Wheelchair

A wheelchair system comprises a substantially rigid seat adapted to be mounted on at least two alternative types of carriages having different drive arrangements each comprising a pair of molded plastic side frames which are interconnected by a bracing mechanism to permit the side frames to be folded together for collapsing the chair or spread apart for erecting the chair. A connecting system including locators which allow the seat to be placed on an approximately fully spread carriage and then to locate the side frames at the correct spacing from one another in order to ensure complete spreading of the carriage, and additionally retainers which allow the seat to be moved into a position in which it cannot be lifted from the thus spread carriage, the retainers allow positive engagement of the seat with the chair with the ability to spread the carriage solely by downward pressure on the seat, thereby enabling a disabled user of the wheelchair to be able to place the seat on the carriage ready for use. A locking assembly prevents the seat from movement so as to disengage the retainers and hence retain the seat and the carriage in engaged configuration.

The present invention relates to a wheelchair. 
Conventional wheelchairs have the capacity to be folded flat when not in 
use, and for this purpose the seat is normally of flexible construction, 
for example comprising canvas panels extending between folding tubular 
side frames of the wheelchair. The wheelchair frame is equipped with means 
for locking the frame erect, usually comprising some form of overcenter 
linkage. 
Although such wheelchairs have the advantage that the seat will readily 
adapt to various different body sizes and may be made more comfortable by 
the addition of seat cushions, the fact remains that the seat is not 
normally sculptured to the anatomy of the human body and thus occupant 
comfort leaves much to be desired. 
Another disadvantage of the conventional wheelchair is that the user of a 
wheelchair is confined to one particular wheel geometry once he has bought 
a chair, so that where the user may wish to have the facility of being 
pushed by an attendant or propel himself or herself by simply grasping the 
rear wheels of the wheelchair for self-propulsion, and additionally to 
have the facility of either motorized self-propulsion or level-operated 
self-propulsion when out-of-doors, various different types of chair need 
to be bought, each costing quite a considerable sum of money and involving 
considerable investment by the wheelchair owner. 
It is an object of the present invention to enable the drawbacks of 
conventional wheelchairs to be overcome. 
Accordingly, the present invention provides a wheelchair system comprising 
a carriage, wheels rotatably supported on the carriage, and a 
substantially rigid seat removably connected to said carriage, 
characterized by further including an alternative carriage having a 
different wheel configuration to the first-mentioned carriage, said 
substantially rigid seat being adapted to be attached alternately to both 
of the two said carriages. 
Preferably the seat is substantially rigid; the carriage is foldable and 
includes first locating means and first retaining means and the seat 
includes second locating means and second retaining means; the various 
said locating means and retaining means are arranged such that engagement 
of the locating means positions the seat relatively to the carriage and 
spreads the carriage to a fully erected configuration merely by resting 
the seat on the carriage; and sliding movement of the seat relative to the 
carriage brings said first and second retaining means into engagement to 
maintain the seat on the carriage. 
Advantageously, on each side of the carriage in the upper portion thereof 
is a horizontal bar; said second locating means comprise a downwardly open 
elongate recess running along each side of the substantially rigid seat of 
the underside thereof and said first locating means comprise a 
co-operating upper surface of said bar on which the recess slidably rests; 
and said second retaining means comprise a bore extending parallel to and 
positioned as an extension of said recess, at each side of the seat, for 
enveloping the entire circumference of said bar; said bar upper surface 
constituting the first locating means and the remainder of the 
circumference constituting said first retaining means.

FIG. 1 illustrates the wheelchair 1 as comprising a seat 2 and a carriage 3 
on which the seat is removably attached. The seat is of molded plastics 
construction, either injection molded or molded with a fibrous reinforcing 
structure such as a glass-fiber reinforced plastics composite, and has a 
hinge 4 connecting the seat back 5 pivotally to the seat base 6. 
For removal of the seat 2 from the carriage 3 the seat back 5 is first of 
all folded flat onto the base 6 and then the flat-folded seat is slid 
forwardly off the carriage. This enables the seat to be used in another 
application (for example allowing the seat to be mounted in a vehicle 
where suitable attachment fittings are provided for the seat, or 
alternatively enables the one seat to be used with different carriages 3). 
