WO1994016249A1

WO1994016249A1 - Sealing ring for hydraulic pistons or connecting rods

Info

Publication number: WO1994016249A1
Application number: PCT/DE1994/000015
Authority: WO
Inventors: Heinz K. MÜLLER; Ulrich Frenzel
Original assignee: Mueller Heinz K; Ulrich Frenzel
Priority date: 1993-01-15
Filing date: 1994-01-11
Publication date: 1994-07-21
Also published as: DE4300889C1

Abstract

A sealing ring for hydraulic pistons or connecting rods made of elastomer material with an annular sealing edge has a plurality of small roughnesses on its rear preferably of spherical shape which project from the relatively smooth rear. The fluid-filled hollows between the roughnesses and the sliding surface in conjunction with the relatively smooth regions in the vicinity of the sealing edge favour the return of fluid by the axially moving sliding surface to the chamber to be sealed and thus improve the dynamic sealing. At the same time, the effective contact area and hence the stiction and sliding friction is substantially reduced as against sealing rings with completely smooth rears.

Description

Sealing ring for hydraulic pistons or piston rods

The invention relates to a sealing ring for hydraulic pistons or piston rods made of elastomer material with an annular sealing edge and a back surface arranged between the sealing edge and a low-pressure-side contact surface, the sealing edge and at least part of the back surface being the cylindrical sliding surface of the piston which can be moved axially relative to the sealing ring Touch the piston or the piston rod, with at least a portion of the back surface being rough. The roughness consists either of crests that protrude at least 20 micrometers from the otherwise smooth back surface and have a distance between their highest points that is four to eight times as large as the height of the smallest adjacent crest, or of irregular roughness surveys in accordance with them VDI guideline 3400 defined roughness has a class with a value of at least 25. A partial area of the back surface of the seal according to the invention is accordingly substantially rougher than the back surfaces of elastomer sealing rings which are smooth according to the prior art. This has the effect that between the sealing ring and the piston or the piston rod produced by the support effect of the crests or the roughness, very narrow, interconnected cavities are formed, which are open to the low-pressure chamber, but are closed to the space to be sealed.

Hydraulic seals are required to be tight under all operating conditions, have little friction, if possible not cause stick-slip movements and can be produced with the least possible economic outlay. Because elastomers are relatively inexpensive sealing materials and elastomer sealing rings can be manufactured as injection molded parts or as pressed parts, they are, especially in large numbers, much cheaper than seals made from reinforced polytetrafluoroethylene. For this reason, sealing rings made of elastomer materials, mainly made of polyurethane, are often used to seal pistons or piston rods in hydraulic devices. Disadvantages of the elastomer sealing rings designed according to the prior art are their adhesiveness and the resulting high friction as well as their often insufficient dynamic tightness as a result of a deformation-related impediment to the return transport of liquid into the space to be sealed.

The requirement for static tightness, that is to say tightness when the sealing ring is not moved relative to the piston or to the piston rod, is satisfactorily met by practically all commercially available sealing rings. The essential prerequisites for this, namely a relatively smooth sealing edge which is pressed against a likewise smooth sliding surface, are state of the art. On the other hand, it is often problematic to achieve a leak-free seal if the sliding surface touched by the sealing ring (in the case of a rod seal the rod surface or in the case of a piston seal the hollow cylinder surface) moves axially relative to the sealing ring. In this case, an elastohydrodynamic lubrication effect basically forms a thin layer of liquid between the sealing ring and the sliding surface that is moved relative to it. This liquid layer is thus first of all dragged out of the space to be sealed. It depends on whether this creates leakage depends on whether during the reciprocating movement of the piston or the piston rod the liquid that has been dragged out, with the opposite direction of movement in each case, is completely transported back through the gap between the sealing ring and the sliding surface into the space to be sealed. According to the prior art, certain requirements must be placed on the course of the surface pressure in the contact zone between the sealing ring and the sliding surface. Since the leakage only occurs when a piston rod protruding into the environment from a hydraulic device is sealed, it is particularly important with piston rod seals that dynamic tightness is achieved during operation. For this purpose, according to the prior art, elastic sealing rings are used in which the sealing surface pressure rises as steeply as possible at that end of the contact zone which adjoins the space to be sealed, and falls as flatly as possible at the other end. The steep rise is achieved according to the prior art by means of an asymmetrical sealing edge formed on the elastic sealing ring, which is flattened when the sealing ring is pressed radially onto the sliding surface. The low-pressure, relatively flat drop in pressure is achieved according to the prior art by arranging a back surface between the sealing edge and a low-pressure side, generally radially flat, contact surface of the sealing ring, which generally has a conical area which only partially rests on the sliding surface when the sealed liquid is at low pressure, but increasingly contacts the cylinder surface when the pressure rises. At high pressure, however, conditions are often unfavorable for the return transport of the liquid if, with the back surface of the sealing ring already lying completely against the sliding surface, the pressure drop at the low-pressure end of the contact zone is very steep. Targeted investigations by the inventors have shown that, under these circumstances, the return transport of the liquid is particularly hindered when the back surface is very smooth and therefore bears almost everywhere on the sliding surface even on a microscopic scale. This is explained by the fact that under these conditions the desired hydrodynamic lubricant film formation when retracting the piston rod into the space to be sealed is considerably restricted. In this case, the liquid adhering to the sliding surface is not transported back into the space to be sealed, but is largely stripped off. This can result in a significant leakage loss.

