US3643980A

US3643980A - Pushchair

Info

Publication number: US3643980A
Application number: US839550A
Authority: US
Inventors: Guenther Kirsch
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-07-05
Filing date: 1969-07-07
Publication date: 1972-02-22
Anticipated expiration: 1989-02-22
Also published as: DE6751042U

Abstract

A pushchair comprising a supporting frame, a body, means for detachably mounting said body on said frame, a pair of sides for said frame, a pair of frame parts for each said side, a pivot interconnecting said frame parts so that at least the lower ends of said parts form an inverted V, an extensible spring unit interconnecting the lower ends of said parts, a pair of coextending hollow levers composing said unit, at least one tensioned spring within said levers, and a bendable joint between said levers.

Description

United States Patent Kirsch Feb. 22, 1972 [S4] PUSHCHAIR [21] Appl.No.: 839,550

FOREIGN PATENTS OR APPLICATIONS 559,228 5/1923 France ..280l4l 218,494 7/1924 Great Britain ....280/4l 872,111 7/1961 Great Britain...........................280/36 Primary Examiner-4&0 Friaglia Assistant Examiner-Winston H. Douglas Attorney-Sughrue, Rothwell, Mion, Zinn 8L Macpeak [30] Foreign Application Priority Data July 5, 1968 Germany ..o 67 51 042 [57] 51mm Mar. 5, 1969 Germany ..G 69 08 738.3 A pushchair comprising a supporting frame, a body, means for detachably mounting said body on said frame, a pair of sides [52] US. CL ..280/4l A for Said frame. a p f fr parts for h said side, a pi [5 l 1 Int. Cl ..B62b 7/06 interconnecting Said frame P so that a! least the lower ends 581 Field of Search .280/36, 36 8, 38,41 A, 41 B 0f said P form an inverted an extensible spring unit terconnecting the lower ends of said parts, a pair of coexlend- 56 R Cited ing hollow levers composing said unit, at least one tensioned I 1 e cums spring within said levers, and a bendable joint between said UNITED STATES PATENTS levers.

2,373,809 4/1945 Bryant X 8 Claims, 9 Drawing Figures p l5 l5 PATENTEDFB22 ma 3'. 643.980

SHEEI 1 0F 4 Fig. 7

Inventor Dr. Guenther Kirsch PATENIE0FB22 1912 3.643880 lnven tor Dr. Guenther Kirsch SHEET 3 BF 4 Inventor Dr. Gu'enther Kirsch PATENTEDFEBZZ i972 SIEHBUFI! Inventor Dr. Gfienlhor Kirsch PUSIICHAIR The present invention relates to a collapsible perambulator or pushchair having a supporting frame which detachably receives a bodywork or seat and which is resiliently mounted relative to the wheel axles.

The object of the invention is to provide an improved perambulator or pushchair subsequently referred to as pushchair which is provided with a spring mounting of the support frame relative to the wheel axles, and is collapsible.

A further object of the invention is to provide a pushchair which is provided with fixing devices for a bodywork, seat or the like of simple structure and which is easy to manipulate.

In one embodiment two spring elements are provided, one on each side, and the levers of each spring element at least partially enclose the spring element and in the position of use, their pivot ends when the support frame is unstressed or only slightly stressed, are in abutment.

The spring unit is provided with two spring elements such as helical springs. interconnected at the pivot end. The levers of each spring unit are enclosed at a hinge region by a gaiter and have their free ends hingedly connected with parts supporting the wheel bearings in the support frame.

In a preferred embodiment, the pivotal connecting point between a bearing part and a support member may be located with clearance, above the connecting region between the bearing part and spring unit. The pivot of the bearing part formed by the wheel axle, the hinged connecting point, and the connecting region of the bearing part with the support frame and spring unit may form the vertices ofa triangle.

In a further feature of the invention, the support frame is provided with lateral receiving devices which detachably hold the pram body or seat, by means of lateral bearing pins or the like. The receiving device may therefore be provided with receiving recesses open at the top and/or the front, and may also have a bearing region which locates the bearing pin of the pram body. Consequently, the pram body having its bearing pins pivotally mounted in the receiving recesses by means of adjusting devices is continuously located in various inclined positions relative to the plane of travel.

