Abstract:
A piston ( 11 ) of an automatic machine for infusion of beverages, such as coffee and the like, the piston having a piston head ( 13   a,    13   b ) divided into two parts connected together and axially movable relative to each other, an annular gasket ( 15 ) being housed between those parts and radially expanding when an axially directed thrust such as to make said two piston parts ( 13   a,    13   b ) approach each other is applied onto said piston ( 11 ), wherein the elastic resistance of said gasket ( 15 ) keeps said two piston parts ( 13   a,    13   b ) spaced apart when no axial thrust is applied onto the piston.

Description:
BACKGROUND 
     The present invention relates to the field of automatic infusion making machines, in particular for making espresso coffee, and more particularly it concerns a piston for the infusion assembly in one such machine. 
     It is known that, for making espresso coffee and other beverages, automatic machines are used, which internally contain a set of components allowing an operator, by simply pressing a push-button, to make the machine perform a group of operations, in a predetermined sequence, until the espresso coffee infusion is directly delivered into a cup. Essentially, such operations are: grinding the exact amount of coffee grains, accumulating the ground coffee powder in a suitable chamber, compacting and compressing the coffee powder until forming a “tablet” with predetermined compactness and size, sending a dosed water flow at a predetermined temperature through said tablet, collecting and sending the coffee infusion downstream the powder tablet towards a group of ducts and from said ducts towards the cup, and ejecting the exhausted powder tablet towards a suitable container. 
     The mechanical and thermal wear the components of such a machine undergo during the operating cycle is made more severe by the extreme aggressiveness, from both the chemical and the mechanical standpoint, of the coffee powder obtained by grinding the grains. 
     From the chemical standpoint, indeed it is known that, during the operating cycle, the oil substances contained in toasted coffee are spontaneously released, mainly because of the high temperatures that are required to produce and maintain at about 90° C. the water necessary for delivery. From the mechanical standpoint, as known, abrasiveness of the ground coffee powder is high as a consequence of the toasted grain hardness and of the very fine grinding granulometry. 
     Therefore, automatic machines for making espresso coffee are to be submitted to periodical maintenance operations, with repair and replacement of the worn parts. 
     Different solutions have been adopted in the art to contrast the above-mentioned aggressiveness, by choosing more resistant materials and/or by various surface treatments, capable of protecting the surfaces in contact with the coffee powder. 
     Greater difficulties have been on the contrary experienced in dealing with the problem of the coffee powder abrasiveness, especially in respect of those components which are to ensure a perfect hydraulic tightness in the different operations during coffee powder infusion and which are therefore to keep their mechanical integrity. 
     Among those components, of particular importance is the gasket mounted onto the pistons which are to compact the tablet of ground coffee, to compress it in the infusion chamber, to keep a constant tightness of the chamber while pressurised hot water is being pumped for the coffee infusion, and lastly to convey the exhausted tablet outside the infusion chamber for evacuation. 
     Referring to  FIG. 1 , there is shown an infusion assembly of a prior-art coffee making machine, as disclosed for instance in EP-A-1260166. Said infusion assembly, generally denoted by reference numeral  71 , comprises a horizontally moving chamber  73 , sliding on guides  75  and driven by a threaded shaft  77 . The chamber has an internal cavity  79 , intended to receive the dose of coffee powder or other powdered substance. A first piston  81   a , also horizontally moving, and a second stationary piston  81   b , facing the first piston, co-operate with said moving chamber  73 . Said moving chamber  73  has ducts  83  for introducing coffee powder or other powdered substance into cavity  79 . Also moving piston  81   a  is slidably mounted on guides, and it is driven by a shaft  85  co-operating with shaft  77  driving moving chamber  73 , in such a manner that moving chamber  73  and moving piston  81   a  are simultaneously translated in opposite directions. Moving piston  81   a  has a head  87   a , which can fit into infusion chamber  73 . Hydraulic tightness between said head and said chamber is ensured by an O-ring  89   a . Said head  87   a  is equipped at its free end with a disc-shaped filter  91   a  for the coffee and it has an axial duct  26  for introducing hot water, supplied through tubes not shown. Stationary piston  81   b  has a head  87   b  equipped with a disc-shaped filter  91   b  for the coffee and an O-ring  89   b  for hydraulic tightness with infusion chamber  73 . The piston is integral with a rod  93  having an axial duct intended to convey the prepared infusion outside infusion assembly  71 . 
