NO342225B1 - Device for protection against leakage from seals between a cylinder housing and a piston rod - Google Patents

Abstract

Device for protection against leakage from seals between a cylinder housing (3) and a piston rod (4). The device comprises holding ring (11), which is arranged to be fixed relative to the cylinder housing (3) so that it projects outside the end of the cylinder housing (3) where the piston rod (4) is inserted into the cylinder housing (3). It also comprises the smallest gasket (13) which is arranged to be placed in the space between the holding ring (11) and the piston rod (4) and a press element (14), which is arranged to hold the gasket (13) in place.

Description

The present invention relates to a device that protects against leakage from seals between a cylinder housing and a piston rod. The invention is both suitable for preventing leakage and sealing a leak that has occurred.

The invention is particularly suitable for use on large cylinders, which are demanding to dismantle and reassemble for maintenance.

A hydraulic cylinder will be actuated by an internal hydraulic pressure, either on one side of a piston or on both sides of the piston. Where the piston rod extends out of the cylinder, the cylinder must therefore be equipped with one or more seals, which seal against the piston rod's outer surface, so that the hydraulic fluid does not leak out of the cylinder along the piston rod. This seal is very prone to leakage, as it must allow room for the piston rod to move out of and into the cylinder while at the same time as little hydraulic fluid leaks out between the seal and the piston rod as possible.

Especially for larger hydraulic cylinders, such as for use on large cranes, such as e.g. crane vessel, it is a very extensive and time-consuming operation to change such seals that leak. Such seals are also exposed to extra high stresses, as these cylinders are located outside and often on or near salt water.

The crane booms with the cylinders will be able to hang over the salt water in use and the piston rod will be exposed to salt water drives and particles from the air. Also, objects, such as objects being lifted, can hit the piston rod and damage it.

A piston rod with damage will obviously wear heavily on the seals when the damaged area repeatedly passes past the seal. Salt from the sea will form crystals on the surface of the piston rod and act like sharp little knives against the seals.

Piston rods that are to be exposed to harsh environments are usually coated with a chrome alloy. This is somewhat porous and it has been shown that over time salt and contaminants will penetrate through the coating. It will thereby corrode between the chrome coating and the piston rod. This can cause pieces of the chrome coating to fall off. This leaves sharp edges on the surface, which destroy the seals.

It also happens that incorrect assembly of the cylinder itself, e.g. if a gasket is inserted the wrong way or parts are assembled in the wrong order, leakage will result. Furthermore, it has been shown that vibrations from the ship's hull and the cranes can cause leakage over time.

Even indoors, damage to the piston rod's surface can occur over time, which in turn wears down the seal and causes leakage of hydraulic fluid.

Whether hydraulic fluid leaks into the sea, on a ship's deck, a quay area or in a factory hall, such an oil spill will entail risks, especially for personnel, who can easily slip and fall, and it can also cause pollution of nature. Leaks can therefore only be accepted to a very small extent and rectification must be started quickly. In the meantime, the crane will often not be able to be used. This causes delays and financial loss.

For large cylinders on board large crane vessels, it will not be possible to carry out this type of repair at sea. The vessel must therefore go to the quay and the cylinder dismantled and brought ashore. This will mean that the crane vessel will remain at the quay for a longer time while waiting for spare parts and carrying out repairs. It has happened several times that crane vessels have had to interrupt operations to go to the quay due to leakage in such seals. This can mean that the support vessel, a large number of workers and a lot of equipment are left waiting until the cylinder is repaired or a replacement crane vessel is in place. In such cases, it has happened that the operator of the crane vessel has lost the contract with the client.

It is therefore clear that leakage in such seals between cylinder and piston rod can have very serious financial consequences.

US 4623152A describes a device for protection against leakage from seals between a cylinder housing and a piston rod as described in claim 1's preamble.

US 528773 shows an example of a conventional seal between a cylinder housing and a piston rod, where a gasket (possibly a stuffing box) is arranged between the cylinder housing and the piston rod and is held in place by an element that lies around the piston rod and is screwed into the cylinder housing. If this gasket begins to leak, the cylinder must first be dismantled on the piston rod side, the retaining element unscrewed and the gasket replaced.