The frictional resistance associated with the relative movement between the sliding surface and the sealing ring is greater, the larger the mutually contacting surfaces of the sealing ring or the sliding surface are. With increasing fluid pressure, friction increases for two reasons. On the one hand, the actual contact surface grows due to the adaptation of the microscopic unevenness of the sealing ring and the sliding surface, which increases with the contact pressure. As a result, the number of interacting atoms in the sealing surfaces increases (van der Waals forces), which is synonymous with an increase in friction. The increase in friction due to this effect is relatively small in the case of elastomer sealing rings, because such sealing rings are manufactured with a very smooth surface with a view to their manufacture in injection or compression molds. However, because of the smooth sealing surfaces and the adhesiveness of these materials, the surface-related frictional forces already relatively high even with low contact pressure. On the other hand, when the fluid pressure increases, the axial thrust on the elastic seal increases, thereby deforming it radially more and the parts of the back surface of the sealing ring that are not yet in contact with the sliding surface come to rest. For this reason, friction is initially observed to increase sharply with the fluid pressure. Finally, when the entire back surface of the sealing ring lies against the sliding surface at high pressure and the microscopic bumps are adapted to one another, the friction increases only insignificantly as the fluid pressure increases further.

DE-OS 2023 675 describes measures for reducing the friction of lubricating sealing surfaces of rotating machine parts (shaft seals). For this purpose, it is proposed to provide the hard sliding surface of the shaft with a large number of densely adjacent, essentially triangular, unevenness which is separated from one another by depressions. The area of the sliding surface on which these bumps are located extends from the space to be sealed under the contact surface of the seal through to the outside of the seal. However, this initially questions the static tightness, since the sealing edge of the sealing ring can no longer conform to the sliding surface in a completely sealing manner. In addition, such a seal fails at low sliding speeds or when there is a temporary lack of lubricant in that the triangular bumps on the hard shaft surface wear and destroy the soft sealing ring. Because of these disadvantages, the features proposed in DE-OS 2023 675 could not prevail in practice.

Another known measure for reducing friction is the so-called "X-sel coating". It consists of crater-like pockets a few micrometers deep, which are located in a layer of polyurethane sprayed onto finished elastomer seals. Lubricant is stored in these pockets. As a result, the breakaway force, ie the force required to initiate a sliding movement as a result of the static friction, is reduced in particular.

In order to alleviate the increase in friction with hydraulic pressure in the case of hydraulic piston or rod seals, there are sealing rings according to the prior art which have a circumferential, bead-like increase on the back surface. In the case of these sealing rings, which are also referred to as double-edge rings, the entire back surface cannot bear against the sliding surface at high pressure because of the supporting effect of the bead, and the friction is accordingly somewhat less. At the same time, however, the bead, at least at higher pressure, hinders recovery in a manner similar to that of a completely fitting back surface. A dragging flow between the sealing edge and a second annular bead resting on the sliding surface can also build up a liquid pressure which disturbs the dynamic tightness. Experiments by the inventors showed that the dynamic sealing effect of double edge rings is often even worse than that of conventional sealing rings with a smooth back and a single sealing edge. In addition, rod seals are known which, for the purpose of reducing the friction in the central region of the back surface, have pyramid-shaped elevations which protrude more than 0.5 millimeters from the back surface and lie close together. Due to their shape, the relatively high and stiff elevations of these seals are only slightly flattened in contact with the sliding surface, as a result of which a relatively large gap is retained in the areas between the elevations. Compared to sealing rings with a smooth back surface, such seals showed a considerably greater dynamic leakage in practice. Therefore, they could not prevail in practice.

