The invention concerns a screw having a head and a shank with a threaded section by means of which the screw may be threaded into a preformed hole having a corresponding threaded section, as well as a nut having a body which is formed with a hole with a threaded section by means of which the nut may be screwed onto a shank having a cor¬ responding threaded section.

Screws and nuts of plastics for constructional building sets involve the problem that the threads may be destroyed by excessive tightening. This destruction may take place either on the screw, the nut or on a preformed threaded shank or hole.

To prevent such loads on the threaded sections on building elements incorporated in a toy building set, the applicant introduced a new toy building set in the autumn of 1992 in which a torque limiting ratchet device is incorporated in a tool having the shape of a screwdriver. The toy building set is marketed under the trademark T00L0® and is de- scribed e.g. in the international patent applications PCT/DK92/0319 and PCT/DK92/0321.

The screws and the screwdriver cooperate in that the tip of the shank on the screwdriver has a triangular cross- section, and that the head of the screw is formed with a hole having a corresponding cross-section. The screwdriver is formed with a torque limiting coupling between shank and handle, said coupling limiting the size of the torque that the screwdriver can apply to the screw.

However, there is a problem in connection with toy build¬ ing sets in which the elements - typically of plastics - can only withstand relatively small torques, making it necessary to specially design the interface between tool and screw to avoid the use of other non-authorized tools without incorporated torque limiting couplings, which results in the destruction of the building element.

Torque limiting couplings are well-known and described in e.g. the British patent applications GB-A-2 022 199 and GB-A-2 141 999, French patent specification FR-B-1 376 669, German patent specifications DE-C-2 114 793, DE-C-2 163 109 and DE-C-2 915 526, British patent specifications GB-B-661 543, GB-B-952 253, GB-B-1 151 544, GB-B-1 330 631 and GB-B-1 472 484, German Offenlegungsschrift DE-A-2 608 304, Danish published applications DK-B-147 118 and DK-B- 155 509, as well as US patent specifications US 2 731 865, US 3 712 433 and US 4 131 039.

Torque limiting couplings are moveover used directly in screws and nuts, and teachings in this respect may be found e.g. in US 2 373 812, US 2 685 812, US 3 789 524 and US 3 425 314. All of these documents teach metal compo¬ nents, and it is not readily feasible to transfer the taught art to plastics components, since plastics injec¬ tion moulding involves quite special conditions concerning core withdrawal, etc.

The present invention is intended to provide a screw that can be manufactured by plastics injection moulding, and which just consists of a few components. The screw is to have an incorporated torque limiting coupling which pro¬ tects the threads of the screw shank against destruction by application of great torques to the head of the screw.

This object is achieved according to the invention in that the screw is constructed as stated in the characterizing portion of claim 1. This ensures that the firm toothed rim may be formed on the inner side of the hollow screw head member, thereby avoiding a double wall structure in said member. This is extremely attractive, precisely because the head is to have a thick outer wall in order to with¬ stand the loads to which the screw head is subjected. Further, the wall in which the toothed rim is formed is to have a thickness which is somewhat greater than the height of the teeth.

Finally, the resilient teeth are then formed on the inter¬ nal part, and since this part will nevertheless be hollow because of the core withdrawal in the moulding, the resi¬ lient teeth can freely move away from the firm toothed rim.

The screw of the invention can therefore be moulded and assembled without complicated process steps, which makes it suitable for inclusion in a constructional building set in which both the tools of the building set and real tools may be used for tightening the screw.

The present invention is intended to provide a screw for cooperation with a threaded hole having corresponding hreads, where tightening of the screw is possible without any risk of destroying the threaded parts.

This object is achieved according to the invention in that the head and the shank of the screw are formed as two parts rotatably mounted with respect to each other, and that a torque limiting coupling is provided between the head and the shank so that the parts can rotate mutually when the torque between them exceeds a predetermined size.

This ensures that the engagement faces on the screw head for cooperation with the tool may be shaped in any manner. Thanks to the use of a torque limiting coupling the threaded parts will not be destroyed when the children play with the constructional building set comprising such screws, even though a real, professional tool is used for the tightening.

