Abstract:
A hydraulic cylinder unit is provided with a rod seal and/or piston seal which has a surface that deviates from the vertical circular cylindrical surface of the piston head throughhole through which the piston rod passes or the vertical circular cylindrical surface of the cylinder interior while forming respective converging gaps with regard to the rod surface and the interior surface of the cylinder, respectively.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a hydraulic cylinder unit, specifically a hydraulic cylinder unit with a cylinder tube having an inner surface with a piston that is guided thereon and that has an outer surface, with a piston rod joined thereto whose rod surface is guided on a cylinder head that closes the rod-side cylinder space, with at least one rod seal arranged therein that seals the piston rod-side cylinder space from the area of the cylinder unit between the rod surface and the cylinder head by means of a rotationally symmetrical inner seal surface, and with at least one piston seal arranged in the piston that seals the piston-rod side cylinder space and the cylinder space formed by the cylinder floor and the piston between the cylinder space inner surface and the outer surface of the piston by means of a rotationally symmetrical outer seal surface.  
         [0002]     Especially low-friction cylinder units are of interest for various purposes. Among these are cylinders for suspension and steering functions, or working cylinders subject to high demands in terms of sensitivity and positionability. The effect of increased friction is a poor ratio of effective force, that is working force of the cylinder, to theoretical pressure force. Among other things, this leads to the fact that the cylinder must be designed larger than theoretically necessary in order to provide adequate effective force.  
         [0003]     In suspension cylinders, the frictional force acts like additional damping. However, the greater the basic damping of the cylinder itself, the lower the portion that can be effectively influenced in the control. However, it is precisely the option for influencing damping that is the basis for a modem, active suspension and damping system.  
         [0004]     In addition to relatively high friction, the ratio of static friction to sliding friction is also of interest because major differences between the two values can lead to undesired oscillations or vibrations (so-called stick/slip effect).  
         [0005]     Finally, unsatisfactory friction values and relatively wide variance of known hydraulic cylinder units in series are also disadvantageous.  
         [0006]     The object of the invention is therefore to design a hydraulic cylinder unit such that it is particularly low in friction.  
       SUMMARY OF THE INVENTION  
       [0007]     This object is inventively attained in a hydraulic cylinder unit in accordance with the invention in that the rod seal and/or piston seal (hereinafter referred to as seal) has, at least by region, on its inner and outer seal surfaces (hereinafter seal surfaces) a shape that deviates from the surface of a vertical circular cylinder while forming a converging gap with regard to the rod surface and the inner surface of the cylinder tube.  
         [0008]     The inventively provided rotationally symmetrical rod seal thus is in contact along a circumferential closed line, hereinafter referred to as the inner equator, on its inner seal surface with the outer surface of the piston, and/or the likewise rotationally symmetrically embodied piston seal is thus in contact along a likewise circumferential closed line, hereinafter referred to as the outer equator, on its outer seal surface with the inner surface of the cylinder tube. The line can also form a certain width while creating a circumferential contacting surface. Thus, as the distance from the equator to the gaps converging in the direction of the equator decreases, the free cross-section of the gaps for the passage of the displaced hydraulic fluid preferably grows increasingly smaller. In this, the shape of the region that is adjacent to each gap and that deviates from the shape of the surface of a vertical circular cylinder can be embodied curved not only as a result of its rotational symmetry in the circumferential direction but also in the axial direction. However, a gap embodied in a wedge-shape without additional curvature is also possible.  
         [0009]     The inventive principle thus utilizes the hydrodynamic effect of the hydraulic liquid in the converging gap in order to minimize the frictional forces between preferably the piston seal and the cylinder tube on the one hand and the rod seal and the piston rod on the other hand. Although the qualities of the running surface (roughness, material, surface treatment) are not the subject of the invention, it must be embodied such that, in combination with the running surface (inner surface of the cylinder tube or outer surface of the piston rod), with the seal it ensures optimum friction and wear behavior.  
         [0010]     It has been found that the frictional force or damping properties of the hydraulic cylinder unit in accordance with the invention can be substantially reduced by using the piston seal and/or rod seal. Numerous possibilities and advantages result from this. Thus, the reduction in the size of the cylinder due to improved force utilization can lead to more numerous employment possibilities and to savings in costs. Furthermore, the improvement in the properties of suspension cylinders, due to reduced basic damping, leads to the fact that a greater portion is available for actively influencing system damping. In working cylinders, controllability and sensitivity can be markedly enhanced. In addition, new areas of application become available for the inventive cylinder units for cylinders that were previously not suitable due to inadequate effective force, frictional forces and thus interfering forces that were too high, or damping that was too high. Double-acting cylinders can also act as the hydraulic cylinder unit.  
         [0011]     In one advantageous embodiment of the invention, the piston seal has two largely wedge-shaped gaps that deviate from the surface of a vertical circular cylinder, of which the one gap is open to the cylinder space and the other is open to the rod-side cylinder space.  
         [0012]     In accordance with the invention, the gap for the rod seal is embodied asymmetrical with respect to its center plane and open to the piston rod-side cylinder space so that at least in the outward-moving direction of the piston rod the hydrodynamic friction-reducing floating effect results.  
         [0013]     If in addition an additional seal is arranged between the rod seal and the area of the cylinder unit and between it and the rod seal a recess acting as a reservoir for the hydraulic fluid that is carried out is arranged in the cylinder head, the extra hydraulic fluid carried out as a lubricating film can be collected there and the hydrodynamic friction-reducing floating effect can also be produced in the inward-moving direction of the piston rod.  
