Abstract:
A force-measuring ring includes an annular housing which contains at least one piezoelectric measuring element, and a pressure transmission element which is attached to the housing via inner and outer circular-annular membrane areas. The inner and outer membrane areas of the force measuring ring are situated on opposite sides of a symmetry plane defined by the at least one piezoelectric measuring element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a force-measuring ring or load cell comprising an annular housing which contains at least one piezoelectric measuring element, and further comprising a pressure transmission element which is attached to said housing, preferably by welding, via inner and outer circular-annular membrane areas. The invention further relates to devices for measuring forces which are acting axially upon a shaft or a stamp. 
     2. The Prior Art 
     Force-measuring rings are used to measure forces, pressures, accelerations and moments, and for this purpose are inserted between force-transmitting means and engine parts. 
     From WO 2004/070334 A1 for instance there is known a measuring sensor with a pre-tensioning device, which essentially consists of an adapter part and an adapter sleeve. The adapter part has an integrated central tensioning sleeve, which extends into a central bore of the adapter sleeve and may be pre-tensioned there by means of a nut. A ring-shaped receiving space of the adapter sleeve carries a force-measuring ring, the measuring elements of the force-measuring ring being disposed in an annular housing and being covered by a membrane, which lies against the adapter part. The membrane is attached to the sensor housing by means of welding and is thus pre-tensioned. 
       FIG. 1  gives a more detailed description of the known force-measuring ring, which is for instance presented in G. GAUTSCHI, “Piezoelectric Sensors”, Springer 2002. The force-measuring ring  1  has an annular housing  2 , which forms a receiving space bounded by an inner flange  8  and an outer flange  9 , and contains two piezoelectric measuring elements  5  and a pick-up electrode  6  disposed between those elements. The pick-up electrode  6  connects to a socket element  7  attached to the outer flange  9 . The force acting on the force-measuring ring  1  is transmitted to the piezoelectric measuring elements  5  via the pressure transmission element  11 . The pressure transmission element  11  has an inner membrane area  12  and an outer membrane area  13 , which are welded to the inner flange  8  and the outer flange  9  of the housing  2 , respectively. Force-measuring rings of this kind are suitable in conditions when the force to be measured is uniformly applied over the contact surface of the pressure transmission element  11 . Such measurement conditions are described for instance in WO 2004/070334 A1 cited above. 
     There are applications, however, in which no uniform force distribution across the surface of the pressure transmission elements may be assumed. Measuring the axial force on a ball bearing as shown in  FIG. 2  would be an example of such an application. In that case the force must be taken off from one of the two ball cages. 
       FIG. 2  shows in cross-section a rotating shaft  20  with a load in axial direction, the shaft axis being indicated by  20 ′. The ball bearing  21  sitting on the rotating shaft has an inner cage  22  and an outer cage  23 , in which the balls  24  are held. In order to transmit force to the force-measuring ring  1 , an annular intermediary part  25  is required to distribute the force uniformly over the force-measuring ring  1 . Disadvantages of this arrangement are the great height and the higher elasticity of the measuring assembly. 
     It is the object of the present invention to improve a force-measuring ring of the kind described above in such a way that it may be used satisfactorily even under space limitations and with asymmetrical force input. In particular, the aim is to develop a robust force-measuring ring for measuring axial forces acting on an axial ball bearing. 
     SUMMARY OF THE INVENTION 
     According to the invention this object is achieved by providing that the inner and outer membrane area of the force-measuring ring lie on opposite sides of a symmetry plane defined by the at least one piezoelectric measuring element. 
     It is especially provided that the housing should have a first housing part with a ring-shaped inner flange and a first pressure transmission element as well as a second housing part with a ring-shaped outer flange and a second pressure transmission element, the inner membrane area being located between the inner flange and the second pressure transmission element and the outer membrane area being located between the outer flange and the first pressure transmission element. A force-measuring ring designed as proposed by the invention will feature high stiffness and small height and will as a consequence eliminate the need for an intermediary part for the force input surface of an axial ball bearing, for example. 
