TECHNICAL FIELD

The present invention relates to a damper arrangement for a railway coupler, the damper arrangement comprising a damper and a plurality of elastomeric element.

BACKGROUND

In railway couplers, a plurality of force absorbing elements are typically provided for absorbing forces in draft mode (i.e. when a coupler is pulled away from a connected coupler) and buff mode (i.e. when the coupler is pushed against the connected coupler).

Typically, a plurality of elastomeric elements or spring pads may be provided for absorbing smaller forces whereas a damper is also provided for absorbing larger forces. Between them, at least one set of elastomeric elements and at least one damper may act during normal operation of a coupler in a train set and ensure that the forces are absorbed without risking damage to components of the coupler or of the railway car on which it is mounted.

Designing the railway coupler so that the elastomeric elements and the damper act as desired is often complicated, in particular since force absorption must function both in buff mode and in draft mode. Space restraints also generally require that force absorbing components are arranged either in parallel or one behind the other and this further complicates the design of the coupler. At the same time, the force absorbing components form a very important part of the coupler due to the ability to protect other components and ensure their continued operation. Whatever design is used must therefore always perform in a highly efficient and reliable way without risking malfunction or breakage.

One prior art solution is disclosed by GB1273440A that presents a combined buff and draw gear for central buffer couplings on railway vehicles. However, this solution is not sufficiently stable and efficient and therefore requires improvement.

It is particularly important in applications involving freight transport on railway that the damper arrangements provided are both space efficient and able to absorb large forces.

There is a need for improvements within this area.

SUMMARY

The object of the present invention is to eliminate or at least to minimize the problems discussed above. This is achieved by a damper arrangement and a coupler according to the appended independent claims.

The damper arrangement according to the present invention comprises a shell with a mounting bracket for mounting on a rear part of a coupler, and a draw bracket for mounting in connection with a pivot pin of a forward part of a coupler. Also, the damper arrangement comprises a damper having a damper body connected to the shell and a piston mounted on a damper head, the piston being axially received in the damper body and biased to extend in a forward direction. The damper arrangement also comprises a plurality of elastomeric elements arranged around the damper body between a first disc and a second disc, wherein the second disc is slidable to a first axial stop arranged on or connected the draw bracket at a rear part of the damper body, and wherein the first disc is slidable to a second axial stop that is arranged on or connected to a forward part of the damper body, such that a movement of the draw bracket in the forward direction in relation to the shell causes a compression of the elastomeric elements against the disc.

The damper arrangement thus provides a combination of a damper and a plurality of elastomeric elements that are arranged around the damper body. This is a highly space efficient solution that combines the operation of the damper and the elastomeric elements while at the same time requiring significantly less space in the coupler than known prior art technologies. Another advantage of the damper arrangement is that the damper and the elastomeric elements are configured to cooperate such that force absorption in both draft mode and buff mode is efficient and reliable. In draft mode, at least the elastomeric elements are operative whereas both damper and elastomeric elements act during buff mode. This is particularly advantageous in avoiding snatch forces that would otherwise cause the damper arrangement to suddenly spring back from a compressed state as soon as the compression force is released. Due to the damper cooperating with the elastomeric elements and acting to dampen the movement back to the neutral position, such sudden movements of the damper arrangement are minimized or in some cases even eliminated entirely.

It is also highly beneficial that the piston is biased to extend in the forward direction, since this renders the damper ready for damping further buff forces again almost immediately. It also enables the damper arrangement to provide a stabilizing force in any situation where a coupler shank of a coupler is pivoted in relation to the damper arrangement.

Suitably, the damper head is arranged with a clearance in relation to the first disc so that the damper head is configured to move a first stroke length in the rear direction towards the first disc when the damper is activated without contacting the disc. Thereby, force absorption in buff mode is divided into two parts where the damper acts alone during a first stroke length without engaging the elastomeric elements. This is particularly advantageous in extending a maximum stroke of the damper arrangement since the total stroke length is not limited by a stroke length of the elastomeric elements. Rather, the combination of available stroke length for a damper and for a stack of elastomeric elements may be selected so that a suitable total stroke length of the damper arrangement is achieved. It is also advantageous to combine the energy absorption of the damper and the energy absorption of the elastomeric elements towards the end of the stroke, since this provides a high force absorption that is generally difficult to achieve by either the one or the other acting alone. By providing the damper to act during the first stroke length, a total number of elastomeric elements may be reduced as compared to prior art solutions while still achieving a damper arrangement having a very high total energy absorption.