In a further feature of the invention, a collapsible supporting frame is adapted to be fixed in a motor car as a child's seat by means ofa locating device at the rear, and to act as support frame for a pram body or seat. This frame is provided in its upper end region with lateral supporting struts having receiving devices formed by recesses so as to suspend a pram body or seat therein.

Further features of the invention are shown in the other subclaims.

The subject matter of the invention not only extends to features of individual claims, but also to combinations thereof.

The perambulator or pushchair in accordance with the invention is of simple structure, readily manufactured and of manifold use. A particularly favorable feature is the spring unit provided in the two lateral regions, which make the supporting frame resilient and allows it to be collapsed readily and quickly. The support frame is collapsible by means of a few manipulations, since the spring units collapse under application of slight foot pressure and hence allow the supporting frame to be collapsed and stored in a small space.

Furthermore, the perambulator or pushchair can be collapsed to a small size when the pram body or seat is removed. This is because the mutual distance between the wheel axles is reduced, which results in a reduction of the volume occupied by the pram body which allows the collapsed pram to be stowed away in a limited space without having to remove the wheels.

A further advantage of the invention comprises the simple, favorable and reliable suspension of the pram body or seat on the supporting frame. Due to the construction of the receiving device, it is possible to mount the pram body on both sides of the frame.

Furthermore, if the supporting frame is collapsible, such that the pram body can be converted into a childs seat in a motorcur, then the frame may be provided with a suspension device for the pram body which allows it to be reliably secured to the seat or the backrest of a car seat.

The invention will now be described with reference to the accompanying drawings, in which:

FIG. I is a side view ofa pushchair having a body adapted to be detachably mounted on a supporting frame, with a spring unit provided between lateral supporting struts;

FIG. 2 is a longitudinal section through a spring unit;

FIG. 3 is a side view of one of the regions where the supporting frame is connected to the pram body, which view is a partial section as shown by the fragmentary section A in FIG. 1;

FIG. 4 is an end view of the fixing region shown in FIG. 3 on the support frame;

FIG. 5 is a side view ofa support frame detachably receiving the pram body such that it can be removably mounted in a mo torcar;

FIG. 6 is a side view of a further embodiment of a perambulator showing a supporting frame receiving the pram body which is spring-mounted on the wheel axles;

FIG. 7 is a side view of the lower part of the pushchair showing bearing parts pivotally mounted on the wheel axles and connected to the support frame at one end and to a spring unit at the other end.

FIG. 8 is a longitudinal section through a spring unit; and

FIG. 9 is a side view of two bearing parts arranged at one side of the body, showing the supporting frame and spring unit in a stressed and/or partially collapsed position of the supporting frame.

In FIG. I, a pushchair is provided with a pair of collapsible inverted V-shaped supporting frames 10 on either side of the chair each having two lateral supporting struts I2 and I3. These two supporting struts are disposed such that they are at an acute angle relative to one another, and are pivotally inter connected by a joint 11. A member 14, bent towards the rear of the body, is formed as an extension to supporting strut 12 and is mounted thereon.

The supporting

struts

12 and 13 diverge downwardly from the joint 11 and are each provided at their lower end with a wheel axle having wheels 15 rotatably mounted thereon. At each side of the supporting frame 10 and between the support struts 12 and I3, a springing unit 16 is provided. This resiliently retains the supporting frame 10 as such in the position of use, and also assists the collapse of the supporting frame 10 by being so formed that it is downwardly hinged. The two supporting struts l2 and 13 are pivotal about the joint ll towards one another as shown by the arrow in FIG. I.

Each spring unit I6 has two

separate springs

21 and 22. These are interconnected at one end by a coupling member 18 provided in the pivotal region I7 and at their other end are each connected to levers l9 and 20 which at least partially enclose each spring the levers being connected to supporting struts I2 and 13. In the position of use of the pushchair, the lever ends nearest the pivot are in abutment with one another.

The levers I9 and 20 are tubular and are at their end mounting regions so as to engage the supporting struts I2 and 13 on two sides. The levers l9 and 20 are pivotally mounted on the support struts l2 and 13 by pivots 23. These levers I9 and 20 may have a cross section which is U-shaped, channel shaped, V-shaped or polygonal and may be open towards the bottom.

The coupling member 18, which is enclosed by the tubular levers, is provided on the top, with a space 24 for the ends of the downwardly hinging levers l9 and 20 such that the lower side assumes a position wherein it has a slight clearance with respect to the wall of the levers l9 and 20 enclosing it (as shown in FIG. 2). This coupling member 18 is a plate, which in cross section assumes an upright position relative to the levers l9 and 20, and is provided in its central region with a wedgeshaped recess space 24.

