GB1589521A - Fluid operated telescoping device eg for use as an elevating device for a gun barrel - Google Patents

Abstract

The gun has a gun barrel, which is not balanced, and a cylinder/piston unit for height adjustment and damping of the gun barrel. This unit has two parts (24, 36) which can move in one another in a telescopic manner. The one part (24) is connected to the frame and the other part (36) to the gun barrel. A stationary piston (33) is attached to one of the two parts (24 or 36). A moving piston (34), together with an end surface of the stationary piston, bounds a cylindrical space (U2) which holds a flow medium under pressure in order to adjust the unit in length and, by means of the stationary piston (33), to produce damping of an abrupt lengthening of the unit. The stationary piston (33) has at least one hole with a narrowed outlet (35a) and/or has an overpressure valve (35b) in order to connect the two ends of this piston to one another, so that an abrupt lengthening of the unit is damped by limiting the piston movement by means of the flow medium which is located in the cylindrical space (U2). <IMAGE>

Description

(54) FLUID OPERATED TELESCOPING DEVICE E.G. FOR USE AS AN ELEVATING DEVICE FOR A GUN BA RREL (71) We, AKTIEBOLAGET BOFORS, a Swedish joint-stock company, acting under the laws of Sweden, of S-690 20 Bofors, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a fluid operated, telescoping device having a damping action and we describe below such a device which can be used for elevating a gun barrel.

According to the present invention, there is provided a fluid operated, telescoping device having a dampened telescoping action, comprising: a first and second part in telescoping relation, a piston unit in one of said parts defining an end wall of a first fluid chamber for receiving fluid under pressure to longitudinally displace the first and second parts, the piston unit comprising a first piston, fixed with respect to one of the parts and a second piston moveable in the longitudinal direction of the telescoping parts to provide with the first piston a damping action against abrupt telescoping extension of the device.

The device of the invention may in a preferred embodiment when used as an elevating device for a gun, permits running with the maximum elevating velocity to the fully elevated position. This may be achieved as a result of coaction between the movable or floating piston and a lower piston on the outer telescopic part and by ensuring that areas subject to the pressure of hydraulic fluid are such that when the barrel has passed the fully elevated position, the effective force transmitted by the hydraulic fluid will become insufficient to maintain continued elevation of the barrel; and the barrel will thus fall back to the fully elevated position.

In a preferred form of the device tensional forces due to expansion because of temperature changes in the fluid used are accommodated when the barrel is in the clamped position by the use of a compensation piston.

One embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a diagrammatic side view of a gun barrel elevation system.

Figure 2 shows in detail the fluid circuit for a gun barrel elevation system.

Figure 3a-3b show a longitudinal section of hydraulic cylinder (figures 3a and 3b should be viewed together).

Figure 4 is a detailed cross-sectional view of a fluid control valve.

Figures 5a-5c are sections of a combined fluid control valve.

Figure 1 shows a gun barrel 1 which is pivotally supported at one end on a trunnion 2 in a known manner. The barrel is supported on an upper mounting 3 which is rotatable in a horizontal plane about a centre axis 4. The barrel and upper mounting are shown in association with an artillery weapon, for instance a field artillery weapon, of a known type having a barrel with a calibre of, for example, 155 mm.

For elevation, the barrel is provided with two elevating cylinders 5, one on each side, of which only one is shown in Figure 1.

Each elevating cylinder has two telescopic parts, of which the first telescopic part 6 is pivotally mounted at its lower end about a supporting journal 7 on the upper mounting 3. The second telescopic part 8 is mounted in a spherical support 9 on the barrel.

The first and second telescopic parts of the elevating cylinder 5 are displacable in relation to each other by hydraulic fluid under pressure, which means that, in principle, the barrel will rest upon a column of hydraulic fluid in the elevating cylinder above the piston unit of the first part. The barrel is unbalanced and hence during ele vation the elevational force is proportional to the angle of elevation.

