EP0346716B1 - Actuator with a non-rotatable piston rod - Google Patents

Abstract

Actuator unit (10) which can be acted upon by compressed air and whose piston rod (12) is secured against rotation by a plurality of ball tracks being provided at least in the longitudinal area (36) passing through the allocated cylinder lid (32), which ball tracks are distributed over the periphery of the piston rod and run in the longitudinal direction of the piston rod. A bearing component (46), through which the piston rod (12) passes, is fastened to the outside of the cylinder lid (32) removed from the piston, which bearing component (46) is in turn provided with ball tracks in alignment with the ball tracks of the piston rod (12), rows of bearing balls (42) being provided in the intermediate space between the outer surface of the piston rod (12) and the inner surface of the bearing component (46), which bearing balls (42) roll on the ball tracks of the piston rod (12) on the one hand and the ball tracks of the bearing component (46) on the other hand. In the connecting area between the cylinder lid (32) and the bearing component (46), a lip sealing ring (60) having a passage opening corresponding in complementary fashion to the cross-sectional shape of the piston rod (12) is provided in an encircling location groove (62) surrounding the piston rod (12) at a radial distance, the sealing lips (68; 70) of which lip sealing ring (60) are directed into the interior of the actuator (10). <IMAGE>

Description

The invention relates to a piston-cylinder unit that can be pressurized with pressure, with a piston rod which, at least in the length range which seals the associated cylinder cover by a seal against the escape of pressure medium, has a cross-section deviating from a circular cross-section, through which a piston-connected piston rod passes Bearing component is assigned, which guides the piston rod in a longitudinally displaceable manner, the piston rod in the area passing through the associated cylinder cover being formed over the largest part of the circumference by surface sections of circular cross section with the same circular radius, which are interrupted at equal angular intervals by sections forming longitudinal raceways , and on the cylinder cover the bearing component penetrated by the piston rod is fastened, which in turn is in alignment with the longitudinally running ball raceways points, and in the space between the outer surface of the piston rod and the inner surface of the bearing component rows of bearing balls rolling on the ball raceways of the piston rod on the one hand and the ball raceways of the bearing component on the other are arranged, and wherein in the cylinder cover in a radial distance surrounding the piston rod Receiving groove is provided with a sealing ring with a passage opening that complements the cross-sectional shape of the piston rod

The piston rod of piston-cylinder units that can be pressurized with pressure medium, that is to say pneumatics or hydraulic cylinders, generally have a cylindrical shape, at least in the length section penetrating the associated cylinder cover during the working stroke, which has the seal of the between the piston rod and the Passage opening in the cylinder cover required gap against pressure loss and leakage of the pressure medium by means of conventional ring seals - be it O-rings or lip seals. Due to the cylindrical shape of the piston rod, the end of the piston rod emerging from the cylinder is not secured against rotation, especially since the cylinder firmly connected to the piston rod has a concentric circular boundary, which is also not secured against rotation in the circumferential direction within the cylinder. If such piston-cylinder units are used in constructions in which they have to advance and retract components which have to be held during the stroke in an orientation which is exactly secured against rotation relative to the longitudinal central axis of the piston-cylinder unit, additional guides must be provided , which secure the piston rod against rotation. Regardless of whether there are guides provided in series or parallel to the piston rod and connected to it, the overall length or width of the piston-cylinder unit increases considerably. In addition, the structure of the piston-cylinder unit becomes more complex and the number of required components and the effort for their processing and assembly increases significantly compared to a simple piston-cylinder unit which is not secured against rotation.

From DE-A-3435 964 a proposal for the design of a piston-cylinder unit of the type mentioned is known, in which the - in cross-section, for example, elliptical piston rod through a bushing inserted in the cylinder cover penetrated by the piston rod with a complementary (elliptical ) Opening cross-section is secured against rotation. Due to the flat abutment of the piston rod in the guide bushing, however, a Especially at the beginning of the piston stroke, ie when the cylinder is actuated from the rest position, effective friction (rest friction), which can lead to a jerky movement of the piston rod, which is undesirable for many applications. Nothing can be found about the design and arrangement of the seal preventing the pressure medium from escaping from the piston-cylinder unit.

