Abstract:
A pressure medium-actuated working cylinder axially displaceable between two end positions in a cylinder space, with opposite changes in, the volumes of two cylinder chambers on its two sides. A damping element is arranged on one side of a piston. A passage aperture is located between one cylinder chamber and a cylinder connection when the piston runs into one end position forming an annular throttle gap with the passage aperture for outflow of pressure medium. For a high damping capacity, the outer surface of the damping element is shaped to have a maximum diameter at the beginning of the passage aperture and, after a surface section, a small diameter, A middle diameter lies between the maximum diameter and the small diameter.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a pressure medium-actuated working cylinder in which the piston, on running into an end position, is braked by throttling of the pressure medium outflow from the shrinking cylinder chamber. As a result of the throttling of the output flow of pressure medium a pressure is built up in the shrinking cylinder chamber that generates a force on the piston that is directed against the movement of the piston. 
     What is referred to as the damping pressure building up in the cylinder chamber should, in this case, not exceed a maximum value which is from 1.5 times to twice as high as the nominal pressure of the working cylinder. On the other hand, the working cylinder has maximum damping capacity if the damping pressure has the maximum value throughout the damping stretch. Even theoretically, this ideal course of the damping pressure can only be achieved by the formation of the throttle cross sections and the throttle length between the damping element and the passage aperture if the same framework conditions are always maintained, in other words if the working cylinder, for example, is always moved at the same speed and moves the same mass. An attempt is then made, for the case of maximum speed and maximum mass, to obtain the ideal end position damping, so that the damping pressure no longer reaches the maximum value at lower speeds and lower masses. 
     Pressure medium-actuated working cylinders with end position damping are known from a number of publications. Thus, for example, EP 0 837 250 A2 shows a working cylinder in which the damping element has throttle grooves extending axially on its outer surface and tapering in their cross section. The throttle cross section over the throttle grooves becomes smaller and smaller as the damping element is inserted into the passage aperture. In addition to the throttle grooves, after the damping element is inserted into the passage aperture between the cylinder chamber and the cylinder connection, a pressure medium connection is switched on via a throttle point whose hydraulic resistance is largely independent of the depth of insertion of the damping element. 
     DE-OS 22 14 032 recites a pressure medium-actuated working cylinder. In this type of working cylinder, the outer surface of the damping element is rotationally symmetrical. In the known working cylinder, an entry whose effect is negligible for the end position damping, is adjoined by a surface section with a smaller diameter, which is followed approximately from the center of the damping element by a surface section with a larger diameter, which extends to the piston end of the damping element. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to develop a pressure medium-actuated working cylinder so that a high damping capacity is achieved, in other words so that a large mass can be braked over a short travel, without any expectation of damage caused by pressure peaks. 
     This object is achieved with a pressure medium-actuated working cylinder which additionally has the features of the invention. In a working cylinder according to the invention, then, viewed in the direction of insertion of the damping element into the passage aperture, the damping element has, before the section of smaller diameter, an average diameter which lies between the maximum diameter at the piston end of the damping element and the smaller diameter. This prevents the damping pressure falling rapidly again after a sharp rise at the start of insertion of the damping element into the passage aperture, so that it remains at a high level. The average diameter exists only over a short stretch relative to the length of the surface section of small diameter, which prevents the damping pressure exceeding the maximum admissible pressure, in other words prevents the working cylinder being damaged by pressure peaks. It has been found that, with the damping element constructed according to the invention, a course of the damping pressure close to the ideal curve can be achieved for a particular speed and mass. 
     According to features of the invention surface sections with, respectively, a fixed large diameter, a fixed small diameter and a fixed average diameter are provided. The diameter of the damping element thus does not change continuously during progression in the axial direction. The second surface section, according to feature of the invention, advantageously makes a transition into further surface sections with a diameter changing continuously during axial progression, into the first surface section and into the third surface section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of embodiment of a pressure medium-actuated working cylinder according to the invention, and a diagram in which, for various speeds, the damping pressure has been plotted over the damping travel, are shown in the drawings. The invention is now explained in detail with reference to these drawings. 
