Abstract:
A filter insert ( 1 ) having an annular filter element ( 2 ) which is connected in a fluid-tight manner to a cover ( 5 ) on one axial end face ( 3 ) and to a base ( 6 ) on an opposite axial end face ( 4 ). The filter element ( 2 ) has a central channel ( 25 ) in which a central tube ( 8, 28 ) is situated. The central tube ( 8, 28 ) extends between the cover ( 5 ) and the base ( 6 ) and communicates with a central opening ( 7 ) in the cover ( 5 ). To ensure high stability of the filter ( 1 ) with a low manufacturing complexity, the central tube ( 8, 28 ) is designed to have a variable length in the direction of its longitudinal central axis ( 10 ).

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a filter insert comprising an annular filter element connected in a fluid-tight manner on one axial end face to a cover and on an opposite axial end face to a base, and having a central channel communicating with a central opening in the cover and having a central tube situated therein extending between the cover and the base. 
   Filter inserts comprising a filter element fixedly connected to a cover and a base are known in general and are used as replaceable filter cartridges. To ensure adequate stability of the filter element, such filters usually have a central tube running between the cover and the base in the central channel of the filter element. The central channel thus communicates with an opening in the cover, which usually forms the outlet from the filter. To ensure adequate stability of the filter insert, the central tube should always extend from the base to the cover of the filter. The filter element usually comprises filter paper in lamellar folds or other filter media. Such a filter bellows may have considerable tolerances in its dimensions. To ensure that the central tube extends between the cover and the base, there must be a narrow tolerance in the filter element or central tubes of different lengths must be made available. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide an improved filter insert. 
   Another object of the invention is to provide a filter insert with a center tube that can automatically compensate for length tolerances. 
   A further object of the invention is to provide a filter insert with a variable length center tube that has a high stability. 
   An additional object of the invention is to provide a filter insert with a variable length center tube that can be manufactured easily. 
   These and other objects are achieved in accordance with the present invention by providing a filter insert comprising an annular filter element connected in a fluid-tight manner at one axial end face to a cover and at an opposite axial end face to a base; said filter element having a central channel in which a central tube is situated extending between the cover and the base with said central channel communicating with a central opening in the cover; wherein the central tube has a variable length in the direction of its longitudinal central axis. 
   The variable length of the central tube in the direction of the longitudinal central axis of the central tube makes it possible to use the same central tube even with filter elements having a considerable deviation in their axial length. When designed appropriately, such a central tube may also be used in filters of different axial lengths, thus permitting a reduction in the number of parts which must be kept on hand. At the same time, the cost of manufacturing the central tube and keeping it in stock is reduced. 
   The central tube advantageously has a length-compensating section, designed to be resilient in the direction of the longitudinal central axis of the central tube. The central tube thus automatically adapts to the axial length of the filter element. Thus, adequate stability of the filter element can be ensured easily at the same time because this assures that the central tube will extend over the entire length of the filter element. 
   The central tube is advantageously made of rings arranged at a right angle to the longitudinal central axis of the central tube, these rings being joined together by stays extending in the axial direction. In particular, one or more stays are designed to be resilient in a length-compensating section. Accordingly, at least one stay may be advantageously designed in a V shape in a length-compensating section. The V-shaped design of a stay ensures an adequate spring path in the longitudinal direction of the central tube due only to the natural elasticity of the material. In the circumferential direction of the central tube, however, torsion is prevented with this design. Alternatively, it may be advantageous for at least one stay to extend in a spiral around the longitudinal central axis of the central tube in a length-compensating section. This makes it possible to realize larger spring paths. 
   At least one stop is advantageously provided to limit the minimum length of the central tube. This makes it possible in particular to prevent the stays, which are designed to be resilient, from being destroyed due to excess compression of the central tube. The stop is advantageously formed by a stay which extends into the length-compensating section and has its end situated at a distance from a ring adjacent to this section. 
