Patent Abstract:
A stroke piston for a stroke piston compressor for generating compressed air for a vehicle, such as a commercial vehicle, includes a piston body with a piston floor, a piston skirt with piston skirt walls and pin hubs for receiving a piston pin, and at least two piston rings, which can be inserted into circumferential ring grooves of the piston body. Ridges of the piston body are provided between each two ring grooves and between the piston floor and the first ring groove arranged closest to the piston floor and the ring groove arranged closest to the piston skirt facing away from the other ring groove or grooves. At least one of the ridges is recessed at least over a partial section of its ridge height, which is measured parallel to the axis of the piston body, with at least one recess relative to the diameter of the piston body.

Full Description:
FIELD OF THE INVENTION 
     The invention generally relates to a reciprocating (stroke) piston for a reciprocating piston compressor for generating compressed air for a vehicle, especially, a commercial vehicle. 
     BACKGROUND OF THE INVENTION 
     In a piston compressor, the reciprocating piston comprises a piston body having a piston head, a piston skirt with piston skirt walls and preferably pin bosses for receiving a gudgeon pin. At least two, preferably three, piston rings, can be introduced into encircling ring grooves in the piston body, of which at least the piston ring closer to the piston head is a compression ring and at least one piston ring, preferably the piston ring closest to the piston skirt, is an oil control ring. Lands are provided on the piston body between each pair of ring grooves and between the piston head and the ring groove arranged closest to the piston head, and on that side of the ring groove arranged closest to the piston skirt that faces away from the other ring groove or grooves. 
     In the case of a reciprocating piston compressor of the general type under consideration, there may be an unpleasant or toxic discharge of oil into the air. This discharged oil, which is carried in the air, can cause contamination of the systems supplied with this air or even of other, downstream systems, which can lead to problems that can be rectified only with great difficulty in servicing terms, and which can shorten the life of the systems. Also, this contaminated air may cause increased environmental pollution. 
     SUMMARY OF THE INVENTION 
     Generally speaking, it is an object of the present invention to improve the cleanliness of the air delivered in respect of the oil discharge described above. 
     According to an embodiment of the present invention, in a reciprocating piston of the general type under consideration, at least one of the lands of the piston body is set back from the diameter of the piston body by at least one recess, at least over a partial section of its land height, measured parallel to the axis of the piston body. This results in an overall improvement in the sealing behavior of the reciprocating piston. In particular, it is possible to achieve compensating volumes or flow cross sections in the zone of the piston rings (ring zone), by which it is possible to set specific ring interspace pressures and/or by which the flow behavior of oil-containing air in respect of oil discharge from the drive-side space of the reciprocating piston compressor is made more difficult. 
     According to an embodiment of the present invention, lands of the piston body are configured with different geometries by means of recesses and/or configured differently by means of recesses. By this, the flow cross sections of the reciprocating piston can be varied particularly well in order to improve the capability of the piston to prevent leaks in respect of any possible oil discharge or transfer or passage. The “fire land” of the reciprocating piston, which is situated between the piston head and the piston ring arranged closest to it, can also be included in these measures. 
     The recess in one of the respective affected lands (i) can be or have a step, (ii) can be or have a groove encircling the piston body, and/or (iii) can be or have a chamfer encircling the piston body. 
     A chamfer of this kind can be adjacent to a ring groove and/or can face or face away from the ring groove. 
     According to another embodiment, at least one recess can encircle the piston body in a rotationally asymmetrical and/or eccentric manner with respect to the axis of the piston body. In such case, different dimensions of the asymmetry and/or eccentricity can be provided, especially in the direction of connecting rod oscillation, in relation to the gudgeon pin direction of the reciprocating piston. 
     The orbital path of the recess can substantially follow an oval or an elliptical shape or a free form. 
     Irrespective of any asymmetry or eccentricity in respect of the overall orbital path of the recess, at least one recess can encircle the piston body with an inconstant recess depth, i.e., to have continuous or even section-wise differences in radial recess depth. It is ultimately also possible to achieve asymmetry or eccentricity itself by means of an inconstant recess depth. 
     For the capability of the reciprocating piston to prevent leaks, measures are also preferably taken as regards the associated piston rings or the selection thereof. It is possible for at least one piston ring to be designed as a compression ring, and for a taper-faced Napier ring to be provided for this purpose. 
     At least one other piston ring can be an oil control ring, and, for this purpose, can be a coil spring loaded slotted oil control ring, preferably a coil spring loaded double beveled oil control ring or a coil spring loaded beveled edge oil control ring. As a preferred option, two compression rings and one oil control ring can be used in order to maintain redundancy, particularly, in respect of the compression effect, if a piston ring breaks, for example. 
     According to a further embodiment, the coil spring loaded oil control ring provided as an oil control ring has at least two outward-projecting lands. Oil tightness in this region is thereby advantageously improved. 
     Preferably, the lands have land heights that, when measured parallel to the axis of the piston ring, correspond jointly to no more than 20% of the height of the piston ring and correspond individually to no more than 10% each of the height of the piston ring. However, further improvements can be achieved if the lands have land heights that, when measured parallel to the axis of the piston ring, are different. 