The disadvantages of conventional elastomer sealing rings mentioned, namely a leak due to a hindrance of the return effect and the relatively high friction are eliminated according to the invention by one and the same measure, namely a large number of small, distributed on the back surface of the elastomer sealing ring, made of the material of the sealing ring existing and closely connected with this roughness surveys. The inventive idea leading to this measure is explained below.

It is known that the return transport of liquid into the space to be sealed is particularly favored if the back surface forms a very flat wedge gap with the sliding surface outside the low-pressure end of the sealing surface. This is because the liquid dragged into this wedge gap by the sliding surface undergoes a hydrodynamic increase in pressure, as in the case of a sliding bearing. However, this effect fails as soon as the wedge gap disappears when the back surface is completely in contact. On the other hand, however, it is known that the pressure of a liquid also increases when it is dragged through a moving wall into a cavity of any shape, but closed at its end. This idea gave rise to the inventive idea of generating a hydrodynamic pressure which favors the return flow in an elastomer sealing ring, however by extending around the periphery, open to the low-pressure chamber and extending into the contact zone between the sliding surface and the sealing ring closed cavities are created towards the space to be sealed. This is achieved according to the invention in that, on the one hand, a portion of the back surface of the sealing ring has a large number of small elevations which prevent the elastic back surface from nestling completely against the sliding surface in practical operation, but on the other hand maintaining a tight osculation in the area of the adjacent sealing edge . The cavities created by the support effect of the roughness elevations are desirably smaller because of the flexibility of the roughness elevations and because the liquid stored between the elevations can flow off, when the liquid pressure in the space to be sealed increases and the radial pressure on the sealing ring increases. The smaller the cavities, the greater the pressure in these cavities when the sliding surface drags liquid into them. This pressure finally becomes large enough at the sealing edge to raise it slightly during the sliding movement and thus to cause liquid to be transported back into the space to be sealed. With the sealing rings according to / 4 / there is obviously such a large gap in the spaces between the relatively high and rigid pyramid-shaped elevations that the drag pressure required for the return flow effect cannot arise to a sufficient extent with these seals. At the same time, the relatively large pyramid-shaped elevations relieve the sealing edge so much that a thick lubricating film forms when the rod is extended. Compared to sealing rings with a smooth back surface, the seals according to / 4 / showed a significantly greater dynamic leakage. They have therefore disappeared from the market again.

The shape of the roughness elevations and their spacing from one another are coordinated according to the invention depending on the hardness of the elastomer used so that the portion of the back surface which has the roughness elevations is not completely leveled even at the highest operating pressure of the liquid to be sealed. The roughness surveys preferably consist of individual crests which protrude from the central profile of the otherwise relatively smooth back surface by a height H of at least 20 micrometers and have a distance between their highest points which is four to eight times the height of the smallest adjacent ones Crest. The domes preferably have approximately the shape of spherical sections. In particular, such domes result in a favorable sealing behavior if the elastomer sealing ring has a hardness in the range from 90 to 98 IRHD in accordance with DIN 53 519, the height H of the domes is between 100 and 200 micrometers and the domes have an ab between their highest points ¬ stand, which is four to eight times the height of the smallest adjacent crest. The sealing ring preferably consists of the wear-resistant material polyurethane, preferably with a hardness in the range from 40 to 70 Shore D in accordance with DIN 53 505.

In contrast to the roughness of the X-sel coating, the rough or crest-bearing portion of the back surface of the sealing ring according to the invention has no craters, so that, in contrast to the X-sel coating, the caverns according to the invention have self-contained caverns cannot form. In contrast to the dome or roughness structure of the sealing ring according to the invention, the cavities of the X-sel coating which adjoin one another like a honeycomb also hinder the hydrodynamic pressure generation which favors the return transport. Furthermore, liquid-filled, self-contained caverns of the X-sel coating are not radially flexible.

