US3929077A

US3929077A - Conveyor systems

Info

Publication number: US3929077A
Application number: US474793A
Authority: US
Inventors: Lars Olov Benndahl
Original assignee: Lars Olov Benndahl
Current assignee: FROST Inc
Priority date: 1973-05-30
Filing date: 1974-05-30
Publication date: 1975-12-30
Anticipated expiration: 1992-12-30
Also published as: FR2231549A1; FR2231549B1; SE411112B; SE7407098L; CA1037315A; GB1469237A; JPS5048679A; DE2425726C2; DE2425726A1

Abstract

A conveyor system comprising an overhead rail and a number of self-propelled driving units, movable along the rail for moving associated load carriers such as floor supported carts along the path of the rail by way of a pulling rod connection between the driving units and the load carriers, wherein the pulling rod holding arrangement of each driving unit is operatively connected with an arrangement for urging the driving wheel or wheels of the unit against the rail for increasing the frictional driving engagement therebetween in response to an increased rearward pull in the pulling rod. The vertical component of the pulling force is applied to the driving wheel through a force moment amplifying arrangement, and the horizontal component of the pulling - or pushing - force is applied to the driving wheel through a force direction converter arrangement.

Description

United States Patent Benndahl Dec. 30, 1975 [5 CONVEYOR SYSTEMS 408,372 12/1944 Ital 104/89 [76] Inventor: Lars Olov Benndahl, Vidtskuegaard,

Aaby Ringvej, 8230 A b h j, Primary Examiner-M. Henson Wood, Jr. Denmark Assistant ExaminerAndres Kashnikow [22] Filed y 30 1974 Attorney, Agent, or FirmCraig & Antonelli 21 A 1. No.: 474 793 1 pp 57 ABSTRACT 30 Foreign Appncation Priority Data A cotnveytgr sg fstem ctfimflgslflg an oxerhead rbalil arid a num er 0 se -prope e r1v1ng un1 s, mova e a ong May 1973 United Kingdom 25696/73 the rail for moving associated load carriers such as [521 [LS Cl l04/93 104/148 105/50 floor supported carts along the path of the rail by way [51 Int. Cllilliilliilillii .1 11010 5/00; 1380M 1/34 a Pulling driving "P [58] Field of Search 104/89 93-95 and the mad carrers l f P 9 hmdmg 104/111, 118 147 R 148 R 148 172 arrangement of each driving umt 1s operatrvely con- H R 148 nected with an arrangement for urging the driving wheel or wheels of the unit against the rail for increas- [56] References Cited ing the frictional driving engagement therebetween in UNITED STATES TS response to an 1ncreased rearward pull in the pulling rod. The vertical component of the pulling force is ap- 2,317,689 4/1943 Medenwald 105/153 plied to the driving wheel through a force moment g ug 105/153 X amplifying arrangement, and the horizontal compo- 3502'O39 3 970 g f g nent of the pulling or pushing t'orce is applied to 382868l 8H9 4/148 R x the driving wheel through a force direction converter arran ement.

FOREIGN PATENTS OR APPLICATIONS g 550,771 5 1932 Germany 105 153 24 8 D'awmg F'gures US. Patent Dec. 30, 1975 Sheet 10f2 3,929,077

US. Patent Dec. 30, 1975 Sheet 2 of 2 3,929,077

CONVEYOR SYSTEMS The present invention relates to a conveyor system of the type comprising an overhead guiding rail supporting a number of driving units each provided with a driving motor so as to be movable along the rail and with means for holding one end of a depending pulling rod operable to be connected with a load carrier for driving the carrier along the path of the rail, each of said driving units being provided with at least one driving wheel connected with said motor and frictionally engaging a first surface portion of the rail, the rail having a second surface portion opposed to said first surface portion, each driving unit further being provided with at least one counter pressure wheel engaging said second surface portion of the rail, clamping means being provided for urging the driving wheel and the counter pressure wheel in opposite directions against the respective surface portions of the rail.

Conveyor systems of this type are used for driving a number of load carriers along the path of the rail in order to move goods from one position to another, e.g. in freight terminals. Normally the load carriers are floor supported carts having a handle rod which can be hooked into and out of engagement with the lower end of a pulling rod extending downwardly from each driving member. However, in some systems the pulling rod hangs down from the driving units so as to carry a hanging load. The shelf propelled driving units include an electric motor energized through a contact rail arrangement. This motor operates under varying load conditions, from driving its associated driving unit idly to pulling a heavily loaded carrier and even pulling it along an upwardly inclined path.