In some embodiments, the first stroke length is at least 20 mm, preferably at least 55 mm, more preferably at least 90 mm. Thereby, the damper is able to act alone during a significant part of the total stroke length of the damper arrangement. The total stroke length of the damper arrangement is also increased by increasing the first stroke length of the damper to avoid an available stroke length of the elastomeric elements limiting the total stroke length unduly.

Also, the first stroke length may be selected so that the damper is configured to have a total energy absorption of at least 100 kJ, preferably at least 120 kJ and more preferably at least 140 kJ, during movement of the damper head. Thereby, the damper is able to absorb a desired energy amount before the elastomeric elements are engaged. This also provides the advantage of achieving a higher total energy absorption due to the high energy absorption before the elastomeric pads are engaged.

Suitably, the damper head is configured to move a second stroke length together with the disc after the first stroke length by pushing against the disc for compressing the elastomeric elements. Thereby, the damper arrangement provides energy absorption by both the damper and the elastomeric elements acting simultaneously during the second stroke length, to increase the total energy absorption of the damper arrangement.

In some embodiments, the second stroke length is at least 10 mm, preferably at least 30 mm, more preferably at least 50 mm. Thereby, the total stroke length of the damper arrangement is formed by the first stroke length and the second stroke length combined. The second stroke length is suitably selected to be a total stroke length of the elastomeric elements, but may in some embodiments suitably be selected to be smaller than the total stroke length of the elastomeric elements.

Suitably, the second stroke length is selected so that an end force on the damper and the elastomeric elements combined is at least 1500 kN, preferably at least 2000 kN. Thereby, the damper arrangement is able to receive a very high end force by the damper and the elastomeric elements acting together during the second stroke length.

Also, the clearance of the damper head in relation to the disc may be adjustable by selecting a length of a contact portion of the damper head to be the first stroke length shorter than a distance from the damper head to the second axial stop. Thereby, the distance that the damper head may move towards the disc before actually contacting the disc can be selected so that the first stroke length is determined. This also allows for selecting the total stroke length, since the total stroke length is decided by the first stroke length combined with the smaller of a remaining available stroke length of the damper or a total available stroke length of the elastomeric elements.

Suitably, the damper is pre-tensioned in the forward direction. Thereby, the damper head moves forward during draft so that a return to the neutral position is dampened by the damper when the draft force is no longer applied. This is advantageous in avoiding snatch of the damper arrangement where the damper arrangement would otherwise return to the neutral position without any damping.

The present invention also relates to a coupler having a damper arrangement according to the invention.

Suitably, the coupler comprises a coupler shank mounted on a pivot pin, wherein the coupler shank further comprises a side contact portion for pushing the damper head of the damper arrangement in a rear direction when the coupler shank is in a pivoted position. Thereby, the damper arrangement acts to center the coupler shank and urge it back to a neutral position in situations where it is pivoted in relation to the damper arrangement.

Many additional benefits and advantages of the present invention will be readily understood by the skilled person in view of the detailed description below. DRAWINGS

The invention will now be described in more detail with reference to the appended drawings, wherein

Fig. 1 discloses a planar view from above of a damper arrangement according to a first embodiment of the invention, the damper arrangement being in a neutral position;

Fig. 2 discloses a planar view from above of the damper arrangement of Fig. 1 with the damper arrangement in a buff mode where the damper arrangement is compressed a first stroke length;

Fig. 3 discloses a planar view from above of the damper arrangement of Fig. 1 with the damper arrangement in a buff mode where the damper arrangement is compressed a total stroke length;

Fig 4a-4e disclose perspective views of the damper arrangement according to the first embodiment at different stages in buff mode, from a neutral position in Fig. 4a to a maximum stroke length;

Fig. 5a-5b disclose perspective views of the damper arrangement according to the first embodiment at different stages in draft mode;

Fig. 6a discloses a perspective view of the damper arrangement with the coupler shank rotated about a vertical axis;

Fig. 6b discloses a planar view from above of the damper arrangement with the coupler shank rotated about the vertical axis;

Fig. 7a discloses a perspective view of the damper arrangement with the coupler shank rotated about a horizontal axis; and

Fig. 7b discloses an enlarged view of the coupler shank, pivot pin and damper head of the damper arrangement of Fig. 7a.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the respective embodiments, whereas other parts may be omitted or merely suggested. Any reference number appearing in multiple drawings refers to the same object or feature throughout the drawings, unless otherwise indicated.

