Steering gear apparatus of rack-and-pinion type

A steering gear apparatus of the rack-and-pinion type including a pinion formed with helical teeth and a rack member formed with helical teeth which are brought into meshing engagement with the helical teeth of the pinion in such a manner that a steering effort is applied to the meshed portion of the helical teeth of the pinion and rack member at a predetermined angle relative to an axis line of the rack member, the rack member being resiliently biased toward the pinion, wherein the helical teeth of the pinion or the rack member each are formed with a tip portion to be brought into engagement with each root of the helical teeth of the rack member or the pinion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a steering gear apparatus of the 
rack-and-pinion type adapted for use in a steering system of an automotive 
vehicle. 
2. Description of the Prior Art 
In a conventional steering gear apparatus of this kind, the toothed portion 
of the pinion is in the form of a helical gear, and the axis line of the 
pinion is inclined at a predetermined angle relative to the normal line of 
the rack member such that a steering effort is applied to the meshed 
portion of the toothed portions of the pinion and rack member at the 
predetermined angle relative to the axis line of the rack member. To 
eliminate looseness at the meshed portion, the rack member is resiliently 
supported by a rack guide which is carried by a spring member to bias the 
rack member toward the pinion. 
In the conventional steering gear apparatus, as shown in FIG. 12, a tooth 
1a (a helical toothed portion the tip circle radius of which is r2 and the 
pitch circle radius of which is r1) of a pinion 1 is disengaged from teeth 
2a of a rack member 2 at its tip and root and is in sliding contact with 
the teeth 2a of rack member 2 at two flanks a and b in a tooth trace 
direction for effecting a wedge effect. In transmission of the steering 
effort at the meshed portion of the teeth 1a and 2a, there will occur a 
frictional force due to sliding at the tooth flanks a and b, and the rack 
member 2 is slightly rotated about its axis line or deviated from its axis 
line due to a counterforce acting against the frictional force when 
applied with a rotational force from the pinion 1. (see FIGS. 9(A), 10(A) 
and 11(A)) This results in deterioration of the steering performance. The 
frictional force Fp caused by sliding at the tooth flanks a and b is 
determined by a biasing force of the spring member (not shown) for biasing 
the rack member 2 toward the pinion 1, a normal line force F1 (a force 
acting in a direction perpendicular to the engaged tooth flanks) defined 
by an angle .theta. (usually, 29.degree.-40.degree.) between the tooth 
flanks a and b and a frictional coefficient .mu., as is represented by the 
following equation; 
EQU Fp=.mu..times.F1=(.mu..times.Fo/2)/sin (.theta./2); 
Where Fo is a numerical representation of the biasing force of the spring 
member. 
To improve such deterioration of the steering performance as described 
above, Japanese Utility Model Laid-open Publication No. 2-33171 discloses 
a steering gear apparatus wherein a rack member is provided with a rib 
portion for restriction of rotation, and wherein a housing containing the 
meshed portion of the rack member and a pinion is provided with a 
restriction means for engagement with the rib portion of the rack member 
to restrict rotation of the rack member in a condition where the rack 
member is slidable only in an axis line direction. In the steering gear 
apparatus, it is, however, required to provide a space for the rib portion 
and restriction means in the housing. As a result, the steering gear 
apparatus becomes large in size and heavy due to the provision of the rib 
portion and restriction means. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide an 
improved steering gear apparatus of the rack-and-pinion type capable of 
enhancing the steering performance without causing the problems discussed 
above. 
According to an aspect of the present invention, the object is accomplished 
by providing a steering gear apparatus of the rack-and-pinion type 
including a pinion formed with a toothed portion, a rack member formed 
with a toothed portion which is brought into meshing engagement with the 
toothed portion of the pinion in such a manner that a steering effort is 
applied to the meshed portion of the toothed portions of the pinion and 
rack member at a predetermined angle relative to an axis line of the rack 
member, and resilient means for resiliently biasing the rack member toward 
the pinion, wherein each tooth of the toothed portion of the pinion or the 
rack member is formed with a tip portion to be brought into engagement 
with a root of each tooth of the toothed portion of the rack member or the 
pinion. 
