Radial piston pump
BACKGROUND OF THE INVENTION 
     The present invention relates to a radial piston pump. More particularly, it relates to a radial piston pump in which base portions of pistons are in sliding engagement with an eccentric of a shaft of the pump. 
     Radial piston pumps in which base portions of pistons are in sliding engagement with eccentric of a pump shaft have been proposed in the art. In the known radial piston pumps, the base portions of the pistons are of one piece with the latter and are constituted by the same material. Since under the action of great bearing forces this material tends to wear, the piston is to be hydraulically equalized for reducing bearing forces between the piston base portions and the eccentric. For this purpose, the piston is provided with radial passage which communicates an inner chamber of a pump housing with the piston base portion, whereas in the eccentric region of the piston base portion a recess is provided, which substantially corresponds to the contact surface of the piston. Such hydrostatic unloading of the piston with the piston base portion and corresponding reduction of the pressure exerted by the piston base portion to the eccentric requires very high manufacturing accuracy, inasmuch as otherwise a working fluid enters the region between the piston base portion and the eccentric and, in addition to increased leakage, makes the hydrostatic unloading ineffective. This results in fast wear of the contact surfaces of the eccentric and piston base portions which are movable relative to one another. 
     It has been also proposed to produce the piston base portions as separate members, and to make the piston base portion of a material having good bearing characteristics, such as of a bronze alloy. In this case, the piston base portion is secured to the piston by means of screws. An additional hydrostatic unloading of the pistons with the piston base portions is thereby not needed. However, this construction also requires accurate manufacture in order to reduce the sliding friction between the piston base portions and the eccentric, since otherwise cavities formed on the piston base portions and the eccentric would result in fast wear of the same. The above-mentioned known bearing of the piston base portions with the eccentric requires also very fine filtering of the oil or the working fluid, in order to halt the wear of the surfaces of the piston base portions and the eccentric sliding relative to one another, in narrow limits so as to assure a practically justified service life of the bearing. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a radial piston pump which avoids the disadvantages of the prior art. 
     More particularly, it is an object of the present invention to provide a radial piston pump whose bearing of the piston base portions on the eccentric requires small manufacturing expenditures, has a high service life and does not require fine filtering of the oil. 
     In keeping with these objects, and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a radial piston pump having a housing, a shaft mounted for rotation in said housing and including an eccentric provided with a cam track, inlet and outlet ports in the housing, valve means installed in the ports, a plurality of radially reciprocable pistons spaced about the eccentric and each having a radially inner surface in sliding engagement with the cam track, in which pump a layer of synthetic plastic material having a low coefficient of friction is provided on each of the surfaces of the pistons, so as to reduce friction between the same and the cam track. In this construction bearing of the pistons on the eccentric requires small manufacturing expenditures, has a high service life, and does not require fine filtering of the oil. 
     Polytetrafluoroethylene may be utilized as the above-mentioned synthetic plastic material. The inner surface provided with the layer of this synthetic plastic material may be formed in a base portion of each of the pistons. A radially outer surface of each of the pistons may contact a radially inner surface of a respective one of valve elements provided in the valve means. The above-mentioned radially outer surfaces of the pistons may be provided in separate cylindrical members associated with the pistons. 
     Another feature of the present invention is that a sheet member is provided, which is secured to the base portion of each of the pistons, and the layer of the synthetic plastic material is secured to the sheet member. The sheet member may be bronze coated and secured to the base portion of the piston by means of glue. 
     A further feature of the present invention is that the sheet member has a thickness equal to substantially 1 millimeter. 
     A still further feature of the present invention is that a surface of the base portion of the piston facing toward the eccentric, and a surface of the sheet member facing toward the piston may be roughened. In this case, the facing surface of the base portion may be etched whereas the facing surface of the sheet member may be sand-blasted. 
     An additional feature of the present invention is that the cam track provided in the eccentric of the shaft may have a peak-to-valley height at most equal to 4 microns. 