This versatility of the wheelchair in accordance with the present 
invention offers considerable advantages in that it enables an economic 
wheelchair system to be built-up in modular form by buying a single rigid 
seat 2 which may or may not require special cushions to adapt it to the 
body of the user, and allowing that one seat to be used with two or more 
separate carriages one of which may, for example, be motorized and the 
others may be intended for attendant operation or self-propulsion manually 
by the occupant of the wheelchair. Thus, instead of having to buy several 
complete chairs, and find storage space for those chairs, the user of the 
wheelchair in accordance with the present invention may economize on both 
cash investment and storage space by a reduction in the number of 
components which are duplicated between two separate types of wheelchair. 
As a further feature of the wheelchair shown in FIG. 1, the same pair of 
side frames 7 constituting the carriage 3 may be used with different wheel 
configurations. FIG. 1 shows a large rear wheel 8 and a small castering 
front wheel 9 on the side frame 7 nearest the observer and there will of 
course be a second pair of such wheels on the opposite side frame (not 
shown) behind the plane of the paper. This large wheel 8 is convenient for 
manual self-propulsion by the user and may, for example, be fitted with a 
hand-grip rim to allow the user to propel the wheelchair without having to 
touch the ground-engaging tires of the wheel. 
An alternative configuration would be for smaller rear wheels to be used 
(for example of the type illustrated at 8a in FIG. 6), where the 
wheelchair is to be attendant-manipulated, or for a lever-operated rear 
wheel configuration (not illustrated in the drawings) to be incorporated. 
With each of these alternative wheel configurations, it is possible for 
the same side frames 7 to be used and for the wheel type to be changed 
simply be removing the wheel carrier 10 (FIG. 5) from the wheel carrier 
socket 11 of the side frame 3 and then substituting a different wheel 
carrier having the alternative wheel type attached. The same exchange, by 
the user or more preferably by a companion, can convert the carriage 3 
from a manually propelled carriage to a motorized self-propelled carriage 
by attaching the motorised wheel carrier 12 of FIG. 7 in place of the 
wheel carrier 10 of FIG. 5. 
FIG. 1 also shows a bolt 13 having an operating handle 14 and engaging in a 
tubular socket 15 of the side frame 7 to prevent the rigid seat base 6 
from sliding forwardly when the seat back 5 is erect. This assists in 
preventing inadvertent release of the seat 2 from the carriage 3. 
An additional aspect of the seat 2, shown in FIG. 1, is the provision of 
arm rests 16 which are pivoted at 17 to the respective sides of the seat 
back 5 and are supported at their free ends 16' by means of support stays 
which are pivotally attached to the seat base 6 and releasably fastenable 
to the end 16' of the respective arm rest. The arm rests 16 clearly fold 
parallel to the seat base 6 as the seat back 5 is folded forwardly (after 
release of the lower end of each stay). 
The two carriage side frames 7 are of molded plastics construction, 
preferably a fiber reinforced plastics composite, and are interconnected 
one with the other by means of a conventional folding mechanism, not shown 
in the drawings. Thus, once the seat 2 has been removed from the carriage 
3 it is possible for the carriage to be folded flat, preferably after 
removal of the wheel carriers 10 (FIGS. 5 and 6) from the wheel carrier 
sockets 11 (FIG. 1) in order to provide a more compact finished folded 
structure. It is an advantageous characteristic of the wheelchair 
illustrated in the drawings, that the substantially rigid seat 2 serves as 
additional bracing means to maintain the side frames 7 at the desired 
"spread" spacing from each other. This requires the need for relatively 
straightforward re-assembly of the seat 2 and carriage 3 once the carriage 
3 has been erected; the means for achieving this accurate alignment of the 
various parts of the connection between the side frames 7 and the seat 2 
can be appreciated from FIGS. 2 and 3. 
FIG. 2 shows a detail section taken on the line 2--2 of FIG. 1 and 
illustrates a front view of a longitudinal recess 17 of inverted, i.e. 
upwarding tapered, trough-shaped form extending along a portion of the 
seat base 6. This recess 17 slidably engages a horizontal tube 18 attached 
to the carriage side frame 7 by way of a bracket 19 which is molded in 
situ in the carriage side frame 7. This front bracket 19 and guide tube 18 
are both shown in FIG. 1. 
