This invention is concerned with preforms for friction elements of the type which are generally annular and contain curable friction material. Such preforms are heated and pressed to form friction elements. Such friction elements are used, for example, as clutch facings.

The preforms used in producing friction elements of the above-mentioned type are at present made by winding a single length of yarn coated with the curable friction material on to a rotating plate. The yarn usually comprises several filaments or strands which may be of different compositions. These filaments may be, for example, made of ara id, metal, rayon or glass. The filaments are coated with a curable friction material which is, thus, incorporated into the preform. The yarn is deposited on to the rotating plate by an applicator which is moved radially during the deposition to deposit the yarn between an inside diameter of the preform and an outside diameter thereof. The applicator oscillates more than once in each revolution of the rotating plate. This results in the yarn being deposited in a lobed pattern on the rotating plate. The term "lobed pattern" is used herein to denote a pattern in which the point of deposit of the yarn moves from an inside diameter to an outside diameter of the pattern or vice versa in less than one revolution. The number of lobes per revolution, ie the number of times the applicator moves back and forth between the inside and outside diameters during one revolution of the plate, may be arranged not to be a whole number. In this case, during the next revolution, the yarn is not deposited directly on

top of the previously-deposited yarn but instead lies adjacent to and crosses over that yarn. The objective is to cover the area between the inside and outside diameters with yarn as uniformly as possible. After a number of revolutions, the pattern may repeat itself so that the yarn forms a new layer on top of that already deposited. A complete preform may be built up as a plurality of overlying layers in an annular shape which provides the preform. In order to form a more even preform, the applicator may be arranged to spend more time adjacent to the outside diameter than adjacent to the inside diameter so that account is taken of the greater distance around the preform at increasing diameters. Once the preform has been wound, it is pressed and heated to form the completed friction element by compressing the yarn and curing the friction material.

Clutch facings used in automotive applications are subject to high rotational speeds and hence to high centrifugal forces. If such clutch facings cannot withstand such forces they burst. Accordingly, clutch facings are subjected to burst tests in which they are heated to simulate operating conditions and spun until they burst. The rotational speed at which bursting occurs gives a measure of clutch performance. It is desirable to increase this burst speed to increase the range of uses of such clutch facings or to increase the safety factor thereof.

It is an object of the present invention to provide a preform which, when formed into a friction element, provides higher burst speeds than conventional preforms.

The invention provides a generally annular preform incorporating curable friction material, the preform being formed from a single length of yarn wound into a plurality of overlying layers, characterised in that at least one

portion of the preform is wound according to a lobed pattern and at least one other portion of the preform is wound according to a spiral pattern.

A preform according to the invention has a lobed portion with lengths of the yarn extending from the inside to the outside diameter of the portion in less than one revolution about the centre of the preform. These lengths serve to hold together the spiral portion of the preform. The spiral portion, in which the point of deposit of the yarn moves radially by the thickness of the yarn or a little more in one revolution, has the yarn lying in approximately circumferential directions in which it is best positioned to resist centrifugal forces. Furthermore, as the yarn in the spiral portion does not cross over itself, it is considerably less likely to be broken during the pressing of the component into a friction element than yarn wound to a lobed pattern. The filaments are, therefore, intact, giving greater strength. Friction elements made from preforms according to the invention can exhibit higher burst speeds than elements with the same length of yarn and the same inside and outside diameters but wound according to a lobed pattern consistently throughout the preform. Preforms according to the invention have been found to provide an improvement of up to 16% in burst speed.

A preform according to the invention may have the lobed portion thereof forming at least one radially- extending layer of the preform. This layer may form a radially-extending face of the preform, the layer advantageously being the first to be wound, since, in this case, the spiral portion will not be deformed by ejector pins pushing the preform off the rotating plate after the preform has been wound. Both radially-extending faces may be formed by lobed portions which have a spiral portion between them. This gives a symmetrical preform with the

spiral portion held together on both sides. A symmetrical preform with a spiral layer between two lobed layers is found to produce flatter clutch facings (ie less likely to warp) than preforms wound entirely according to a lobed pattern. Alternatively, one lobed layer may be between two spiral layers.

The spiral portion may, alternatively, form a radially-inner portion of at least one layer of the preform, and the lobed portion form a radially outer portion of said layer. The lobed portion may, alternatively, be between a radially inner spiral portion and a radially outer spiral portion of a single layer of the preform.

The number of lobes per revolution selected for the lobed portion depends on the diameters concerned but may be between 1.2 and 9.6.

Advantageously, to avoid an end of the yarn coming loose, the initial and/or the final end of the yarn may be deposited away from the circumferential edges of the preform, thereby "tucking" these ends in.

The invention also provides a clutch facing formed by pressing and heating a preform according to the invention.

