Patent Publication Number: US-2022235750-A1

Title: Piston type pump drive arrangement

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2019/063110, filed on May 21, 2019. The International Application was published in English on Nov. 26, 2020 as WO/2020/233796 A1 under PCT Article 21(2). 
    
    
     FIELD 
     The present disclosure relates to a piston type pump, comprising a pump housing having at least one pump inlet and one pump outlet and a piston arrangement connected to a drive shaft which, when driven, sets the piston arrangement into movement, wherein the drive shaft comprises a first eccentric, and the piston arrangement comprises a first primary stage piston connected to a first sliding block guide slidably seated on the first eccentric, said first sliding block guide having a main axis, a minor axis and an inner surface. Such pumps may be used to induce a vacuum at the pump inlet and/or to provide pressurized fluid at the pump outlet. 
     BACKGROUND 
     Vacuum pumps are for example known from WO 2017/137144 A1 or WO 2017/137141 A1. Such vacuum pumps are generally referred as piston type vacuum pumps in distinction from so-called rotary vane vacuum pumps. Piston type pumps include at least one piston which reciprocatingly moves inside a cylinder. The pump inlet usually is connected with the working chamber formed by the cylinder such that when the piston moves inside the cylinder for increasing the working volume of the working chamber the vacuum is induced at the inlet. A first piston rod of a first piston is driven by a drive shaft of the pump and performs a combined linear and pendulum motion. A second piston rod of the pumps is rotatable connected to the first piston rod via a connecting bolt and thus also performs a combination of a linear and a pendulum motion. However, the drive assembly of such pumps requires close manufacturing tolerances and comprises multiple parts, resulting in increased time for assembly and increased manufacturing cost. Furthermore, the non-rectilinear movement of the pistons might lead to increased wear on the piston and/or cylinder of the pump. Pumps of this type are used in passenger vehicles or trucks as in particular vacuum pumps to supply specific modules of the vehicle with a vacuum. Generally manufacturing cost and good wear characteristics are of utmost importance for pumps used in vehicles. 
     A drive assembly of a compressed fluid motor, which provides a linear piston motion is presented in US 2017/0350249 A1. Pistons of said motor drive an output shaft via a crank pin. A rolling bearing positioned on the crank pin is slidably supported in between guide plates connected to respective piston rods of the pistons. During operation of said motor, the crank pin is rotated through a linear movement of the guide plates, wherein the outer ring of said rolling bearing slides relative to the guide plates and thereby eliminating a rocking motion of the piston rods. However, the presented drive assembly still comprises multiple parts requiring close manufacturing tolerances and therefore increasing a required time for assembly as well as manufacturing cost. Furthermore, the pistons are not fixed rotationally to the crank pin which can lead to irregular wear on piston seals. Moreover, drive assemblies are known, wherein a crank pin directly interacts with a guide slot without a bearing. While such assemblies may have fewer components highly accurate machining of the parts is necessary, in order to provide acceptable vibrational and noise levels. 
     SUMMARY 
     In an embodiment, the present disclosure provides a piston type pump. The piston type pump includes a pump housing having a pump inlet, a pump outlet, and a piston arrangement connected to a drive shaft. The drive shaft is configured to drive the piston arrangement, and the drive shaft includes a first eccentric. The piston arrangement has a first primary stage piston connected to a first sliding block guide slidably seated on the first eccentric. The first sliding block guide has a main axis, a minor axis, and an inner surface. A first sliding bush is arranged between the first eccentric and the first sliding block guide. The outer surface of the first sliding bush corresponds to the inner surface of the first sliding block guide. Translational movement of the first sliding bush relative to the first sliding block guide is allowed along the main axis and restricted along the minor axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following: 
         FIG. 1  shows a perspective simplified cut view of a piston type pump comprising an eccentric and sliding block guide drive arrangement; 
         FIG. 2  shows a simplified perspective view of a piston arrangement of a piston type pump comprising an eccentric and sliding block guide drive arrangement; 
         FIG. 3  shows a perspective view of a second primary stage piston and a first secondary stage piston attached together; 
         FIG. 4  shows the arrangement of  FIG. 3  in a cut view; 
         FIG. 5  shows a cut view of a first primary stage piston and a second secondary stage piston attached together; 
         FIG. 6  shows a more detailed cut view of  FIG. 1  in the area of the first primary stage piston; 
         FIG. 7  shows a detailed cut view of a drive arrangement; 
         FIG. 8  shows a detail of a slot in the first sliding block guide; 
         FIG. 9  shows a cut view of the first embodiment perpendicular to the cut view of  FIG. 7 ; 
         FIG. 10  shows a detailed cut view of a drive arrangement; 
         FIG. 11  shows a cut view of the second embodiment at a different angle than  FIG. 10 ; 
         FIG. 12  shows a detailed cut view of a drive arrangement according to a third embodiment; and 
         FIG. 13  shows a schematic view of a vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a piston type pump which allows for reduced manufacturing cost, improved wear characteristics, compact design and/or low noise emissions. 
     The present disclosure provides for a first sliding bush arranged between a first eccentric and a first sliding block guide, such that an outer surface of said first sliding bush corresponds to said inner surface of the first sliding block guide and that translational movement of said first sliding bush relative to said first sliding block guide is allowed along a main axis and restricted along a minor axis. The inner surface of said first sliding block guide forms a slot, wherein the first eccentric and the first sliding bush are located. The first sliding bush comprises an inner bore which is positioned on a corresponding outer surface of the first eccentric. Preferably, the first eccentric is rotatable in the first sliding bush about a rotational axis, which is parallel to a rotational axis of the drive shaft. A first direction of the first eccentric forms its central axis which is parallel to the rotational axis of the drive shaft und performs an orbital motion around said rotational axis when the drive shaft is driven. The first sliding bush is slidably seated in the first sliding block guide such that movement of the sliding bush relative to the first sliding block guide is allowed along the main axis. The main axis of the sliding block guide is perpendicular to said rotational axis of the drive shaft. Preferably, the main axis is perpendicular to a cylinder axis of a first primary stage cylinder, which corresponds to the first primary stage piston. The minor axis of the first sliding block guide is perpendicular to the said rotational axis as well as the main axis. It is preferred that the minor axis is parallel or coaxial to the cylinder axis of the first primary stage cylinder. Movement of the first eccentric along the main axis results in a sliding motion of the eccentric and the first sliding bush along the inner surface of the sliding block guide. When the first eccentric and the first sliding bush move parallel to the minor axis a relative motion between the sliding guide block and the sliding bush is restricted and a motion of the first eccentric is transferred to the first sliding block guide via the first sliding bush. Thus, an orbital motion of the eccentric around the rotational axis of the drive shaft is converted to a rectilinear motion of the first sliding block guide parallel to the minor axis. The sliding bush allows for minimum friction between the first sliding block guide and the first sliding bush and/or between the first sliding bush and the first eccentric. A first material forming the first sliding bush is preferably one of carbon filled polymers, PTFE, ABS, PEEK, Nylon, PPS, Xylonite, fiber reinforced plastics, bronze, ceramics and/or white metal. 