The inner, or first telescopic part 6 is provided with a piston unit which will be described in more detail below, and the elevating cylinder is provided with first and second connectors 10 and 11 for the supply of hydraulic fluid under pressure.

Passage of hydraulic fluid through connectors 10, 11 to the elevating cylinder from a fluid circuit (to be described below) is controlled by valve elements 12a, 12b respectively of a blocking valve 12. Connector (10) communicates with an operating pressure supply point A through the valve element 12a; and connector (11) can communicate either with drain pipe 14 via a constriction 13e, or the pressure supply line from point A or be sealed off from the rest of the fluid circuit by pressure distribution valve 13. The pressure distribution valve 13 has two control inlets 1 3a and 13 b, the former being connected at all times to operating pressure supply point A and the latter being connected via the second valve element 12b and constriction 13f to the pressure chamber on the lower side of the piston unit (space U, in Figures 3a, 3b): the pressure distribution valve 13 is also connected, via a constriction 13e, to an outlet, in this illustration drain pipe 14, which is connected to a drain or reservoir 15.

Depending upon the values of the fluid pressures at 13a and 13b, which thus control the position of its slide valves (to be described in more detail below) the valve 13 will connect line 13d (from connector 11) with either line 13c (the condition existing at the instant shown in Figure 1) or to line 14 and thus tank or reservoir 15 (by moving the spool to the right from the position shown in Figure 1, or will isolate connector 11 from the rest of the fluid circuit by moving the slide valves to a position where they blank off the opening to line 13d.

Connector (11) communicates through line 11 with the space below a floating piston 34 (space U in Figure 3a, b); and connector (10) communicates through line 10 with the space above fixed piston 33, between that piston and compensating piston 38 (space U3 in Figure 3a, b).

The purpose of the pressure distribution valve 13 is to ensure a predetermined relationship between the hydraulic fluid pressure in spaces U3 and U2. In the following description the arrangement of the fixed piston 33 and floating piston 34 is referred to as the piston unit, and it will be apparent, from the above description that the pressure distribution valve functions to control the relationship of the hydraulic fluid pressure on each side of the piston unit.

Opening and closing of valve elements 12a, 12b is effected by control of hydraulic fluid pressure from point B, which may be connected to either a drain point D or a system pressure supply point C by selective actuation of a first operating valve 17 (see Figure 2). Operating valve 17 may be a hydraulic three-way two-position valve of known type, which is spring-loaded towards its one position and caused to adopt its alternative position by an electromagnet.

Valve elements 12a and 12b open or close together.

When elevating or depressing the barrel the valve elements 12a, b will be open; when the barrel is depressed or clamped in position after depression, the valve 12a, b will be closed. The arrangement of blocking valve elements 12a, b and pressure distribution valve 13 permits firing with open blocking valve elements 12a, b, which is essential for efficient target tracking up to and including the instant of firing, or with closed blocking valve elements 12a, b.

Figure 2 shows the hydraulic circuit in more complete form than Figure 1. Operating unit 16 comprises the first operating valve 17 which connects pressure supply point B with either drain point D or system pressure C; at the instant shown in Figure 2, B is connected with drain point D and valve elements 12a, b will, therefore be closed.

Elevation and depression control is effected through control valves 18a and 18b. Valve 18a permits connection of drain point D to pressure supply point A and valve 18b permits connection of system pressure supply point C. Valve 18a cannot be open when valve 1 8b is open, and vice versa; though both may be closed at the same time. These valves 18a, b are described in more detail below in relation to Figure 4.

A traverse control system is included in the illustrated hydraulic circuit and comprises a second operating valve 19 and a second pair of control valves 20a and 20b.

The traversing control mechanism (not shown) is in fluid communication with valves 19 and 20a, b at connection points F and E.

A hand pump 21 permits manual pressurization of the system, enabling elevating and traversing to be carried out manually if the system supply pressure fails. The hand pump 21 is connected to the system pressure line, for communication with points A and B, at G, and to a hydraulic fluid tank or reservoir at H.

