The present invention relates to a steering system, particularly a centre take off type steering system, for a road vehicle .

In general, centre take off type steering systems are used to meet packaging requirements without compromising on rack stroke and/or where long steering tie rod length is required so as to meet the suspension and steering kinematic characteristics for a given vehicle. This is typically observed in vehicles with narrow wheel track and suspension with large travel.e.g Subaru 360.

A centre take off steering system typically comprises of a rack and pinion arrangement and a pair of Inner Ball Joints (IBJs), typically in form of twin ball joints, located close to the vehicle longitudinal centre plane in vehicle straight ahead condition based on suspension and steering kinematic characteristics. The steering rack, comprising a rack portion provided with a plurality of teeth meshing with a pinion, is accommodated within a tubular structure or rack housing. The IBJs ideally need to be mounted coaxially to the steering rack, adjacent a centre line of the steering system or vehicle (in use), but due to packaging constraints it is most commonly mounted offset from the steering rack axis. A pair of tie rods then connects the IBJs to the steering arm (i.e. at wheel end) through the outer ball joints (OBJs). These Inner ball joints (IBJs)need to be secured to the steering rack with suitable locking arrangement(s).

In case of centre take off steering systems where the IBJs are not aligned with the rack axis, i.e IBJ's are offset with respect to a rack axis, the bending load/force exterted on the rack is at higher side, and additional torsion load is exerted on the rack about its axis due to the IBJ offset. This load is transferred to mounting structure through pinion and rack housing. To withstand these loads, the pinion and rack needs to be strengthened and additional supports need to provided on the rack housing or structure which not only increases the weight, cost and size of the overall system but also results in load increase at mountings and frictional resistance during operation thus reducing its efficiency.

Such a support may consist, in one form, of a slider block which is fastened to the steering rack. The assembly of slider block and steering rack generally happens after the assembly of the steering rack inside the tubular structure or rack housing. The slider block is then fastened to the steering rack through a cutout provided on the steering rack housing.

The Inner ball joints (IBJs which may also be in form of twin ball joints) are also fastened to the slider block as are the tie-rods which connect the IBsJ to the steering arm or wheel end through the outer ball joints (OBJs).

Since the location of fastening means for the IBJs on the slider block is typically offset to the rack axis, the mountings and pivot points of the IBJs are likewise offset to the rack axis.

Attempts have been made in the past to align axes of ball pins for the IBJs with the axis of steering rack with the objective of addressing the load issues described above. One of such arrangement is illustated , as background, in US Patent No 4473240. GB 1449016 also seems to illustrate a similar kind of arrangement.

This arrangement of center take off steering system, where IBJs are mounted coaxial to the steering rack axis, requires a multi-part rack assembly of some complexity. One part is in the form of a solid shaft provided with a plurality of teeth, the rack portion. The steering rack is press fitted or fastened to a second part in the form of a tube. This rack tube comprises cut outs to insert the ball pins for the IBJs and these ball pins are secured with the assistance of three supporters or inserts (one on LHS ,one in the mid of two ball pins and a third at RHS) as shown in Figs. 1 and 2. All three inserts, with ball pins, are spring loaded, or tightened by a theaded or other suitable fastening arrangement, to ensure acceptable fitting of the ball joints.

The above described assembly is then accommodated within a steering rack housing .The rack housing, in turn, has mountings for mounting it to a vehicle frame structure.

Such construction of centre take off type steering system has many disadvantages chief of which is created by the significant number of parts, and space required to accommodate these parts, which makes the system bulky and the manufacturing process time consuming, complex and costly thus affecting productivity and profitability.

There are also mechanical disadvantages. The cut outs must not be so small as to excessively limit articulation angle of the ball pins. On the other hand, if cut outs are too large, the rack tube is weakened and a higher tube wall thickness is required to compensate. In turn, a larger ball pin diameter must then be adopted for a given pullout load requirement due to limited area of contact with the restricting inserts as shown by Fig, 2b.

Further, spring loading of the arrangement requires tight control on manufacturing process for achieving desired amount of spring load and also potential difficulties in fitting.

It is an object of the present invention to provide a centre take off steering system less subject to the load and packaging issues described above.