     Furthermore, the first material forming the first sliding bush can have a hardness that is lower than a hardness of a material forming the eccentric and/or the first sliding block guide. The first sliding bush can compensate for existing manufacturing deviations of the first sliding block guide and/or the first eccentric and thus allows for lower manufacturing costs of said components. Preferably the first sliding block guide and the first primary piston are integrally formed. For example, the first primary piston, the first sliding block guide, and a first piston rod connecting the first primary piston and the first sliding block guide can be casted in one piece. Casting production allows for low manufacturing costs at high quantities but in general reachable manufacturing tolerances are low. The rough tolerances can be compensated by the sliding bush and thus lower manufacturing costs can be achieved. It should be understood that the disclosure is not limited to casted first sliding block guides. Rough tolerances can also result from other manufacturing methods such as milling, turning, casting and/or additive manufacturing. In order to reduce manufacturing cost and time or to improve process stability rough tolerances are often accepted. 
     Preferably, said inner surface of the first sliding block guide comprises a first inner wall parallel to said main axis and a second inner wall parallel to said main axis and spaced from said first inner wall by a slot width. A first dimension of the sliding bush perpendicular to the first direction, thus a dimension along the minor axis of the sliding block guide, is smaller or equal to said slot width such that the sliding bush can be placed in between the first and second inner wall. For example, the outer surface of the sliding bush can have a rectangular cross-section, wherein a first side of the sliding bush is parallel to said first inner wall of the sliding block guide and a second side of the sliding bush is parallel to said second inner wall of the sliding block guide. Preferably, the first inner wall and the second inner wall of said first sliding block guide are symmetrical to a plane perpendicular to the minor axis. 
     In a preferred embodiment of the piston type pump said outer surface of the first sliding bush is tapered along a first direction parallel to a rotational axis of said drive shaft towards a first end of said drive shaft and said inner surface of the first sliding block is correspondingly tapered along the first direction towards said first end. Thus, the first sliding bush becomes gradually smaller along the first direction starting from an end of the drive shaft opposite to the first end towards said first end. Preferably a first dimension of the first sliding bush, which is measured parallel to the minor axis, gradually becomes smaller, while a second dimension of the first sliding bush, which is perpendicular to the first dimension and the first direction, may stay constant. Accordingly, a free inner surface or slot width of the first sliding block guide gradually becomes smaller starting form said opposite end of the drive shaft towards the first end of the drive shaft. Thus, a width of the slot, measured parallel to the minor axis gradually decreases along the first direction. A minimum width of the slot is larger than a diameter of the first eccentric. The inner surface of the first sliding block guide and the outer surface of the first sliding bush are tapered correspondingly such that the inner surface of the first sliding block guide forms a negative of the corresponding outer surface of said first sliding bush. Preferably there is surface contact between said inner surface of the first sliding block guide and said outer surface of the first sliding bush and gaps are reduced or eliminated. Gaps often result in unwanted noise levels, increased wear and/or stability problems of the pump and therefore should be reduced or eliminated. The inner surface and/or outer surface can taper at a constant rate and/or can be variable. For example, the first sliding bush can be shaped as a truncated cone or bell wherein the inner surface of said first sliding block guide resembles a surface line of said cone or bell. In a particularly preferred embodiment the first sliding bush comprises four perpendicular sides forming the outer surface, wherein two opposing sides of said four sides are tapered and the two remaining sides are not tapered. Due to manufacturing tolerances an inner width of said inner surface of the first sliding block guide is variable and gaps between said first sliding bush and said first sliding block guide may exist. By tapering the inner and outer surface, areal contact between the first sliding bush and the first sliding block guide can be achieved by relative movement of the first sliding bush and the first sliding block guide parallel to the rotational axis of said drive shaft. Increased wear resulting from line contact as well as gaps can be avoided. A first axial length of said inner surface of the first sliding block guide, measured along the first direction, can be smaller than or equal to a corresponding second axial length of said outer surface of the first sliding bush. Preferably said first axial length is 50% to 100%, particularly preferred 70% to less than 100%, of said second axial length. Thus, the first sliding bush protrudes from the first inner surface at one or two sides. 
     In another preferred embodiment the first sliding bush is biased towards said first end of said drive shaft by a first biasing member, such that said outer surface of the first sliding bush contacts said inner surface of the first sliding block guide. A first tapered end of the first sliding bush and a first tapered end of the first sliding block guide are oriented towards said first end of the drive shaft. Thus, the first sliding bush is continuously pushed into the inner surface or slot of the first sliding block guide and areal surface contact can be ensured. During operation of the pump the first sliding bush slides relative to the first sliding block guide and wear occurs on contacting surfaces of the first sliding bush and the first sliding block guide. The slot width of the first sliding block guide increases while at least a first dimension of the first sliding bush is decreased. Hence, gaps between the first sliding bush and the first sliding block guide are created. By biasing the tapered first sliding bush towards the tapered inner surface of said first sliding block guide, wear can be compensated and contact between the components can be ensured. The first sliding bush is pushed or pulled into the first sliding block guide along the first direction. Since wear is compensated, a movement of the first primary stage piston can be controlled effectively. The formation of gaps between the first sliding bush and the first sliding block be reduced or avoided. Such gaps generally lead to undesirable noise generation during pump operation. Furthermore, a travel of the piston during operation is constant, resulting in stabile working properties of the pump. Preferably the first tapered end of the first sliding bush is located adjacent to the first tapered end of the first sliding block guide. Preferably, the biasing member is spaced from said inner surface of the first sliding block guide in the first direction. As has been described above, a second axial length of the first sliding bush in said first direction can be larger than a first axial length of said first sliding block guide. Thus, the first sliding bush protrudes from the first inner surface on at least a first end. Preferably the biasing member is located on said first end. 
     According to a further preferred embodiment said first biasing member applies a first biasing force in a range of larger 0 N to 40 N, preferably 5 N to 30 N, particularly preferred 12 N to 20 N, on said first sliding bush in said first direction. 
