Abstract:
A reciprocating positive displacement compressor unit comprises a central body, a motor having a shaft which rotates about an axis of rotation, two cylinders, two pistons, each of which moves with alternating motion in a respective cylinder to compress the air contained in it, a first, axial fan for generating an air flow for cooling the compressor unit and a second, radial fan for circulating air inside the central body, the first, axial and second, radial fans both being driven by the shaft.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a reciprocating positive displacement air compressor unit.  
         [0002]     In particular, the present invention relates to a high pressure compressor unit which can be used to supply air to pneumatic tools.  
         [0003]     Pneumatic tools are used in a lot of technical work due to their practicality and strength. For example, such work may include the application of rivets to sheets metal, nails in wood, tightening and unscrewing the screws which secure the wheels on motor vehicles.  
         [0004]     Above all in the professional field, increasing levels of performance are required of such types of tools.  
         [0005]     Obviously, improved performance normally corresponds to an increase in the overall dimensions of the tools and the compressor units which power them.  
         [0006]     In recent years, to avoid such an unwanted increase in tool dimensions, the direct consequence of which is a loss of tool practicality, the production of high pressure compressors began. The forces required to perform the above-mentioned work being the same, these compressors, developing air pressure values close to 30 bar, allow a considerable reduction in the dimensions and weight of the tools which until now were normally used with air operating pressures varying between 10 and 15 bar.  
         [0007]     The use of high pressure compressed air, a term which in the present text refers to the above-mentioned pressure values close to 30 bar, has brought a series of important problems both to the use and construction of the compressor units.  
         [0008]     In particular, the high pressure compressors currently made have many disadvantages, most of which are linked to the difficulty of achieving efficient cooling of the various parts of the compressor. Reaching high air pressures for lengthy periods such as those normally required during professional use of the compressor results in considerable heating of the compressor components and of the air itself, with an evident reduction in the device&#39;s overall performance.  
         [0009]     Another disadvantage of the known high pressure compressors is their complexity and the dimensions and weight of their mechanical components. Such complexity means that production costs are high, constituting another disadvantage of known compressors.  
       SUMMARY OF THE INVENTION  
       [0010]     The aim of the present invention is to overcome the above-mentioned disadvantages by providing a high pressure compressor unit with efficient cooling, which is functional, simple to produce and practical to use.  
         [0011]     The technical features of the present invention, in accordance with the above-mentioned aims, are clearly indicated in the claims herein, in particular claim  1  and, preferably, by any of the claims which are directly or indirectly dependent on claim  1 . 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Moreover, the advantages of the invention are more clearly indicated in the detailed description which follows, with reference to the accompanying drawings which illustrate a preferred non-restricting embodiment of it and in which:  
         [0013]      FIG. 1  is a perspective top view with some parts cut away for the sake of clarity, of a preferred embodiment of the compressor unit made according to the present invention;  
         [0014]      FIG. 2  is a schematic plan view with some parts in cross-section, of the compressor unit illustrated in  FIG. 1 ;  
         [0015]      FIG. 3  is a schematic side elevation of a portable compressor fitted with a compressor unit illustrated in the previous figures;  
         [0016]      FIG. 4  is a schematic front view of the compressor unit illustrated in  FIG. 1 ;  
         [0017]      FIG. 5  is a schematic perspective top view, of a detail of the compressor unit made according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     As illustrated in  FIGS. 1 and 2 , the numeral  1  denotes a positive displacement compressor unit for compressing air at high pressure, designed for integration, for example, in a portable compressor of the type illustrated in  FIG. 3  and labeled  2 .  
         [0019]     The compressor unit  1  is of the two-stage reciprocating type.  
         [0020]     In this text the term high pressure refers to air pressures between approximately 23 and approximately 30 bar, that it to say, pressures significantly higher than those normally supplied by most compressors available on the market.  
         [0021]     As illustrated in  FIGS. 1 and 2 , the compressor unit  1  comprises a central body  3  which has a substantially cylindrical shape, from the sides of which there extend a first and a second cylinder, labeled  4  and  5  respectively.  
         [0022]     The central body  3  is connected, at its first end  3   a  to an electric motor  6  which has a shaft  7  rotating about an axis of rotation A. The shaft  7  is supported by two bearings of the known type and, therefore, not described in further detail.  
