This invention relates to dampers, and in particular though not exclusively to linear piston and cylinder type dampers.

According to the invention there is provided a piston and cylinder type damper having an elongate cylinder with a longitudinal axis, a piston assembly mounted in the cylinder for reciprocal linear movement along said axis and dividing the cylinder into separate chambers with a restricted flow path therebetween for passage of damping fluid contained within the cylinder, and an elongate piston rod arranged coaxially with the cylinder and connected at one end to the piston assembly with the other end protruding out of the cylinder, wherein the damper further comprises a retaining mechanism for releasably holding the piston rod in a position where it is housed substantially within the cylinder.

By way of example, embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a form of damper according to the invention,

Figures 2a, 2b and 2c are detail views of the damper of Figure 1 showing operation of the piston rod retaining mechanism,

Figure 3 is a detail view of a modified form of damper according to the invention, and Figure 4 is a detail view of another form of damper according to the invention.

The damper seen in Figure 1 is a linear piston and cylinder type damper of circular cross-sectional shape having a piston assembly 10, an elongate piston rod 11 and an elongate cylinder 12 with a longitudinal axis x. The cylinder 12 is closed off at one end 12a by a plug 13. At its other end 12b, the cylinder 12 is closed off by a cap assembly 14. The cylinder contains a damping medium, such as oil or silicone.

The piston rod 11 is mounted coaxially with the cylinder 12 for linear reciprocal movement along the axis x. The piston rod 11 extends through the cap assembly 14 and has a free end 11a that protrudes out of the cylinder 12. The cap assembly 14 provides support to guide the linear reciprocal movement of the piston rod 11 and is suitably sealed off to prevent leakage of damping medium out of the cylinder 12. The piston rod 11 extends into the interior of the cylinder 12 where its inner end lib is attached to the piston assembly 10.

The piston assembly 10 divides the interior of the cylinder 12 into two separate chambers A, B and has an annular seal 17 for controlling passage of damping medium between them, in generally known manner. A compression spring (not shown in Figure 1) may typically be mounted in the cylinder 12 between the plug 13 and the piston assembly 10 acting to bias the piston assembly towards the cap assembly 14, ie tending to cause the damper to assume its most extended condition, with the free end 11a of the piston rod 11 protruding out of the cylinder 12 to its greatest extent. With linear piston and cylinder type dampers such as that seen in Figure 1, if they are left free to assume their most extended condition, this will leave the piston rod exposed to potential damage from knocks or stresses experienced during handling or transit, which can lead to problems of misalignment and/or leaky seals. In recognition of this, the damper according to the present invention incorporates a mechanism for retaining the piston rod for purposes of handling and transit.

The retaining mechanism for the damper shown in Figure 1 is seen more clearly in Figure 2a. The mechanism in this case is provided as an internal feature of the cylinder 12 and includes a small rib 15 on the inner bore 16 of the cylinder. This extends circumferentially around all or part of the bore. The rib 15 is designed to protrude into the interior of the cylinder 12 by a sufficient distance to engage the seal 17 on the piston assembly 10 with an intereference fit, but not so far as to engage the piston assembly itself, which is free to travel past it without interference.

The seal 17 here is in the form of a standard circular-section O-ring of a resiliently deformable material such as rubber. By virtue of its resiliently deformable nature, the seal 17 is capable of being forced past the rib 15. This is seen occuring in Figure 2a, as an axial force C applied to the end of the piston rod 11 is transmitted to the seal 17 via a first flange 18 on the piston assembly 10. Once the seal 17 has been forced past the rib 15 in this manner, the rib will then act as a stop to oppose its return. This is the position shown in Figure 2b. This is a preferred condition of the damper for purposes of handling and transit, because its piston rod is retracted and hence largely out of harm's way. The retaining mechanism is released by forcing the seal 17 back past the rib 15. This is achieved by applying a force D in the opposite direction to the end of the piston rod 11, which will then be transmitted to the seal 17 via a second flange 19 on the piston assembly 10. This is seen occuring in Figure 2c. Once the seal 17 has cleared the rib 15 in this manner, the damper will be free to operate normally.