The piston and cylinder arrangement is shown in more detail in Figure 3a, b. The first telescopic part is in the form of a pipe 23 one end of which has stepped portions at 24, with a pronounce shoulder 25, and its other end being received in a support 28.

Pipe 23 has a hollow interior, with a first section 26 thereof of larger diameter than a second section 26l extending within stepped portion 24. A small diameter pipe 27 is force fitted into section 26l and extends throughout the first section 26 of pipe 23 and is sealed into support 28 where it communicates with a first duct 30 connected to blocking valve element 12a. A second duct 32 connects the interior of pipe 23 with second blocking valve element 12b.

A fixed piston 33 is secured on the outside of stepped portion 24 and sealingly engages the inner face of cylinder or outer pipe 36. Floating piston 34 is mounted also in the stepped portion 24 at the wider end portion thereof, for movement in the region defined by a face of fixed piston 33 and the wider end of stepped portion 24, which is formed by shoulder 25. The floating piston 34 sealingly engages the outer surface of stepped portion 24 and the inner surface of cylinder or outer pipe 36. The space between fixed piston 33 and floating piston 34 is space U2 which is of variable volume, the maximum volume being shown in Figure 3a, b. In principle the minimum volume can be zero; this would occur if the floating piston 34 came into contact with the lower side of fixed piston 33. It is here assumed that the situation will not occur. Cylinder or outer pipe 36 is supported on pipe 23 by piston 33 and 34 and a cylinder end 37 mounted on cylinder or pipe 36 in its end portion closest to the mounting 28. Space U1 is located below floating piston 34 and between outer pipe 36 and pipe 23, communicating with the interior of pipe 23 via passage 50. In the other end portion of cylinder or pipe 36 is mounted a pressure compensation piston 38 which is urged by a strong spiral compression spring 39, acting on the end cap 43 of cylinder or pipe 36 and the piston 38, to move in the direction of fixed piston 33, thereby to reduce the volume (space U3) between the two pistons 33, 38. Piston 38 will normally be kept forced against the lower face of end cap 43, with its flange 42 engaging therewith.

End cap 43 is bolted to a sleeve 41 which is screwingly mounted in the end of cylinder or pipe 36. The compensation piston 38 is located for movement within the sleeve, its longitudinal displacement being defined by the co-action of flange 42 with respectively the end face of cap 43 and a shoulder on sleeve 41.

The compensation piston 38 has a recess 40 in which is received the narrower end portion of the stepped portion 24 of pipe 23 when the barrel is in the depressed position; this is the position illustrated in Figure 3a, b. In the fully depressed position of the barrel compensation piston 38 is in mechanical contact with the fixed piston 33.

Longitudinally spaced apart openings 44 are provided in the narrower end portion of the tapered portion 24 and end of pipe 27 and connect the interior of pipe 27 with space U3 via a longitudinal passage 49 formed in the thickness of end portion 24.

It will be apparent that the space U3 is in communication with the operating pressure or drain from the control valve 1spa, or b respectively via longitudinal passage 49, openings 44, pipe 27, duct 30, when blocking valve element 12a is open. Recess 40 communicates with the interior of pipe 27 through opening 40l.

The fixed piston 33 has one or more constriction 35a through which hydraulic fluid can pass from space U2 on the upper side of floating piston 34 into space U3, or vice versa. In addition fixed piston 33 has a shock valve 35b which comprises a spring loaded slide, normally sealed against a seat but which can be forced therefrom to permit the passage therethrough of hydraulic fluid when the pressure in space U2 exceeeds by a predetermined value, the pressure in space U3. By virtue of this arrangement it is possible to smooth out the initial damping effect provided by the piston unit.

The contol valves 18a, b and 20a, b are similar and therefore only valves 18a, b are described in detail and with reference to Figure 4. The control valve comprises two valve spindles 51 and 52 moveable longitudinally in a chamber by the action of an eccentric cam 53 turned by a handle 56.

