EP2558403A1

EP2558403A1 - Method for locking operating position of boom, personnel hoist and lift cylinder

Info

Publication number: EP2558403A1
Application number: EP11768513A
Authority: EP
Inventors: Kari Raitmaa
Original assignee: Bronto Skylift Oy AB
Current assignee: Bronto Skylift Oy AB
Priority date: 2010-04-14
Filing date: 2011-04-14
Publication date: 2013-02-20
Anticipated expiration: 2031-04-14
Also published as: EP2558403A4; FI122539B; EP2558403B1; FI20105387A0; FI20105387A; PL2558403T3; WO2011128510A1; ES2552434T3

Abstract

The invention relates to a method for locking an operating position of a boom, to a personnel hoist and a lift cylinder for a boom. The boom (3) may be raised up to an operating position (A) by the lift cylinder (8). The movement between a piston rod (13) of the lift cylinder (8) and the cylinder body (14) may be locked at least in the return direction by means of a locking device (17). The locking device comprises a first locking member (10) moving together with the piston and a second locking member (15) coupled to the cylinder body.In the locking position of the locking device the relative longitudinal movement between the locking members is prevented mechanically, which allows the boom to be locked to its operating position.

Description

Method for locking operating position of boom, personnel hoist and lift cylinder

Background of the invention

[0001] The invention relates to a method for locking a boom into a desired operating position. To lock the boom, a lift cylinder used for raising the boom is locked mechanically to prevent the cylinder from retracting. At least one of the boom lift cylinders is provided with a locking device to perform the locking.

[0002] The invention further relates to a personnel hoist and a boom lift cylinder. The field of the invention is described in greater detail in the preambles of the independent claims of the application.

[0003] Personnel hoists are used e.g. in fire fighting and rescue operations as well as in building construction works and different maintenance duties. A personnel hoist comprises a boom that is raised from a transport position to a vertical position by one or more lift cylinders. If the boom is long, also a lift cylinder with relatively long dimensions is to be provided. In that case a hydraulic lift cylinder also has fluid spaces of a large volume. Although the compressibility of fluid is small, a long lift cylinder has flexibility due to its large fluid volumes. It is also to be noted that since the lift cylinder is at the bottom end of a long boom, even minor flexibility in the lift cylinder causes large movements at the top of the long boom. Such flexibility impairs significantly the positioning of the boom. To solve this problem, a locking device fastened to the cylinder body of the lift cylinder has been designed to mechanically lock the piston rod. A prior art locking device wedges the piston rod to lock it immovable. A disadvantage of this solution is that the wedge members are pressed at an extremely great force against the piston rod, which may damage the rod surface and thus cause leakage in the lift cylinder.

Brief description of the invention

[0004] An object of the invention is to provide a novel and improved method for locking the operating position of a boom, a personnel hoist, and, further, a novel and improved lift cylinder.

[0005] The method of the invention is characterized by providing a first locking member and a second locking member with a matching, rotational- ly non-symmetric shape in the longitudinal direction of the lift cylinder; and selecting the operation of the locking device between a released position and a locking position by changing the respective positions of the locking members about the longitudinal axis of the lift cylinder, the rotationally non-symmetric shapes of the locking members thus occupying a matching or a non-matching position, respectively.

[0006] The personnel hoist of the invention is characterized in that the first locking member comprises a tubular part and an end piece arranged around the lift cylinder; that the end piece comprises a locking aperture having a rotationally non-symmetric shape; that the second locking member is fastened to the outermost end portion of the cylinder body and the shape of the outer surface of the second locking member substantially corresponds to the shape of the locking aperture, whereby the second locking member and the locking aperture match in the direction of the longitudinal axis of the lift cylinder; and that the locking device comprises at least one turning device allowing the first locking member to be rotated about its longitudinal axis so that in the released position the locking aperture is placeable to match the second locking member and, on the other hand, in the locking position the locking aperture is placeable to a non-matching position in relation to the second locking member.