Further back along the seat 2 is a second such bracket, referenced 20, 
carrying a guide tube 21 which is integral with the socket 15 for the bolt 
13. Like the front bracket 19, the rear bracket 20 is also molded in situ 
in the side frame 7. The section line 3--3 of FIG. 1 passes through a lug 
22 of the seat base 6 which, instead of exhibiting the downwardly open 
longitudinal guide recess 17 defines a fully enveloping circular bore to 
receive the rear horizontal guide tube 21. 
In practice, the front and rear brackets 19 and 20 with their respective 
guide tubes 18 and 21 are very similar and so also are the co-operating 
parts of the seat base 6 in that not only does the part of the 
substantially rigid seat base engaging the rear guide tube 21 have a front 
lug 22, but so also does the front part of the seat base have a similar 
lug 23 with its fully enveloping bore to receive the front end of the 
guide tube 18. 
It will be appreciated from the above that the configuration of the two 
guide tubes 18 and 21 on their respective brackets 19 and 20 gives them a 
T-shaped configuration with the guide tubes serving as the cross-bar of 
the T in each case, and the brackets serving as the stem of the T. 
In order to mount the seat base 6 on the spread carriage 3, it is necessary 
firstly to place the seat base 6 on the guide tubes 18 and 21 in such a 
way that only the trough-shaped downwardly open recesses 17 of the front 
and rear parts of the seat base 6 engage the guide tubes 18 and 21. In 
other words, the seat will be in a position displaced forwardly from that 
shown in FIG. 1 and will be resting on the front ends only of the guide 
tubes 18 and 21. It is then a simple matter to ensure that the degree of 
spreading of the side frames 7 of the carriage 3 is appropriate to allow 
the accurate alignment of the two guide tubes 18 and 21 in their 
respective recesses 17, and it is envisaged that this manipulation may 
well be within the capability of the wheelchair user provided of course he 
has some alternative means of bodily support while effecting this 
operation (assuming he is unable to stand unsupported). 
The mere act of pressing downwardly on the seat base 6 will itself help to 
align the side frames 7 with the respective sides of the seat base, by 
virtue of the V-shaped tapered configuration of the side walls of the 
recesses 17. 
Once the seat has been thus pressed firmly downwardly against the carriage 
side frames 7, the seat 2 as a whole is pushed rearwardly so that the 
front ends of the two guide tubes 18 and 21 enter the bores in the 
respective lugs 22 and 23 of the seat base 6 and then it will not be 
possible for the seat to be lifted vertically off the carriage or for the 
seat to be displaced sideways off the carriage during rough use. However, 
it is then necessary to lock the seat in this rearward position to hold 
the lugs 22 and 23 on the guide tubes 18 and 21, by operation of the bolt 
13. However, this bolt can only be used once the seat back 5 has been 
erected and erection of the seat back 5 is therefore the last manipulation 
before final locking of the seat. 
Although not shown in the drawings, it is possible for some means of 
variable inclination of the seat back 5 to be provided, so as to give the 
seat some degree of reclining ability. 
FIG. 1 shows clearly a handle 24 to facilitate pushing of the wheelchair by 
a companion or attendant. 
FIG. 1 also illustrates quite clearly a central cutaway 25 of each side 
frame 7 of the carriage 3, thereby allowing the wheelchair to be 
relatively lightweight construction, particularly bearing in mind the fact 
that the side frames 7 are manufactured from plastics materials. As shown 
in FIG. 1, the side frame is of a generally Z-shaped configuration and the 
profile of the upper and lower parts 7a and 7b of the molded seat 
construction at the bend in the Z can be shown from the dotted line 
outline of these components illustrated in FIG. 4 which is a detail of the 
seat side frame 7 to show the wheel carrier socket 11. 
As shown in FIG. 4, the molded plastics body of the seat side frame 7 
includes a lug 26 defining a vertical cylinder and swaged internally 
within this lug 26 is a metal liner 27 to allow repeated insertion and 
withdrawal of a wheel carrier 10 (FIGS. 5 and 6) for the manually 
propelled wheelchair, or the wheel carrier 12 (FIGS. 7 and 8) of the 
motorized self-propelled wheelchair, without undue wear of the interior of 
the socket 11. It is of course important that the wheel carrier be a 
secure fit in the wheel carrier socket 11, in order to ensure that the 
chair has the desired degree of accurate steerability over a prolonged 
useful life of the chair. 