Solutions have been proposed which will be able to remedy the situation to some extent. One of these is shown in NO 333699, and concerns a protective sleeve that extends around part of a piston rod. One end of the sleeve is attached to the cylinder housing and the other end of the sleeve is attached to the piston rod. This last attachment point must be outside the part of the piston rod that is fed into the cylinder. The sleeve thereby covers the entire area of the piston rod that is passed past the seal.

Before assembly, the sleeve can be present as a flap, which during assembly is placed around the area to be covered.

After installation, the sleeve will collect leakage of hydraulic fluid from the cylinder. A drainage device is also provided to drain out the collected liquid in a controlled manner.

There are several disadvantages with this known solution. Firstly, this protective sleeve does not prevent leakage. If it is applied before the damage occurs, it can to some extent protect the piston rod from external influences, but once the damage has occurred, it will only act to collect leaking hydraulic fluid. There is a limit to how much liquid it is able to collect, which limits how much leakage you can allow and lays down guidelines for how often you have to drain away liquid from the sleeve. It can be difficult to keep track of how much liquid has accumulated. The leakage can increase without being able to see this in a simple way. The sleeve can therefore be overfilled with liquid and there is a risk that it may burst either due to overpressure or an external stress on the sleeve. If this happens, a critically large amount of hydraulic oil leaks onto the deck or into the sea. Especially if the crane works with underwater installations, leakage to the sea could be a major risk.

A further major disadvantage of the known solution is that the sleeve must extend over practically the entire length of the piston rod when it is in a fully extended position. On the other hand, this means that the sleeve must fold up significantly in the longitudinal direction when the piston rod is fully inserted into the cylinder. This puts great stress on the sleeve and it must be made of a very flexible and elastic material.

If the sleeve has collected a quantity of leaked hydraulic oil, it will no longer fold neatly, but a bulge may occur at the bottom of the sleeve. This bagging will be able to hook into other parts, so that the sleeve is punctured.

US 4945724 describes a leakage collection device which is adapted to collect the small amount of hydraulic fluid which remains on the piston rod as a thin film of oil as the rod moves out of the cylinder. This is to prevent this thin oil film from accumulating over time and dripping onto the factory floor. A wiper ring is therefore arranged around the piston rod. At this ring, an annular groove has been made where hydraulic fluid can collect. From this groove, a line leads the fluid back to a hydraulic tank.

This device is only suitable for collecting the very small amounts of liquid that settle as a film on the surface of the piston rod. If a leak occurs in the seal, this wiper ring will not prevent the liquid from leaking out to the surroundings.

A similar device is known from JP H6241209. Here, a combined seal and scraper ring is fitted together in the end cap of the cylinder which encloses the piston rod. A drain line leads hydraulic fluid, which is wiped off, back to the hydraulic sump. This requires an end cap specially designed for the purpose and this device is not able to prevent major leaks.

Other solutions in this area are known from NOK 20090116010, RU 2232295, CN 102734263, DE 2903333, GB 1082790, CN 201281126YY, CN104006029, WO 2004044437, JP H07218, JP H072, JPE H072, JP1. None of these represent perfect solutions to the problem.

It is thus an object of the present invention to provide a leak sealing device that can be mounted on a hydraulic cylinder when a leak has occurred and preferably before the leak becomes so large that it poses a danger to the use of the cylinder, as the device is able to contain the leak back while the cylinder is in use.

It is a further object of the invention that the sealing device should be able to be mounted with the cylinder in place, and thereby without having to dismantle the cylinder or associated equipment.

It is also a purpose that the sealing device should be able to be mounted on the cylinders of a crane vessel while the vessel is at sea.

It is a further object of the invention that the sealing device should be able to be mounted on the cylinder as a safeguard in the event of leakage, e.g. before the cylinder is mounted in place, e.g. on a faucet.

