DK146274B - Apparatus for the manufacture of friction elements - Google Patents

Description

09) DENMARK VJj!

("IN PRESENTATION OR 146274 B

DIRECTORATE OF THE PATENT 06 BRAND

(21) Patent Application No. 0236/73 (51) IntCI.3: B29G 1/00 (22) Filing Date: 16 Jan 1973 (41) Aim. available: 18 Jul 1973 (44) Submitted: 22 Aug 1983 (86) International Application No: - (30) Priority: 17 Jan 1972 US 218313 (71) Applicant: * ABEX CORPORATION; New York, US.

(72) Inventor: Richard H. 'Gilbert; US, Arvon M. Griffith; US.

(74) Prosecutor: Hofman-Bang & Boutard Patent Office (54) Friction Elements Manufacturing Device

The invention relates to an apparatus of the kind set forth in claim 1.

A blend material is known for producing low friction brake shoes for rail vehicles. This blend material contains 12-30% by weight cast iron particles, 20-55% by weight graphite particles, 8-17% by weight asbestos fibers, 14-24% by weight thermosetting phenol formaldehyde resin and up to 30% by weight of a mixture of friction modifying agents such as barites, alumina, coke and more. A relatively large proportion of asbestos fibers DO Up to 35 - 50% by weight is also characteristic of brake pads for automobile road vehicles.

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For the manufacture of such friction elements, a compression molding technique is used, in which, after the homogeneous mixing of the components, the blend material is filled into a mold cavity and for several hours at high temperature and under considerable pressure partially cured. Hereby the mixture is compressed and the curing of the partially cured molded product is terminated by subjecting it to a temperature of about 10 hours or more. 180 ° or more.

It has been proposed to avoid the high cost of the traditional manufacture of friction elements according to the known pressure casting method described, ie. by using high pressure and high temperature simultaneously by compressing at room temperature first a practical liquid-free so-called dry mixture to provide a molding having the desired particle density, and then leaving the plastic-binder heat cure outside the mold, avoiding the costly simultaneous application of heat and high pressure. Theoretically, this principle should be applicable, but experience has shown in practice that it is not possible to obtain a coherent brake material alloy with the desired final density with certainty, since the pressed bodies tend to fall apart even when the binder is sticky.

The present invention therefore aims to compress a solvent-free, dry, very fluffy and dissolved material at room temperature to obtain a final-element brake element blank in a single press operation, i.e. by rapidly advancing a press plunger, and then performing the curing operation outside the press mold and without applying pressure. This is achieved by an apparatus of the kind specified and with the peculiarities of claim 1. Namely, these achieve that even a brake element material with a relatively high proportion of dry asbestos can be pressed to a final density body without the risk of the body falling apart when withdrawn from the mold or later before curing, and this effect is achieved by allowing the air between the particles of the material during the pressing operation to quickly escape from the material. The present invention is based on the recognition that the preparation of two-stage friction element bodies, namely the compression step and the thermoset step from a dry starting material, is practicable only if the apparatus for pressing the material mixture can be arranged so that all air from the spaces between the material particles can freely flow away from the cavity in which the material mixture is compressed. This is achieved precisely by the characterizing part of the new main claim.

For nearly 10 years, the problem has been how to produce friction elements from a substantially dry blend material containing a thermosetting adhesive in practically dry state without having to apply heat and pressure simultaneously to cause the binder to harden.

An attempt has been made to solve this problem by subjecting the friction material mixture containing the binder in a mixing chamber to such a strong mechanical effect that the binder is transferred in a partially melted, plastic and sticky but far from hardened state under the effect of the internal friction between the particles. The resulting mixture was still substantially solvent-free, however, the binder was sufficiently softened that a metered amount of the mixture in a mold could be compressed to the final density of the friction element, with the thus-pressed body to adhere and the density of the body to be maintained during removal from the body. the mold despite the binder not yet hardened.