     The invention will now be described in more detail with reference to the enclosed drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  an axial section of a state-of-the-art force-measuring ring; 
         FIG. 2  a state-of-the-art force-measuring ring in an assembly for measuring the axial forces acting on an axial ball bearing; 
         FIG. 3  an enlarged sectional view of the force-measuring ring of  FIG. 2 ; 
         FIG. 4  a force-measuring ring according to the invention in sectional view; 
         FIG. 5  a sectional view of a force-measuring ring according to the invention in an assembly such as shown in  FIG. 2 ; 
         FIG. 6  and  FIG. 7  further variants of the force-measuring ring of the invention in sectional view as in  FIG. 1 ; and in 
         FIG. 8  a sectional view of the force-measuring ring of the invention in a further assembled situation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The state-of-the-art force-measuring ring shown in  FIGS. 1 and 2  and its assembled situation has already been described above. 
       FIGS. 3 and 4  present an illustrative comparison on a magnified scale of the differences between the invention ( FIG. 4 ) and the state of the art ( FIG. 3 ). In the case of the state-of-the-art force-measuring ring  1  the inner membrane area  12  and the outer membrane area  13  are located on one and the same side of the symmetry plane ε defined by the piezoelectric measuring element  5 , respectively the stack of two or more measuring elements  5 . In this way the two sides of the pressure transmission element  11  will be connected to the inner flange  8  and the outer flange  9  of the housing  2  only via the thin membrane areas  12 ,  13 . To achieve uniform force input, an annular intermediary part  25  is required. 
     With the force-measuring ring  1  of the invention presented in  FIG. 4  the situation is different. Its housing  2  has a first, lower housing part  3  with an annular inner flange  8  and a first pressure transmission element  10 , as well as a second, upper housing part  4  with an annular outer flange  9  and a second pressure transmission element  11 . The inner membrane area  12  is located between the inner flange  8  and the second pressure transmission element  11 . The outer membrane area  13  is located between the outer flange  9  and the first pressure transmission element  10 . In contrast to the state of the art the inner  12  and outer membrane area  13  lie on opposite sides of the symmetry plane ε defined by the piezoelectric measuring elements  5 . 
     According to the invention the inner flange  8  together with the first pressure transmission element  10  and the outer membrane are  13  may be designed as an integral unit. The same holds for the outer flange  9 , the second pressure transmission element  11  and the inner membrane area  12 . This will result in a robust force-measuring ring which is torsionally stiff even under one-sided force input as shown in  FIG. 4  (arrows  26 ) and does not require an annular intermediary part  25 . 
     Furthermore the outer flange  9  may be made stronger than the inner flange  8 , thus further enhancing the stiffness of the force-measuring ring. 
     In the example shown the outer membrane area  13  is welded to the outer flange  9  on its outer rim, while the inner membrane area is welded to the inner flange  8  on its inner rim. The inner flange  8  and the first pressure transmission element  10  are essentially L-shaped in cross-section. The same applies to the outer flange  9  and the second pressure transmission element  11 . 
     In the same way as shown in  FIGS. 1 to 3  for the state-of-the-art version, the force-measuring ring of the invention shown in  FIGS. 4 to 7  may also be provided in the two-part housing  2  with two piezoelectric measuring elements  5 , whose adjacent contact surfaces are in contact with a pick-up electrode  6 . A socket element  7  (see  FIGS. 6 and 7 ) may be located on the outer flange  9 , in the same way as shown in  FIG. 1 . 
     In  FIG. 5  the force-measuring ring  1  of  FIG. 4  is shown in an assembly for measuring the axial forces of an axial ball bearing  21 . The intermediary ring  25  required for uniform force input with state-of-the-art force-measuring rings (see  FIG. 2 ) need not be included in this case and the height of the measuring assembly will be substantially reduced. Via the interposed axial ball bearing  21  the shaft  20  is supported by the force-measuring ring  1 , for instance by means of a shoulder  26  on the shaft  20 . 
     In  FIGS. 6 and 7  there are shown variants of the force-measuring ring  1  of the invention with different dimensions and construction details. 
       FIG. 8  shows a further advantageous example of an application of the present invention. The force-measuring ring  1  of the invention is here used in an assembly for measuring the force acting axially on a stamp  27 . The shaft  28  of the stamp  27  on which the force to be measured is acting, passes through the force-measuring ring  1 , with the shaft  28  touching the inner flange  8  of the force-measuring ring  1  and thereby being centered. In contrast to a state-of-the-art force-measuring ring the design of the force-measuring ring  1  according to the invention avoids relative movement between the shaft  28  and the inner flange  8 , thus eliminating hysteresis effects during force measurement. The force-measuring ring  1  is supported by an interior flange  29  of the housing  30 .