This is achieved by a device for protection against leakage from seals between a cylinder housing and a piston rod, where the cylinder housing is equipped with a first barrier in the form of a gasket that seals against the piston rod to keep fluid enclosed in the cylinder housing, the protection device constituting a further barrier and comprises a retaining ring, which is arranged to be fixed in relation to the cylinder housing so that it projects beyond the end of the cylinder housing where the piston rod is inserted into the cylinder housing; at least one gasket adapted to be placed in the space between the retainer ring and the piston rod; and a pressing element, which is designed to hold the gasket in place, which device is characterized in that it further comprises a fastening device (e.g. clamps or welded ears) which is designed to be attached to the cylinder housing at a distance from the retaining ring and a the number of tension units that extend between the retaining ring and the clamp.

In one embodiment, the pressing element is inserted into the space between the retaining ring and the piston rod and is attached to the retaining ring. Thereby, the pressure against the gasket can be adjusted by adjusting the connection between the retaining ring and the pressure element.

In that the tensioning units are of a type that is designed to be able to be tightened after assembly, such as e.g. tensioners or actuators, the pressure against the gasket can be tightened.

In one embodiment, the retaining ring and the pressing element are continuous rings which are arranged to be threaded on or cylinder housing and piston rod from the outer end of the piston rod. This provides a simple and robust device that can be put on the cylinder already when it is produced.

In another embodiment, the retaining ring and the pressing ring are split rings, which can be placed around or the cylinder housing and the piston rod and are assembled together to form complete rings. Thereby, the device can also be retrofitted while the cylinder is in e.g. the crane.

As the gasket has a first and a second end which during installation are placed against each other and possibly glued together, the gasket can be installed or replaced while the cylinder is in place, e.g. in the faucet.

In an alternative embodiment, the device has a first and a second gasket and a chamber device which defines a chamber between the first and second gasket, the chamber having a drainage opening. This provides additional protection against leakage and leakage can be detected at the innermost of the seals,

Leaks along scratches in the piston rod can be stopped by the retaining ring being equipped with at least one pressing screw, which is arranged to be placed roughly flush with a longitudinal scratch in the piston rod, in order to press the gasket down into the scratch.

In a further embodiment, the device comprises a cap which is arranged to cover the piston rod and at least part of the retaining ring. Thereby, leaks past the device can be caught.

In one embodiment of the invention, a drainage pipe extends from the drainage opening and through the cap, the drainage pipe having an opening on the inside of the cap to drain away liquid that has accumulated on the inside of the cap.

The invention will now be explained in more detail with the help of concrete embodiments, which can be seen from the attached drawings, where:

Figure 1 shows an example of a lifting cylinder in longitudinal section,

Figure 2 shows a detail of the lifting cylinder in Figure 1,

Figure 3 shows the same area of the cylinder as in Figure 2b with a sealing device according to the invention fitted,

Figure 4 shows an alternative embodiment of the sealing device according to the invention,

Figure 5 shows a detail of the embodiment in Figure 4 with a tightening device for the gaskets,

Figure 6 shows an alternative embodiment of the invention

Figure 7 shows individual parts of the sealing device according to the invention in a split drawing,

Figure 8 shows an embodiment of the invention with two gaskets and a chamber between them,

Figure 9 shows an embodiment of the invention with compression screws in cross section,

Figure 10 shows the embodiment in Figure 9 in perspective,

Figure 11 shows an embodiment with an inspection hatch,

Figure 12 shows an embodiment with actuators to tighten the sealing device,

Figure 13 shows an embodiment with a cap covering the piston rod,

Figure 14 shows a detail of Figure 13,

Figure 15 shows an embodiment with a split retaining ring and pressure ring and

Figure 16 shows the split rings in the assembled state and in cross-section.

The present invention is particularly well suited for use on large cylinders in a lifting crane. This can be on land or on board a floating vessel. It is equipped with a number of lifting cylinders, which are caused to extend or contract by means of hydraulic pressure. These functions are well known to those skilled in the art. However, the sealing device according to the invention is well suited for use on hydraulic cylinders in general and is therefore not limited to crane cylinders. In the subsequent design examples, however, crane cylinders will be used as an example.

