FR2516859A3 - Double-tapered fibre reinforced plastics mast - thickest at point of greatest stress, used for wind surf board - Google Patents

Abstract

A process for mfg. fibre-reinforced plastics masts involves one or more pieces of pre-impregnated woven reinforcement, esp. glass-fibre mesh, around a slightly conical mandrel, more mesh being used as appropriate to create a hollow mast thicker at its point subject to greatest stress and progressively thinner towards each end. The slope of the mandrel may vary along its length, but its dia. always decreases towards the top of the mast to allow removal after heat-curing of the thermosetting resin impregnation. The prodn. is used for conventional masts, esp. for masts for windsurfing boards which are subject to high flexional stress at a certain point above their base where the "wishbone" is attached. The masts can be made simply, in one piece, with additional wall thickness only where needed and no unnecessary thickness and wt. at the base.

Description

 The invention relates to a method of manufacturing a mast made of reinforced plastic and to the mast obtained using this method.

 It applies more particularly but not exclusively to the manufacture of conventional masts but also of beams and similar elements which, at a certain distance from their ends, undergo a bending stress as is for example the case for board masts sailing, up to the "wishbone".

 To date, it is already known to manufacture a windsurf mast made of reinforced plastic (French patents A-2,455,980 and A-2,075,042).

 In this known method, use is made of an at least slightly conical mandrel, and this, according to a constant slope all along the mast or gradually increased towards the top of the mast but although it is always oriented in the same direction so to form a draft face allowing when the mast is finished the withdrawal of this mandrel and its reuse.

 On this mandrel, is then wound a fabric formed from the fibers chosen to make the frame, such as glass fibers.

 Before this winding, the fabric will have been impregnated with a binder made of thermosetting resin and may have undergone a pre-polymerization giving it a stiffness similar to that of a cardboard.

 Similarly, to take into account the width of the weaving, this fabric could have been formed in several pieces wound successively, and to take into account the taper of the mandrel and therefore the shape that the said mandrel would have after its development, the pieces may have a trapezoidal configuration whose large and small base will respectively form the base and the head of the mast.

 After winding the impregnated fabric, before the complete polymerization of the assembly, around the impregnated fabric is wound, with tightening and in a partially overlapping helix, a ribbon of plastic material which is heat shrinkable and not capable of becoming one with the impregnated fabric after polymerization.

When put in place, this coating ensures the regularization of the binder layer
Then, during the polymerization, by its retraction, this coating subjects the entire lateral surface of the roll of impregnated fabric to a pressure force improving the hardness and standardizing the appearance of the mast.

 After this polymerization and after the coating and the mandrel have been removed, the mast, consisting essentially of reinforced plastic, is finished and can then be machined and / or receive the accessories which it must carry.

 Thus produced according to the known technique, the mast is in the form of a tubular element which, as a whole, is slightly conical with its large base at the foot of the mast and which has a wall of constant thickness at least between its base and the attachment point of the "wishbone".

 So that at this point of attachment of the "wishbone" the mast is able to withstand the stresses of it it is necessary that at this point it offers a sufficient moment of inertia.

 As, by definition, the moment of inertia is substantially directly proportional on the one hand to the thickness of the wall and on the other hand to the cube of its distance from the axis, in the known technique which leads to a wall d constant thickness, at this point of attachment of the "wishbone", the mast must have a sufficiently large diameter.

 By observing the stress diagram, along the mast we notice that the bending moment which is maximum at the point of attachment, has a value which decreases gradually not only towards the head of the mast or thanks to the taper of the mandrel used to its manufacture, the mast can have a very small diameter but also at the foot of the mast where unfortunately to allow the withdrawal of the mandrel, the diameter is still greater than that measured at the point of attachment of the "wishbone".

 In addition to leading to a mast which is heavy, this diameter gives the mast a moment of excessive inertia which is perfectly useless.

To the identical problems arising in the field of duralumin masts, a satisfactory solution having been given by the adoption of a biconical mast, the manufacturers of masts in reinforced plastic material, then attempted to adopt the same solution and to resolve the problem of the withdrawal of the manufacturing mandrel, they produced separately the parts of the mast located on one side and the other of the attachment point, which parts, only after retraining, are then assembled by a sleeve fitting into the great basis of each of them
If these two parts do form a biconical mast and solve the problem of tilting while allowing to give the mast a larger diameter at the point of attachment of the "wishbone" as well as at the head than at the foot of the mast, unfortunately , it turns out that it is very complicated and expensive a sleeving sufficiently resistant to be carried out at the same place where the bending moment is maximum.