DETAILED DESCRIPTION

The damper arrangement of the invention will be described below with reference to the appended drawings. The damper arrangement is configured to be mounted in a railway coupler and it is to be noted that the damper arrangement as such is suitable for various types of railway couplers.

A railway coupler generally comprises a rear part for mounting on a railway car or a locomotive and a front part for coupling to a similar coupler in order to form a train set. The front part comprises a coupler shank that is connected to a front end where at least a mechanical coupler but suitably also at least an electrical coupler is provided. The coupler shank is pivotably arranged on a pivot pin that connects the coupler shank to the rear part of the coupler.

During operation of the railway vehicles, the relative position of two railway cars in relation to each other changes continuously, in particular during acceleration and braking of the railway vehicles. When one railway vehicle is pulled away from another, for instance during acceleration of the train set, this is referred to as a draft mode. The draft mode is defined herein as the front end of the coupler being pulled away from the rear end. On the other hand, when one railway vehicle is pushed against another, for instance during braking of the train set, this is referred to as a buff mode. The buff mode is defined herein as the front end of the coupler being pushed towards the rear end. The present invention is particularly useful in freight applications where heavy goods are transported on railway and where efficient damping is particularly important.

The damper arrangement of the present invention is provided for energy absorption in draft mode as well as in buff mode. The invention will now be described in detail with reference to the appended drawings.

Fig. 1 discloses a damper arrangement 1 in a neutral position according to a first embodiment of the invention for mounting in a coupler 100. The neutral position is a rest position in which the damper arrangement 1 is subjected to neither draft nor buff, i.e. where no energy is absorbed by the damper arrangement.

Components of the coupler 100 itself are shown in the figures only where they are relevant for explaining the configuration and operation of the damper arrangement 1. It is to be noted that the coupler 100 apart from the inclusion of the damper arrangement 1 according to the invention may be a conventional coupler having conventional components as is commonly known within the art. The coupler 100 in Fig. 1 is shown with a coupler shank 120 rotatably arranged on a pivot pin 110.

The damper arrangement 1 comprises a shell 2 connected to a mounting bracket 21 for mounting on the rear part (not shown) of the coupler 100. The damper arrangement 1 also comprises a draw bracket 3 that is connected to the pivot pin 110 as will be explained below with reference to Fig. 4a onwards.

In the damper arrangement 1 , a damper 4 is provided with a damper body 41 that is connected to the shell 2. In the first embodiment, this is achieved by a rear end 411 of the damper 4 being fixedly attached to the shell 2. The damper 4 also comprises a piston 42 mounted in the damper body 41 and connected to a damper head 43. The piston 42 is biased to extend in a forward direction FD, i.e. an axial direction away from the damper body 41. The forward direction FD is a direction towards the left-hand side of Fig. 1 and is indicated by an arrow.

The damper 4 may be a hydraulic damper, a gas-hydraulic damper, or any other type of damper that is suitable for use in a railway coupler. The configuration and operation of the damper itself is well-known in the art and will not be described in more detail herein. Suffice it to say that the damper 4 operates by absorbing energy as the piston 42 is pushed into the damper body 41. Since the piston 42 is biased in the forward direction FD, the damper 4 strives to return to a neutral position or even an extended position of the piston 42 at any time when a force in a rear direction RD, i.e. to the right in Fig. 1 or towards the damper body 41, is not acting on the piston 42. This is particularly beneficial since it causes the piston 42 to extend at least to the neutral position when not subjected to buff forces, so that it is always ready to absorb additional buff forces without requiring a draft force applied to the piston 42 to bring it back to the neutral position.

The damper 4 of the damper arrangement 1 is preferably pre-tensioned in the forward direction FD so that the damper head 43 is extended forward during draft loads. This is particularly beneficial in causing the damper 4 to dampen movement back to the neutral position when the draft load is no longer applied and thus avoids sudden undamped movement of the damper arrangement when moving from the draft mode to buff mode. In other embodiments, the piston 42 is instead biased in the forward direction FD to the neutral position but not pre-tensioned towards an extended position in the forward direction FD.

