Abstract:
A method and a lubricant dosing pump for accurately dosing the small lubricant quantities. It is provided that at least two pistons are moved from an initial position relative to one another, thereby enlarging a dosing volume situated between them, and that thereby lubricant is conveyed through an inlet into the closing volume. Subsequently, the inlet is sealed and the two pistons situated opposite to each other in the stroke direction are moved together with the dosing volume until in a discharge position the dosing volume is connected with a discharge opening spaced apart from the inlet in the stroke direction. Then, the pistons are moved relative to each other, thereby reducing the dosing volume, and convey a predetermined dosing quantity of the lubricant through the discharge opening. Finally, the pistons are moved back into their initial positions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to German Application Serial No. 102006038389.3, filed on Aug. 15, 2006. 
     The invention relates to a method for dosing a lubricant as well as a lubricant dosing pump. 
     BACKGROUND 
     In the field of dosing pumps, it is problematic to accurately dose small quantities in the range of 0.5 mm 3  to approximately 15 mm 3 , preferably 1 mm 3  to 10 mm 3 . However, it is just the dosage of small quantities that is becoming increasingly important due to the increasingly wide-spread minimal quantity lubrication methods, such as oil-air lubrication. 
     SUMMARY 
     It is therefore an object of the invention to accurately dose small quantities of lubricant. 
     According to the invention, this object is achieved for the method mentioned in the beginning in that at least one piston is moved from an initial position relative to at least one other piston with enlargement of a dosing volume located between the pistons, and thereby the lubricant is conveyed through an inlet into the dosing volume, subsequently the inlet is sealed and the pistons with the dosing volume are moved towards a discharge opening, until the dosing volume is connected with the discharge opening in a discharge position, then at least one piston is moved relative to at least one other piston with reduction of the dosing volume and a predetermined dosing quantity of the lubricant is discharged through the discharge opening and finally the pistons are moved back into their initial positions. 
     For the lubricant dosing pump mentioned in the beginning, this object is achieved according to the invention by an embodiment in which at least two pistons disposed in a cylinder so as to be movable relative to one another and being opposed in the stroke direction, an inlet disposed in the area of a piston stroke of at least one of the pistons and ending in the cylinder, and a discharge opening spaced from the inlet in the stroke direction and ending in the cylinder which is disposed in the area of the piston stroke of at least one of the pistons, are provided, wherein a dosing volume exists between the pistons, the volume of which is variable and which can be moved from the inlet to the discharge opening. 
     The solution according to the invention is simple and permits a surprisingly accurate dosage of the lubricant. 
     The invention can be further developed by various embodiments which are each advantageous by themselves. 
     In one advantageous further development, for example, a particularly accurate dosage can be enabled when the dosing volume is reduced to zero during the discharge of the lubricant. This is achieved, for example, in that both pistons are moved relative to one another in the discharge position until they touch each other. Correspondingly, in the lubricant dosing pump, at least in the discharge position, the piston strokes can overlap. This embodiment permits the complete discharge of the lubricant from the dosing volume. 
     At least one piston can be driven hydraulically or pneumatically into at least one direction. In particular the hydraulic drive by the lubricant itself represents a simple embodiment that can be realized at low costs and can do without additional pressure lines and results in a self-sustaining, independently operating lubricant dosing pump that does not need any further energy supply from outside. The hydraulic drive of at least one piston can be effected against a spring force which is generated by at least one spring element and provides for an automatic return. In this case, the spring force can, in one embodiment, be guided over the dosing volume to the hydraulically driven piston, that means the one piston can be moved against the spring force acting on the other gap piston. 
     Preferably, the piston circumferential surfaces represent gap rings which can seal the pistons in particular without rubber gaskets against the piston accommodation, so that no air volumes can occur in the area of the dosing volume which would affect the dosing accuracy. 
     Furthermore, the piston strokes can overlap in the area between the inlet and the discharge opening. During the movement of the dosing volume into the stroke direction from the initial position into the discharge position, the inlet can preferably be sealed by the one piston situated at the inlet. During the back movement, the discharge opening is preferably sealed by the other piston situated at the discharge opening. Due to a longer stroke movement, in particular in an embodiment where the two piston circumferential surfaces are designed as gap rings, the respective sealing surfaces between the dosing volume and the respective openings are large enough for reliably avoiding leakage and thus a change of the dosing volume. Preferably, the front face of the piston sealing the discharge opening is moreover in the initial position further apart from the discharge opening than from the inlet. 