With this object in view, the present invention provides a steering system of centre take off type-comprising:

a steering rack including a rack portion provided with a plurality of teeth and a rack tube;
and a plurality of ball joints comprising ball pins for connecting said steering rack to tie rods of said steering system;

wherein each of said rack portion and said rack tube are connected together through at least one module comprising said ball joints, said module being located proximate to a vehicle longitudinal centre plane in vehicle straight ahead condition based on suspension
and steering kinematic characteristics, and such that pivot points of said ball joints, said steering rack and said rack tube also being located along a common transverse axis.

The expression "pivot points" may be used interchangeably, in this specification and in other contexts, with the expression "pivot centers".

The module comprising the ball joints, typically termed the inner ball joints (IBJs) or twin ball joint assembly, may conveniently be connected to the rack portion and/or rack tube by jointing, particularly through rigid joints. The joints may be made by welding such as by friction welding or capacitive welding.

The module may be provided with connection means allowing connection to the rack portion and/or rack tube. Conveniently, the module may include connection portions fitting telescopically within, and locked to, the steering rack and rack tube. Such fitting simplifies assembly of the steering system though further fasteners may be required to make the connection secure. Alternatively, a threaded or other connection between module, rack portion and/or rack tube could be used. The module may be connected to the rack portion on one side, and to the rack tube on its other side. The module may have a tubular body portion comprising the ball joints and, on each side of the body, tubular portions allowing for connection to rack portion and rack tube. The tubular portions are conveniently cylindrical in shape.

Any connection, between module, rack portion or rack tube advantageously includes a locking means, preferably a locking nut, with profile complementary to, and mating with, a matching profile of rack portion and/or rack tube. This allows closer control over the required axial alignment of ball joints, steering rack and steering tube along the common transverse axis. A suitable profile is a conical profile with the locking means having a conical protrusion which engages and locks within a conical complementary recess.

The module may be manufactured with ball joints at the steering system assembly plant or at another location. That is, the ball joints may be integrated within the module at manufacture so no separate assembly step using inserts and other supports is required when assembling the steering system. There is no requirement for multiple inserts or biasing springs to locate the ball pins. At the same time, the inserts do not unreasonably restrict the articulation angle of the ball pins. The module may be manufactured with precision or tolerance such that there is little or no offset of the ball joint pivot points from the steering rack axis thus minimizing and even practically eliminating the torsional load issues above.

The precision also means that smaller cut outs to locate the ball pins may be provided, reducing the wall thickness required for the body of the module. Cost is saved in this manner but assembly is also made considerably easier. Where a telescope type connection is made, little time needs to be spent, during steering system assembly, on locating the ball joints or connecting the various components of the steering system. Productivity may therefore be significantly enhanced. Still further, the steering system allows more efficient packaging.

In another aspect of the present invention, there is provided a module for a steering system of centre take off type including a steering rack comprising a rack portion provided with a plurality of teeth and a rack tube, said module comprising at least one ball joint and ball pin for connecting said steering rack to a tie rod of said steering system and said module being connectable with said rack portion, said rack tube or a further said module, such connection resulting in rack portion, pivot points for said ball joint and rack tube to be located along a common axis.

The module preferably comprises a bush correspondent with each ball joint and ball pin. Accommodated within a housing of the module, the bush has a circular, part or substantially spherical surface which contacts and engages a ball pin over a significant portion of its surface area. The larger contact area, together with an absence of a requirement for bulky carefully positioned inserts, allows a smaller ball pin diameter to be adopted than previously. The circular, part or substantially spherical bush surface mates with a complementary surface of the ball pin. The bush may otherwise be fitted into, or connected to, the module in any desired manner. This allows reduction of ball joint/ball pin diameter and significant reduction of size and weight of over steering rack to be achieved.

In yet another aspect of the present invention, there is provided a method for assembling a steering system of centre take off type including a steering rack comprising a rack portion provided with a plurality of teeth and a rack tube and ball joints comprising ball pins for connecting said steering rack to tie rods of said steering system, said method comprising the step of connecting said steering rack to said rack tube through at least one module comprising said ball joints wherein, following connection, said pivot points of said ball joints, said steering rack and said rack tube are located along a common transverse axis.