     In the drawings: 
     FIG. 1 shows a longitudinal section through a hydraulic working cylinder according to the invention, 
     FIG. 2 shows a longitudinal section, turned through 90° relative to FIG. 1, through a damping bush used in the working cylinder shown in FIG. 1, 
     FIG. 3 shows a section of FIG. 2 with a partially vertically exaggerated representation of the outer surface of the damping bush, and 
     FIG. 4 is a diagram in which the measured damping pressure is plotted over the damping travel for two piston speeds. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The hydraulically operated working cylinder shown in FIG. 1 is a cylinder of what is known as the circular construction type. The cylinder housing  10  has, as essential components, a cylinder tube  11  and a cylinder head  12  placed on one end and a cylinder base  13  placed on the other end of the cylinder tube  11 . To fix the cylinder tube, cylinder head and cylinder base to one another, a flange  14  is screwed onto each of the two ends of the cylinder tube  11 , provided with an outer thread, the flange  14  having threaded axial holes distributed over 360° into which screws  16  tensioning the cylinder head and the cylinder base, respectively, against the cylinder tube are screwed. 
     In the interior of the cylinder tube  11 , a piston  20  is guided to slide axially in close contact and divides the interior of the cylinder tube into two cylinder chambers  21  and  22  whose volumes change in opposite directions when the piston moves. Hydraulic pressure medium can be fed to the cylinder chamber  21  and removed from that cylinder chamber via a cylinder connection  23  in the cylinder head  12 . The radially arranged cylinder connection  23  here opens initially into a chamber  24  in the cylinder head  12 , which chamber  24  is in fluid connection with the cylinder chamber  21  via an axial passage aperture  25  of a particular diameter. Similarly, a pressure medium path runs from a radial cylinder connection  26 , a chamber  27  and an axial passage aperture  28  in the cylinder base  13  to the cylinder chamber  22 . The two passage apertures  25  and  28  in the cylinder head and in the cylinder base, respectively, have the same diameter. 
     The piston  20  is combined with a piston rod  35 , which emerges to the outside through the cylinder head  12  and converts the chamber  24  and the passage aperture  25  of the cylinder head  12  to annular spaces. The piston  20  is pushed from the inner end via a reduced-diameter section of the piston rod  35  and tensioned against a shoulder of the piston rod  35  with the interposition of a flanged bush  36  and with the aid of a nut  37  screwed onto the threaded end of the piston rod  35 . 
     A damping bush  40  is arranged axially between the piston  20  and the flange  38  of the flanged bush  36 , on the latter and with axial and radial play, performing the function of a throttle body and of a return valve body. An identical damping bush  40  is arranged with axial and radial play between the piston  20  and a flange  39  of the nut  37 . 
     The shape of the damping bushes is more clearly apparent from FIGS. 2 and 3. A damping bush  40  has a constant internal diameter over its entire length, with the exception of a turned recess  41  on its end face  42  facing the piston  20 , this internal diameter being adapted to the external diameter of the flanged bush  36  and of the nut  37  so that a radial play of, for example, 0.5 mm results. In terms of length, the damping bush  40  is, for example, 0.3 mm shorter than the clear distance between the piston  20  and a flange  38 ,  39 . On its end face  42 , the damping bush has two diametrically opposite recesses  43  in the form of circular segments which are not quite as deep as the turned recess  41 . On the end face  44  remote from the piston  20 , the damping bush has a run-up ramp  45 , which guarantees that the damping bush is threaded into the passage aperture  25  or  28 , despite the radial play. Even if the run-up ramp  45  is omitted, the diameter of the outer surface  50  of the damping bush  40  is not constant over its length. The diameter is largest in a first surface section  51  beginning directly at the end face  42 . In the example of embodiment considered, the diameter in the surface section  51  is 30 μ less than the 48 mm diameter of the passage holes  25  and  28 . Axially, the first surface section  51  extends further, about 1 mm in the present example of embodiment, from the end face  42  of the damping bush  40  than the recesses  43 . In a second surface section  52 , the outer surface  50  of the damping bush  40  has a constant smallest diameter over a length of about 8 mm which is approximately 110 μ less than the diameter of the passage apertures  25  and  28 . Furthermore, a third axially extending surface section  53  with a constant diameter is provided. Specifically, the diameter in the surface section  53  is about 80 μ less than the diameter of the passage apertures  25  and  28 . The surface section  53  directly adjoins the run-up ramp  45  and extends over a length of approximately from 1 to 2 mm. Its diameter lies between the diameters in the surface section  52  and in the surface section  51 . Between the two surface sections  52  and  53  is a frustum-like surface section  54 , in which the diameter increases from the diameter in surface section  52  to the diameter in the surface section  53 . Finally, in a frustum-shaped surface section  55 , the diameter of the outer surface  50  of the damping bush  40  increases from the diameter in the surface section  52  to the diameter in the surface section  51 . In this case, the surface section  55  is axially longer than the surface section  54 . Its length is approximately from 6 to 7 mm, while the surface section  54  is only approximately 3 mm long. The overall length of the damping bush in the present case is approximately 24.5 mm. 