     Moreover, it is preferred that a biasing force, applied by said first biasing member, is substantially symmetrical to said main axis. The amount of friction and wear between said outer surface of the first sliding bush and said inner surface of the first sliding block guide is at least dependent on a contact area, material properties on the inner surface and the outer surface as well as the biasing force applied to the sliding bush. Due to the tapered shape, the biasing force leads to a reaction force perpendicular to the inner surface and/or outer surface. If the biasing force is chosen excessively wear on the first sliding bush and/or the first sliding block guide is increased. By choosing the biasing force in the preferred range, friction and/or wear between said inner surface and said outer surface can be minimized while allowing for constant wear compensation. By applying the biasing force symmetrically, symmetrical wear on the inner surface and/or outer surface can be ensured. Uneven wear might lead to local overloads and result in increased wear, functional loss and/or stability problems of the pump. Preferably, the biasing member pushes said first sliding bush into said first sliding block guide. However, it should be understood that the first biasing force can be a pulling force such that the first sliding bush is pulled into the first sliding block guide. 
     In a further preferred development of the piston type pump said first biasing member is a spring clip attached to said first sliding block. Preferably, the spring clip comprises a first recess such that the first eccentric and/or the drive shaft can protrude through said recess. The spring clip comprises a biasing section contacting said first sliding bush, wherein the first sliding bush slides along the spring clip parallel to the main axis. It is preferred that the spring clip applies a uniform biasing force to the first sliding bush independently of a relative position of the first sliding bush in the first sliding block guide. Preferably the spring clip covers an opening of said first sliding block guide perpendicular to said inner surface. By attaching said spring clip to the first sliding block guide, a relative movement of the spring clip relative to the first slid block guide is inhibited. Since the spring clip is attached to the first sliding block guide it biases the first sliding bush towards said first end without applying torque on the first sliding block guide. Therefore, a rotation of the first primary stage piston in the first primary stage cylinder is avoided. It is preferred that the spring clip comprises a first hook section engaging a corresponding attachment section of said first sliding block guide. Preferably, the spring clip comprises a second hook section engaging a second corresponding attachment section of said first sliding guide block, wherein the first and second hook sections are formed on opposing ends of said spring clip such that the biasing section is located between the first and second hook section. The spring clip is fixed to the first sliding block guide via the first hook section and/or the second hook section. It is further preferred that the spring clip can be attached to the first sliding block guide in a one-hand operation and/or tool-less operation. By using a spring clip arrangement, a part count of the piston type pump can be reduced. It is also preferred that the spring clip prevents the first sliding bush from slipping out of a second end of said first sliding block guide along said first direction. It should be understood that other methods for fixing the spring clip to the first sliding block guide are also preferred. For example, the spring clip can be attached to the first sliding block guide by screwing, bolting, welding and/or bonding. In a particularly preferred embodiment the spring clip is integrally formed with the sliding block guide. 
     According to a preferred embodiment said first biasing member is a wave spring, a flat coil, a Belleville washer and/or a wave washer connected to said first eccentric by a first retaining screw. The retaining screw is screwed into a corresponding internal thread of said first eccentric. Furthermore, one or multiple washers can be located between the wave spring, flat coil, Belleville washer and/or wave washer and the first sliding bush and/or between the wave spring, flat coil, Belleville washer and/or wave washer and a screw head of said retaining screw. It is further preferred that the wave spring, flat coil, Belleville washer and/or wave washer is connected to said first eccentric by a first threaded nut screwed to a corresponding external thread of the first eccentric. Again, one or more washers can be located between the biasing member and said threaded nut and/or between the biasing member and the first sliding bush. In this embodiment, the biasing member is not fixed to the first sliding block guide. 
     Moreover, it is preferred that said first sliding bush comprises an end stop larger than a maximum slot width of said inner surface. Said slot width is measured parallel to the minor axis of the first sliding block guide. Preferably, said slot width is constant in a contacting are of said inner surface, wherein the first sliding bush contacts the first sliding block guide in said contacting area. The end stop can be formed as a protrusion on the first sliding bush extending perpendicular to the first direction. The end stop prevents the first sliding bush from being pushed or pulled entirely through the first sliding block guide by the first biasing member. It is preferred that the first biasing member contacts said end stop. The end stop provides an increased surface area for contacting the biasing member. Surface pressure induced on the first sliding bush by the biasing force decreases and wear on the first sliding bush is reduced. 
     In a further preferred embodiment said end stop is spaced from a corresponding stop face of said first sliding block guide. Preferably the corresponding stop face is perpendicular to said inner surface of the first sliding block guide. For example, the corresponding stop face and the end stop can be opposing flat surfaces. By spacing the corresponding stop face and the end stop, minimum wear during operation of the piston type pump can be ensured. When wear occurs on the inner surface of the first sliding block guide and the outer surface of the first sliding bush, the first sliding bush is pushed or pulled into the slot. A distance between the end stop and the corresponding stop face is reduced over time until the end stop and the corresponding stop face contact each other. Thus, emergency running characteristics can be ensured. It is preferred that the end stop is spaced from the corresponding stop face of said first sliding block guide by a range of larger 0% to 20% of a length of said inner surface, measured in the first direction. If the outer surface of the first sliding bush and/or the inner surface of the first sliding block guide is worn during operation of the pump, the first sliding bush is pushed or pulled into the first sliding block guide by the biasing member. The end stop is spaced apart from the corresponding stop face such that the first sliding bush can move relative to the first sliding block guide along the first direction. In a worn out state, the end stop contacts the corresponding stop face of said first sliding block guide and prevents the sliding bush from being pushed or pulled entirely through the first sliding block guide. A range of larger 0% to 20% of a length of said inner surface allows for wear compensation possibility and reduced friction while ensuring stable operation and compact design. 
     Moreover, it is preferred that the drive shaft further comprises a second eccentric, the piston arrangement further comprises a second sliding block guide slidably seated on the second eccentric and wherein a second sliding bush is arranged between the second eccentric and the second sliding block guide. It should be understood that the second eccentric, the second sliding block guide and/or the second sliding bush can have similar properties as described above in respect to the first eccentric, the first sliding block guide and the first sliding block guide. Reference is made to above described features. Preferably the first eccentric and the second eccentric are phase-shifted by 180°. 
     According to a particularly preferred embodiment, an outer surface of the second sliding bush facing the second sliding block guide is tapered along a second direction towards a second end of said drive shaft, an inner surface of the second sliding block guide corresponding to the outer surface of the second sliding bush is tapered along the second direction towards a second end of said drive shaft. The second end of said drive shaft and the first end of said drive shaft can be located on opposing sides or on the same side of said drive shaft. Preferably, the second sliding bush and the first sliding bush are identical or symmetrical and the first sliding block guide and the second sliding block guide are identical and or symmetrical. Thus, manufacturing cost can be reduced. Preferably, the second sliding bush is biased towards said second end of said drive shaft by a second biasing member, such that said outer surface of the second sliding bush contacts said inner surface of said second sliding block guide. The second biasing member preferably has features as described above for the first biasing member. However, the first biasing member can be formed as a spring clip, while the second biasing member is a wave spring, flat coil, Belleville washer and/or wave washer or vice versa. Forming them identical is particularly preferred to reduce costs. 