Following connection of the ball joint module to the steering rack to complete the steering rack/ball joint assembly, the assembly is located within a rack housing; and assembly to the frame structure, chassis, of a vehicle may proceed.

The steering system may be used in a range of different vehicle types. Such vehicles may include three and four wheeler vehicles including vehicles of narrow track. Such vehicles may implement the suspension systems of the Applicant's co-pending Indian Provisional Patent Application Numbers 3990/CHE/2010 and 3991/CHE/2010.

Preferred embodiments of the steering system of the invention are now described with reference to the following figures in which:

Figure la Shows the prior art as described in US Patent No 4473240.

Figure lb shows one form of centre take off steering system according to prior art 6B 1449016.

Figure 2a shows a sectional view of Fig. la.

Figure 2b is a detail view showing construction of a ball joint shown in Fig, 2a

Figure 3 shows another form of centre take off steering system with offset mounted IBJs according to prior art.

Figure 4 shows an assembly view of a centre take off steering system according to one embodiment of the present invention

Figure 5a is a sectional view of the centre take off steering system as shown in Fig, 4.

Figure 5b is a detail view showing mounting of a ball joint in the system of Fig. 5a.

Figure 6a and 6b show a schematic showing part of the assembly of the centre take off steering system as shown in Figs. 4 and 5.

Figure 6c shows a schematic showing a further method of assembly of the steering system of the invention.

Figure 7 shows detailed view housing incorporating ball pins according to invention.

Figs. 1 to 3 show prior art centre take off steering systems. In the steering systems of

Figs. 1 and 2, attempt is made to align the ball pins (103) along the axis of the steering rack (105) to reduce bending loads. The attempt requires a multi-part rack assembly of some complexity. One part is in the form of a solid shaft provided with a plurality of teeth, the rack portion. The steering rack (105) is press fitted or fastened to a second part in the form of a rack tube (112). This rack tube (112) comprises cut outs (113) to insert the ball pins (103) for the inner ball joints (IBJs) and these ball pins (103) are secured into position with the assistance of three supporters or inserts (one on LHS ,one in the mid of two ball pins and a third at RHS). All three inserts, with ball pins (103). are spring loaded, or tightened by a theaded or other suitable fastening arrangement, to ensure acceptable fitting of the ball joints.

Such construction of centre take off type steering system has many disadvantages chief of which is created by the significant number of parts which makes the system bulky and the manufacturing process time consuming, complex and costly thus affecting productivity and profitability.

There are also mechanical disadvantages. The cut outs (113) must not be so small as to excessively limit articulation angle of the ball pins (103). On the other hand, if cut outs (113) are too large, the rack tube is weakened and a higher tube wall thickness is required to compensate. In turn, a larger ball pin diameter must then be adopted for a given pullout load requirement due to limited area of contact with the restricting inserts. Further, spring loading of the arrangement requires proper selection of spring load and potential difficulties in fitting and manufacturing springs within close stiffness and size tolerance.
Referring now to Figs 4 to 7, there is shown a centre take off steering system (10) comprising a steering rack (2, 5) including a rack portion (2) provided with a plurality of teeth and a rack tube (5) and two ball joints (7), including ball pins (3), located proximate to vehicle longitudinal centre plane in vehicle straight ahead condition based on suspension and steering kinematic characteristics ,for connecting said steering rack (2,5) to two respective tie rods (4) of the steering system (10). Each of the rack portion (2) and rack tube (5) are connected together through a module or housing (6) comprising the two inner ball joints (7). It will be noted that the pivot points (7a) of the inner ball joints (7), the rack portion (2) and the rack tube (5) are located along a common transverse axis (28).

The module or housing (6) may be friction welded to rack portion (2) and rack tube (5) such that these parts are rigidly jointed together. Alternatively, housing (6) may be provided with connection means (6a) allowing connection to the rack portion (2) and rack tube (5). These connection means (6a) could be tube portions or stubs (6a) allowing the housing (6) to fit telescopically over correspondent portions (2a, 5a) of the rack portion (2) and rack tube (5) located on respective sides of the housing (6). Such fitting simplifies assembly of the steering system. To this end, tubular portions (6a) have greater inner diameter than the outer diameters of the correspondent tubular portions (2a, 5a) of the rack portion (2) and rack tube (5) to which they are connected. The mentioned tubular portions (6a) are cylindrical in shape.