The screw may be finger-operated, but is preferably formed with an engagement face by means of which it may be ope¬ rated by the tool. This engagement face may e.g. be a slot in the end face of the screw or be a screw head shape of a polygonal cross-section.

The torque limiting coupling used in a screw according to the invention comprises an annular toothed rim mounted on the one part and a plurality of resilient teeth on the other part. It is preferably the screw head that is formed with the annular toothed rim, whose teeth protrude in- wards. Thus, the shank of the screw will have a plurality of radially outwardly directed, resilient teeth which are adapted to engage said toothed rim. In a preferred embodi¬ ment in which the screw is used for reinforming a frictio- nal coupling between two elements incorporated in a toy building set, the screw is preferably formed such that a certain travel of the shank with respect to the rest of the screw is allowed. For this purpose the screw part with the screw head is formed with two shoulders in an axial direction between which projections on the shank part may move.

When the screw is manufactured by plastics injection moulding, the shank is advantageously formed with a hollow cylinder body having a threaded section at one end and projections at the other end. The resilient teeth are pre¬ ferably connected with the cylinder body through thin ribs

and form part of these projections. The slidable shank is preferably biassed by a resilient element which is located interiorly in the hollow cylinder body.

According to the invention, the toothed rim may be formed asymmetrically so that the pitch of the teeth is different clockwise and counterclockwise, respectively. The torque limitation during a screwing-in operation and a screwing- out operation will thus be of different magnitudes so that loosening a screw will never pose any problem. In a pre¬ ferred embodiment, the screw of the invention serves as a base for a coupling head for interconnection with other elements incorporated in a toy building set. The coupling head may then be secured to the screw as stated in claim 11. When the screw is constructed as stated in claim 12, it may be incorporated in a toy building set having box- shaped elements, e.g. of the type marketed under the trade marks DUPLO® or T000®, so that the box-shaped ele¬ ment with a screw may be interconnected with a box-shaped element both with and without a screw hole.

Claim 13 concerns a nut with a body which is formed with a hole having a threaded section for threading on to a pre¬ formed shank having a corresponding threaded section. The hole is formed in a bushing, said body and bushing being formed on two separate parts. A torque limiting coupling is provided between the body and the bushing so that the parts can rotate mutually when the torque between the two parts exceeds a predetermined size. The invention as de- fined in this claim is based on the same basic principle as the screw described above. In general terms, this prin¬ ciple may be expressed in the manner that the torque li¬ miting coupling has been moved from the tool to the screw or the nut which is incorporated in a construction.

The nut is preferably formed with an engagement face for a tool, enabling tightening to take place. In a preferred embodiment, the torque limiting coupling of the nut com¬ prises an annular toothed rim provided on one part and a plurality of radially protruding teeth which are resili- ently provided on the other part to engage said toothed rim. Since the bushing has a cylindrical body, the annular toothed rim is preferably provided exteriorly on it, while the resilient teeth are provided on the head and are di- rected inwards to engage the toothed rim. The teeth of the toothed rim as well as the resilient teeth are preferably shaped asymmetrically so that the torque limitation will occur at lower torques during tightening than during loosening.

Finally, the invention concerns a building element for a constructional building set, said building element com¬ prising a substantially box-shaped member which is formed with an integral threaded part. Interconnection can hereby take place with another element having a corresponding threaded part incorporated in the constructional building set. A torque limiting coupling is provided between the member and the threaded part so that the threaded part may rotate with respect to the member when the torque between them exceeds a predetermined size. The threaded part may be provided in the form of exterior threads on a protrud¬ ing shank or shaft, or as interior threads in a bushing.

The invention moreover concerns the use of a torque limit- ing coupling having a cylindrical toothed rim in a screw or nut, wherein a head and a shank/bushing provided with threads are allowed to rotate mutually at torques which exceed a predetermined size.

The invention will be described more fully below in con¬ nection with preferred embodiments and with reference to

the drawing.

Fig. 1 is a perspective view of a screw according to the invention screwed into a building element incorporated in a toy building set, said screw being formed with a coup¬ ling means in the form of a gripper claw for interconnec¬ tion with other building elements.

Fig. 2 is a perspective view of a screw according to the invention corresponding to the one shown in fig. 1, with the gripper claw replaced by a coupling means complemen¬ tary thereto.