Each spindle is combined with a valve seat 54, 55 respectively.

The control valve has an inlet connection Cl to the system pressure supply (refer to C in Figure 2) which will usually be, say, 110 bar. Connection Al will be connected to a pressure supply point (point A in Figure 2) and connection Dl will be connected to a drain tank or reservoir for the hydraulic circuit (refer to D in Figure 2).

Actuation of valve spindle 52 (18b) moves seat valve 55 off its seat enabling a fluid connection between connections C' and Al.

Because of the eccentric configuration of cam 53 valve seat 54 (18a) remains in a closed position. The greater the degree of actuation of spindle 52, that is the higher valve seat 55 is lifted from its seat, the greater the rate of hydraulic fluid flow from Cl to A'. It follows that the elevational velocity of the gun can be determined through the control valve, by varying the degree of rotation of the cam 53.

On actuation of valve spindle 51 (18a) the operation is more or less identical to that for spindle 52 (18b) except that fluid will then pass from Al to Dl. This will have the effect of lowering the barrel (overall operation of the illustrated circuit is described below). Valve seat 55 (18b) remains closed.

In the position illustrated (the neutral position) of the cam 23 both valve seats 54, 55 are in the closed position: this prevents unwanted drifting of the barrel for reasons which will be clear from the following description of the operation of the circuit.

The blocking valve 12 is shown in detail in Figure 5a and comprises blocking valve elements 12a, 12b each of which is urged by an associated spiral compression spring 57, 571 into a closed position. Each valve element 12a, 12b is coupled via a shaft to a piston 58 on which springs 57, 571 act to effect the aforesaid urging of the valve elements. The lower faces (as seen in the drawings) of the pistons 58 are subjected to pressure from hydraulic fluid in chamber Bll which is connected to pressure supply point B. When B is connected to the system pressure supply through first operating valve 17 pistons 58 are moved upwardly (as seen in the drawings) lifting valve seats 56, 56l to allow fluid to flow from line 60 to 32 (and then to connector 11) and from line 30 (from connector 10) to line 59. When fluid pressure in B" is reduced or removed springs 57, 571 will again close seat valves 56, 561.

Referring to Figure 5b and c, the distribution valve 13 is shown in a closed position i.e. a position in which there is no connection of 13 with either 13C or 14 and the pressure on the lower side of the piston unit is one eighth of that above it. The distribution valve 13 comprises two slide valves 13g, 13h which can be displaced either downwardly (as seen in the drawings) to bring 13d into connection with 13c, or upwardly to bring 13d into connection with line 14 depending, as mentioned above, on the relative pressures at 13a and 13b. The slide valves 13g, 13h, each work against their respective control edges 61 and 62, and are arranged with their ends in neutral contact.

Valve 1 3g is arranged in a fixed lining formed with the control edge 61; the lining is arranged at an end of a recess in which the slide valve 13h is reciprocably located.

The slide valve 13g has a piston area which is 18 of the piston area of the slide valve 13h.

Items Sac may be attached as a unit (29 in Figure 1) to support 28 of the gun.

On the outer pipe 36, there is a support comprising a spherical bearing 45, contained in a bearing housing 46. The bearing has a dust cover 47 to prevent dirt from entering the support 9.

The components described above can be sealed using any known seal. The sleeve 41 can be secured to the outer pipe 36 by means of threads or the like. Figures 3a and 3b show the completely telescoped position of the telescopic first and second parts of the elevating cylinder. During elevation, the outer pipe is extended in the direction shown by the arrow 48.

Operation of the apparatus described above is as follows.

1. To elevate the barrel.

Operating valve 17 is actuated from the position shown in Figure 2, in which position the supply point B is connected to drain point D, to a position in which supply point B is connected to system pressure supply point C. The pressure at supply point C is then transmitted to chamber B11 (Figure 5a) causing pistons 58 to move upwardly, thereby opening valve seats 56, 56l; valve elements 12a, b are then opened.