The motor and the driving wheel arrangement, of course, should be designed so as to be operative at maximum load conditions, this involving a firm frictional engagement between the driving wheel and the rail. Preferably the driving units should be as light as possible, so normally the weight of the driving unit is not sufficient to provide the necessary firm engagement. conventionally, therefore, the driving units are provided with the said counter pressure wheels, and a spring arrangement is interposed between these wheels and the driving wheels so as to urge the driving wheel against the rail with increased force for providing the necessary friction therebetween. However, the frictional engagement thus being permanently adapted to maximum load conditions, this will involve a mechanical construction which is in fact amply dimensioned, since maximum loaded conditions are normally actual sporadically only. Also the self friction of the driving unit along the rail will be kept at a constantly high level.

It is the purpose of this invention to provide a conveyor system of the type referred to in which the described disadvangages are counteracted or eliminated.

According to the invention the pulling rod holding means are operatively connected with said clamping means so as to effect an increase of the clamping force of said wheels against the opposed rail portions in response to the pulling force in said pulling rod exerting an increased resistance against movement of the driving unit. It will be appreciated that in such a system the actual clamping force will automatically adjust itself so as to be low when no very firm frictional engagement is required and high only when required, as given by the actual counter traction force acting on the driving unit through the pulling rod. When the pulling rod holding means form part of the clamping system the said counter traction force is transferable directly into an actual clamping force, whereby springs or other permanent sources of high clamping forces can be entirely avoided so as to condition a simple construction of the driving unit.

In the following the invention is described in more detail with reference to the accompanying drawing, in which:

FIG. 1 is a si'deview of a portion of the conveyor system according to the invention,

FIG. 2 is perspective view of a driving units and the rail means,

FIG. 3 is a cross sectional view thereof,

FIG. 4 is a more detailed side view of the driving unit,

FIG. 5 is a corresponding view of a modified embodiment of the driving unit,

FIG. 6 is a view corresponding to FIG. 5, but showing the driving unit assuming an inclined position,

FIG. 7 is a side view of a further embodiment of the driving unit, and

FIG. 8 is a similar view of still a further embodiment of the driving unit.

The system illustrated in FIGS. 14 comprises a rail 2 mounted under the ceiling and serving to support a number of driving units 4 each provided with a depending pulling rod 6 the lower end of which is connectable to the outer end of a handle rod 8 of a load carrier cart 9 adapted to rollalong the floor. The pulling rods 6 are normally spring loaded so as to swing upwardly when they are not actually coupled together with a handle rod 8. The rail 2 is made of profiled plate material which is self carrying between suspension brackets secured to the ceiling. In the embodiment shown the plate rail 2 is generally of inverted U cross section, and the lower side portions thereof are profiled so as to have inwardly protruding box flanges 10 each having a top side 12 and a bottom side 14. Preferably the rail side portions are extended further downwardly at 16 and provided with inwardly protruding bottom flanges 18. The interior vertical surfaces of the box flanges l are designated 20. The flange top sides 12 const tute the surfaces on which the driving units are rollmgly supported, as described in more detail below. In operation the driving units will be practically totally enclosed within the plate rail, i.e. they are to a large degree protected from dust, and besides the entire rail system will have a very clean appearance, also because the current rails for energizing the driving motors may be mounted inside the hollow rail, preferably unde the ceiling thereof as shown at 22, whereby the Current rails are mounted or placed in a highly protected manner. The rail is of course usable also ingp y tem including rail junctions and rail sw Controlled rail branchings.

At intervals the rail is secured to suspension brackets 24 each comprising a top piece 26 connected to the ceiling in any suitable manner and a P of slde Pleces 28 made of flat iron bent according to the main shape of the rail sides and having a reinforcing flange 30. The flat iron 28 is secured to the plate rail side by means of bolts 32. i

The rail 2 holds a number of driving un ts of which two are shown in FIG. 1. Each driving unit comprises an elongated base chassis 34 which is situated m e Space between the lower rail side portions 16 and pro} vided with an upstanding post 36 to the upper end 0 which there is pivotally secured one end of an elongated upper chassis portion 38, the other end of which is pivotally secured to a vertical lever 40 to be described in more detail below. On the upper chassis portion 38 is mounted a gear box 42 associated with an electric driving motor 44 and having a crosswise arranged driving shaft 46 provided with a driving wheel 48 at either end thereof. These opposed driving wheels and therewith the entire driving unit are carried on the respective opposed top side portions of the box flanges with which they are in frictional driving engagement.