Thus, the pressed friction material should be transferable to a furnace and maintained at a high temperature until the thermosetting resin has been transferred from the soft state to its hardened immiscible state. To this end, the dry mixture has been compressed without simultaneous application of heat and cured the body thus pressed without the use of pressure, except for any low pressure to prevent casting. However, as mentioned, it has been found that in this way pressed friction element blanks tended to break before curing.

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There are several restrictive factors that limit the number of possible measures to achieve the desired solution. First and foremost, it is desired to avoid the simultaneous use of pressure and heat to cure the plastic binder in the manufacture of friction elements, since pressure and heat are desired to be used successively to provide a product whose essential properties (coefficient of friction, abrasion resistance, impact resistance, etc.) correspond to the properties of a similar product made by the simultaneous application of pressure and heat. In addition, the method of separate application of pressure and heat must be suitable for a mixture material of varying density and containing a significant proportion of asbestos and up to .80% air. A mixture of this type is set forth in the following example for a preferred blend material for the manufacture of a brake tray for a rail vehicle.

A method similar to these limiting criteria has been developed. Before discussing this in more detail below, a preferred material composition should be described in the manufacture of a brake shoe for a rail vehicle. Those skilled in the art will readily be able to change this composition with a view to preparing road vehicle brake pads or even clutch disc pads of a similar dry blend.

In this case, the blend material should contain a larger proportion of asbestos and possibly instead of cast iron particles brass shavings.

The exact composition of the blend material to be pressed into the apparatus of the invention will vary depending on the intended function of the friction element, i.e. whether the friction element should be in the form of a brake pads for a road vehicle, a brake shoe for a rail vehicle, a clutch disc or the like. The friction and wear properties will vary widely depending on the use and the frictional properties as well as the wear resistance are determined by adapting or varying the components of the blend material. However, the material which is suitable for pressing in the apparatus according to the invention must in any case contain at least 5% by weight of asbestos and as a binder a thermosetting resin. The exact chemical nature of the resin is not critical as the binder can be modified in a variety of ways for hardness.

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However, novolak or a two-stage heat-curable phenol formaldehyde resin is preferably used, although a one-step heat-curable phenol-formaldehyde resin or a mixture of these two types of resins can also be used. A rail vehicle brake shoe can e.g. is made from a material according to the following example:

EXAMPLE

Ingredients Weight percent- Preferred range weight percent phenol formaldehyde resin (novolac) 14 - 24 24 cast iron particles (+ 120 mesh) 12 - 30 17.5? Electrite particles -60, + 325 mesh) 20 - 55 21 asbestos fibers 8-17 8.8 friction modifiers (barites or alumina, 95% - 325 mesh; coke, - 20 mesh) 0-30 29

The novolac is heat curable (curing by hexamethylene tetramine, hereinafter "hexa") to a heat cured state.

In the following, the invention is explained in more detail with reference to the drawing, in which fig. 1 is a schematic and sectional view of one embodiment of the apparatus according to the invention; FIG. 1A is a perspective view of a piston belonging to the apparatus, FIG. 1B is a bottom view of the stamp, and FIG. 2 is a section through a brake shoe made in the apparatus according to the invention for a rail vehicle.

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The constituents of the blend material in the example are homogeneously mixed together in a dry state. A metered quantity of 10, cf. FIG. 1, of the material mixture is placed on top of a suitable iron plate 12 in a cavity in a press mold 15, the shape of which corresponds to the shape of the upper surface of a fixed bottom part 13 of the press mold, which also has sidewalls 15. On the plate 12's facing surface of the mixing material For example, a heat-activatable adhesive may be applied to promote the adhesion between the friction mixture 10 and the plate 12. In most cases, the necessary adhesion between the mixture and the plate is secured by openings 12A in the plate into which blend material penetrates during the pressing operation.