     During the operation cycle of the infusion assembly, chamber  73  and moving piston  81   a  are translated relative to each other in order to pass from a phase of coffee powder loading to a compression and infusion phase and from the latter to a phase of used coffee ejection. In the loading phase, moving chamber  73  and moving piston  81   a  are in such a position that cavity  79  forms an infusion chamber closed by heads  87   a ,  87   b  of pistons  81   a ,  81   b , and the coffee powder dose can be introduced through ducts  83 . Once the desired coffee dose has been introduced, the coffee powder compaction and compression phase is performed, by actuating shafts  77 ,  85  so that said chamber  73  and said moving piston  81   a  are translated by such an extent that heads  87   a ,  87   b  are brought to the minimum possible distance, and by compressing said powder between said heads. Under such conditions, the machine is ready for the introduction of infusion water. Hot water is introduced into cavity  79  through filter  91   a  and the infusion then flows out through filter  91   b . At the end of the percolation time, shafts  77 ,  85  are again actuated, but in reverse directions with respect to the compression phase, thereby spacing apart heads  87   a ,  87   b  of the pistons. When shafts  77 ,  85  reach the end of that new displacement, infusion assembly  71  is in the condition of coffee ejection, in which cavity  79  is open on the side if moving piston  81   a  and the used coffee is ejected thanks to the thrust of stationary piston  81   b  due to the backward movement of moving chamber  73  towards said stationary piston  81   b.    
     Several solutions have been developed in the past to limit the O-ring wear, in particular in the piston moving into and out of the infusion chamber at each operating cycle. 
     According to prior art solutions, the piston head is formed in two parts and the O-ring is located between said parts, so that only when the two piston parts approach each other an O-ring expansion takes place such that O-ring sealingly adheres to the infusion chamber wall. Examples of such solutions can be found in CH570145, EP0608805 and FR2202668. 
     Another example of two-part piston according to the prior art is denoted by reference numeral  51  in  FIG. 2 . 
     In the illustrated example, piston  51  has a head divided into two parts  53   a ,  53   b  kept joined by retaining screws  55 . A spring  57  is located between the two piston parts  53   a ,  53   b  so as to keep the two piston parts spaced apart and to allow them to approach each other only when the compression force exerted by the piston on the coffee powder exceeds the elastic resistance of the spring. An O-ring  59  is located between the two piston parts  53   a ,  53   b  in an annular groove defined by an internal cylindrical abutment and by two facing conical abutments. When the piston is in idle condition, O-ring  59  lies upon cylindrical abutment  59  and has an external circumference with substantially the same diameter as piston  51 . As the two piston parts move towards each other, the two facing conical abutments cause O-ring  59  to expand towards the outside and press it against the wall of the infusion chamber. 
     The prior art solutions have a number of drawbacks. 
     First of all, constructing the piston head by using a plurality of telescopic members results in an increase of the number of components and requires using additional gaskets to ensure the hydraulic tightness between such components, in order to avoid seepage into the piston. 
     Moreover, the proposed arrangements do not prevent all or part of the individual components from undergoing contact with water during the infusion phase and consequently from undergoing a continuous deposition of oil substances released by the coffee powder. With continuing use, the constant accumulation of said substances entails an increased friction between the individual parts, which may lead to system blockage and consequent damages to the O-ring, until a complete breaking thereof. 
     SUMMARY 
     It is the main object of the present invention to overcome the above drawbacks, by making a piston for a beverage infusion machine that is of simple construction and that is made of a limited number of components. 
     It is another object of the present invention to make a piston in which the wear of both the mechanical piston parts and the sealing gasket associated therewith is reduced. 
     It is a further object of the present invention to make a piston for an infusion assembly in which the components are protected from infusion water seepage, so as to avoid the accumulation of oil substances released by the coffee powder. 
     The above and other objects are achieved by a piston for an infusion assembly of a coffee-making machine as described herein. 
     According to the invention, the above objects are achieved by using a single gasket that ensures tightness between the piston and the internal wall of the infusion chamber during the phases of compression and infusion of the coffee tablet and that, at the same time, opposes an elastic resistance to the relative movement of the two piston parts, keeping them spaced apart when the piston is not compressed against the coffee powder. 
     Thus, it is possible to avoid using springs between such elements, thereby making the piston structure simpler. 
     Advantageously, according to the invention, said gasket further ensures tightness between the two piston parts during the whole operating cycle of the infusion assembly, as well as during the periods in which said assembly is not operating. 
     Thanks to such a gasket, the use of additional seals can therefore be avoided, thereby further reducing the number of components. 