From US-A-3 994 539 a piston-cylinder unit of the type mentioned at the outset is known, the piston rod of which, in principle, has a circular cross-section can be provided with ground flats on opposite sides. It is not clear from the publication whether these flattenings only serve as raceways for the longitudinal ball bearings that guide the piston rod in the housing, or whether they should also provide rotational locking of the piston rod in cooperation with a complementary passage opening in the cylinder cover or the outer housing of the longitudinal ball bearings.

In contrast, the invention has for its object to provide a piston-cylinder unit in which an external dimension of the piston-cylinder unit which at most slightly increases, and which protects the piston rod against rotation, is integrated, the anti-rotation device ensuring no sluggishness or loss of compressed air.

This object is achieved in that the bearing component is arranged in a rotationally fixed manner on the outside of the cylinder cover facing away from the piston, that the ball raceways are shaped in a circular concave cross section, and that the sealing ring is arranged in the connecting region between the cylinder cover (18) and the bearing component (46) and as Lip sealing ring is formed, the sealing lips are directed inside the piston-cylinder unit. So the piston rod is fundamentally similar to a longitudinally displaceable shaft that is secured against rotation by a linear movement bearing with ball guides, but the problem of sealing the piston rod, which does not occur with such shafts, against the pressure built up by the pressure medium inside the piston-cylinder unit by means of a special lip Sealing ring with the cross-sectional shape of the piston rod adapted passage opening is released.

In a preferred embodiment of the invention, the piston rod is provided with a number of equally spaced, radially protruding longitudinal wedges running in the working stroke direction, which are each provided with the ball raceways on both sides in the transition area to the adjoining, cross-sectionally circular surface section, the bearing component being in alignment to the longitudinal wedges is provided with longitudinal grooves in which the ball raceways of the bearing component are provided. This configuration with a linear motion bearing that has become known as the "Ball Spline" appropriate anti-rotation lock has proven itself. When three circumferential longitudinal wedges are arranged on the piston rod, a total of six rows of balls aligned on the circumference on the one hand ensure easy longitudinal displacement, while on the other hand an angular play between the piston rod and the cylinder is prevented even at high acting moments.

Alternatively, the configuration can also be such that the piston rod is provided with a number of equally spaced, radially recessed, longitudinal grooves forming the ball raceways, to which the longitudinal grooves forming the ball raceways are associated with a corresponding angular displacement in the inner surface of the bearing component. This simplified exemplary embodiment is particularly suitable in cases in which there are comparatively small moments acting in the circumferential direction and trying to twist the piston rod.

In an advantageous development of the invention - again in accordance with the linear motion bearings mentioned - in the circumferential direction offset to the ball raceways, deeper longitudinal return grooves are provided in the inner surface of the bearing component, which are connected at their two ends to the adjacent ends of the respectively assigned ball raceway, so that the bearing balls arriving at one end of the associated ball raceway in the bearing component during a stroke of the piston rod are displaced into the return longitudinal grooves as the piston rod movement continues and are returned via the latter in the opposite direction to the stroke direction of the piston rod.

It is then expedient if a cage which holds the bearing balls in the bearing component is assigned to the rows of bearing balls rolling on the ball raceways and the rows of bearing balls which are returned in the return longitudinal grooves and which is opposite the ball raceways of the piston rod lying areas each have a slot through which the bearing balls can only partially pass through, while it is closed in the area of the return longitudinal grooves in each case towards the piston rod. This makes it possible to disassemble the bearing component from the piston rod without the balls being able to escape from the bearing component.

If the piston rod has projecting longitudinal wedges, the lip sealing ring provided according to the invention is preferably designed such that it has a circularly delimited outer diameter which is larger than the diameter measured over the end face of the longitudinal wedges of the piston rod, and at the edge of which one on the cylindrical inner surface the receiving groove-sealing circumferential elastic sealing lip is provided, and that along the cross-sectional shape of the piston rod complementarily shaped inner passage opening, a further all-round closed and continuously sealing elastic sealing lip is provided on the outer surface of the piston rod, with a circumferential sealing lip between the two pointing in the same direction , pressure chamber open towards the inside of the cylinder is formed, whose radial dimensions measured between the outer and the inner sealing lip in the area assigned to the longitudinal wedges of the piston rod hen are less than in the regions lying between them in the circumferential direction.

