This invention relates generally to a method of producing an .article having a bore with a groove therein, means for securing a member to such an article, and such an article with a member secured thereto, and is particularly concerned with a method of producing a hub, means for securing a pair of wheels to said hub to form a twin-wheel castor, and a castor so produced. Twin-wheel castors are well known and are commonly used, for example, on chairs, trolleys and stands.

One type of twin-wheel castor comprises an upright boss, for reception of a castor stem about which the castor can rotate, and a hub portion, a central longitudinal axis of which is perpendicular to the axis of the. boss. The wheels of the castor have respective central cylindrical spigots which extend into the hub portion at opposite sides thereof, and each wheel is secured against axial movement relative to the hub portion.

Many different means of.securing the wheels to the hub portion are known. In British Patent No. 1416991 there is shown a twin-wheel castor wherein one of the spigots fits within the other and a plug is fitted into the inner spigot at the end thereof remote from the wheel from which it extends. An annular projection of the shank of the plug engages as a snap-fit into a groove within the inner spigot and a head of the plug is received in a recess in the wheel carrying the outer spigot, thereby locking the two wheels together.

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This arrangement is unsatisfactory in that an additional member, the plug, is required and in that three thicknesses of material, namely the hub portion and the two spigots, are present, thus increasing the cost of the item. An alternative arrangement, shown in the patent, with a projection on the outer surface of the inner spigot engaging in a groove in the inner surface of the outer spigot, eliminates the plug, but still suffers from the cost disadvantage caused by having the three thicknesses of material.

It has thus been proposed instead to provide the inner surface of the hub with either a pair of annular projections or a pair of annular grooves spaced axially apart at respective opposite sides of the center of the hub -portion. The wheels are provided with respective annular grooves or annular projections in or on the outer surfaces of their respective spigots and by pressing the wheels onto opposite ends of the hub portion the spigots engage therein, with ^' the projections snap-fitting into the grooves to retain the wheels against axial movement.

With this arrangement the spigots of the wheels do not interfit with each other but only interfit with the hub portion and as a result material is saved as . compared with the arrangement shown in British Patent No. 1416991 since the spigots can be made shorter and there is only two thicknesses of material. The castor is thus less expensive to produce and could be of reduced weight.

However this snap-fit arrangement of an annular groove and an annular projection for each wheel is not problem free. Castors are conveniently moulded from plastics material. Using simple tooling, in order to

try to minimise costs, the depth of the annular groove and the height of the annular projection is each limited to a maximum value of the order of 10/1000" (approximately 0.254mm). If the groove is made deeper than this limit, removal of the core pin used to form the groove becomes difficult and can lead to material at the edge of the groove being ripped away by said core pin, resulting in a radiussed edge rather than the sharp edge desired. Similar removal problems occur if the height of the projection is made larger than this above-mentioned limit. In practice a groove depth and projection height of about 7/1000" is commonly . used.

With this simple tooling, there are other disadvantages to the conventional moulding method. Firstly due to the need for relative axial movement upon withdrawal between the mould parts and the component being moulded, the component must be made of a material which can be compressed so that, for example, the annular projection on a core pin, which forms the groove in the hub, can compress the wall of the hub adjacent, but outside, the groove, as the core pin is axially withdrawn. Once the projection on the core pin passes the wall, it springs back to its undeformed state. Thus it is not possible to produce, for example, the hub from a non-compressible material such as a metal, e.g. aluminium.

Secondly the need ^' for this axial withdrawal limits the shape of the groove and the projection to an arcuate form, e.g. semi-circular. Such a shape is less desirable than a square or rectangular groove, as the snap-fit of the projection is less secure therein due to the lack of a sharp edge and generally vertical inner wall to the groove which resists any axial forces

tending to pull the projection out of the groove.

Accordingly it is an object of the present invention to overcome the problems and disadvantages referred to.