Turning now to FIGS. 5 and 6, there will be seen the structure of the wheel 
carrier 10 and from this it can be seen that the same U-shaped carrier 10, 
having upper and lower hand grips 28 and 29, respectively can be provided 
with a lower hub 30 for the smaller diameter wheel 8a shown in FIG. 6, or 
a higher hub 31 for the larger diameter wheel 8 shown in FIG. 6. For this 
purpose, the U-shaped wheel carrier 10 will be manufactured with two 
separate mounting bolt sockets 32 and 33 (FIG. 5) to receive the lower and 
upper hubs 30 and 31, respectively. It is envisaged that a specialist will 
be responsible for securing the appropriate wheel type 8 or 8a to the 
wheel carrier 10 and that this would not normally be a task undertaken by 
the wheelchair user, or the attendant or companion, because of the 
importance of achieving positive mounting of the wheel hub 30 or 31 on the 
wheel carrier to ensure absolute safety of the wheelchair occupant at all 
times. However, the mere fact that a single wheel carrier 10 can take the 
two different kinds of wheels will simplify the stock control of a 
wheelchair supplier because with a reduced number of wheels, covering the 
two different sizes of wheel shown in FIG. 6, he needs only to carry a 
limited number of the U-shaped wheel carriers 10 to make it possible to 
supply customers with finished wheel carriers equipped with either wheel 
size. 
At the mid-point of the vertical bridge 34 serving as wheel support bar of 
the U-shaped wheel carrier 10 is a horizontally extending cylindrical bar 
35 which is welded to the wheel support bar 34 and which is intended to be 
snugly received in a hemi-cylindrical recess 36 (FIGS. 1 and 4) of the 
seat side frame 7. This provides a first location of the wheel carrier 10 
and serves to prevent "wheel wobble". 
The hand grips 28 and 29 at either end of the vertical wheel support bar 34 
enable the person engaging the wheel carrier 10 with the carriage 3 to 
grip the wheel carrier securely when locating the wheel carrier stem 37 in 
the socket 11 and when withdrawing the stem 37 from the socket 11. Thus 
the one hand grip 28 is used when the wheelchair is being assembled and 
the other hand grip 29 is used when the wheelchair is being dismantled. 
It is an advantageous feature of the present invention that the location of 
the cylindrical bar 35 connecting the wheel support bar 34 with the stem 
portion 37 is mid-way between the mountings 32 and 33 for the two 
different wheels 8a and 8, thereby minimizing any effects of instability 
caused by lack of co-axial relationship between the axis of rotation of 
the wheel hub 30 or 31 and the axis of the cylindrical bar 35. 
Naturally, further steadying of the wheel is required and this is achieved 
by way of the vertical stem 37 of the wheel carrier, rigidly secured to 
the cylindrical bar 35. This stem 37 engages snugly in the swaged liner 27 
of the wheel carrier socket 11 and completes the accurate location of the 
wheel carrier 10 in relation to the carriage side frame 7. 
At its upper end, the stem 37 has a ball catch 38 which will prevent 
accidental dropping of the wheel carrier 10 out of the socket 11 in the 
event of the weight of the wheelchair being taken by the attendant or 
companion, for example when negotiating stairways. It is considered a very 
important aspect of the wheelchair that the wheel carrier 10 is simply a 
plug-in fit in the socket 11 of the carriage but it must of course be 
ensured that accidental disconnection of this wheel carrier 10 from the 
side frame 7 cannot occur in use of the wheelchair. Thus the strength of 
the ball catch 38 is required to be just sufficient to prevent the wheel 
carrier 10 from dropping out of the socket 11 under its own weight (given 
some degree of frictional engagement between the stem 37 and the liner 27) 
and should not be so stiff that a partially handicapped wheelchair user 
would not be able to remove that wheel from the carriage 3 for collapsing 
of the carriage when desired. 
FIG. 5 also shows that the bottom end of the socket 15 for the bolt 13 
(FIG. 1) may engage the top end of the stem 37 for further steadying of 
the wheel and wheel carrier. 