     In the position of the piston  20  shown in FIG. 1, the damping bush  40  is axially seated on the flange  39  of the nut  37 . If pressure medium is now fed to the cylinder connection  26 , the damping bush  40  is displaced by the force generated by the rising pressure by the amount of its axial play toward the piston  20  until its end face  42  rests against the piston. Pressure medium can now flow through the axial gap between the end face  44  of the damping bush  40  and the flange  39  of the nut  37 , through the radial gap, existing as a result of the radial play, between the damping bush  40  and the nut  37 , and through the recesses  43  of the damping bush  40  into the cylinder chamber  22 . The hydraulic resistance of the flow path described along the inner wall of the damping bush  40  is much less than the hydraulic resistance between its outer surface and the wall of the passage aperture  28 . The piston  20  now moves toward the cylinder head  12  at a speed corresponding to the volume of pressure medium flowing in via the cylinder connection  26 , pressure medium from the shrinking cylinder chamber  21  being forced via the passage aperture  25  and the cylinder connection  23 . At a determined distance between the piston  20  and the cylinder head  12 , the other damping bush  40 , which is guided on the flanged bush  36 , begins to become inserted into the passage aperture  25 . As a result, the flow cross section available for the outflow of pressure medium from the cylinder chamber  21  through the passage hole  25  is reduced. The pressure in the cylinder chamber  21  thus becomes higher than the pressure in the chamber  12  and in the cylinder connection  23  of the cylinder head  12 , so that the damping bush  40  is moved away from the piston  20  onto the flange  38  of the flange bush  36  and rests with its end face  44  on the flange. Like the movable body of a return valve, the damping bush  40  thus blocks the flow path along the radial gap between itself and the flange bush  36 . The surface section  53  of the damping bush  40  then enters the passage aperture  25 . The flow path along the outside of the damping bush  40  becomes very narrow, and the pressure in the cylinder chamber  21  rises relatively quickly. After only a short further travel of the piston  20 , of course, the surface sections  54  and  52  of the damping bush  40  enter the passage aperture  25 , which prevents the damping pressure in the cylinder chamber  21  rising beyond the intended maximum value. As a result of the rising damping pressure in the cylinder chamber  21 , the piston  20  is braked. The surface sections  55  and  51  of the damping bush  40  then enter the passage aperture  25 , as a result of which the hydraulic resistance along the flow path on the outside of the damping bush  40  once again rises sharply and a damping pressure close to the maximum pressure is maintained in the cylinder chamber  21 , although, because the speed of the piston  20  is now already low, the volume of pressure medium to be forced out of the cylinder chamber  21  per unit of time is also low. Finally, the piston  20  arrives at very low speed in its end position on the cylinder head  12 . Travel out of this end position into the starting position shown in FIG. 1 takes place in a similar way to travel out of the starting position. 
     In the diagram shown in FIG. 4, various curves are illustrated which show a damping pressure in a cylinder chamber plotted over the damping travel, the zero point of the damping travel being located at the start of the insertion of the damping bush into a passage aperture. The broken-line curve  60  represents an ideal damping curve. The damping pressure rises quickly to the maximum value, remains at that value almost throughout the entire damping travel, and falls sharply only at the end. The curve  61  is calculated on the basis of certain framework conditions, such as for example a maximum speed of the piston, and assuming a particular mass of a damping bush formed according to the invention. The curve  62  has been plotted in an experiment based on the same framework conditions as were assumed in calculating the curve  61 , in particular the same speed of the piston and the same moved mass. The curve  63  has been plotted with a lower piston speed at the beginning of damping. The speed here was approximately 300 mm/s, while the plotting of the curve  62  was based on a speed of 500 mm/s. It is noticeable that the damping pressure rises less quickly at lower speed, with the same damping bush and the same passage aperture. This is not at all surprising, since in accordance with the lower speed, a lower volume of pressure medium is initially forced through the throttle flow path between the damping bush and the wall of the passage aperture.