There now follows a detailed description, to be read with reference to the accompanying drawings, of three preforms which are illustrative of the invention.

In the drawings:

Figure 1 is a view of approximately half of the first illustrative preform;

Figure 2 is a diagrammatic plan view of a layer of the first illustrative preform;

Figure 3 is a diagrammatic plan view of a different layer to that shown in Figure 2 of the first illustrative preform;

Figure 4 is a view similar to Figure 3 but showing only one turn of yarn of said layer;

Figure 5 is a view similar to Figure 1 but of a second illustrative preform; and

Figure 6 is a view similar to Figure 2 but of a layer of a third illustrative preform.

The first illustrative preform 10 is generally annular being formed between an inside edge 12 which is generally circular about a centre 14 (Figures 2 to 4) and an outside edge 16 which is also generally circular about the centre 14. The edges 12 and 16 are represented by circles in Figures 2 to 4. The preform 10 is formed by winding a single length of yarn 20 coated with curable friction material on to a plate (not shown) which rotates about the centre 14. A description of the machine used for winding the yarn 20 to form the preform 10 can be found in WO 91/15354. The preform 10, thus, incorporates curable friction material and can be pressed and heated to form a friction element, specifically a clutch facing.

The yarn 20 is wound into a plurality of overlying layers. These layers comprise a first layer 22 (shown in Figures 1 and 3) wound according to a lobed pattern of 3.2 lobes per revolution, two layers 24 and 26 wound according to a spiral pattern (Figure 2 illustrates the layer 24) and a fourth layer 27 wound according to a lobed pattern. In this case, the patterns of the layers 22 and 27 are identical but they may be mirror images of one another. Figures 3 and 4 illustrate the formation of the layers 22 and 27. Figure 2 illustrates the spiral pattern of layers 24 and 26. Thus, the preform 10 comprises a portion, the layer 22, wound according to a lobed pattern, a portion, the layers 24 and 26, wound according to a spiral pattern.

and a further portion, the layer 27, wound according to a lobed pattern.

Figure 4 illustrates how the yarn 20 is deposited during the first revolution of the rotating plate. An initial end 28 of the yarn 20 is deposited at a point between the diameters of the edges 12 and 16 so that this end is "tucked in" away from the circumferential edges 12 and 16. The yarn 20 (which is tacky) adheres to the rotating plate. The yarn applicator (not shown) moves outwardly to the diameter of the edge 16 which it reaches at point 30. The applicator then moves to the diameter of the inner edge 12 which is reaches at point 32. The applicator continues sweeping between the diameters of the inner and outer edges 12 and 16 at a rate of 3.2 lobes per revolution. In other words, the point of deposit of the yarn 20 moves from the diameter of the outer edge 16, to the diameter of the inner edge 12, and back to the diameter of the outer edge 16, 3.2 times in each revolution of the rotating plate. This produces a lobed pattern.

After a little over one revolution, the yarn 20 crosses over itself for the first time, and from then on, crosses over itself repeatedly. After 5 revolutions (see Figure 3) the yarn 20 returns to the point occupied by the initial end 28 having completed the pattern designed to cover the area between the edges 12 and 16. This completes the first layer 22 which is of conventional construction except that the initial end 28 is usually at the inner or the outer edge. In this case, the layers 22 and 27 have 5 turns of yarn but they may have more.

After the first layer 22 has been completed, the movement of the applicator is changed. Having first reached the point 30 at the diameter of the outer edge 16, the applicator begins to sweep more slowly moving only by the thickness of the yarn 20 in each revolution of the

rotating plate. This results in the yarn 20 being deposited in the layer 24, on top of the layer 22 to which it adheres, in a tight flat spiral pattern. Figure 2 illustrates the spiral pattern diagrammatically but, for the purposes of illustration, shows a looser spiral of only 6 turns. As shown in Figure 1, the yarn 20 actually makes 12 turns in forming the complete spiral. Point 34 in Figure 2, is the starting point of the spiral and corresponds to point 30 in Figure 3. The spiral is deposited inwardly from the point 34 and forms the layer 24 on top of the layer 22.

When the applicator reaches a point 36 at the diameter of the inner edge 12, its direction of movement is reversed but its speed continues at the thickness of the yarn 20 in each revolution. The deposits the layer 26 as a tightly- wound spiral of 12 turns on top of the layer 24. The layer 26 is terminated when the applicator reaches the diameter of the outer edge 16 which occurs after 12 revolutions of the rotating plate.

The applicator then changes back to the lobed pattern of 3.2 lobes per revolution and deposits the layer 27 on top of the layer 26. After about 5 revolutions, the applicator "tucks in" the final end 29 of the yarn by moving to a point intermediate the inner and outer edges 12 and 16.