The base chassis 34 is provided with two upstanding rollers 50 generally or almost filling out the cross sectional space between the opposed side wall portion of the box flanges 10 so as to act as guiding rollers guiding the driving unit longitudinally along the rail. Moreover, the base chassis 34 is provided with laterally arranged rollers 52 projecting under the bottom surfaces 14 of the box flanges 10 and rollingly engaging these surfaces so as to constitute counter pressure wheels for the driving wheels 48.

The lever extends from the upper chassis 38 downwardly through a recess 54 in the lower chassis, and at its lower end it is connected to a bracket 56 to which the top end of the pulling rod 6 is pivotally secured. Preferably a hinge 58 is interposed between the

elements

40 and 56 is order to allow for a limited transverse pivoting of the bracket 56. On the upper chassis may be mounted on control box shown in dotted lines at 60 and serving to house various control elements and circuits such as destination code responsive motor switch means, which, however, shall need no further description in the present connection. The box 60 may be provided with upper sliding shoes 62 (see FIG. 3) engaging the current rails 64 and with a control signal transmitter and receiver unit which is moved along a control rail 66 with which it communicates in any convenient manner in order to be able to receive start/stop signals and to transmit e.g. rail switch actuating signals for automatic operation.

The weight distribution about the transverse line of engagement between the driving wheels 48 and the rail surface portions 12 is such that the upper chassis 38 tends to tilt about this line so as to lift the post 36 and therewith the entire lower chassis, whereby the counter pressure wheels 52 engage the rail surface portions 14 with an upward pressure, which is only small as long as the driving unit is unloaded. When the pulling rod 6 is engaged to a cart and the driving unit is started the pull transferred to the lever 40 will serve to increase the force with which the post 36 and therewith the wheels 52 are urged upwardly, or correspondingly the force with which the driving wheels 48 are urged downwardly, whereby the frictional engagement between the wheels 48 and the rail will increase.

The pulling direction being inclined, only the vertical component of the pull will be active for producing the said increase of the pressure of the driving wheels against the rail. As shown in FIG. 4, however, the driving unit is provided with means for converting the horizontal component of the pull into a downwardly directed force on the lever 40, these means comprising a pin or roller 68 mounted on a side portion of the lever 40 and a block element 70 secured to the lower chassis 34 inside the recess 54 so as to have its inclined front end 72 situated just above the pin or roller 68.

It will be understood that the horizontal component of a counter traction pull in the pulling rod 6 will cause the pin or roller 68 to be urged horizontally against the inclined cam surface 72 of the block 70, whereby the lever 40 will be urged correspondingly downwardly and thus contribute substantially to the desired increase of the said frictional driving engagement in response to the pulling force being increased.

As shown in FIG. 4 another block member 74 is mounted in front of the pin or roller 68 so as to have an inclined cam surface portion 76 situated just above the pin or roller 68 opposite to the cam portion 72. This arrangement shows the important effect that the lever 40 will be drawn downwardly also in response to a push being transferred through the pulling rod such as can be actual by a sudden stop of the driving unit or when the cart is moved along a downwardly sloping floor portion. In these cases the motor 44 will act as a positive braking device when the driving wheels 48 do not slide along the rail, and the increased wheel friction caused by the pin or roller 68 engaging the cam surface 76 will thus secure a good braking effect.

FIG. 5 shows a modified embodiment in which the means for converting a horizontal pull or push in the lever 40 into a vertical force are inverted of those shown in FIG. 4, viz. comprising a cam member 68' mounted on the lever 40 and a pair of pins or rollers 72' and 76 mounted in the recess 54.