     The conveyance of the lubricant into the dosing volume through the inlet and/or the conveyance of the lubricant from the dosing volume through the discharge opening can be effected under the influence of hydraulic or pneumatic pressure or under the influence of a spring element on at least one piston. 
     In a further advantageous embodiment, the drive of at least one piston, for example during the movement from and/or to the discharge position and/or the initial position, can be effected by the other piston, for example in that the one piston is pressed against the other piston under spring force or by means of hydraulic or pneumatic pressure acting on a piston. The motive force acting on the one piston can be passed to the other piston via the lubricant contained in the dosing volume, so that the same is passively carried along. In this case, the lubricant is automatically discharged from the dosing volume through the discharge opening when the dosing volume becomes connected to the discharge opening and the motive force moves the one piston towards the other piston thereby displacing the lubricant from the dosing volume through the discharge opening. Preferably, the other piston is provided with an element, such as for example a spring element, which generates a counterforce opposed to the motive force. 
     The operational reliability can be increased if in the initial position and/or in the discharge position, at least one piston each is retained in an end position, that means the respective piston stroke ends in the area of the inlet and/or the discharge opening. Such an end position can be achieved, for example, if one piston has driven against a stop under a force acting on the piston. 
     Below, two embodiments of the invention are illustrated by way of example with reference to the drawings. The differing features in the embodiments can be arbitrarily combined according to the above explanations to give the various further developments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  shows a first embodiment of the lubricant dosing pump according to the invention in a schematic sectional view; 
         FIGS. 2 to 6  show other schematic sectional views of the embodiment of  FIG. 1  in other operational positions; 
         FIG. 7  shows a second embodiment of the lubricant dosing pump according to the invention in a schematic sectional view; 
         FIGS. 8 to 12  show other schematic sectional views of the embodiment of  FIG. 2  in other operational positions. 
     
    
    
     DETAILED DESCRIPTION 
     First, the construction of a lubricant dosing pump according to the invention is illustrated by way of example with reference to the schematic sectional view of  FIG. 1 . 
     Accordingly, the lubricant dosing pump  1  comprises at least two pistons  2   a ,  2   b  which are movably disposed in a piston accommodation  4 . The stroke directions H of the two pistons  2   a ,  2   b  extend in parallel or preferably coincide. The pistons  2   a ,  2   b  are situated opposite one another in the stroke direction H. 
     The pistons  2   a ,  2   b  each have a dosing section  5   a ,  5   b  and a control section  6   a ,  6   b , where the control section  6   a ,  6   b  can be provided with a larger cross-sectional area than the respective dosing section  5   a ,  5   b  essentially extending transversely to the stroke direction H. 
     Correspondingly, the piston accommodation  4  can be provided with a smaller clear cross-section in the area of the dosing sections  5   a ,  5   b  and with a larger clear cross-section in the area of the control sections  6   a ,  6   b , which is in each case adapted to the outer contours of these sections. The tolerances between the piston accommodation  4  and the dosing sections  5   a ,  5   b  are here preferably selected such that a gap ring  7   a ,  7   b  is formed at the circumferential surfaces between the dosing sections  5   a ,  5   b  of the pistons  2   a ,  2   b  and the piston accommodation  4  each. 
     At least one drive surface  8   a ,  8   b  of the pistons  2   a ,  2   b  situated at the front in the stroke direction H serves as drive surface upon which hydraulic or pneumatic pressure can act for driving the respective piston. The drive surfaces  8   a ,  8   b  are preferably disposed at the control section  6   a ,  6   b.    
     Furthermore, the pistons  2   a ,  2   b  can be provided with spring elements  9   a ,  9   b  which act upon the pistons  2   a ,  2   b  each with a spring force F. In particular, the spring forces generated at each of the two pistons can act in opposite directions, so that the two pistons  2   a ,  2   b  are pressed towards each other in the non-operative state when no pressure acts on the drive surfaces  8   a ,  8   b . At least one spring element, in  FIG. 1  spring element  9   a , acts against the pressure acting on the drive surfaces  8   a ,  8   b.    