[0007] The lift cylinder of the invention is characterized in that the first locking member and the second locking member comprise matching rotationally non-symmetric shapes in the longitudinal direction of the lift cylinder; and that the locking device comprises at least one turning device allowing the relative position of the locking members to be rotated in relation to the longitudinal axis of the lift cylinder so that in the released position the rotationally non- symmetric shapes of the locking members are placeable to match each other and, on the other hand, in the locking position they are placeable to non- matching positions.

[0008] According to an idea at least one of the boom lift cylinders comprises a locking device provided with at least one first locking member and at least one second locking member which are independent of the basic structure of the lift cylinder. The first locking member is an elongated piece arranged to move with the piston rod and in relation to the cylinder body and to a second locking member fastened thereto. The locking device further comprises means for coupling the first and the second locking member to be immovable in relation to each other in the longitudinal direction at least in the return direction of the lift cylinder. In the released position of the locking device the lift cylinder may be freely extended and retracted. The basic idea of the locking is that when seen in the longitudinal direction of the lift cylinder, the locking members are provided with rotationally non-symmetric yet matching shapes or shaped surfaces and therefore rotating the locking members in relation to one another allows a locking position and a released position to be selected. The locking reflects the rotating positions and compatibility of the locking members in relation to each other in the longitudinal direction of the lift cylinder.

[0009] An advantage of this is that locking forces caused by the operation of the locking device act on the locking members, which are independent of the basic structure of the lift cylinder and therefore locking does not damage the lift cylinder in any way. For example, the locking causes no forces that could damage the piston rod and its sealing surfaces. Moreover, the locking device is strong and both its structure and operation are simple.

[0010] According to an embodiment, the first locking member and the second locking member are locked in a shape-locked manner in relation to each other in at least one pre-determined locking position. The shape-locking is firm and allows the boom to be locked to a pre-determined position with precision. In addition, the shape-locking may have a simple structure.

[0011] According to an embodiment, the first locking member is a tubular piece, and the second locking piece is arranged to act on the tubular first locking member. The tubular first locking member is arranged to surround the piston rod of the lift cylinder, thus protecting the piston rod against damages. The tubular locking member may have an outer surface which is impermeable and has no openings and therefore it is capable of protecting the piston rod also against impurities.

[0012] According to an embodiment, the first locking member is a tubular piece with a rectangular, preferably square, cross-section. This embodiment facilitates the manufacturing of the locking device.

[0013] According to an embodiment, the first locking member is a tubular piece with an impermeable outer casing.

[0014] In addition, the top end of the tubular locking member is sealed with sealing members. This prevents impurities, dust and water, such as fire fighting water, from entering the locking device from above. In some cases also the bottom end may be sealed, the locking device being thus protected also against impurities entering from below. On the other hand, the bottom end may be open, whereby an aired structure is provided and no humidity due to condensation, for example, forms inside. [0015] According to an embodiment, the first locking member serves as a security lock and protection for the piston rod of the lift cylinder.

[0016] According to an embodiment, the first locking member is arranged to support the structure of the lift cylinder, which is a feature that may be taken into account in the design of the structural parts of the lift cylinder.

[0017] According to an embodiment, the tubular first locking member is arranged around the lift cylinder and moved together with the piston rod in relation to the cylinder body. The second locking member is immovably arranged on the outermost end portion of the cylinder body. The first locking member and the second locking member have matching, rotationally non- symmetric shapes when seen in the longitudinal direction of the lift cylinder. The tubular first locking member may be rotated about its longitudinal axis with respect to the second locking member to select a released position or a locking position.

[0018] According to an embodiment, there are two adjacent lift cylinders associated with the boom, both cylinders being provided with a tubular first locking member with one common rotating device provided between them. The turning device allows the first locking members of the adjacent lift cylinders to be rotated to select the released position and the locking position of the lift cylinders.

[0019] According to an embodiment, the locking device only locks the return movement of the lift cylinder.