     In the case of a coil spring loaded double beveled oil control ring or coil spring loaded beveled edge oil control ring, which are provided as preferred options, the lands have connection angles, preferably, of 0 to 60 degrees. Connection angles of the lands could furthermore have a different and/or asymmetric angle size. 
     According to another embodiment, the oil control ring has at least one oil drainage facility, preferably, a plurality of oil drainage facilities, for the radial passage of oil. The oil drainage facility (facilities) can be embodied as a hole (holes) or as a slot (slots), for example. Oil transfer is thereby advantageously further reduced. 
     If, in addition, the ring groove provided for an oil control ring has at least one oil drainage facility, preferably, a plurality of oil drainage facilities, this respective oil drainage facility can be closed with respect to a hollow interior of the piston body. That is, for example, it can be designed to resemble a bowl or hay, or can lead into a hollow interior of the piston body, and, at the same time, an oil drainage facility of the oil control ring can preferably correspond at least partially to at least one oil drainage facility of the ring groove. The oil drainage facility of the ring groove can extend in the form of a slot in the circumferential direction of the ring groove. 
     Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
     The present invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, all as exemplified in the constructions herein set forth, and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below using exemplary embodiments and with reference to the accompanying drawing figures, in which: 
         FIG. 1  shows a section through a reciprocating piston according to an embodiment of the present invention; 
         FIG. 2  shows the detail  1   b  in  FIG. 1  on an enlarged scale; 
         FIG. 3  shows a cross section through a compression ring designed as a taper-faced Napier ring; 
         FIGS. 4   a ) to  d ) show further illustrative embodiments of details according to  FIG. 2 ; 
         FIGS. 5   a ) to  d ) and  FIGS. 6   a ) to  d ) show different possibilities for recess shapes in lands according to  FIG. 1 or 4 ; 
         FIG. 7  shows a detail of a land with a recess in section; 
         FIG. 8  shows a circumferential line of the recess according to  FIG. 7  in a plan view of the section line indicated by A, A in  FIG. 7 ; 
         FIGS. 9   a ) to  c ) show an exemplary oil control ring in various sectional and detail views; 
         FIG. 10  shows a sectional view of another illustrative embodiment of an oil control ring; and 
         FIG. 11  and  FIG. 12  show two different examples of a ring groove for an oil control ring in section. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a section through a reciprocating piston according to an embodiment of the present invention. The reciprocating piston has a piston head  1   a , pin bosses  1   c , a piston skirt with loadbearing piston skirt walls  1   d  and box walls  1   e , which connect the piston skirt walls  1   d  to the pin bosses  1   c.    
     Moreover, the reciprocating piston has a ring zone  1   b  with three ring grooves, which are described in greater detail below. However, as shown in  FIG. 1 , some of the lands delimiting the ring grooves can have recesses. 
       FIG. 2  show the ring zone  1   b  according to  FIG. 1  on an enlarged scale. The ring grooves are numbered as a first ring groove  1 , a second ring groove  2  and a third ring groove  3 , starting from the piston head  1   a . The lands delimiting the three ring grooves are likewise numbered consecutively as a first land  3   a   1 , a second land  3   a   2 , a third land  3   a   3  and a fourth land  3   a   4 . The first land  3   a   1  is the “fire land”. Lands  3   a   2  to  3   a   4  show examples of encircling recesses  3   b , which are set back or notched radially relative to the outer circumferential surfaces  3   c  of the reciprocating piston. In the illustrative embodiment shown in  FIG. 2 , the recesses  3   b  are each set back in the form of steps. 
     The first ring groove  1  and the second ring groove  2  are provided for compression rings, which, as shown in section by way of example in  FIG. 3 , can be designed as taper-faced Napier rings. 
     The third ring groove  3  is provided for an oil control ring of the kind shown, for example, in  FIG. 9 or 10 . 
       FIGS. 4   a ) to  d ) show further possible illustrative embodiments of a ring zone  1   b , in which the respective lands  3   a   2  to  3   a   3  show different possible cross sectional shapes of recesses  3   b . The fire land  3   a   1  can also have a recess. The lands  3   a   1  to  3   a   4  shown in  FIGS. 4 a   ) to  d ) can also be varied independently of one another. Here, the reference signs correspond to those in  FIG. 2 . 
       FIGS. 5   a ) to  d ) show further possible cross sections of the recesses  3   b , in particular of the land below the compression piston ring grooves (first ring groove  1  and/or second ring groove  2 ). 
       FIGS. 6   a ) to  d ) show further possible cross sections of the recesses  3   b , in particular for the land below the oil control piston ring groove (third ring groove  3 ). 
