Patent Publication Number: US-2013237355-A1

Title: Low-noise belt drive

Description:
The present invention relates to electromechanical power-assisted steering having the features of the pre-characterising clause of claim  1 . 
     In electromechanical power-assisted steering toothed belt drives are preferably used where a high level of support and low noise emission are required. An example of such steering is known by way of example from document DE 10 2007 004 521 A1. The toothed belt drive connects the output shaft of an electric motor to the input shaft of a gear transmission such that the rotational movement is converted into a linear movement of a rack. A ball screw is conventionally used here. The conventional design of the toothed belt drive provides that the belt is provided on its inner side with teeth arranged transversely to the running direction and these mesh with corresponding grooves on the belt pulley. The envelope of the belt pulley is exactly cylindrical, so the toothed belt is not fixed in the axial direction of the two shafts. To prevent shifting of the toothed belt on the discs the smaller belt pulley is outwardly provided with what are known as flanged discs which limit shifting of the toothed belt in the axial direction. 
     In belt drives of this kind vibrations of the belt occur which are produced by the teeth provided at regular intervals transversely to the running direction. At certain rotational speeds of the belt drive the belt can be caused to vibrate by the contour of the belt pulley. Great efforts are therefore being made to make a belt drive of this kind of a low-noise character. The conventional measures relate to the form of the individual teeth on the inner side of the toothed belt and their orientation. Toothed belts by way of example are also known therefore which have a herringbone gearing. In this connection the two sections of an arrow-shaped tooth can also be mutually offset in the running direction of the toothed belt. Toothed belts of this kind are very expensive. The matching belt pulleys are similarly very expensive to produce. 
     Document BE 520 967 A relates to an endless gear having rollers via which a belt, which automatically centres itself, is tensioned.  FIGS. 3 to 8  show various rollers with corresponding belts, wherein  FIG. 6  shows a roller with convex external contour and a belt with a concave internal contour. 
     It is therefore the object of the present invention to improve generic power-assisted steering in relation to its noise emission. 
     This object is achieved by power-assisted steering having the features of claim  1 . 
     Because in the case of a belt drive with a belt, which, transversely to its running direction, has a concave internal contour, viewed from the direction of the belt pulley, and with corresponding belt pulleys, which are provided transversely to the rotational direction with a corresponding convex external contour, an edge region of the belt outwardly encompasses the convex external contour of the belt pulley, the tendency of the belt to vibrate in the region of its free sides is reduced. The flexural stiffness of the belt is increased. If the belt is a toothed belt and the belt pulleys are matching toothed belt discs, the stimulation of the belt when a tooth strikes the corresponding groove in the belt pulley is less pronounced. 
     In a preferred embodiment the flanged discs for centring the toothed belt may be omitted. Potential kinematic problems, which could occur owing to the different circumferential speeds of the belt wheels in the axial direction of the teeth, can be compensated by an enlarged face clearance between the tooth and the groove in the edge region. It may also be provided that the edge region is designed free of teeth. 
     Particularly good suppression of the tendency to vibrate is achieved if in the edge region the toothed belt has inwardly-pointing, encircling ribs which engage in correspondingly freed regions of the belt pulley or outwardly encompass the belt pulley. 
    
    
     
       Exemplary embodiments of the present invention will be described in more detail below with the aid of the drawings, in which: 
         FIG. 1 : shows a cross-section through a belt pulley with convex external profile and a corresponding belt in a schematic diagram; 
         FIG. 2 : shows a cross-section through a belt pulley with roughly rectangular cross-section and a belt which comprises inwardly-pointing guide ribs; 
         FIG. 3 : shows a schematic side view of a belt drive using the belt from  FIG. 2 ; and 
         FIG. 4 : shows a cross-section through a belt pulley with a concave, roughly semi-circular running surface and a belt with a round cross-sectional profile. 
     
    
    
       FIG. 1  shows a cross-section [of] a belt pulley  1  which has an outer convex running surface  2  for a corresponding concave toothed belt  3 . The toothed belt  3  is provided in a central region close to a plane of symmetry  4 , which is perpendicular to the drawing plane, with teeth which engage positively and without clearance in corresponding teeth of the belt pulley  1 . To the right and left of the plane  4  the toothed belt  3  is pulled around the convex region  2  of the belt pulley  1 . The toothed belt  3  thus automatically centres itself on the belt pulley  1  during rotation. A second belt pulley with a different diameter is provided with a corresponding contour. A characteristic feature is that the conventionally required lateral flanged discs can be omitted due to the contour of the toothed belt  3  and that the illustrated cross-sectional contour of the belt with the height a indicated in  FIG. 1  ensures particular stability with respect to induced vibrations perpendicular to the running direction in the region of the free sides of the belt, thus minimising noise emission. 
       FIG. 2  shows in a corresponding diagram another belt pulley  11  which has a substantially rectangular cross-section. An outer circumferential surface  12  is provided with teeth which mesh with corresponding teeth of a belt  13 . The belt  13  is fitted with ribs  14  on the side and these project parallel to its direction of rotation inwardly over the plane of the teeth. On the one hand the ribs  14  centre the belt  13  on the belt pulley  11  and on the other hand the ribs  14  stiffen the belt  13 , so minimisation of noise emission is in turn achieved. 
     The ribs  14  form an edge region of the belt which outwardly encompasses the convex external contour of the belt pulley. 
       FIG. 3  shows the arrangement from  FIG. 2  in a side view. The belt pulley  11  is round and mounted so as to rotate about a centre of rotation  15 . The belt  13  is guided over the belt pulley  11 . The ribs  14  overlap the belt pulley  11  laterally by an amount which matches the illustrated height a. 
     Finally  FIG. 4  shows a belt pulley  16  which on its outer circumferential surface has a concave recess  17  with roughly semi-circular cross-section. A circular belt  18  is placed over the belt pulley  16 . The circular belt  18  is provided with teeth in the region of the plane  4 , which is the plane of symmetry of the belt pulley  16  and of the belt  18 , and the teeth engage in correspondingly compatible teeth on the belt pulley  16 . 
     Due to the teeth, the illustrated belt designs can be placed in a belt drive so as to be slip-free. The contour of the belt suppresses natural vibrations in the regions between the two belt pulleys. In the edge region the belt can have a certain elasticity which supports the encircling process of the belt when it engages with the belt pulleys. For this purpose it may be provided that highly tensile, non-lengthenable fibres are provided only in the centre of the belt  3  or  13 , i.e. where the teeth are arranged in the region of the plane  4 . 
     It may also be provided that, in particular in the case of the embodiment according to  FIG. 1 , the teeth are guided over the entire convex surface  2  of the belt pulley  1 . It is then advantageous to provide the teeth with a certain clearance in the edge regions by way of appropriate design of the grooves on the belt pulley  1  or the teeth on the inner side of the belt  3 , which play allows the required deformation. In the case of low heights a in the region of a few millimetres it may also be provided that the profile shift of the teeth varies from the centre of the plane  4  to the edges. 
     Overall the illustrated belt designs make it possible to construct a belt drive in electromechanical motor vehicle power-assisted steering without the conventional flanged discs and to consequently save space in the radial direction of the belt pulleys. In addition, the noise emission of the belt drive is minimised by the increased flexural stiffness of the illustrated belt.