If, on the other hand, the piston rod is provided with a number of longitudinal grooves forming the ball raceways, the lip sealing ring is designed in such a way that it has a circularly limited outer diameter which is larger than the maximum diameter of the piston rod, and at the edge of which one on the cylindrical one The inner surface of the receiving groove is provided with a circumferential elastic sealing lip, and along the inner passage opening which is complementarily shaped in accordance with the cross-sectional shape of the piston rod Another closed all around and continuously on the outer surface of the piston rod sealing elastic sealing lip is provided, whereby a circumferential pressure chamber is formed between the two sealing lips pointing in the same direction, the radial dimensions of which are measured between the outer and the inner sealing lip in the the areas assigned to the longitudinal grooves of the piston rod are larger than in the areas lying between them in the circumferential direction.

• Fig. 1 einen Längsmittelschnitt durch ein erstes Ausführungsbeispiel einer als Pneumatikzylinder ausgebildeten Kolben-Zylinder-Einheit, deren Kolbenstange in der erfindungsgemäßen Weise gegen Verdrehung gesichert ist;
• Fig. 2 eine Schnittansicht, gesehen in Richtung der Pfeile 2-2 in Fig. 1;
• Fig. 3 eine Draufsicht auf den zur Abdichtung der Kolbenstange vorgesehenen Lippen-Dichtring, gesehen in Richtung des Pfeils 3 in Fig. 4;
• Fig. 4 eine Schnittansicht des Lippen-Dichtrings, gesehen in Richtung der Pfeile 4-4 in Fig. 3;
• Fig. 5 ein vergrößertes Detail des in Fig. 1 innerhalb des strichpunktierten Kreises 5 gelegenen Teils der Kolben-Zylinder-Einheit;
• Fig. 6 einen Längsmittelschnitt durch ein zweites Ausführungsbeispiel einer als Pneumatikzylinder ausgebildeten Kolben-Zylinder-Einheit, deren Kolbenstange in der erfindungsgemäßen Weise gegen Verdrehung gesichert ist;
• Fig. 7 eine Schnittansicht, gesehen in Richtung der Pfeile 7-7 in Figur 6;
• Fig. 8 eine Draufsicht auf den zur Abdichtung der Kolbenstange vorgesehenen Lippen-Dichtring, gesehen in Richtung des Pfeils 8 in Figur 9;
• Fig. 9 eine Schnittansicht des Lippen-Dichtrings, gesehen in Richtung der Pfeile 9-9 in Figur 8; und
• Fig. 10 ein vergrößertes Detail des in Figur 6 innerhalb des strichpunktierten Kreises 10 gelegenen Teils der Kolben-Zylinder-Einheit.

The invention is explained in more detail in the following description of two exemplary embodiments in conjunction with the drawing, which shows:

• 1 shows a longitudinal central section through a first exemplary embodiment of a piston-cylinder unit designed as a pneumatic cylinder, the piston rod of which is secured against rotation in the manner according to the invention;
• Fig. 2 is a sectional view seen in the direction of arrows 2-2 in Fig. 1;
• 3 shows a plan view of the lip sealing ring provided for sealing the piston rod, viewed in the direction of arrow 3 in FIG. 4;
• Fig. 4 is a sectional view of the lip seal ring, seen in the direction of arrows 4-4 in Fig. 3;
• 5 shows an enlarged detail of the part of the piston-cylinder unit located within the dash-dotted circle 5 in FIG. 1;
• 6 shows a longitudinal central section through a second exemplary embodiment of a piston-cylinder unit designed as a pneumatic cylinder, the piston rod of which is secured against rotation in the manner according to the invention;
• Figure 7 is a sectional view seen in the direction of arrows 7-7 in Figure 6;
• 8 shows a plan view of the lip sealing ring provided for sealing the piston rod, viewed in the direction of arrow 8 in FIG. 9;
• 9 is a sectional view of the lip sealing ring, seen in the direction of arrows 9-9 in FIG. 8; and
• FIG. 10 shows an enlarged detail of the part of the piston-cylinder unit located within the dash-dot circle 10 in FIG. 6.

The piston-cylinder unit shown in FIG. 1, designated in its entirety by 10, is a double-acting pneumatic cylinder with end position damping that can be acted upon with compressed air as the working medium, which corresponds to known pneumatic cylinders of this type in terms of its basic structure, so that its structure is not described in detail to this extent must be, especially since the protection according to the invention against rotation of its piston rod 12 can also be realized in the same way in other types, for example single-acting and undamped pneumatic cylinders, as well as in piston-cylinder units which can be acted upon by a hydraulic fluid as the working medium.