     The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a partially sectioned plan view of a radial piston pump in accordance with the present invention; and 
     FIG. 2 is an enlarged view showing a piston with a piston base portion and an eccentric of the radial piston pump in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a rotary piston pump in accordance with the present invention wherein a pump shaft is identified by reference numeral 1 and has an eccentric identified by reference numeral 1a. Three radial bores 7 are provided in a housing 4 and uniformly distributed over a circumferential surface thereof. A plurality of pressure chambers 12 are formed between pressure valve bodies 10 and locking screws 8 or threaded sleeve members 8a. The threaded sleeve members 8a form pressure conduits. Channels 22a and 22b which are casted in the housing 4 communicate the pressure chambers 12 with one another. 
     The housing 4 bounds an inner chamber 13. An end portion 10d of each of the pressure valve bodies 10 facing toward the inner chamber 13 of the housing is ball-shaped. A cylindrical member 23 has a sealing surface 23a which sealingly abuts against the ball-shaped end portion 10d of a respective one of the pressure valve bodies 10. Therefore, the ball-shaped end portion 10d of the piston valve member 10 and the sealing surface 23a of the cylindrical member 23 together form a universal joint. Each of pistons 24 has a sliding member 26 which is in contact with a contact surface 1a1 of the eccentric 1a. The above-mentioned universal joint formed by the respective surfaces of the pressure valve bodies and the cylindrical members compensate for the manufacturing inaccuracy between the eccentric contact surface 1a1 and the piston sliding portion 26. 
     The sliding portion or a base piston 26 of each of the pistons has a surface 26a facing toward the eccentric, which is roughened, for instance by sand-blasting. A sheet member 40 is provided on the surface 26a of the piston base portion 26. A layer 40a of a synthetic plastic material such as polytetrafluoroethylene is provided on the sheet member 40. The sheet member 40 has a surface 40b facing toward the piston base portion, which surface is roughened, for instance by etching. The sheet member 40 is secured to the piston base portion 26 by means of gluing, for example by a layer of a synthetic plastic glue. The roughened surface 26a of the piston base portion 26 and the roughened surface 40b of the sheet member 40 assures durable securing of the sheet member 40 to the piston base portion 26 by means of the above-mentioned glue. 
     During polymerization of the synthetic plastic glue, the piston base portion 26 with the sheet member 40 are subjected to pressure and temperature. During this process a contact surface of the layer of the synthetic plastic material 40a, which is in operation in sliding engagement with an outer surface of the eccentric 1a, abuts against a jig which is shaped in correspondence with the shape of the eccentric. Therefore, exact correspondence of the contact surface of the layer of the synthetic plastic material 40a to the outer surfaces of the eccentric 1a of the shaft 1 of the pump is guaranteed. The contact surface 1a1 of the eccentric has the maximum peak-to-valley height equal to 4 microns, so as to provide a wearless sliding engagement between the layer of the synthetic plastic material 40a and the eccentric 1a. Slight roughness of the contact surface of the eccentric 1a is smoothed by slight wear of the layer of the synthetic plastic material 40a and thereby the contact surface 1a1 of the eccentric additionally assumes low friction characteristics. The layer of the synthetic plastic material constituted by polytetrafluoroethylene which is mixed in known manner with tetrafluoropropylene, guarantees that low friction sliding moment between the piston base portion and the eccentric is provided not only during hydrodynamic lubrication, but also during semi-dry start-up phase of the pump operation. The sheet member 40 which carries the synthetic plastic layer 40a has a thickness equal to substantially 1 millimeter, so that the sheet member can be mounted on the piston base portion without preforming and tension-free. 
     The piston 24 has an axial passage 24a which is in an operative communication with a groove 32 provided in the contact surface 1a1 of the eccentric and extending to the lowest reversal point, so that when the eccentric rotates between the upper dead point and the lower dead point in the direction shown by the arrow 33 a communication with the inner chamber 13 is established, which, in turn, communicates with a reservoir through an opening 33a. This rotation from the upper to the lower dead points of the eccentric corresponds to the suction stroke when the inner chamber of the pump is filled with working fluid through a not-shown valve located in a piston head 24a. During the subsequent discharge stroke of the piston when the piston is displaced in the direction of the pressure valve members 10, the communication between the axial passage 24a and the inner chamber 13 is interrupted. In this region of the eccentric 1a there is no groove, so that the entire contact surface 1a1 of the eccentric 1a is available for transmission of the operative force to the piston 24. 
     It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
     While the invention has been illustrated and described as embodied in a radial piston pump, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.