Fig. 3 is an exploded view of the screw shown in fig. 2.

Fig. 4 is a longitudinal section of the screw shown in fig. 2.

Fig. 5 shows an alternative embodiment of a screw accord- ing to the invention.

Fig. 6 shows a torque limiting coupling for use in a screw or nut according to the invention.

Fig. 7 shows the coupling of fig. 6 when the teeth do not mesh because the applied torque exceeds a predetermined level.

Fig. 8 is a longitudinal section of a preferred embodiment of a nut according to the invention.

Fig. 9 is a perspective view of the nut shown in fig. 8.

Fig. 10 is an exploded view of the nut shown in fig. 8.

Fig. 11 shows a structure in which four of the nuts shown in fig. 8 are used for securing a plate to a frame.

Fig. 12 shows a torque limiting coupling used in a build- ing element according to the invention.

Reference is first made to fig. 4. This shows a longitudi¬ nal section of a screw according to the invention, which comprises a head 1 and a shank 2 having a threaded section 3. The screw may hereby be screwed into a preformed hole having a corresponding threaded section, e.g. made in a plastics injection moulded building element incorporated in a toy building set. It will be seen that the head 1 and the shank 2 are provided on two separate parts 10, 20. A torque limiting coupling 30 is provided between the head 1 and the shank 2, allowing the parts 10, 20 to rotate mutu¬ ally when the torque between the two parts exceeds a pre¬ determined size. The torque limiting coupling shown along a cross-section VI-VI corresponds to the coupling shown in figs. 6-7.

In the preferred embodiment, the shank part 20 is manufac¬ tured by plastics injection moulding in its entirety, and is has a cylindrical portion 23 which, via a conical step 22, merges into the shank 2 provided with threads 3. The shank 2 is terminated with an end wall. The inner wall of the step 22 serves as a shoulder 21 for a screw spring 25 which extends in the longitudinal direction of the shank. At the other end of the cylindrical portion 23, the shank part 20 is formed with an annular bead 24 which, via bosses 36 extending in an axial direction, is terminated in resilient wings 37 provided with teeth to engage the annular toothed rim of the main part 10. This will be ex¬ plained in greater detail in connection with figs. 6 and 7.

The main part 10 likewise comprises a cylindrical portion 12, which is exteriorly formed with an engagement face 11 for a tool (not shown), and the engagement face 11 may have a hexagonal cross-section so as to fit clamping wrenches known per se. Through a stepped part 13 the cy¬ lindrical portion 12 merges into a further cylindrical portion 14 having a diameter fitting the cylinder part 23 of the shank part 20, thereby enabling rotation between the parts, and providing a good guide between the parts.

The cylindrical portion 14 is exteriorly formed with an annular bead 15 having an axially extending wall and an inclined or conical guide wall. A box-shaped skirt 80 having a circular central opening may hereby be guided inwards over the bead 15 of the portion 14 and be retained rotatably between the bead and the stepped part 13. For this purpose the stepped part 13 is formed with an annular bead 13a to increase the size of the engagement face with the skirt 80. The skirt 80 is locked to the cylinder part by means of an irreversible snap lock provided with the asymmetrical bead 15.

Two beads 16, 17 are provided at the other end of the cy¬ linder portion 12, one bead 16 of which serving as a stop for a coupling part 90, which will be described in connec¬ tion with figs. 1-3. The other bead 17 has a guide wall so that the coupling part 90 may be moved over the guide wall of the bead 17 and be retained in an irreversible snap coupling between the beads 16 and 17.

Interiorly, the cylindrical portion 12 is formed with an annular toothed rim 31 corresponding to the one that will be described in connection with figs. 6 and 7. The teeth in the toothed rim 31 has an axial extent which corres- ponds to the axial travel of the shank part 20 in the chamber defined by the cylindrical part 12. The shank part

20 is biassed by the spring 25 in the position shown in fig. 4, said spring 25 exerting a pressure between the internal wall 21 in the step 22 and a plug 91 on the coup¬ ling part 90. The plug 91 may advantageously have a cen- tral pin 92 so that the spring 25 is kept in position. The bead 24 with the wings 37 on the shank part 20 permits travel of the shank part 20 between the engagement of the bead with the shoulder including the step 13 and with the plug 91, respectively. The shank part 20 can therefore be pressed into the main part 10, if a force greater then the spring force of the spring 25 is applied in an axial di¬ rection.