     Advantageously, said gasket undergoes a strong compression only during the phases of compression and infusion of the coffee tablet, whereas during the remaining phases of the operation cycle of the infusion assembly, as well as during the periods in which said assembly is not operating, the only stress said gasket undergoes is the light compression necessary to ensure the tightness between the piston members, so that the wear of said gasket is significantly reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the piston for an infusion assembly according to the invention will be now described by way of non-limiting example, with reference to the accompanying drawings, in which: 
         FIGS. 1 and 2  are longitudinal sectional views of an infusion assembly and a piston, respectively, according to the prior art; 
         FIGS. 3   a  and  3   b  are perspective sectional views of a piston according to the invention, in idle and compression conditions, respectively; 
         FIG. 4  is a perspective view of the piston shown in  FIG. 3   a , according to a variant embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 3   a  and  3   b , there is shown a piston  11  of an automatic machine for beverage infusion, for instance for making espresso coffee, which piston comprises a piston head divided into two parts  13   a ,  13   b , connected to each other and axially displaceable relative to each other. 
     An annular gasket  15  is located between the two piston parts and radially expands when an axial thrust such as to cause said two piston parts  13   a ,  13   b  to approach each other is applied to said piston  11 . 
     Advantageously, according to the invention, the elastic resistance of said gasket  15  keeps said two piston parts  13   a ,  13   b  spaced apart when no axial thrust is applied to said piston  11 , without the need to provide a spring or other resilient means between said two piston parts. 
     According to the invention, when no axial thrust is applied to said piston  11 , said gasket  15  lies within the diameter of said two piston parts  13   a ,  13   b  ( FIG. 3   a ). On the contrary, when an axial thrust is applied to said piston  11  in order to cause said two piston parts  13   a ,  13   b  to approach each other, said gasket projects outside said diameter ( FIG. 3   b ). 
     Thus, tightness of gasket  15  against internal wall  25  of infusion chamber  27  is ensured when the piston exerts a compression onto the coffee powder, and the piston can move into and out of chamber  27  and slide within the latter without any rubbing of gasket  15  against wall  25  of said infusion chamber. 
     Always according to the invention, gasket  15  ensures liquid tightness between said two piston parts  13   a ,  13   b  both when said piston is in idle condition, i. e. it is not submitted to an axial thrust, and when said piston exerts a compression onto the powder from which the infusion is to be extracted, i.e. when the piston is submitted to an axial thrust causing said two piston parts  13   a ,  13   b  to approach each other. 
     According to the invention, a circumferential seat  17  receiving said annular gasket  15  is formed between said two piston parts  13   a ,  13   b . An internal zone  17   a , intended to retain and axially compress gasket  15 , and an external zone  17   b , communicating with said internal zone  17   a  and intended to allow radial expansion of gasket  15  towards the outside of piston  11 , are defined in said seat  17 . 
     Preferably, seat  17  has a T-shaped section, in which the crossbar of the “T” and the stem of the “T” correspond with said internal and external zones  17   a ,  17   b , respectively. Correspondingly, said gasket  15  also has a T-shaped section, in which internal portion  15   a , corresponding with the crossbar of the “T”, is housed in said internal zone  17   a  of said seat  17 , and external portion  15   b , corresponding with the stem of the “T”, is housed in said external zone  17   b  of said seat  17  and can freely radially expand towards the outside of said seat  17  when said gasket  15  is axially compressed. 
     Preferably moreover the tolerances between the walls of internal zone  17   a  and gasket  15  are so determined that innermost portion  15   a  of said gasket is submitted to a stronger axial compression, whereas outermost portion  15   b  undergoes a lower stress, so as to facilitate its outward radial expansion, i.e. towards wall  25  of infusion chamber  27 . However, seat  17  could be even made so that said gasket  15  is axially compressed in uniform manner, without thereby affecting the good operation of the piston according to the invention. 
     In the illustrated embodiment, said two piston parts  13   a ,  13   b  comprise a first, outer piston part  13   a  intended to compress the powder for the infusion of said beverage, and a second, inner piston part  13   b . Said second piston part  13   b  in turn comprises an axial cavity  19  receiving a portion  21  of said first piston part  13   a . Said piston portion  21  is slidable in said axial cavity  19  when an axial thrust is exerted onto piston  11 , i.e. during the compression phase of the beverage powder, e.g. coffee powder, and when said thrust is released, i.e. in the phase in which the piston is moving away from the infusion chamber. 
     According to the invention, said two piston parts  13   a ,  13   b  are kept spaced apart thanks to said gasket  15  only and said two piston parts  13   a ,  13   b  are always kept axially spaced apart during the whole operating cycle of the piston. 
     Referring to a coffee-making machine, the operation of the piston according to the invention is as follows. 
     When coffee delivery is requested, after an initial loading phase of the coffee powder into infusion chamber  27 , thanks to the relative displacement of infusion chamber  27  and piston  11 , piston  11  begins penetrating into chamber  27 , and collects the coffee powder and compacts it until a point where the compression phase of the tablet formed by said powder starts. By continuing the thrust, piston parts  13   a ,  13   b  tend to approach each other, by overcoming the elastic resistance of gasket  15 . As a result of that movement, gasket  15  is made to expand in the only possible direction, i.e. in radial direction towards the outside of seat  17 , towards wall  25  of infusion chamber  27 , until arriving in contact with said wall. In such manner, said gasket  15  ensures hydraulic tightness between piston  11  and wall  25  of said infusion chamber  27  as long as the gasket undergoes compression between said two piston parts  13   a ,  13   b.    