Figure 1 shows a typical lifting cylinder 2 in longitudinal section. Its main components are a cylinder housing 3 and a piston rod 4. The inner end of the piston rod 4 is received in the interior of the cylinder housing 3 through an opening 5 in the outer end of the cylinder housing. Figure 2 shows this in greater detail. The piston rod 4 is equipped with a piston 6 at its inner end. The piston 6 seals against the cylinder housing 3 walls. At the opening 5, an end cover 7 is arranged with a gasket 8, which seals against the outer surface of the piston rod 4. A head 20 is arranged at the piston rod's outer end, via which the piston rod can be attached to e.g. a crane arm.

When a hydraulic pressure is applied in the chamber 9 inside the piston 6, also called the piston side, the piston rod is pushed out of the cylinder housing 3. When a hydraulic pressure is applied in the chamber 10 outside the piston 6, also called the rod side, the piston rod 4 will be pushed into the cylinder housing 3 .

This means that the gasket 8 must hold tight against the very high pressure applied to the rod side 10. If a leak occurs at this gasket 8, as indicated by the arrow y, you will not only lose hydraulic pressure, but also risk oil spillage, as it is explained in the introduction.

A first embodiment of the invention will now be described with reference to Figure 3.

Figure 3 shows the area at the end cover of the cylinder housing, as in Figure 2.

The device according to the invention comprises several parts, which will be explained in the following:

A retaining ring 11 is arranged on the outer end (i.e. the end where the piston rod comes out of the cylinder housing) of the cylinder housing 3 at the end cover 7. The retaining ring 11 projects partly into the cylinder housing 3 and partly outside the cylinder housing 3.

The retaining ring 11 can be one continuous ring, and will then be threaded onto the cylinder 2 before it is mounted in place in e.g. the faucet, or it may be split lengthwise. In the latter case, the retaining ring 11 can be attached to the cylinder 2 while it is mounted in e.g. the crane. The two halves of the retaining ring 11 are fastened together in alternative conventional ways, e.g. using bolts.

The retaining ring 11 can be equipped with a seal 50, e.g. an O-ring that is placed in a custom groove. This prevents any leakage between the retaining ring 11 and the cylinder housing 3.

The retaining ring 11 is equipped with a pressing element 14, which can be integrated with the retaining ring 11, so that the retaining ring 11 and the pressing element form one unit, and project radially inwards from the part of the retaining ring 11 that lies around the cylinder housing 3. A gasket is placed inside the pressing element 13. This is located between the pressing element 14 and the cylinder housing 3. This gasket 13 can be any type of gasket, e.g. a felt gasket inserted with lubricating grease, a rubber or synthetic rubber gasket or a tallow gasket. It can be equipped with a reservoir of lubricating grease.

The retaining ring can also be equipped with at least one grease nipple 39 for supplying grease or other suitable liquids to the seal 13.

When the retaining ring 11 is in place at the end of the cylinder housing 3, it is attached to the cylinder housing 3 using suitable means, such as e.g. a row of set screws 12. Thereby the retaining ring 11 is held firmly in the cylinder housing 3.

To further secure the position of the retaining ring 11 on the cylinder housing 3, a clamp 16 is arranged around the cylinder housing 3 at a distance from the retaining ring 11. The clamp 16 is fastened tightly around the cylinder housing 3 and is equipped with ears 17 to which tension units 18 are attached. The tension units 18 are attached at their opposite end to lugs 19 on the retaining ring 11. The tension units 18 can e.g. be turnbuckles that can be tightened after they have been installed. The stretching units 18 can also be remotely operated actuators.

By means of the stretching units 18, the gasket 13 can be put under pressure, so that it effectively seals against leakage from the cylinder.

The tensioners/actuators can also be attached to another place on the cylinder housing, e.g. at the very rear of the cylinder housing.

The retaining ring 11 can also be equipped with a drain to drain away any leakage.

The retaining ring is also equipped with press screws 22, which will be explained in more detail below.