Handle 56 is then rotated clockwise to open valve seat 55, connecting system pressure supply point C with supply point A and thereby hydraulic fluid is fed under pressure through open valve element 12a, connection point 10, duct 30, interior 31 of pipe 27, openings 44 and cut away portion 49 to space U3 between the compensation piston 38 and fixed piston 33. At the same time the lower side of piston 34 (space U1) is connected, through open valve element 12b with valve 13. The feeding of fluid under pressure in space U3 displaces compensation piston 38 with respect to fixed piston 33.

This results in a decrease in the volume of space U1 and a consequential pressurization of the fluid therein. The pressurized fluid acts, through line 13b (Figure 1 and 5b) to displace slide valve 13h upwardly so as to bring line 13d into connection with 14, through 13e (Figure 1 and 5b) and drain 15.

Draining of hydraulic fluid may then take place via the abovementioned lines past the control edge 62 of the valve 13.

As a result of the feeding of fluid under pressure to space U3 and draining of fluid under pressure from space U1 pipe or cylinder 23 and outer cylinder 36 are telescopically extended. Constriction 13e causes a back pressure to be obtained which is dependent upon the fluid velocity through the valve 13 from 13d to 14. This back pressure acts through channel 13i as the upper side of slide valve 13h so as to tend to close the valve against line 14: the effect of this is that the relation between the pressures acting on the upper and lower sides of slide valve 13h will change according to elevating velocity, and at the highest elevating velocities will approach 1:1.

2. To depress the barrel.

In order to depress the barrel handle 56 is rotated anticlockwise opening valve seat 54 (18a): this action connects supply point A with drain D, thus allowing draining of fluid from space U3 above the piston unit.

In this condition the weight of the barrel causes the fluid in space U3 to be forced to drain through open valve element 12a.

When this occurs the pressure in the fluid below the piston unit (in space U1) is decreased and the resulting differential between the pressure at 13a and 13b causes slide valves 13g and 13h to move downwardly (as seen in Figure Sb) bringing line 13d into connection with 13c and, thus point A, whereby some of the fluid from above the piston unit will be recirculated to space U1 below the piston unit. The pressure distribution valve 13 (13', 13ill in Figure 2) thus maintains a fluid pressure ratio between the lower side (space U1) and upper side (space U3) of the piston unit of, for example 1:8 during depression i.e. the valve is in a closed position when this ratio prevails, creating a recirculating fluid circuit. If the ratio varies from this figure the valve 13 will reopen allowing drainage from space U1 until the pressure below the piston unit is such that the ratio is again achieved. During elevation of the barrel the valve is open and the fluid pressure ratio is determined by constriction 13e (13e', 13elm in Figure 2): as explained above the greater the velocity the closer the pressure ratio will be to 1:1.

Due to the imbalance in the suspension of the barrel, the magnitude of the vertical recoil is dependant on the angle of elevation of the barrel. The higher the angle of elevation, the greater the vertical recoil. It will be understood that the characteristics of the pressure distribution valve 13 can be selected so that the drain 14 is not closed off from line 13d due to recoil when firing at low angles of elevation, but the pressure relationship above and below the piston assumes values close to 1:1. The pressure distribution valve also ensures that there will never be a pressure drop on the lower side (space Ul) and thereby prevents air release from the fluid.

As the barrel is unbalanced the pressure level of the hydraulic fluid in spaces U2 and U3 will vary in proportion to the ande of elevation of the barrel. In one embodiment the pressure will be approximately 25 bar at the maximum angle of elevation, and approximately 50 bar at an angle of 0 , as the column of fluid on which the barrel rests supports approximately 5 tons.

The pressure on the lower side of the floating piston 34 (space U1) is maintained, by the pressure distribution valve, at a value which, where the barrel is clamped or being depressed, is approximately X the pressure on the upper side. As explained above, during elevation of the barrel the pressure in U1 is between X and 1/1 of the pressure on the upper side of the piston unit U3 and during depression is 9 of the pressure on the upper side.