Next, the dry blend material 10 in the mold is compressed by means of a press piston 20 forming in the mold and the top wall of the mold cavity to provide a pressing material body of the same shape as the final shape of the finished brake shoe. The brake shoe is shown in FIG. 2 and denoted by 30. The final maximum compressive pressure used is of the order of 3700 kg / cm and is terminated as soon as it is obtained and the blend material is compressed to its final density corresponding to 85 - 90% of the theoretically possible greatest density.

Next, the plunger 20 is pulled out of the mold, after which the compression friction block 10A and the plate 12 consisting of brake shoes 30, FIG. 2, is removed from the mold cavity. The shape and the finished product can of course be designed differently than shown in the drawing.

The FIG. 2 is then transferred to a furnace in which it is heat treated for approximately 10 hours, in which the temperature is gradually increased from approximately 150 ° C to approx. 230 ° C. During this heat treatment, the pressed material is subjected to a slight pressure of

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the order of 1.5 kg / cm. This pressure is merely to prevent the pressed body from falling apart before the binder is cured. After the said 10 hours, the material is affected for approximately another 6 hours with the latter temperature of 230 ° C, completing the curing of the phenolic formaldehyde binder. The phenol binder in the example is a novolac containing about 6% of an accelerator, such as nhexa ", to accelerate curing. However, a one-stage, hardenable phenol formaldehyde resin or a combination of this resin with a novolac of the specified type may also be used. The major and essential component of the binder however, must be a known, heat-curable phenol formaldehyde resin of the type used so far in the manufacture of friction elements.

In one of the confining walls of the mold cavity, passages are provided to facilitate the expulsion of the air between the particles of the friction material during the pressing operation, whereby the duration of the compression operation can be substantially shortened so that the performance of the compression molding apparatus can be increased. These passages through which it between it originally only solved. packed friction material particles enclosed relatively large amount of air can more rapidly escape under the pressure of pressure, preferably groove passages 32, fig. 1A and 1B, in the side faces and end faces of the piston 20. These notes extend from the piston surface adjacent to the press material to the opposite surface of the piston. Through the passages, trapped air can escape into the open air. Filter elements 34 are interposed between the friction material in the press mold and the plunger, which prevents, together with the escaping air, material particles from entering the grooves 32 and clogging them. The filter elements consist of a perforated metal plate placed at the top, ie. closest to the piston 20, which abuts against the piston's convex downwardly against the curved printing surface of the mold cavity. This filter plate has relatively large perforations 34A as shown in the upper right of FIG. 1. Underneath the filter element 34 is a further filter element 35, and between this and the friction material there is a paper foil 36. The filter element 35 has perforations 35A which are substantially smaller than the perforations 34A of the filter element 34. The paper wrap is porous. The two filter plates may be attached to the plunger pressure surface.

The paper film has openings through which the air can pass but is so small that particles from the friction material 10 are retained so that such material particles cannot clog the perforations in the filter elements 35 and 34. As a paper film, a kraft paper film is preferably used.

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The filter element 35 serves to support the paper foil and prevent pieces of paper from being detached and clogging the relatively large perforations of the upper filter element 34.

After the pressing operation, the plunger is pulled out of the mold, after which the pressing member is transferred to the oven mentioned above. In most cases, the paper sheet will adhere to the pressed friction body 10A. However, this does not matter because the foil is burnt during curing.

During the pressing operation, the plunger 20 abuts closely against the sidewalls 15 of the mold.

In the manufacture of brake shoes for rail vehicles, the density of the friction element material in the original dry, loosely packed state of the material corresponds to 0.35 - 0.40 g per meter. cnr *, while the density of the material in the compressed state corresponds to 1.90 - 2.20 g / cm cm ^. The difference between these two values is a measure of the degree of compression achieved by applying a compressive pressure of the order of 3300 - 3750 kg / cm 2.

For the manufacture of road vehicle brake pads, the friction material in the initially loosely packed condition has a density of between 0.24 and 0.29 g / cm 2 and a final density after the pressing operation of between 1.90 and 2.20 g / cm 2. In this case, a working pressure of between 3000 and 3750 kg / cm2 is used.