In order to prevent, in particular in the case of a conical back surface, which in some cases has or has ridges, that the ridges or roughness elevations which are arranged in the vicinity of the sealing edge and are more strongly pressed due to the overlap, the height of the The ridges or the roughness depth near the sealing edge is larger than at a greater distance from the sealing edge. The sealing ring is therefore preferably designed in such a way that the height of the crests or the roughness, as seen from the sealing edge, decreases toward the low-pressure-side contact surface. In a preferred embodiment of the sealing ring, in which the roughness elevations are crests, each of which is otherwise essentially smooth by the height (H) Protrude from the back surface, the height (H) is preferably graded so that it decreases from the sealing edge to the low-pressure side contact surface.

Due to the crests or the roughness, according to the invention the effective contact surface and thus the friction is at the same time significantly reduced in comparison to a smooth back surface which has been customary to date. The drag pressure stabilizes the cavities. In this way, the radial pressure on the sealing ring is compensated hydrodynamically by the drag pressure generated by the sliding surface. In addition, liquid is permanently stored in the cavities remaining between the crests or the roughness elevations and the piston or the piston rod. As a result, the crests or roughness elevations of the sealing ring which bear against the piston or the piston rod are lubricated even with a slight axial displacement of the sliding surface. For these reasons, the static friction, the sliding friction and in particular the tendency to slip (stick-slip) of the sealing ring according to the invention is significantly reduced.

In the case of sealing rings, the rough partial area of the back surface consists of elastomer, in particular of polyurethane or nitrile-butadiene rubber, according to the invention of uneven or uniform roughness elevations. So that the hydrodynamic recovery takes effect over the entire circumference of the sealing ring, the rough or crested portion of the back surface is preferably closed in a ring. With regard to good static and dynamic sealing action, it is advantageous if the sealing ring lies as close as possible to the flattened sealing edge and in its immediate vicinity of the sliding surface. For this purpose, the average roughness depth R of the back surface in the immediate vicinity of the sealing edge is preferably less than 10 micrometers, that is to say that the rough or cresting portion of the back surface does not extend over the entire back surface. Adequate static tightness with a very favorable return conveying effect is also achieved according to the invention in that the rough or couping portion of the back surface directly adjoins the sealing edge and that on the side of the space to be sealed directly adjoins the sealing edge when the sealing ring is pressed against the sliding surface, the surface of the sealing ring is smooth. In the latter case, the entire back surface has crests or roughness elevations.

Another preferred method of producing the rough partial area is that the negative of the roughness elevations is worked into the molding tool with which the sealing ring is formed by means of electroerosive machining. According to the invention, good sealing behavior is achieved if the class K of roughness according to VDI guideline 3400 has at least the value 25. The sealing ring is preferably designed in the range of classes from 25 to 45. Another particularly favorable manufacturing method for producing roughness elevations of almost any shape on the back surface of the sealing ring according to the invention is that the negative of the roughness elevations is incorporated into the mold with which the sealing ring is formed by means of laser beam processing. In order to prevent the sealing ring, which is radially deformed under high pressure and thereby contacts the sliding surface on the low pressure side, from touching the sliding surface with a smooth part of its contact surface, a part of the contact surface of the sealing ring adjoining the back surface or a transition surface between the back surface and the contact surface with the roughness structure according to the invention.

In the following, the invention is explained on the basis of a number of exemplary embodiments shown in the drawings. Show in the drawings

Fig. 1 shows a cross section of a sealing ring with the features according to the invention.

FIG. 2 shows a cross section of the sealing ring according to FIG. 1 in the operating state together with other components of the sealing system.

3 shows a cross section of a design of the sealing ring referred to as a groove ring with the features according to the invention.

4 shows a cross section of a design of the sealing ring referred to as a sealing edge ring with the features according to the invention.

5 shows a partial section through the portion of the back surface of the sealing ring which has the crests.

Fig.l shows in cross section a sealing ring 1, the sealing edge 11, the back surface 3 with the crests or roughness sub-area 5, the contact surface 2 with a rough area 52 and a conical face 12 facing the space to be sealed in the operating state.

Fig. 2 shows the sealing ring according to FIG. 1, installed in a groove of the machine part 9 (piston or housing), the space 91 containing the liquid to be sealed, the machine part 41 axially movable relative to the machine part 9 with the sliding surface 4, on which the sealing ring with the sealing edge, the back surface and part of the portion 5 having ridges or roughness elevations and the low-pressure chamber 92.