         [0023]     A circular removable lid  8  is fixed to the second end  3   b  of the central body  3 , opposite the first end  3   a.    
         [0024]     The first and second cylinders  4 ,  5  are opposite one another and have respective axes B and C which are parallel with one another.  
         [0025]     The axes B, C of the cylinders  4 ,  5  and the axis of rotation A of the shaft  7  substantially lie in the same plane, labeled P in  FIG. 4 .  
         [0026]     The first cylinder  4 , which is larger than the second cylinder  5  and has a bigger cylinder capacity, provides a first stage of air compression for the unit  1 . The second cylinder  5  provides a second stage of air compression.  
         [0027]     Inside each cylinder  4 ,  5  there are, respectively, a first and a second piston  9  and  10  which slide along the axes B, C.  
         [0028]     The rotary shaft  7  is connected to the first and the second pistons  9 ,  10  by a first and a second connecting rod, respectively  11  and  12 .  
         [0029]     At a respective small end,  11   a ,  12   a , each connecting rod  11 ,  12 , is rotatably connected to the respective piston  9 ,  10  by a pin  13 . Around each piston  9 ,  10 , in suitable hollows made in its cylindrical side surface, two shoes  14  are keyed, which guide the alternate motion of the piston  9 ,  10  in the cylinder  4 ,  5 . Therefore, the guide shoes  14  are inserted between the piston  9 ,  10  and the relative cylinder  4 ,  5 .  
         [0030]     The shoes  14  are advantageously made of polytetrafluoroethylene, a material with a low friction coefficient.  
         [0031]     As illustrated in  FIG. 2 , on each of the pistons  9 ,  10 , close to the crown, there is at least one compression ring  15 , of the known type and therefore, not described in further detail.  
         [0032]     At a first end  7   a  connected to the central body  3 , the shaft  7  has a counterweight  16  to balance the rotation.  
         [0033]     As illustrated in  FIG. 2 , at the first end  7   a , two cylindrical elements  17 ,  18  are attached to the shaft  7 , the elements mounted off-center on the shaft  7  so that, when roller bearings  19  of the known type are inserted between them, they form cranks for the connecting rods  11  and  12 .  
         [0034]     The cylindrical elements  17 ,  18  are positioned and attached to one another and on the rotary shaft  7  by two pegs  20  and more fastening parts of the known type and not illustrated.  
         [0035]     At a second end  7   b  of the shaft  7  longitudinally opposite the first end  7   a  and outside the motor  6 , a first, axial fan  21  is fitted on the shaft  7 , this fan forming a first rotary ventilation part  22  for the unit  1 .  
         [0036]     The first, axial fan  21  has a plurality of blades, of the known type and not illustrated, and an external ring  23  which connects the individual blades, the ring  23  constituting a flywheel mass for the rotary shaft  7  to which the fan  21  is connected.  
         [0037]     A second, radial fan  24  which forms a second rotary ventilation part  25  for the unit  1  is attached on the cylindrical element  17  relative to the first connecting rod  11  and not adjacent to the rotary shaft  7 , the fan being attached coaxially to the shaft  7 .  
         [0038]     As illustrated in  FIG. 1 , on a cylindrical side surface  26  of the central body  3 , at the second, radial fan  24 , there is a plurality of openings  27  for the exchange of air with the outside.  
         [0039]     In particular with reference to  FIGS. 1 and 2 , the compressor unit  1  comprises a conveyor  28  of a flow  29  of cooling air generated by the first, axial fan  21  which forms the first rotary ventilation part  22 .  
         [0040]     The conveyor  28  comprises a laminar structure, extends longitudinally along the axis A and has a first air inlet hole  30  and a second outlet hole  31 . The first hole  30 , is close to the first, axial fan  21  and the second outlet hole  31  gives onto the cylinders  4 ,  5 .  
         [0041]     As it extends, the conveyor  28  surrounds the electric motor  6  and the latter is struck at a tangent by the cooling air flow  29  conveyed by the conveyor  28 .  
         [0042]     As illustrated in  FIGS. 1 and 4 , the conveyor  28  has a substantially octagonal cross-section close to the first, axial fan  21  and, along its longitudinal length, from its two opposite side faces there extend side portions  32 ,  33 , each designed to direct the cooling air flow  29  onto a cylinder  4 ,  5 .  