This space 24 may be a semicircular recess or groove. One end of each of the

springs

21 and 22 is secured to the coupling member, these springs 2| and 22 being helical tension springs. The other end of each spring 2| and 22 is detachahly secured to the levers I9 and 20 by means of bolts 25.

A gaiter 26 made ofa resilient and/or flexible material, such as rubber or plastics material is arranged around the pivot region of the levers l9 and 20. This gaiter, having cylindrical end regions, engages around the levers l9 and 20 and is annularly outwardly enlarged in the region of the pivot (as shown in FIG. 2).

When the supporting frame 10 is stressed, the springing units 16 impart a resilient property thereto and absorb the impact stresses which occur. The two spring elements 2i and 22 of each springing unit 16 are drawn outwards and the two levers l9 and with their adjacent end faces also move out wards. in the pivotal region between the two

levers

19 and 20, a gap which is variable in accordance with the stresses arising is set up.

By virtue of the abutment of the levers l9 and 20 and the arrangement of the coupling member 18, the levers l9 and 20 are prevented from pivoting downwards or upwards when stresses arise so that the supporting frame 10 is automatically locked in the position of use.

Levers 20 of both spring units (one located on each side of the pram) are interconnected by a connecting axle 27. When this axle is raised or lowered, levers 20 are forced to pivot about their respective hinge point 23 thus causing synchronized movement between levers on both sides of the pram. if the supporting frame is to be collapsed, the axle 27 connecting the two levers 20 is depressed manually. It is also possible to collapse the supporting frame by directly depressing either or both of the levers l9 and 20. However, it is more convenient to collapse the supporting frame by depressing the axle 27. Both levers l9 and 20 of each spring unit 16 pivot downwardly at their hinge points 23 and engage with their ends adjacent to the pivot side, in the space 24. The springs 2], 22 because of the downward pivotal movement of the levers l9 and 20 are slightly tensioned. When levers l9 and 20 of one of the spring units 16 are moved about their pivot points 23 on the supporting frame, the corresponding levers l9 and 20 on the other side of the pram move in synchronism therewith causing these corresponding levers to pivot in the same vertical direction (upwards and downwards) about its respective pivot point. When subsequently unfolding the supporting frame 10 the tension springs 21 and 22 assist the procedure. In the collapsed state of the supporting frame 10, the levers l9 and 20 assume an acute angle relative to one another and the supporting struts l2 and 13 are close together. The collapsed supporting frame 10 may be located in this position by means of an arresting device or stop. The supporting frame 10 may therefore be collapsed so as to occupy a very small space.

A receiving device 28 (see FIGS. 3 and 4) for a body or seat 29 to be detachably mounted on the supporting frame 10 is provided above the joint 11 on the inside of the support struts 12. This receiving device 28 has a bearing 30 mounted rigidly on the inside surface of each supporting strut l2, and is provided in its upper region with a recess 31 open at the front (in the direction of travel) or at the top, into which a bearing pin 32 mounted on the side of the pram body or seat 29 is inserted.

This recess 31 has a T-shaped outline and the downwardly directed recess portion 33 receives the bearing pin 32 of the pram body 29 suspended detachably therein. The bearing pin 32 is secured against slipping out accidentally. The pram body 29 is suspended in the receiving devices 28 of the supporting frame 10 both in the direction of travel and against the direction of travel and is removably mounted therein. By means of a locating stirrup 34, mounted on the side of the body and an adjusting screw mounted on the frame which is adapted to be positively and nonpositively connected to the stirrup, the pram body 29 may be disposed in various inclined positions relative to the plane of travel (see FIG. I).

FIG. 5 of the drawings shows a further collapsible supporting frame 34 for receiving the pram body or seat 29, which is used for mounting the pram body 29 in a motorcar, so that the pram body 29 may be converted into a child's seat. This frame 34 is provided at one end region with a runnerlike upturned erecting frame 35 which is telescopic. On each side region a supporting strut 36 is provided which is pivotally mounted on the frame 35. This is located in the position of use by a toggle linkage 37 hingedly mounted on the supporting strut 36 and one erecting frame 35.