[0020] According to an embodiment, the locking device locks the lift cylinder in both directions, i.e. in the return direction and the impact direction.

[0021] According to an embodiment, one or more detection members, such as sensors, are provided in association with the locking device to allow the locking state and the released position of the locking device to be detected. The personnel hoist has a control unit that may comprise a control strategy to prevent a normal use of the boom until the locking position is detected. This embodiment improves safety. When in operation, the boom is always secured by a mechanical locking device as a precaution against hose breaks and other damages.

Brief description of the figures

[0022] Some embodiments of the invention will be described in greater detail with reference to the accompanying drawings, in which Figure 1 is a schematic view of a personnel hoist with its boom raised up to the operating position and its telescoping boom portions extended;

Figure 2 is a schematic view of a personnel hoist with its boom in transport position supported by a chassis;

Figure 3 is a schematic view of a boom of the invention in the transport position;

Figure 4 is a schematic view of the boom of Figure 3 raised up in its operating position;

Figure 5 is a schematic view of a detail of a locking device in its locking position and its released position; and

Figure 6 is a schematic view, seen from the longitudinal direction of the hoist cylinder, of the released position and the locking position of the locking device in the arrangement of Figure 4.

[0023] For the sake of clarity, some embodiments of the invention have been simplified in the figures. Like parts are denoted with like reference numerals.

Detailed disclosure of some embodiments of the invention

[0024] Figure 1 shows a personnel hoist 1 comprising a chassis 2 provided with a boom 3. The chassis 2 may be a movable chassis, such as a vehicle or a trailer. The chassis 2 may be provided with a pivoting frame 4 to which the bottom end of the boom 3 is attached by a first joint 5. The top end of the boom 3 may be provided with a man cage 6 accommodating one or more persons to be lifted. The man cage 6 may also is provided with tools, such as equipment needed for spraying fire extinction medium. The boom 3 may comprise only one boom section or two or more articulated boom sections. The boom 3 in Figure 1 comprises a first boom section 3a and a second boom section 3b, with a second joint 7 between them. The bottom part of the first boom section 3a is fastened to the pivoting frame 4 by a first joint 5 to allow it to be raised and lowered in relation to the joint 5. By pivoting the pivoting frame 4 the boom 3 may be moved in a lateral direction. The second boom section 3b may be raised and lowered in relation to a second joint 7. In Figure 1 reference numeral 3b' designates the second boom section raised in the top position. The length of the first boom section 3a and the second boom section 3b may be changed telescopically, which allows the boom 3 to be retracted for transport position B shown in Figure 2 and extended into a desired length in operating situation A. As shown in Figure 1 , the personnel hoist can have a very long reach. As numbers of high-rise buildings increase, personnel hoists used in rescue operations in particular must allow ever greater heights to be reached. The reach may be as much as 100 meters. Figure 2 shows the boom 3 of the personnel hoist driven to the transport position on the chassis 2.

[0025] Figures 1 and 2 further show a lift cylinder 8 with its bottom end fastened to the pivoting frame 4 and its top end to a fastening lug 9, or the like, located at a distance from the bottom end of the first boom section 3a. The lift cylinder 8 may be a hydraulic cylinder. Only one lift cylinder 8 may be sufficient to raise the boom 3 from a transport position B to an operating position A, although typically two, three or even more lift cylinders are used. The lift cylinder 8 has a long stroke to allow the first boom section 3a to be raised to an almost vertical operating position A. In addition, the lift cylinder 8 is dimensioned to enable it to move the long boom 3 in a controlled manner to raise and lower it. Since the force created with the lift cylinder 8 is high and its stroke is long, the lift cylinder has a large fluid volume. Consequently, hydraulic flexibility may appear in the lift cylinder, which the present application aims at preventing by a mechanical locking device disclosed herein. For the sake of clarity, the locking device is not shown in Figures 1 and 2 but the different embodiments are presented later with reference to Figures 3 to 6.