The housing (6) may be manufactured with ball joints (7) at the steering system assembly plant or at another location. Ball pins (3) may then easily be connected to the ball joints (7). There is no requirement for multiple inserts to locate the ball pins (3). At the same time, the inserts do not unreasonably restrict the articulation angle of the ball pins. The module may be manufactured with precision or tolerance such that there is little or no offset of the ball joint pivot points from the steering rack axis thus minimizing and even practically eliminating the torsional load issues above.

The precision also means that smaller cut outs (13) to locate the ball pins (3) may be provided, reducing the wall thickness required for the body of the module (6). Cost is saved in this manner but assembly is also made considerably easier. With the steering system as here described, little time needs to be spent, during steering system assembly, on locating the ball joint (7)s or connecting the various components of the steering system. Productivity may therefore be significantly enhanced. Still further, the system allows more efficient packaging.

The housing (6), as conveniently shown in Fig. 5b, comprises, or accommodates, a bush (12) correspondent with each ball joint (7), the bush (12) having a substantially spherical inner surface (13) which contacts, and mates with, each ball joint (7) over a significant portion of its complementary surface area. Portion of the bush (12) also has a part spherical outer surface. The larger contact area (690 mm2), together with an absence of a requirement for carefully positioned and bulky inserts, allows a smaller ball pin diameter (16mm) to be adopted than previously (22mm). The bush (12) may be fitted into the module (6) in any desired manner.

The housing of module (6) can be fabricated through an integrated forging/casting route having two openings or cut outs (13) to accommodate the ball pins (3). The one end of opening then may be sealed by standard process called as crimping or any other methods can be employed. The other ends are kept open so as to connect the tie rods (4) to ball pins.

The joining of the forged ball pin housing (6), rack portion (2) and rack tube (5) can be carried out by processes like friction welding, capacitive welding, metal adhesives, suitable fastening arrangements and so on. Three examples are provided here.

Example 1:

The connection means (6a) of the finished forged ball joint housing (6) may be threaded for connection to the rack portion (2) and rack tube (5), the complementary portions (2a, 5a) having the same threading type/specification as for the connection means (6a). The threaded connection of housing (6) with rack portion (2) and rack tube (5) can then be locked by lock washer or lock nut.

Advantageously, as shown in Fig 6(c), the threaded connections, between module (6), rack portion or rack tube include locking nuts (26) which are rotated over connection means (6a) to make the required connections. The locking nuts (26) have a profile complementary to, and mating with, a matching profile of rack portion (24) and rack tube (25). This allows closer control over the required axial alignment of pivot points of ball joints (7), steering rack portion (24) and steering tube (25) along common transverse axis (28). Locking nut (26),shown in detail for connection between connection means (6a) of module (6) and rack tube (25), includes a conically profiled protrusion (26a) which engages, and locks within a conical complementary recess (25a) of rack tube (25). Connection between module (6) and rack portion (24) is made in the same way.

The conically profiled locking means (25a, 26a) ensures accurate alignment of module (6), rack portion (2) and rack tube (5), which ensures accurate alignment of pivot points of ball joints (7) of rack portion (2) and rack tube (5) along the required common transverse axis (28).

Example 2:

The rack portion (2) and rack tube (5) can be press fitted to the connection means (6a) of housing (6) and then riveting can be done in the press fitted area to strengthen the joint.

Example 3:
The ball joint housing, rack and rack tube can be joined by metallic adhesives, or press fit or friction/capacitive welding etc. On completion of the steering rack/ball joint assembly (15), the assembly (15) is located within a rack housing (20) and assembly of the whole to the frame structure or chassis (not shown) of a vehicle may proceed. Rack housing (20) has a two part construction involving a pinion casing (21) and a rack casing (22). First the steering rack/ball joint assembly (15) is assembled to the pinion casing (21) having a recess accommodating almost the whole of the assembly (15). Then the rack casing (22) is fastened/press fitted to the pinion casing (21) over the steering rack assembly (22). This assembly design facilitates ease of service and part replacement of internal detail part instead of complete rack assembly replacement in field and thus reduces service/ maintenance cost for the customer.

