Asbestos out: M1301 in new liner for '84
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Our engineering editor learns about Mintex's efforts to find a safe replacement material for brake linings ASBESTOS features in the news quite a lot because of its associated potential health hazards. One area more affected than most by any suggestions of a ban on the material is the brake lining industry. The major reason is that asbestos is a very useful material indeed for the engineer.
Mintex has for many years been heavily involved in the development of friction materials — asbestos based or otherwise — as I found when I talked to the company's product engineering manager, Bert Smales. One of the main problems facing the brake lining manufacturer is that the vehicle makes a number of demands upon the lining material and not all of them are compatible.
Basically, a brake lining has three separate tasks to perform. It must: • Generate a torque at the interface; • Transmit that torque mechanically into the body of the brake; • Survive the temperatures generated at the interface and assist in conducting them away.
In this context, while an average operating temperature is around 400°C (750°F) it should be noted that local hot spots can occur where the temperature can reach some 700°C (1,290°F).
Asbestos has played a substantial part in meeting these requirements. Not only can it withstand the temperatures, but it gives good resistance to wear and is a good reinforcing fibre.
The chief sources of raw asbestos are the Urals, South Africa and Canada, which supply the vast majority of material required for friction linings. (Asbestos has in fact been in use for a tong time — the Romans used asbestos strands for lamp wicks).
Linings have traditionally been made from asbestos, thermo-setting resins, fillers (which are used to increase the bulk, improve the thermal conductivity and structural strength) and friction modifiers. Asbestos is a good material from the production point of view as the fibres will readily "wet" when the resin is applii This gives a good adhes bond.
Asbestos is thus very useful the lining manufacturer — "it' God-given material for ul according to Mintex's indust chemist — and as such is vi difficult to replace. So why place it? The answer is that 1 use of asbestos represents major health hazard.
Regulations relating to the t of asbestos were first torn lated in 1932, but only recer has a major environmental pi. started. Most EEC countri work to a limit of two fibres asbestos per millilitre of measured over a four-ha period. This refers to white bestos — there is already a untary ban on the use of b asbestos. In January of this yl the UK standard was tighter up to one fibre/ml and there i proposed Health and Sat standard aimed at mid-1.c. which will halve this limit again.
These regulations apply to situations, but they have tb greatest effect where brake I ings are produced rather It they operate. The ba incentive remains the sar however — find an asbestos-f material which will equal or t ter the multi-functional perfot ance of asbestos.
Companies like Mintex are ways on the look-out for n materials, anyway, and Miii. started looking very hard to fi an alternative some six or se‘, years ago. But as Bert Sma explained: "If you want to move asbestos, you must real that it's not just a case of taki one ingredient out and putti another one in."
Work on asbestos-free () linings started out using ya based materials as used clutches. (Mintex being terested in friction materials general is obviously also hew involved in clutch linings). Th( were yarns impregnated vk. small metal particles; yarns still used but "with other t in," in the words of Bert Smal( Metallic wire has been usec a similar configuration to thai 3rillo pad along with inert ers, the compositions of ich tend to be trade secrets. is "Brillo pad" idea of mgthening material is also ng used to reinforce piston wns as mentioned in "From Drawing Board" (CM August or metallic linings the raw terials are bought by weight, the final product is sold by ume. Thus the metal-filled es tend to be expensive beIse of the amount of metal in m.
loving to an AF lining is not aggravation, however. Minhas found some advantages, even these are not clear-cut. general, it can improve the Ivy-duty wear characteristics he lining. It is also possible to rease the thermal conductivof the friction material and s reduce the temperature at interface, but this has a wback.
k high thermal conductivity is ter for drum brakes, but, with 3s, so much heat is conducted Dugh the brake that the hyattic fluid vaporises. This ans that with an hydraulic braked arrangement, some ii of insulating underlay is uired.
his underlay is an asbestos3., high-strength resin-based
terial designed to display•cific physical properties: ..ow thermal conductivity;
• Good bonding ability; • Resonance damping.
The latter property means that the underlay can be used to modulate the damping coefficient of the pad to help with reducing noise.
On the machining side, linings can be jig ground on the shoes without the health hazards associated with asbestos. It is still a dusty job so extraction equipment is still necessary, but no more so than in, say, a joinery shop.
Research into producing satisfactory AF linings is expensive and the raw materials tend to be dearer, too. This is where the weight vs volume argument crops up. It is all very well saying that Material X weighs only half as much as Material Y, but when you need four times as much of it . .
Manufacturers like Mintex have for years been geared up to produce asbestos-based linings, so any new change in material or production technology is bound to add to the cost.
While development costs are high, this applies to most commercial vehicle components. So
far as Mintex is concerned, this
is where the heavy truck side is at a disadvantage compared with the passenger car, because of the size of the components in question.
Sets of tyres and drums are expensive and it is no good trying to get anything homologated with worn drums. Having said that, Mintex does carry out a lot of development work on linings using worn drums, because after all, that is what the operator will be doing.
Mintex will be offering its M1 3 01 lining material for original equipment or for retrofit by the end of the year. It is a rigid-moulded material which has a medium friction value (a} in the range of 0.36 to 0.40 and will be available initially for Scam brakes only. Wedge brakes demand a heavier-duty material whereas 1301 is aimed initially for general-duty application.
Cost, however, is a problem; M1301 is about twice as expensive as an asbestos-based lining. The environment, however, has powerful friends and thus the asbestos-free lining is likely to become a legal requirement in the future. Certainly. Mintex would argue that it is well placed to satisfy future technical and marketing requirements in this environmentally sensitive area.