[0026] For the sake of clarity, Figure 3 only shows the first boom section 3a, which is in a substantially horizontal transport position B. Hence the lift cylinders 8, two for example, are driven in their most retracted position where the piston rod is almost entirely inside the cylinder body. As shown in the figure, the outermost end of at least one lift cylinder 8 has fastened thereto a first locking member 10 that may be an elongated tubular piece capable of moving with the piston rod and sliding on the cylinder body. When the lift cylinder 8 is in the retracted position, the tubular first locking member 10 may cover the lift cylinder 8 substantially entirely. The first end of the lift cylinder 8, i.e. its bottom end, is fastened to the pivoting frame 4 by a first coupling member 1 1 and its second end, i.e. the top end, is fastened to the fastening lug 9 in the boom by a second coupling member 12. As indicted in Figure 3, the pivoting frame 4 may be pivoted in the direction of arrow C about a vertical joint, and the lift cylinder 8 may be extended and retracted in linear direction D to raise and lower the boom portion 3a in direction G in relation to the joint 5. [0027] In Figure 4 the boom section 3a has been raised to the operating position A, where it points directly upward or diagonally upward at a desired angle. In that case the lift cylinders 8 may be driven to their other extreme positions with the piston rods 13 pushed out of the cylinder bodies 14. When inside the tubular first locking member 10, the piston rods 13 are out of sight but, for the sake of clarity, Figure 4 shows with a broken line the piston rod of the right-hand side lift cylinder. The tubular first locking member 10 has its top part fastened to the outermost end of the piston rod 13 and therefore it moves with the piston rod 13 and in relation to the cylinder body 14. The first locking member 10 and the cylinder body 14 have suitable guide surfaces between them to allow the first locking member 10 to slide on the cylinder body 14 when the length of the lift cylinder 8 is changed. The top part of cylinder body 14 is further provided with a second locking member 15 which may comprise a rotationally non-symmetric outer shape, for example a substantially square shape. The second locking member 15 may be immovably fastened to the cylinder body 14. The cross-section of the inner surface of the tubular first locking member 10 may have a rotationally non-symmetric shape corresponding to the outer shape of the second locking member 15. Alternatively, the bottom part of the first locking member 10 is provided with a separate piece fastened to the tubular part and provided with a shape corresponding to the outer shape of the second locking member 15. In addition, the first locking member 10 is arranged to rotate about its longitudinal axis in direction E for a predetermined limited angle by means of a turning device 16. The turning device 16 may be a hydraulic cylinder, for example, coupled between the first locking members 10 of two adjacent lift cylinders 8, which allows it to rotate both the locking members 10 simultaneously although in opposite directions. When the boom is raised to the operation position A, the first locking member 10 may be turned from the released position to the locking position, a shape-locking being thus formed between the locking members 10 and 15, and the first locking member 10 is no longer capable of extending onto the second locking member 15. This provides a firm mechanical locking between the locking members 10 and 15 belonging to the locking device 17, the locking preventing retraction of the lift cylinder 8 and lowering of the boom. The locking device 17 thus forms a security locking, which prevents unintentional lowering of the boom due to faults and damages, for example. When the boom is to be lowered, the lift cylinder 8 may be extended a little to reduce friction between the locking mem- bers 10 and 15. Next, the first locking member 10 may again be rotated to the released position so that it sets into a matching position with the second locking member 15. The first locking member 10 is thus able to move again past the second locking member 15 towards the bottom end of the lift cylinder 8.

[0028] The length of the first locking member 10 and the position of the second locking member 15 are selected so that when they are in the locking position, distance L between the fastening members 1 1 and 12 of the lift cylinder 8 matches to the desired operating position A of the boom part 3a. It is also possible to arrange two or more operating positions for the boom part 3a. In fact, Figure 4 shows with broken lines alternative locking members 15a belonging to the locking device 17, the locking members being located at a distance from the locking members 15 at the top of the cylinder tube 14. This allows the lift cylinder 8 to be locked to an alternative second length with the locking members 15a. Obviously, coupling to the locking position in the second locking position cannot be based on the rotation of the first locking member 10, but the operation of the locking members 15a may be based on some other locking principle.