         [0014]     One exemplary embodiment of the invention is explained in greater detail in the following, with reference to the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  depicts a schematic section through a cylinder unit, in partial break-away;  
         [0016]      FIG. 2  is detail II in accordance with  FIG. 1 , using a larger scale;  
         [0017]      FIG. 3  is detail III in accordance with  FIG. 1 , using a larger scale;  
         [0018]      FIG. 4  is  FIG. 2  using a larger scale. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The cylinder unit illustrated in  FIG. 1  and labeled  5  overall has a cylinder tube  6  on whose inner surface  7  a piston  8  having an outer surface  9  is guided. On the piston, a piston rod, labeled  10  overall, is joined to a rod surface  11  that extends out of the cylinder unit  5  via a cylinder head labeled  12  overall that is driven out when hydraulic fluid that is under pressure acts upon the cylinder space  14  that is formed between the piston  8  and the cylinder floor  13 . If the cylinder unit  5  is embodied with double action, pressure can also act on the annular piston rod-side cylinder space  15  that is formed by the piston rod  10 , the cylinder head  12 , and the piston  8 , driving the piston rod  10  in.  
         [0020]     A piston seal labeled  16  overall is provided on the piston  8  between the cylinder space  14  and the piston rod-side cylinder space  15 . Correspondingly, a rod seal labeled  17  overall is provided on the cylinder head  12  between the external area of the cylinder unit  5  and the piston rod-side cylinder space  15 .  
         [0021]      FIG. 2  illustrates the piston seal  16  in greater detail. This piston  8  has three circumferential grooves  181 ,  182  spaced at intervals from one another on the outer surface  9  of the piston. A guide ring  19  is inserted in each of the two external, outwardly open grooves  181 . Provided in the center groove  182  is the seal that is labeled  16  overall and that is arranged from two parts, specifically the piston seal  161  embodied as a sliding and sealing element and the pre-stress element  162  arranged thereunder, e.g. in the form of an elastomer ring that when installed assures that the sliding and sealing element  161  exerts a certain basic pressure against the cylinder&#39;s inner surface  7 .  
         [0022]     The piston seal  161  has an outer seal surface  163  that is symmetrically embodied with respect to the center plane  164  ( FIG. 4 ) of the piston seal  161 , while forming with respect to the inner cylinder surface  7  of the cylinder tube  6  two converging gaps  165  ( FIG. 4 ), deviating from the surface of a vertical circular cylinder. The highest point and at the same time the longest circumferential, closed line of contact, the outer equator  166 , between the seal outer surface  163  and the inner surface  7  of the cylinder tube  6  is located in the region of the center plane.  164 . Thus, as the distance from the outer equator  166  to the two gaps  165  that converge in the direction of this outer equator  166  decreases, the free cross-section of the gaps for the passage of the displaced hydraulic fluid grows increasingly smaller. In the exemplary embodiment depicted, the region that is adjacent to the gap  165  and that deviates from the shape of the surface of a vertical circular cylinder is embodied in a wedge-shape with curvature in the axial direction. The wedge-shaped gap  165  between the outer seal surface  163  and the inner cylinder surface  7  of the cylinder tube  6  results in a hydrodynamic effect such that the piston seal  161  lifts from the hydraulic fluid carried into the wedge-shaped gap  165  over the highest point, the outer equator  166 , that is, the seal outer surface  163 , which is overall ball-shaped, lifts from the inner cylinder surface  7  of the cylinder tube  6 . This substantially reduces the mechanical frictional forces of the seal.  FIG. 4  depicts this part of the piston  8  from  FIG. 2  in greater detail.  
         [0023]     The rod seal, labeled  17  overall, that is inserted in the cylinder head  12 , is illustrated in greater detail in  FIG. 3 . In interiorly situated circumferential grooves that are spaced at intervals from one another, a wiping element  20  is provided in the outermost groove and a rod guide  21  is provided in the groove next closest to the piston rod-side cylinder space  15 . The rod seal  17  is likewise provided in two parts, with a seal  171  embodied as a sliding and sealing element and with a pre-stress element  172  located thereunder that corresponds to the element  162  already discussed in accordance with  FIG. 2 .  
         [0024]     Deviating from the piston seal  161 , the inner seal surface  173  of the rod seal  171  is embodied asymmetrical with respect to the center plane and has either only one wedge-shaped gap or two gaps with different gap angles and/or length. In the former case the gap is preferably open to the piston rod-side cylinder space  15 .  
         [0025]     An additional seal labeled  22  overall is arranged between the rod seal  171  and the outer area of the cylinder unit. Arranged between the latter and the rod seal  171  is a recess  23  in the cylinder head that acts as a reservoir for the hydraulic fluid, in which recess is collected hydraulic fluid that is carried out as a lubricating film under the rod seal  171  during the outward stroke of the piston rod. During the subsequent inward stroke of the piston, this quantity of oil can be carried with its piston rod using the return ability of the rod seal back into the rod-side cylinder space  15 .  
         [0026]     The asymmetrical shape of the seal inner wall  173  is provided in order to promote the return using the rod seal  171 , this shape ensuring the return of the quantity of hydraulic fluid previously carried out.  
         [0027]     The piston seal and the rod seal may be comprised of a polyurethane or a dimensionally stable material, such as polytetrafluoroethylene, polyamide, polyethylene or polyoxymethylene. These polymers may be admixed with a filler, such as subdivided (e.g., particulate or powdered) bronze or graphite, before being fabricated into the seals.