in fig1 the reference 1 identifies a gas - spring which is shown in chain lines and which includes a substantially tubular , outer casing 2 , a rod 3 which is guided for axial movement into and out of the hollow casing 2 , between two terminal positions and is dampened herewith by means of gas constrictions ( not shown ), said gas - spring further including attachment parts 4 , 5 for attachment of the gas - spring between relatively movable devices or structures , not shown . fig1 and 2 - 5 illustrate an embodiment of a friction device 6 which is effective in braking or retarding the relative axial movement of the casing 2 and the rod 3 , and the device , in appropriate circumstances , can be used to fixate the casing and the rod in selective relative positions , substantially positions located between said terminal positions . the illustrated friction device comprises a housing 7 which is intended to be fitted externally onto the gas - spring casing 2 , preferably at one end 2 &# 39 ;, and held firmly in position at said one end , and includes clamping members8 arranged around the rod 3 and connected to said housing , these clamping members preferably being intended to be brought into clamping engagement with the rod 3 , with the aid of screw - type devices . in the case of the illustrated embodiment , the screw devices comprise a nut arrangement 9 which coacts with the outer face 7 &# 39 ; of the housing through the intermediary of the screwthreads 9 &# 39 ;. the illustrated nut arrangement is such that when said nut arrangement is screwed toward the housing and unscrewed therefrom , the clamping members will generate continuous , variable clamping forces , for example through the intermediary of conical faces 10 . in the case of the illustrated embodiment , the clamping members 8 define a substantially cylindrical configuration around the rod 3 , and are slightly conically shaped externally , wherewith the clamping members will be displaced radially to a given extent by means of axially movable ring - part 11 on the nut arrangement 9 , these ring - part acting externally on the clamping members . the illustrated clamping members 8 comprise four mutually identical parts separated by axially extending slots . the housing 7 has an internal flange 12 which coacts with a corresponding groove 13 in the casing , such as to enable the housing to be snapped firmly into engagement with the casing . illustrated in fig6 - 8 is an embodiment in which a housing 14 is intendedto be fitted externally to the casing 2 and firmly secured thereto . a radial slot 15 extends axially along the housing 16 , to form housing partsor clamping parts 16 extending substantially around the rod 3 , wherewith a screw 17 or some like device is provided for increasing or decreasing the width of the slot 15 such as to vary continuously the clamping forces exerted by the clamping parts 16 . this embodiment is also provided with aninternal flange , referenced 18 , corresponding to the aforedescribed flange 12 of the fig1 embodiment . fig9 - 14 illustrate schematically a sun - bed 19 . the upper part or canopy 20 of the sun - bed is raised and lowered with the aid of an electric hoist arrangement which is complemented with the inventive gas - spring arrangements , such as the gas - spring illustrated in fig1 . the gas - springarrangements are primarily intended to replace the hoist arrangement as canopy positioning means during breakdowns in operation . the sun - bed of the illustrated embodiment includes a box - like base 21 , a lower part 22 which rests on the base 21 , an upright panel construction 23 which is mounted on the base 21 , the lower part 22 and the canopy 20 , said canopy being rotatably journalled on the panel 23 for pivotal movement towards and away from the lower bed part . as will be seen from fig1 - 14 , the major part of the hoist arrangement is located in the panel . the reference 24 identifies a rotational shaft which is rotatably journalled in the panel structure and extends through and along said panel structure and which is connected with the canopy 20 , preferably externally of the side walls 25 of said panel , for swinging of the canopy , e . g ., with the aid of end parts 26 extending transversely of the shaft 24 . the shaft 24 is rotated by means of an electric motor 27 , via a winding shaft 28 rotated by means of said motor , a winding element 29 , such as a belt or strap 29 , arranged between the shaft 28 and an auxiliary arm 30 projecting transversely to the shaft 24 and intended to be wound onto and unwound from said shaft , so as to shorten or lengthen , respectively , the free belt lengths between the shaft and an outer part 31 , such as an end part 31 , of the auxiliary arm 30 , such as to rotate theshaft 24 . thus , when a canopy raise - pulse is delivered to the motor , preferably by a control means 32 on the panel , the hoist arrangement will function to wind the belt or strap 29 onto the shaft 28 , so as to swing the auxiliary arm 30 and rotate the shaft 24 , and therewith raise the canopy 19 . when the motor receives a canopy lowering pulse , through the aforesaid control means 32 , the belt or strap is unwound from the shaft and the canopy lowers gravitationally , under its own weight . although not shown , position limit switches are provided for defining the highest and lowest level to which the canopy can move . in order to enable the canopy to be manoeuvered and positioned without the aid of the electric hoist arrangement , two gas - spring arrangements are mounted between the canopy and the lower bed part , one at each end wall ofthe sun - bed and preferably relatively close to said panel structure . the gas - spring is referenced 1 and the friction device is referenced 6 . when the electric hoist arrangement is in active use , the gas - spring arrangements are adjusted to a non - friction mode . adjustment of the gas - spring arrangements to &# 34 ; manual &# 34 ; will enable the canopy of the sun - bed to be lowered manually to its lowermost position and then raised to a position substantially midway between the lowest and the highest canopy positions , the friction device of respective gas - spring arrangements beingadjusted so that the canopy is braked and the weight of said canopy counter - balanced , such that said canopy will remain in its manually adjusted position unless acted upon by forces other than those generated by its own weight . the control panel structure 23 preferably also incorporates four fans 33 , with individual controls 34 , for blowing ventilating air onto the sun - bed and also houses a timer , control means for automatic cooling of the sun - bed , and the requisite cables . the method of operation of the inventive arrangement will be understood in all essentials from the aforegoing . through the intermediary of the continuous , variable clamping forces generated by the friction device , it is possible to achieve a desired braking force with respect to mutual movement between the casing 2 and the rod 3 . the braking force can be increased to an extent such as to lock the casing and the rod relative to one another . it will be understood from the aforegoing that the invention affords important advantages in the form , inter alia , of increased flexibility when using gas - springs , and also novel possibilities of use . the invention has been described in the aforegoing with reference to an exemplifying embodiment thereof . it will be understood , however , that modifications and minor changes can be made , without departing from the scope of the inventive concept . for instance , embodiments are conceivable in which the friction device is attached to the rod 3 instead of the casing 2 and has clamping members or the like which act on said casing . the clamping members may be located externally , as with the clamping members 8 , 16 , or internally , wherewith radially displaceable parts are arranged to act against the inner wall or corresponding surface of the casing , such as to achieve braking of relative movement between the casing and the rod , and when appropriate to lock or fixate said casing rod against relative movement . when the friction device is attached to the casing , the clamping members acting against the rod may be located inside the casing 2 instead of outside the casing as , for instance , in accordance with fig1 . it will be understood , that when using the embodiment illustrated in fig9 - 14 , the construction of both the gas - spring arrangement and the hoist arrangement can be varied . consequently , the invention is not restricted to the aforedescribed embodiments but modifications can be made within the scope of the following claims .