[0029] Figure 5 shows the lift cylinder 8 in the locked position and in the released position. The lift cylinder 8 on the left has been locked to the operating position by rotating the first locking member 10 belonging to the locking device 17 in direction E in relation to the second locking member 15, which is fixedly fastened to the top of the cylinder body 14. The first locking member 10 may comprise an elongated tube part 10a that may have a planar end piece 10b, known as a coulisse, at the bottom end thereof. The end piece 10b is provided with an opening 10c whose shape substantially corresponds to the shape of the outer surface of the second locking member 15. This allows the first locking member 10 to be rotated about its longitudinal axis to a released position, where the opening 10c and the second locking member 15 fit together, and to a locking position, where the end surfaces of the end piece 10b and the second locking member 15 are against one another. The shape of the inner side of the cross-section of the tube part 10a may substantially correspond to the shape of the second locking member 15 to allow it to rest on the second locking member 15 when the tube part 10a has slid onto the cylinder body 14. Further, the bottom end of the first locking member 10 may be provided with a guide sleeve 10d or a similar guide surface that may be supported to the outer surface of the cylinder body 14 by slide pieces 10e or similar guide pieces. The guide sleeve 10d and its guide pieces 10e may hold the first locking member 10 parallel with the longitudinal axis of the lift cylinder 8 after the lift cylinder 8 has been extended and the end piece 10b has moved above the second locking piece 15. This ensures that the locking members 10 and 15 are parallel with the longitudinal axis of the lift cylinder 8 and one may penetrate into the other when the locking device 17 has been rotated to the released position. The second locking member 15 may be a planar piece with a rotationally non- symmetric outer surface presenting a substantially square shape, for example.

[0030] In Figure 5 the locking device 17 on the right is in the released position and the first locking member 10 has slid onto the cylinder body 14 because the lift cylinder 8 has been retracted. As seen in the figure, the slide pieces 10e rest against the cylinder body 14 and hold the first locking member 10 parallel with the longitudinal axis of the lift cylinder 8. The slide pieces 10e may be of a suitable slide bearing material, such as plastic suitable for the purpose.

[0031] Figure 6 shows the operation of the locking device 17 as seen the direction of the piston rod of the lift cylinder. Locking devices 17a and 17b of adjacent lift cylinders are operated with the same rotating device 16, whose linear movement D is transmitted by lugs 18 to a rotating movement of the first locking member 10. When the turning device 16 is retracted, the opening in the end piece 10a of the first locking member 10 and the second locking member 15 fit together, the locking devices 17a and 17b being in the released position. When the turning device 16 is extended, the lugs 18' move outward to the position shown with a broken line and the first locking member 10 rotates to the locking position. Also the rightmost partial figure shows a locking device 17c in the locking position. In that case the second locking member 15 and the end piece 10b are interleaved and the inward movement of the piston rod is prevented.

[0032] Unlike in Figures 3 to 6, it is also possible to arrange the second locking member 15 to rotate in relation to the first locking member 10. The second locking member 15 may be rotated in relation to the cylinder body 14 by a suitable rotating motor, for example, or by some other actuator. In addition, it is possible to rotate the cylinder body 14 with a suitable actuator, which allows the second locking member 15 to rotate in relation to the first locking member 10. In some cases it is also possible to rotate both locking members 10 and 15 about the longitudinal axis of the lift cylinder 8. In other words, this is a question of the position of the locking members 10 and 15 in relation to one another and of the compatibility of their shapes. This allows the released position and the shape-locked locking position of the lift cylinder 8 to be selected.

[0033] A point to be noted is that the arrangement of the lift cylinder 8 may be vice versa to that shown in the previous figures, i.e. the piston rod 13 may be below and the cylinder body 14 above. This aspect is not essential to the operation of the lift cylinder 8 and the locking device 17. The first locking member 10 is coupled to the piston rod 13 and arranged to move in relation to the cylinder body 14 when the lift cylinder 8 is extended or retracted.

