GB2258657A - Friction materials for railway brake pads - Google Patents

Abstract

Railway brake pads or blocks comprising a friction material which contains particulate material, fibre and binder. The binder makes up only 2% to 17% by volume of the material, and can be thermoset rubber or resin or mixtures thereof.

Description

Improvements in or relating to friction materials for railways brakes This invention relates to friction materials for railway brakes and particularly to brake pads and blocks of the kind which are used in brakes for railway applications.

Friction materials for railway brakes constitute a separate field within the friction materials art because the particular conditions which they have to fulfil are very different from those for automotive applications such as on cars and lorries.

In general friction materials for railway applications utilise similar ingredients to those used in many automotive applications but they generally contain high proportions of organic binder, eg 20 to 55 per cent by volume and rubber generally constitutes a substantial proportion of the organic binder used.

Because of their massive size relative to automotive brake pads, conformability is of greater significance in railway materials and this is one of the major reasons which it is usual to employ substantial proportions of organic binders.

Organic binder materials are in widespread use in the manufacture of friction materials for both railway and automotive purposes, the common types being phenolic resins or modified phenolic resins with which a proportion of rubber such as SBR or Nitrile rubber is also often used. Many published patent specifications mention organic binder materials of this kind, for example UK Patent No 1,604,828 discloses non-asbestos brake pads which are made from a friction material containing a thermoset binder making up 20% to 45% by volume of the friction material at least half of which is phenol formaldehyde resin. UK Patent No 1,604,839 discloses a friction material for brake pads which contains a thermoset binder making up 20% to 60% by volume of the friction material at least half of the thermoset binder being phenol formaldehyde resin.

European Patent No 0,184,708 discloses a friction material for automotive uses which contains steel fibres and organic binder, the amount of organic binder being 10% to 35% by volume of the friction material. Although this latter specification mentions rubber as a possible binder, all the examples of the invention used resin. There is nothing in the specification which relates to friction materials for railway brake applications.

We have now found that friction materials for railway applications with advantageous properties may be manufactured using organic binders at low volumes.

Thus according to the invention a railway brake pad or block comprises a friction material containing fibres, particulate materials and an organic binder, wherein the binder makes up 2% to 17% by volume of the friction material.

Conventionally it has been believed that such low proportions of binder were not satisfactory for railway brake materials because of the conformability needed in such materials. In the friction materials of the present invention we have found remarkably good properties, with good conformability. It appears that the conformability of such materials improves again when the proportion of binder is sufficiently low.

The binder used may be rubber, resin, or a mixture of such materials.

The rubber used is preferably nitrile rubber or SBR, although other rubbers may be used as the whole or part of the binder, for example bromobutyl rubber, chlorobutyl rubber.

The resin used is preferably a phenolic resin or modified phenolic resin although other thermosetting resins such as epoxy resins and polyester resins may also be used. Where a mixture of rubber and resin is used it is preferred that at least 50% by volume of the binder is rubber.

The other ingredients of the friction material may be selected from the wide range of available fibres, fillers and friction and wear modifiers.

Examples of possible fibres include metal fibres such as steel, mineral fibres such as glass or basalt, and organic fibres such as aramid. The amount of fibre used is not critical but will typically be in the range 7 to 25% by volume.

The balance of the friction material apart from binders and fibres is of particulate materials. The particulate materials used as fillers and friction and wear modifiers may be taken from a vast range of possible options. For example alumina, antimony trisulphide, zirconia, crushed coke, graphite, molybdenum disulphide, powdered metals such as copper, tin, brass, reinforcing fillers such as mica, vermiculite, wollastonite, and other fillers such as barytes, silica, calcium carbonate etc.Owing to the low volumes of binder used in this invention the particulate materials are preferably used in the larger sizes of the conventional range eg 300,um to 600m, and substantial amounts of finely divided materials (ie particle -sizes below 50mum) are to be avoided in order to ensure that there is not too much particle surface area to be wetted out by the binder.

The invention will be described in more detail, by way of example only, in the following examples.

EXAMPLE 1 to 3 Friction materials were prepared according to the formulations given below in parts by volume in Table 1. In each case the dry ingredients were pre-blended, then intimately mixed with a solution of binder rubber in trichloroethylene. The mix was dried and disintegrated and then charged to a die where brake pads were cold moulded to shape under a pressure of 5.5 tons/in2. The thus shaped railway brake pads were cured by baking whilst held under a pressure of over 9 psi in an oven for 1 hour at 1600C, plus 1 hour at 2000C and 3 hours at 235 0C.

TABLE I Example No 1 2 Nitrile Rubber 10.00 10.00 Curing Agents 3.00 3.00 Steel Fibre 15.00 15.00 Glass Fibre 4.00 Petroleum Coke 15.00 15.00 Fillers 1 21.00 20.50 Friction & Wear Modifiers 32.00 36.50 Testing Railway brake pads made according to Examples 1 and 2 were tested for friction and wear in machine tests against cast iron brake discs. The behaviour of each material was tested in a sequence which consists of a bedding in phase of 30 successive stops from about 90 kph, followed by a dry friction test of 18 successive stops from speeds varying cyclically between 80kph and 200kph followed by a wet friction test of 27 successive steps from speeds varying cyclically over the same range but with a wet brake disc. The results of these tests are given below in tabular form in Table II.

TABLE II Example No ~ 2 Bedding in phase Max. mean coefficient of friction* 0.32 0.34 Min. mean coefficient of friction* 0.21 0.21 Overall average coefficient of friction 0.296 0.310 Dry phase Max. mean coefficent of friction* 0.33 0.34 Min. mean coefficient of friction* 0.27 0.29 Overall average coefficient of friction 0.293 0.312 Wet phase Max. mean coefficient of friction* 0.28 0.27 Min. mean coefficient of friction* 0.23 0.23 Overall average coefficient of friction 0.254 0.250 Ratio of wet friction coefficient to dry 0.865 0.801 * In this table the expression "mean" coefficient of friction is used to refer to the average value of the coefficient of friction during a single stop, because it is normal to see some variation of the coefficient with time during each stop as the pad temperature rapidly rises.

It can be seen from Table II that the brake pads made according to each example showed excellent resistance to fade and consistency of friction during all phases of the test.

Claims (6)

1. A railway brake pad or block which comprises a friction material containing fibres, particulate materials and an organic binder, wherein the binder makes up 2% to 17% by volume of the friction material.

2. A railway brake pad or block according to claim 1 in which the binder comprises a thermoset rubber selected from nitrile rubber, SBR, chlorobutyl rubber and bromobutyl rubber and mixtures thereof.

3. A railway brake pad or block according to claim 1 or 2 in which the binder comprises a thermoset resin selected from phenolic resins, modified phenolic resins, epoxy resins and polyester resins.

4. A disc brake pad according to claim 1, 2 or 3 in which the friction material contains fibres in an amount of 7% to 25% by volume.

5. A disc brake pad according to claim 3 in which the fibres are selected from steel fibres, glass fibres, mineral fibres and aramid fibres and mixtures thereof.

6. A railway brake pad substantially as described herein in any one of the foregoing examples.