Also provided in the damper arrangement 1 are a plurality of elastomeric elements 5 that are arranged around the damper body 41 between a second disc 52 and a first disc 53. The second disc 52 is arranged on or connected to a first axial stop 32 on the draw bracket 3 at the rear end 41 1 of the damper body 41. On a forward part 412 of the damper body 41, a second axial stop 44 is provided to prevent the first disc 53 from sliding past the second axial stop 44 in the forward direction FD. The second axial stop 44 may be a nut or a disc, or alternatively a plurality of stops distributed on a circumference of the damper body 41. Such a plurality of stops may be attached to the damper body 41 by welding or bolting, or alternatively they may be extruded. Other designs of the second axial stop 44 are also possible as long as a reliable stop is provided for the first disc 53. The plurality of elastomeric elements 5 suitably comprise individual elastomeric elements 51 that are arranged in a stack and that are suitably separated from each other by separation discs 54, but other configurations of elastomeric elements 5 are also possible within the scope of the invention. The elastomeric elements, also known as spring pads, themselves typically comprise a polymer material and / or rubber.

The damper head 43 further comprises or is connected to a contact portion 45 that extends in the rear direction RD towards the first disc 53. The contact portion 45 suitably extends also in a circumferential direction to form a contact end 431 configured to contact the first disc 53 evenly. Between the contact end 431 of the contact portion 45 of the damper head 43 is a distance in the neutral position that forms a first stroke length S 1.

When the damper arrangement 1 is subjected to a buff force that pushes the coupler shank 120 of the coupler 100 in the rear direction RD, the piston 42 is pushed into the damper body 41 by the coupler shank 120 pushing against the damper head 43. During a first part of the stroke of the damper 4, the damper 4 absorbs energy without the damper arrangement 1 also engaging the elastomeric elements 5. This is achieved through the damper head 43 being pushed in the rear direction RD along the first stroke length SI to compress the damper 4 without contacting the first disc 53.

Fig. 2 discloses the damper arrangement 1 during buff mode when the damper head 43 has been pushed to a position where the contact end 431 contacts the first disc 53. In this position, any further movement of the damper head 43 in the rear direction RD also transfers force to the first disc 53 so that the elastomeric elements 5 are compressed. Also shown in Fig. 2 is a cross-section of the damper 4 to show that the piston 42 is pushed into the damper body 41 but that there is still space for the piston 42 to move further in the rear direction RD.

Fig. 3 discloses the damper arrangement 1 during buff mode when a total stroke length SL of the damper arrangement 1 has been reached. From the position of the damper head 43 and the first disc 53 in Fig. 2, they have now moved a second stroke length S2 in the rear direction RD to reach a position where a maximum stroke length of the damper 4 or of the elastomeric elements 5 or both has been reached. This corresponds to a total stroke length SL of the damper arrangement.

As soon as the compressive force applied through the coupler shank 120 pushing the damper head 43 in the rear direction RD is removed, the elastomeric elements 5 will return to their neutral position shown in Fig. 1, and the piston 42 of the damper 4 will also return to the neutral position due to the bias.

The buff mode will now be described in more detail with reference to Fig. 4a- 4e.

Fig. 4a discloses the neutral position also shown in Fig. 1. The draw bracket 3 is shown as connected to the pivot pin 110 by the pivot pin extending into an opening 33 in the draw bracket 3. In the neutral position, the pivot pin 110 is held in a front end of the opening 33 so that any draft forces on the pivot pin 110 is transferred to the draw bracket 3, whereas a clearance or play in the opening 33 allows the pivot pin 110 to move in the rear direction RD without a force transfer to the draw bracket 3. The play is achieved by the opening 33 being larger than a diameter of the pivot pin 110 so that the pivot pin is allowed to move in the rear direction RD without contacting a rear end of the opening 33.

Furthermore, the pivot pin 110 is also arranged with a clearance or play in relation to the coupler shank 120 so that the opening in the coupler shank 120 is larger than the pivot pin 110 and allows the coupler shank 120 to move without transferring this movement to the pivot pin 110. In the neutral position, the pivot pin 110 is in a rear end of the opening in the coupler shank 120 so that a draft movement of the coupler shank 120 in the forward direction FD is transferred to the pivot pin 110, whereas a buff movement of the coupler shank is not transferred to the pivot pin 110.

