support means being mounted by the housing
and slidably engaging the slideway substantially in a plane which extends
radially of the drive member and which includes the region of engagement
between the rack gear and the pinion gear.
Although an apparatus constructed in accordance with the present
invention is advantageously utilized to turn the steerable wheels of a vehi-
cle, it is contemplated that the apparatus could be utilized in other envi-
ronments if desired. For example, the apparatus could be used to control the
movement of a traverse slide of a machine tool.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features of the present invention will become
more apparent upon a consideration of the following description taken in con-
nection with the accompanying drawings wherein:
Figure 1 is a fragmentary sectional view of a variable ratio steer-
ing apparatus constructed in accordance with the present invention;
Figure 2 is a fragmentary sectional view of a portion of the steer-
ing apparatus of Figure 1 and illustrating the relationship between an array
of rack gear teeth and a cam track disposed on a rotatable drive member;
Figure 3 is a sectional view, taken generally along the line 3-3 of
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Figure 2, and illustrating the relationship between the drive member and a
pinion gear which is disposed in meshing engagement with the rack gear teeth,
a cam follower which engages the cam track, and a support member which pres-
ses the rack gear teeth into engagement with the pinion gear;
DESCRIPTION OF SPECIFIC PREFERRED
EMBODIMENTS OF THE INVENTION
A variable ratio steering apparatus 10 constructed in accordance
with the present invention is illustrated in Figure 1. The steering appara-
tus 10 is connected with a pair of steerable wheels (not shown) of a vehicle
in a well known manner by a suitable linkage. This linkage includes a tie
rod 12 connected with one of the steerable vehicle wheels and a second tie
rod 14 connected with the other steerable wheel. The two tie rods 12 and 14
are connected with opposite ends of a longitudinally extending force trans-
mitting member or bar 16 by a pair of ball joints 18 and 20. Upon axial move-
ment of the bar 16 relative to a housing 24, the steerable wheels of the ve-
hicle are turned to effect a desired steering action.
A tubular drive member or sleeve 28 is rotatably mounted on a central
portion of the bar 16 and can be rotated without rotating the bar 16. To
this end a bearing assembly 30 is disposed between a cylindrical inner sur-
face 32 of the drive member 28 and the bar 16. The bearing assembly 30 co-
operates with a needle bearing assembly 34 to rotatably support the drive
member 28 on the bar 16 in a coaxial relationship with the bar. The bearing
assembly 30 is effective to hold the drive member 28 against axial movement
relative to the bar 16.
A longitudinally extending array 38 of helical rack gear teeth 40
is formed on the drive member 28 (see Figures 1 and 2). The rack gear teeth
40 extend only part way around the cylindrical outer surface of the drive
member 28 and are disposed in meshing engagement with a rotatable pinion gear
42. Upon rotation of the pinion gear 42 about its central axis in response
to turning of a steering wheel, the pinion gear 42 and rack gear teeth 40 co-
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operate to cause the drive member 28 and bar 16 to be moved along their com-
mon central axis 46. This effects turning movement of the steerable vehicle
wheels in a known manner.
It is contemplated that during parking and other low speed operations
in which the vehicle wheels are sharply turned, it may be desirable to de-
crease the effort required to turn the vehicle wheels by increasing the steer-
ing gear ratio. In order to increase the steering gear ratio, the drive
member 28 is rotated about its central axis 46. This is accomplished by the
interaction between a cam track 50 (Figure 2) and a follower 52 ~Figure 3).
Rotation of the drive member 28 causes the helical rack gear teeth
40 to slide relative to the helical teeth on the pinion gear 42. This slid-
ing action displaces the drive member 28 longitudinally relative to the hous-
ing 24. This longitudinal movement of the drive member 28 is in a direction
opposite to the direction in which the drive member is being moved relative
to the housing 24 by the pinion gear 42 to thereby increase the steering gear
ratio. If the pinion gear 42 is rotated in a counterclockwise direction as
viewed in Figure 2, the drive member 28 and bar 16 move toward the right (as
viewed in Figure 2). During this rightward movement, the follower 52 engages
a downwardly (as viewed in Figure 2) curving end portion 54 of the cam track
50. This rotates the drive member 28 upwardly (as viewed in Figure 2) or in
a clockwise direction (as viewed in Figure 3).
The helical rack gear teeth 40 extend at an acute angle to the lon-
gitudinal central axis 46 of the drive member 28. Therefore, rotation of the
drive member 28 about its central axis 46 causes the flanks on the rack gear
teeth to effect a sliding or camming action against the flanks of the helical
teeth of the pinion gear 42. This camming action moves the drive member
toward the left (as viewed in Figure 2) with a screw type action. This re-
sults in a reduction in the rate of rightward (as viewed in Figure 2) move-
ment of the drive member 28 and bar 16 to thereby effect an increase in the -
mechanical advantage provided by the steering gear assembly.