     The lubricant dosing pump  1  in the embodiment which is shown in  FIG. 1  comprises at least one control opening  10  per piston  2   a ,  2   b  ending in a drive chamber  11   a ,  11   b  adjacent to one of the drive surfaces  8   a ,  8   b . The drive chambers  11   a ,  11   b  and the drive surfaces  8   a ,  8   b , respectively, are situated at the front side of the pistons  2   a ,  2   b  each opposite the spring elements  9   a ,  9   b  with respect to the stroke direction H. 
     An inlet  12  and a discharge opening  13  are spaced apart in the stroke direction H and end in the piston accommodation  4 . For example, the inlet  12  is disposed in a section of the piston accommodation  4  over which the front end surface  14   a  of the dosing section  5   a  of the piston  2   a  situated at the inlet  12  passes in the course of a stroke H 1  (cf.  FIG. 5 ). The discharge opening  13  is correspondingly disposed in an area of the piston accommodation  4  over which the front end surface  14   b  of the dosing section  5  of this piston  2   b  passes in the course of a stroke H 2  (cf.  FIG. 5 ) of the other piston  2   b  situated at the discharge opening. The stroke H 2  of the piston  2   b  can end in the area of the discharge opening  13  or pass beyond the discharge opening  13  into the direction away from the other piston  2   a . Equally, the stroke of the piston  2   a  can end in the area of the inlet  12  or pass beyond the inlet  12  into the direction away from the other piston. 
     A dosing volume  16  is disposed between the two pistons  2   a ,  2   b , that is movable into the stroke direction H and has a variable size and which is connected to the inlet  12  in the initial position of  FIG. 1 . 
     Preferably at the control section  6  of each piston, stops  17   a ,  17   b ,  17   c  pointing into the stroke direction H or opposite to the stroke direction H and being rigidly connected to the pistons are provided which cooperate with stationary counter-stops associated to the piston accommodation and limit the stroke H 1 , H 2  each of the pistons  2   a ,  2   b  into at least one direction. In particular, one of the pistons, in particular the piston  2   a  adjacent to the inlet  12 , the stroke H 1  can be limited in both directions by stops. This is also true for piston  2   b.    
     Preferably, the inlet  12  as well as the control openings  10  are connected to each other and preferably with a lubricant line  18  upon which pressure acts intermittently. Thereby, a self-sustained, automatically operating lubricant pump  1  is achieved. 
     The discharge opening  13  can be connected to a non-depicted lubricant line, which leads to one or several neither depicted lubrication points. The inlet  12  or the lubricant line  18 , respectively, can be connected to a neither depicted lubricant storage or a non-depicted lubricant feed pump. 
     With reference to the schematic sectional views of  FIGS. 1 to 6 , now the function of the embodiment of  FIG. 1  is illustrated.  FIGS. 1 to 6  show the same embodiment at various points of time of an operating cycle wherein the lubricant quantity contained in the dosing volume is discharged. 
     In the initial position which is shown in  FIG. 1 , pressure acts upon the lubricant in the lubricant line  18 , as indicated by arrow  19 . The pressure acts via the control openings  10  onto the drive surfaces  8   a ,  8   b  of the pistons  2   a ,  2   b , so that each of them is traversed against the action of at least the one spring element  9   a  to their end positions situated at the inlet, and the corresponding stops  17   a ,  17   c  adjoin the counter-stops at the side of the piston accommodation. In the initial position, the front faces  14   a ,  14   b  of the dosing sections  5   a ,  5   b  are spaced apart in the stroke direction H, and the dosing volume  16  is filled with lubricant through the inlet  12 . As in particular the piston  2   b , which is the lower one in  FIG. 1  and seals the discharge opening is designed as double diameter piston the one front face of which is formed by the dosing section  5  and the other front face of which is formed by the drive surface  8   b , the pressure acting upon the front face  14   b  in the dosing volume  16  is not sufficient for overcoming the compressive force acting on the drive surface  8   b.    
     By an adjustment of the stop  17   c  of the piston  2   b  sealing the discharge opening  13  towards the piston  2   a  or away from it, preferably from outside the lubricant dosing pump  1 , the dosing volume can be easily reduced or increased. 
     In the next step, as represented in  FIG. 2 , no more pressure acts upon the lubricant line  18 . This is for example the case because a lubricant feed pump not shown in  FIGS. 1 to 6  which is connected to the lubricant line  18  is switched off. The lubricant line  18  is in  FIG. 2 , for example, connected to a lubricant reservoir. 