As indicated above, the same wheelchair carriage 3 can, if desired, be 
equipped with self-propulsion units. One of these could have a 
lever-operated self-propulsion unit incorporated on a wheel carrier not 
too dissimilar from that illustrated at 10 in FIGS. 5 and 6. However, 
another possible variation would be for two of the wheel carriers 12 of 
FIG. 8 to be attached, one at each side of the carriage 3, and for an 
appropriate wheelchair control unit to be mounted in one of the arm rests 
16 of the seat. Bearing in mind that each of the wheel carriers 12 is 
provided with its own electric motor 39, it is desirable for the 
interengagement of the wheel carrier 12 with the side frame 3 to include 
provision (not shown) for plug and socket connection of control leads for 
the motor 39. This could, for example, be incorporated on or in 
association with the cylindrical body 40 which serves the same purpose as 
the bar 35 of the wheel carrier 10 of FIGS. 5 and 6 and engages in the 
recess 36 of the side frame 7. Similarly, a further plug-in-connection 
system will be desirable at the interconnection of the seat 2 with the 
carriage 3 so that where the control unit for the motorized wheel carriers 
12 is already installed in one of the seat arm rests 16, the connection of 
the leads from that control unit to the leads of the seat side frame can 
be ensured as the seat is attached to the carriage 3. These various 
plug-and-socket connectors at each location may, for example, be an 
integral part of the three components, namely the wheel carrier 12, the 
side frame 7, and the seat base 6, so that no separate electrical 
connection is required. Alternatively, it may be possible for a "loose 
lead" connector to be provided on the seat base 6 on the one hand and on 
the wheel carrier 12 on the other hand so that one plug-in connection is 
made at each side of the seat once the seat and the wheel carriers have 
been connected to the carriage side frames 7. 
It is envisaged that the motorized wheel carriers 12 will provide adequate 
controlability of the wheelchair 1 given the castering ability of the 
front wheels and the possibility of accurate independent control of the 
respective righthand and lefthand motors 39 on the righthand and lefthand 
wheel carriers 12. 
As shown in FIG. 7, the motorized wheel carrier 12 also includes an 
attachment 41 for a battery support plate 42 to mount an electric storage 
cell 43 in a housing in the space between the two side frames 7 of the 
carriage 3. For this purpose, each side of the support plate 42 
additionally includes a part-cylindrical boss 44 forwardly of the wheel 
carrier 12, and adapted to rest in the part-cylindrical cradle formed by 
the upper portion 7a (FIG. 1) of the carriage side frame 7. 
It will be clear from the above that the conversion of a carriage 3 from 
manually-propelled to motorized type is a more time consuming and involved 
process than the simple conversion of the carriage 3 from 
attendant-manipulated form (using the wheels 8a of FIG. 6) to manually 
self-propelled form (using the wheels 8 of FIGS. 1 and 6) and it is 
therefore envisaged that once a carriage 3 has been converted to 
self-propelled motorized format it will not normally be regularly 
converted back to manual propulsion. However, it does not require a lot of 
technical skill to make such a conversion either way and this therefore 
brings the modification of the chair from manual to motorized format and 
vice versa within the realms of the skill of the average do-it-yourself 
expert. 
From the above it will be understood that the wheelchair described and 
illustrated herein does not merely constitute a very complex system of 
exchangeable parts which, when purchased by the user, can enable him to 
convert his chair between attendant-propulsion, motorized self-propulsion, 
simple hand wheel self-propulsion, and lever-operated self-propulsion, but 
additionally enables a wheelchair user to start off with the basic 
wheelchair illustrated in FIG. 1, possibly with the smaller diameter wheel 
8a of FIG. 6, and then to expand the capabilities of his wheelchair by 
buying "bolt-on" conversion accessories as his financial resources improve 
or as his degree of dependence on the chair increases. Thus, starting from 
the simple configuration shown in FIG. 1, the user may well finish up with 
one seat 2 and a pair of carriages 3, one of which is permanently fitted 
with motorized wheel carriers 12 and the other of which can accommodate 
two alternative sets of wheel carriers 10, one with the self-propulsion 
large wheels 8, and the other with the attendant-propulsion small wheels 
8a which offer the advantage of making the folded chair much more compact, 
for example in order to allow the wheelchair carriage to be folded into 
the trunk space of even the smallest saloon automobile. 
FIG. 9 shows an alternative embodiment of the chair, embodying the 
principles illustrated in FIGS. 1 to 8, and described above, and 
incorporates additional improvements which will be described below. 