According to a first aspect of the invention, there is

5 provided a method of producing a hub by moulding or casting, characterized by providing in a mould or die a cavity in which is disposed a first core pin of circular cross-section having its axis in a first direction, an abutment surface of said first core pin

10 abutting against an abutment surface of a second core pin extending into the cavity, a surface or a plurality of surfaces of said second core pin being disposed in a plane perpendicular to the direction of said first core pin axis, and said abutment surface of

15. said second core pin at least partly surrounding said abutment surface of said first core pin, supplying to said cavity the material from which the hub is to be made, forming said hub in said cavity by the moulding or casting process, withdrawing said first core pin

20 from said cavity in said first direction and with¬ drawing said second core pin from said cavity in a direction perpendicular to said first direction, so as to leave said hub formed with a hollow, generally cylindrical interior portion formed by said first core

25 pin, and a single groove or a plurality of grooves, in the interior wall of said hub, formed by said second core pin, and effecting relative separation between the hub and the mould or die.

According to a further aspect of the invention a 30 castor comprises a body portion having a hub, and a wheel having a central spigot and an annular projection or a plurality of projections on its exterior surface, the body portion having a boss for

reception of a swivel pin, and the hub mounting the wheel for rotation relative to the body portion, the hub having a hollow generally cylindrical interior, characterized in that the hub interior is provided with an annular groove or a plurality of grooves having a part thereof extending radially through the hub to the exterior thereof, the wheel being secured to the hub by the snap-fit of said projection or projections in said annular groove or grooves.

According to a still further aspect of the invention, means for securing a wheel, to a hub is characterized by the hub having a hollow generally cylindrical interior portion, a groove or a plurality of grooves in the interior wall of said portion, part of said groove or one of said plurality of grooves extending radially through the hub to the exterior thereof to form an. opening, a wheel disposed on said hub at one end thereof and having a spigot extending into said interior portion of the hub, the spigot having a projection or a plurality of projections on its outer surface, the arrangement being such that said spigot is a snap-fit engagement with the hub by virtue of said projection or projections on the outer surface of the spigot snap-fitting into said groove or grooves in the interior hub wall.

The invention also resides in a method of producing an article having a bore with a groove therein, characterized by providing in a cavity in a. mould or die, a first core pin extending in a first direction, and a second core pin extending in a second direction at an angle to said first direction, with respective surfaces of said first and second pins abutting, and the abutment surface of one of the pins at least partly surrounding the abutment surface of the other

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pin, supplying to said cavity the material from which the article is to be made, forming said article in •said cavity, withdrawing said first core pin from the cavity in said first direction and withdrawing said second core pin from the cavity in said second direction, so as to leave sa d article formed with an interior bore and at least one groove in the interior wall of the bore, and effecting relative separation between the article and the mould or die.

The invention still further resides in means for securing a member to an article having a bore with a groove therein, characterized by the bore in the article having a groove or a plurality of grooves in the interior wall thereof, part of said groove or one of said grooves extending through the bore wall to the exterior thereof to form an opening, a member having at least part thereof extending into said bore and having a projection or a plurality of projections on the exterior surface thereof, the arrangement being such that said member is secured to the article by said projection or projections snap-fittingly engaging in said roove or grooves in said bore.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a side view of a body of a twin-wheel castor, to which, in use, a pair of wheels are secured by means according to the present invention; Figure 2 is a cross-sectional view on the line 2-2 of Figure 1;

Figure 3 is a cross-sectional view on the line 3-3 of Figure 1, with a pair of wheels shown fitted to the castor body;

Figure 4 is a view on arrow A of Figure 1, again with a pair of wheels shown fitted to the castor body, and Figures 5 and 6 show part of a mould, for carrying out the method of the invention to produce a twin-wheel castor body, in closed and partly open positions respectively.

The twin-wheel castor shown in Figures 1 to 4, comprises a one-piece body portion 10. and a pair of wheels 11 attached to the body portion.

The body generally has a central hub portion 12 which externally is generally barrel-shaped (as best shown in Figure 2) and which internally is of hollow generally cylindrical form, an internal wall 13 of the hub portion tapering symmetrically from each end towards an enlarged center portion- 14. In this example,. the portion 10 and the wheels 11 are moulded from plastics material.

Extending from the outer surface of the hub portion 12 is a horizontal support arm 15 which is disposed just above a horizontal plane through the axis of the hub portion 12. At its end remote from the hub portion, the arm 15 is integrally joined to one end of a generally semi-cylindrical cover element 16. A further support arm 17 extends from the top of the outer surface of the hub portion 12 at a position approximately 90° therearound from the arm 15. The arm 17 extends away from the arm 15 at an obtuse angle and integrally joins the cover element 16.

