MULTI-LAYER BRAKE ELEMENT WITH ENHANCED THERMAL INSULATION

Abstract: An intermediate layer (4) of a relatively low thermal conductivity, high resistance to thermal degradation elastomeric material is interposed between a friction lining (3) and a relatively soft elastomeric layer (5), the latter being bonded to a backing plate (2).

Description: MULTI-LAYER BRAKE ELEMENT WITH ENHANCED THERMAL INSULATION This invention relates to brake elements particularly useful for railway braking applications. It is known from PCT/GB94/01964 (WO95/07418) that modern high speed train systems routinely impose extremely severe loading on the brake pads, especially on disk brake pads. This is due to the substantial amounts of energy which have to be dissipated every time the brakes are applied, resulting in the attainment of extremely high working temperatures, typically well in excess of 1000°C. WO95/07418 discloses a friction pad composition suitable for this type of heavy duty braking service. It is also known from PCT/GB92/02055 (WO93/10370) to interpose a layer of a relatively soft elastomeric material between a layer of friction material and a backing plate in order to confer a degree of resilience onto the assembly, thereby enabling the working face of the friction lining to conform more uniformly to the confronting surface of the moving element of the system which is to be braked. The elastomer material should exhibit a Rockwell hardness value below that of the friction lining in order to ensure satisfactory conformability. In relatively moderate braking applications the technique disclosed in WO93/10370 has proved valuable in considerably extending brake element working life. Unfortunately, attempts to apply the concept of WO/93/10370 to severe duty braking applications of the kind addressed by WO95/07418 have not proved successful, due to the extremely high temperatures developed in use. According to the present invention a brake element comprising a friction material lining and a backing plate, together with a relatively soft layer of elastomeric material being interposed between said friction material lining and said backing plate, is characterised in that a further layer of elastomeric material of relatively low thermal conductivity and relatively high resistance to thermal degradation is interposed between the friction material lining and the relatively soft layer of elastomeric material. The further layer is preferably semi-conformable relative to the soft underlayer. That is to say it is more conformable than the friction layer, but less conformable than the underlayer. It has been found that the incorporation of the further elastomeric material layer with relatively poor thermal conductivity allied to high resistance to thermal degradation makes it possible to utilise the technique of WO93/10370 to very considerably enhance the useful service life of severe duty disk brake pads of the kind disclosed in WO95/07418. In a typical case the service life may be extended from an unacceptable 1.5 months to as much as 10 or 11 months before replacement is necessary. The further layer apparently serves to protect the soft elastomeric material layer from the heat generated in the top layer of friction material. The further layer is preferably formulated to have a thermal conductivity on the order of only 8-12% of that of the friction layer. This will normally be greater than that of the relatively soft elastomeric material layer. All three layers are preferably formulated so as to avoid or at least minimise interface shear stresses due to thermal expansion developed in response to the temperatures attained in braking service. This may be done by appropriate choice of materials such as fillers and/or metallic ingredients for the two elastomeric material layers. For practical purposes it has proved sufficient to formulate the elastomer material layers for a thermal expansion of the order of 0.01mm to 0.05mm, over a temperature range of, say, ambient to 200°C and determined in accordance with the test method described below, using a sample length of 8mm. Advantageously, the longitudinal and/or transverse grooving of WO95/07418 is employed, but with the grooves extending through both the friction material layer and through the further elastomeric material layer to the interface between the latter and the relatively soft elastomeric material layer. This has been found to enhance the conformability of the friction lining in use against the confronting moving surface of a brake disk. It is preferred that the friction material lining has a modulus of elasticity in the range 1000-2000 N/mm 2 , the further layer has a modulus of elasticity in the range 300-800 N/mm 2 and the relatively soft elastomeric material layer has a modulus of elasticity in the range 50-100 N/mm 2 , as determined for temperatures in the range from ambient to 200 β C. In the present context, modulus of elasticity is measured by applying a load of 312.5 N to a material sample of 25mm cube seven times, at a loading rate of 2mm/minute. The deflection is measured by an LVDT transducer and the modulus is calculated as maximum stress divided by total strain, (otherwise known as the secant modulus) . This test can be carried out at different temperatures if desired; the figures quoted herein are for the seventh loading cycle. In order that the invention be better understood a preferred embodiment of it will now be described by way of example with reference to the accompanying drawing. In the drawing the sole Figure is a schematic view of a disk brake pad constructed in accordance with the present invention. The drawing shows a metal backplate 2 and a friction layer 3. A relatively soft elastomeric material layer 5 is in contact with the backplate and is disposed between the latter and the friction material layer 3. A further layer of elastomeric material 4 is interposed between the layers 3 and 5. Transverse and longitudinal grooves 6, 7 are provided extending from the working face 1 of the friction material to the interface between the layers 4 and 5. In this case, the relatively soft elastomeric material layer 5 was formulated to exhibit a Rockwell penetration hardness at least twice that of the friction material. The further layer of elastomeric material 4 was formulated to have a thermal conductivity not much greater than that of the soft elastomeric material layer 5, but very much lower than that of the friction material layer 3. It was also formulated to have high resistance to thermal degradation. To further illustrate the invention, examples of typical formulations are given below, although it will be appreciated that these are examples only; those skilled in the art will realise that numerous permutations are possible without departing from the invention. All percentages are by volume. Friction material laver Phenolic resin 15% Metallic (copper) fibres 7% Other fibres (mineral fibres) 5% Friction modifiers 10% Fillers/load bearing agents 48% Graphite 15% Relatively soft elastomeric material laver Phenolic resin 9% SBR Rubber 44% Metallic (steel) fibres 11% Other fibres (organic) 4% Friction modifiers 4% Fillers (barytes) 25% Activators (sulpher etc) 3% Further elastomeric material layer Nitrile rubber 20% Metallic (steel) fibres 10% Abrasives 12% Friction modifier 4% Veπniculite 30% Fillers (barytes) 22% Activators (sulpher etc) 2% Processing of these mixes into the final disk brake pad product was carried out conventionally. The measured physical properties of the layers were as follows:- 1. Modulus of elasticity fN/mm 2 ) (determined by the method described earlier) Room Temperature 50 β C 100°C 200°C Friction layer 1700 1600 1200 1100 Further layer 500 450 400 300 Soft layer 80 70 70 70 2. Thermal expansion from room temperature to 200°C Friction layer 0.02mm Further layer 0.01mm Soft layer 0.03mm 3. Thermal conductivity iW/mlO Friction layer 8.455 Further layer 0.801 Soft layer 0.536 For present purposes, thermal conductivity was determined by testing a standard disk of material 1.6 inches (40.6mm) in diameter, 0.1875 inches (5mm) thick in the conventional Lees' conductivity apparatus. After measuring the thickness, the faces of the sample were coated with a thermally conductive paste prior to assembling three copper disks and two samples as a stack which was heated from one end over a period of at least six hours whilst monitoring the copper disk temperatures, and prior to calculating the conductivity, in the usual way. In service, a disk brake pad as just described exhibited good service life in terms of wear rate and absence of "hot spots" due to uneven pressure. It was highly conformable to an associated brake disk in use and it had excellent resistance to thermal damage, even under severe operating temperatures. The thermal expansion figures quoted in 2 above were determined by heating a sample 5mm by 8mm square and 5mm thick at controlled rate to 200°C. The change in the 8mm dimension was measured by a probe. The sample was then cooled at the same rate and the contraction measured. The test was repeated with a second sample, to determine an average value. The rate of heating/cooling was 5 β C per minute.