The upper end region of the supporting struts 36 are formed as receiving devices and are provided with a bearing 39 having a receiving recess 38 formed therein. The pram body or seat 29 may be inserted from above into these receiving recesses 38 with its lateral bearing pins 32 and is located in position by means of locking hinged levers 40. This supporting frame 34 may be placed on the seat of a motor car and located with its upturned runnerlike region of the erecting frame 35 below the backrest of the car seat. Furthermore, this supporting frame 34 may be provided with a device which allows the seat 29 to be adjusted into an inclined position. In the normal position, the lower region of the seat 29 is supported against an upper axis 370 of the toggle lever linkage 37 In a further embodiment of a perambulator and/or pushchair as shown in FIGS. 6 to 9, there is a downwardly diverging inverted V or scissorlike and collapsible supporting frame 42 hingedly receiving end regions of a springing unit 43 arranged between the wheel axles 41 and connected to pivotal bearing parts 44, so that the supporting frame 42 and the lateral springing units 43 resiliently mounted on the wheel axles 41 by means of the pivotal bearing parts 44. The bearing parts 44 thus constitute the parts supporting the wheel bearings.

The frame sections 42:: which face downwards in the position of use from their common pivot 45 at an angle, are each connected at a connecting point 46 to the bearing parts 44. and the springing unit 43 at the end face is secured to the hearing part 44.

Each bearing pan 44 is provided in the outer region between supporting frame 42 and lever 48a or 42th with a wheel 47 and wheel axle 41, the wheel axle passing therethrough with clearance.

The bearing parts 44 are formed by U-shaped bearing yokes, with a hinge point 46 being provided between this bear ing yoke 44 and supporting frame 42 in the end region of the upper shank 44a. A further hinge point is provided in the connecting region between the bearing yoke 44 and the spring unit 43 in the end region of the lower U-shaped shank 44b. The wheel axle 4] also extends through the lower bearing shank of the yoke and has a clearance from the connecting region between bearing yoke 44 and springing unit 43, such that the wheel axle 41, the hinged connecting point 46, and the rigid connecting region of each bearing yoke 44 with the spring unit 43 form the vertices of a triangle.

The connecting points 46 and the connecting regions of two bearing parts 44 arranged on the sides of the perambulator and/or pushchair extend between the wheel axles 41 in every position of the supporting frame and their U-shapecl shanks 44a, 44b face towards the center of the pram body and face one another.

The hinge point 46 is a bolt which passes through the sup porting frame part 420. The bearing part 44 is formed as a flat body, such as a U-shaped yoke, and extends upright or parallel to the pivotal plane of the wheel and also extends between wheel 47 and support frame 42. Each bearing part 44 has a wheel axle 41 passing therethrough and connected rigidly thereto as by welding.

The spring unit 43 extends along the straight line connecting the two wheel axles 41 and is so made as to be pivotal both upwardly and downwardly.

The pivotal spring unit 43 is provided at its pivot 43a with two

levers

48a and 48b. The adjacent end faces of the two levers 48 tubularly abut in the pivot region, whilst the remote ends of the levers are rigidly connected to the bearing parts 44. These levers are held together under tension (as shown in FIG. 8) by means ofa helical spring 49.

The helical spring 49 is fixed at both ends to the levers 48a and 48!) by means of fixing means 50 such as screws or bolts passing through the hollow levers.

The free end ofeach tubular lever 50 is connected by means ofa sleeve 44c having a screw 51 mounted therein to the bear ing part 44.

In the abutment region 430 of the two tubular levers 50, each spring unit 43 is disposed around the helical spring 49 of an annular damping part 52 made of plastics material, rubber or the like provided for the abutting lever ends, which clamp ing part is maintained in position in the pivot region 43a by widening an end face of the levers.

In the pivot region 430 of the spring units 43, a gaiter made of flexible resilient material is disposed around each lever 50.

When the supporting frame 42 is stressed by the pushchair load or by uneveness in the road over which the chair travels, the impact loads are absorbed by the spring units 43. The bearing parts 44 holding the support frame 42 pivot about their bearings (i.e., the wheel axles 41) and act on the spring units 43. With compressive loads or uneveness, the supporting frame 42 or the wheel axles 41 cause the stressed bearing parts 44 to pivot downwards, (as seen in H0. 9), so that the tension spring 49 of the spring units 43 is subjected to a tensile stress which compensates and absorbs the impact loads.