 A result which the invention aims to obtain is a method of manufacturing a plastic mast which is in one piece, light and which while having an interior shape slightly conical in a constant direction to allow the withdrawal of its mandrel , offers to the right of the attachment point of the "wishbone" a moment of inertia markedly greater not only than at the head but also at the foot of the mast.

 To this end, the subject of the invention is a method of manufacturing a mast made of reinforced plastic of the type mentioned above, in particular characterized in that, at the point of the point of maximum stress on the mast, a total width of fabric, which is more important than that of the fabric which, independently of any reinforcements, is wound at the foot and at the head of the mast, and in that between each of these ends of the mast and the point of maximum stress, this width varies gradually.

 It also relates to the mast obtained using this process.

The invention will be better understood with the aid of the description below given by way of nonlimiting example, with regard to the drawing which represents, schematically, and without the proportions between length and diameter and slope being respected
- Figure 1: view during its manufacture, a
mast perspective,
- Figure 2: a half-section of a completed mast,
- Figure 3: seen in profile the mandrel,
- Figure 4: plan views, the pieces of impregnated fabric
which will be wound on the mandrel,
- Figure 5: the diagram of the bending moments at
along the mast.

 Referring to the drawing (FIG. 3), it can be seen that the mandrel 1 is a body having a conicity of constant direction but of slope which can be increasing, for example the part 2 extending over approximately three quarters of its length from from its base 3 will have a taper of five per ten thousand, the next tenth of length will be split into four parts 4, 5, 6, 7 which will have conicities of fifty six for ten thousand, one hundred and seven for ten thousand, one hundred and fifty respectively nine for ten thousand and two hundred and ten for ten thousand. Finally, the remainder 8 forming approximately fifteen percent of its length and therefore extending to the head 9 of the mandrel will have a taper of two hundred ninety per ten thousand.

 Around this mandrel 1 is wound a fabric 10 formed of more warp 11 than weft 12 and for example from eighty five to ninety percent warp 11 and from ten percent to fifteen percent weft 12 of fibers or bundles of fibers of the chosen material such as fiberglass, which fabric has previously been impregnated with thermosetting resin.

 Given the width in which it can be manufactured, the impregnated fabric 10 is generally formed from several main parts 14, 15 wound successively.

 This winding takes place by placing the chain 11 parallel to the longitudinal axis 13 of the mandrel 1.

 Obviously, independently of these main parts, small reinforcing parts 16, 17 can be provided locally which, for their part, can be oriented with their chain perpendicular to the axis of the mandrel.

 Referring to Figure 5, we see that the diagram 18 of the bending moments has substantially the shape of a triangle whose base corresponds to the axis 13 of the mast and whose top 19 is located on the line 20 which, perpendicular to this axis, goes through the attachment point 21 of the "wishbone".

 According to an essential characteristic of the invention, the total width L1 of fabric wound in the attachment area of the "wishbone" is much greater than that L2, L3 of the fabric which, independently of any reinforcements, is wound both at the foot and 'at the head of the mast and between each of which ends and the attachment zone, this length varies gradually.

 To this end, in a preferred embodiment of the invention, at least part of the impregnated fabric 10, and for example the part 15, is cut into a substantially triangular shape, the base 22 of which is parallel to the chain 11 and of which, after winding, the top 23 will be located at the point 21 of attachment of the "wishbone" so that at this point the fabric winding has a thickness (el) greater than the thickness (e2) measured at the foot 24 of the mast and that that (e3) measured at the head 25 of said mast 26.

 This difference in thickness will be all the more significant as the height of the triangle will be large and that its winding will therefore be done over several, for example, four turns.

 This difference can go so far as to give the mast, in the attachment zone, a diameter greater than that which it presents at the foot, which mast will thus acquire a substantially biconical external shape.

 To take into account the possible variation in height of the attachment point, the top of the triangle can be truncated, the mast will then present in the area of the attachment point, a cylindrical part separating the two zones of reverse taper.