         [0043]     The conveyor  28  is rigidly attached to the motor  6  and to the central body  3  by a plurality of fastening elements  34 , of which one is illustrated in  FIG. 1 .  
         [0044]     As illustrated in  FIG. 5 , the compressor unit  1  comprises an intermediate cooling part  35  for cooling the compressed air exiting the first cylinder  4 , before it enters the second cylinder  5  which forms the second stage.  
         [0045]     The intermediate cooling part  35  comprises a tubular  36  pipe for the passage of the compressed air. The pipe extends along a curved trajectory T and close to the central body  3  and the electric motor  6 , outside them.  
         [0046]     For a stretch of the curved trajectory T, the tubular pipe  36  comprises two tubular portions  37 ,  38  through which the compressed air exiting the first cylinder  4  runs in parallel.  
         [0047]     The two tubular portions  37 ,  38  into which the pipe  36  is divided advantageously allow an increase in the heat exchange surface area of the pipe  36 , improving the cooling of the compressed air.  
         [0048]     The intermediate cooling part  35  is at least partially inserted between the motor  6  and the conveyor  28 .  
         [0049]     The first and second ventilation parts  22 ,  25 , the conveyor  28  and the intermediate cooling part  35  together form cooling means  39  for the compressor unit  1 .  
         [0050]     In practice, during a normal compressor  2  operating cycle, for example, to supply compressed air to one or more pneumatic tools which are not illustrated, the compressor unit  1  compresses the air in its cylinders  4 ,  5 , driven by the electric motor  6 .  
         [0051]     By means of its rotary shaft  7 , the electric motor  6  not only drives the alternate motion of the pistons  9 ,  10 , but at the same time drives the rotation of the first rotary ventilation part  22  and the second rotary ventilation part  25 .  
         [0052]     The first rotary part  22 , consisting of the first, axial fan  21 , generates the above-mentioned cooling air flow  29 , the flow  29  being channeled into the conveyor  28 , and heading towards the central body  3  of the unit  1 . On this path, the air flow  29  strikes the electric motor  6  at a tangent, carrying heat away from it. Moreover, since the electric motor  6  has radial fins, the heat exchange between the motor  6  and the outside is further increased by the speed at which the air flow  29  travels thanks to the first, axial fan  21 .  
         [0053]     The cooling air flow  29  is also directed, by each of the side portions  32 ,  33 , onto a respective cylinder  4 ,  5 .  
         [0054]     As described above with reference to the motor  6 , the air flow  29  strikes each of the two cylinders  4 ,  5  carrying heat away from them.  
         [0055]     The cylinders  4 ,  5  also have cooling fins, of the known type, designed to increase the surface area for heat exchange with the outside and, therefore, the extent and efficiency of the heat exchange.  
         [0056]     Advantageously, the cylinders  4 ,  5 , being opposite one another, allow optimization of the cooling action exerted on them by the air flow  29 , achieving improved ventilation.  
         [0057]     Moreover, the cooling air flow  29  strikes the intermediate cooling part  35 , located and extending close to the electric motor  6 , inserted between the latter and the air flow  29  conveyor  28 .  
         [0058]     The air flow  29 , striking the tubular pipe  36  of the intermediate cooling part  35 , carries heat away from it and cools the partially compressed air exiting the first cylinder  4  before it enters the second cylinder  5  where the second stage of compression takes place.  
         [0059]     This intermediate cooling of the compressed air optimizes the compression cycle and its efficiency is increased by the presence of the two tubular portions  37 ,  38  into which the pipe  36  is divided, the portions  37 ,  38  advantageously increasing the pipe  36  heat exchange surface area.  
         [0060]     The second rotary ventilation part  25 , consisting of the second, radial fan  24 , circulates air in the central body  3 , with relative exchange with the outside through the openings  27  made in the cylindrical side surface  26  of the central body  3 .  
         [0061]     Advantageously, to optimize compression unit  1  compression, it has been proved that the optimum ratio between the respective cylinder capacities of the first cylinder  4  and the second cylinder  5 , that is to say, the cylinder capacity ratio between the first and the second stage, is between 5.37 and 5.40.  
         [0062]     The present invention is suitable for evident industrial applications, it can also be subject to modifications and variations without thereby departing from the scope of the inventive concept. Moreover, all the details of the invention may be substituted by technically equivalent elements.