   The central tube and the cover together forms a receptacle for a gasket. A connection for supplying or removing a medium to be filtered may be provided on this gasket. Particularly when the central tube and cover are made of molded synthetic resin material (i.e., plastics), it is essential for the parts to separate well from the molds in which they are formed. To facilitate this, the receptacle is designed as a circumferential groove which is divided at a right angle to the longitudinal central axis of the central tube, with part of the receptacle being integrally molded on the central tube and part on the cover. To ensure a good seal, the receptacle must have a predetermined size. Due to the variable length of the central tube, which is particularly designed to be resilient, it is possible to ensure that the two parts forming the receptacle are in tight contact with one another. It is thus possible to assure that a predetermined size of the receptacle will be maintained. 
   The central tube, the base and/or cover may advantageously be made of synthetic resin material, in particular POM or polyamide (PA). The filter element is attached to the cover and the base in particular by hot tool welding or by butt welding. Such a weld can be produced easily and quickly. At the same time, a tight connection of the filter element to the cover and base can be ensured without any additional means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention ill be described in further detail hereinafter with reference to illustrative preferred embodiments shown in the accompanying drawing figures in which: 
       FIG. 1  is longitudinal sectional view through a filter according to the invention taken along the line I—I in  FIG. 2 ; 
       FIG. 2  is a top view of the filter from  FIG. 1 ; 
       FIG. 3  is a perspective view of the central tube of the filter of  FIG. 1 ; 
       FIG. 4  is a side elevational view of the central tube of  FIG. 3 ; 
       FIG. 5  is a longitudinal sectional view of the central tube of  FIG. 3 ; 
       FIG. 6  is a sectional view taken along line VI—VI; 
       FIG. 7  is an enlarged detail view of the area VII of  FIG. 4 ; 
       FIG. 8  is an enlarged view of the detail VIII of from  FIG. 5 ; 
       FIG. 9  is a enlarged view of the detail IX from  FIG. 5 , and 
       FIG. 10  is a perspective view of a central tube according to the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The filter  1  shown in  FIG. 1  has a filter element  2 , which separates an unfiltered fluid side from a clean side in a filter housing. The filter element  2  may be, for example, filter bellows made of filter paper in lamellar folds or some other filter medium. The filter element  2  is cylindrical and has a cylindrical central channel  25 . On one end face  3  of the filter element  2 , a cover is provided having a central opening  7  as also shown in  FIG. 2 . On the opposite end face  4 , there is a base  6 , which is connected to the filter element  2 . Base  6  is designed in the shape of a disk which seals the central channel  25 . 
   The cover  5  and the base  6  are joined in a fluid-tight seal to the filter element  2  by hot tool welding or butt welding. To join the cover  5  and the base  6  to the filter element  2 , the cover  5  and the base  6 , which are made of synthetic resin material, in particular POM or polyamide (PA), are heated until the material on the side facing the filter element  2  is plastically deformable. In this state, the filter element  2  is pressed tightly against the cover  5  and the base  6  so that the end faces  3  and  4  of the filter element  2  are joined to the material of the cover  5  and the base  6 . The annular filter element  2  is thereby sealed in a fluid-tight connection at its axial end faces  3  and  4 . 
   A central tube  8  which is situated in the central channel  25  is in contact with the base  6  at its end  17  and is welded to the cover  5  on its opposite end  16 . In the area of the cover  5 , the central tube  8  and the cover  5  together form a receptacle  12  which is designed as a peripheral groove in which a gasket  11 , in particular an O-ring, is situated. The receptacle  12  is designed so that it is divided in a direction perpendicular to the longitudinal central axis  10  of the central tube  8  to ensure that the central tube  8  and the cover  5  can be unmolded. The part  13  of the receptacle  12  integrally molded on the central tube  8  is formed by a disk-shaped shoulder on the central tube  8 . The disk-shaped shoulder extends radially inward from the filter element  2  and has a central opening  27 , which communicates with the opening  7  in the cover  5 . The second part  14  of the receptacle  12 , which is integrally molded on the cover  5 , is formed by a shoulder  33 , which extends from the cover  5  in the axial direction to the base  6  in an area radially outside of the opening  7 . The shoulder  33  is designed in a ring shape here and forms the base of the receptacle  12 . 
   To ensure a good seal with a connection situated in the openings  7  and  27  by the gasket  11 , the receptacle  12  must have a predetermined size. This may be achieved by having the central tube  8  in tight contact with the cover  5 . Therefore, length-compensating sections  9  are provided so that tolerances in the axial length of the filter element  2  can be compensated. 