The assembly steps are conveniently illustrated by Figures 6 a and 6 b. In the initial stages, the ball joint housing (6) is connected to the rack portion (2) and rack tube (5) as above described (see figure 6a). Then, the assembly (15) is located into its final position within the rack and pinion casings (21, 22) as above described (see figure 6b).

Modifications and variations to the steering system of the invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention.

WE CLAIM:

1. A steering system of centre take off type comprising:

a steering rack including a rack portion provided with a plurality of teeth and a rack tube; and

a plurality of ball joints comprising ball pins for connecting said steering rack to tie
rods of said steering system;

wherein each of said rack portion and said rack tube are connected together through at least one module, comprising said ball joints, said module being located proximate to a vehicle longitudinal centre plane in vehicle straight ahead condition based on suspension and steering kinematic characteristics and such that pivot points of each said ball joint, said steering rack portion and said rack tube are located along a common transverse axis.

2. The steering system of claim 1 wherein said at least one module comprising said ball joints is connected to at least one of said rack portion and said rack tube by jointing through rigid joints.

3. The steering system of claim 2 wherein said ball joints are inner ball joints (IBJs) or twin ball joints.

4. The steering system of any one of the preceding claims wherein said module comprising is provided with connection means allowing connection to at least one of said rack portion and said rack tube.

5. The steering system of claim 4 wherein said module connection means allows connection to both rack portion and rack tube.

6. The steering system of any one of the preceding claims wherein said module fits telescopically and is locked within the steering rack portion and rack tube.

The steering system of any one of claims 1 to 5 wherein said module is connected to said rack portion or rack tube by threaded connection.

The steering system of claim 6 or 7 wherein said threaded connection includes a locking means with profile complementary to, and mating with, a matching profile of said rack portion or said rack tube.

The steering system of claim 8 wherein said profile is a conical profile.

The steering system of any one of the preceding claims wherein said module comprises a tubular or cylindrical body portion comprising the ball joints and, on each side of said tubular body, tubular portions allowing for connection to said rack portion and said rack
tube.

The steering system of any one of the preceding claims wherein said module comprises a housing accommodating a bush, and ball pin correspondent with each ball joint, said bush having a circular, part or substantially spherical surface which mates with a complementary surface of said housing over a significant portion of its surface area.

A module for a steering system of centre take off type including a steering rack comprising a rack portion provided with a plurality of teeth and a rack tube, said module comprising at least one ball joint and ball pin for connecting said steering rack to a tie rod of said steering system and said module being connectable with said rack portion, said rack tube or a further said module comprising at least one ball joint, such connection resulting in rack portion, pivot points for said ball joint and said rack tube being located along a common transverse axis.

13. The module of claim 12 comprising a tubular or cylindrical body portion comprising the ball joints and, on each side of said tubular body, tubular portions allowing for connection to said rack portion and said rack tube.

14. The module of claim 12 or 13 wherein said module comprises a housing accommodating a bush and ball pin correspondent with each ball joint, said bush having a circular, part or substantially spherical surface which mates with a complementary surface of said ball pin over a significant portion of its surface area.

15. The module of claim 14 wherein said housing is an integrated forged housing.

16. A method for assembling a steering system of centre take off type including a steering rack comprising a rack portion provided with a plurality of teeth and a rack tube and ball joints comprising ball pins for connecting said steering rack to tie rods of said steering system, said method comprising the step of connecting said steering rack to said rack tube through at least one module comprising said ball joints wherein, following connection, said pivot points of said ball joints, said steering rack portion and said rack tube are located along a common transverse axis.

17. The method of claim 16 wherein said module comprises a tubular body portion comprising the ball joints and, on each side of said tubular body, tubular portions allowing for connection to said rack portion and said rack tube.

18. The method as claimed in claim 16 or 17 wherein said module is connected to said steering rack and rack by threaded connection.

19. The method of claim 18 wherein said threaded connection includes a locking means with profile complementary to, and mating with, a matching profile of said rack portion or said rack tube.

20. The method of claim 19 wherein said profile is a conical profile.

21. The method of any one of claims 16 to 20 wherein said module comprises a housing accommodating a bush and ball pin correspondent with each ball joint, said bush having a circular, part or substantially spherical surface which mates with a complementary surface of said ball pin over a significant portion of its surface area.