The embodiment of FIG. 5 is used in FIG. 6 for showing that the invention is applicable also to conveyor systems in which the driving units are adapted to convey hanging loads. In such systems, of course, there are no problems as to the frictional driving engagement during movement in the horizontal direction, but the problems occur if the driving unit shall move along upwardly or downwardly sloping rail sections. As illustrated in FIG. 6, however, the pull excerted by a load hanging in the rod 6 will cause the lever 40 to be subjected to a pulling force component parallel to the moving direction of the driving unit, whereby an increased frictional driving engagement will be automatically obtained in the same manner as described in connection with FIGS. 4 and 5. Correspondingly the frictional braking engagement will be increased when the driving unit moves through a downwardly sloping path. The wedge shaped cam surfaces may be substituted by other arrangements, of which an example is shown in FIG. 7. In this embodiment of the driving unit the pulling rod 6 is connected to the downwardly extending arm 80 of a T-lever 82 which at 84 is pivotally connected with the base chassis 34. The outer ends of the horizontal top lever portion 86 of the T-lever are provided with pins engaging in elongated slots 88 formed in the lower ends of two opposed levers 90 which are both at their top ends pivotally connected to the upper chassis 38' at a common pivot point 92. It will be understood that when the lower lever portion 80 is subjected to a horizontal push or pull the top lever 86 will be tilted so as to pull one of the levers 90 downwardly while the other of these levers, due to the slot 88, will be unable to counteract this pull, and the same result as described hereinafter will be obtained.

For illustrating a further possibility FIG. 7 shows that the upper chassis 38' may be provided with an extra set of wheels 94 so as to constitute a carriage resting on the rail portions 12, while the lower chassis 34 is suspended in the upper chassis by way of the lever or lever system transferring the said self adjusting clamping force, by way of the lever system 82-90. The lever system, of course, should be so adapted that the lower chassis is suspended with its wheels 52 situated very close to the rail surface portions 14.

FIG. 8 illustrates a further embodiment in which the top end of the post 36' is provided with a bracket 96 in which a vertical lever 40' is pivotally secured at 98. The lever 40' extends down through a recess in the lower chassis and is connected to the pulling rod 6 while adjacent its upper end the lever 40' is secured to the gear box 42 so as to carry the motor and gear box assembly. It will be seen that when a pull is transferred to the lower end of the lever 40' through the pulling rod 6 the lever 40 will tend to pivot anticlockwise so as to thereby cause the driving wheels 48 to be pressed more firmly against the rail, assisted by a corresponding increase of the upward pressure of the lower wheels 52' against the underside of the rail. This embodiment, however, is not able to produce an increased braking friction, unless the driving wheels are mounted exactly above the pivot point 98.

The invention is not limited to the embodiments shown, since it will be obvious that many modifications will be possible within the scope of the invention in order to obtain the self adjusting clamping force between the driving wheels and the rail; Thus, the driving wheels and the counter pressure wheels should be urged away from each other if they are arranged so as to engage with interfacing surface portions of the rail.

It will be understood that in the embodiments of FIGS. 1-5 the desired amplification of the frictional engagement between the driving wheel and the rail in response to an increased pull in the pulling rod is obtainable already by virtue of the vertical force compo nent in the member 40, since the horizontal distance between the member 40 and the post 36 is larger than the distance between the engagement point of the driving wheel 48 and the post 36. A very high force amplification will 'be obtained if the wheel 48 is mounted very close to the post 36, but in practice this arrangement involves that the thickness of the rail should be extremely uniform throughout the length thereof, as otherwise the upper chassis 38 and therewith the member 4 andthe rod 6 would oscillate violently up and down. Placing the driving wheel 48 in larger distance from the post 36 involves a reduced force amplification, but this reduction is compensated for by means of the particular force converter arrangement enabling also the horizontal force component to contribute to the clamping force, and then the almost inevitable thickness variations of the rail become less critical. The said converter arrangement, of course, is especially important for causing a braking action-in case of a push being transferred to the member 40 through the pulling rod 6.

What is claimed is:

l. A conveyor system of the type comprising an overhead guiding rail supporting a number of driving units each provided with a driving motor so as to be movable along the rail and with means for holding one end of a depending pulling rod operable to be connected with a load carrier for driving the carrier along the path of the rail, each of said driving units being provided with at least one driving wheel connected with said motor and frictionally engaging a first surface portion of the rail, the rail having a second surface portion opposed to said first surface portion, each driving unit further being provided with at least one counter pressure wheel engaging said second surface portion of the rail, clamping means being provided for urging the driving wheel and the counter pressure wheel in opposite directions against the respective surface portions of the rail, characterized in that the pulling rod holding means are operatively connected with said clamping means so as to effect an increase of the clamping force of said wheels against the opposed rail portions in response to the pulling force in said pulling rod exerting an increased resistance against the movement of the driving unit.