     In a further preferred embodiment a tapered end of said first sliding bush faces a tapered end of said second sliding bush. A tapered end of a sliding bush is oriented in the first direction and reduced in thickness when compared to an opposite end. Preferably the tapered end of said first sliding bush and the tapered end of said second sliding bush are oriented towards a middle section of said drive shaft positioned between the first eccentric and the second eccentric. The first end of the drive shaft and the second end of the drive shaft are opposite ends of the drive shaft. If a first biasing member contacts the first sliding bush on a second end opposite to the tapered end, an assembly process can be improved, since the first biasing member can be assembled after the first sliding bush and the first sliding block guide. In an analogue manner if a second biasing member contacts the second sliding bush on a second end opposite to the tapered end, an assembly process can be improved, since the second biasing member can be assembled after the first sliding bush and the first sliding block guide. A first biasing force applied by a first biasing member and a second biasing force applied by a second biasing member are preferably oriented in opposite directions. Moreover, it is preferred that said first biasing force and said second biasing force are equal. 
     Preferably the first sliding bush is rotationally fixed to the first sliding block guide. Furthermore, also a second sliding bush can be rotationally fixed to a second sliding block guide. The sliding motion between the first sliding block guide and the first sliding bush and/or between the second sliding block guide and the second sliding bush can be limited to a linear sliding motion. For example, an outer cross-section of the first sliding bush can be substantially rectangular, such that a longer side of the outer cross-section of the first sliding bush is larger than a smaller side of the inner cross-section of the first sliding block guide and/or wherein an inner cross section of the second sliding block guide can be substantially rectangular and a corresponding outer cross-section of the second sliding bush is substantially rectangular such that a longer side of the outer cross-section of the second sliding bush is larger than a smaller side of the inner cross-section of the second sliding block guide. 
     According to further preferred embodiment the piston arrangement further comprises a first primary stage cylinder formed in the pump housing, in which said first primary stage piston is slidably seated, and a first secondary stage piston being slidably seated in a first secondary stage cylinder formed in the first primary stage piston. Preferably, the first primary stage piston and the first secondary stage cylinder are integrally formed. For example, the first secondary stage cylinder is machined in the first primary stage piston. They are, thus, preferably formed in a one-piece construction. Due to this arrangement, the overall size of the pump can be reduced. The second stage is formed inside the first stage and not adjacent to it or at any other position. While the first primary stage piston moves relative to the pump housing inside a first primary stage cylinder formed inside the piston housing, the first secondary stage piston moves inside the first primary stage piston. 
     According to a further preferred embodiment the piston type pump is formed as a so-called twin piston type pump and therefore comprises a second primary stage piston and a second secondary stage piston, wherein the second primary stage piston is slidably seated in a second primary stage cylinder formed in the pump housing, and the second secondary stage piston is slidably seated in a second secondary stage cylinder formed in the second primary stage piston. Depending on how the different cylinders communicate with each other the second primary stage piston may also form a first tertiary stage piston and the second secondary stage piston may form a first quaternary stage piston. In this manner, the piston type pump, which, according to this embodiment, in total includes four pistons, can form a four-stage piston pump. However, particularly preferred is a two stage twin pump which includes two stages, with four pistons and thus a first and second first stage and a first and second secondary stage. As it has been described with respect to the first primary stage piston and the first secondary stage cylinder, also the second primary stage piston and the second secondary stage cylinder preferably are integrally formed, in particular preferred as a one-piece. It is further preferred that the minor axis of the first sliding block guide and the minor axis of the second sliding block guide are parallel to each other. Moreover, it is preferred that a main axis of the first primary stage cylinder and the second primary stage cylinder are coaxial such that the piston type pump is a boxer type pump. Preferably the first secondary stage piston is attached to the second primary stage piston and the second secondary stage piston is attached to the first primary stage piston. Further it is particularly preferred that the first secondary stage piston and the second primary stage piston are integrally formed and the second secondary stage piston and the first primary stage piston are integrally formed. 
     According to a second aspect, a piston type pump is provided, the piston type pump comprising a pump housing having at least one pump inlet and one pump outlet and a piston arrangement connected to a drive shaft which, when driven, sets the piston arrangement into movement, wherein the drive shaft comprises a first eccentric, and the piston arrangement comprises a first primary stage piston connected to a first sliding block guide slidably seated on the first eccentric, said first sliding block guide having a main axis and a minor axis characterized in that a first rolling bearing is being arranged between the first eccentric and the first sliding block guide, such that translational movement of said first rolling bearing relative to said first sliding block guide is allowed along the main axis and restricted along the minor axis, wherein the piston arrangement further comprises a first primary stage cylinder formed in the pump housing, in which said first primary stage piston is slidably seated, and a first secondary stage piston being slidably seated in a first secondary stage cylinder formed in the first primary stage piston. 
     According to a third aspect, a vehicle is provided, in particular a passenger car, comprising a piston type pump according to any of the aforementioned preferred embodiments of a piston type pump according to the first aspect or according to the second aspect. 
     It shall be understood that the pump may also be used in applications other than vehicles, and in particular other than braking systems. Other uses of pumps for generating a vacuum on a vehicle can include engine mounts, compressor waste-gate and bypass valves actuation. This type of pump could also feasibly be used to evacuate a housing for a KERS (Kinetic Energy Recovery System) for example. Furthermore, the pump can be used as a diagnostic pump for an automotive Evaporative Emissions Circuit (EVAP). 
     For a more complete understanding of the present disclosure, the embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. The detailed description will illustrate and describe what is considered as a preferred embodiment. It should of course be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the present disclosure. It is therefore intended that the present disclosure may not be limited to the exact form and detail shown and described herein. The wording “comprising” does not exclude other elements or steps. The word “a” or “an” does not exclude the plurality. The wording “a number of” items comprising also the number 1, i.e. a single item, and further numbers like 2, 3, 4 and so forth. In the accompanying drawings: 
     A piston type pump  1  according to the present disclosure is suitable to be mounted within a vehicle  100  (see  FIG. 12 ) and used as a vacuum pump to provide vacuum for a braking system or any other consumer in this vehicle. The piston type pump  1  in particular is suitable to be driven by an electric motor which for simplicity is not shown in the drawings. 