     The spring element  9   a  acting upon the piston  2   a  moves the same now from the end position towards the other piston  2   b , thereby reducing the dosing volume  16 . In the course of this movement, lubricant is conveyed out of the drive chamber  11   a  through the control opening  10  and out of the dosing volume  16  through the inlet  12  back into the lubricant line  18 , as is schematically shown in  FIG. 2  by arrow  20 . In the course of this movement, the front face  14   a  of the dosing section  5   a  of the piston  2   a  passes over the inlet  12 , so that the following dosing section  5   a  seals the inlet  14  via the gap ring  7   a  and prohibits a backflow of lubricant  16  through the inlet  12 . This position is shown in  FIG. 2 . The distance between the two front faces  14   a ,  14   b  and their cross-section in the moment when the inlet  12  is sealed governs the size of the dosing volume  16 . 
     As the spring element  9   a  has a stronger design than the counter-acting spring element  9   b  and as the lubricant in the dosing volume  16  is incompressible, the piston  2   a  moves, after the inlet  12  has been sealed, the piston  2   b  against the action of the spring element  9   b  associated to the piston  2   b . Correspondingly, the dosing volume  16  moves, together with the movement of the two pistons  2   a ,  2   b , into the stroke direction H towards the discharge opening  13 . As the piston  2   a  carries the passive piston  2   b  along during this stroke movement, the piston  2   a  can also be referred to as drive piston in the embodiment of  FIG. 1 . 
     In  FIG. 3 , the point of time at which the front face  14   b  of the piston  2   b  situated at the discharge opening  13  is just starting to pass over the discharge opening  13  is schematically shown. Until this point of time, the discharge opening  13  was sealed by the dosing section  5   b  of the piston  2   b . If the front face  14   b  passes the discharge opening  13 , the same is opened and connected with the dosing volume  16 . The lubricant in the dosing volume can now yield the pressure of the spring element  9   a  and/or the pressure of the spring element  9   b  by flowing through the discharge opening  13  out of the lubricant dosing pump  1 , as is indicated in  FIG. 4  by arrow  21 . 
     As is furthermore shown in  FIG. 4 , at the end of its stroke H 1 , the piston  2   a  sealing the inlet  12  is situated in the area of the discharge opening  13 , so that the same remains open. This end position ensures that all of the lubricant contained in the dosing volume  16  can be discharged. As the piston  2   b  discharges the lubricant out of the dosing volume with its stroke movement against the piston  2   a , the piston can be referred to as dosing piston in the embodiment of  FIG. 1 . 
     In the position which is shown in  FIG. 4 , the front faces  14   a ,  14   b  adjoin each other and the dosing volume is reduced to zero. This position of the pistons  2   a ,  2   b  is stable if the lubricant line  18  is free from pressure, as the piston  2   a  is pressed by the stronger pressure spring  9   a  against its stop  17   b  facing towards the other piston  2   b , and the other piston  2   b  with the weaker spring element  9   b  presses against the piston  2   b , so that the piston  2   a  forms the stop for the piston  2   b.    
     If in a next lubrication cycle the lubricant pressure in the lubricant line  18  is now built up again, as is represented in  FIG. 5  by arrow  19 , the lubricant pressure acts upon the drive chambers  11   a  and  11   b  via the control openings  10 , and the pistons  2   a ,  2   b  are moved back from the discharge position into the initial position, wherein the front faces  14   a ,  14   b  can adjoin each other at least at the beginning of this movement. In the process, the piston  2   b  seals the discharge opening  13  again, as is shown in  FIG. 6 . 
     If the piston  2   b  reaches its end position facing the piston  2   a , the piston  2   a  moves further, at the same time enlarging the dosing volume  16  disposed between the pistons  2   a ,  2   b . As soon as the front face  14   a  of the dosing section  5   a  passes the inlet  12 , the same is opened and lubricant is conveyed into the dosing volume  16 , as is represented in  FIG. 1 . Consequently, the operating cycle of the lubricant dosing device  1  starts again. 
     By the gap ring  7   b , it is avoided that due to pressure differences between the dosing volume  16  and the discharge opening  13  lubricant can flow through the discharge opening  13  and change the discharged dosing quantity. To this end, in the initial position, the front face  14   b  of the one piston  2   b  situated at the discharge opening  13  is further spaced apart from the discharge opening  13  than from the inlet  12 . 