Fig. 3 shows the same structure, but in an exploded view. It will be seen that the skirt 80 is box-shaped, and in a preferred embodiment it may be interconnected with four coupling studs on a constructional building element of the type commercially available under the trade mark DUPL0®. If this building element moreover has screw holes of the type indicated by the reference numeral 66 in fig. 12, the coupling between the coupling studs 67 and the skirt 80 may be reinforced by screwing-in of the shank of the screw.

It will be seen in figs. 2 and 3 that the coupling part 90 has two prongs 93, each having respective round projec¬ tions 94 on the inner side. Fig. 1 shows another type of coupling part 90', which is secured to the screw in the same manner as the coupling part 90, but which has a beak- shaped portion including a beak 97 with a slot 96 and two partly circular depressions 98. When the coupling part 90' is coupled to the coupling part 90, the projections 94 can be received in the depressions 98 by deflection of the prongs 93 while the tightening screw 95 is received in the slot 96 of the beak 97. The interconnection of the two coupling parts 90 and 90', respectively, can be maintained

by tightening the screw 95, and deflection of the prongs is no longer possible until the screw 95 is loosened again. The screw of fig. 1 is coupled together with one of the construction elements 100 of the toy building set, and this is known from the international patent application

PCT/DK92/00319 and is moreover formed with a coupling part corresponding to the coupling part 90' .

Fig. 5 shows another embodiment of a screw according to the invention, and in contrast to the embodiments shown in figs. 1-4 this screw has a rather conventional appearance, since the head 1 does not serve as a basis for one of the coupling mechanisms of the toy building set. In addition to the head 1, the screw has a shank 2 with threads 3. The screw is screwed into a complementary screw hole of another building element, whereby a plate member can be retained with the shoulder 66 of the head 1. The head 1 is moreover formed with gripping faces 11 for a tool. Alter¬ natively, the screw may be formed with wings and serve as a finger-operated wing screw. The shank 2 and the head 1 are two mutually rotatable parts which are interconnected through a torque limiting coupling corresponding to the one shown in figs. 6 and 7. In addition, the head 1 has a cylindrical collar 64 which serves as a bearing for the shank 2, and which stabilizes the mutual movement of the parts.

Fig. 6 is a cross-sectional view of a preferred embodiment of a torque limiting coupling 30 for use with a nut or a screw according to the invention. The coupling 30 com¬ prises an outer part 31 having a annular, internal toothed rim. The outer part 31 is in the shape of a bushing. The length of the bushing depends upon whether the inner part 32 of the coupling 30 is to be allowed to perform a pre- determined travel in an axial direction. If travel of the part 32 is allowed, the toothed rim of the part 31 has an

axial length corresponding to at least the travel plus the width of the contact portion of the part 32 in engagement with the toothed rim.

The teeth 33 of the toothed rim are shown to be asymmetri¬ cal, since, in a preferred embodiment, their pitch forms an angle with the radius R of the bushing of between 0 and 10°, preferably between 2 and 5°, in one direction, and between 45 and 80°, preferably between 55 and 65°, in another direction.

The inner part 32 comprises a bushing part 34 which is mounted co-axially with the outer part 31. In the pre¬ ferred embodiment, the bushing part is formed with two protruding bosses 36 in an axial direction. The wings 37, which extend in the peripheral direction of the part, are secured to said bosses 36 at one end 35, while their oppo¬ site end is free. Outwardly directed teeth 38 on the wings 37 thus resiliently engage the teeth 33 on the toothed rim of the outer part 31.

