Abstract:
A rack-and-pinion steering system, in particular for lighter commercial vehicles, has a housing, a toothed rack, a driven element and a servo unit. The driven element is arranged between the toothed rack and the servo unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International Application No. PCT/EP2005/001158 filed Feb. 4, 2005, the disclosures of which are incorporated herein by reference in entirety, and which claimed priority to German Patent Application No. DE 20 2004 001 740.7 filed Feb. 5, 2004, the disclosures of which are incorporated herein by reference in entirety. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a rack-and-pinion steering system, in particular for lighter commercial vehicles, comprising a housing, a toothed rack, a driven element and a servo unit. 
   For economical reasons the use of rack-and-pinion steering systems has meanwhile been pushed ahead in commercial vehicles having a partial load on the front axle of up to approx. 6 tons. In so doing, the problem arises that on account of the high tooth rack forces of up to 60 kN and the large stroke of up to more than 125 mm special loads act on the toothed rack. 
   BRIEF SUMMARY OF THE INVENTION 
   A feature of the invention is to provide a steering system which is capable of withstanding the loads but at the same time cheap regarding the manufacturing costs. 
   In order to achieve this feature, the invention provides a rack-and-pinion steering system, in particular for lighter commercial vehicles, comprising a housing, a toothed rack, a driven element and a servo unit. The driven element is arranged between the toothed rack and the servo unit. Such a so-called mid-tapped rack-and-pinion steering has proved to be particularly advantageous in the case of the high loads occurring, in particular because a symmetrical or at least near symmetrical construction may be achieved. 
   In accordance with a preferred embodiment of the invention it is provided for that the driven element is a sliding block arranged in the housing. Such a sliding block may be used to pass the transverse forces occurring directly into the housing so that a toothed rack having smaller dimensions may be used. If the developing transverse forces were directly transferred to the toothed rack, the latter would have to be dimensioned considerably larger, in particular with a larger diameter, in order to be able to resist the loads occurring. However, a toothed rack dimensioned to be so large would result in considerable costs and a very high weight of the rack-and-pinion steering. 
   In accordance with a preferred embodiment of the invention it is also provided for that the housing comprises a pair of expansion bellows closing off the housing at the end at which the toothed rack is arranged. In this manner the housing proper may be configured to be considerably shorter than usually, which results in a higher degree of freedom with regard to the arrangement of the rack-and-pinion steering. There is in particular not the risk of the housing protruding into the space in which one of the steerable vehicle wheels can pivot into. 
   Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a bottom view of a steering system in accordance with the invention; 
       FIG. 2  shows a schematic section through the housing of the steering system of  FIG. 1 ; and 
       FIG. 3  shows an enlarged detail of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a front axle  10  of a commercial vehicle of which a frame  12  is shown in this arrangement as essential component. At the front axle  10  there is mounted a steerable vehicle wheel  14  which is connected via a tie rod  16  to a rack-and-pinion steering  18 . The rack-and-pinion steering  18  is firmly mounted relative to the frame  12 . The driving direction is indicated with the arrow P. 
   The rack-and-pinion steering  18  (see  FIG. 2 ) comprises a housing  20  in which a sliding block  22  is arranged so as to be movable in translation. With the sliding block  22  there is connected a connecting element  24  to which the two tie rods  16  are attached. The connecting element  24  comprises a bearing section  26  resting against two supporting surfaces  28  which are configured as the edges of a slot in the housing  20 . The bearing section  26  is configured as a plain bearing so that it slides with low friction at the supporting surfaces  28 . 
   At the left end of the housing  20  referring to  FIG. 1  there is arranged a servo unit  30  which is designed as a hydraulic cylinder. The hydraulic cylinder comprises a piston rod  32  which is connected via a pin arrangement  34  to the sliding block  22 . 
   On the side of the sliding block  22  opposite to the servo unit  30  there is arranged a toothed rack  36  which is connected via a ball-and-socket joint  35  to the sliding block  22 . A pinion (not shown) engages the toothed rack  36 , this pinion being arranged in a gearbox housing  38 . The pinion is connected to the steering wheel of the vehicle. 
   On the side of the steering gear  38  the housing  20  is constructed so as to be shorter than would be necessary for accommodating the toothed rack  36  in each of its positions. The end of the housing is formed by a pair of expansion bellows  40  mounted at the housing and moved by the toothed rack by the end face of the toothed rack to the right, if the latter is shifted to the right referring to  FIG. 1 . When the toothed rack is again shifted to the left, the expansion bellows  40  automatically return to the left on account of the fixation at the toothed rack. Thus, the expansion bellows  40  may be considered a variable part of the housing  20  which only assumes large dimensions, if required. In this manner it is ensured that with a complete lock of the wheel  14  for a left turn, there is a clearance F between the wheel and the end of the housing  20 . 
   The rack-and-pinion steering described offers the following advantages: On account of the arrangement of the sliding block centrally between the two vehicle wheels a symmetrical arrangement of the two tie rods  16  is achieved. All tilting forces transferred by the two tie rods via the connecting element  24  to the steering are passed into the latter via the bearing section  26  of the connecting element  24  and the supporting surface  28  of the housing  20 . In this manner these forces are prevented from acting onto the toothed rack  36 , the dimensioning of which has thus only to be effected substantially in view of the loads acting in longitudinal direction. The use of the ball-and-socket joint  35  for connecting the toothed rack  36  to the sliding block  22  additionally ensures that the toothed rack is not acted upon with transverse forces. The use of the expansion bellows results in a particularly compact rack-and-pinion steering so that there is a high degree of freedom in the arrangement of the rack-and-pinion steering at the vehicle. 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.