Figure 4 shows an alternative embodiment of the sealing device according to the invention. This differs from the embodiment in Figure 3 in that it is equipped with two seals 13a and 13b and a chamber unit which defines a chamber 24 between the seals 13a and 13b. A drainage line 25 runs from the chamber 24. The purpose of this is that if hydraulic oil should nevertheless leak past the first gasket 13a, e.g. because of. rust craters on the piston rod, then this oil will collect in the chamber 24 and drain away in a controlled manner. The drainage line 25 can be connected to a separate tank (not shown) or the hydraulic system's main tank (not shown). The drain line 25 can also be equipped with a shut-off valve 26. In this case, you can open this and drain oil from the chamber at regular intervals. If you drain the oil into a suitable container, you can easily check how much leakage there is.

The chamber unit which defines the chamber 24 can be provided by two pressure washers 27, 28 which are kept at a distance from each other by a number of set screws 29 (see figure 5), which are placed around the circumference of the retaining ring 11. With the help of these set screws 29, the distance between the disks 27, 28 and thereby also the pressure against the seals 13a, 13b can be adjusted.

Figure 5 shows a press screw in detail, which can be used in connection with the device in Figure 4.

Figure 6 shows a further embodiment of the invention. This differs from the embodiment in Figure 3 in that the pressing element 14 is not integrated with the holding ring 11, but is a separate ring. Figure 7 shows the embodiment in Figure 6 in its individual parts.

Unlike the previous embodiments, the gasket 13 can be put in place after the retaining ring 11 has been fitted. Gasket 13 is inserted into the space formed between the retaining ring 11 and the piston rod 4, where the retaining ring protrudes above the cylinder housing 3.

As shown by the designations L1 and L2, where L1 is the section where the retaining ring protrudes into the cylinder housing 3 and L2 is the section where the retaining ring protrudes outside the cylinder housing 3. These sections L1 and L2 will be adapted, among other things. according to the dimensions of the cylinder and the hydraulic pressure inside the cylinder.

The gasket 13 can be an elongated body with a first and a second end, which is placed around the piston rod 4 so that the first and second ends are adjacent to each other. Some such gaskets are delivered by the meter and can be cut to size as required. The first and second ends of the gasket can preferably be glued to each other to form an endless gasket 13.

The pressing ring 14 has a first part 14a with a dimension that fits into the space between the retaining ring 11 and the piston rod 4, and a second part 14b, which has approximately the same outer diameter as the retaining ring 11. The pressing ring 14 is guided with its part 14a into the space between the retaining ring 11 and the piston rod 4. The pressure ring 14 is pressed against the gasket 13 by e.g. screwed inwards by means of bolts 15 which attach it to the retaining ring 11. Alternatively, the first part 14a of the pressing ring can be equipped with external threads that engage with internal threads in the retaining ring 11, so that the pressing ring 14 is fixed by turning the entire pressing ring 14 in relation to the retaining ring 11. Regardless of which of these solutions is chosen, the pressure from the pressure ring 14 against the gasket 13 can be adjusted by either adjusting the bolts 15 or screwing on the pressure ring 14 itself.

Figure 8 shows an embodiment of the invention which is similar to the embodiment in Figure 4. It differs from this in that a separate pressing ring 14 is used, as in the embodiment in Figure 6. Another difference is that the pressing discs 27, 28 are fixed on the inside of the retaining ring 11. Thereby the set screws 29 are not needed. The inner gasket 13a is put under pressure by tightening the tension units 18 and the outer gasket 13b is put under pressure by tightening the bolts 15.

Figure 8 shows that the one disk 27 is designed with an axial part 27a, which extends outside the first gasket 13a. this will protect the first gasket 13a against excessive pressure from the side.

Figure 8 also shows that the compression ring 14 has been given an axial part 14c, which extends outside the second gasket 13b. This has also been done to protect the second gasket 13b against excessive pressure from the side, e.g. if the piston rod head hits the sealing device.

The axial portions 27c and 14c need not be continuous, but may also be in the form of lips extending over part of the circumference of the gaskets 13a and 13b.