At the side of the hub portion towards which the arm 17 is directed, an upstanding boss 18 passes through the cover element 16. The boss 18 is generally cylindrical with a blind circular-section bore 19

therein. The lower part of the boss merges with the side of the hub portion disposed 180° therearound from the arm 15 (Figure 2) and terminates in a flat under- surface 20. The arm 15 is of constant width, but the width of the arm 17 increases regularly from the cover element 16 to the outer surface of the hub portion 12. A narrow rib 2.1 connects the arm 17 to the boss 18.

The undersurface 20 is flush with the lowermost part of the outer surface of the hub portion and the cover element 16 also terminates at said surface 20.

The castor body so far described is of generally conventional form and it is known to mould such a body in plastics" material. However as stated in the introduction, moulding processes with only simple tooling can only produce annular grooves with a depth of 10/1000" (0.254mm) in the hub portion inner wall for respective annular projections on spigots on the wheels 11, to snap-fit into.

With the hub portion of the invention the internal wall of the hub is formed with a pair of annular projections 22, 23 respectively at respective opposite sides of an annular groove 24, which has a flat base and arcuate sides. The center of the groove 24 lies on a central plane of symmetry for the castor body.

Each projection has a - frusto-conical surface 25 leading to a cylindrical surface 26, the surface 26 terminating at the edge of the groove 24. The groove 24 and projections at opposite sides thereof together constitute the enlarged center portion 14 of the hub portion inner surface.

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The groove 24 is annular, as stated, but over a 90° portion, it extends through the thickness of the hub portion 12, so that over this portion the groove 24 is open to the outside of the hub portion a denoted at 27. The 90° portion extends from the junction of the hub portion with the arm 15 to the junction of the undersurface 20 with the hub portion, as indicated by the dashed lines 27a_ in Figure 1, and has parallel sides as shown in Figure 4.

As explained, with conventional moulding processes the maximum depth of grove 24 obtainable would be 10/1000" (approximately 0.254mm). However with the present inention, the castor body is produced with the opening 27 as well as with the hollow interior of the hub portion. As will be explained the opening 27 allows the groove 24 to be formed with a depth of in exces of 10/1000" (0.254mm), such a depth not being possible with moulding processes using simple tooling, where no opening would be provided.

Each of the wheels 11 has a dished interior surface with a central spigot 28 upstanding therefrom. Each spigot has a blind co-axial bore 29 therein. The bore 29 widens towards its open end, whilst the outer diameter of the spigot slightly decreases from its junction with the dished interior surface of the wheel. However just before the free end of the spigot, the outer surface is formed with a frusto-conical portion 30, having approximately the same inclination as the surfaces 25. This portion leads to an annular groove 31 of cylindrical form, this groove being wider than the surfaces 26.

A short frusto-conical surface 32 leads from the groove 31 to a small annular projection 33 having an outer flat surface 33a_ and frusto-conical end surface 34. A short surface 35 parallel to the interior surface of the wheel joins the surface 34 to the surface of interior bore 29 in the spigot. However the precise shape of the external surfaces of the projection can be varied as required provided it securely engages in the groove 24.

The depth of the projection 33 is 10/1000" (0.254mm) or as close to this maximum as is. possible, since the groove 24 is now sufficiently deep to receive a projection of this depth. With moulding processes using simple tooling the groove depth is limited, thereby similarly limiting the depth of the projection.

On assembly, each- wheel is merely presented upto the hub. portion and pressed on.. The projection 33 flexes inwardly as it is passed through the reduced section formed by the projection 22 or 23 and then springs back to snap-fit in the groove 24 as shown in Figure 3. Simultaneously any similar flexing of the projection 22 or 23 ceases as the projection snap-fits ^■ in the groove 31.

Once snap-fitted on the hub portion, each wheel is able to rotate freely, but is secured against axial movement. The reception of each projection 33 in the increased depth of groove 24 in the interior wall of the hub portion ensures an improved securement of the wheels, thus reducing any chances of them becoming loose. In addition, the edges of the sides of the groove 24 are sharp-, thereby also producing an improved snap-fit resisting any pull of the wheels in an axial direction.