According to another aspect of the present invention, the object is 
accomplished by providing a steering gear apparatus of the rack-and-pinion 
type including a pinion formed with a toothed portion, a rack member 
formed with a toothed portion which is brought into meshing engagement 
with the toothed portion of the pinion in such a manner that a steering 
effort is applied to the meshed portion of the toothed portions of the 
pinion and rack member at a predetermined angle relative to an axis line 
of the rack member, and resilient means for resiliently biasing the rack 
member toward the pinion, wherein one of the toothed portions of the 
pinion and the rack member is formed larger in width than the other 
toothed portion, and wherein the other toothed portion smaller in width is 
formed in its full width with a plurality of teeth while the toothed 
portion larger in width is formed with the corresponding teeth in 
transcription for engagement with the teeth of the other toothed portion 
at least at their opposite ends in a tooth width direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Illustrated in FIG. 1 of the drawings is an embodiment of a steering gear 
apparatus in accordance with the present invention, which includes a 
pinion 11 rotatably mounted in place within a gear housing 14 by means of 
a pair off axially spaced bearings 12 and 13. The pinion 11 is mounted 
within the gear housing 14 in such a manner that an axis line L1 of pinion 
11 is inclined at a predetermined angle relative to a normal line (not 
shown) of an axis line L2 of a rack member 15 in the form of a lateral rod 
arranged to be translated. The pinion 11 is formed with helical teeth 11a 
which are brought into meshing engagement with helical teeth 15a formed on 
one side of the rack member 15 such that a steering effort is applied to 
the meshed portion of the helical teeth 11a, 15a at a predetermined angle 
relative to the axis line L2 of rack member 15. The rack member 15 is 
resiliently supported by a rack guide 17 which is carried by a spring 
member 18 to bias the rack member 15 toward the pinion 11. The spring 
member 18 is disposed between a closure cap 18 threaded into the housing 
14 and the rack guide 17. The rack guide 17 is slidably mounted within a 
mounting hole 14a formed in the housing 14. The closure cap 18 acts as a 
spring receiver and as an element for defining a displacement amount of 
the rack guide 17 in a biasing direction of the spring member 16 and is 
locked to the housing 14 by means of a lock nut 19. 
As clearly shown in FIG. 2, the helical teeth 11a of pinion 11 are formed 
with a tip circle radius r2 which is the same as a pitch circle radius r1 
of the helical teeth 11a. The teeth 15a of rack member 15 are formed in 
such a manner that each root 15a1 of the rack teeth 15a is in rolling 
contact with each tip portion 11a1 of the pinion teeth 11a in an axial 
direction of the rack member 15. At the meshed portion of the teeth 11a 
and 15a, the tip potion 11a1 of each of the pinion teeth 11a is constantly 
brought into contact with the root 15a1 of each of the rack teeth 15a in a 
tooth width direction. Thus, a frictional force Fp (Fp=.mu..times.Fo) 
defined by a frictional coefficient .mu. at the meshed portion is 
obtainable by the biasing force Fo of spring member 16 in the axial 
direction of rack member 15. 
In the above-described embodiment the construction of which is the same as 
the conventional steering gear apparatus except for the toothed portions 
11a and 15a, unwanted looseness at the meshed portion can be reduced by 
the rolling frictional force Fp obtained by engagement of the tip portion 
11a1 of the respective pinion teeth 11a with the root 15a1 of the 
respective rack teeth 15a. As a result, a clearance in a rotational 
direction of the pinion 11 can be formed at the meshed portion to effect 
sliding contact at one tooth flank in a tooth trace direction without 
causing sliding contact at two tooth flanks as in the conventional 
steering gear apparatus discussed above. This is effective to decrease a 
sliding frictional force caused by sliding contact at the engaged tooth 
flanks in the tooth trace direction. Thus, even if the rack member 15 is 
slightly rotated about its axis line or deviated from its axis line due to 
a counterforce acting against the sliding frictional force when applied 
with a rotational force from the pinion 11, the rotation or deviation of 
the rack member 15 can be greatly restrained in comparison with the 
conventional steering gear apparatus as shown in FIGS. 9(B), 10(B) and 
11(B). This is useful to restrain deterioration of the steering 
performance. 