   The central tube  8  is shown in detail in  FIGS. 3 through 9 . The central tube  8  is formed by rings  15  which extend in the peripheral direction to the longitudinal central axis  10  of the central tube  8 . The central tube  8  here is formed by nine rings  15 . Instead of a ring  15 , a ring flange  18  having a greater axial extent is provided on the end  16  facing the cover  5 . The rings  15  are held in the direction of the longitudinal central axis  10  by stays  20  which extend between adjacent rings  15  in parallel with the longitudinal central axis  10  and connect them. Four stays  20  are distributed uniformly around the circumference between two rings  15 . As shown in  FIG. 6  in particular, the rings  15  have a width b, measured in the radial direction of the longitudinal central axis  10 , which is greater than the width c of the stays  20 , also measured in the radial direction. The stays  20  are arranged on the inside circumference of the rings  15  so that only the rings  15  are in contact with the filter element  2 . 
   The central tube  8  has two length-compensating sections  9 . The length-compensating sections  9  are each at a distance from the base  6  and the cover  5  of the filter  1  corresponding approximately to the length of three stays  20  with the rings  15  provided between them. 
   The length-compensating sections  9  are designed to be resilient in the direction of the longitudinal central axis  10 . Therefore, stays  21  which are resilient in the direction of the longitudinal central axis  10  are provided. The stays  21  are designed in a V shape. Four V-shaped stays  21  are arranged in one section  9 , with the V of adjacent stays  21  in the peripheral direction opening in the opposite direction. 
   Between two stays  21 , there is a stay  23  which is secured on a ring  15  and extends in the direction of the adjacent ring  15  in the section  9 . As shown in  FIG. 7 , a distance a is formed between the stay  23  and a ring  15 . The stay  23  together with the corresponding ring  15  forms a stop  26  which defines the minimum length in the length-compensating section  9 . This prevents a stay  21  from breaking due to excessive deformation and thereby being destroyed. 
   As shown in  FIG. 7 , each stay  21  is secured with a leg  34  on a ring  15  and with a leg  35  on the opposite ring  15  bordering the section  9 . The legs  34  and  35  are designed to be symmetrical with an axis of symmetry  22  which runs perpendicular to the longitudinal central axis  10  of the central tube  8 . 
   On the end  16  facing the cover  5 , the central tube  8  has an annular flange  18  which is shown in an enlarged sectional view in  FIG. 8 . In the area of the end  16 , the annular flange  18  has a peripheral groove  24  on its outside circumference, extending over approximately half of the thickness e of the annular flange  18  as measured in the radial direction. The groove  24  ensures a secure fixation of the central tube  8  in the cover  5 . 
   As shown in  FIG. 9  on an enlarged scale, the ring  15  situated on the end  17  facing the base  6  is provided at the end  17  on its outside circumference with a recess  19  which ensures a secure footing of the central tube  8  on the base  6 . 
     FIG. 10  shows an illustrative embodiment of a central tube  28 . Like the central tube  5 , the central tube  28  is formed by rings  15 . The rings  15  are joined by stays  30  running in the direction of the longitudinal central axis  10 . The width b measured in the radial direction corresponds to the width d of the stays  30  as measured in the radial direction. The central tube  28  has an edge  32  designed in a ring shape on the end  16  facing the cover  5  as well as on the end  17  facing the base. The central tube  28  has a length-compensating section  29 , which is situated in the center of the central tube  28 . A spiral stay  31  is situated in the section  29 , its width measured in the radial direction corresponding to the width b of the rings  15 . The stays  31  extend in a spiral pattern around the longitudinal central axis  10  approximately once around the circumference of the central tube  28 . The spiral-shaped stays  31  permit a resilient compensation of the length of the central tube  28 . 
   Other configurations of the stays  21  and/or the stay  31  may also be advantageous. In particular, more or fewer stays  21  and more stays  31  than in the exemplary embodiments depicted here may be provided. 
   The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the described embodiments incorporating the sprit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations within the scope of the appended claims and equivalents thereof.