The piston-cylinder unit 10 thus has the actual cylinder 20, which is closed at one end by a closed cylinder cover 14 and at the other end by a cylinder cover 18 provided with a circular through-opening 16 in cross section for the piston rod 12. In the cylindrical bore 22 of the cylinder is a bore in two size-adjustable working spaces 24, 24 'dividing and on the inner wall delimiting sealing piston 26 slidably mounted. The piston 26, which is assembled from a plurality of parts not described in detail, is penetrated centrally by the reduced-diameter cylindrical inner end 28 of the piston rod 12 and held at the end of the piston rod by a sleeve 30 screwed onto the free end.

The compressed air connections 32, 34 provided in the cylinder covers 14 and 18 are only indicated by dashed lines in FIG. 1.

The piston rod 12 has, in the region 36 adjoining the reduced-diameter cylindrical end section 28 - deviating from the customary circular cross-sectional shape - the cross-section recognizable in FIG 38 radially step forward, forming 40 ball raceways for rows of bearing balls 42 (FIG. 1) in transition regions rounded into the circular cross-sectional wave area, of which one row lies on each side of each longitudinal wedge. On the other hand, each row of balls runs in a longitudinal groove 44 in a surrounding bearing component 46, which is held centered in the cylinder cover 18 and is screwed to the cylinder cover and secured against rotation. This screwing takes place - in the case shown - with fastening screws 48, Only one of which is shown in FIG. 1 and which at the same time also serve to fasten the piston-cylinder unit 10 as a whole in a through bore 50 in a mounting plate 52 forming part of a larger construction, only one of which is shown in FIG. 1 - dash-dotted hatched - partial area is shown.

In Fig. 1, the piston rod 12 is shown in the fully retracted position in the piston-cylinder unit 10, in which the working space 24 has its maximum and the working space 24 'its minimum size. The length of the section 36 of the piston rod 12 provided with the longitudinal wedges 38 is selected such that it still extends through the bearing component 46 and at the end on the left in FIG. 1 something emerges from the bearing component 46. At the section 36 then adjoins a cylindrical piston rod section 54, on which one of the piston rod 12 depending on the loading of the working spaces 24, 24 'with compressed air to be moved back and forth - not shown - component can be assembled from that it is assumed that it must not change its relative rotational position with respect to the longitudinal central axis of the piston-cylinder unit 10 because, for example, it may have projecting pins which have to engage in the corresponding, suitable bores in the fully extended end position of the piston rod 12.

The rotation of the piston rod 12 is thus ensured by the rows of balls supporting the longitudinal wedges 38 laterally and supported in turn in the longitudinal grooves 44 in the bearing component 46. During the working stroke, the bearing balls 42 roll on the associated raceways of the section 36 and the bearing component 46 and thus also change their position in the longitudinal direction. In order to make this possible and to nevertheless bring the balls to bear over the entire length of the bearing component 46, deeper longitudinal return grooves 56 are offset in the circumferential direction to the raceways formed in the bearing component 46 provided, which are connected at their ends via circular arc-shaped connecting sections, not shown, so that during the rolling movement at one end bearing balls 42 emerging from the longitudinal grooves 44 pass through the respective connecting section into the associated return longitudinal groove and in this in the opposite rolling direction are returned to the other end of the longitudinal groove 44. These return longitudinal grooves 56 are dimensioned such that they completely accommodate the bearing balls, which therefore do not roll on the piston rod section 36 in these areas. A suitably designed plastic cage 58 holds the bearing balls 42 in such a way that in the area of the longitudinal grooves 44 they can partially pass through a slot provided there to the ball races on the piston rod section 36 while opposing the deeper return longitudinal grooves 56 the piston rod closes. Even when the piston rod 12 is completely pulled out of the bearing component 46, the bearing balls 42 are thus held back by the cage 58 in the bearing component 46, so that there is no risk of loss of individual bearing balls when dismantling or assembling the piston-cylinder unit 10 42 exists. To the extent described so far, the anti-rotation device of the piston rod 12 corresponds to the linear motion bearings supplied by the company THK Europe GmbH, Dusseldorf under the name "Ball Spline", which therefore enable a shaft to be moved centrally in a bearing component and secured against rotation, but none Allow axial sealing against gaseous or liquid pressure medium.