Especially if the piston rod has a significantly smaller diameter than the cylinder housing, the retaining ring can also have a radial part that extends towards the piston rod and then transitions into an outer axial part that extends along the piston rod and at a distance from it. The gasket or gaskets will then be placed between the piston rod and this outer axial portion. Thereby, a gasket with a significantly smaller diameter than the diameter of the cylinder housing can be used. This solution can be used whether the pressing element is an integral part of the retaining ring or is a separate part.

Figures 9 and 10 show and illustrate the function of the press screws 22. From time to time you may experience that grains of sand or other foreign elements get between the piston rod 4 and the seal 8 (see figure 2). Such particles can get stuck here and create scratches in the piston rod 4 when it moves in and out of the cylinder housing 3. Such a scratch 21 is shown greatly exaggerated in figures 9 and 10.

Hydraulic oil will easily leak out of the cylinder housing 3 along such scratches.

To prevent this, the retaining ring 11 is equipped with a number of press screws 22. These are shown in figures 3, 4, 6, 7, 8, 910, 11, 12, 13, 15 and 16. In figures 4, 8, 11 and 13 two sets of press screws 22 are shown, one set for each of the two gaskets. In Figures 9 and 16, every second pressing screw 22 belongs to its own set.

When the retaining ring 11 is placed on the cylinder housing 3, it is rotated so that one of the pressing screws 22 aligns with the notch 21. After the gasket 13 is placed in the space between the retaining ring 11 and the piston rod 4, this pressing screw 22 is screwed in so that it presses against the gasket 13 and presses this into the groove 21. When the pressing screw 22 has been screwed in sufficiently, this is locked with a counter nut 23, so that the pressing screw stays in this position. The gasket 13 will now seal effectively also in the scratch 21.

If there are several scratches, the retaining ring 11 is oriented so that press screws 22 come as close as possible to these scratches. Even if a pressing screw 22 should not end up exactly flush with a scratch, it will still be able to press the gasket 13 sufficiently into the scratch 21.

When you have checked that the gasket has been pressed sufficiently into the scratch, the pressing ring 14 is fitted in the same way as explained above.

The pressing screws 22 can of course also be used if a scratch occurs after the sealing device according to the invention has been mounted. You then screw in the screw or screws closest to the scratch until the leak stops.

The retaining ring 11 can be equipped with an inspection hatch 30, as shown in figure 11. This can be fixed with screws 31. The inspection hatch can also be used to replace gaskets 13.

Figure 12 shows an embodiment where small actuators 32 are arranged between the retaining ring 11 and the pressure ring 14. Thereby the pressure on the gasket 13, or the gaskets 13a, 13b can be adjusted as needed, e.g. is increased to stop a leak past the seals. The stretching units 18 can also be actuators. This makes it possible to adjust the pressure on the seals 13, 13a, 13b remotely.

Figures 13 and 14 show an embodiment where a cap 33 is pulled over the piston rod 4 and the sealing device according to the invention. This hood can be of the type shown in NO 333699, but can also be a continuous hood without longitudinal division.

The cap 33 can be attached by means of a clamp 37 to the cylinder housing 3 and to an end piece 38, which is attached to the piston rod 4 near its outer end.

In order to avoid hydraulic oil leaking from the sealing device and collecting in the cap 33, the sealing device is preferably equipped with a drainage pipe 25, which leads oil from the chamber 24 between the seals 13a and 13b out through a hole 34 in the cap 33. The hole 34 for the drainage pipe 25 is preferably made in a slightly stiffer bulge 35 of the cap 33. This bulge will collect any oil that may have leaked into the cap 33. There may be an opening 36 in the drainage pipe 25 near the bottom of the bulge 35, so that the oil can flow into the drain pipe 25. The drain pipe can, as in the above embodiments, lead the oil to a separate collection tank, the main tank of the hydraulic system or be equipped with a valve for periodic emptying.

