FRAME STRUCTURES

Abstract: A space frame structure, particularly suitable for use in high accuracy coordinate measuring machines, is formed of a plurality of hollow frame components (10) comprising unidirectional fibres which lie parallel to the longitudinal axes of the respective components. A connector (11), formed from an isotropic material, has sockets (12), into which ends of respective ones of the frame components (10) may be inserted and mechanically secured, the socket arrangement being such that the respective longitudinal axes of the components (10) intersect at a common point. Such a structure restrains and/or minimises the effects of hygrothermal deflection of the frame components.

Description: Frame Structures This invention concerns frame structures, and relates in particular to structures exhibiting minimal changes in size and shape under conditions of varying temperature and humidity, such as are suitable for use in high accuracy co-ordinate measuring machines, machine tools and the like. Co-ordinate measuring machines require frameworks, for example in the form of space frames, which remain substantially rigid both under conditions of accele¬ ration and when subjected to changes in temperature and humidity. The material used in constructing the frame components may therefore be chosen to have a low mass density and a high modulus of elasticity (to resist accelerat ional forces and minimise any deflection), and low coefficients of thermal and of hygro (humidity- induced) expansion. The materials which may be used include carbon fibre reinforced plastics, which possess all the required properties, albeit only in the direction of the fibres. In directions normal to the fibres such materials have poor stiffness, and exhibit relatively high hygrothermal expansion. Accordingly, the structure must be adapted so that the material is used unidi rect iona_ 1 y , with changes in shape and dimension normal to the fibres being of little practical consequence. These considerations are particularly relevant when considering the manner in which the various frame components are to be interconnected. The present invention seeks to provide a connector which both confers on the frame structure the necessary properties of stiffness and stability just described and is relatively simple to manufacture. The term "hygro-thermal deflection" appearing in this specification is used to designate changes in size, shape and/or spatial arrangement induced by changes in humidity and/or temperature, and related expressions are to be interpreted accordingly. According to the present invention, there is provided a connector for use in a frame structure, the connector being formed of an isotropic material, and having means for mounting each of a plurality of frame components thereon with the longitudinal axes of the components intersecting at a common point, the mounting means being adapted for mounting the frame components in a manner whereby the effect on the frame structure of hygro-thermal deflection of any or all of the frame components is minimised. The mounting means may comprise a plurality of sockets on the connector into each of which may be inserted an end of a respective frame component. At least two of the sockets may be axially aligned whereby to constitute a through passage on the connector. There may be provided on the connector at least one aperture directly opposite a respective socket whereby a frame component can be inserted into the socket from either side of the connector. In an alternative arrangement, the mounting means comprises a socket defining a passage through the connector and a plurality of outwardly directed flanges on the connector providing for the mounting thereon of a respective end of each of a plurality of frame components. Preferably the position of the frame components may be initially adjusted relative to the connector before being secured thereto. In a further alternative arrangement, the mounting means comprises a plurality of interconnected substanti¬ ally planar mounting plates for mounting a respective end of each of a plurality of frame components. According to a further aspect of the invention there is provided a frame structure comprising a plurality of frame components and at least one connector as defined in any of the preceding six paragraphs. Preferably the frame components and the sockets are axially symmetrical. Preferably also the frame components are each constructed from a carbon fibre composite material. The carbon fibres may be oriented parallel to the longitudi¬ nal axis of the respective frame component. Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:- Fig. 1 is a diagrammatic sectional view of part of a frame structure utilising a first embodiment of a connector according to the invention; Fig. 2 is a view similar to Fig. 1 showing a second embodiment of connector; Fig. 3 is a diagrammatic perspective view of part of a frame structure incorporating a third embodiment of connector according to the invention; Fig. 4 is a diagrammatic perspective view showing a fourth embodiment of connector; Fig. 5 is a side view of a frame structure according to the invention; and Fig. 6 is a section on line F-F in Fig. 5. Referring to Fig. 1, a space frame structure comprises a plurality of generally cylindrical hollow frame components 10 each constructed of carbon fibre reinforced plastics so that the fibres lie parallel to the longitudinal axis of the frame component. The frame components 10 are interconnected at a nodal joint by way of a connector 11, which is formed from an isotropic material having a minimal coefficient of hygro-thermal expansion, such as one of those nickel-iron alloys sold under the name Invar. The connector 11 provides four sockets 12 of substantially circular cross-section, such that an end of each frame component 10 may be mounted on the connector 11 by insertion into a respective socket 12. The frame components 10 may be mechanically secured within the sockets 12, for example by clamping; alterna¬ tively they may be bonded therein by means of a suitable adhesive such as an epoxy resin. The sockets 12 are so positioned on the connector 11 that when the frame components 10 are mounted thereon the respective longi¬ tudinal axes of the frame components intersect at a common point , as identified by the letter X in Fig. 1. Two of the sockets 12 are axially aligned to constitute a through passage on the connector 11. Fig. 2 illustrates an arrangement similar to that of Fig. 1, and corresponding parts are identified by the reference numerals used previously. The connector shown in Fig. 2 has formed therein an aperture 13 directly opposite an angled socket 12a such that, during assembly of the frame structure, a frame component 10a may be incorporated into the structure either by insertion into the socket 12a directly to the position shown in Fig. 2, or by insertion through the aperture 13 and the body of the connector 11 into the socket 12a, and thus in the reverse direction. It is thus possible to utilise on the connector socket configurations into which it would otherwise be difficult or impossible to insert the final frame components during assembly of the frame structure. Fig. 3 illustrates an alternative form of connec¬ tor 14 adapted for use with one tubular frame component 15 and a plurality of further frame components 30, at least one end of each of which comprises a pair of spa¬ ced parallel and substantially planar bars 16, 17. The components 30 are formed from carbon fibre reinforced plastics with the fibres parallel to the longitudinal axes of the bars. The connector 14 incorporates a socket 21 of gen¬ erally circular cross-section which provides a through passage on the connector 14, together with outwardly directed radial flanges 18, 19, 20. The frame component 15 which, in use, represents the most highly loaded or stressed component of those shown, extends through the socket 21, and may be secured thereto in any convenient manner, for example by clamping or with a suitable adhesive. The remaining frame components 30 are mounted on the connector 14 by attaching each pair of plates 16, 17 to respective sides of one of the flanges 18, 19, 20. The bars 16, 17 may initially be pivotally mounted on the flanges 18, 19, 20 such that the relative position of each frame component may be adjusted until the longitudinal axes intersect at the common point X. Thereafter the respective joints may be fixedly secured, for example by means of pins 22 and a suitable adhesive. The connector 14 may therefore be manufactured without compound angle machining, and so may be produced relatively inexpensively. A further alternative form of connector 25 (Fig. 4) is suitable for mounting a plurality of the frame components 30. The connector 25 comprises a substant¬ ially circular plate 23 of an isotropic material such as Invar having a further plate 24 projecting at a right angle therefrom. The bars 16, 17 of each respective frame component 30 may be mounted around the plate 23 and on the plate 24 as described hereinbefore with reference to the mounting of frame components 30 on flanges 18, 19, 20 in Fig. 3. Figs. 5 and 6 illustrate a further frame structure according to the invention. The structure comprises tubular hollow frame components 50 formed from a carbon fibre reinforced plastics material. The fibres of the material are orientated to lie parallel to the longitudinal axes of the frame components. Other frame components 52 comprise pairs of spaced parallel and substantially planar bars, again formed from a carbon fibre reinforced plastics material with the fibres parallel to the longitudinal axes of the bars. The frame components 50, 52 are mounted on appropriate connectors, for example as indicated at 54, 56, 58, each constructed in a generally similar manner to that of the connector of Fig. 3. Each connector is formed from an isotropic material having a minimal coefficient of hygro thermal expansion. There is thus described a connector for space frame structures which restrains and/or minimises the effects of hygro-thermal deflection of the frame components. In the embodiments shown in Figs. 1 and 2 the frame components 10 and sockets 12 are axially symmetrical, so that any hygro-thermal expansion of the frame components in directions normal to the fibres, and thus to the longitudinal axis, will be resisted by the sockets. The connector itself is formed of an isotropic material such that any expansion or contraction will occur equally in all directions. In order further to minimise any expansion effects, the thickness of the connector walls may be relatively large compared to the thickness of the walls of the frame components. In the embodiments shown in Figs. 3 and 4, the construction of the connector permits hygro-thermal expansion of the bars of the frame components to occur, but only where this will be of no practical consequence to the overall shape and size of the frame structure. Modifications may be made without departing from the invention. The connector and the frame components may be made from materials other than those described, and the frame components may be secured within the sockets or fixed to the flanges or plates by any suit¬ able method. The number and arrangement of sockets and flanges and connecting plates may be other than that described and shown, depending upon the requirements of the particular frame structure. Although the invention is described herein with reference to space frame structures for use in machine structures, and is particularly suitable for high accuracy coordinate measuring machines, it will be appreciated that application of the invention is not limited thereto.