The overall shape of the side frames 107 in FIG. 9 is considerably 
different from that of side frames 7 of FIG. 1, although the same method 
of fixing the seat base 106 to the side frames 107 has been retained, 
namely the use of a pair of forwardly projecting pegs 118 and 121 of the 
side frame, engaging sleeves 122 and 123 of the seat base, with a 
V-section guide groove (not shown in FIG. 9) behind each of the sleeves 
122 and 123 to ensure that as the seat base 106 is pressed down on to the 
completely or almost completely spread carriage structure before the seat 
is pushed back towards the pegs 118 and 121, the camming action between 
the V-shaped grooves of the seat base 106 and the pegs 118 and 121 of the 
side frames serves to ensure that the carriage side frames are completely 
spread apart ready for the seat base 106 to be pushed rearwardly to anchor 
it safely on the pegs 118 and 121. 
However, this embodiment is different from that of FIG. 1 in that the 
sleeves 122 and 123 can be a friction fit on the respective pegs 121 and 
118, and the final rearward movement of the seat base 106 relative to the 
side frames 107 is actuated by pivoting the seat back 105 into the upright 
configuration (automatically raising the arm rests 116, in so doing) thus 
causing a downwardly projecting end portion 125 of the seat back to engage 
cammingly against a surface 126 of the back of the side frame and as a 
result a levering action is applied (bearing in mind the position of the 
fulcrum 104 between the seat back 105 and the seat base 106) which pulls 
the seat base 106 slightly further rearwardly into its fully home 
position. 
As the seat base 106 comes into its FIG. 9 or "fully home" position, a 
locking toggle 127 on each of the side frames catches the rear of the 
projection 125 to hold the seat back 105 upright. 
Quite clearly, in view of the camming engagement between the projecton 125 
and the surface 126 of each side frame, release of the locking toggles 127 
is necessary before the seat back 105 can be folded forwardly to permit 
the seat base 106 to move forwardly for subsequent removal. Thus, the 
entire chair is held as a rigid structure until such time as the locking 
toggles 127 have been deliberately released and the seat back 105 has been 
folded down to the seat base 106. 
As shown in FIG. 9, the foot rests are mounted on swinging supports 101 
pivoted on the support shaft 128 for the front castering wheels 109. This 
pivoting action of each foot rest support 101 allows it to be folded 
backwardly into contact with the side frame 107, to render the wheelchair 
more compact. 
The wheelchair shown in FIG. 9 has both the large diameter 22 inch (56 cm) 
wheels 108 illustrated, and also the alternative smaller diameter 
rearwheels 108'. To receive these wheels, each removable wheel carrier 110 
has two alternative wheel spindle bearing recesses, 132 for the smaller 
wheels 108' and 133 for the larger wheels 108. An additional wheel spindle 
bearing recess 134 is provided for yet a third form of wheel, if desired. 
As in the case of the wheel carriers 10 of the FIG. 1 embodiment, the wheel 
carriers 110 are formed separately from the main side frame members 107 
and are in this case bolted in place thanks to tabs 111 of the wheel 
carriers 110, and bolts (not shown) which pass through the tabs 111 and 
co-operating lugs of the side frames 107. 
To help to keep the entire assembly of wheel carriers 110 and side frames 
107 rigid, the cross-section of the wheel carrier 110 is, as illustrated 
in FIG. 10, formed with an upwardly open groove 135 which extends along 
the top of the wheel carrier 110 and down the entire inclined front edge 
of the wheel carrier 110, and which receives a corresponding bead 109 of 
the side frame 107. Thus, even before fastening the bolts through the tabs 
111 and the corresponding lugs of the side frames 107, the assembly of the 
wheel carriers 110 and the side frames 107 is already a firm friction fit 
thanks to the interengagement of the bead 109 with the groove 135. This 
structure provides the same degree of wobble prevention which is exhibited 
by the engagement of the wheel carriers 10 in the side frames 7 in the 
FIG. 1 embodiment. 
The wheelchair is completed by the addition of a one piece handle 124 which 
fits in cylindrical sockets 129 of the seat back 105, and furthermore by a 
pair of cross braces 130 which constitute a scissor-action folding 
mechanism to the chair. 
Although, in FIG. 10, the wheel carrier 110 is shown as having a recess 135 
to receive the rib 109, it is of course possible for each of the wheel 
carriers 110 to be provided with a projection, analogous to the rib 109, 
to engage in a corresponding recess, analogous to the groove 135, in the 
side frame 107.