In the preceding embodiments, the retaining ring 11 and the pressing ring 14 are complete rings. These are mounted while the cylinder 2 is dismantled, by threading them over the piston rod's outer end (after the piston rod's head 20 has been removed). However, these parts can also be split, e.g. divided into two equal halves, so that they can be attached even when the cylinder is mounted on e.g. a crane. In this case, the device can be attached when a leak is detected. Such a variant is shown in Figures 15 and 16.

As shown in Figure 15, the retaining ring 11 is in two parts 11a and 11b. Each of these parts 11a, 11b is provided with either lugs 40 or a longitudinal flange at the opposite edges. During assembly, a gasket 41 is preferably placed along the opposite edges and bolts are passed through the ears 40 and tightened so that the two halves form one unit, as shown in figure 16.

The pressing ring 14 is also divided into two halves. Here, however, protruding parts must be avoided. Accordingly, depressions 42 are formed at the opposite edges of the two halves, so that the bolts which fasten them together lie below the surface of the pressure ring 14. Here, too, the joint can be equipped with gaskets 43.

A sensor (not shown) can be arranged between the cylinder housing's original seal 8 and the sealing device according to the invention. This is designed to measure leakage past the original seal 8 and keep the operator informed of developments.

If the leakage exceeds certain limits, replacement of the original seal can be planned at the next inspection.

In some cases, the piston rod 4 does not have a circular cross-section. In such cases, the pressure ring 14 will be designed with an opening adapted to the cross section of the piston rod 4.

A person skilled in the field will understand that the above-described embodiments can be combined in various ways, which is within the scope of the present invention.

Claims (9)

Patent requirements 1. Device for protection against leakage from seals between a cylinder housing (3) and a piston rod (4), where the cylinder housing (3) is equipped with a first barrier in the form of a gasket (8) that seals against the piston rod to keep fluid contained in the cylinder housing (3), which device constitutes a further barrier and includes a holder ring (11), which is designed to be fixed in relation to the cylinder housing (3) so that it projects outside the end of the cylinder housing (3) where the piston rod (4) is guided into the cylinder housing (3); at least one gasket (13) which is adapted to be placed in the space between the holder ring (11) and the piston rod (4); and a pressing element (14), which is designed to hold the gasket (13) in place, characterized in that the device further comprises a fastening device (16) (e.g. clamps or welded ears) which is designed to be attached to the cylinder housing ( 3) at a distance from the holder ring (11) and at a number of tension units (18) that extend between the holder ring (11) and the clamp (16). 2. Device according to claim 1, characterized in that the pressing element (14) is introduced into the space between the holder ring (11) and the piston rod (4) and is attached to 3. Device according to claim 1, characterized in that the tensioning units (18) are of a type that is designed to be able to be tightened after assembly, such as e.g. drawbars or actuators. 4. Device according to one of the preceding claims, characterized in that the holder ring (11) and the pressing element (14) are continuous rings which are arranged to be mounted on the the cylinder housing (13) and the piston rod from the outer end of the piston rod (4). 5. Device according to one of the claims 1-4, characterized in that the holder ring (11) and the pressing ring (14) are split rings, which can be placed around or the cylinder housing (3) and piston rod (4) and assembled together until the entire ring. 6. Device according to one of the preceding claims, characterized in that the device has a first (13a) and a second seal (13b) and a chamber device which defines a chamber (24) between the first and second seal (13a, 13b), the chamber ( 24) has a drainage opening (25). 7. Device according to one of the preceding claims, characterized in that the holder ring (11) is equipped with at least one pressing screw (22), which is arranged to be placed approximately flush with a longitudinal scratch (21) in the piston rod (4), in order to press the gasket (13) into the groove (21). 8. Device according to one of the preceding claims, characterized in that it comprises a cap (33) which is designed to cover the piston rod (4) and at least part of the retaining ring (11). 9. Device according to claims 6 and 8, characterized in that a drainage pipe (25) extends from the drainage opening and through the hood (33) and that the drainage pipe (25) has an opening (36) on the inside of the hood (33) to drain away liquid that has accumulated on the inside of the hood (33).