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Similarly, the pinion 42 is rotated in a clockwise direction (as
viewed in Figure 2) to effect movement of the drive member 28 and bar 16
toward the left (as viewed in Figure 2). As this occurs, an arcuately up-
wardly curving end portion 58 of the cam track 50 engages the cam follower
52 to rotate the drive member 28 downwardly (as viewed in Figure 2). The
resulting interaction between the helical rack and pinion gear teeth reduces
the rate of leftward (as viewed in Figure 2) movement of the drive member 28
and bar 16.
It should be noted that the cam track 50 has a central portion 62
which extends parallel to the longitudinal axis 46 of the drive member 28.
Therefore when the cam follower 50 is engaging the central portion 62 of the
cam track 50, the cam follower and cam track are ineffective to cause rota-
tion of the drive member 28 relative to the housing 24. Therefore the steer-
ing ratio remains constant at this time.
In accordance with another feature of the present invention, the cam
track 50 is axially coextensive with the array 38 of rack gear teeth 40.
Thus, the oppositely curving end portions 54 and 58 of the cam track 50 are
disposed adjacent to axially opposite ends of the array 38 of rack gear teeth.
By having the length of the cam track 50 coextensive with the axial length of
the array 38 of rack gear teeth 40, the overall length of the drive member 28
is minimized. It is believed that this feature will be particularly advanta-
geous when the steering apparatus 10 is utilized in association with a rela-
tively small or compact vehicle.
In order to maintain solid meshing engagement between the pinion
gear 42 and the array 38 of rack gear teeth 40 during rotation of the drive
member 28, the array 38 of rack gear teeth has a curving configuration which
is the same as the curving configuration of the cam track 50. Thus, a lon-
gitudinally extending edge portion 66 of the array 38 of rack gear teeth has
a central portion which extends parallel to the straight central portion 62
of the cam track 50. The opposite ends of the edge portion 66 of the array
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38 of rack gear teeth curve in opposite directions in the same manner as do
the opposite end portions 54 and 58 of the cam track 50. Accordingly, the
edge portion 66 of the array 38 of rack gear teeth is parallel to the longi-
tudinal central axis of the cam track 50. The opposite edge portion of the
array 38 of rack gear teeth (not shown) has the same configuration as the
edge portion 66. Therefore the array 38 of rack gear teeth has a substantial-
ly constant circumferential extent of about 140 about the drive member 28.
In accordance with still another feature of the present invention,
the cam follower 52 engages the cam track 50 at a location which is axially
aligned with the location at which the pinion gear 42 meshingly engages the
rack gear teeth 40 (see Figure 3). By having the location at which the cam
follower 52 engages the cam track 50 axially aligned with the area where the
pinion gear 42 meshingly cngages the rack gear teeth, the application of
torsional loads to the drive member 28 is minimized. In addition, sidewise
loads and bending moments applied to the drive member 28 are minimized. If
the cam follower 52 engaged the cam track 50 at a location which was axially
offset from the location where the pinion gear 42 meshes with the rack gear
teeth 40, the forces applied to the drive member 28 by the cam follower 50
would be offset from the forces applied to the drive member by the interaction
between the rack and pinion gear teeth. By having these forces aligned with
each other so that there are no bending moments tending to twist the drive
member about a transverse axis, the loading applied to the support structure
for the drive member 28 and the bar 16 is minimized.
The rack gear teeth 40 and the drive member 28 are continuously ~-
pressed into meshing engagement with the pinion gear 42 under the influence
of a support yoke 70. The support yoke 70 is pressed upwardly against a
lower side of the drive member 28 under the influence of a biasing spring 72
(see Figure 1). It should be noted that the support yoke 70 engages the drive
member 28 at a location which is directly opposite from the location where
the pinion gear 42 meshingly engages the rack gear teeth 40 so that the gear
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tooth forces which tend to separate the rack and pinion gears are offset by
the support yoke. The cam follower 52 engages the cam track 50 at a location
between the support yoke 70 and the area of meshing engagement between the
rack and pinion gears.
In Figures 1-3, an apparatus constructed in accordance with the
present invention is utilized to effect turning movement of the steerable
wheels of a vehicle. ~owever, it is contemplated that the apparatus could
be utilized in environments other than in association with the steerable
wheels of a vehicle. For instance, it is contemplated that the apparatus
could be utilized to move the traverse slide of a machine tool relative to
the bed of the machine tool.
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