       FIG. 1  shows the piston type pump  1  prepared to be used as a vacuum pump and to induce a vacuum at a pump inlet  4 . However, the same construction may also be used as a compressor. The embodiment shown in  FIG. 1 to 6  is used to describe possible features of a piston type pump. Possible drive arrangements for a piston type pump are described with reference to  FIG. 7 to 11 . It should be understood that all combinations of the drive arrangement according to the embodiments shown in  FIG. 7  to  FIG. 11  with the features of a piston type pump presented with reference to  FIG. 1 to 6  are preferred. Furthermore, a piston type pump having different drive arrangements as described according to the embodiments shown in  7  to  FIG. 11  for different pistons is preferred. 
     The piston type pump  1  comprises a pump housing  2  which in the embodiment shown in  FIG. 1  substantially is cylindrical. The pump housing  2  has a pump inlet  4  which can be connected to a consumer. Moreover, the pump housing comprises a pump outlet  6  which opens into the environment. The pump outlet  6  is formed as a simple opening in the pump housing  2 . Fluid, in particular air, which is drawn away from the pump inlet  4 , is not used and only discharged to the environment instead of being provided to any consumer, when the piston type pump  1  is used as a vacuum pump. Within the pump housing  2  a piston arrangement  8  is provided, which will be described in more detail below. The piston arrangement  8  is connected to a drive shaft  10  which, when driven, sets the piston arrangement  8  into movement for inducing a vacuum at the pump inlet in this embodiment. The drive shaft  10  is rotatable about a rotational axis A and may be connected to an electric motor. 
     The piston arrangement  8  according to the embodiment shown in  FIG. 1  comprises a first primary stage piston  12  which is slidably seated in a first primary stage cylinder  14  formed in the pump housing  2 . The first primary stage piston  12  in  FIG. 1  is shown in its first end position, which is the position furthest away from the rotational axis A, however might travel within the first primary stage cylinder  14  to the left-hand side direction with respect to  FIG. 1 , thus closer to the rotational axis A. 
     The piston type pump  1  according to the shown embodiment is formed as a twin type two-stage piston pump and therefore also comprises a first secondary stage piston  16 , which is provided within a first secondary stage cylinder  18 , which is formed within the first primary stage piston  12 . The first primary stage piston  12 , therefore, is formed in a hollow manner, to form the first secondary stage cylinder  18 . The first primary stage piston  12  comprises a first primary stage piston wall  13  which defines the first secondary stage cylinder  18 . The first secondary stage cylinder  18  in particular is formed by an inner circumferential surface of the first primary stage piston wall  13  within the first primary stage piston  12 . 
     In a similar fashion the piston arrangement  8  according to this embodiment also comprises a second primary stage piston  40 , which is slidably seated in a second primary stage cylinder  42 , which again is formed inside the pump housing  2 . The complete interior  3  of the pump housing  2  can be formed as a cylindrical hollow portion to form both, the first primary stage cylinder  14  and the second primary stage cylinder  42 . 
     Also a second secondary stage cylinder  44  is provided which is slidably seated in a second secondary stage cylinder  46  formed within the second primary stage piston  40 . Again the second primary stage piston  40  comprises a second primary stage piston wall  41  which defines the second secondary stage cylinder  46  by its inner circumferential surface restricting a second hollow space  47 . 
     Moreover, in  FIG. 1  it can be seen that the first primary stage cylinder  14  comprises a first central axis B 1  and the second primary stage cylinder  42  comprises a second central axis B 2 , which are coaxial. Thus, the first and the second central axes B 1 , B 2  form a single axis on which the first primary stage piston  12  and the second primary stage piston  40  move. When the first secondary stage cylinder  18  and the second secondary stage cylinder  46  are formed concentrically within the respective first primary stage piston  12  and the second primary stage piston  40 , also the first secondary stage piston  16  and the second secondary stage piston  44  move coaxially with the first and second central axes B 1 , B 2 . Thus, the overall design of the piston type pump  1  is a boxer type piston type pump in which the single pistons move in opposing directions. This may lead to a well-balanced design. 
     The first secondary stage piston  16  comprises a first piston rod  54  which extends through a first assembly opening  60  in the first primary stage piston wall  13 . The portion of the hollow space  19  which is on the opposite side of the piston rod  54  with respect to the first secondary piston face  30  can be named the first secondary stage working chamber. In the same manner the second secondary stage piston  44  comprises a second piston rod  56  which extends through a second assembly opening  58  formed in the second primary stage piston wall  41  of the second primary stage piston  40 . 
     Now beginning with  FIG. 6 , the flow of fluid will be described in more detail. 
     The pump inlet  4  (see  FIG. 6 ) is here only shown as a single opening which is in fluid connection with a first conduit  74  formed in the pump housing  2 . The conduit  74  is surrounded by a protuberance  75  of the pump housing  2 , which can be seen in  FIG. 1  also. This first conduit  74  substantially extends in a parallel way to the first and second central axes B 1 , B 2 . The first conduit  74  on the one hand leads to a second conduit  76  formed in a first housing lid  78  which closes the pump housing  2  and also closes the first primary stage cylinder  14 . This second conduit  76  terminates in a first inlet chamber  80  which is closed to the environment by means of a first chamber lid  82 . The first inlet chamber  80  comprises a first inlet check valve  84  which allows fluid to flow through the first conduit  74 , the second conduit  76 , the inlet chamber  80  into the first primary stage cylinder  14 , but not vice versa. This is indicated by the arrows in  FIG. 6 . The first inlet check valve  84  can be formed as a leaf valve and comprises a leaf  86  which is flexible and might be formed out of any flexible material, such as thin metal, elastomer or the like. 
     A similar arrangement is provided on the other end of the pump housing  2  (see  FIG. 1 ). Even though  FIG. 1  is not as detailed as  FIG. 6 , it shall be understood that the same arrangement is provided. In particular, the pump housing  2  comprises a second housing lid  88  comprising a second inlet chamber  90  with a second inlet check valve  92  and a respective second leaf of the second inlet check valve  92 . A third conduit  96  is provided in the second housing lid  88 , however, not shown in cut view in  FIG. 1 , but connected to the first conduit  74  in a similar manner as it has been described with respect to the second conduit  76 . Again the second inlet check valve  52  allows fluid to enter the second stage cylinder  46  through the first conduit  74 , the third conduit  96 , the second inlet chamber  90  and the second inlet check valve  92 . The first housing lid  78  and the second housing lid  88  may be formed identical to each other or in a mirrored fashion. In any case manufacturing of the piston type pump  1  is simplified. 