The first illustrative preform 10, thus, comprises a portion, formed by the layer 22, which is wound according to a lobed pattern, another portion, formed by the layers 24 and 26, which is wound according to a spiral pattern, and another portion, formed by the layer 27 which is wound according to a lobed pattern. The layers 22 and 27 form radially-extending faces of the preform 10.

In the case of the preform 10, the layers 22 and 27 are wound at 3.2 lobes per revolution but for other sizes or types of preform, eg made of less or more flexible yarn or of different sizes, the number of lobes may be different. For example, 1.2, 2.2, 5.4 or 9.6 lobes per revolution may be employed. The number of lobes will normally be between 1.2 and 9.6 per revolution. It is also possible to omit the layer 27. Another possibility is for the layers 22 and 27 to be wound according to a spiral pattern with the layers 24 and 26 being wound according to a lobed pattern. Where there are two or more layers wound according to a lobed pattern, they may be wound at different numbers of lobes per revolution.

The first illustrative preform 10, has considerable advantages over a conventional preform wound entirely according to a lobed pattern. Firstly, the area between the inner and outer edges 12 and 16 is more efficiently and uniformly packed with yarn. This enables more uniform and flatter clutch facings to be formed. Secondly, the yarn 20 does not cross over itself as often so that pressing into a clutch facing does not break the yarn into as many pieces. This increases the strength of the facing as there are fewer short pieces of yarn therein. Thirdly, more of the yarn 20, ie that in the spiral layers 24 and 26, extends approximately circumferentially and so is better aligned to resist bursting. The lobed layers 22 and 27 hold the preform 10 against unravelling before pressing and curing, especially when the preform is ejected from the rotating plate by ejectors which pass through openings in the plate to break the adhesion between the preform and the rotating plate.

In order to test the advantages of preforms according to the invention, sample preforms were prepared having an outside diameter of 242mm and inside diameter of 155mm. These preforms were pressed and cured into clutch facings

3.9mm in thickness and were drilled with the usual rivet holes used for mounting such facings. The samples were then subjected to burst speed tests at 150°C. Test samples were made using various winding patterns according to the invention. Further samples of the same size were wound entirely according to a conventional lobed pattern at 3.2 lobes per revolution for comparison purposes.

The conventional samples were found to burst at an average of 10,738 revolutions per minute. In contrast to this the preform 10 burst at an average of 11,985 revolutions per minute. The increase was, therefore, 11.6%. With the layer 22 wound at 1.2 lobes per revolution, the burst speed averaged at 12,462. Winding the lobed layer at 2.1 lobes per revolution, produced a value of 12,203. All these samples, according to the invention, therefore, showed significantly improved burst speeds over the conventional samples.

The second illustrative preform 40 is shown in Figure 5. It is similar to the preform 10 in that it has layers 42 and 47 which are identical to the layers 22 and 27 (ie they are wound according to a lobed pattern) . However, instead of two central spiral layers, there is only one spiral layer 44. The preform 40, thus, comprises a central spiral between two lobed portions. A clutch pressed from the preform 40 was found to have adequate burst speed and not to be as prone to warping as conventional clutches.

Figure 6 illustrates a layer of the third illustrative preform 50 according to the invention. The preform 50 has a first and a last layer (not shown but similar to the layer 22) wound according to a lobed pattern between an inside edge 52 and an outside edge 56 of the preform 50 which is formed about a centre 54. The remaining two layers of the preform 50 are wound according to the pattern shown diagrammatically in Figure 6. This pattern comprises

a spiral portion 58 extending from the inner edge 52 to a diameter intermediate the diameters of the inner and outer edges 52 and 56. This intermediate diameter is selected as the diameter at which the radially inner edge of the rivet holes (not shown) of the clutch facing are formed. These rivet holes are drilled through the facing formed from the preform. The portion 60 of each of these layers formed between the intermediate diameter and the outer edge 56 is formed according to a lobed pattern at 3.2 lobes per revolution. The applicator used to deposit the yarn for the preform 50, thus, moves at the thickness of the yarn per revolution until it reaches the intermediate diameter. Then, it sweeps between the intermediate and outer diameters at 3.2 lobes per revolution, although other numbers of lobes are also possible. Thus, the preform 50 has a portion (the whole of two layers and the outer part of the other two layers) wound according to a lobed pattern and another portion (the inner part of two layers) wound according to a spiral pattern.

The preform 50 has the advantages that the spirally- wound yarn is concentrated at its inner edge 52 which is where bursting is most likely to take place. Furthermore, the rivet holes are drilled into the more open winding of the lobed pattern so that the drilling is less likely to break the filaments of the yarn so that more of the yarn's strength is retained in the facing.