Fig. 4a also shows that the pivot pin 110 is fixedly attached to the damper head 43, suitably by bolts or screws 111, and that the damper body 41 is attached to the shell 2 by bolts or screws 413.

In Fig. 4b, the coupler shank 120 is pushed in the rear direction RD, moving the damper head 43 towards the first disc 53 but still with a clearance between the contact end 431 and the first disc 53. The movement of the damper head 43 is still within the first stroke length S 1. In Fig. 4c, the damper head 43 has moved the first stroke length S 1 from the neutral position so that the contact end 431 contacts the first disc 53, and in Fig. 4d the damper head 43 pushes both the piston 42 and the first disc 53 in the rear direction RD. This creates a distance between the second axial stop 44 and the first disc 53.

Fig. 4e discloses the damper arrangement 1 in a buff end position where the damper head 43 has moved both the first stroke length S 1 , where the damper 4 acts alone to absorb energy, and the second stroke length S2, where the damper 4 and the elastomeric elements 5 act together to absorb energy. This means that the total movement of the damper head 43 forms the total stroke length SL of the damper arrangement 1 , formed by the sum of the first stroke length SI and the second stroke length S2.

It is advantageous if the pivot pin 110 does not reach the rear end of the opening 33 until the buff end position of Fig. 5e is reached, to avoid pushing against the draw bracket 3.

In some embodiments, the total stroke length SL corresponds to a total stroke length of the damper 4 with the second stroke length S2 corresponding to a total stroke length of the elastomeric elements 5. This means that the full available stroke of both the damper 4 and the elastomeric elements 5 is used in the damper arrangement 1. However, in other embodiments the total stroke length SL may be shorter than the total stroke length of the damper 4 or the second stroke length S2 may be shorter than the total stroke length of the elastomeric elements 5. In particular, the second stroke length S2 may be shorter than the total stroke length of the elastomeric elements 5 to allow for a stroke length in draft mode, where only the elastomeric elements 5 are compressed, that is larger than the second stroke length S2 in buff mode. In one such embodiment, the total stroke length of the elastomeric elements 5 may be 50 mm but the second stroke length S2 may be less, suitably about 30 mm. This is achieved by selecting the first stroke length SI to be a desired second stroke length S2 less than the total stroke length of the damper 4. As a result, in buff mode the damper 4 is compressed its total stroke length but the elastomeric elements are not. This is particularly useful in ensuring a high energy absorption in buff mode. In one embodiment, the total energy absorption in buff mode may be 2000 kN provided by the damper 4 and the elastomeric elements 5, while the total energy absorption in draft mode may be 1000 kN provided by the elastomeric elements 5 alone.

Operation of the damper arrangement 1 in draft mode will now be described with reference to Fig. 5a-5b and also to Fig. 4a that discloses the neutral position.

In Fig. 5a, the coupler shank 120 is pulled in the forward direction FD and the pivot pin 110 transfers a force to the draw bracket 3 by contacting the forward end of the opening 33. This in turn causes the draw bracket 3 to move in the forward direction FD and move the second disc 52 by the first axial stop 32 pulling on the second disc 52. This causes the elastomeric elements 5 to be compressed between the second disc 52 and the first disc 53 that is held against the second axial stop 44. Thus, the elastomeric elements 5 act to absorb the energy during draft mode, whereas the damper 4 is suitably pretensioned to allow the damper head 43 to follow the movement of the pivot pin 110.

In Fig. 5b, the damper arrangement 1 is in a draft end position where the second disc 52 has moved the second stroke length S2 while the first disc 53 is held immobile as compared with the neutral position. In the draft end position, the elastomeric elements 5 are compressed and the piston 42 is extended in the forward direction FD.

One particular advantage of the damper arrangement 1 according to the invention is that the damper 4 dampens movement of the damper arrangement 1 back to the neutral position when the draft force ceases to act. This ensures a smooth operation of the damper arrangement 1 and minimizes the snatch when going quickly from draft to buff forces.