Although in the above-described embodiment, the tooth tip circle radius r2 
at the pinion teeth 11a has been determined to be the same as the pitch 
circle radius r1, the tooth tip circle radius r2 may be determined to be 
different from the pitch circle radius r1. In the case that the tooth tip 
circle radius r2 is larger than the pitch circle radius r1, lateral 
movement of the rack member 15 is accelerated in steering operation, and 
the tooth flank of the respective rack teeth 15a at its forward side is 
maintained in meshing engagement with the tooth flank of the respective 
pinion teeth 11a at their reverse rotation sides. Accordingly, even when 
the rotation of pinion 11 is suddenly stopped or inverted, any engagement 
noise of the tooth flanks does not occur. Even if the pinion 11 is applied 
with a counterforce from the rack member 15, noises caused by engagement 
of the tooth flanks can be reduced by the frictional force between the 
root 15a1 of the respective rack teeth 15a and the tip portion 11a1 of the 
respective pinion teeth 11a. 
Although in the above-described embodiment, the root 15a1 of the respective 
rack teeth 15a has been formed to be brought into sliding engagement with 
the tip portion 11a1 of the respective pinion teeth 11a in the axial 
direction of rack member 15, the tip portion of the respective rack teeth 
15a may be formed to be brought into sliding engagement with the root of 
the respective pinion teeth 11a in the axial direction of rack member 15. 
In this case, lateral movement of the rack member 15 is delayed by the 
rolling frictional force, and the backward side flank of the respective 
rack teeth 15a is maintained in engagement with the forward side flank of 
the respective pinion teeth 11a. Accordingly, even when the pinion 11 is 
applied with a counterforce from the rack member 15, any engagement noise 
of the flanks does not occur. Even if the pinion 11 is suddenly stopped or 
rotated in a reverse direction in steering operation, noises caused by 
engagement of the flanks can be reduced by a frictional force of the root 
15a1 of the respective rack teeth 15a with the tip portion 11a1 of the 
respective pinion teeth 11a. 
Illustrated in FIGS. 3 to 5 is another embodiment of the present invention 
wherein width Wp of the toothed portion of a pinion 111 is formed smaller 
than width Wr of the toothed portion of a rack member 115. The toothed 
portion of pinion 111 is formed in its full width with helical teeth 111a, 
while the toothed portion of rack member 115 is formed with the 
corresponding teeth 115a in transcription for engagement with the helical 
teeth 111a without machining the opposite end portions thereof. The other 
construction is substantially the same as that of the steering gear 
apparatus shown in FIG. 1. 
In the embodiment shown in FIGS. 3 to 5, the teeth 111a and 115a can be 
brought into engagement with each other at least at their opposite ends in 
the tooth width direction. Accordingly, even if the rack member 115 is 
slightly rotated about its axis line or deviated from its axis line due to 
a counterforce against a sliding frictional force caused by a sliding 
contact at its tooth flanks in the tooth trace direction, the rotation or 
deviation of the rack member 115 is reduced by an engagement frictional 
force caused at the opposite ends of the teeth 111a and 115a in the tooth 
width direction. This is effective to restrain deterioration of the 
steering performance. 
Although in the above embodiment, the width Wp of the toothed portion of 
pinion 11 has been determined smaller than the width Wr of the toothed 
portion of rack member 115, the embodiment may be modified as shown in 
FIGS. 6 to 8. In this modification, the width Wp of the toothed portion of 
a pinion 211 is determined larger than the width Wr of the toothed portion 
of a rack member 215, and the toothed portion of rack member 215 is formed 
in its full width with helical teeth 215a while the toothed portion of 
pinion 211 is formed with the corresponding teeth 215a in transcription 
without machining at its opposite end portions. The other construction is 
substantially the same as that in the above embodiment. 
Although in the above embodiments shown in FIGS. 3 to 5 and 6 to 8, the 
toothed portion smaller in width is formed in its full width with a 
plurality of teeth while the other toothed portion larger in width is 
formed with the corresponding teeth in transcription for full engagement 
with the teeth of the toothed portion smaller in width, the corresponding 
teeth of the toothed portion larger in width may be formed in 
transcription for partial engagement in depth with the teeth of the 
toothed portion smaller in width at least at their opposite ends in the 
tooth width direction.