[0034] One or more sensors 30 may be provided in association with the locking device 17 to enable the locking position and the released position of the locking device 17 to be detected. This detection data may be transmitted to one or more control units 31 of the personnel hoist. The control unit 31 may prevent the moving and normal use of the boom until data of a detected locking has been received. This embodiment is illustrated in Figure 4.

[0035] The top end of the tubular first locking member 10 may be provided with sealing members 32. Since the structure of the first locking member 10 is an impermeable flange sealed at the top end, entry of dirt and water to the lift cylinder 8 can be effectively prevented. This is advantageous in rescue operations in particular, because it enables to avoid freezing of the cylinder 8 due to extinction water. The bottom end of the first locking member 10 may be airing, i.e. the bottom end may be open 33 and comprise a gap or openings to allow impurities that have entered the locking member 10 and water entered or condensed there to leave the structure. This improves the strength and reliability of the construction.

[0036] A further point to be mentioned is that it is possible to arrange a first locking member 10 which is moved with a separate actuator, in which case the locking member is driven to the locking position by means of an actuator after the lift cylinder has be driven to its predetermined position where the locking is to take place. The actuator for moving the first locking member may be a pressure medium cylinder or motor, for example.

[0037] In some cases features disclosed in this application may be applied as such, independently of other features. On the other hand, features disclosed in this application may be combined, when necessary, to provide different combinations. [0038] The drawings and the related specification are only intended the idea of the invention. The details of the invention may vary with¬e of the claims.

Claims

1 . A method for locking an operating position of a boom, comprising raising the boom (3) by means of at least one lift cylinder (8) by extending and retracting the length of the lift cylinder (8), the piston rod (13) of the lift cylinder (8) moving in relation to the cylinder body (14); and

raising the boom (3) to an operating position (A) and preventing the at least one lift cylinder from retracting in the return direction by at least one locking device (17);

using the locking device (17) provided with a first locking member (10) and a second locking member (15), a relative movement taking place between them in the longitudinal direction of the lift cylinder (8), when the lift cylinder (8) is extended or retracted in the released position of the locking device (17); and

preventing the relative longitudinal movement between the locking members (10, 15) at least in the return direction of the lift cylinder, when the locking device (17) is in the locking position;

c h a r a c t e r i z e d by providing the first locking member (10) and the second locking member (15) with a matching rotationally non-symmetric shape in the longitudinal direction of the lift cylinder (8); and

selecting the operation of the locking device (17) between a released position and a locking position by changing the respective positions of the locking members (10, 15) about the longitudinal axis of the lift cylinder (8), the rotationally non-symmetric shapes of the locking members (10, 15) thus occupying a matching or a non-matching position, respectively.

2. A method according to claim 1 , c h a r a c t e r i z e d by arranging a tubular first locking member (10) around the lift cylinder

(8) and moving it together with the piston rod (13) in relation to the cylinder body (14);

holding the second locking member (15) immovably on the outermost end portion of the cylinder body (14); and

rotating the tubular first locking member (10) about its longitudinal axis in relation to the second locking member (15), thus allowing the released position and the locking position to be selected.

3. A personnel hoist comprising:

a chassis (2); a pivoting frame (4) arranged to pivot on the chassis (2);

a boom (3) with its bottom part coupled to the pivoting frame (4) by a horizontal joint (5);

at least one lift cylinder (8) to raise and lower the boom (3) in relation to the horizontal joint (5), the lift cylinder (8) comprising a cylinder body (14) and a piston rod (13), which are arranged to move in relation to each other when the lift cylinder (8) extends or retracts during its operation;

at least one locking device (17) arranged to at least one lift cylinder (8), the locking device (17) being arranged to mechanically lock the cylinder body (14) and the piston rod (13) in relation to one another so that they are immovable at least in the return direction of the lift cylinder when the boom (3) has been raised up to its at least one operating position (A); and