In order to ensure this seal, which is essential when a piston rod is secured against rotation, the lip seal 60, which is shown separately in FIGS. 3 and 4, is arranged in a circumferential receiving groove 62 (FIGS. 1 and 5) which is cylindrical on the groove base and which is on the one hand in the cylinder cover 18 and on the other hand seals on the outer surface of the piston rod section 36.

For this purpose, the lip sealing ring has an outer circular and an inner radial section 64 which is delimited in accordance with the cross-sectional shape of the piston rod section 36 on the side opposite the working space 24 of the piston-cylinder unit by a metallic support disk 66 in the receiving groove. The support disk is held clamped between a radial surface of the cylinder cover 18 and the bearing component 46. A circumferential sealing lip 68, which is attached to the outer circumference of the radial section 64 and points in the direction of the working space 24, rests under prestress on the circumferential or bottom surface of the receiving groove 62 and a sealing lip 70 following the outline shape of the inner boundary rests on the piston rod section 36. The lip sealing ring 60 made of elastomeric plastic thus forms a circumferential pressure chamber 72 open in the direction of the working chamber 24, in which the pressure prevailing in the pressure chamber 24 builds up and additionally presses the sealing lips 68, 70 against the assigned sealing surfaces. However, the sealing lip 70 is only strongly pressed against the piston rod section 36 when the working space 24 is pressurized with compressed air, i.e. when the piston rod is withdrawn inside the piston-cylinder unit. The relative movement between the section 36 and the sealing lip 70 is then directed in the direction of the free end of the sealing lip, whereby its wear is only relatively small, provided the surface of the piston rod section 36 is designed with sufficient smoothness. On the other hand, when the working space 24 'is pressurized with compressed air to extend the piston rod 12, the pressure space 24 is vented, i.e. there is no overpressure applied to the sealing lip 70 with additional pressure on the surface of the piston rod. In this respect, too, there is no fear of premature wear of the sealing lip.

FIGS. 6 to 10 show a piston-cylinder unit compared to the piston-cylinder unit described above in connection with FIGS. 1 to 5 10 modified piston-cylinder unit 10 'shown, which, however, only in relation to the - from the circular cross-section - cross-sectional shape of the cylinder cover 18 passing length range of the piston rod 12 and the resulting different arrangement of the ball raceways on the piston rod and a correspondingly changed shape of the lip seal ring differs from the piston-cylinder unit 10. To avoid unnecessary repetition, only the modifications mentioned are described below, while it is sufficient to refer to the previous description of the piston-cylinder unit 10, especially since functionally identical parts of the two exemplary embodiments in the drawing figures 1 to 5 on the one hand and 6 to 10 are otherwise provided with the same reference numerals.

An essential difference of the piston-cylinder unit 10 'compared to the piston-cylinder unit 10 is that instead of the longitudinal wedges 38 of the piston rod 12 of the first embodiment, concave longitudinal grooves 39 (FIGS. 6 and 7) are provided in cross section, which form the ball raceways on the piston rod side. The length region 36 '(FIG. 7) of the piston rod 12 provided with these ball raceways accordingly has a somewhat larger diameter than the region 36 of the piston rod 12 of the piston-cylinder unit 10. Accordingly, the sealing lip 70' of the lip sealing on the piston rod has Sealing rings 60 (Figures 8, 9 and 10) also have a complementary shape corresponding to the cross-sectional shape of the region 36 '.

Otherwise, the two exemplary embodiments 10 and 10 'largely coincide, the design taken in the piston-cylinder unit 10 being fundamentally suitable for securing the piston rod against rotation against higher torques seeking to turn it, on the other hand in the design according to the piston Cylinder unit 10 'of Lip sealing ring 60 'has a less complex and therefore simpler to manufacture.

Claims (7)