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To produce the castor body, Figures 5 and 6 show how a mould 36 is provided with a rectangular cross-section, elongated core pin 37 having opposite flat sides 38, 39 respectively and an arcuate end surface 40

5 extending over approximately 180°. Each of said flat sides is abutted by the end of a generally cylindrical further core pin 41, each of which is externally shaped to produce the configuration of the hub interior wall upto the edge of the groove 24. The

10 further core pins 41 are axially aligned and are per¬ pendicular to said elongated core pin 37. The mould or die itself is shaped to produce the exterior shape of the hub and remainder of the castor body. As shown in Figure 5, the abutment surfaces 38, 39 of the pin 37

15 _. surround the respective flat end surfaces of the pins 41 which abut the pin 37.

The material to form the body is then poured into the mould or die .and any application of heat and/or pressure takes place. Thereafter when the body has 0 been formed, the further core pins 41 are withdrawn from the mould along their axes and the elongated core pin 37 is withdrawn at 90° thereto. The two axially aligned core pins 41 produce the hollow interior of the hub .whilst the elongated, arcuate ended core pin 5 37 produces the annular groove 24, and also the opening 27, through which the core pin 37 is withdrawn frpm the groove and thereafter out of the mould.

The body portion and the wheels can all be produced without maching. This is an advantage since it is 0 expensive to achieve required tolerances with machining processes, in this case for the groove and wheel configurations.

The method of the invention thus allows the body to be produced from metal, instead of plastics material. This is because there is no axial withdrawal from the mould of the core pin part which forms the groove in the hub. Since the core pin forming the groove is withdrawn in a direction in the same plane as the groove, the body material need not be compressible to allow withdrawal, as with the prior art. Thus the body can be die cast. The wheels would however still be moulded from plastics material.

The invention also allows a groove deeper than 10/1000" to be formed without any withdrawal problems. In addition it allows any shape of groove to be produced, with the groove having a sharp edge or sharp edges to provide an improved, secure, snap-fit of the or each wheel projection.

The projections 22 and 23 and grooves 31 can be omitted is desired. All that is needed is for the projection 33 to be compressed inwardly by some part of the hub. interior wall before it reaches the groove 24, whereupon it springs.back to its undeformed state and snap-fits in the groove. In addition the groove 24 need not be continuous. It could be replaced by a plurality of axiall aligned grooves preferably, but not necessarily on a common circle formed by the arcuate ended core pin being gapped, the plurality of grooves being formed angularly spaced around the hub inner wall. This would allow corresponding gapped projections on the wheel spigot to be greater than 10/1000" in height, but would possibly provide a less secure fit, than with an annular groove and annular projection arrangement.

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Finally, instead of a single groove 24, a separate groove for each wheel could be formed. These grooves need not, moreover, be near the center of the hub, but could each be anywhere between the center and free end of the hub, with the projections on the wheel spigots being correspondingly disposed. To form such grooves, a pair of core pins 37 could be employed with a middle core between them which abuts against the respective flat sides 39. The pins 41 would abut the respective other sides 38 of the pins 37 and would be capable of withdrawal from the mould in their axial direction. The pins 37 and the middle core could be connected so that all three members could be withdrawn together at 90° to said direction of withdrawal of the pins 41, thereby creating a relatively larger opening than the opening 27 formed by one core pin 37 alone.

Although the oening 27 is described as extending over 90° from the surface 20 to the arm 15, it could extend over a different angle and could begin and end at different positions from those shown.

The invention could also be applied to single wheel . castors with one side of the groove 24 preferably being closed and the hub portion at said side preferably being absent.

Moreover this invention in its broadest aspect is not limited to castors, i.e. the production of a grooved hub and the securement of a wheel or wheels thereto, but includes a method for the production of an article having a bore with a groove therein and also means for securing a member to such an article. Thus the article need not have the member secured to it capable of rotation, only restrained against movement in a direction out of the bore. It will be appreciated that

the groove need not be perpendicular to the bore. Thus the securement of a wheel is merely a specific example of the general applicability of the method and means of the invention.

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