The spring units 43 are downwardly pivoted and accordingly the pivotal movement of the bearing part is stressed to a greater or lesser extent. Each spring unit 43 draws the bearing parts 44, into the normal position of use (see FIG. 7) when the stresses diminish.

During the pivotal movement of the bearing parts 44 the tubular member 48 of the spring unit 43 which is comprised of

hollow levers

48a and 48b through which the tension spring passes collapses. When the inner ends of

levers

48a and 48b are forced in a downward direction, the hollow lever 48!) which has a smaller diameter than the adjacent lever 480 which is widened at its inner end surface, is supported at the upper periphery of its end face against the damping part 52. This causes the corresponding region of the damping part 52 into the enlargement of the hollow lever 48a. Simultaneously, the opposite region of the end face of the hollow lever 48a lifts off of the damping part 52. In response to a folding movement upwardly, the lower region of the damping part is accordingly forced into the enlargement of the hollow lever 480 while the opposite or top part of the end face of lever 48b lifts off of the dampening part.

When the supporting frame 42 is to be collapsed, the bearing parts 44 are pivoted upwards due to the increase of the angle between the frame sections 42a when they are swung about their pivot 45. The spring unit 43 also pivots upwards, whereby the distance between the wheel axles 41 is reduced (as shown in the chain-dotted lines in FIG. 9). Consequently when the chair is collapsed and the body has been removed, the chair only occupies a very small amount of space and is easily stowed in vehicles.

The pushchair in accordance with the invention is provided with low-cost spring units of simple construction, and with simple and reliable receiving devices for a removable pram body or seat.

A particularly advantageous feature of the invention is that the spring units are formed of pivotable spring elements which absorb widely varying impact stresses. These spring units fold up when subjected to pressure loads and hence permit the supporting frame to be simply and readily collapsed.

We claim:

1. A perambulator with a reclining or seat part including:

a. a mobile supporting frame,

bv a receiving body for carrying a child in a reclining or seated position,

c. means for detachably mounting said receiving body to said supporting frame,

d. said supporting frame comprising two interconnected side portions each of said side portions being collapsible and formed of two frame portions which at their lower ends assume the form of an inverted V relative to each other with a hinge point at the apex thereof,

e. first and second wheel axles, said lower ends of the frame portions being coupled to said wheel axles,

f. each of said side portions including a longitudinally extendably and foldable spring suspension unit mounted below the supporting frame hinge point enabling the supporting frame to be folded,

g. each of said spring suspension units including two hollow levers located, in the unfolded position, in a straight line one end of each of said levers abutting an end of the other, a spring suspension means within the hollows of said two levers, said spring suspension means having its ends closest to the supporting frame mounted to the ends of said hollow levers, said spring suspension means comprising at least one tension spring.

2. The perambulator according to claim 1 in which each of said spring suspension units contains two tubular hollow levers pivotably connected to said supporting frame, the nonabutting ends of said levers being bifurcated and provided with a transverse bore, and fixing means located in said transverse bore.

3. A perambulator as claimed in claim 2 in which said abutting ends of said two hollow levers, in the uncollapsed position, are supported flush one against the other, the abutting region forming the folding point for the spring unit, said spring unit further comprising a flexible gaiter em bracing said abutting region.

4. A perambulator as claimed in claim 1 in which one of the two hollow levers of each of said spring suspension units are interconnected by means of a transversely extending rigid connecting means.

5. A perambulator as claimed in claim 1 in which each of said spring units contains two hollow levers, spring means located in the hollows of said levers, said spring means comprising two tension springs located serially in an axial direction, a connecting means connecting the inner ends of said springs in the region ofthe folding point of the spring unit, the outer ends of said springs being fixed to fastening means located on the hollow levers,

6. A perambulator as claimed in claim 1 in which said spring means is comprised of two tension springs positioned in mm dem arrangement in an axial direction, the inner ends of said springs being interconnected in the abutting region of the hollow levers forming the folding point by means ofa connecting member, the outer ends of said springs being fixed to fixing means provided on the hollow levers, said connecting means being formed of a plate part containing a V-shaped groove enabling the spring unit to fold.