1. Piston/cylinder unit (10;10′) which may be actuated by a pressure medium including a piston rod (12), which has a cross-section deviating from the circular at least in the longitudinal region which passes through the associated cylinder head (18) and is sealed against the discharge of pressure medium, associated with which cylinder head is a bearing member (18) which is connected to the cylinder (20) and through which the piston rod (12) passes and which longitudinally movably guides the piston rod (12), whereby the piston rod (12) is defined over the major portion of its periphery in the region (36;36′) passing through the associated cylinder head (18) by circular section surface portions of the same radius which are interrupted at uniform angular spacings by portions defining ball races (40;39) extending in the longitudinal direction and secured to the cylinder head (18) is the bearing member (46), through which the piston rod (12) passes, and which for its part has ball races (44) aligned with the longitudinally extending ball races (40,39) and rows of ball bearings (42) are disposed in the space between the outer surface of the piston rod (12) and the inner surface of the bearing member (46) running on the ball races (40;39) of the piston rod on the one hand and the ball races (44) of the bearing member (46) on the other hand, and whereby a sealing ring (60) with a hole corresponding to the cross-sectional shape of the piston rod (12) is provided in the cylinder head (18) in a peripheral receiving groove (62) surrounding the piston rod with a radial clearance, characterised in that the bearing member (46) is rotationally fixedly arranged on the outer surface of the cylinder head (18) directed away from the piston, that the ball races (40;39) are of concave circular shape in cross-section and that the sealing ring is arranged in the connecting region between the cylinder head (18) and the bearing member (46) and is constructed as a lip sealing ring (60), whose sealing lips (68;70;70′) are directed into the interior of the piston/cylinder unit (10;10′).
2. Piston/cylinder unit as claimed in claim 1, characterised in that the piston rod (12) is provided with a number of longitudinal splines (38) which extend in the direction of the working stroke and project radially and are offset at uniform angular spacings and are each provided on both sides with the ball races (40) in the transition region to the adjacent surface portion of circular cross-section and that the bearing member (46) is provided with longitudinal grooves which are in alignment with the longitudinal splines and in which the ball races (44) of the bearing member are constructed.
3. Piston/cylinder unit as claimed in claim 1, characterised in that the piston rod (12) is provided with a number of longitudinal grooves which constitute the ball races (39) and are radially recessed and which are offset at uniform angular spacings and associated with which at a corresponding angular spacing in the inner surface of the bearing member (46) are longitudinal grooves constituting its ball races (44).
4. Piston/cylinder unit as claimed in one of claims 1 to 3, characterised in that provided in the inner surface of the bearing member (46) there are deeper longitudinal return grooves (56) offset from the ball races in the peripheral direction which are connected at their two ends with the adjacent ends of the respective associated ball race (44) so that the ball bearings (42) which arrive at one end of the associated ball race in the bearing member (46) when the piston rod (12) performs a stroke are displaced into the longitudinal return grooves (56) as the movement of the piston rod continues and are returned via them against the direction of movement of the piston rod.
5. Piston/cylinder unit as claimed in claim 4, characterised in that associated with the rows of ball bearings (42), which run on the ball races (40;44;39) and are returned in the longitudinal return grooves (56), is a cage (58) which retains the ball bearings in the bearing member (46) and which, in each region opposed to the ball races of the piston rod (12), has a slot through which the ball bearings (42) can only partially pass whilst it is closed towards the piston rod (12) in the region of the longitudinal return grooves (56).
6. Piston/cylinder unit as claimed in claim 2, characterised in that the lip sealing ring (60) has a circular external diameter which is larger than the diameter measured over the end faces of the longitudinal splines on the piston rod (12) and on whose edge a peripheral, elastic sealing lip (68) is provided which seals on the cylindrical internal surface of the receiving groove (62) and that provided along the inner hole, which is shaped to correspond to the cross-sectional shape of the piston rod (12) there is a further elastic sealing lip (70) which is annularly closed and forms an uninterrupted seal on the outer surface of the piston rod, whereby defined between the two sealing lips (68;70), which are directed in the same direction, there is a peripheral pressure space (72) open to the interior of the cylinder whose radial dimensions measured between the outer and the inner sealing lip (68;70) in the regions associated with the longitudinal keys (38) on the piston rod are smaller than in the regions lying therebetween in the peripheral direction.
7. Piston/cylinder unit as claimed in claim 3, characterised in that the lip sealing ring (60′) has a circular external diameter which is larger than the maximum diameter of the piston rod (12) and on whose edge there is provided a peripheral elastic sealing lip (68) forming a seal on the cylindrical internal surface of the receiving groove (62) and that provided along the inner hole, which is shaped to correspond to the cross-sectional shape of the piston rod (12) there is a further annularly closed elastic sealing lip (70′) which forms an uninterrupted seal on the external surface of the piston rod, whereby defined between the two sealing lips (68;70′), which are directed in the same direction, there is a peripheral pressure space (72), which is open to the interior of the cylinder and whose radial dimensions measured between the outer and the inner sealing lip are larger in the regions associated with the longitudinal grooves (39) of the piston rod (12) than in the regions lying therebetween in the peripheral direction.

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