 After this characteristic phase of cutting and winding of the impregnated fabric, the manufacturing process continues according to the traditional technique by coating with hot shrink film then by polymerization and finally by peeling of the shrink film and removal of the mandrel.

 The mast 26 thus obtained which has an outwardly substantially biconical shape, offers, at the point of attachment 21 of the "wishbone", a moment of inertia markedly greater than at its head 25 and at its foot 24, while having been formed in one piece on a mandrel whose conicity in a constant direction and whose withdrawal remains therefore possible.

 It also has the advantage of being more resistant while being lighter and at least as rigid as the masts known to date.

 In an alternative embodiment, the variation in the width of wound fabric is obtained by successively winding, by superimposing them, pieces which may be, for example rectangular, trapezoidal or other, but of increasingly smaller dimensions in the longitudinal direction, each of them being more or less centered with respect to the previous one and with respect to the attachment zone of the "wishbone", so that the thickness (el) of material there is greater than that (e2) and (e3) at foot and at the top of the mast and so that between the attachment zone of the "wishbone" and, both the foot and the head of the mast, the said thickness varies fairly gradually.

Claims (9)

 1. A method of manufacturing a mast (26), made of plastic reinforced with fibers, whether this mast is a conventional mast or a beam or the like, and more particularly a method of manufacturing a mast which, in one point (21) situated at a certain distance from its ends (24, 25), undergoes a bending stress, according to which method a mandrel (1) is used which is at least slightly conical, the conicity of which may be constant or vary throughout of the mast but whose orientation is always the same in order to form an undercut face, on which mandrel a fabric (10) is wound which is formed of fibers (11, 12) chosen to make the reinforcement such  glass fibers, which has been pre-impregnated with a binder, such as thermosetting resin, and which can be formed from one or more main parts (14, 15) or even possible local reinforcements (16, 17) which are wound successively, after which the assembly, the wound fabric and the mandrel, is inserted into a coating of heat shrinkable material and then the thermosetting resin is polymerized hot, after which not only the coating is removed, but also the mandrel (1 ), this process being CHARACTERIZED in that in the zone of maximum stress of the mast, a total width (L1) of fabric is wound which is greater than that (L2, L3) of the fabric (10) which, independently of any reinforcements (16,17) , is wound at the foot (24) and the head (25) of the mast (26) and in that between each of these ends (24,25) of the mast and the zone (21) of maximum stress this width varies gradually.  2. Method according to claim 1 characterizedaen that in order to wind a width (L1) of fabric (10) more and more important from the ends (24,25) of the mast (26) to its area ( 21) of maximum stress, at least a portion (15) of the triangle-shaped fabric is cut, the base (22) of which is parallel to the axis (13) of the mandrel and the top (23) of which after winding is height of point (21) of maximum stress.  3. Method according to claim 2 characterized in that the triangle is truncated at its top (23).  4. Method according to claim 1 characterized in that in order to wind a width (L1) of fabric (10) more and more important from the ends (24,25) of the mast  (26) up to the maximum loading area, successively winding, by superimposing them, pieces of fabric (10) having warp (11) of increasingly smaller dimensions and in that each of them is substantially centered. 'them with respect to the previous part and with respect to the point (21) of maximum stress.  5. Method according to any one of claims 1 to 4 characterized in that the fabric is woven (10) with more warp than weft and in that during its winding, we have this fabric (10) with its chain (11) exclusively in the longitudinal direction of the mandrel.  6. Method according to claim 5 characterized in that the fabric is woven with about ten to fifteen percent weft (12) and eighty five to ninety percent warp (11).  7. Mast made of plastic reinforced with fibers obtained according to the method of any one of claims 1 to 4, CHARACTERIZED in that after removal of the manufacturing mandrel (1), it has in the zone (21) of maximum stress , a thickness (el) of material greater than that (e2, e3) which it presents not only at the head (25) but also at the foot (24) of the mast.  8. Mast according to claim 7 characterized in that its diameter at the point (21) of maximum stress is greater than that at the foot- (24) of the mast (26) which thus acquires a biconical external shape.  9. Mast according to claim 7 characterized in that its diameter in the zone of maximum stress is greater than that at the foot (24) of the mast which thus acquires externally a shape composed of two inverted cones separated by a cylindrical zone.