Abstract:
Disclosed is a pump, in particular a high-pressure fuel pump, comprising at least one pump element ( 10 ) that has a roller tappet ( 20 ) inside which a roller ( 42 ) is rotatably mounted on a bearing bolt ( 44 ) by means of a bearing sleeve ( 46 ), said roller ( 42 ) rolling off a cam ( 22 ) of an input shaft ( 24 ). The bearing sleeve ( 46 ) is made of a plastic material, especially polyether ether ketone (PEEK) or polyphthalamide (PPA).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention proceeds from a pump, in particular a high-pressure fuel pump. 
         [0002]    Such a pump in the form of a high-pressure fuel pump is disclosed by DE 10 2010 063 363 A1. This pump comprises a pump element having a roller tappet, via which a pump piston is supported on a cam of a drive shaft. In the roller tappet a roller, which bears on the cam, is rotatably supported on a bearing pin. The roller is supported on the bearing pin by way of a bearing bush. The bearing bush usually comprises a body composed of metal, in particular steel, which is provided with a coating of friction-bearing material. The highly accurate machining required and the application of the coating make manufacturing of the bearing bush very exacting. In addition, an uneven pressure distribution can occur in the bearing bush between this and the roller and/or the bearing pin, resulting in greater pressures in the edge areas of the bearing bush, so that increased wearing of the bearing bush occurs in these areas. The known steel bearing bushes have only poor emergency running characteristics in the event of inadequate lubrication, so that in this case heavy wearing of the bearing bush and/or the roller and/or the bearing pin can occur. 
       SUMMARY OF THE INVENTION 
       [0003]    The pump according to the invention by contrast has the advantage that the bearing bush is easy to produce, and due to the greater elasticity of the plastic material compared to steel allows a more even pressure distribution. 
         [0004]    Advantageous embodiments and developments of the pump according to the invention are specified in the dependent claims. Suitable plastic materials for the bearing bush are specified in the claims, as well as a development that allows good emergency running characteristics in the event of inadequate lubrication of the bearing bush, and an embodiment that facilitates manufacturing of the bearing bush. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    An exemplary embodiment of the invention is represented in the drawing and is explained in more detail in the following description. 
           [0006]      FIG. 1  shows a simplified representation of a pump in a longitudinal section; and 
           [0007]      FIG. 2  shows an enlarged representation of a detail of a roller tappet of the pump. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]      FIG. 1  represents a pump, which is in particular a high-pressure fuel pump for a fuel injection device of an internal combustion engine. The pump comprises at least one pump element  10 , which comprises a pump piston  16 , displaceably and tightly guided in a cylinder bore  12  of a housing part  14 , which is referred to hereinafter as a cylinder head. With its end projecting into the cylinder bore  12  the pump piston  16  defines a pump working chamber  18 . The end of the pump piston  16  projecting from the cylinder bore  12  is connected to a roller tappet  20 . The roller tappet  20  is supported on a cam  22  of a drive shaft  24 , which cam under the rotational movement of the drive shaft  24  produces a reciprocating movement of the pump piston  16  in the cylinder bore  12 . The drive shaft  24  may be part of the pump or part of the internal combustion engine, for example its camshaft or another shaft. 
         [0009]    The pump working chamber  18  can be connected via an inlet valve  26  to a low-pressure inlet  27  to the pump and via an outlet valve  28  to a high-pressure outlet, which leads, for example, to a high-pressure accumulator  30 . The low-pressure inlet  27  may be fed, for example, by a feed pump, which draws in fuel from a storage tank. 
         [0010]    The roller tappet  20  comprises a hollow cylindrical tappet body  40 , into which the end of the pump piston  16  protruding from the cylinder bore  12  projects on the side thereof remote from the cam  22 . On the side of the tappet body  40  facing the cam  22  a roller  42 , which rolls on the cam  22 , is rotatably supported in said tappet body. A bearing pin  44 , on which the roller  42  is rotatably supported by a bearing bush  46 , is fixed in the tappet body  40 . The tappet body  40  has a bore  48 , which runs at least approximately perpendicular to the longitudinal axis  17  of the pump piston  16 , wherein the diameter of the bore  48  is enlarged in its middle area, viewed in the longitudinal direction, and the bore  48  in its middle area is open to the cam  22 . The bearing pin  44  may be pressed into the bore  48  at its end areas or may be secured in the bore  48  by means of a sprung retaining clip  50 , for example, so that it cannot be pushed out of the bore  48  in the direction of its longitudinal axis. 
         [0011]    The bearing bush  46  is of hollow cylindrical design and is arranged with a slight radial play on the middle area of the bearing pin  44 . The hollow cylindrical roller  42  is supported with a slight radial play on the bearing bush  46 . Here the roller  42  is arranged with its larger diameter in the middle area of the bore  48  and protrudes out through the open side of the middle area of the bore  48  towards the cam  22 . The bearing bush  46  is produced from a plastic material, preferably from polyether ether ketone (PEEK) or from polyphthalamide (PPA). In addition, polyimide, polyamide imide or polyphenylene sulfide (PPS) may also be used as plastic material. 
         [0012]    Fillers, which serve in particular to improve the emergency running characteristics of the bearing bush  46  in the event of inadequate lubrication, may be added to the plastic material. Carbon fibers, for example, and/or glass fibers and/or potassium titanate and/or polyaramid may be used as fillers. In addition, fillers which improve the anti-frictional characteristics of the bearing bush may be added to the plastic material. Solid lubricant particles, which may contain graphite, may be used as fillers for this purpose. Titanium dioxide and/or zinc sulfide and/or polytetrafluoroethylene may also be used as additives. 
         [0013]    The bearing bush  46  is preferably produced by an injection molding method, wherein no further production operation, or at least only a machining of the inside and/or outside diameter of the bearing bush  46 , is necessary following the injection molding process. Alternatively, the bearing bush  46  may also be produced by a method of pressing. In addition, the bearing bush  46  may also be produced from a bar material, from which portions of the required width are cut off.