3 shows in cross section a sealing ring 1, which is usually referred to as a grooved ring, the sealing edge 11, the back surface 3 with the portion 5 having ridges or roughness elevations, and the contact surface 2. As is usual with Nutringen, the back surface near the sealing edge has a conical shape Part to which a cylindrical part adjoins which merges with the contact surface. In the sealing ring according to FIG. 3, a first part of the back surface in the vicinity of the sealing edge is "smooth", followed by the portion 5 having ridges or roughness elevations, which extends over the cylindrical part to over the chamfer of the back surface. The one to be sealed Space facing conical end face 12 is also "smooth". Here, "smooth" means that the roughness is significantly smaller in comparison to the portion having ridges or roughness elevations.

FIG. 4 shows in cross section a sealing ring 1 designated as a sealing edge ring, the sealing edge 11, the back surface 3 with the portion 5 having ridges or roughness elevations, and the contact surface 2. Again, a part of the back surface 3 is near the sealing edge and the conical end face 12 "smooth".

5 finally shows a partial section through a portion of the back surface having crests. The crests 71, 72 protrude from the middle profile of the "smooth" back surface 3 between the crests by the height H1 or H2 and their highest points ZI or Z2 are at a distance K from one another.

Claims

1. Sealing ring (1) for hydraulic pistons or piston rods made of elastomer material with an annular sealing edge (11) and a back surface (3) arranged between the sealing edge and a low-pressure side contact surface (2), the sealing edge and at least one during operation Part of the back surface touches the sliding surface (4) of the piston or piston rod, which is axially movable relative to the sealing ring, and has at least a partial region (5) of the back surface of crests (71, 72), the back surface between the crests being smooth in such a way that it conforms to DIN 4762 / ISO 4287/1 has an average roughness depth R _z of less than 10 micrometers, and that the material forming the dome is identical to and is related to the material of the sealing ring, characterized in that the height (H) around the one dome protrudes from the central profile (31) of the back surface between the crests, is at least 20 micrometers and that the distance ( K) between the highest points (ZI, Z2) of adjacent crests is four to eight times as large as the height of the smallest adjoining crest, in such a way that between the sealing ring and the piston or the piston rod, the support effect of the crests produces very narrow crests , interconnected cavities are formed, which are open to the low pressure chamber (92), but are closed to the space to be sealed (91).

2. Sealing ring according to claim 1, characterized in that the height of the tips is between 100 and 200 microns.

3. Sealing ring according to claim 1 or 2, characterized in that the height (H) of the tips decreases from the sealing edge to the low-pressure side contact surface.

4. Sealing ring (1) for hydraulic pistons or piston rods made of elastomer material with an annular sealing edge (11) and a back surface (3) arranged between the sealing edge and a low-pressure-side contact surface (2), the sealing edge and at least one during operation Part of the back surface touching the sliding surface (4) of the piston or the piston rod, which is axially movable relative to the sealing ring, characterized in that at least a portion (5) of the back surface contacting the piston or the piston rod is rough, in such a way that the class K is its Roughness according to VDI guideline 3400 has at least the value 25 and that the material forming the roughness is identical to the material of the sealing ring and is connected to it in such a way that between the sealing ring and the piston or piston rod the support effect of the roughness surveys produced, very narrow, interconnected cavities are formed that are open to the low pressure room, but are closed to the room to be sealed.

5. Sealing ring according to claim 4, characterized in that the value of class K of the roughness decreases from the sealing edge to the low-pressure side contact surface.

6. Sealing ring according to claim 4 or 5, characterized in that the negative of the rough portion (5) is incorporated by means of electroerosive machining in the mold with which the sealing ring is formed.

7. Sealing ring according to one of claims 1 to 5, characterized in that the negative of the domes having or rough portion (5) by means of laser beam processing in the mold with which the sealing ring is formed, is incorporated.

8. Sealing ring according to one of the preceding claims, characterized in that there is a smooth area of the back surface between the crests or rough portion (5) and the sealing edge, which according to DIN 4762 / ISO 4287/1 a mean roughness depth R less than 10 microns.

9. Sealing ring according to one of the preceding claims, characterized in that the domes or rough portion (5) adjoins the sealing edge and that on the side of the space to be sealed directly adjacent to the sealing edge, when pressing the sealing ring on the sliding surface partially flattened end face (12) of the sealing ring has a roughness depth R of less than 10 micrometers averaged in accordance with DIN 4762 / ISO 4287/1.

10. Sealing ring according to one of the preceding claims, characterized in that additionally has at least a partial area (52) of the contact surface according to the definitions in one of claims 1 or 4 crests or is rough.