The first stroke length SI is suitably at least 20 mm, preferably at least 55 mm, and more preferably at least 90 mm. In some embodiments, the first stroke length SI may even be 95 mm or more. The second stroke S2 on the other hand is suitably at least 10 mm, preferably at least 30 mm, more preferably at least 50 mm. In some embodiments, the second stroke length S2 may even be 55 mm or more. This gives a total stroke length SL of the damper arrangement 1 that may be up to 140 mm and in some applications even 150 mm or in some cases up to 160 mm.

It is also advantageous if the damper 4 is configured to absorb at least 100 kJ, preferably at least 120 kJ and more preferably at least 140 kJ, during movement of the damper head 43 the first stroke length S 1. This ensures that the energy absorption is high even before the elastomeric elements 5 are engaged.

Furthermore, it is an advantage if second stroke length is selected so that an end force on the damper 4 and the elastomeric elements 5 combined is at least 1500 kN, preferably at least 2000 kN. Thereby, the total force that the damper arrangement 1 is able to absorb is very high and renders the damper arrangement 1 highly efficient in many applications.

Another advantage of the damper arrangement 1 is shown in Fig. 6a-6b and Fig. 7a-7b that disclose pivoting of the coupler shank 120 in relation to the damper arrangement 1.

In Fig. 6a-b, the coupler shank 120 is pivoted in a horizontal direction, i.e. around a vertical axis extending along the pivot pin 110. This may be in response to a curve in railway tracks during operation of the train set in which the coupler 100 is mounted. The coupler shank 120 is pivoted a horizontal first angle a, causing a side contact portion 121 of the coupler shank to push against the damper head 43 and cause a buff stroke of the damper arrangement 1. Due to the bias of the piston 42, the damper 4 strives to push the damper head 43 back to the neutral position and thereby also to push the coupler shank 120 back to its neutral position where the first angle a is zero and the coupler shank 120 extends along a first axis A. This is particularly advantageous since the damper arrangement 1 is able to provide a stabilizing force in any situation where the coupler shank 120 is pivoted in relation to the damper arrangement 1. Fig. 7a-7b disclose a vertical pivoting of the coupler shank 120, i.e. a pivoting around a horizontal axis extending perpendicular to the pivot pin 110 and also perpendicular to the first axis A. In a vertically pivoted position, the coupler shank 120 extends at a vertical second angle p to the first axis A, and the side contact portion 121 of the coupler shank 120 pushes the damper head 43 to cause a buff stroke of the damper arrangement 1. Due to the bias of the piston 42, this also causes a stabilizing force in the damper arrangement 1 that strives to return the coupler shank 120 to the neutral position where the coupler shank 120 extends along the first axis A.

Fig. 8 discloses a force diagram showing the stroke of the damper arrangement 1 in relation to a force in buff mode and to a force in draft mode. The strokes and forces given in the diagram are to be seen as exemplary only and serve to further explain the present invention.

Starting from the neutral position, a dynamic buff stroke is shown in an upper curve and corresponds to a sudden impact or a crash, whereas a quasi-static stroke is shown in a lower curve and corresponds to a slow compression of the damper caused by a slow braking of the train set.

In the dynamic buff stroke, the buff force causes a stroke of the damper arrangement 1 and during a first part of the curve that corresponds to the first stroke length SI (given as 80 mm in the example), the damper 4 acts alone and absorbs forces up to a first level that is in the diagram given as 1500 kN. After the first stroke length SI, the elastomeric elements 5 are activated and act together with the damper 4 during the second stroke length S2 (given as 30 mm to arrive at a total stroke length of 110 mm). This allows for end forces of of up to 2000 kN.

In the quasi-static buff stroke, the damper 4 acts alone during the first stroke length SI, given as 80 mm, and the elastomeric elements 5 are then engaged during the second stroke length S2 as the damper 4 and elastomeric elements 5 are compressed to the total stroke length of 110 mm. The absorbed energy and applied force are smaller than in a dynamic buff stroke. Returning to the neutral position, the lower curve discloses the damper acting to dampen the movement of the piston 42 and the expansion of the elastomeric elements 5 back to their original position.

In draft mode, the force diagram shows the elastomeric elements 5 acting alone to a maximum draft stroke of 30 mm in the example. In the example shown in Fig. 8, the maximum draft stroke is equal to the second stroke length S2 but in some embodiments the maximum draft stroke may instead be larger, as is explained in the description above. The return from the draft stroke is dampened by the damper 4 due to the pre-tension of the piston 42. It is to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.