2. A conveyor system according to claim 1, characterized in that said drive unit includes a base chassis, said at least one counter pressure wheel being mounted on said base chassis, in that said clamping means includes an upper chassis pivotally mounted on said base chassis, said driving motor and said at least one driving wheel is mounted on said upper chassis, and in that said clamping means further includes at least one lever having one end thereof pivotally mounted to said upper chassis, the other end of said at least one lever being pivotally mounted to said pulling rod holding means.

3. A conveyor system according to claim 2, characterized in that said base chassis includes a post for pivotally supporting said upper chassis, and in that said lower chassis includes a recess through which said lever extends to said pulling rod holding means.

4. A conveyor system according to claim 3, characterized in that means are provided on said base chassis for guiding the drive unit longitudinally of the guiding rail.

5. A conveyor system according to claim 4, characterized in that the means for guiding the drive unit includes at least one guiding roller provided on said base chassis.

6. A conveyor system according to claim 5, characterized in that said guiding rail has a substantially U- shaped profile opening toward the load carrier, the lower portion of said guiding rail being provided with box flanges along each leg thereof directed inwardly toward each other, each of said box flanges having a top surface portion, bottom surface portion and a side surface portion connecting said top and bottom surface portions, said top and bottom surface portions respectively defining the first and second surface portions of the guiding rail.

7. A conveyor system according to claim 6, characterized in that said guiding roller means is disposed between said third surface portion.

8. A conveyor system according to claim 6, characterized in that means are provided for dividing said guide rail into an upper and lower portion, said drive motor being disposed in said lower portion of said guide rail above said first surface portion, current rail means are mounted on said dividing means, sliding shoe means engaging said current rail means operatively connected with said driving motor, and in that a control rail means is disposed in said lower portion of said guide rail for controlling the operation of said drive motor.

9. A conveyor system according to claim 3, characterized in that means are provided at said recess for converting a force component transfered to said pulling rod holding means through said pulling rod.

10. A conveyor system according to claim 9, characterized in that said converting means includes at least one cam surface provided on said recess, and a roller means cooperable with said at least one cam surface.

11. A conveyor system according to claim 9, characterized in that said converting means includes a first cam means provided at said recess, and a further cam means mounted on said lever cooperating with said first cam means.

12. A conveyor system according to claim 11, characterized in that said first cam means includes at least two spaced rollers mounted in said recess.

13. A conveyor system according to claim 9, characterized in that said clamping means includes a second lever pivotally mounted on said upper chassis, and in that said converting means includes a further lever pivotally connected to said two levers of said clamping means.

14. A conveyor system according to claim 13, characterized in that said further lever is a substantially T-shaped lever pivotally mounted in said recess, said pulling rod is pivotally connected to the stem of said T-shaped lever, and said two levers of said clamping means are pivotally connected at the free ends of the top portion of said T-shaped lever.

15. A conveyor system according to claim 1, characterized in that said drive unit includes a gear box operatively connected with the drive motor and a base chassis, said at least one driving wheel is connected to said gear box, said at least counter pressure wheel is mounted on said base chassis, and in that said clamping means includes at least one lever pivotally mounted on said base chassis, one end of said at least one lever is pivotally mounted to said pulling rod, the other end of said at least one lever having said gear box mounted thereon.

16. A conveyor system according to claim 1, characterized in that said guiding rail has a substantially U- shaped profile opening toward the load carrier, the lower portion of said guiding rail being provided with box flanges along each leg thereof directed inwardly toward each other, each of said box flanges having a top surface portion, bottom surface portion, and third surface portion connecting said top and bottom surface portions, said top and bottom surface portions respectively defining the first and second surface portions of the guiding rail.

17. A conveyor system according to claim 16, characterized in that means are provided for dividing said guide rail into an upper and lower portion, said drive motor being disposed in said lower portion of said guide rail above said first surface portion thereof, current rail means are mounted on said dividing means, sliding shoe means engaging said current rail means are operatively connected with said driving motor, and in that a control rail means is disposed in said lower portion of said guide rail for controlling the operation of said drive motor.