Claims: CLAIMS 1. A brake element comprising a friction material lining and a backing plate, together with a relatively soft layer of elastomeric material being interposed between said friction material lining and said backing plate, characterised in that a further layer of elastomeric material of relatively low thermal conductivity and relatively high resistance to thermal degradation is interposed between the friction material lining and the relatively soft layer of elastomeric material. 2. A brake element according to claim 1 having grooves extending transversely and/or longitudinally with respect to the major plane of the element, characterised in that said grooves extend through the friction material lining and through said further layer to the relatively soft elastomeric layer. 3. A brake element according to claim 1 or claim 2 characterised in that the thermal conductivity of the further layer is in the range of rom 8-12% of that of the friction material lining, but greater than that of the relatively soft layer. 4. A brake element according to any of claims 1-3 characterised in that the friction material lining and the elastomeric material layers are formulated to exhibit approximately the same amount of thermal expansion when heated from ambient temperature to 200 β C. 5. A brake element according to claim 4 wherein said expansion is in the range 0.01mm to 0.05mm, referred to a sample dimension of 8mm. 6. A brake element according to any preceding claim characterised in that the friction material lining has a modulus of elasticity in the range 1000-2000 N/mm 2 , the further elastomeric material layer has a modulus of elasticity in the range 300-800 N/mm 2 and the relatively soft elastomeric material layer has a modulus of elasticity in the range 50-100 N/mm 2 , as determined for temperatures in the range from ambient to 200°C. 7. A brake element substantially as herein described with reference to and as illustrated by the accompanying drawing.

Inventors: LOTFIPOUR MAHMOUD, LOTFIPOUR, MAHMOUD
Applicants: FERODO LTD, LOTFIPOUR MAHMOUD, FERODO LIMITED, LOTFIPOUR, MAHMOUD
Application year: 1997
Main classification: F16D69
IPC codes: F16D65, F16D69