Claims: Claims : 1. A connector for use in a frame structure, the connector being formed of an isotropic material, and having means for mounting each of a plurality of frame components thereon with the longitudinal axes of the components intersecting at a common point, the mounting means being adapted for mounting the frame components in a manner whereby the effect on the frame structure of hygrothermal deflection of any or all of the frame components is minimised. 2. A connector according to Claim 1, wherein the mounting means comprises a plurality of sockets on the connector into each of which can be inserted an end of a respective frame component. 3. A connector according to Claim 2, wherein at least two of the sockets are axially aligned whereby to constitute a through passage on the connector. 4. A connector according to Claim 2 or 3, wherein there is provided on the connector at least one aperture directly opposite a respective socket whereby a frame component can be inserted into the socket from either side of the connector. 5. A connector according to Claim 1, wherein the mounting means comprises a socket defining a passage through the connector and a plurality of outwardly directed flanges on the connector providing for the mounting thereon of a respective end of each of a plurality of frame components. 6. A connector according to Claim 5, wherein the flange mountings provide for initial adjustment of the frame components before being permanently secured to the connector. 7. A connector according to Claim 1, wherein the mounting means comprises a plurality of interconnected substantially planar mounting plates for mounting a respective end of each of a plurality of frame components. 8. A frame structure comprising a plurality of frame components and at least one connector according to any of the preceding Claims. 9. A frame structure according to Claim 8, wherein the frame components and the sockets are axially symmetrical . 10. A frame structure according to Claim 8 or 9, wherein the components each comprise unidirectional fibres . 11. A frame structure according to any of Claims 8 to 10, wherein the frame components are each constructed from a carbon fibre composite material. 12. A frame structure according to Claim 11, wherein the carbon fibres are oriented parallel to the longitudinal axis of the respective frame component.

Inventors: BURY JAMES, STOTT CHRISTOPHER, BURY, JAMES, STOTT, CHRISTOPHER
Applicants: LK LTD, LK LIMITED
Application year: 1992
Main classification: E04B1
IPC codes: E04B1