7. A perambulator as claimed in claim 1 in which said spring unit includes two hollow levers affixed non-moveably to U- shaped parts connected to the wheel axles and pivotably connected to the frame portions of the supporting frame, one hollow lever being provided at its inner end with a larger cross sectional area than the adjacent end of the other hollow lever, a resilient damper layer being positioned between the adjacent inner ends of said hollow levers, said tension spring within the hollows of said levers being fixed at its outer ends to a fixing means connected to said hollow levers,

8. A perambulator with a reclining or seat part including:

a. mobile support frame,

b. a receiving body for carrying a child in a reclined or seated position,

c. means for detachably mounting said receiving body on said supporting frame,

. first and second wheel axles, said lower ends of the frame portions being coupled to said wheel axles,

. each side portion including a longitudinally extendable and foldable spring suspension unit, said spring units being hingedly mounted between the supporting frame hinge point and the wheel axles on said side portions and being subject to tensile stress causing the front and rear wheels to be moved towards one another.

. each of said spring units including two hollow levers which in the unfolded, normal position are located in tandem in a straight line in an axial direction, an end of each of said levers being provided with mounting means for

Claims

1. A perambulator with a reclining or seat part including: a. a mobile supporting frame, b. a receiving body for carrying a child in a reclining or seated position, c. means for detachably mounting said receiving body to said supporting frame, d. said supporting frame comprising two interconnected side portions each of said side portions being collapsible and formed of two frame portions which at their lower ends assume the form of an inverted V relative to each other with a hinge point at the apex thereof, e. first and second wheel axles, said lower ends of the frame portions being coupled to said wheel axles, f. each of said side portions including a longitudinally extendably and foldable spring suspension unit mounted below the supporting frame hinge point enabling the supporting frame to be folded, g. each of said spring suspension units including two hollow levers located, in the unfolded position, in a straight line one end of each of said levers abutting an end of the other, a spring suspension means within the hollows of said two levers, said spring suspension means having its ends closest to the supporting frame mounted to the ends of said hollow levers, said spring suspension means comprising at least one tension spring.

3. A perambulator as claimed in claim 2 in which said abutting ends of said two hollow levers, in the uncollapsed position, are supported flush one against the other, the abutting region forming the folding point for the spring unit, said spring unit further comprising a flexible gaiter embracing said abutting region.

5. A perambulator as claimed in claim 1 in which each of said spring units contains two hollow levers, spring means located in the hollows of said levers, said spring means comprising two tension springs located serially in an axial direction, a connecting means connecting the inner ends of said springs in the region of the folding point of the spring unit, the outer ends of said springs being fixed to fastening means located on the hollow levers.

6. A perambulator as claimed in claim 1 in which said spring means is comprised of two tension springs positioned in tandem arrangemenT in an axial direction, the inner ends of said springs being interconnected in the abutting region of the hollow levers forming the folding point by means of a connecting member, the outer ends of said springs being fixed to fixing means provided on the hollow levers, said connecting means being formed of a plate part containing a V-shaped groove enabling the spring unit to fold.

7. A perambulator as claimed in claim 1 in which said spring unit includes two hollow levers affixed non-moveably to U-shaped parts connected to the wheel axles and pivotably connected to the frame portions of the supporting frame, one hollow lever being provided at its inner end with a larger cross sectional area than the adjacent end of the other hollow lever, a resilient damper layer being positioned between the adjacent inner ends of said hollow levers, said tension spring within the hollows of said levers being fixed at its outer ends to a fixing means connected to said hollow levers.

8. A perambulator with a reclining or seat part including: a. mobile support frame, b. a receiving body for carrying a child in a reclined or seated position, c. means for detachably mounting said receiving body on said supporting frame, d. said supporting frame comprising two interconnected side portions each of said side portions being collapsible and formed of two frame portions which at their lower ends assume the form of an inverted V relative to each other with a hinge point at the apex thereof, e. first and second wheel axles, said lower ends of the frame portions being coupled to said wheel axles, f. each side portion including a longitudinally extendable and foldable spring suspension unit, said spring units being hingedly mounted between the supporting frame hinge point and the wheel axles on said side portions and being subject to tensile stress causing the front and rear wheels to be moved towards one another. g. each of said spring units including two hollow levers which in the unfolded, normal position are located in tandem in a straight line in an axial direction, an end of each of said levers being provided with mounting means for pivotably mounting the levers on the supporting frame, the hollows of said levers containing a spring means which is fixed at its ends facing the supporting frame to said hollow levers and is formed by two helical springs located in series in the axial direction of the spring unit, hingedly interconnected in the folding region of the spring unit.