     When now the drive shaft  10  begins to rotate due to operation of an electric motor attached to the drive shaft  10 , the first and second primary stage pistons  12 ,  40  (see  FIG. 1 ) will move along the respective first and second central axes B 1 , B 2  toward the rotational axis A. Thus, the working chamber, which is formed between the piston housing  2 , the respective housing lids  78 ,  88  and the respective first and second primary stage pistons  12 ,  40  will be enlarged and therefore fluid will be drawn through the pump inlet  4 , the first conduit  74 , the second and third conduits  76 ,  96 , the first and the second inlet chambers  80 ,  90  and the first and second inlet check valves  84 ,  92 . Due to the movement of the first and second primary stage pistons  12 ,  40  the primary stage vacuum is induced at the pump inlet  4 . 
     When now the drive shaft  10  continues to rotate, the first and second primary stage pistons  12 ,  40  will again be pushed outwardly, i.e. away from the rotational axis A. The respective first and second working chambers will become smaller and residual fluid, which is in these working chambers, will be compressed. The first and second inlet check valves  84 ,  92  prevent this fluid from flowing toward the pump inlet  4  again. However, this fluid needs to exit the piston type pump  1 . To achieve this, the first primary piston face  24  is provided with a first primary outlet  26  which in turn is provided with a first primary check valve  28 . Thus, the fluid contained in the first working chamber can flow through the first primary outlet  26  and the first primary check valve  28  into the first secondary stage cylinder  18 . 
     In the same manner also a second primary piston face  48  of the second primary stage piston  40  is provided with a second primary outlet  50  which in turn is provided with a second primary check valve  52 . Thus, fluid contained in the second working chamber may flow through the second primary outlet  50  and the second primary check valve  52  into the second secondary stage cylinder  46  upon movement of the second primary piston  40  away from the rotational axis A. 
     Both, the first and second primary check valves  28 ,  52  again might be formed as leaf valves and comprise respective first and second primary check valve leaves  96 ,  98  which can be identical to leaves  86 ,  94 . 
     For a more easy manufacturing and assembly, the first primary stage piston face  24  is defined by a first primary stage piston lid  70  attached to the first primary piston wall  13 . This first primary stage piston lid  70  carries the first primary check valve  28 . Also the second primary stage piston face  48  is defined by a second primary stage piston lid  72  attached to the second primary piston wall  41 . This second primary stage piston lid  72  carries the second primary check valve  52 . 
     When the first and second primary stage pistons  12 ,  40  are in the central position, thus proximal to the rotational axis A, the first and second secondary stage pistons  16 ,  44  are at the outermost position, thus most distal to the rotational axis A, due to their connection to the first and second eccentrics  20 ,  21 . In this position the first and second secondary stage pistons  16 ,  44  are proximal to the first and second primary check valves  28 ,  52  and the respective working chamber is small. Upon rotation of the central drive shaft  10  and movement of the first and second primary stage pistons  12 ,  40  outwardly, the first and second secondary stage pistons  16 ,  44  are drawn inwardly toward the rotational axis A, therefore enlarging the respective first and second secondary stage working chambers. A vacuum is induced and additional fluid may flow from the pump inlet  4  through the first and second inlet check valves  84 ,  92 , the first and second primary check valves  28 ,  52  into the first and second secondary stage working chambers. 
     On the other hand, when the drive shaft  10  rotates further, the first and second secondary stage pistons  16 ,  44  are pushed outwardly again, thus decreasing the respective first and second secondary stage working chambers. The fluid, contained in these first and second secondary stage working chambers needs to exit the piston type pump  1 . 
     To achieve this, the first secondary piston face  30  is provided with a first secondary outlet  32 , which in turn is provided with a first secondary check valve  34  (see  FIGS. 3, 4 and 6 ). As shown in  FIG. 6 , fluid can pass through this first secondary check valve  34  and out of the pump outlet  6 . 
     In the same manner, also the second secondary stage cylinder  46  is provided with a second secondary outlet  49  in a second secondary piston face  45  and a second secondary check valve  51 . Again, fluid may pass through this second secondary check valve  51  and out of the pump outlet  6 . 
     Afterwards, the drive shaft  10  rotates further and again moves the first and second secondary stage pistons  16 ,  44  toward the rotational axis A. It shall be understood that dependent on how the first, second and third conduits  74 ,  76 ,  96  are arranged, also the, for example, first secondary outlet  32  may be guided into the second primary stage working chamber, thus into the second primary stage cylinder  42  and the vacuum may be further decreased. In such an arrangement the piston type pump  1  would be a four stage vacuum pump instead of a two stage twin type vacuum pump as shown in the embodiments in the attached figures. 
     Beginning with  FIG. 7  the drive assembly according to a first embodiment is explained in more detail. The first primary stage piston  12  is connected to a first sliding block guide  62  seated on a first eccentric  20  of the drive shaft  10 . The first eccentric  20  is integrally formed with the drive shaft  10  comprising a first eccentricity e 1 , measured with respect to the rotational axis A of the drive shaft  10 . In this embodiment the first eccentric  20  is formed as a crank pin  112  having a circular cross section. An inner surface  114  of the first sliding block guide  62  forms a slot  116  in the sliding block guide. A first sliding bush  118  is arranged between the first eccentric  20  and the first sliding block guide  62 . An outer surface  120  of the first sliding bush  118  is directed towards said inner surface  114  of the first sliding block guide  62 . In an analogous manner an outer surface  121  of the second sliding bush  119  is directed towards an inner surface  115  of the second sliding block guide  64 . Furthermore, the first sliding bush  118  comprises an inner bore  122  for receiving the first eccentric  20 . 
     A minor axis C 2  of the first sliding block guide  62  is perpendicular to a main axis C 1  ( FIG. 8 ) and to a first direction D 1  (which is perpendicular to the plane of the drawing in  FIG. 8 ). The inner surface  114  comprises a first inner wall  124  parallel to the main axis C 1  and a second inner wall  126  parallel to said first inner wall  124 . Preferably, the first inner wall  124  and the second inner wall  126  are symmetrical to a first plane Si which is defined by the main axis C 2  and the first direction D 1 . The first inner wall  124  and the second inner wall  126  are spaced from each other at a slot width SW measured parallel to the minor axis C 2 . A slot height SH of the slot  116  measured parallel to the main axis C 1  is preferably chosen such that the first sliding bush  118  does not contact a third inner wall  128  and a fourth inner wall perpendicular to the first inner wall  124  and the second inner wall  126 . Corners  132  of the slot  116  can be rounded or angular. Also, the third and fourth walls  128 ,  130  may be curved. 