     Once the infusion phase is over, the relative displacement of piston  11  and infusion chamber  27  brings piston  11  back to the idle condition, and the elastic force due to axial expansion of gasket  15  moves again the two piston parts  13   a ,  13   b  away from each other. Gasket  15  returns into seat  17  and takes again a diameter lower than that of the internal cavity of chamber  27  and substantially identical to that of piston  11 . 
     Preferably, according to the invention, when the maximum axial thrust is applied to the piston, i.e. at the end of the compression phase on the powder from which the beverage infusion is to be obtained, base  21   a  of part  13   a  of piston  11  does not abut against bottom  19   a  of cavity  19  in part  13   b  of piston  11 , but it remains always spaced apart therefrom. Such a feature allows recovering the wear of gasket  15  that unavoidably occurs in time. Yet, it is also possible to make base  21   a  contact bottom  19   a  of cavity  19  during compression of gasket  15 , without thereby affecting the good operation of the piston according to the invention. 
     Advantageously, according to the invention, gasket  15 , besides ensuring tightness between piston  11  and wall  25  of infusion chamber  27  while the piston is compressing the beverage powder, also ensures tightness between parts  13   a ,  13   b  forming the head of piston  11 . Such second sealing function is exerted not only during the compression and infusion phases, but also during the whole operating cycle of the infusion assembly, as well as during the periods in which said assembly is not operating. In this manner, use of additional internal gaskets can be dispensed with, and the piston structure is simplified. 
     Advantageously moreover, the only stress gasket  15  undergoes while the piston is in idle condition is the light compression necessary to ensure the tightness between piston parts  13   a ,  13   b . Consequently, the wear of said gasket is significantly reduced. 
     In the illustrated embodiment, said permanent light compression is obtained through a connecting means  14  (e.g. a screw) passing through central axial bore  12   a ,  12   b  formed in both piston parts  13   a ,  13   b . Said screw  14  allows keeping the two piston parts connected together while maintaining a light compression onto gasket  15 . 
     It is clear from the above description that the seat for gasket  15  and gasket  15  itself may have a cross sectional shape different from the disclosed one, provided that said gasket  15  resiliently contrasts the approaching of said piston parts  13   a ,  13   b  and provided that the same gasket, when submitted to a compression sufficient to cause its deformation, can expand outside piston  11 , towards the wall of infusion chamber  27 . 
     In a variant embodiment of the invention, shown in  FIG. 4 , piston  111  has a seat  117  for gasket  115  between parts  113   a ,  113   b . That seat has an internal zone  117   a  with a C-shaped profile and an external zone  117   b  with a rectangular trapezoidal profile joined along its minor side with the C-shaped section of said internal zone  117   a . Two opposed circumferential projections  118  are moreover provided between said internal zone  117   a  and said external zone  117   b , which projections axially extend between said two zones of seat  117  to keep said gasket  115  in said seat. 
     Correspondingly, said gasket  115  has an internal portion  115   a  housed in said internal zone  117   a  of said seat  117 , and an external portion  115   b  housed in said external zone  117   b  of said seat  117 . The latter portion  115   b  can freely radially expand outside said seat  117  when said gasket  115  is axially compressed. 
     Advantageously moreover, said external portion  115   b  of said gasket  115  has a convex surface to improve tightness against internal wall  25  of infusion chamber  27 . 
     The piston shown in  FIG. 4  is an infusion piston, i. e. it is equipped with an axial duct  120   a ,  120   b  allowing the passage of liquid, usually water, intended for beverage infusion. Similarly, the illustrated piston could even be employed as a contrasting piston in which axial duct  120   a ,  120   b  is intended to allow the passage of the beverage, e.g. espresso coffee, obtained as a result of the infusion. 
     The piston according to the invention could even be a blind piston, i.e. without the duct for the passage of liquid, if such a duct is provided e.g. in the infusion chamber itself. 
     The piston according to the invention can be advantageously employed in coffee or other beverage making machines, equipped with either a horizontally or vertically developing infusion chamber. It can be used both in machines equipped with a stationary piston and a moving infusion chamber displacing relative to the piston, and in machines equipped with a stationary infusion chamber inside which a moving piston slides. 
     It is clear that the above description is given only by way of non limiting example and that changes and modifications are possible without departing from the scope of the invention, and that all changes and modifications suggested by the experience of the skilled in the art lie within the scope of the invention.