It is preferred that the retaining mechanism will act to hold the damper in its most compressed condition, ie with the free end 11a of the piston rod 11 protruding out of the cylinder 12 to its least extent, as this will afford it maximum protection against possible damage.

It is also preferred that the retaining mechanism will operate when the piston assembly 10 is at a position that is outside its normal range of travel on its working stroke. This will ensure that the retaining mechanism does not interfere with the normal operation of the damper.

The retaining mechanism for the damper seen in Figure 3 is again provided as an internal feature of the cylinder 12. In this case, the bore 16 of the cylinder 12 is provided with a shallow circumferentially extending groove 20. When the seal 17 is moved axially into alignment with the groove 20, its resiliently deformable nature will tend to cause it to expand outwardly into engagement with the groove. This engagement will effectively act as a stop to oppose axial movement of the seal 17. Hence, the piston assembly 10 and also the piston rod 11 will effectively be held in this position.

It is preferred that this retaining position corresponds to the damper being in its most compressed condition, thus affording it maximum protection. It is also preferred that this is a position that lies outside the normal range of travel of the piston assembly 10 on its working stroke, ensuring that the retaining mechanism will not interfere with the normal operation of the damper.

The damper seen in Figure 4 has a form of retaining mechanism that is again provided as an internal feature of the cylinder 12. In this case, the mechanism takes the form of a releasable connection between a spigot 21, here provided on the piston assembly 10, and a hole 22, here provided in the plug 13. It will be understood that these features could of course be arranged the other way round.

The engagement of the spigot 21 in the hole 22 is designed to be in the nature of a releasable interference fit, with one or both parts being of material with suitable resilient flexibility to allow this. As with the dampers described above, the retaining mechanism here is activated and de-activated by applying suitable axial forces to the piston rod 11 and hence to the piston assembly 10.

As with the dampers described above, the retaining mechanism here is preferably designed to hold the piston assembly 10 in a position that is outside its normal range of travel on its working stroke, and to retain the damper in its most compressed condition.

A further arrangement is shown in figure 5. This arrangement is similar to that shown in figures 2a-2c, except for a variation in the diameter of the bore 16 as described in more detail below.

The cylinder 12 comprises a small rib 15, as in the prior embodiment. However, the inner bore 16 has a different diameter on either side of the rib 15. Specifically, the diameter of the bore 16 between the plug 13 and the rib 15 is greater than the diameter of the bore 16 between the rib 15 and cap assembly 14.

Figure 5 shows three representative diameters, Di, D ₂ and D ₃ . Di is the diameter of the bore 16 between the plug 13 and the rib 15. D ₂ is the diameter of the bore 16 between the rib 15 and the cap assembly 14. D ₃ is the diameter of the seal 17 (in this case an O-ring).

It is particularly preferred that diameter Di is larger than or, at least, equal to the diameter D ₃ of the seal 17. The diameter D ₂ , of the bore 16 between the rib 15 and the cap assembly 14, is smaller than diameter D ₃ - the diameter of the seal 17. Specifically, diameter Di is larger than diameter D ₃ to allow the seal 17 to relax during transit. If diameter Di was similar to that of diameter D ₂ , the seal 17 could be plastically deformed during transportation, particularly if subjected to high temperatures.

If diameter Di is smaller than diameter D ₃ , the seal 17 could deform plastically over time, and may be caused to pass over rib 15, by, for example, vibrations during transit.

It will be understood that a retaining mechanism for a damper could take many different forms in addition to those described above. For example, it would be possible to design a retaining mechanism that works externally of the cylinder, for example between the free end of the piston rod and the cylinder. It would also be possible to design retaining mechanisms with releasable connections in forms other than those described above, for example ones that use magnetic attraction or a frangible connector.