When firing with open blocking valves the pressure distribution system is completely sealed with recirculation through valve 13 (valves 18a, b being closed) except possibly at the lowest angles of elevation as mentioned above.

The arrangement described above gives good damping of vertical recoil of the barrel when firing with closed blocking valves 12 (12a, 12b). When a round is fired the barrel and piston 37 are lifted upwards and the hydraulic fluid enclosed in space U1 is pressurized with the result that floating piston 34 is displaced upwardly. The hydraulic fluid in space U2 is forced through constriction 35a into the space U3 and the movement of the floating piston 34 is resisted, thus resulting in a damping of the movement of piston 37 and thus of the barrel. The shock valve 35b is opened when the pressure in space U2 becomes too high to limit the damping to a predetermined level. When the kinetic energy of recoil has been absorbed, the barrel falls back, and damping takes place in the opposite direction, until the floating piston has resumed its original position against shoulder 25 of pipe 23. Compensation piston 38 equalizes the change in volume in space U3 and, as explained above, thereby avoids any decrease in pressure which might result in air being released from the hydraulic fluid.

It should also be understood that the barrel may recoil vertically on firing particularly at high angles of elevation to such a decree that, even if firing takes place while blocking valves 12 (12a, b) are open, the back pressure created at constriction 13e will assume a value sufficient to completely close drain 14. Piston unit (33, 34) will then carry out the damping function described above despite firing taking place with open blocking valves.

When the barrel is in its fully elevated position, the cylinder end 37 is in contact with the floating piston 34 so that at an elevation beyond the maximum elevation position floating piston 34 will be lifted away from shoulder 25. This movement, pressurizes fluid in space U2 and it will tend to flow into space U3. However as the fluid pressure in U3 and U2 will quickly equalise in value it will be apparent that, with the pressure in both faces of fixed piston U3 being the same, the effective lifting force area will simply be the end area of the tapered section. The sustaining force due to the action of fluid pressure on that area will be insufficient to support the barrel which will depress until the floating piston rests again against shoulder 25. In this way the barrel can be run at full elevating velocity to the fully elevated position with an efficient braking system at the limit of elevation.

The slide valves 13g and 13h work with long overlapping sections 13k and 131, respectively, which prevents undesirable flow of hydraulic fluid when the valves are in the closed position.

When the barrel is secured or clamped, the arrangement shown also accommodates any stresses which may arise due to variations in temperature. When the barrel is secured it is depressed with control valves 18 onto a barrel support. The column of fluid is connected between the drain pipe 14 and the tank via control valve 18a and pressure relief will occur with the aid of the compensation piston 38 where the piston forces fluid into the tank, resulting in a loss of pressure in the hydraulic circuit before the control and blocking valves are closed.

If the remaining fluid expands due to an increase in temperature the floating piston 34 and the compensation piston 38 are displaced to absorb the change in overall volume of the fluid and prevent any forces from being transferred from the cylinders to the damping device. The secured position is approximately 5" higher than the position in which the configuration in Figure 3a, b subsists.

The length of the stroke of the illustrated telescopic unit is of the order of 0.8m.

The reader is referred to our co-pending application No. 28171/77 (Serial No. 1,589,522).

WHAT WE CLAIM IS:- 1. A fluid operated, telescoping device having a dampened telescoping action, comprising: a first and second part in telescoping relation, a piston unit in one of said parts defining an end wall of a first fluid chamber for receiving fluid under pressure to longitudinally displace the first and second parts, the piston unit comprising a first piston, fixed with respect to one of the parts and a second piston moveable in the longitudinal direction of the telescoping parts to provide with the first piston a damping action against abrupt telescoping extension of the device.