     In the following, the construction of the second embodiment is illustrated with reference to  FIG. 7 , where for the sake of shortness, only the differences to the embodiment of  FIG. 1  are discussed. In the second embodiment, the same reference numerals are used as in the first embodiment, as far as the correspondingly designated elements correspond to each other with respect to construction and/or function. 
     The embodiment of  FIG. 7  has a simpler construction than the first embodiment. Only one of the pistons, in this embodiment piston  2   a  situated at the inlet, is driven hydraulically. Consequently, only one control opening  10  is provided. Different to the embodiment of  FIG. 1 , the control opening  10  moreover does not end at the side of the piston  2  facing the other piston  2   b , but at its side facing away, so that by a pressure in the drive chamber  11   a , the piston  2   a  is moved towards the piston  2   b . The spring element  9   a  is disposed at the other side of the piston  2   a  with respect to the drive chamber  11   a  and acts against the pressure in the drive chamber  11   a.    
     Only the spring element  9   b  acts upon the piston  2   b  and exerts a compressive force towards the other piston  2   b . The compressive force is smaller than the motive force of the piston  2   a  generated by the lubricant pressure. 
     In the following, the function of the second embodiment is explained with reference to  FIGS. 7 to 12 , where the positions of  FIGS. 7 to 12  correspond to the positions in  FIGS. 1 to 6  with respect to the position of the dosing volume  16 . By the different arrangement of drive chambers and spring elements, the movement of the dosing volume  1   b  is, however, phase-shifted with respect to the pressure build-up and reduction in the control opening  10 . 
     In the initial position of  FIG. 7 , where piston  2   b  is in its end position moved towards the other piston  2   a , and piston  2   a  is in its end position moved away from piston  2   b , the lubricant line  18  is free from pressure. The pistons  2   a ,  2   b  are pressed against corresponding stops by the respective spring elements  9   a ,  9   b  acting on them, at the same time forming the dosing volume  16 , and the dosing volume  16  is filled with lubricant. If pressure acts now upon the lubricant line  18  via a non-depicted lubrication pump at the beginning of a lubrication cycle, the same acts upon the drive chamber  11   a  of the piston  2   a  ( FIG. 8 ). As in the first embodiment, the piston  2   a  moves against the action of the spring element  9   a  with its front face  14   a  over the inlet  12  and seals the same. As soon as the inlet is sealed, the piston  2   b  is shifted against the action of the spring element  9   b  by the piston  2   b , so that the dosing volume  16  moves together with the pistons  2   a ,  2   b  towards the discharge opening  13 . 
     As soon as the front face  14   b  of the piston  2   b  passes over the discharge opening  13 , lubricant can flow away from the dosing volume  16  through the discharge opening  13 . In its end position, the piston  2   a  is situated with its front face  14   a  in the area of the discharge opening  13  without sealing the same, so that the remaining lubricant is pressed through the discharge opening  13  (arrow  21 ) out of the dosing volume  16  by the action of the spring element  9   b  by means of the piston  2   b  disposed at the discharge opening  13  until the front faces  14   a ,  14   b  adjoin each other. 
     At the end of the lubrication cycle, no more pressure acts upon the lubricant line  18 , and under the action of the spring elements  9   a ,  9   b , the pistons  2   a ,  2   b  move towards the end positions associated to the initial position situated at the inlet  12 , while the lubricant flows back from the drive chamber  11   a  through the lubricant line  18  ( FIGS. 11 and 12 ). 
     If the piston  2   b  situated on the side of the discharge opening  13  reaches its end position at the discharge opening  13 , the piston  2   a  is moved from the spring element  9   a  further into its end position remote from the other piston  2   b , at the same time enlarging the dosing volume  16 . In the process, as in the first embodiment, lubricant is conveyed through the inlet  12  into the dosing volume  16 . 
     Further modifications of the above-described embodiment and the above-described functionality are possible. For example, in the different embodiments, the spring elements  9   a ,  9   b  can each be exchanged with the drive chambers  11   a ,  11   b  without basically changing the functionality in any way, only the phase relation to the pressure in the lubricant line is changed. 
     Furthermore, in the discharge position, the piston  2   b  situated at the discharge opening  13  can be moved against a stop, so that the piston  2   a  situated at the inlet  12  is moved against the stationary piston  2   b  and discharges the lubricant out of the dosing volume in this manner.