As long as the torque between the inner part 32 and the outer part 31 in the direction A is below a given thres¬ hold value, the coupling locks the two parts to each other. When the torque exceeds this threshold value, the wings 37 are pressed inwards in the direction indicated by the arrows B in fig. 7, and the teeth 38 are disengaged from the teeth 33 of the toothed rim 39. As long as the torque exceeds this threshold value, the parts 32 and 31 can be rotated mutually in the direction indicated by the arrow C, with the wings 37 performing a ratchet function. The inner part 32 and the outer part 31 are preferably manufactured by plastics injection moulding and therefore consist of a plastics material, such as ABS. The threshold value of the torque as to when the teeth do not mesh, is determined by parameters such as the pitch of the teeth as

well as the wings 37 and the resilient properties of the bosses 35, the latter depending on the modulus of elasti¬ city and moulding parameters of the material.

It is clear that the torque limitation can also be achieved in the opposite direction by changing the pitch of the teeth. Further, the coupling may be formed with an external toothed rim, with the resilient teeth facing ra¬ dially inwards.

Fig. 9 is a perspective view of a preferred embodiment of a nut according to the invention. The nut, which is gene¬ rally designated by the reference numeral 40, comprises a nut body 41 which is exteriorly formed with an operating face 42 for cooperation with a tool (not shown), e.g. in the form of a spanner. In a cross-section, the shown nut has a hexagonal cross-section. To obtain a substantially round cross-section at the ends of the nut in an axial direction, it is formed with an annular bead 43. Both ends of the nut 40 are provided with bushings having internal threads 45 for threaded engagement with the shank of an external screw. A torque limiting coupling of the type shown in figs. 6 and 7 is provided between respective bushings 44 and the body 41 of the nut. Fig. 8 shows a longitudinal section of the nut shown in fig. 9. It will be seen that the body of the nut is divided into two cham¬ bers 46 by a partition 47 integral with the body. A bush¬ ing 44 is arranged to the left in the chamber 46, and it will be seen that resilient wings 48 with teeth 49 engage the toothed rim 50 of the body. This toothed rim 50 is shown to the right in the chamber 46, the bushing 44 being omitted here for the same reason. The bushings 44 are re¬ tained in the chambers 46, said parts being respectively formed with a bead 51 and a groove 52 so as to provide an irreversible snap lock after mounting.

Fig. 10 shows the same nut as is shown in figs. 8 and 9, but in an exploded view. The annular bead 51 is hereby visible with greater clarity. It will also be seen that the bushing 44 is formed with a reduced diameter section 54, which is to provide clearance for the toothed rim 50, while an adjoining section 53 serves as a bearing wall against the internal cylinder wall of the chamber 46.

Fig. 11 shows the use of a screw according to fig. 5, and this is indicated by the reference numeral 60 here. Four screws 60 of this type retain a plate-shaped building ele¬ ment 61 for a frame structure 62 built with beam-shaped building elements 63 of a constructional building set. Building elements 63 are formed with threaded holes cor- responding to the shank on the screw 60, and the plate 61 has holes through which the shank of the screw 60 may pass unobstructedly. Only a few of these holes are shown and may moreover be located as required, but expediently the holes are arranged in a modular measure corresponding to the other building elements of the toy building set.

A vehicle bottom 65 is shown in fig. 12, and this is known from an international patent application PCT/DK92/00319. The vehicle bottom 65 differs from the vehicle bottom shown in said application in that the threaded holes along the sides of the element are formed with a torque limiting coupling of the type shown in figs. 6 and 7. A longitudi¬ nal section along the line VI-VI thus corresponds to the torque limiting coupling shown in figs. 6 and 7, and the outer part 31 has outwardly directed axial ribs or grooves so that it can be retained with the vehicle bottom member or is integrated in it. For manufacturing reasons it is usually preferred that the torque limiting coupling is formed as an independent plastics injection moulded insert element. In the use shown in fig. 12, the inner part 32 is formed with internal threads in the axial direction of the

bushing. The vehicle bottom is moreover formed with coup¬ ling studs 67 on the upper side and screw holes 66 pro¬ vided between these.

Since the teeth in the torque limiting coupling are asym¬ metrical, it is ensured that the nut or the screw is screwed on by a torque which is below a predetermined threshold value, and can subsequently be loosened by ap¬ plying a considerably greater torque.

The invention lends itself for performance in connection with plastics injection moulded products, but is not re¬ stricted to this, since it will e.g. be advantageous to perform the invention in connection with composite toys where e.g. screws of plastics are screwed into blocks of wood with threaded holes.