18. A conveyor system of the type comprising an overhead guiding rail supporting a number of driving units each provided with a driving motor so as to be movable along the rail and with means for holding one end of a depending pulling rod operable to be connected with a load carrier for driving the carrier along the path of the rail, each of said driving units being provided with at least one driving wheel connected with said motor and frictionally engaging a first surface portion of the rail, the rail having a second surface portion opposed to said first surface portion, each driving unit further being provided with at least one counter pressure wheel engaging said second surface portion of the rail, clamping means being provided for urging the driving wheel and counter pressure wheel in opposite directions against the respective surface portions of the rail, characterized in that the pulling rod holding means are operatively connected with said clamping means so as to effect an increase of the clamping force of said wheels against the opposed rail portions in response to the pulling force in said pulling rod exerting an increased resistance against the movement of the driving unit, the pulling rod holding means are additionally operatively connected with said clamping means so as to effect an increase of said clamping force in response to the pulling force in the pulling rod exerting an increasing forwardly directed pull in the driving unit.

19. A conveyor system according to claim 18, characterized in that between the driving wheels and the counter pressure wheels there is interposed a force direction converting arrangement operable to convert a force component transferred to said pulling rod holding means through said pulling rod and directed parallelly to the rail direction into a force component directed perpendicularly to said rail direction so as to cause said wheels to be urged against their respective cooperating rail surface portions.

20. A conveyor system according to claim 19, in which the force direction converter means comprise a wedge surface.

21. A conveyor system according to claim 19, in which the force direction converter means comprise a pivot lever system.

22. A conveyor system of the type comprising an overhead guiding rail supporting a number of driving units each provided with a driving motor so as to be movable along the rail and with means for holding one end of a depending pulling rod operable to be connected with a load carrier for driving the carrier along the path of the rail, each of said driving units being provided with at least one driving wheel connected with said motor and frictionally engaging a first surface portion of the rail, the rail having a second surface portion opposed to said first surface portion, each driving unit further being provided with at least one counter pressure wheel engaging the second surface portion of the rail, clamping means being provided for urging the driving wheel and the counter pressure wheel in opposite directions against the respective surface portions of the rail, characterized in that the pulling rod holding means are operatively connected with said clamping means so as to effect an increase of the clamping force of said wheels against the opposed rail portions in response to the pulling force in said pulling rod exerting an increased resistance against the movement of the driving unit, and in that between the driving wheels and the counter pressure wheels there is interposed a force direction converting arrangement operable to convert a force component transfered to said pulling rod holding means through said pulling rod and directly parallely to the rail direction in a force component directly perpendicularly to said rail direction so as to cause said wheels to be urged against their respective cooperating rail surface portions.

23. A conveyor system according to claim 22, in which the force direction converter means comprise a wedge surface.

24. A conveyor system according to claim 22, in which the force direction converter means comprise a pivot lever system.

Claims

1. A conveyor system of the type comprising an overhead guiding rail supporting a number of driving units each provided with a driving motor so as to be movable along the rail and with means for holding one end of a depending pulling rod operable to be connected with a load carrier for driving the carrier along the path of the rail, each of said driving units being provided with at least one driving wheel connected with said motor and frictionally engaging a first surface portion of the rail, the rail having a second surface portion opposed to said first surface portion, each driving unit further being provided with at least one counter pressure wheel engaging said second surface portion of the rail, clamping means being provided for urging the driving wheel and the counter pressure wheel in opposite directions against the respective surface portions of the rail, characterized in that the pulling rod holding means are operatively connected with said clamping means so as to effect an increase of the clamping force of said wheels against the opposed rail portions in response to the pulling force in said pulling rod exerting an increased resistance against the movement of the driving unit.

6. A conveyor system according to claim 5, characterized in that said guiding rail has a substantially U-shaped profile opening toward the load carrier, the lower portion of said guiding rail being provided with box flanges along each leg thereof directed inwardly toward each other, each of said box flanges having a top surface portion, bottom surface portion and a side surface portion connecting said top and bottom surface portions, said top and bottom surface portions respectively defining the first and second surface portions of the guiding rail.

16. A conveyor system according to claim 1, characterized in that said guiding rail has a substantially U-shaped profile opening toward the load carrier, the lower portion of said guiding rail being provided with box flanges along each leg thereof directed inwardly toward each other, each of said box flanges having a top surface portion, bottom surface portion, and third surface portion connecting said top and bottom surface portions, said top and bottom surface portions respectively defining the first and second surface portions of the guiding rail.