     According to this embodiment, the inner surface  114  of the first sliding block guide  62  and the outer surface  120  of the first sliding bush  118  are tapered along the first direction D 1  towards a first end  134  of the drive shaft  10  ( FIG. 7 ). Here the second sliding bush  119  is tapered towards a second end  135  of the drive shaft  10  in an analogous manner. The inner surface  114  substantially forms a negative of the outer surface  120 , such that the first sliding bush  118  contacts the inner surface  114  along the first direction D 1 . A tapered end  136  of the first sliding bush  118  is located adjacent to a tapered end  138  of the slot  116 . An opposite end  140  of the first sliding bush  118 , opposite to the tapered end  136 , protrudes from the inner surface  114 . The outer surface  120  of the first sliding bush  118  is substantially bell shaped in a first cross-section perpendicular to the main axis C 1  such that said outer surface  120  is variably tapered in the first direction D 1 . Preferably, an outer width OW of the first sliding bush  118  is essentially equal to the corresponding slot width SW of the slot  116  along the first direction D 1 . In a second cross-section perpendicular to the first cross-section the first sliding bush  118  is preferably rectangular such that surfaces of the first sliding bush which are perpendicular to the main axis C 1  are parallel to the first direction D 1 . 
     A first biasing member  142  applies a biasing force F 1  to the first sliding bush  118 . Here the first biasing member  142  is formed as a spring clip  144 . The spring clip  144  applies the biasing force F 1  to an end stop  146  of the first sliding bush  118 , which is positioned at the opposite end  140  of the first sliding bush  118 . Preferably, the biasing force F 1  is parallel to the first direction D 1  such that the first sliding bush  118  is pushed into the slot  116 . It is further preferred that the biasing force F 1  is applied equally to the first sliding bush  118  such that a first reaction force F 2  on the first inner wall  124  and a second reaction force F 3  on the second inner wall  126  are equal. The end stop  146  of the first sliding bush  118  is spaced from a stop face  148  of the first sliding block guide  62  in the first direction D 1 . The end stop  146  is formed as a circumferential protrusion  150  or bead of the first sliding bush  118  which has an end stop width ESW that is larger than a maximum slot width MSW of the slot  116  ( FIG. 7 ). Thus, the first sliding bush  118  is prevented from being entirely pushed through the slot  116  when the outer surface  120  and/or the inner surface  114  are worn. 
     The spring clip  144  comprises a first hook section  152  engaging a first attachment section  154  of the first sliding block guide  62  and a second hook section  156  engaging a second attachment section  158  of the first sliding block guide  62 . The first hook section  152  and the second hook section  156  are arranged such that even when the biasing force F 1  is applied by the spring clip  144 , the spring clip  144  is fixed to the first sliding block guide  62 . In this embodiment the first attachment section  154  and the second attachment section  158  are formed as planes perpendicular to the first direction D 1 . Preferably, the first hook section  152  and the second hook section  156  are formed on opposing ends of the spring clip  144  while a biasing section  160  is positioned in between the hook sections  152 ,  156 . The biasing section  160  contacts the end stop  146  of the first sliding bush  118 . Furthermore, the biasing section  160  covers the slot  116  such that the first sliding bush  118  is prevented from slipping out of the slot  116 . 
     When the drive shaft  10  is rotated about the rotational axis A, the first eccentric  20  performs an orbital movement around said rotational axis A. Since the first sliding bush  118  is located on the first eccentric  20  said orbital movement is transferred to said first sliding bush  118 . The first sliding bush  118  is rotationally fixed within the first sliding block guide  62  such that the first eccentric  20  rotates relative to first sliding bush  118  in the inner bore  122  and only translational movement is transferred to the first sliding block guide  62 . The orbital movement comprises a first component parallel to the main axis C 1  and a second component parallel to the minor axis C 2 . It should be understood that since the first eccentric  20  performs an orbital movement, movements parallel to the main axis C 1  and parallel to the minor axis C 2  occur simultaneously. If a component of the orbital movement along the main axis C 1  is transferred to the first sliding bush  118 , said first sliding bush  118  slides relative to the first sliding block guide  62  along the main axis C 1  and no movement is transferred to the first sliding block guide  62 . Since the outer surface  120  of the first sliding bush  118  contacts the first inner wall  124  and the second inner wall  126  of the first sliding block guide  62 , translational movement of said first sliding bush  118  relative to said first sliding block guide  62  is not possible along the minor axis C 2 . Thus, the component of said orbital movement of the first eccentric  20  parallel to the minor axis C 2  is transferred to the first sliding block guide  62  via the first sliding bush  118 . The orbital movement of the first eccentric  20  is transformed into a rectilinear movement of the first sliding block guide  62  along its minor axis C 2 . 
     In a second embodiment (see  FIG. 10 ) the first sliding bush  118  is biased towards the first sliding block guide  62  by a wave spring  162 . Again, the outer surface  120  (not shown in  FIG. 10 ) of the first sliding bush  118  and the inner surface  114  of the first sliding block guide  62  are tapered in the first direction D 1 . An upper surface  164  and a lower surface  166  of the outer surface  120  of said sliding bush  118  are parallel to each other and to the first direction D 1 . The wave spring  162  is positioned at the end stop  146  and evenly applies the biasing force F 1 . First eccentric  20  comprises an internal thread  168  which corresponds to a retaining screw  170 . Said retaining screw  170  is screwed into the internal thread  168  and fixes the wave spring  162  to the first eccentric  20 . A thrust washer  172  and a slip washer  174  are arranged between a screw head  176  of said retaining screw  170  and the wave spring  162 . The thrust washer  172  is positioned adjacent to the screw head  176  and has a larger outer diameter than said screw head  176 . Using a thrust washer  172  it is possible to use a standardized screw as retaining screw  170  while allowing for adequate material thickness of the first eccentric  20 . The wave spring  162  acts against the thrust washer  172  and imparts a torque T (indicated by arrows in  FIG. 11 ) to the first sliding block guide  62  via the first sliding bush  118 . Such a torque is absent in the arrangement according to the first embodiment as presented in  FIG. 7  to  FIG. 9 . 
     During operation the first eccentric  20  rotates inside the inner bore  122  of the first sliding bush  118  while the retaining screw  170  is fixed to the first eccentric  20 . Relative motion between a first opposing end  178  and a second opposing end  180  of the wave spring  162  might cause damage to said wave spring  162  and/or change the biasing force F 1 . The slip washer  174  allows for relative rotational movement between said slip washer  174  and the thrust washer  172  about and parallel to the first direction D 1 . During operation the screw head  176  and the thrust washer  172  rotate in the first sliding bush while the slip washer  174  slips relative to said thrust washer  172 . Thus, torsional forces on the wave spring  162  are avoided. 