     In  FIG. 5 a   ), the diameter ØY of the recess  3   b  is set back in the form of a step from the diameter ØX of the piston skirt or piston skirt wall  1   d  (see  FIG. 1 ), i.e., the diameter ØX of the piston skirt is greater than the diameter ØY of the recess  3   b . The configuration in  FIG. 5 b   ) is similar to that according to  FIG. 5 a   ), but, here, a chamfer at an angle α is provided between the diameter ØX and the diameter ØY. According to  FIG. 5 c   ), the recess  3   b  is in the form of a groove, wherein the diameter ØY of the groove is less than the diameter ØX of the piston skirt.  FIG. 5 d   ) shows a combination of  FIG. 5 a   ) or  5   b ) and  FIG. 5 c   ), in which both the diameter ØZ of a groove of the recess  3   b  and a region of the recess situated below the groove and having a diameter ØY are less than the diameter ØX of the piston skirt. 
     According to  FIG. 6 a   ), the diameter ØA of the recess  3   b  is set back from the diameter ØB of the piston skirt or of the piston skirt wall  1   d  (see  FIG. 1 ) in the form of a step, i.e., the diameter ØB of the piston skirt is greater than the diameter ØA of the recess  3   b . The configuration in  FIG. 6 b   ) is similar to that according to  FIG. 6 a   ), but, here, a chamfer at an angle β relative to the third ring groove  3  is provided at the diameter ØB.  FIG. 6 e   ) is similar to  FIG. 6 a   ), but, here, the depth of the recess  3   b  is less than in  FIG. 6 a   ),  FIG. 6 d   ) is a combination of  FIG. 6 a   ) or  6   c ) and  FIG. 6 b   ). Here, the diameter ØB of the piston skirt is once again greater than the diameter ØA of the recess  3   b  and, in addition, a chamfer at an angle β is arranged between the diameter ØB of the piston skirts and the diameter ØA of the recess  3   b.    
       FIG. 7  shows a cross sectional view of any land between ring grooves having a recess  3   b  with a section line A, A. If this section line A, A is viewed in accordance with  FIG. 8 , it is clear from this example that the recess  3   b  can also be formed asymmetrically and/or eccentrically with respect to the axis of the reciprocating piston. That is, the cutting depth of the recess  3   b  can vary in size, for example, in the course of its orbit. In particular, an oval shape or elliptical shape of the recess orbit is conceivable. Here, the direction of the gudgeon pin is indicated by an arrow K and the direction of connecting rod oscillation is indicated by an arrow P. 
       FIGS. 9 and 10  show in section, on the one hand, an example of a coil spring loaded double beveled oil control ring ( FIG. 9 ) and, on the other hand, of a coil spring loaded beveled edge oil control ring ( FIG. 10 ) as a possible embodiment of an oil control ring. The examples of an oil control ring each have a main body  5   d  or  9   d  with two radially projecting lands  5   a, b  and  9   a, b . Moreover, both illustrative embodiments have a spring element  5   e  or  9   e . Respective holes  5   c  and  9   c  are provided between the lands  5   a , hand  9   a, b  as oil drains in both examples. As  FIGS. 5 b   ) and  5   c ) show, these oil drains can be designed as holes  5   c   1  distributed over the circumference of the oil control ring or, alternatively, as more extended slots  5   c   2 . 
     The coil spring loaded double beveled oil control ring shown in  FIG. 9  can, for example, be of a geometrical design such that the land height  5   d   1  of land  5   a  added to the land height  5   d   2  of land  5   b  is less than 20% of the ring height  5   f  of the overall coil spring loaded double beveled oil control dog ring, wherein the respective land heights  5   d   1  and  5   d   2  are each only 10% of the ring height  5   f . If the ring height  5   f= 4 mm, for example, it follows that the land height  5   d   1  of land  5   a  and the land height  5   d   2  of land  5   b  must each be less than 0.4 mm. 
     The coil spring loaded beveled edge oil control ring shown in  FIG. 10  can, for example, be of a geometrical configuration such that the land height  9   d   1  of land  9   a  added to the land height  9   d   2  of land  9   b  is less than 20% of the ring height  9   f  of the overall coil spring loaded double beveled oil control ring, wherein the respective land heights  9   d   1  and  9   d   2  are each only 10% of the ring height  9   f . If the ring height  9   f= 4 mm, for example, it follows that the land height  9   d   1  of land  9   a  and the land height  9   d   2  of land  9   b  must each be less than 0.4 mm. 
     Preferably, lands  5   a, b  and  9   a, b  should have an axial height such that the respective land height is no more than 10% of the total height of the oil control ring and the land heights together are no more than 20% of this total height of the oil control ring. 
     The connection angles can vary, as is likewise indicated, merely by way of example, in  FIGS. 9 and 10 . 
     Yet another possible example of a third ring groove  3  for an oil control ring is shown in section in  FIGS. 11 and 12 . In these figures, the third ring groove  3  can also have at least one, preferably a plurality of, oil drain(s)  11   c  and  12   c , which can pass through the piston skirt wall  1   d  (in the case of  11   c ) or be designed as a blind hole (in the case of  12   c ) and can correspond at least partially to the oil drains  5   c  ( FIG. 9 ) and  9   c  ( FIG. 10 ) of the oil control ring. 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.

Technology Classification (CPC): 5