at least one man cage (6) arranged to the top part of the boom (3); and where the locking device (17) comprises at least one first locking member (10) which is arranged to move together with the piston rod (13) but is a separate piece therefrom and, further, at least one second locking member (15) connected to the cylinder body (14), whereby when the lift cylinder is extended or retracted, a relative movement between the first locking member (10) and the second locking member (15) takes place in the longitudinal direction of the lift cylinder (8);

and where the locking device (17) comprises at least one locking position, where the locking members (10, 15) are locked immovably to one another at least in the return direction of the lift cylinder, and a released position, where the relative longitudinal movement between the locking members (10, 15) is allowed;

c h a r a c t e r i z e d in that

the first locking member (10) comprises a tubular part (10a) and an end piece (10b) arranged around the lift cylinder (8);

the end piece (10b) comprises a locking aperture (10c) having a ro- tationally non-symmetric shape;

the second locking member (15) is fastened to the outermost end portion of the cylinder body (14) and the shape of the outer surface of the second locking member (15) substantially corresponds to the shape of the locking aperture (10c), whereby the second locking member (15) and the locking aperture (10c) match in the direction of the longitudinal axis of the lift cylinder (8); and the locking device (17) comprises at least one turning device (16) allowing the first locking member (10) to be rotated about its longitudinal axis so that in the released position the locking aperture (10c) is placeable to match the second locking member (15) and, on the other hand, in the locking position the locking aperture (10c) is placeable to a non-matching position in relation to the second locking member (15).

4. A personnel hoist according to claim 3, c h a r a c t e r i z e d in that

the boom (3) has two adjacent lift cylinders (8) coupled thereto, both the cylinders being provided with a tubular first locking member (10) with one common turning device (16) provided between them.

5. A lift cylinder for a boom, comprising:

a cylinder body (14) and a piston rod (13), the length of the lift cylinder (8) being arranged to extend and retract during the operation thereof; and at least one locking device (17) arranged to mechanically lock the cylinder body (14) and the piston rod (13) in relation to one another so that they are immovable at least in the return direction of the lift cylinder;

and the locking device (17) comprising at least one first locking member (10) which is coupled to the free end of the piston rod (13) and arranged to move together with the piston rod (13) in relation to the cylinder body (14) and at least one second locking member (15) coupled to the cylinder body (14), whereby during the operation of the lift cylinder (8), a relative movement takes place between the first locking member (10) and the second locking member (15) in the longitudinal direction of the lift cylinder (8);

and the locking device (17) comprising a locking position, where the locking members (10, 15) are locked together at least in the return direction of the lift cylinder, and a released position, where the relative movement between the locking members caused by the operation of the lift cylinder is allowed;

c h a r a c t e r i z e d in that

the first locking member (10) and the second locking member (15) comprise matching rotationally non-symmetric shapes in the longitudinal direction of the lift cylinder (8); and

the locking device (17) comprises at least one turning device (16) allowing relative positions of the locking members (10, 15) to be rotated in relation to the longitudinal axis of the lift cylinder (8) so that in the released position the rotationally non-symmetric shapes of the locking members (10, 15) are placeable to match each other and, on the other hand, in the locking position they are placeable to a non-matching position.

6. A lift cylinder according to claim 5, characterized in that the turning device (16) is arranged to act on the first locking member

(10); and

the second locking member (15) is fixed.

7. A lift cylinder according to claim 5 or 6, characterized in that

the first locking member (10) is a tubular structure with a rotationally non-symmetric cross-section.

8. A lift cylinder according to any one of preceding claims 5 to 7, characterized in that

the first locking member (10) is a tubular structure, its end on the cylinder body side being provided with an end piece (10a) that has a rotationally non-symmetric locking aperture (10c).

9. A lift cylinder according to any one of preceding claims 5 to 8, characterized in that

the first locking member (10) is a tubular structure arranged about the lift cylinder (8) and arranged to continuously cover the piston rod (13) substantially entirely.