     It should be understood that the second eccentric  22 , second sliding block guide  64  and a second sliding bush  119  can have similar features as described regarding the first eccentric  20 , the first sliding block guide  62  and the first sliding bush  118 . 
     With reference to  FIG. 12  a third embodiment of a piston type pump  1  is described. Elements that are similar or identical to other embodiments have the same reference numerals used in the previous figures. Again, all features described with reference to  FIG. 1  to  FIG. 6  can be present in this embodiment. The drive shaft  10  comprises a first eccentric  20  which is located in a slot  116  of a first sliding block guide  62 . In this embodiment a rolling bearing  182  is located between the first eccentric  20  and the first sliding block  62 . It shall be understood that the rolling bearing  182  can be one of: roller bearing, needle roller bearing, barrel roller bearing and/or any other type of rolling bearing. An inner ring  184  of the rolling bearing  182  is fixed to the first eccentric  20  via a fixing member  186 . Preferably, the fixing member  186  is screwed to the first eccentric  20  via a fixing screw (not shown in  FIG. 12 ). For fixing the inner ring  184  of the rolling bearing  182  the first eccentric  20  comprises a first bearing stop face  188  and the fixing member  186  comprises a second bearing stop face  190 . The first bearing stop face  188  protrudes perpendicular to the first direction D 1 . In an analogous manner the second bearing stop face  188  protrudes perpendicular to the first direction D 1 . The inner ring  184  of the rolling bearing  182  is fixed between the first bearing stop face  188  and the second bearing stop face  190 . An outer ring  192  of the rolling bearing  182  is positioned adjacent to the inner surface  114  of the first sliding block guide  62  and slidable to said inner surface  114 . During operation drive shaft  10  is driven and eccentric  20  as well as the inner ring  184  of said rolling bearing  182  perform an orbital movement around the rotational axis A. The outer ring  192  is rotatable supported relative to the inner ring  184  around the first direction D 1  by multiple rollers  194  (shown schematically in  FIG. 12 ). Thus, wear on the first eccentric  20  can be reduced. With respect to  FIG. 12  the outer ring  192  of the rolling bearing  182  slides up and down in said slot  116  of the first sliding block guide  62 . A motion of the first eccentric  20  along the minor axis C 2  is transferred to the first sliding block guide by the rolling bearing  182  such that a pendulum motion of the first primary stage piston  12  is eliminated. Preferably, the outer ring  192  of said rolling bearing  182  and/or the inner surface  114  of said sliding block guide  62  are made of a highly wear resistant material. Preferably, the inner surface  114  and/or the outer ring  192  are hardened. 
       FIG. 13  now depicts a schematic drawing of a vehicle  100 . Vehicle  100  preferably is formed as a passenger car, or a light truck and comprises a pneumatic braking system  102 . The braking system  102  is shown by lines  104  leading to wheels  106   a ,  106   b ,  106   c ,  106   d  for providing the wheels  106   a ,  106   b ,  106   c ,  106   d  with the respective braking pressure. Lines  104  are connected to a central module  108 . The vehicle  100  moreover comprises an engine  110  and a piston type pump  1 , which is herein used as a vacuum pump  1 . piston type pump  1  provides the braking system  102  with vacuum, which e.g. could be used by a brake booster of the braking system  102 , which could be implemented in the central module  108 . 
     While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above. 
     The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C. 
     LIST OF REFERENCE CHARACTERS 
     
         
         
           
               1  piston type pump 
               2  pump housing 
               3  interior 
               4  pump inlet 
               6  pump outlet 
               8  piston arrangement 
             drive shaft 
               12  first primary stage piston 
               13  first primary stage piston wall 
               14  first primary stage cylinder 
               16  first secondary stage piston 
               18  first secondary stage cylinder 
               19  hollow space 
             first eccentric 
               22 —second eccentric 
               24  first primary piston face 
               26  first primary outlet 
               28  first primary check valve 
             first secondary piston face 
               32  first secondary stage piston outlet 
               34  first secondary check valve 
             second primary stage piston 
               41 —second primary stage piston wall 
               42 —second primary stage cylinder 
               44 —second secondary stage piston 
             second secondary piston face 
               46 —second secondary stage cylinder 
               47 —second hollow space 
               48 —second primary piston face 
               49 —second secondary outlet 
               50 —second primary outlet 
               51 —second secondary check valve 
               52 —second primary check valve 
               54  first piston rod 
               56 —second piston rod 
               58 —second assembly opening 
               60  first assembly opening 
               62  first sliding block guide 
               64 —second sliding block guide 
               70  first piston lid 
               72 —second primary stage piston lid 
               74  first conduit 
               75  protuberance 
               76 —second conduit 
               78  first housing lid 
               80  first inlet chamber 
               82  first chamber lid 
               84  first inlet check valve 
               86 ,  94  leaf 
               88 —second housing lid 
               90 —second inlet chamber 
               92 —second inlet check valve 
               96  third conduit 
               100  vehicle 
               102  pneumatic braking system 
               104  lines 
               106   a ,  106   b ,  106   c , wheels 
               108  central module 
               110  engine 
               112  crank pin 
               114  inner surface 
               115  inner surface (second sliding block 
               116  slot 
               118  first sliding bush 
               119 —second sliding bush 
               120  outer surface 
               121  outer surface (second sliding bush) 
               122  inner bore 
               124  first inner wall 
               126 —second inner wall 
               128  third inner wall 
               130  fourth inner wall 
               132  corner 
               134  first end (drive shaft) 
               135 —second end (drive shaft) 
               136  tapered end (first sliding bush) 
               138  tapered end (slot) 
               140  opposite end 
               142  first biasing member 
               144  spring clip 
               146  end stop 
               148  stop face 
               150  protrusion 
               152  first hook section 
               154  first attachment section 
               156 —second hook section 
               158 —second attachment section 
               160  biasing section 
               162  wave spring 
               164  upper surface 
               166  lower surface 
               168  internal thread 
               170  retaining screw 
               172  thrust washer 
               174  slip washer 
               176  screw head 
               178  first opposing end 
               180 —second opposing end 
               182  rolling bearing 
               184  inner ring 
               186  fixing member 
               188  first bearing stop face 
               190 —second bearing stop face 
               192  outer ring 
               194  roller 
             e 1  first eccentricity 
             e 2  second eccentricity 
             A rotational axis 
             B 1  first central axis 
             B 2  second central axis 
             C 1  main axis 
             C 2  minor axis 
             D 1  first direction 
             ESW end stop width 
             F 1  biasing force 
             F 2  first reaction force 
             F 3  second reaction force 
             MSW maximum slot width 
             OW outer width 
             Si first plane 
             SW slot width 
             SH slot height 
             T torque