2. A device as claimed in Claim 1, wherein said first piston is attached to the first telescoping part at one end thereof, and said second piston is slideably located on the first part, on a stepped section thereof, and confined to move between the first piston and a shoulder on said stepped section; the first piston having at least one constricted opening and/or a shock valve for connec tion between each side of said piston, whereby abrupt telescoping extension of the device is damped by the restriction of the movement of the second piston through fluid contained in a second fluid chamber defined between the pistons.

3. A device according to Claim 1 or 2, comprising a compensation piston urged by a spring to press fluid in the device against said first and second pistons in order to reduce cavitation in the fluid during abrupt telescoping movements of the barrel.

4. A device according to Claim 3, including a fluid circuit with valves therein through which fluid can be drained from the first fluid chamber so as to cause contraction of the device, wherein the compensation piston is urged in a direction so as to drive fluid out of the chamber, thereby relieving pressure in the chamber after contraction of the parts, and when the valves are actuated to close the fluid circuit, thereby sealing the chamber, the com

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. any stresses which may arise due to variations in temperature. When the barrel is secured it is depressed with control valves 18 onto a barrel support. The column of fluid is connected between the drain pipe 14 and the tank via control valve 18a and pressure relief will occur with the aid of the compensation piston 38 where the piston forces fluid into the tank, resulting in a loss of pressure in the hydraulic circuit before the control and blocking valves are closed. If the remaining fluid expands due to an increase in temperature the floating piston 34 and the compensation piston 38 are displaced to absorb the change in overall volume of the fluid and prevent any forces from being transferred from the cylinders to the damping device. The secured position is approximately 5" higher than the position in which the configuration in Figure 3a, b subsists. The length of the stroke of the illustrated telescopic unit is of the order of 0.8m. The reader is referred to our co-pending application No. 28171/77 (Serial No. 1,589,522). WHAT WE CLAIM IS:-

1. A fluid operated, telescoping device having a dampened telescoping action, comprising: a first and second part in telescoping relation, a piston unit in one of said parts defining an end wall of a first fluid chamber for receiving fluid under pressure to longitudinally displace the first and second parts, the piston unit comprising a first piston, fixed with respect to one of the parts and a second piston moveable in the longitudinal direction of the telescoping parts to provide with the first piston a damping action against abrupt telescoping extension of the device.

2. A device as claimed in Claim 1, wherein said first piston is attached to the first telescoping part at one end thereof, and said second piston is slideably located on the first part, on a stepped section thereof, and confined to move between the first piston and a shoulder on said stepped section; the first piston having at least one constricted opening and/or a shock valve for connec tion between each side of said piston, whereby abrupt telescoping extension of the device is damped by the restriction of the movement of the second piston through fluid contained in a second fluid chamber defined between the pistons.

3. A device according to Claim 1 or 2, comprising a compensation piston urged by a spring to press fluid in the device against said first and second pistons in order to reduce cavitation in the fluid during abrupt telescoping movements of the barrel.

4. A device according to Claim 3, including a fluid circuit with valves therein through which fluid can be drained from the first fluid chamber so as to cause contraction of the device, wherein the compensation piston is urged in a direction so as to drive fluid out of the chamber, thereby relieving pressure in the chamber after contraction of the parts, and when the valves are actuated to close the fluid circuit, thereby sealing the chamber, the compensation piston may compress the spring to increase the volume of the chamber whereby any expansion of the fluid is absorbed due to temperature changes.

5. A device as claimed in any of Claims 1 to 4 having a third piston mounted at an end of said second part, and said third piston being adapted to slideably receive said first part; wherein, upon extending the device, said third piston eventually abuts said second piston and said second piston is displaced thereby from said stepped section, and the arrangement is such that displacement of the second piston from the stepped section reduces or prevents any increase in the force sustaining the telescopic extension of the two parts, thereby defining the maximum extension of the two parts.

6. A fluid operated telescoping device substantially as hereinbefore described with reference to the accompanying drawings.

7. A gun having an unbalanced barrel and a fluid operated telescoping device according to any one of the preceding claims for elevating the barrel, one of said telescoping parts being pivotally attached to the base of the gun, the other to the barrel.